Regional patterns of Mesozoic-Cenozoic magmatism in western Alaska revealed by new U-Pb and 40Ar/39Ar ages

Bradley, Dwightbradleyorchard2@gmail.com, Miller, Marti L.- mlmiller@usgs.gov, Friedman, Richard M., Layer, Paul W. · 2017 · Report

In support of regional geologic framework studies, we obtained 50 new argon-40/argon-39 ( 40 Ar/ 39 Ar) ages and 33 new uranium-lead (U-Pb) ages from igneous

This 2017 document, Regional patterns of Mesozoic-Cenozoic magmatism in western Alaska revealed by new U-Pb and 40Ar/39Ar ages, is preserved in the Mountain Man Mining Library for research and reference. Original source: pubs.usgs.gov.

Professional Paper 1814-D U.S. Department of the Interior U.S. Geological Survey Studies by the U.S. Geological Survey in Alaska, Volume 15 Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages

Cover.  Photograph of the Tonzona pluton in the Alaska Range dated at 58 million years old in the present study (U.S. Geological Survey photograph by Dwight Bradley).

Studies by the U.S. Geological Survey in Alaska, Volume 15 Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages By Dwight C. Bradley, Marti L. Miller, Richard M. Friedman, Paul W. Layer, Heather A. Bleick, James V. Jones III, Steven E. Box, Susan M. Karl, Nora B. Shew, Timothy S. White, Alison B. Till, Julie A. Dumoulin, Thomas K. Bundtzen, Paul B. O'Sullivan, and Thomas D. Ullrich Professional Paper 1814-D U.S. Department of the Interior U.S. Geological Survey

U.S. Department of the Interior SALLY JEWELL, Secretary U.S. Geological Survey Suzette M. Kimball, Director U.S. Geological Survey, Reston, Virginia: 2017 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit https://www.usgs.gov/ or call 1-888-ASK-USGS (1-888-275-8747). For an overview of USGS information products, including maps, imagery, and publications, visit https://www.usgs.gov/pubprod/ or https://store.usgs.gov. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Suggested citation: Bradley, D.C., Miller, M.L., Friedman, R.M., Layer, P.W., Bleick, H.A., Jones, J.V., III, Box, S.E., Karl, S.M., Shew, N.B., White, T.S., Till, A.B., Dumoulin, J.A., Bundtzen, T.K., O'Sullivan, P.B., and Ullrich, T.D., 2017, Regional patterns of Mesozoic-Cenozoic magmatism in western Alaska revealed by new U-Pb and 40Ar/39Ar ages, in Dumoulin, J.A., ed., Studies by the U.S. Geological Survey in Alaska, vol. 15: U.S. Geological Survey Professional Paper 1814-D, 48 p., https://doi.org/10.3133/pp1814D. ISSN 2330-7102 (online)

Contents Abstract 1 Introduction 1 Geochronology 21 Candle Quadrangle 21 CA1. Granite Mountain Pluton, Candle B5 Quadrangle 21 Tanana Quadrangle 21 TN1. Gabbro in Rampart Group, Tanana B1 Quadrangle 21 Kantishna River Quadrangle 21 KH1. Sischu Igneous Complex, Kantishna River A6 Quadrangle 21 Medfra Quadrangle 21 MD1. Telida Pluton, Medfra C1 Quadrangle 21 Holy Cross Quadrangle 21 HC1. Volcanic Rocks of the Yetna River Area, Holy Cross D1 Quadrangle 21 Iditarod Quadrangle 21 ID1. Ruby Terrane Orthogneiss, Unit PzPg, Iditarod D4 Quadrangle 21 ID2. Gabbro in Dishna River Mafic-ultramafic Complex, Iditarod D3 Quadrangle 22 McGrath Quadrangle 22 MG1. Stock South of Lone Mountain, McGrath B4 Quadrangle 22 MG2 and MG3. Tephras in Sheep Creek Volcanic Field, McGrath B2 Quadrangle 22 MG4. Windy Fork Pluton, McGrath A3 Quadrangle 22 Talkeetna Quadrangle 23 TL1. Intrusion at BM Sheep, Talkeetna D5 Quadrangle 23 TL2. Tonzona Pluton, Talkeetna D5 Quadrangle 23 Sleetmute Quadrangle 23 SM1. Intrusion at Hill 1195, Sleetmute D4 Quadrangle 23 SM2. Aghaluk Stock, Sleetmute C7 Quadrangle 23 SM3. Intrusion at Hill 1662, Sleetmute C6 Quadrangle 23 SM4. Intrusion at Hill 1908, Sleetmute C6 Quadrangle 23 SM5. Intrusion in Oskawalik River Drainage, Sleetmute C5 Quadrangle 23 SM6. Henderson Stock, Sleetmute C5 Quadrangle 23 SM7. Tephra in Kuskokwim Group, Sleetmute C4 Quadrangle 24 SM8 and SM9. Barometer Pluton, Sleetmute C4 Quadrangle 24 SM10. Intrusion in Vreeland Creek Drainage, Sleetmute C4 Quadrangle (97AM81a) 24 SM11. Red Mountain Pluton, Sleetmute C4 Quadrangle 24 SM12. Felsite in Victoria Creek Headwaters, Sleetmute B8 Quadrangle 24 SM13 and SM14. Buckstock Pluton, Sleetmute B8 and B7 Quadrangles 24 SM15. Kaluvarawluk Igneous Complex, Sleetmute B7 Quadrangle 24 SM16. Intrusion Near Holokuk Mountain, Sleetmute B6 Quadrangle 25 SM17. Chuilnuk Pluton, Sleetmute B5 Quadrangle 25 SM18, TA1 and TA2. Timber Creek Pluton, Sleetmute A8 and Taylor Mountains D8 Quadrangles 25 SM19. Intrusion in Timber Creek Headwaters, Sleetmute A8 Quadrangle 25 SM20. Intrusion Near Hill 2639, Sleetmute A7 Quadrangle 25

SM21. Intrusion at Hill 1764, Sleetmute B7 Quadrangle 25 Lime Hills Quadrangle 25 LH1. Stock Near Windy Fork Pluton, Lime Hills D4 Quadrangle 25 LH2. Sill at Gagaryah Barite Deposit, Lime Hills D4 Quadrangle 26 Bethel Quadrangle 26 BH1. Gemuk Mountain Pluton, Bethel C1 Quadrangle 26 BH2. Unnamed Igneous Rocks in Tikchik Terrrane, Bethel A1 Quadrangle 26 BH3. Dike Near Shadow Bay, Bethel A1 Quadrangle 26 Taylor Mountains Quadrangle 26 TA3. Felsite in Gemuk Group, Taylor Mountains D8 Quadrangle 26 TA4. Flat Top Basalt, Taylor Mountains D8 Quadrangle 27 TA5. Dike at Hill 2164, Taylor Mountains D8 Quadrangle 27 TA6. Sill at Hill 1768, Taylor Mountains D8 Quadrangle 27 TA7. Intrusion at Hill 2240, Taylor Mountains D7 Quadrangle 27 TA8 and TA9. Sills in Kuskokwim Group, Taylor Mountains D7 Quadrangle 27 TA10. Little Taylor Pluton, Taylor Mountains D4 Quadrangle 27 TA11. Intrusion in Enatalik Creek drainage, Taylor Mountains C8 Quadrangle 28 TA12. Intrusion Near BM Tippy, Taylor Mountains C8 Quadrangle 28 TA13 and TA14. Sills in Kuskokwim Group, Taylor Mountains C1 Quadrangle 28 TA15 and TA16. Shotgun Hills Pluton, Taylor Mountains C6 and B6 Quadrangles 28 TA17. Intrusion South of Shotgun Airstrip, Taylor Mountains B6 Quadrangle 28 TA18 and TA19. Intrusions in Winchester Claim Area, Taylor Mountains B6 Quadrangle 28 TA20. Intrusion Near BM Reach, Taylor Mountains Quadrangle B5 29 TA21. Intrusion at Hill 1311, Taylor Mountains B4 Quadrangle 29 TA22. Dike at Hill 1272, Taylor Mountains B3 Quadrangle 29 TA23, TA24, and TA25. Dikes in Red Bluff Area, Taylor Mountains B1 Quadrangle 29 TA26. Intrusion Near Keefer Creek, Taylor Mountains B1 Quadrangle 29 TA27. Chaufchivak Pluton, Taylor Mountains A7 Quadrangle 29 TA28. Intrusion at Hill 1623, Taylor Mountains A6 Quadrangle 30 TA29. Intrusion Near Hill 1635, Taylor Mountains A6 Quadrangle 30 TA30 and TA31. Tikchik Mountain Pluton, Taylor Mountains A6 Quadrangle 30 TA32. Arrow Pluton, Taylor Mountains A5 Quadrangle 30 TA33. Dike Near Sleitat Mountain, Taylor Mountains A3 Quadrangle 30 TA34. Sleitat Pluton, Taylor Mountains A3 Quadrangle 30 TA35. Old Man Pluton, Taylor Mountains A3 Quadrangle 31 TA36. Intrusion at Hill 1004, Taylor Mountains A3 Quadrangle 31 TA37. Intrusion at Hill 1422, Taylor Mountains A2 Quadrangle 31 TA38. Rhyolite at Hill 1678, Taylor Mountains A1 Quadrangle 31 TA39. Intrusion at Hill 982, Taylor Mountains A1 Quadrangle 31 Lake Clark Quadrangle 31 LC1. Overlook Pluton, Lake Clark B8 Quadrangle 31 Goodnews Bay Quadrangle 31 GO1. Intrusion Near Jacksmith Creek, Goodnews Bay B8 Quadrangle 31 GO2. Dike at Island Mountain, Goodnews Bay B7 Quadrangle 31 Dillingham Quadrangle 32 DI1. Tikchik Narrows Pluton, Dillingham D7 Quadrangle 32

Implications for the Magmatic History of Western Alaska 32 Pennsylvanian 32 Triassic 32 Jurassic 32 Early Cretaceous 32 Mid-Cretaceous 32 Late Cretaceous 33 Paleocene and Early Eocene 33 Mid-Eocene to Pliocene 36 Acknowledgments 37 References Cited 37 Appendix 1.—Analytical Methods 44 U-Pb Zircon TIMS Analytical Methods, University of British Columbia 44 U-Pb Zircon SHRIMP-RG Analytical Methods, USGS-Stanford University 44 40Ar/39Ar Analytical Methods, University of Alaska, Fairbanks 45 40Ar/39Ar Analytical Methods, University of British Columbia 45 U-Pb Zircon LA-ICP-MS Analytical Methods, Apatite to Zircon, Inc 45 Data Modeling 46 Pb/U Fractionation Factor 46 Fractionation Factor Adjustment for Integrated alpha-damage 47 Common Pb Correction 47 Preferred Age 47 Appendix 2.—New Geochronology Data for Igneous-Rock Samples from Western Alaska 48 Figures

1.  Map of western Alaska showing key physiographic features and sample locations identified by number 3

2.  Simplified geologic map showing the context of samples in the Sleetmute, Taylor Mountains, Bethel, Dillingham, Lake Clark, and Goodnews Bay, Alaska 5

3.  Photograph of the Tonzona pluton in the Alaska Range (Talkeetna quadrangle) dated at 58 million years old in the present study 5

4.  Simplified geologic maps, derived from Wilson and others (2015), showing the context of sample locations in western Alaska 7

5.  Concordia diagrams for samples of zircon and monazite dated by uranium-lead (U-Pb) thermal ionization mass-spectrometry (TIMS) at the University of British Columbia 14

6.  Plots showing weighted average lead-206/uranium-238 (206Pb/238U) ages of zircon samples and individual zircon ages 15

7.  Plots of age spectra showing the results of incremental step heating of argon-40/argon-39 (40Ar/39Ar) samples 17

8.  Maps (A to N) showing the distribution of isotopic ages from igneous rocks in western Alaska in 2-million-year increments from 78 to 50 million years ago (Ma) 34

9.  Map showing Late Cretaceous paleogeographic elements and igneous rock samples with ages of 72-68 million years (Ma) in western Alaska 36 Table

1.  Summary of new geochronology data for igneous-rock samples from western Alaska 8

Studies by the U.S.Geological Survey in Alaska, volume 15 Edited by Julie A. Dumoulin U.S. Geological Survey Professional Paper 1814-D [Also see https://pubs.usgs.gov/AK_studies/] Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages By Dwight C. Bradley,1 Marti L. Miller,1 Richard M. Friedman,2 Paul W. Layer,3 Heather A. Bleick,4 James V. Jones III,1 Steven E. Box,1 Susan M. Karl,1 Nora B. Shew,1 Timothy S. White,1 Alison B. Till,1 Julie A. Dumoulin,1 Thomas K. Bundtzen,5 Paul B. O'Sullivan,6 and Thomas D. Ullrich2 Abstract In support of regional geologic framework studies, we obtained 50 new argon-40/argon-39 (40Ar/39Ar) ages and 33 new uranium-lead (U-Pb) ages from igneous rocks of southwestern Alaska. Most of the samples are from the Sleetmute and Taylor Mountains quadrangles; smaller collections or individual samples are from the Bethel, Candle, Dillingham, Goodnews Bay, Holy Cross, Iditarod, Kantishna River, Lake Clark, Lime Hills, McGrath, Medfra, Talkeetna, and Tanana quadrangles. A U-Pb zircon age of 317.7±0.6 million years (Ma) reveals the presence of Pennsylvanian intermediate igneous (probably volcanic) rocks in the Tikchik terrane, Bethel quadrangle. A U-Pb zircon age of 229.5±0.2 Ma from gabbro intruding the Rampart Group of the Angayucham-Tozitna terrane, Tanana quadrangle, confirms and tightens a previously cited Triassic age for this intrusive suite. A fresh mafic dike in Goodnews Bay quadrangle yielded a 40Ar/39Ar whole rock age of 155.0±1.9 Ma; this establishes a Jurassic or older age for the previously unconstrained (Paleozoic? to Mesozoic?) sandstone unit that it intrudes. A thick felsic tuff in the Gemuk Group in Taylor Mountains quadrangle yielded a U-Pb zircon age of 153.0±2.0 Ma, extending the age of magmatism in this part of the Togiak terrane back into the Late Jurassic. We report three new U-Pb zircon ages between 120 and 110 Ma—112.0±0.9 Ma from syenite in the Candle quadrangle, 114.9±0.3 Ma from orthogneiss assigned to the Ruby terrane in Iditarod quadrangle, and 116.6±0.1 Ma from a gabbro of the Dishna River mafic-ultramafic complex in Iditarod quadrangle. The latter result requires a substantial age revision, from Triassic to Cretaceous, for at least some rocks that have been mapped as the Dishna River mafic-ultramafic complex. A tuff in the Upper Cretaceous Kuskokwim Group yielded a U-Pb zircon (sensitive high-resolution ion microprobe, SHRIMP) age of 88.3±1.0 Ma; we speculate that the eruptive source was an arc along the trend of the Pebble porphyry copper deposit along the Gulf of Alaska continental margin. More than half of the new ages fall between 75 and 65 Ma, confirming the existence, based on conventional potassium-argon (K-Ar) ages, of a 70-Ma igneous flare-up across southwestern Alaska. Our new ages hint that during this pulse, the locus of magmatism shifted toward the Gulf of Alaska, that is, toward a more outboard position. This shift is consistent with the hypothesis that magmatism was the product of rollback of a subducted slab, which at that time would have been the Resurrection Plate. Intrusive rocks in the Taylor Mountains and Sleetmute quadrangles in the age range of 63 to 59 Ma were emplaced shortly before the onset of ridge subduction as dated by near-trench plutons in the adjacent part of the Chugach accretionary complex. Southwestern Alaska at this time would have been positioned above a very young subducted slab belonging to the Resurrection Plate; magmas, in this scenario, were generated near the edge of the slab window related to ridge subduction. A 56.3±0.2 Ma granite in Taylor Mountains quadrangle and a 54.7±0.7 Ma ashfall tuff in McGrath quadrangle were likely emplaced above the Resurrection-Kula slab window, which by this time is inferred to have entered the region. Another ashfall tuff in McGrath quadrangle, at 42.8±0.5 Ma, likely belongs to the Meshik Arc, the product of renewed subduction after inferred passage of the slab window. A 49.0±0.3-Ma rhyolite in Taylor Mountains quadrangle is about the age of the transition from slab window to renewed subduction. Two plutons in the western Alaska Range, at 31.8±0.4 and 30.9±0.6 Ma, belong to a suite of gabbro to peralkaline granite of unknown origin. Finally, a 4.6±0.1-Ma basalt from a flow in Taylor Mountains quadrangle belongs to the Neogene basaltic province of western Alaska. These rocks were erupted in a distal retroarc setting; the cause of magmatism is unknown. Introduction In this paper we report 50 new argon-40/argon-39 (40Ar/39Ar) ages and 33 new uranium-lead (U-Pb) ages from Late Paleozoic, Mesozoic, and Cenozoic igneous rocks of western Alaska (fig. 1). Most of the samples were collected 1U.S. Geological Survey. 2Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, BC. 3Department of Geology and Geophysics, University of Alaska, Fairbanks, AK. 4Deceased; former U.S. Geological Survey. 5Pacific Rim Geological Consulting, Inc. 6Apatite to Zircon, Inc.

2    Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages CANDLE MELOZITNA NORTON BAY NULATO RUBY UNALAKLEET OPHIR MT MCKINLEY MEDFRA HOLY CROSS IDITAROD TALKEETNA MCGRATH RUSSIAN MISSION SLEETMUTE LIME HILLS BETHEL TAYLOR MTS LAKE CLARK DILLINGHAM GOODNEWS BAY ILIAMNA TANANA KANTISHNA RIVER 150° 153° 156° 159° 162° 66° 65° 64° 62° 63° 61° 60° 59° Fig. 4A Fig. 4B Fig. 4C Fig. 4F Fig. 4G Fig. 4H Fig. 4I Fig. 4K Fig. 2 Fig. 4J Fig. 4D Fig. 4E A L A S K A A L A S K A CA1 TN1 MD1 HC1 ID1 ID2 TL1 TL2 SM1 SM7 SM11 SM14 SM16 SM19 SM20 BH1 BH2 BH3 TA5 TA9 TA12 TA14 TA16 TA17 TA21 TA22 TA26 TA36 TA38 TA39 GO2 KH1 MG2 MG3 SM2 SM3 SM4 SM5 SM6 SM8 SM9 SM10 SM12 SM13 SM15 SM17 SM18 SM21 TA1 TA2 TA3 TA4 TA6 TA7 TA8 TA10 TA11 TA13 TA15 TA18 TA19 TA20 TA23 TA24 TA25 TA27 TA29 TA30 TA31 TA32 TA33 TA34 TA35 TA37 GO1 DI1 MG1 MG4 LH1 LH2 TA28 100 KILOMETERS 60 MILES ALASKA Map area U-Pb zircon Locations of geochronology samples EXPLANATION 40Ar/39Ar hornblende 40Ar/39Ar whole rock 40Ar/39Ar biotite 40Ar/39Ar muscovite

Introduction    3 Figure 1.  Map of western Alaska showing key physiographic features and sample locations identified by number. The shaded relief was generated from the U.S. Geological Survey National Elevation Dataset (NED) 2-arc-second Digital Elevation Model (DEM). The black boxes indicate the areas of figures 2 and 4A-I. The gray grid shows the outlines of 1:250,000-scale quadrangles. The samples are identified by a two-letter abbreviation for the 1:250,000-scale quadrangle, followed by a number. Abbreviations for quadrangles are as follows: BH, Bethel; CA, Candle; DI, Dillingham; GO, Goodnews Bay; HC, Holy Cross; ID, Iditarod; KH, Kantishna River; LC, Lake Clark; LH, Lime Hills; MD, Medfra; MG, McGrath; SM, Sleetmute; TA, Taylor Mountains; TL, Talkeetna; TN, Tanana. Within a given 1:250,000-scale quadrangle, sample locations are numbered from west to east in the D-tier of 1:63,360-scale quadrangles (that is, the most northerly tier), then the C-tier, B-tier, and A-tier. 40Ar/39Ar, argon-40/argon-39; U-Pb , uranium-lead. in the course of geologic mapping by the U.S. Geological Survey (USGS) of what had been the largest unmapped swath in the United States—the Sleetmute and Taylor Mountains 1:250,000-scale quadrangles (fig. 2). Samples were collected in the Sleetmute quadrangle in 1993, 1994, 1997, 1998, and 1999 and were dated during that era by 40Ar/39Ar. In the Taylor Mountains quadrangle, samples were collected in 2004, 2005, 2006, and 2008; these were dated by either 40Ar/39Ar or U-Pb, or in a few cases both. The rest of the samples are from miscellaneous smaller collections from the Bethel, Candle, Dillingham, Goodnews Bay, Holy Cross, Iditarod, Kantishna River, Lake Clark, Lime Hills, McGrath, Medfra, Talkeetna, and Tanana quadrangles (fig. 3 and 4A-K). Samples from the Tanana, Bethel, and Taylor Mountains quadrangles provide new age constraints for magmatism in rocks assigned to the Angayucham-Tozitna, Tikchik, and Togiak terranes, respectively. All of the other samples are from magmatic episodes that postdate the Paleozoic and Mesozoic assembly of these and other Alaskan terranes, including the Farewell, Kilbuck, and Yukon-Tanana. Table 1 summarizes the geochronological results and identifies the analytical lab for each sample. The data reported here were obtained at four different labs. U-Pb zircon ages were obtained by thermal ionization mass-spectrometry (TIMS) at University of British Columbia (fig. 5; appendix 2, table A1) and by secondary ion mass-spectrometry (SIMS) at the USGS-Stanford sensitive high-resolution ion microprobereverse geometry (SHRIMP-RG) facility at Stanford University (fig. 6; appendix 2, table A2). Almost all of the 40Ar/39Ar analyses (fig. 7; appendix 2, table A3) were done at the University of Alaska Fairbanks. A single 40Ar/39Ar analysis (sample MG2: 01ADw7a) was performed at the University of British Columbia (fig. 7; appendix 2, table A4). Analytical methods are detailed in appendix 1. Each sample is identified using two schemes. The USGS field number uniquely identifies the year, project area, geologist, and station number. Thus, sample 85AM16a was collected in 1985, in Alaska, by Marti Miller, and the station was Miller's 16th. Any trailing letters a, b, c, and so on correspond to different rock types from that location. We also use a simplified scheme organized by quadrangle, wherein samples are identified by a two-letter abbreviation (fig. 1, caption) followed by numbers 1, 2, 3, and so on for each sample in the quadrangle. For example, sample ID2 is the second sample in Iditarod quadrangle. Petrographic descriptions are based on thin section analysis unless specifically noted. For relating isotopic ages to stratigraphy, we use the time scale of Gradstein and others (2012). Obscure geographic features that are mentioned in the writeups of individual samples (for example hill 1632 or BM Sheep) can be found by referring to the corresponding 1:63,360-scale USGS topographic map, given in table 1.

4    Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages

IDITAROD HOLY CROSS D8 D7 D6 D5 D4 D3 D2 D1 B8 B7 B6 B5 B4 B3 B2 B1 A8 A7 A6 A5 A4 A3 A2 A1 SLEETMUTE LIME HILLS BETHEL TAYLOR MOUNTAINS LAKE CLARK DILLINGHAM

RUSSIAN MISSION MCGRATH GOOD-NEWS BAY SM1 SM7 SM11 SM14 SM16 SM19 SM20 BH1 BH2 BH3 TA5 TA9 TA12 TA14 TA16 TA17 TA21 TA22 TA26 TA36 TA38 TA39 SM2 SM3 SM4 SM5 SM6 SM8 SM9 SM10 SM12 SM13 SM15 SM17 SM18 SM21 TA1 TA2 TA3 TA4 TA6 TA7 TA8 TA10 TA11 TA13 TA15 TA18 TA19 TA20 TA23 TA24 TA25 TA27 TA29 TA28 TA30 TA31 TA32 TA33 TA34 TA35 TA37 DI1 MULCHATNA FAULT DENALI FAULT 156° 159° 62° 61° 60° 50 KILOMETERS 30 MILES Unconsolidated sediments (Quaternary) Water Other rocks, undifferentiated Fault—long dash where approximate; short dash where concealed Thrust fault—long dash where approximate; short dash where concealed; sawteeth on upper plate Volcanic and intrusive rocks (Cret.-Tertiary) Kuskokwim Group (Cretaceous) U-Pb zircon Locations of geochronology samples EXPLANATION 40Ar/40Ar hornblende 40Ar/40Ar whole rock 40Ar/40Ar biotite 40Ar/39Ar muscovite

Introduction    5 Figure 2.  Simplified geologic map showing the context of samples in the Sleetmute, Taylor Mountains, Bethel, Dillingham, Lake Clark, and Goodnews Bay, Alaska, 1:250,000-scale quadrangles, the boundaries of which are bold gray lines. A grid of thin gray lines shows the outlines of 1:63,360-scale quadrangles. These are labeled A1 to D8 for regions south of 62° N latitude (as shown for Taylor Mountains quadrangle), or A1 to D6 north of that line. Geology derived from Wilson and others (2015). Faults are shown by solid or dashed lines. 40Ar/39Ar, argon-40/argon-39; U-Pb , uranium-lead. Figure 3.  Photograph of the Tonzona pluton in the Alaska Range (Talkeetna quadrangle) dated at 58 million years old in the present study (U.S. Geological Survey photograph by Dwight Bradley).

6    Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages 63.5° 63.75° 62.75° 63° 65.25° 65.25° 62.5° 62.75° 63° 64.25° 64° 153.5° 154° 161.5° 161° 150.5° 150° 65.5° 65.5° 62.25° 153.5° 152.5° 153° 153.5° 153° 159.5° 159° 157.5° 158° 154.5° 155° 62.5° MD1 HC1 MG2 MG3 ID2 ID1 MG1 CA1 TN1 KH1 Granite Mountain pluton Sischu pluton Sischu volcanic field Telida pluton Yetna volcanic field Ruby terrane Kuskokwim Basin Kuskokwim Basin Farewell terrane Farewell terrane Farewell terrane Manley Basin Koyukuk Basin Dishna River complex Sheep Creek volcanic field Lone Mountain pluton DENALI SYSTEM FAULT A B D E F G H CANDLE B5 RUBY A1 MEDFRA D1 MT. MCKINLEY D6 MEDFRA C1 MT. MCKINLEY C6 MEDFRA B1 MT. MCKINLEY B6 KANTISHNA RIVER A6 HOLY CROSS D1 MCGRATH B4 MCGRATH B5 MCGRATH B2 MCGRATH B1 MCGRATH B3 MCGRATH C3 MCGRATH C4 MCGRATH C5 HOLY CROSS D2 IDITAROD D6 IDITAROD D3 IDITAROD D4 RUBY A4 RUBY A3 CANDLE B6 CANDLE B4 TANANA B1 LIVENGOOD B6 TANANA B2 TANANA C1 TANANA C2 CANDLE C5 CANDLE C4

Introduction    7 62° 59.25° 59.5° 62.75° 63° 152.5° 152° 161.5° 154.5° 154° MG4 LH1 LH2 GO2 GO1 TL1 TL2 Farewell terrane Windy Fork pluton Stock near Windy Fork pluton Farewell terrane Kilbuck terrane BERING SEA Tonzona pluton DENALI SYSTEM FAULT Metasedimentary rocks of the Ruby terrane (mainly Paleozoic) Unconsolidated sediments (Quaternary) Water EXPLANATION Sedimentary rocks of the Farewell and Livengood terranes, and probable correlatives (Neoproterozoic to Jurassic) Volcanic rocks (Quaternary) Volcanic rocks (Cretaceous-Tertiary) Granitic intrusive rocks (Cretaceous-Tertiary) Intermediate intrusive rocks (Cretaceous-Tertiary) Mafic and ultramafic intrusive rocks (mainly Triassic and Jurassic; locally Early Cretaceous) Turbidites (Cretaceous) Nonmarine sediments and sedimentary rocks (Tertiary) Undivided rocks, mainly sedimentary (Paleozoic) Metasedimentary and metaigneous "basement" rocks of the Farewell terrane (Mesoproterozoic to Paleozoic) Metasedimentary and metaigneous rocks of the Kilbuck terrane (Paleoproterozoic to Neoproterozoic) Metasedimentary rocks of the Yukon-Tanana terrane (mainly Paleozoic) Sedimentary and igneous rocks of the Pingston, Tozitna, Innoko, Goodnews, and Togiak terranes (late Paleozoic and Triassic) Volcanic rocks (Permian) Ice U-Pb zircon Locations of geochronology samples: 40Ar/39Ar hornblende 40Ar/39Ar whole rock 40Ar/39Ar biotite Fault—dashed where approximate Thrust fault—dashed where approximate; sawteeth on upper plate J K MCGRATH A2 LIME HILLS D3 LIME HILLS D2 LIME HILLS D4 TALKEETNA D6 TALKEETNA D5 TALKEETNA C5 GOODNEWS BAY B7 GOODNEWS BAY B8 GOODNEWS BAY C7 GOODNEWS BAY C8 TALKEETNA C6 MCGRATH A3 Figure 4.  Simplified geologic maps, derived from Wilson and others (2015), showing the context of sample locations in western Alaska. A, Granite Mountain area in Candle quadrangle, from Patton and others (2009). B, Yukon River corridor in eastern Tanana quadrangle, from Wilson and others (1998). C, Sischu Mountain area in the four corners of Kantishna River, Ruby, Medfra, and Mt. McKinley quadrangles, from Wilson and others (1998) after Chapman and others (1975) and Patton and others (1980). D, Telida pluton area, Medfra quadrangle, from Wilson and others (1998) after Patton and others (1980). E, Yetna volcanic field in Holy Cross and Iditarod quadrangles, from Patton and others (2006) and Miller and Bundtzen (1994). F, Part of the Iditarod quadrangle, from Wilson and others (1998) after Miller and Bundtzen (1994). G, Lone Mountain area, McGrath quadrangle, from Wilson and others (1998). H, Sheep Creek Volcanic Field, McGrath quadrangle, from Wilson and others (1998) after Bundtzen and others (1997). I, Part of the western Alaska Range in Lime Hills and McGrath quadrangles, from Wilson and others (1998). J, Tonzona pluton in Talkeetna quadrangle, after Wilson and others (1998) after Reed and Nelson (1980). K, A part of the Goodnews Bay quadrangle, from Wilson and others (2013) after Hoare and Coonrad (1978). 40Ar/39Ar, argon-40/argon-39; U-Pb , uranium-lead.

8    Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages Table 1.  Summary of new geochronology data for igneous-rock samples from western Alaska. [Abbreviations: SHRIMP-RG—Sensitive high-resolution ion microprobe, reverse geometry at Stanford University; UBC, TIMS—University of British Columbia, thermal ionization mass spectrometry; UAF—Uni­ versity of Alaska Fairbanks; UBC, argon lab—University of British Columbia argon lab; A to Z LAICPMS—Apatite to Zircon, Inc., laser ablation inductively coupled plasma mass spectrometry; U-Pb, uranium-lead; 40Ar/39Ar, argon-40/argon-39; 206Pb/238U, uranium-206/uranium-238; %, percent; MSDW, mean square of weighted deviates; Mtns., Mountains] Map no. Sample No. Location Latitude Longitude Rock unit dated Rock type Age (Ma) Error Method Mineral Lab Notes CA1 09ADw2 Candle B5 -161.2358 Granite Mountain pluton Syenite U-Pb Zircon SHRIMP RG Weighted average of 10 206Pb/238U ages. TN1 03ATi22a Tanana B1 -150.2414 Gabbro in Rampart Group Gabbro U-Pb Zircon UBC, TIMS Weighted average of 4 overlap­ ping, concordant 206Pb/238U ages. KH1 97ADw126 Kantishna River A6 -152.9778 Sischu igneous complex Granite 40Ar/39Ar Hornblende UAF Weighted averages of two plateau ages. Hornblende #1, 9 fractions, 86% 39Ar released, MSWD=1.7. Hornblende #2 (not illustrated in fig. 7), 9 fractions, 86% 39Ar released, MSWD=1.5. MD1 97ADw118 Medfra C1 -153.1783 Telida pluton Granite 40Ar/39Ar Biotite UAF Plateau age, 6 fractions, 81% 39Ar released, MSWD=4.5. Slight hump, perhaps due to recoil effects. HC1 85AM16a Holy Cross D1 -159.1275 Volcanic rocks of the Yetna River area Rhyolite U-Pb Zircon SHRIMP RG Weighted average of 10 overlapping, concordant 206Pb/238U ages; 3 analyses rejected. ID1 84AM279b Iditarod D4 -157.6517 Ruby terrane orthogneiss Orthogneiss U-Pb Zircon UBC, TIMS 206Pb/238U age of a single concor­ dant zircon. ID2 85AM87c Iditarod D3 -157.4456 Gabbro in Dishna River mafic-ultramafic complex Gabbro U-Pb Zircon UBC, TIMS Weighted average of four concor­ dant, overlapping fractions. MG1 11ADw109a McGrath B4 -154.7537 Satellite of Lone pluton Granite U-Pb Zircon SHRIMP RG Weighted average of 8 206Pb/238U ages. MG2 01ADw7a McGrath B2 -153.8294 Tephra in Sheep Creek volcanic field Felsic tuff U-Pb Zircon UBC, TIMS Age is a lower intercept defined by one concordant and three slightly discordant zircons. MSDW=0.96. MG2 01ADw7a McGrath B2 -153.8294 Tephra in Sheep Creek volcanic field Felsic tuff 40Ar/39Ar Hornblende UBC, Argon lab Plateau age, 12 fractions, 100% of 39Ar released, MSDW=0.58. MG3 01ADw8a McGrath B2 -153.8517 Tephra in Sheep Creek volcanic field Felsic tuff U-Pb Zircon SHRIMP RG Weighted average of 8 206Pb/238U ages. MG4 11ADw117a McGrath A3 -154.0945 Windy Fork pluton Granite U-Pb Zircon SHRIMP RG Weighted average of 7 of 8 206Pb/238U ages. TL1 03AM10a Talkeetna D5 -152.4281 Intrusion at BM Sheep Granodiorite 40Ar/39Ar Hornblende UAF Plateau age, 4 fractions, 58% 39Ar released, MSWD=1.2.

Introduction    9 Table 1.—Continued Map no. Sample No. Location Latitude Longitude Rock unit dated Rock type Age (Ma) Error Method Mineral Lab Notes TL1 03AM10a Talkeetna D5 -152.4281 Intrusion at BM Sheep Granodiorite 40Ar/39Ar Biotite UAF Plateau age, 8 fractions, 90% 39Ar released, MSWD=4.3. TL2 03AM3a Talkeetna D5 -152.1901 Tonzona pluton Granite U-Pb Zircon and monazite UBC, TIMS Weighted average of three zircon 206Pb/238U dates and two mona­ zite 207Pb/235U dates. SM1 97AM130a Sleetmute D4 -157.2369 Intrusion at hill 1195 Granite porphyry 40Ar/39Ar Biotite UAF Plateau age, 11 fractions, 97% 39Ar released, MSWD=0.8. SM2 97AM40b Sleetmute -158.2622 Aghaluk stock Granite porphyry 40Ar/39Ar Biotite UAF Plateau age, 13 fractions, 99% 39Ar released, MSWD=1.1. SM3 97AM61a Sleetmute -158.0797 Intrusion at hill 1662 Quartz diorite 40Ar/39Ar Biotite UAF Plateau age, 9 fractions, 90% 39Ar released, MSWD=1.7. SM4 98AM62a Sleetmute -158.0511 Intrusion at hill 1908 Granite porphyry 40Ar/39Ar Biotite UAF Plateau age, 13 fractions, 98% 39Ar released, MSWD=0.4. SM5 98AM192a Sleetmute -157.6400 Intrusion in Oskawalik River drainage Granite porphyry 40Ar/39Ar Biotite UAF Plateau age, 5 fractions, 59% 39Ar released, MSWD=0.9. SM6 97AM139a Sleetmute -157.7097 Henderson stock Monzonite 40Ar/39Ar Biotite UAF Plateau age, 6 fractions, 80% 39Ar released, MSWD=1.0. SM7 98ADw62e Sleetmute -157.2308 Tephra in Kuskokwim Group Tuff U-Pb Zircon SHRIMP RG Weighted average of 7 of 10 overlapping 206Pb/238U dates. SM8 98AM274a Sleetmute -157.3758 Barometer pluton Granite to granodiorite 40Ar/39Ar Biotite UAF Plateau age, 11 fractions, 85% 39Ar released, MSWD=1.2. SM9 97RJ062 Sleetmute -157.3264 Barometer pluton Granite to granodiorite 40Ar/39Ar Biotite UAF Plateau age, 9 fractions, 90% 39Ar released. SM10 97AM81a Sleetmute -157.3767 Intrusion in Vreeland Creek drainage Granite porphyry 40Ar/39Ar Biotite UAF Plateau age, 6 fractions, 52% 39Ar released, MSWD=2.2. SM11 97ARJ57 Sleetmute -157.2644 Red Mountain pluton Andesite 40Ar/39Ar Biotite UAF Plateau age, 4 fractions, 52% 39Ar released, MSWD=3.1. SM12 99AM315a Sleetmute B8 -158.6983 Felsite in Victoria Creek headwaters Felsite (max age) 40Ar/39Ar Biotite UAF Pseudo-plateau age, 5 frac­ tions, 30% 39Ar released, MSWD=0.7. SM13 99AM390a Sleetmute B8 -158.6661 Buckstock pluton Granite 40Ar/39Ar Biotite UAF Plateau age, 9 fractions, 93% 39Ar released, MSWD=0.8. SM14 93AM46d Sleetmute B7 -158.5858 Buckstock pluton Granite 40Ar/39Ar Biotite UAF Plateau age, 9 fractions, 83% 39Ar released, MSWD=0.8. SM15 99AM450a Sleetmute B7 -158.2600 Kaluvarawluk igneous complex Granite porphyry 40Ar/39Ar Biotite UAF Plateau age, 11 fractions, 89% 39Ar released, MSWD=0.4. SM16 98ARJ102 Sleetmute B6 -158.1431 Intrusion near Holukuk Moountain Felsite 40Ar/39Ar White mica UAF Plateau age, 10 fractions, 99% 39Ar released, MSWD=0.8. SM17 98ADw42a Sleetmute B5 -157.6780 Chuilnuk pluton Granite U-Pb Zircon SHRIMP RG Weighted average of 11 overlap­ ping 206Pb/238U dates. SM17 98ADw42a Sleetmute B5 -157.6780 Chuilnuk pluton Granite 40Ar/39Ar Biotite UAF Plateau age, 9 fractions, 86% 39Ar released, MSWD=1.9.

10    Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages Map no. Sample No. Location Latitude Longitude Rock unit dated Rock type Age (Ma) Error Method Mineral Lab Notes SM18 93AM94a Sleetmute A8 -158.9406 Timber Creek pluton Granite porphyry 40Ar/39Ar Biotite UAF Plateau age, 6 fractions, 71% 39Ar released, MSWD=0.5. SM19 99BT187 Sleetmute A8 -158.8300 Intrusion in Timber Creek headwaters Px-hb intrusive 40Ar/39Ar Hornblende UAF Pseudo-plateau age, 5 fractions, 79% 39Ar release MSWD=4.3. SM20 94AWK353b Sleetmute A7 -158.4181 Intrusion near hill 2639 Px-hb intrusive 40Ar/39Ar Hornblende UAF 4 fractions 62% 39Ar release MSWD=2.5. SM21 94AM309a Sleetmute B7 -156.0314 Intrusion at hill 1764 Granite porphyry 40Ar/39Ar White mica UAF 10 fractions 65% 39Ar release MSWD=0.3. LH1 11ADw115a Lime Hills D3 -153.9439 Stock near Windy Fork pluton Granite U-Pb Zircon SHRIMP RG Weighted average of 7 of 9 overlapping 206Pb/238U dates. LH2 11AD1a Lime Hills D4 -154.4622 Sill at Gagaryah deposit Intermediate dike U-Pb Zircon A to Z, LAICPMS Weighted average of 5 overlapping 206Pb/238U dates. BH1 05AM126a Bethel C1 -159.0119 Gemuk Mountain pluton Quartz monzonite U-Pb Zircon UBC, TIMS Concordia age based on four concordant, overlapping zircon analyses. MSWD (of concordance)=0.47; probability (of concordance)=0.50. BH2 08AM612a Bethel A1 -159.1193 Unnamed igneous rocks in Tikchik Complex Intermediate volcanic rock U-Pb Zircon UBC, TIMS Weighted average of four concor­ dant, overlapping 206Pb/238U dates. BH3 08AM615c Bethel A1 -159.1764 Dike at Shadow Bay Andesite 40Ar/39Ar Hornblende UAF Loss spectra; no plateau. Iso­ chron age (preferred) is 73.8 ± 1.2, 40Ar/36Ari=277.3 ± 2.4, N=10, MSWD=1.19. TA1 05BT148 Taylor Mtns. D8 -158.9359 Timber Creek pluton Granite porphyry U-Pb Zircon SHRIMP RG Weighted average of 9 overlapping 206Pb/238U dates. TA2 05AM247a Taylor Mtns. D8 -158.9533 Timber Creek pluton Granite porphyry 40Ar/39Ar Biotite UAF Plateau age, 9 fractions, 92% 39Ar released, MSWD=2.7. TA3 05ADw158a Taylor Mtns. D8 -158.9432 Felsite in Gemuk Group Felsic tuff U-Pb Zircon SHRIMP RG Weighted average of 9 of 11 overlapping 206Pb/238U dates. TA4 05AM236a Taylor Mtns. D8 -158.8476 Flat Top basalt Basalt 40Ar/39Ar Whole rock UAF Plateau age, 5 fractions, 93% 39Ar released, MSWD=1.4. TA5 05ADw29c Taylor Mtns. D8 -158.7453 Dike at hill 2164 Intermediate intrusive 40Ar/ 39Ar isochron Hornblende UAF Isochron age (preferred) is 69.6 ± 0.3, 40Ar/36Ari=281.3 ± 3.7, N=11, MSWD=1.51. Sample does not meet criteria for plateau. TA6 05AM239a Taylor Mtns. D8 -158.8699 Sill at hill 1768 Intermediate intrusive 40Ar/39Ar Biotite UAF Plateau age, 7 fractions, 74% 39Ar released, MSWD=0.9. Table 1.—Continued

Introduction    11 Map no. Sample No. Location Latitude Longitude Rock unit dated Rock type Age (Ma) Error Method Mineral Lab Notes TA7 05AM224c Taylor Mtns. D7 -158.5767 Intrusion at hill 2240 Quartz monzonite 40Ar/39Ar Biotite UAF Sample does not meet criteria for plateau. Quoted age is based on the weighted mean of the six highest-temperature steps. TA8 94ADw59b Taylor Mtns. D7 -158.4538 Sill in Kuskokwim Group Intermediate intrusive 40Ar/39Ar Whole rock UAF Plateau age, 4 fractions, 76% 39Ar released, MSWD=2.3. TA9 05AM215a Taylor Mtns. D7 -158.4517 Sill in Kuskokwim Group Intermediate intrusive 40Ar/39Ar Hornblende UAF Isochron age, 40Ar/36Ari=285.4 ± 1.5, N=12, MSWD=2.16 (preferred over plateau age). TA10 04AM45a Taylor Mtns. D4 -157.2393 Little Taylor pluton Granite porphyry U-Pb Zircon UBC, TIMS Mean of 3 concordant and overlapping 206Pb/238U ages. TA11 05AM144d Bi#2 Taylor Mtns. -158.7407 Intrusion in Enatalik Creek drainage Granite 40Ar/39Ar Biotite UAF Plateau age, 10 fractions, 98% 39Ar released, MSWD=0.7. A second biotite split did not yield a plateau age. TA12 08AM611a Taylor Mtns. -158.7456 Intrusion near BM Tippy Granite porphyry U-Pb Zircon SHRIMP RG Weighted average of 9 of 10 overlapping 206Pb/238U dates. TA13 05AM115a Taylor Mtns. -156.1376 Sill in Kuskokwim Group Granite porphyry 40Ar/39Ar White mica UAF Plateau age, 12 fractions, 99% 39Ar released, MSWD=0.2 TA14 05ADw17a Taylor Mtns. -156.1220 Sill in Kuskokwim Group Granodiorite U-Pb Zircon SHRIMP RG Weighted average of 9 of 10 overlapping 206Pb/238U dates. TA15 05ADw59c Taylor Mtns. -158.0222 Shotgun Hills pluton Granodiorite 40Ar/39Ar Biotite UAF Plateau age, 6 fractions, 85.7% 39Ar released, MSWD=1.03 TA16 05AM153a Taylor Mtns. B6 -158.0215 Shotgun Hills pluton Granodiorite U-Pb Zircon SHRIMP RG Weighted average of 9 of 10 overlapping 206Pb/238U dates. TA17 05AM154a Taylor Mtns. B6 -158.0679 Intrusion south of Shotgun airstrip Granite porphyry 40Ar/39Ar Biotite UAF Plateau age, 12 fractions, 98% 39Ar released, MSWD=1.6. TA18 08AM630a Taylor Mtns. B6 -158.1642 Intrusion in Winchester claim area Granite porphyry U-Pb Zircon SHRIMP RG Weighted average of 8 overlapping 206Pb/238U dates. TA19 08SB129b Taylor Mtns. B6 -158.1764 Intrusion in Winchester claim area Gabbro 40Ar/39Ar Hornblende UAF Isochron age, 40Ar/36Ari=293.1 ± 2.7, N=12, MSWD=3.40 (preferred over plateau age). TA20 08AM654b Taylor Mtns. B5 -157.7768 Intrusion near BM Reach Granite porphyry U-Pb Zircon SHRIMP RG Weighted average of 12 206Pb/238U dates. TA21 06ADW537a Taylor Mtns. B5 -157.4571 Intrusion at hill 1311 Granite porphyry 40Ar/39Ar plateau Muscovite UAF Plateau age, 6 fractions, 97% 39Ar released, MSWD=0.21. TA22 08SK290a Taylor Mtns. B3 -156.8417 Dike at hill 1272 Granite or granodiorite U-Pb Zircon SHRIMP RG Weighted average of 4 of 8 206Pb/238U dates (not reliable) TA23 08SK316a Taylor Mtns. B1 -156.0143 Dike in Red Bluff area Granite porphyry U-Pb Zircon SHRIMP RG Weighted average of 7 of 9 overlapping 206Pb/238U dates. Table 1.—Continued

12    Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages Map no. Sample No. Location Latitude Longitude Rock unit dated Rock type Age (Ma) Error Method Mineral Lab Notes TA24 06AK42 Taylor Mtns. B1 -156.0439 Dike in Red Bluff area Granite porphyry 40Ar/39Ar Muscovite UAF Plateau age, 8 fractions, 96.4% 39Ar released, MSWD=0.96. TA25 06AM447a Taylor Mtns. B1 -156.0099 Dike in Red Bluff area Granite porphyry 40Ar/39Ar plateau White mica UAF Plateau age, 9 fractions, 96% 39Ar released, MSWD=0.1. TA26 06AM453a Taylor Mtns. B1 -156.0835 Intrusion near Keefer Creek Gabbro U-Pb Zircon SHRIMP RG Weighted average of 7 of 8 overlapping 206Pb/238U dates. TA27 08AM614a Taylor Mtns. A7 -158.5323 Chaufchivak pluton Granodiorite 40Ar/39Ar Biotite UAF Plateau age, 5 fractions, 81.9% 39Ar released, MSWD=0.44. TA28 08SB127c Taylor Mtns. A6 -158.1664 Intrusion at hill 1623 Quartz diorite or tonalite 40Ar/39Ar White mica UAF Plateau age, 7 fractions, 99% 39Ar released. TA29 08ADw825a Taylor Mtns. A6 -158.1484 Intrusion near hill 1635 Diorite U-Pb Zircon SHRIMP RG Weighted average of 7 of 8 overlapping 206Pb/238U dates. TA29 08ADw825a Taylor Mtns. A6 -158.1484 Intrusion near hill 1635 Granite 40Ar/39Ar Hornblende UAF Plateau age, 3 fractions, 96.6% 39Ar released, MSWD=0.14. TA30 08ADw822a Taylor Mtns. A6 -158.2426 Tikchik Mountain pluton Granite U-Pb Zircon SHRIMP RG Weighted average of 8 of 10 overlapping 206Pb/238U dates. TA31 08SB117a Taylor Mtns. A6 -158.2053 Tikchik Mountain pluton Granite 40Ar/39Ar Biotite UAF Plateau age, 9 fractions, 98.3% 39Ar released, MSWD=1.84. TA32 08SB147a Taylor Mtns. A5 -157.8410 Arrow pluton Granite U-Pb Zircon SHRIMP RG Weighted average of 10 overlapping 206Pb/238U dates. TA32 08SB147a Taylor Mtns. A5 -157.8410 Arrow pluton Granite 40Ar/39Ar Biotite UAF Plateau age, 5 fractions, 82.6% 39Ar released, MSWD=0.21. TA32 08SB147a Taylor Mtns. A5 -157.8410 Arrow pluton Granite 40Ar/39Ar Hornblende UAF Plateau age, 4 fractions, 96.6.0% 39Ar released, MSWD=2.9. TA33 08AM676a Taylor Mtns. A3 -157.0999 Dike near Sleitat Mountain Granite porphyry U-Pb Zircon SHRIMP RG Weighted average of 9 of 10 overlapping 206Pb/238U dates. TA34 08SB169a Taylor Mtns. A3 -157.0750 Sleitat pluton Granite U-Pb Zircon SHRIMP RG Weighted average of 6 of 8 overlapping 206Pb/238U dates. TA35 06AK69a Taylor Mtns. A3 -156.8562 Old Man pluton Granite 40Ar/39Ar Biotite UAF Plateau age, 9 fractions, 86% 39Ar released, MSWD=0.2. TA36 08ADw896a Taylor Mtns. A3 -156.8036 Intrusion at hill 1004 Quartz monzonite to granite U-Pb Zircon SHRIMP RG Weighted average of 8 overlapping 206Pb/238U dates. TA37 06ADw570e Taylor Mtns. A2 -156.6045 Intrusion at hill 1422 Quartz monzonite U-Pb Zircon UBC, TIMS Concordia age based on five concordant, overlapping zircon analyses. MSWD (of concordance)=0.56; probability (of concordance)=0.46 Table 1.—Continued

Introduction    13 Map no. Sample No. Location Latitude Longitude Rock unit dated Rock type Age (Ma) Error Method Mineral Lab Notes TA38 06AJ062a Taylor Mtns. A1 -156.0311 Rhyolite at hill 1678 Rhyolite U-Pb Zircon UBC, TIMS Concordia age based on two concordant, overlapping zircon analyses. MSWD (of concordance)=0.18; probability (of concordance)=0.67. TA39 06ADw568a Taylor Mtns. A1 -156.0417 Intrusion at hill 982 Granite U-Pb Zircon UBC, TIMS Concordia age based on five concordant, overlapping zircon analyses. MSWD (of concordance)=0.00039; probability (of concordance)=0.98. 06ADw558a Lake Clark B8 -155.7922 Overlook pluton Gabbro U-Pb Zircon UBC, TIMS Concordia age based on five concordant, overlapping zircon analyses. MSWD (of concordance)=1.8; probability (of concordance)=0.17. GO1 06ADw603b Goodnews B8 -161.6497 Intrusion near Jacksmith Creek Granite porphyry 40Ar/39Ar biotite UAF Plateau age, 6 fractions, 94% 39Ar released, MSWD=0.9. GO2 06ADw602c Goodnews B7 -161.3786 Dike at Island Mountain Basalt 40Ar/39Ar whole rock UAF Weighted mean of two plateau ages: 7 fractions, 43% 39Ar released, MSWD=1.3; and 8 fractions, 77% 39Ar released, MSWD=1.6. DI1 08ADw826a Dillingham D7 -158.3367 Tikchik Narrows pluton Granite 40Ar/39Ar biotite UAF Plateau age, 3 fractions, 62.0% 39Ar released, MSWD=1.44. Table 1.—Continued

14    Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages 03ATi22a 229.5±0.2 Ma 06ADw558a 76.5±0.2 Ma

05AM126a 71.6±0.3 Ma

06ADw568a 56.3±0.2 Ma

06ADw570e 61.0±0.1 Ma

06AJ062a 49.1±0.3 Ma 04AM45a 68.6±0.1 Ma 84AM279b 114.9±0.3 Ma monazite 01ADw7a 56.0±1.9 Ma 206Pb/238U 207Pb/235U 03AM3a 58.0±0.1 Ma

116.6±0.1 Ma 317.7±0.6 Ma 08AM612a 85AM87c BH1 BH2 TA10 ID1 TN1 TL2 ID2 MG2 TA37 TA38 TA39 Figure 5.  Concordia diagrams for samples of zircon (blue ellipses) and monazite (green ellipses) dated by uranium-lead (U-Pb) thermal ionization mass-spectrometry (TIMS) at the University of British Columbia. Each analysis is represented by an ellipse representing the 2-sigma uncertainty. Concordia curve is the brown line in each plot; these lines are of variable thickness depending on the scale of the plot. For most plots, ages (in millions of years, Ma) along the concordia curve are demarked by white ellipses, which convey the uncertainties in the decay constants of uranium-238 and uranium-235. The dashed line in the plot for sample MG2 meets the concordia curve at the lower intercept age. 206Pb/238U, lead-206/uranium-238; 207Pb/235U, lead-207/uranium-235.

Introduction    15 206Pb/238U age, in Ma 09ADw02 112.0±0.9 Ma 05AM153a 67.9±1.2 Ma 98ADW42a 70.2±1.0 Ma 85AM16 66.8±0.4 Ma 98ADw62e 88.3±1.0 Ma 08AM611a 69.8±1.6 Ma 05ADw158a 153.0±2.0 Ma 05BT148 63.2±0.6 Ma 08AM630a 69.5±1.0 Ma 05ADw17a 68.1±1.9 Ma TA16 TA18 CA1 HC1 01ADw8a 42.8±0.5 Ma MG3 SM7 TA3 SM17 TA1 TA12 TA14 Lone, 11ADw109a 61.8±1.1 Ma MG1 Windy Fork pluton 11ADw117a 31.8±0.4 Ma MG4 11AD1a Dike at Gagaryah deposit 63.2 ± 2.2 Ma LH2 LH1 Stock near Windy Fork pluton 11ADw115a 30.9 ± 0.6 Figure 6.  Plots showing weighted average lead-206/uranium-238 (206Pb/238U) ages of zircon samples (horizontal gray bars) and individual zircon ages (vertical black and white bars, with lengths corresponding to age ±1-sigma uncertainties). Analyses represented by black bars were used to calculate each weighted average age. Analytical data are given in appendix 2 (table A1 and table A5, sample LH2 only). Ma, millions of years.

16    Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages 06AM453a 78.2±0.8 Ma 08SK290a 80.2±1.3 Ma 08AM676a 68.7±1.6 Ma 08SK316a 66.7±0.4 Ma 08SB169a 61.4±0.7 Ma 08ADw896a 69.1±0.9 Ma 08ADw825a 79.4±0.4 Ma 08SB147a 74.3±1.1 Ma 08ADw822a 62.2±0.8 Ma 08AM654b 69.9±2.9 Ma TA30 TA33 TA32 TA34 TA22 TA36 TA20 TA23 TA26 TA29 206Pb/238U age, in Ma Figure 6.—Continued

Introduction    17 Percentage of 39Ar released 97AM130a Intrusion at hill 1195 biotite #1 97AM40b Aghaluk stock biotite #1 97RJ062 Barometer pluton biotite 97AM61a Intrusion at hill 1162 biotite 97AM81a Intrusion in Vreeland Creek drainage biotite #1 98AM62a Intrusion at hill 1908 biotite #1 97AM139a Henderson stock biotite #1 98AM192a Intrusion in Oskwalik River drainage biotite #1 98AM274a Barometer pluton biotite #1 97ADW118 Telida pluton biotite 97ADW126 Sischu igneous complex hornblende #2 03AM10a Intrusion at BM Sheep biotite 03AM10a Intrusion at BM Sheep hornblende 01ADw7a Tephra in Sheep Creek volcanic field hornblende MG2 Age, in Ma 65.9 ± 0.7 Ma 69.8 ± 0.4 Ma 70.5 ± 0.4 Ma 70.9 ± 0.4 Ma 71.8 ± 0.4 Ma 71.2 ± 0.4 Ma 70.6 ± 0.4 Ma 74.6 ± 0.5 Ma 71.9 ± 0.8 Ma 72.1 ± 0.5 Ma 72.4 ± 0.4 Ma 54.7 ± 0.7 Ma 58.7 ± 0.6 Ma 58.0 ± 0.3 Ma TL1 KH1 MD1 TL1 SM1 SM2 SM5 SM9 SM3 SM4 SM6 SM8 SM10 Figure 7.  Plots of age spectra showing the results of incremental step heating of argon-40/argon-39 (40Ar/39Ar) samples. Red rectangles indicated steps used in the weighted mean plateau ages; gray rectangles were rejected. Ma, millions of years.

18    Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages 97ARJ57 Red Mountain pluton biotite #1 Pseudo-plateau steps 75.3 ± 0.5 Ma 73.7 ± 0.5 Ma 61.2 ± 0.4 Ma 59.9 ± 0.4 Ma 60.2 ± 0.5 Ma 60.5 ± 0.4 Ma 69.8 ± 0.4 Ma 62.9 ± 0.5 Ma 62.8 ± 0.8 Ma 62.4 ± 0.4 Ma 66.0 ± 1.0 Ma 99AM390a Buckstock pluton biotite #1 Felsite in Victoria Creek headwaters biotite #1 05AM247a Timber Creek pluton biotite 05AM236a Flat Top basalt whole rock #1 4.6 ± 0.1 Ma 99AM450a Kaluvarawluk igneous complex 93AM46d Buckstock pluton biotite #1 98ARJ102 Felsite near Holokuk Mountain white mica #1 Intrusion in Timber Creek headwaters hornblende #1 93AM94a Timber Creek pluton biotite #1 94AM309a Intrusion near hill 1764 white mica #1 94AWK353b Intrusion near hill 2639 hornblende #1 80.3 ± 0.9 Ma 08AM615c Dike at Shadow Bay hornblende 73.8 ± 1.2 Ma 05ADw29c Dike at hill 2164 hornblende 98ADW42a Chuilnuk pluton biotite 99AM315a 99BT187 69.6 ± 0.3 Ma SM18 SM19 SM20 SM21 BH3 TA2 TA4 TA5 SM12 SM13 SM14 SM15 SM16 SM17 SM11 Age, in Ma Percentage of 39Ar released Figure 7.—Continued

Introduction    19 Weighted mean age 87.7 +/- 1.7 Ma

05AM239a Sill at hill 1768 biotite #1 Intrusion at hill 2240 biotite #1 Intrusion south of Shotgun airstrip biotite #1 06AK42 Dike in Red Bluff area white mica 08ADw825a Intrusion near hill 1635 hornblende #1 Intrusion in Enatalik Creek drainage biotite #2 05ADW59C Shotgun pluton biotite #1 Intermediate sill in Kuskokwim Group biotite 08AM614a Chaufchivak pluton biotite 06ADw537a Intrusion at hill 1311 white mica 05AM115a Sill in Kuskokwim Group white mica #1 Intermediate sill in Kuskokwim Group whole rock 06AM447a Dike in Red Bluff area white mica 08SB129b Intrusion in Winchester claim area hornblende 87.7 ± 1.7 Ma 05AM224c 05AM215a 05AM144d 05AM154a TA8 TA15 TA17 TA19 TA21 TA6 TA7 TA9 TA11 TA13 TA24 TA25 TA27 TA29

08SB127c Intrusion near hill 1632 muscovite TA28 74.7 ± 0.4 Ma 74.2 ± 1.1 Ma 68.7 ± 0.3 Ma 69.2 ± 0.3 Ma 79.1 ± 0.3 Ma 76.9 ± 0.9 Ma 67.2 ± 0.2 Ma 67.3 ± 0.2 Ma 67.6 ± 0.2 Ma 66.1 ± 0.3 Ma 70.1 ± 0.4 Ma 77.6 ± 0.7 Ma 68.3 ± 0.3 Ma 69.3 ± 0.4 Ma Age, in Ma Percentage of 39Ar released 94ADw59b Figure 7.—Continued

20    Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages

08SB117a Tikchik Mountain pluton biotite #1 08SB147a Arrow pluton biotite #1 06AK69a Old Man pluton biotite 74.0 ± 0.2 Ma 08ADw826a Tikchik Narrows pluton biotite 155.0 ± 1.9 Ma 08SB147a Arrow pluton hornblende #1 06ADw602c Dike at Island Mountain whole rock #1 60.2 ± 0.2 Ma 155.0 ± 1.9 Ma 06ADw602c Dike at Island Mountain whole rock #2 06ADw603b Intrusion near Jacksmith Creek biotite 70.7 ± 0.3 Ma TA32 GO1 TA32 TA35 GO2 GO2 DI1 TA31 61.5 ± 0.2 Ma 66.7 ± 0.3 Ma 73.8 ± 0.3 Ma Age, in Ma Percentage of 39Ar released

Figure 7.—Continued

Geochronology    21 Geochronology Candle Quadrangle CA1. Granite Mountain Pluton, Candle B5 Quadrangle The Granite Mountain pluton is a composite body of Early Cretaceous granite, syenite, and nepheline syenite that intrudes Early Cretaceous volcanic rocks of the Koyukuk terrane (Patton and others, 2009) (fig. 4A). The geochronology sample (09ADW2), collected by Tom Donley in 2008, is a coarse-grained syenite containing abundant hornblende crystals that define a comagmatic lineation. Using the USGSStanford SHRIMP-RG, we obtained a U-Pb zircon age of 112.0±0. 9 million years (Ma). The age is the weighted average of 10 overlapping lead-206/uranium-238 (206Pb/238U) analyses (fig. 6). This age is consistent with previous U-Pb zircon age determinations of 109 and 112 Ma from elsewhere along this magmatic belt (Patton and others, 2009). Tanana Quadrangle TN1. Gabbro in Rampart Group, Tanana B1 Quadrangle The Rampart Group of Mertie (1937) is an assemblage of mafic sills and flows and interlayered sedimentary rocks along the Yukon River corridor in Tanana quadrangle (fig. 4B). It has been included in the Angayucham-Tozitna terrane (Silberling and others, 1994; Till and others, 2006). The geochronology sample (03ATi22a) is a gabbro from a fault sliver of Rampart Group just north of the Victoria Creek strike-slip fault, in rocks assigned to unit Trmg of Reifenstuhl and others (1997). The gabbro is pale orange-weathering, medium to dark green, and medium grained. In thin section, the rock has a subophitic texture and the major minerals are plagioclase and brown amphibole (some cored with relict clinopyroxene); there is minor anhedral quartz. Rims of actinolite on hornblende, fine-grained epidote in plagioclase, and small patches of chlorite indicate partial recrystallization at greenschist-facies conditions. We obtained a precise 206Pb/238U zircon age of 229.5±0.2 Ma by TIMS. The quoted age is the mean of four overlapping, concordant 206Pb/238U ages (fig. 5). Kantishna River Quadrangle KH1. Sischu Igneous Complex, Kantishna River A6 Quadrangle The Sischu Mountain igneous complex spans the northeastern corner of Medfra (Patton and others, 1980) and southwestern corner of the Kantishna River (Chapman and others, 1975) quadrangles (fig. 4C). It intrudes the Farewell terrane. Our sample (97ADw126) is from a coarse-grained, equigranular, nonfoliated biotite-hornblende granite. Two splits of hornblende were analyzed by 40Ar/39Ar (fig. 7). Both splits yielded essentially identical ages of 65.9±0.5 Ma as detailed in appendix 2 (table A3). Rhyolite samples from a different part of the igneous complex in Medfra quadrangle yielded potassiumargon (K-Ar) whole-rock ages of 71.0±2.8 and 69.9±2.7 Ma and a sanidine age of 66.3±2.0 Ma (Moll and others, 1981). Medfra Quadrangle MD1. Telida Pluton, Medfra C1 Quadrangle The Telida pluton, mapped as unit TKg by Patton and others (1980) in the eastern Medfra quadrangle, intrudes metasedimentary rocks of the Yukon-Tanana terrane (fig. 4D). It is a coarse-grained, A-type biotite granite (Moll and others, 1981). We obtained a 40Ar/39Ar biotite age of 72.1±0.4 Ma (fig. 7) from a sample (97ADw118) of the granite. The age is based on a six-step plateau representing 81 percent of the 39Ar released; the slight hump may be due to recoil effects. Within 2-sigma uncertainty, this overlaps the K-Ar age of 70.5±2.8 Ma on biotite that was previously reported by Moll and others (1981). Holy Cross Quadrangle HC1. Volcanic Rocks of the Yetna River Area, Holy Cross D1 Quadrangle The volcanic rocks of the Yetna River area crop out in a broad area of low hills in eastern Holy Cross quadrangle (fig.  4E). Miller and Bundtzen (1994) reported lava flows and tuffs of andesite and rhyolite, and minor flows of basalt. They reported eight K/Ar ages (on biotite, whole rock, and sanidine) ranging from 68.7±2.1 to 54.4±1.6 Ma. Our sample (85AM16a) is a welded rhyolite tuff, containing phenocrysts of embayed quartz, sanidine, and minor white mica. We obtained a U-Pb zircon SHRIMP-RG age of 66.8±0.4 Ma (fig. 6). The zircon age is the weighted average 206Pb/238U age of 10 single-crystal analyses; three zircon analyses were discounted, one for suspected lead loss and two for suspected inheritance. The zircon age is very close to a K-Ar age of 67.0±2.0 Ma on sanidine from the same sample (Miller and Bundtzen, 1994). Iditarod Quadrangle ID1. Ruby Terrane Orthogneiss, Unit PzPg, Iditarod D4 Quadrangle A narrow belt of schist and associated deformed granitoids in Iditarod quadrangle was correlated by Miller and Bundtzen (1994) with the Ruby terrane (fig. 4F). A sample of

22    Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages granitic orthogneiss (84AM279a) yielded a K-Ar biotite age of 108±3 Ma (Miller and Bundtzen, 1994, p. 11), who interpreted the result as a post-metamorphic cooling age. We obtained a U-Pb zircon TIMS age from an archived sample (84AM279b) from the same outcrop. A single concordant zircon yielded the igneous age that we pick for this rock—114.9±0.3 Ma (fig. 5). Other zircon fractions, some of which are not shown, showed variable amounts of inheritance. The presence of about 115 Ma granitic rocks is entirely consistent with the interpretation favored by Miller and Bundtzen (1994) that their unit PzPg is part of the Ruby terrane. The main belt of Ruby terrane, north of the Yukon River, is intruded by a number of Early Cretaceous plutons, which have yielded U-Pb zircon ages of 118-113 Ma (Roeske and others, 1998). ID2. Gabbro in Dishna River Mafic-ultramafic Complex, Iditarod D3 Quadrangle The Dishna River mafic-ultramafic complex in Iditarod quadrangle (fig. 4F) is a poorly exposed, fault-bounded package of rocks along the Dishna River Fault Zone (Miller, 1990; Miller and Bundtzen, 1994). The following conventional K/Ar ages were reported by Miller and Bundtzen (1994). One gabbro sample yielded hornblende ages of 228±25 and 222±23 Ma; excess argon was suspected in both cases. Another gabbro sample yielded a hornblende age of 92.2±2.8 Ma; partial resetting during the Late Cretaceous was suspected. We obtained a U-Pb zircon TIMS age of 116.6±0.1 Ma from a third, archived sample (85AM87c) of hornblende gabbro (fig. 5). The age is the 206Pb/238U weighted average of four concordant, overlapping fractions, which establishes an Early Cretaceous intrusive age. Together, the available K-Ar and U-Pb results suggest that the Dishna River mafic-ultramafic complex includes rocks of two ages, Triassic and Early Cretaceous. McGrath Quadrangle MG1. Stock South of Lone Mountain, McGrath B4 Quadrangle This sample is from a small stock that is presumably a satellite to the Lone Mountain pluton about 4 kilometers (km) to the north; it is much smaller than the dot used to locate the sample in figure 4G. Most of the bedrock in the Lone Mountain area is late Neoproterozoic to Paleozoic platformal carbonate deposits assigned to the Farewell terrane. The stock is leucocratic fine-grained granitic rock, or aplite, with sprays of black tourmaline. The sample (11ADw109a) is a medium to fine-grained equigranular granite composed mainly of intergrown equant 1-to 2-millimeter (mm) grains of quartz, plagioclase and microcline with less than 5 percent Fe-oxide stained biotite and 0.5-to 1.0-centimeter (cm) sprays of black tourmaline needles making up much less than 1 percent. Much smaller skeletal grains of brown pleochroic biotite are replaced in cleavage-parallel strips by green biotite or clear muscovite, and elsewhere by pale green chlorite. Zircon is present in trace amounts. Zircons were dated using the SHRIMP-RG and indicate an age of 61.8±1.1 Ma (fig. 6). The age is the weighted average of 8 overlapping 206Pb/238U ages. The new age overlaps with error with a biotite K-Ar age of 59.8±1.6 Ma from the nearby Lone Mountain pluton (biotite granite) as reported by Reed and Lanphere (1972) and recalculated by Solie and others (1991). MG2 and MG3. Tephras in Sheep Creek Volcanic Field, McGrath B2 Quadrangle The Sheep Creek volcanic field of Bundtzen and others (1997) is a small, Paleogene eruptive center in the western Alaska Range (fig. 4H). We report ages from near the base and from the very top of the section. The lower sample, MG2 (01ADw7a), is a felsic tuff from what Bundtzen and others (1997) mapped as unit Tvt—"intermediate and felsic air-fall tuff." The tuff is several meters thick and includes fossil wood. Sample MG2 (01ADw7a) yielded very similar 40Ar/39Ar and U-Pb ages, both done at University of British Columbia. TIMS analysis of four zircon fractions yielded a discordant-concordant array with a lower intercept age (56.0±1.9 Ma) (fig. 5). This overlaps within error with a 40Ar/39Ar plateau hornblende age of 54.7 ± 0.7 Ma (fig.  7). The argon age, which is based on 12 steps representing 100 percent of the 39Ar released, is probably more reliable in this case because the zircon population is complex and largely xenocrystic, and the volcanic rocks appear to have lain undisturbed, and certainly not reheated, since they crystallized. Sample MG3 (01ADw8a) is from the top of the Sheep Creek volcanic field, a reworked felsic tuff 10-20 cm thick interbedded with andesite-cobble conglomerate, possibly a lahar deposit. The sample is from what Bundtzen and others (1997) mapped as unit Tvs—"volcaniclastic sandstone and lacustrine silt." The sampled horizon contains abundant, well-preserved fossil leaves. Clear, prismatic zircons were dated using the SHRIMP-RG and indicate an age of 42.8±0.5 Ma (fig. 6). The age is the weighted average of 8 overlapping 206Pb/238U ages; one outlying analysis was discounted. The new U-Pb zircon age is within error of, but much more precise than, a conventional K-Ar age of 43.5±7.8 Ma on biotite from andesite in the Sheep Creek volcanic field (Solie and others, 1991). Together, results from our two samples show that the Sheep Creek volcanic succession includes strata of two substantially different ages—(1) near the Paleocene-Eocene boundary and (2) mid-Eocene. MG4. Windy Fork Pluton, McGrath A3 Quadrangle The Windy Fork pluton is a peralkaline intrusion that cuts folded deep-water strata of the Paleozoic Dillinger subterrane (Gilbert and others, 1988). It has been targeted for closer scrutiny than most other intrusive complexes in the Alaska Range because

Geochronology    23 of its rare-earth-element (REE) resource potential (Bundtzen, 1999). The geochronology sample (MG4: 11ADw117a) is from a prominent cirque in the core of the pluton (fig. 4I). The sample is medium-coarse grained granite that has about 5 percent dark green amphibole. The amphibole, which is most likely hornblende, is 0.5-1 mm and lies in a groundmass of feldspar and quartz as large as 6 mm with trace zircon. Prismatic zircons were dated by SHRIMP-RG and indicate an age of 31.8 ± 0.4 Ma (fig. 6). The age is the weighted average of seven overlapping 206Pb/238U ages; one other analysis was rejected on suspicion of minor lead loss. Our result is similar to two published K-Ar ages from the pluton—30.9±0.9 on biotite and 29.7±0.9 Ma on hornblende (Reed and Lanphere, 1972, as recalculated by Wilson and others, 1994, using 1976 decay constant). Talkeetna Quadrangle TL1. Intrusion at BM Sheep, Talkeetna D5 Quadrangle This is a narrow, elongate intrusion near the northern front of the Alaska Range near Mystic Pass in Talkeetna quadrangle (fig. 4J) is a fine-to medium-grained biotite granodiorite to diorite. Our geochronology sample (03AM10a) is from near the summit of Peak 5753 (BM Sheep). We obtained a 40Ar/39Ar hornblende age of 58.7±0.6 Ma. The age is based on a four-step plateau representing 58 percent of the 39Ar released. A very similar but slightly more robust 40Ar/39Ar biotite age of 58.0±0.3 Ma was also obtained (fig. 7). This age is based on an eight-step plateau representing 90 percent of the 39Ar released. The biotite age, with its better defined plateau, is preferred for this sample. TL2. Tonzona Pluton, Talkeetna D5 Quadrangle This is a prominent, spectacularly exposed granite pluton along the northern front of the Alaska Range near Mystic Pass in Talkeetna quadrangle (fig. 4J). It is an unfoliated, mediumgrained, biotite granite. Miarolitic cavities suggest relatively shallow-level emplacement. We obtained a U-Pb zircon TIMS age of 58.0±0.1 Ma from sample 03AM3a (fig. 5). The age is a weighted average of three zircon 206Pb/238U dates and two monazite lead-207/uranium-235 (207Pb/235U) dates; one zircon date (dashed ellipse in fig. 5, plot TL2) was suspected of minor lead loss and was not included in the age calculation. Sleetmute Quadrangle SM1. Intrusion at Hill 1195, Sleetmute D4 Quadrangle This sample (97AM130a) is from rubble crop of granite porphyry on hill 1195 in the drainage of the South Fork George River, Sleetmute D4 quadrangle (fig. 2). The surrounding region is underlain by Kuskokwim Group, which the granitic rock presumably intruded; the porphyry body itself is too small to show on the map. The dated sample is a granite porphyry with biotite, quartz, and feldspar phenocrysts. A 40Ar/39Ar biotite age of 70.9±0.4 Ma is based on a 11-step plateau representing 97 percent of the 39Ar released (fig. 7). SM2. Aghaluk Stock, Sleetmute C7 Quadrangle This intrusion, which we refer to as the Aghaluk stock, is a biotite-quartz-feldspar granite porphyry in Sleetmute C6 and C7 quadrangles (fig. 2). We obtained a 40Ar/39Ar biotite age of 72.1±0.5 Ma from sample 97AM40b (fig. 7). The age is based on a 13-step plateau representing 99 percent of the 39Ar released. SM3. Intrusion at Hill 1662, Sleetmute C6 Quadrangle Hill 1662 in Sleetmute C6 quadrangle (fig. 2) is underlain by a north-northwest-striking dike or sill of medium-to fine-grained quartz diorite containing biotite, feldspar, and minor quartz. The dike or sill crosscuts hornfels assigned to the Kuskokwim Group. We obtained a 40Ar/39Ar biotite age of 70.5±0.4 Ma from sample 97AM61a (fig. 7). The age is based on a nine-step plateau representing 90 percent of the 39Ar released. SM4. Intrusion at Hill 1908, Sleetmute C6 Quadrangle Hill 1908 in Sleetmute C6 quadrangle (fig. 2) is underlain by a stock of hypabyssal biotite-quartz granite porphyry. We obtained a 40Ar/39Ar biotite age of 71.8±0.4 Ma from sample 98AM62a (fig. 7). The age is based on a 13-step plateau representing 98 percent of the 39Ar released. SM5. Intrusion in Oskawalik River Drainage, Sleetmute C5 Quadrangle A stock of unknown extent was encountered as a rubble pile along a creek bottom in the Oskawalik River drainage in Sleetmute C5 quadrangle (fig. 2). It is a garnet-biotite-feldspar porphyry of felsic to possibly intermediate composition. We obtained a 40Ar/39Ar biotite age of 71.9±0.4 Ma from sample 98AM192a (fig. 7). The age is based on a five-step plateau representing 59 percent of the 39Ar released. SM6. Henderson Stock, Sleetmute C5 Quadrangle Henderson Mountain, a prominent peak in Sleetmute C5 quadrangle (fig. 2), is underlain by plutonic rocks. The geochronology sample (97AM139a) is a monzonite or quartz

24    Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages monzonite containing clinopyroxene, biotite, plagioclase, and lesser potassium feldspar and quartz. We obtained a 40Ar/39Ar biotite age of 69.8±0.4 Ma (fig. 7), based on a six-step plateau representing 80 percent of the 39Ar released. SM7. Tephra in Kuskokwim Group, Sleetmute C4 Quadrangle Zircons from a tuff within the Kuskokwim Group were dated using the USGS-Stanford SHRIMP-RG. The tuff (sample 98ADw62e) is a 1-to 2-cm-thick white-weathering, soft, clayey layer interbedded with turbidites of the Kuskokwim Group in cutbanks along the Kuskokwim River in Sleetmute C4 quadrangle (fig. 2). It yielded a U-Pb zircon (SHRIMP) age of 88.3±1.0 Ma. This is the 206Pb/238U weighted average of seven zircon analyses; three zircon analyses were discounted, two on suspicion of inheritance and one on suspicion of lead loss (fig. 6). The age falls in the Coniacian stage of the Late Cretaceous according to the time scale of Gradstein and others (2012) and is broadly consistent with fossil evidence summarized by Elder and Box (1992). SM8 and SM9. Barometer Pluton, Sleetmute C4 Quadrangle Two samples were dated from either end of the prominent pluton that underlies Barometer Mountain near the village of Sleetmute (fig. 2). The pluton intrudes the Kuskokwim Group. From the west end of the summit ridge, sample SM8 (98AM274a) yielded a 40Ar/39Ar biotite age of 71.2±0.4 Ma (fig. 7); the age is based on a 11-step plateau representing 85 percent of the 39Ar released. From the east end, sample SM9 (97RJ062) is a rhyolite porphyry (SiO2=73 percent) having a phenocryst assemblage of biotite, hornblende, quartz, and plagioclase. It yielded a 40Ar/39Ar biotite age of 70.6±0.4 Ma (fig. 7); the age is a nine-step plateau which represents 90 percent of the 39Ar released. SM10. Intrusion in Vreeland Creek Drainage, Sleetmute C4 Quadrangle (97AM81a) A granite porphyry body of unknown extent was discovered in the watershed of Vreeland Creek, about 6 km south of Barometer Mountain (fig. 2). The sample is a porphyry with phenocrysts of biotite, quartz, feldspar, and garnet. We obtained a 40Ar/39Ar biotite age of 74.6±0.5 Ma (fig. 7). The age is based on a six-step plateau representing 52 percent of the 39Ar released. SM11. Red Mountain Pluton, Sleetmute C4 Quadrangle The Red Mountain pluton underlies a prominent ridge about 10 km south-southwest of the village of Sleetmute (fig. 2). The geochronology sample (97ARJ57) is a porphyritic andesite with plagioclase and biotite phenocrysts in a flinty, medium-gray groundmass. Nearby, the intrusion consists of a fine-grained, equigranular diorite. We obtained a 40Ar/39Ar biotite age of 75.3±0.5 Ma (fig. 7). The age is based on a four-step plateau representing 52 percent of the 39Ar released. SM12. Felsite in Victoria Creek Headwaters, Sleetmute B8 Quadrangle This sample is from the headwaters of Victoria Creek in Sleetmute B8 quadrangle (fig. 2). The geochronology sample (99AM315a) is a pyrite-bearing felsite containing phenocrysts of potassium feldspar and biotite. We obtained a poor 40Ar/39Ar biotite age of 73.7±0.5 Ma (fig. 7). The quoted age is a pseudoplateau based on the five oldest fractions, which represent only 30 percent of the 39Ar released. SM13 and SM14. Buckstock Pluton, Sleetmute B8 and B7 Quadrangles The Buckstock pluton is a 5 by 25 km pluton of granite to granodiorite that broadly parallels the strike of bedding in the host Kuskokwim Group in southwestern Sleetmute quadrangle (fig. 2). Sample SM13 (99AM390a) yielded a single concordant U-Pb zircon with an age of 59.5±0.5 Ma, as reported by Miller and others (2002) (appendix 2, table A5). From the same sample, we here report a 40Ar/39Ar biotite age of 61.2±0.4 Ma (fig. 7). The argon age is based on a nine-step plateau representing 93 percent of the 39Ar released. The argon geochronology sample is a biotite granite to granodiorite containing feldspar phenocrysts to 1 cm. The discrepancy between the 40Ar/39Ar biotite and U-Pb zircon ages has no obvious explanation, because typically, U-Pb ages are older not younger than 40Ar/39Ar ages from the same sample. Sample SM14 (93AM46d), from about 5 km to the east, is a granite containing quartz, potassium feldspar phenocrysts to several centimeters, plagioclase, and biotite. We obtained a 40Ar/39Ar biotite age of 59.9±0.4 Ma (fig. 7). The age is based on a nine-step plateau representing 83 percent of the 39Ar released. SM15. Kaluvarawluk Igneous Complex, Sleetmute B7 Quadrangle Hypabyssal and volcanic rocks underlie Kaluvarawluk Mountain in northeastern Sleetmute B7 quadrangle (fig. 2). The dated sample (99AM450a), from a prominent northeast-striking dike off the northeastern flank of the mountain, is a granite porphyry containing phenocrysts of feldspar (to 2 cm), biotite, and quartz. We obtained a 40Ar/39Ar biotite age of 60.2±0.5 Ma (fig. 7), based on an 11-step plateau representing 89 percent of the 39Ar released.

Geochronology    25 SM16. Intrusion Near Holokuk Mountain, Sleetmute B6 Quadrangle This sample is from a felsite plug east of, and along strike from, Holokuk Mountain in Sleetmute B6 quadrangle (fig. 2). Small phenocrysts of smoky quartz, feldspar, and white mica are set in a fine-grained pinkish-gray groundmass. We obtained a 40Ar/39Ar white mica age of 60.5±0.4 Ma from sample 98ARJ102 (fig. 7). The age is based on a 10-step plateau representing 99 percent of the 39Ar released. SM17. Chuilnuk Pluton, Sleetmute B5 Quadrangle The Chuilnuk pluton is part of a prominent volcanicplutonic complex near the Denali Fault in Sleetmute quadrangle (fig.  2). The sample (98ADw42a) is a medium-to coarse-grained biotite granite containing relict garnet, epidote and zircon in trace quantities, and secondary chlorite, white mica, and calcite. The rock was dated by two methods. We obtained a U-Pb zircon age of 70.2±1.0 Ma using the SHRIMP-RG. The age is the weighted average 206Pb/238U age of 11 single-crystal zircon ages (fig. 6). We also obtained a 40Ar/39Ar biotite age of 69.8±0.4 Ma (fig. 7), based on a nine-step plateau that accounts for 86 percent of the 39Ar released. Similarly, Decker and others (1995) reported four K-Ar biotite ages from the Chuilnuk pluton that range from 68.7±2 to 67.5±2 Ma. We interpret the U-Pb age as the crystallization age and the 40Ar/39Ar and K/Ar ages as cooling ages. SM18, TA1 and TA2. Timber Creek Pluton, Sleetmute A8 and Taylor Mountains D8 Quadrangles This intrusion, which we refer to as the Timber Creek pluton, straddles the boundary between Sleetmute A8 and Taylor Mountains D8 quadrangles. It was sampled in three locations (fig. 2). Sample SM18 (93AM94a) is from low hills west of Timber Creek in southwestern Sleetmute A8 quadrangle. This sample is a granite porphyry containing phenocrysts of quartz, potassium feldspar, plagioclase, biotite, and rare garnet. We obtained a 40Ar/39Ar biotite age of 62.9±0.5 Ma (fig. 7). The age is based on a six-step plateau representing 71 percent of the 39Ar released. Sample TA1 (05BT148), from Taylor Mountains D8 quadrangle, is a granite porphyry containing phenocrysts of quartz, biotite, plagioclase, and potassium feldspar. We obtained a U-Pb zircon SHRIMP-RG age of 63.2±0.6 Ma (fig. 6). The age is the mean of nine overlapping 206Pb/238U ages. Sample TA2 (05AM247a), from Taylor Mountains D8 quadrangle, is a granite porphyry containing phenocrysts of biotite and plagioclase, in a groundmass of quartz, plagioclase, biotite, and potassium feldspar. This sample is from an apophysis or dike just south of the main Timber Creek pluton. Biotite yielded a 40Ar/39Ar age of 62.4±0.4 Ma, based on a nine-step plateau representing 92 percent of the 39Ar released (fig. 7). All three ages agree within 2-sigma error; we cite the U-Pb age as the preferred age for the Timber Creek pluton. SM19. Intrusion in Timber Creek Headwaters, Sleetmute A8 Quadrangle This sample (99BT187) of intermediate intrusive rock from the headwaters of Timber Creek in Sleetmute A8 quadrangle is from an area mainly underlain by the Gemuk Group of Miller and others (2007c) (fig. 2). The dated sample is a porphyritic pyroxene-hornblende intermediate intrusive. Phenocrysts are medium grained in a fine-to medium-grained groundmass of feldspar and quartz. Brown hornblende phenocrysts (as long as 4 mm) are relatively fresh looking. Altered clinopyroxene (?) phenocrysts are broken and rimmed by secondary chlorite. The groundmass shows secondary chlorite, calcite, and local white mica. We obtained a 40Ar/39Ar hornblende age of 66.0±1.0 Ma (fig. 7), based on a five-step plateau representing 79 percent of the 39Ar released. SM20. Intrusion Near Hill 2639, Sleetmute A7 Quadrangle This small stock intrudes the Gemuk Group in Sleetmute A7 quadrangle (fig. 2). The rock is a pyroxene-hornblende intrusive. We obtained a 40Ar/39Ar hornblende age of 80.3±0.9 Ma from sample 94AWK353b (fig. 7). The age is based on a four-step plateau representing 62 percent of the 39Ar released. SM21. Intrusion at Hill 1764, Sleetmute B7 Quadrangle This is a small granite porphyry body at hill 1764 in southwestern Sleetmute quadrangle (fig. 2). It intrudes the Kuskokwim Group. The sample (94AM309a) consists of phenocrysts of muscovite, biotite, quartz, and possible tourmaline in a very fine grained matrix. We obtained a 40Ar/39Ar white mica age of 62.8±0.8 Ma (fig. 7), based on a 10-step plateau representing 65 percent of the 39Ar released. Lime Hills Quadrangle LH1. Stock Near Windy Fork Pluton, Lime Hills D4 Quadrangle This is a small stock a few kilometers east of the much larger Windy Fork pluton and is presumably a satellite of it (fig. 4I). The stock cuts folded deep-water strata of the Paleozoic Dillinger subterrane (Gamble and others, 2013). The sample is biotiteamphibole(?), medium grained equigranular granite. The biotite is fine-grained and ratty looking. The amphibole phenocrysts are as much as 1 mm in long dimension and appear to have been hornblende but are altered to green biotite(?). The quartz and feldspar in the groundmass are mostly 2-3 mm in size. Trace zircon is noted in thin section. Prismatic zircons from sample

26    Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages 11ADw115a were analyzed using the SHRIMP-RG and indicate an age of 30.9±0.6 Ma (fig. 6). The age is the weighted average of seven overlapping 206Pb/238U ages; one analysis was rejected on suspicion of minor lead loss, and another was rejected on suspicion of inheritance. The new U-Pb zircon age overlaps within 2-sigma uncertainty with the U-Pb zircon age reported above for the main Windy Fork pluton (31.8±0.4 Ma, sample MG4). Our result is similar to a published K/Ar biotite age of 29.3±0.8 Ma (Reed and Lanphere, 1972; recalculated by Wilson and others, 1991, using the 1976 decay constant). LH2. Sill at Gagaryah Barite Deposit, Lime Hills D4 Quadrangle The Gagaryah sedimentary barite deposit is hosted in a shale-dominant Upper Devonian succession in the Mystic subterrane of the Farewell terrane (Bundtzen and Gilbert, 1991). The mineralized section is exposed in a gully in the headwaters of the Gagaryah Creek drainage. We dated a 1-m-thick intermediate sill near the top of the exposed section (fig. 4I). The sample (11AD1a) is a plagioclase porphyry with millimeter-long altered plagioclase and smaller probable pyroxene and scraps of brown pleochroic hornblende mostly replaced by a fine aggregate of chlorite, calcite and epidote with irregular calcite veins. Zircons were dated by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) by Apatite to Zircon, Inc., at Washington State University (fig. 6). The yield was poor; only 13 zircons were recovered, and only 11 of these produced concordant ages. A cluster of five Paleogene zircons yielded a weighted average 206Pb/238U age of 63.2±2.2 Ma; we interpret this as the crystallization age of the sill. In addition, the sample yielded concordant zircons with ages of 159, 191, 1,243, 1,260, and 1,432 Ma (appendix 2, table A5). These are interpreted as xenocrysts. The Mesoproterozoic zircons were likely derived from Farewell terrane sedimentary rocks into which the sill was emplaced; the two Jurassic zircons presumably were derived from Jurassic igneous rocks at depth. Bethel Quadrangle BH1. Gemuk Mountain Pluton, Bethel C1 Quadrangle The Gemuk Mountain pluton spans the boundary between the Taylor Mountains and Bethel 1:250,000-scale quadrangles (fig. 2). It intrudes dark, hornfelsed metasedimentary rocks that yielded Turonian bivalves (about 90 Ma, Late Cretaceous; Karl and others, 2011). The sample (05AM126a) is a hypidiomorphic-granular, medium-grained quartz monzonite. The mafic phases are biotite and pseudomorphs after pyroxene (now chlorite and calcite). We obtained a U-Pb zircon age of 71.6±0.3 Ma by TIMS (fig. 4). This concordia age is based on four concordant and overlapping single crystal ages. BH2. Unnamed Igneous Rocks in Tikchik Terrrane, Bethel A1 Quadrangle The oldest rock reported here is an intermediate volcanic rock from Bethel quadrangle. It is part of an accretionary complex termed the Tikchik terrane (Box and others, 1993) (fig. 2). The sample (08AM612a) is a porphyritic volcanic rock containing phenocrysts of feldspar, one or more former mafic phases (now chlorite and calcite), and minor quartz. The groundmass is recrystallized and shows secondary calcite and white mica. We obtained a U-Pb zircon age of 317.7±0.6 Ma by TIMS (fig. 4). The age is the weighted average of four concordant and overlapping single crystal 206Pb/238U ages. An additional concordant zircon age is somewhat younger, and was not included in the weighted average calculation on suspicion of minor lead loss. Implications of this sample are discussed below. BH3. Dike Near Shadow Bay, Bethel A1 Quadrangle A fine-grained, holocrystalline, felsic to intermediate dike cuts greenstone and sandstone of the Tikchik complex north of Shadow Bay on Lake Chauekuktuli in Bethel A1 quadrangle (fig. 2). The sample (08AM615c) is an ophitic, fine-grained hornblende-clinopyroxene-plagioclase andesite. We obtained a 40Ar/39Ar isochron age of 73.8±1.2 Ma on hornblende. The agerelease spectrum shows evidence of argon loss (fig. 7) and a true plateau cannot be defined. Taylor Mountains Quadrangle TA3. Felsite in Gemuk Group, Taylor Mountains D8 Quadrangle Cutbanks along the Atsaksovluk River in Taylor Mountains D8 quadrangle (fig. 2) expose a thick sequence of buff-weathering felsic tuff. The sampled tuff (05ADw158a), at water's edge, is about 20 meters thick; two other tuffs of similar thickness are exposed in inaccessible cliffs above, interbedded with black, green, and iron-stained fine-grained sedimentary rocks. The felsites contain altered glass shards and hence are volcanic rocks, not sills. Clear, prismatic zircons were dated by SHRIMP-RG and suggest an age of 153.0±2.0 Ma (fig. 6). The age is the mean of nine overlapping 206Pb/238U analyses; two other analyses were discounted on suspicion of inheritance. We assign the felsic tuff to the restricted Gemuk Group (of Miller and others, 2007c) on the basis of age, lithology, and location. This extends the known age range of explosive volcanism in this unit. In a previous study, Miller and others (2007c) presented a generalized stratigraphic section of the (restricted) Gemuk Group in Taylor Mountains and Sleetmute quadrangles. It consists of Triassic pillow lavas, Triassic to Jurassic cherts and mudstones, and Lower Cretaceous sandstone plus minor tuff. The 153-Ma rhyolite ignimbrite is older than the

Geochronology    27 previous U-Pb zircon ages of 145.9±1.4 and 136.5±2.2 Ma from Gemuk Group tuffs (Miller and others, 2007c), extending the age of magmatism in this part of the Togiak terrane farther back in the Late Jurassic. The thickness of this tuff section as whole, the thicknesses of individual ignimbrites, and the welded texture together suggest that the eruptive center, and by inference the axis of the Togiak arc, could not have been far away, probably within several tens of kilometers. TA4. Flat Top Basalt, Taylor Mountains D8 Quadrangle Flat Top Mountain, in the upper reaches of Atsaksovluk River drainage in Taylor Mountains D8 quadrangle (fig. 2), is underlain by Neogene basalt. The rock is a porphyritic olivine-clinopyroxene-plagioclase basalt with 48 weight percent SiO2. Phenocrysts are mostly subhedral and make up about 30 percent of the rock by volume. We obtained a 40Ar/39Ar whole-rock age of 4.6±0.1 Ma from sample 05AM236a. The age is based on a five-step plateau representing 93 percent of 39Ar released (fig. 7). The new result agrees with previously published K-Ar ages of 4.64±0.14 Ma (plagioclase) and 4.72±0.14 Ma (whole rock) (Reifenstuhl and others, 1985). TA5. Dike at Hill 2164, Taylor Mountains D8 Quadrangle This sample is from an intermediate dike cutting the restricted Gemuk Group (of Miller and others, 2007c) at hill 2164 in the easternmost headwaters of Waterboot Creek, Taylor Mountains D8 quadrangle (fig. 2). The sample (05ADw29c) is a hornblende-bearing intermediate rock consisting of a felted lathwork of hornblende and plagioclase with interstitial quartz and much secondary chlorite. We obtained a 40Ar/39Ar hornblende age of 70.1±0.4 Ma from a five-step plateau representing 52 percent of the 39Ar released (fig. 7). The age spectrum shows evidence of argon loss. The corresponding isochron age (not shown) is 69.6±0.3 Ma and is preferred. TA6. Sill at Hill 1768, Taylor Mountains D8 Quadrangle This sample is from hill 1768 on the divide between Waterboot and Cinnabar Creeks in Taylor Mountains D8 quadrangle (fig. 2). It intrudes rocks assigned to the restricted Gemuk Group of Miller and others (2007c), which ranges in age from Triassic to Early Cretaceous and is part of the Togiak terrane. The sample (05AM239a) is a fine-grained, intermediate rock with granular to subophitic texture; plagioclase, minor quartz and secondary biotite are present. Primary clinopyroxene has been altered to chlorite and calcite. We obtained a 40Ar/39Ar biotite age of 74.7±0.4 Ma, based on a seven-step plateau representing 74 percent of the 39Ar released (fig. 7). TA7. Intrusion at Hill 2240, Taylor Mountains D7 Quadrangle A small intermediate intrusive body was discovered at hill 2240 in the northwestern part of the Taylor Mountains D7 quadrangle (fig. 2). It intrudes the Gemuk Group. The sample (05AM224c) is a fine-grained, granular to subhedral, altered quartz monzonite. Mafic phases are early clinopyroxene, later hornblende, and even later biotite. A biotite separate yielded a disturbed 40Ar/39Ar age spectrum (fig. 7). A plateau age cannot be determined. We suggest an age of 87.7±1.7 Ma based on the weighted mean of the six highest-temperature steps. TA8 and TA9. Sills in Kuskokwim Group, Taylor Mountains D7 Quadrangle Sample TA8 (94ADw59b) is from a vesicular, biotitebearing intermediate sill near the northern edge of Taylor Mountains D7 quadrangle that intrudes the Kuskokwim Group (fig. 2). A baked, concordant basal contact and the lack of pillow structures together suggest that it is more likely a sill than a flow that erupted onto a submarine fan. The geochronology sample is a subophitic textured andesite or basaltic andesite with primary hornblende, minor biotite, and possible minor relict clinopyroxene. Sample 94ADw59B yielded a whole rock sample plateau age of 69.3±0.4 Ma; the age is based a four-step plateau representing 76 percent of 39Ar released (fig. 7). A lithologically similar igneous rock (sample TA9, 05AM215a) was collected roughly along strike, about 0.5 km to the south. The sample is a fine-grained, subophitic porphyry. Relict clinopyroxene is present; later brown hornblende is commonly rimmed by lesser biotite. We obtained a 40Ar/39Ar hornblende age of 70.1±0.4 Ma (fig. 7). The age is based on a six-step plateau representing 71 percent of the 39Ar argon released. The age spectrum shows evidence of argon loss. An isochron age is calculated at 74.2±1.1 Ma (not shown). We feel that the isochron age is more representative of the cooling age of this sample, but should be considered a possible minimum age. TA10. Little Taylor Pluton, Taylor Mountains D4 Quadrangle The Little Taylor Mountains are underlain by a series of steeply dipping, tabular bodies, which are too small to show on figure 2. These granite porphyry intrusions strike roughly 300°, parallel to regional bedding strike. Minor associated gold mineralization has been reported (Hudson, 2001). The sample (04AM45a) is a granite porphyry containing abundant, blocky phenocrysts of quartz, plagioclase, and potassium feldspar, smaller phenocrysts of white mica, and a fine-grained groundmass of quartz and feldspar. We obtained a U-Pb zircon TIMS age of 68.6±0.1 Ma, based on the mean of three concordant and overlapping single crystal 206Pb/238U ages (fig. 5).

28    Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages TA11. Intrusion in Enatalik Creek drainage, Taylor Mountains C8 Quadrangle A small stock was discovered near the southern edge of the Taylor Mountains C8 quadrangle (fig. 2) in the headwaters of Enatalik Creek, 2-3 km north of BM Tippy (see TA12 below). The sample (05AM144d) is a granite with large crystals of potassium feldspar, quartz (rounded), plagioclase, and less abundant biotite; a finer grained groundmass of the same minerals fills interstices. We obtained a 40Ar/39Ar biotite age of 68.7±0.3 Ma (fig. 7). The age is based on a 10-step plateau representing 98 percent of the 39Ar released. TA12. Intrusion Near BM Tippy, Taylor Mountains C8 Quadrangle Rubble of a medium-grained intrusive rock of intermediate composition was sampled along the southern boundary of the Taylor Mountains C8 quadrangle, about 0.5 km south of BM Tippy (fig. 2). Abundant hornfels in the area of BM Tippy suggests the presence of a sizeable pluton at depth, of which the present sample is the only surface exposure studied. The sample (08AM611a) is a granite porphyry. Phenocrysts are of plagioclase, quartz, and an altered mafic mineral that was probably hornblende. We obtained a U-Pb zircon SHRIMP-RG age of 69.8±1.6 Ma (fig. 6). The age is the mean of nine overlapping 206Pb/238U analyses; one zircon analysis was suspected of lead loss and was discounted. A nearby detrital zircon sample from contactmetamorphosed Kuskokwim Group yielded zircons as young as 77, 78, and 79 Ma (Miller and others, 2007b). These ages constrain the time between deposition and plutonism in this part of the Kuskokwim Basin to only about 7 million years. TA13 and TA14. Sills in Kuskokwim Group, Taylor Mountains C1 Quadrangle Two narrow, apparently sill-like intrusive bodies were mapped along the eastern edge of Taylor Mountains quadrangle (fig. 2). The more northerly of the two (TA13, 05AM115a) is a granite porphyry containing phenocrysts of quartz, feldspar, and white mica, and what appear to be xenocrysts of a chloritized mafic mineral and altered garnet. The fine-grained groundmass consists of quartz, feldspar, and sericite. We obtained a 40Ar/39Ar white mica age of 68.3±0.3 Ma (fig. 7). This is a 12-step plateau age representing 99 percent of the 39Ar released. The more southerly intrusion (TA14, 05ADw17a) is a coarse-grained granodiorite. The sample contains large, randomly oriented crystals of plagioclase, quartz, potassium feldspar, and chlorite in a somewhat finer groundmass that includes a population of smaller quartz crystals. We obtained a U-Pb zircon SHRIMP-RG age of 68.1±1.9 Ma (fig. 6). The age is the mean of nine overlapping 206Pb/238U analyses; one zircon analysis was suspected of minor inheritance and was discounted. TA15 and TA16. Shotgun Hills Pluton, Taylor Mountains C6 and B6 Quadrangles The Shotgun Hills pluton is one of the most conspicuous intrusive bodies in Taylor Mountains quadrangle (fig. 2). The more northerly of two geochronology samples (TA15, 05ADw59c) is from near the northern contact where the pluton intrudes and bakes Kuskokwim Group strata that were already tight to isoclinally folded prior to intrusion. The sample is an unfoliated granodiorite containing coarse crystals of plagioclase, quartz, potassium feldspar, and abundant biotite. We obtained a 40Ar/39Ar biotite age of 67.2±0.2 Ma (fig. 7). The age is based on a six-step plateau representing 86 percent of the 39Ar released. Sample TA16 (05AM153a) is from about 5 km to the south. The sample is an unfoliated granite containing coarse crystals of quartz, potassium feldspar, plagioclase, and biotite. We obtained a U-Pb zircon SHRIMP-RG age of 67.9±1.2 Ma. The age is the mean of nine 206Pb/238U ages; one zircon analysis was suspected of lead loss and was discounted (fig. 6). These results are quite similar to a 40Ar/39Ar biotite plateau age of 68.7±0.2 Ma from the Shotgun Hills pluton reported by Rombach and Newberry (2001; sample location not given). [Rombach and Newberry (2001) also reported 40Ar/39Ar biotite plateau ages from four samples of a separate body, known, confusingly, as the Shotgun granite porphyry. This irregular, slightly older stock, which hosts quartzstockwork gold mineralization, is located about 0.5 km southwest of the much larger Shotgun Hills granodiorite pluton. Rombach and Newberry (2001) reported a 40Ar/39Ar biotite age of 69.7±0.3 Ma from the granite porphyry, the mean age from the four least-altered samples.] TA17. Intrusion South of Shotgun Airstrip, Taylor Mountains B6 Quadrangle This intrusion forms a conspicuous, glacially streamlined hill south of the airstrip and mineral exploration camp in the Shotgun Hills (fig. 2). It is a biotite-quartz porphyry. The sample (05AM154a) is characterized by a fine-grained groundmass of quartz, feldspar, and sericite. Large crystals of resorbed quartz, plagioclase, potassium feldspar, and biotite are interpreted as granitic xenocrysts, as the enclosing groundmass is finer grained. We obtained a 40Ar/39Ar biotite age of 70.1±0.4 Ma (fig. 7). This is a 12-step plateau age representing 99 percent of the 39Ar released. TA18 and TA19. Intrusions in Winchester Claim Area, Taylor Mountains B6 Quadrangle We report two new ages from the Winchester claim area, a gold prospect between the King Salmon and Klutuspak Rivers in Taylor Mountains B6 and A6 quadrangles (fig. 2). Sample TA18 (08AM630a), from between hills 1690 and 2036, is a granite porphyry. Phenocrysts are plagioclase (some glomerophyric) to

Geochronology    29 3 mm, rounded to embayed quartz to 2 mm, and biotite to 1 mm that has been altered to chlorite, sphene, and locally, white mica. The groundmass is fine-grained potassium feldspar, quartz, and plagioclase. We obtained a U-Pb zircon SHRIMP-RG age of 69.5±1.0 Ma, the mean of eight 206Pb/238U results (fig. 6). Sample TA19 (08SB129b), from near hill 2169, is an altered gabbro with late hornblende. A hornblende age spectrum (fig. 7) shows evidence of argon loss; accordingly, we regard the isochron age (77.6±0.7 Ma) as the best measure of the cooling age of this sample. Field relations between the two intrusive bodies were not determined. TA20. Intrusion Near BM Reach, Taylor Mountains Quadrangle B5 This sample is from a granite porphyry dike or sill cutting an extensive area of hornfelsed Kuskokwim Group north of BM Reach (fig. 2). The sample (08AM654b) is characterized by a finegrained groundmass of quartz, feldspar, and sericite. Aggregates of large crystals of quartz, plagioclase, potassium feldspar, and muscovite are interpreted as granitic xenoliths, and individual large crystals of these minerals are considered xenocrysts. The groundmass next to many of these is finer grained. We obtained a poorly constrained U-Pb zircon SHRIMP-RG age of 69.9±2.9 Ma. The age is the mean of 12 206Pb/238U results (fig. 6). It is possible that the youngest zircons in this dataset experienced minor lead loss, and the oldest have minor inheritance—but none of the individual analyses are obvious outliers, so all are included in the calculation of the mean. TA21. Intrusion at Hill 1311, Taylor Mountains B4 Quadrangle This sample (06ADw537a) is from a granite porphyry dike or sill intruding sedimentary rocks of the Kuskokwim Group in the highlands immediately northwest of the confluence of the Nushagak and King Salmon Rivers (fig. 2). As observed in thin section, the fine-grained groundmass consists of quartz, feldspar, and sericite. What appear at the outcrop scale as phenocrysts are seen in thin section to be aggregates of quartz, plagioclase, potassium feldspar, muscovite, and rare biotite; these aggregates are thus interpreted as xenoliths, and individual large crystals of these minerals are considered xenocrysts. We obtained a 40Ar/39Ar muscovite age of 67.3±0.2 Ma. The age is based on a six-step plateau representing 97 percent of the 39Ar released (fig. 7). TA22. Dike at Hill 1272, Taylor Mountains B3 Quadrangle This sample is from a 6-meter-thick dike that cuts the Kuskokwim Group at hill 1272 in eastern Taylor Mountains B3 quadrangle (fig. 2). The sample (08SK290a) is a highly altered granite or granodiorite containing quartz, plagioclase, and potassium feldspar; one or more relict mafic phases are present, probably originally biotite and hornblende. We obtained a U-Pb zircon SHRIMP-RG age of 80.2±1.3 Ma. This is the mean of 4 overlapping 206Pb/238U ages and is rather poorly constrained. We discounted one zircon analysis that was suspected of lead loss, and three suspected of inheritance (fig. 4). TA23, TA24, and TA25. Dikes in Red Bluff Area, Taylor Mountains B1 Quadrangle A swarm of east-to northeast-striking felsic dikes cuts a folded turbidite section in the Red Bluff area, just north of the Mulchatna Fault in easternmost Taylor Mountains quadrangle (fig. 2). The three dated samples contain 76-77 weight percent SiO2. Sample TA23 (08SK316a), from hill 2086, has a fine grained groundmass of quartz, feldspar, and sericite and abundant, large, anhedral crystals and aggregates of quartz, muscovite, and plagioclase, and potassium feldspar. The aggregates are granitic rock fragments and thus would be termed xenoliths; the solitary crystals are most likely xenocrysts from the same granitic source. We obtained a U-Pb zircon SHRIMP-RG age of 66.7±0.4 Ma. The age is the mean of seven overlapping 206Pb/238U ages; two zircon analyses were suspected of lead loss and were discounted (fig. 6). Sample TA24 (06AK42), from hill 1841, is lithologically similar to sample TA23. We obtained a 40Ar/39Ar muscovite age of 67.6±0.2 Ma (fig. 7). The age is based on an eight-step plateau representing 96 percent of the 39Ar released. Sample TA25 (06AM447a) is a granite porphyry that has an extremely fine-grained groundmass of quartz, feldspar, and sericite. Large euhedral crystals of quartz are present and these appear to be phenocrysts; some polymineralic aggregates of quartz, muscovite, and feldspar may be xenoliths. We obtained a 40Ar/39Ar muscovite age of 66.1±0.3 Ma (fig. 7). The age is based on a nine-step plateau representing 96 percent of the 39Ar released. TA26. Intrusion Near Keefer Creek, Taylor Mountains B1 Quadrangle Plutonic rocks underlie Hill 1483, southwest of Keefer Creek in Taylor Mountains B1 quadrangle (fig. 2). The sample (06AM453a) is a coarse-grained gabbro containing plagioclase, hornblende, and altered clinopyroxene. We obtained a U-Pb zircon SHRIMP-RG age of 78.2±0.8 Ma. The age is the mean of seven 206Pb/238U ages; one zircon analysis was suspected of lead loss and was discounted (fig. 6). TA27. Chaufchivak Pluton, Taylor Mountains A7 Quadrangle Mount Chaufchivak is a low but prominent hill of hornfels rising above the eastern end of Lake Chauekuktuli in Taylor Mountains A7 quadrangle; associated plutonic rocks are exposed on a lower knob to the east (fig. 2), where the geochronology sample (08AM614a) was collected. The sample is a granodiorite

30    Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages containing plagioclase in large crystals, quartz, potassium feldspar, biotite, and lesser hornblende. We obtained a 40Ar/39Ar biotite age of 69.2±0.3 Ma (fig. 7). The age is based on a five-step plateau representing 82 percent of the 39Ar released. TA28. Intrusion at Hill 1623, Taylor Mountains A6 Quadrangle This small intrusion is exposed around the top of hill 1623 in the headwaters of Koneruk Creek in Taylor Mountains A6 quadrangle. It intrudes the Kuskokwim Group. The sample (08SB127c) is a medium-grained hypidiomorphic granular quartz diorite or tonalite. Plagioclase is partly altered to sericite. Quartz is not abundant. Primary muscovite is present. What was probably primary biotite is now represented by clots of chlorite and opaque minerals. Sphene, apatite, and partly hematized opaque minerals are present. Potassium feldspar is absent. We obtained a 40Ar/39Ar muscovite age of 76.9±0.9 Ma (fig. 7). The age is based on a seven-step plateau representing 99 percent of the 39Ar released. This could either represent an igneous cooling age or cooling from a subsequent hydrothermal system. TA29. Intrusion Near Hill 1635, Taylor Mountains A6 Quadrangle This intrusive body was encountered in a single outcrop in Taylor Mountains A6 quadrangle along an unnamed creek between hill 1405 on the west and hill 1635 on the east (fig.  2). The sample (08ADw825a) is an undeformed diorite featuring large plagioclase crystals (some of them rimmed by plagioclase, presumably of a different composition), hornblende, clinopyroxene, and sphene. Zircons yielded a U-Pb SHRIMP-RG age of 79.4±0.4 Ma. The age is the mean of 7 overlapping 206Pb/238U ages; one zircon analysis was suspected of lead loss and was discounted (fig. 6). We also obtained a 40Ar/39Ar hornblende age of 79.1±0.3 Ma (fig. 7), based on a three-step plateau representing 97 percent of 39Ar released. TA30 and TA31. Tikchik Mountain Pluton, Taylor Mountains A6 Quadrangle The Tikchik Mountain pluton, in Taylor Mountains A6 quadrangle, may or may be part of the same body as what we refer to as the Tikchik Narrows pluton (sample DI1) in Dillingham quadrangle (fig. 2). It was dated by U-Pb zircon at one location and by 40Ar/39Ar biotite at a second location. Sample TA30 (08ADw822a) is a fine-to medium-grained, hypidiomorphic granular biotite granite containing accessory epidote and allanite. Locally, biotite has been slightly altered to chlorite. Zircons yielded a U-Pb SHRIMP-RG age of 62.2±0.8 Ma (fig. 6). This is the mean of eight overlapping 206Pb/238U ages; two zircon analyses were suspected of lead loss and were discounted. A sample of biotite granite (TA31, 08SB117a), from about 2 km to the east, was dated by 40Ar/39Ar (fig. 7). It yielded an age of 61.5±0.2 Ma, based on a nine-step plateau representing 98 percent of the 39Ar released. The two results overlap within uncertainties. TA32. Arrow Pluton, Taylor Mountains A5 Quadrangle The Arrow pluton, which takes its informal name from nearby Arrow Creek (fig. 2), was dated by multiple methods. The sample (08SB147a) is a medium-grained, hypidiomorphic hornblende-biotite granite. Tourmaline and sphene are present as accessory phases. We obtained a U-Pb zircon age of 74.3±1.1 Ma using the SHRIMP-RG. The age is the mean of 10 overlapping 206Pb/238U ages (fig.  6). 40Ar/39Ar ages were obtained at University of Alaska Fairbanks (fig. 7) from both biotite and hornblende. A biotite age of 74.0±0.2 Ma is based on a five-step plateau representing 83 percent of the 39Ar released. A very similar hornblende age of 73.8±0.3 is based on a four-step plateau representing 97 percent of the 39Ar released. All three ages agree within uncertainty and suggest that the pluton cooled to <400 °C in less than a million years. TA33. Dike Near Sleitat Mountain, Taylor Mountains A3 Quadrangle This dike, on Sleitat Mountain, is the most southwesterly of a line of intrusive bodies along the Mulchatna Fault (fig. 2). The country rocks are hornfels that we have assigned, on the basis of detrital zircon ages, to the Kuskokwim Group (Miller and others, 2007b). The sample (08AM676a) is a quartzbiotite-muscovite porphyritic granite containing phenocrysts to 2 mm across in a very fine-grained groundmass. We obtained U-Pb zircon SHRIMP-RG age of 68.7±1.6 Ma. The age is the mean of 9 206Pb/238U ages; one zircon analysis was suspected of inheritance and was discounted (fig. 6). TA34. Sleitat Pluton, Taylor Mountains A3 Quadrangle The Sleitat pluton is a peraluminous granite along the Mulchatna Fault trend (fig. 2). The sample (08SB169a) is a fine-to medium-grained hypidiomorphic granular biotite granite. Secondary white mica and late (or secondary?) tourmaline fill interstices. We obtained a U-Pb zircon age of 61.4±0.7 Ma using the SHRIMP-RG. The age is the mean of six overlapping 206Pb/238U ages; we discounted one zircon analysis that was suspected of lead loss, and one suspected of inheritance (fig. 6). Previously, Burleigh (1991) reported a conventional K-Ar age of 56.8±2.8 Ma on muscovite, whereas Layer and Bundtzen (2010) reported 40Ar/39Ar biotite ages for two intrusive phases, both 59.4±0.2 Ma. Tin-silver-tungsten deposits are associated with the pluton; muscovite from

Geochronology    31 greisen yielded a 40Ar/39Ar age of 58.2±0.4 Ma (Layer and Bundtzen, 2010). TA35. Old Man Pluton, Taylor Mountains A3 Quadrangle This pluton does not appear to crop out but its presence is inferred from abundant, large boulders strewn across hill 1180 near Old Man Creek in Taylor Mountains A3 quadrangle. It is one of a line of intrusive bodies that lie along or very close to the Mulchatna Fault trend (fig. 2). The sample (06AK69a) is a fine-to medium-grained, hypidiomorphic granular biotite granite. Some but not all of the biotite has been replaced by chlorite. We obtained a 40Ar/39Ar biotite age of 66.7±0.3 Ma, based on a nine-step plateau representing 86 percent of the 39Ar released (fig. 7). TA36. Intrusion at Hill 1004, Taylor Mountains A3 Quadrangle This unnamed intrusive body does not crop out, but its presence is revealed by a field of granite boulders east of Sleitat Mountain (fig. 2). The sample (08ADw896a) is a medium-grained, hypidiomorphic hornblende-biotite quartz monzonite to granite. Clinopyroxene cores are identifiable in some hornblende crystals. We obtained a U-Pb zircon SHRIMP-RG age of 69.1±0.9 Ma; the age is the mean of eight overlapping 206Pb/238U ages (fig. 6). TA37. Intrusion at Hill 1422, Taylor Mountains A2 Quadrangle This small unnamed stock crops out along or near the Mulchatna Fault in southeastern Taylor Mountains quadrangle (fig. 2). The top of hill 1422 is underlain by complexly deformed, contact metamorphosed metasedimentary rocks; quartz  monzonite is exposed on the northern flank of the hill. The sample  (06ADw570e) is a fine-to medium-grained, hypidiomorphic orthopyroxene-clinopyroxene-biotite quartz monzonite. We obtained a U-Pb TIMS zircon age of 61.0±0.1 Ma, based on five concordant and overlapping single crystal ages (fig.  5). TA38. Rhyolite at Hill 1678, Taylor Mountains A1 Quadrangle This small body of rhyolite crops out in low hills in the northeastern corner of the Taylor Mountains A1 quadrangle (fig. 2). The sample (06AJ062a) is a flow-banded, porphyritic rhyolite containing phenocrysts of quartz, plagioclase, and K-feldspar in a very fine-grained groundmass of the same minerals. Angular lithic clasts of country rock are present. The flow banding suggests a pyroclastic origin. We obtained a U-Pb TIMS zircon age of 49.1±0.3 Ma (fig. 5), based on two concordant and overlapping single crystal ages. TA39. Intrusion at Hill 982, Taylor Mountains A1 Quadrangle This small unnamed stock crops out on low hills near the eastern edge of the Taylor Mountains A1 quadrangle (fig. 2). The sample (06ADw568a) is a biotite granite. Phenocrysts of K-feldspar (to 5 mm), plagioclase (2-5 mm) and biotite (1 mm) occur throughout a finer groundmass of the same minerals. Biotite is locally altered to chlorite. We obtained a U-Pb TIMS zircon age of 56.3±0.2 Ma (fig. 5), based on five concordant and overlapping single crystal ages. Lake Clark Quadrangle LC1. Overlook Pluton, Lake Clark B8 Quadrangle This pluton crops out at BM Overlook just south of the Mulchatna Fault trace in western Lake Clark quadrangle (fig. 2) and appears to be truncated by the fault. It intrudes Mesozoic turbidites assigned by Nelson and others (1983) to their map unit KJs and by Wallace and others (1989) to their Jurassic-Cretaceous Koksetna River sequence. The sample (06ADw558a) is a coarse-grained gabbro containing large crystals of altered plagioclase, hornblende, clinopyroxene, an opaque phase inferred to be an Fe-Ti oxide, and sphene. We obtained a U-Pb TIMS zircon age of 76.5±0.2 Ma (fig. 5), based on five concordant and overlapping single crystal ages. Goodnews Bay Quadrangle GO1. Intrusion Near Jacksmith Creek, Goodnews Bay B8 Quadrangle Separate rubble patches of Cretaceous granite porphyry and Paleoproterozoic granite of the Kilbuck terrane were sampled near hill 820 in the headwaters of Jacksmith Creek (fig. 4K). The older granite, which yielded a U-Pb zircon age of 2,084.9±5.8 Ma, is the oldest tightly dated rock in Alaska (Bradley and others, 2014). Presumably, the younger granite porphyry (sample 06ADw603b) intruded the older granite. The granite porphyry has medium grained phenocrysts of quartz, plagioclase, biotite, and minor very altered mafics (former hornblende?), in a fine-grained groundmass of quartz, plagioclase, and potassium feldspar. We obtained a 40Ar/39Ar plateau age of 70.7±0.3 Ma from the granite porphyry (fig. 7). The age is based on a six-step plateau representing 94 percent of the 39Ar released (fig. 7). GO2. Dike at Island Mountain, Goodnews Bay B7 Quadrangle On the north flank of Island Mountain (fig. 4K), an altered dike (sample 06ADw602c) cuts green, deformed, pebbly sandstone that Hoare and Coonrad (1978) assigned to their

32    Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages undifferentiated mapping unit, MzPz. The dike is medium grained and consists primarily of plagioclase and clinopyroxene, accessory opaques, and very minor quartz. The plagioclase is altered to finegrained white mica(?); the edges of the clinopyroxene are altered to chlorite. We obtained a 40Ar/39Ar whole rock age of 155.0±1.9 Ma based on the weighted mean of two overlapping plateau ages (155.7±3.0 and 154.4±2.6 Ma) (fig. 7). Dillingham Quadrangle DI1. Tikchik Narrows Pluton, Dillingham D7 Quadrangle The Tikchik Narrows pluton crops out along the shores and islands of Tikchik Lake in Dillingham D7 quadrangle (fig. 2). The sample (08ADw826a) is a coarse-grained granite containing alkali feldspar (now perthite), quartz, plagioclase, biotite, and conspicuous accessory zircon. We obtained a 40Ar/39Ar biotite age of 60.2±0.2 Ma. This is a three-step plateau age representing 62 percent of the 39Ar released (fig. 7). The biotite age of this pluton is 1-2 million years younger than the biotite (61.5±0.2 Ma) and zircon (62.2±0.8 Ma) ages of the Tikchik Mountain pluton a few kilometers to the north (samples TA30 and TA31). Owing to lack of exposure, it is not known whether the Tikchik Mountain and Tikchik Narrows plutons are parts of a single igneous body. Implications for the Magmatic History of Western Alaska Pennsylvanian The new U-Pb zircon age of 317.7±0.6 Ma reveals the presence of Pennsylvanian intermediate igneous (probably volcanic) rocks in the Tikchik terrane in the Bethel quadran­ gle. Igneous rocks of this general age are not well represented in the Alaskan geologic record and the locations of various terranes at the time is conjectural. The new age supports the idea (Box and others, 2015) that the Tikchik terrane represents part of the elusive collider with the Farewell terrane during the Late Pennsylvanian to Early Permian Brown's Fork orogeny (of Bradley and others, 2003b). Triassic Our U-Pb zircon age of 229.5±0.2 Ma from gabbro intruding the Rampart Group in the Tanana quadrangle is the most reliable isotopic age yet obtained from anywhere in the AngayuchamTozitna terrane. It confirms the suspected Triassic age of mafic intrusive rocks that make up a significant part of the terrane (Siwieck and Till, 2006). The new age is somewhat older than a K-Ar hornblende age of 210±6 Ma (recalculated by Wilson and others, 1994, using 1976 decay constant) from gabbro from the main outcrop belt of Rampart Group (Brosge and others, 1969), roughly 30 km to the north of our sample site. Jurassic A mafic dike in Goodnews Bay quadrangle yielded a 40Ar/39Ar whole rock age of 155.0±1.9 Ma. This establishes a Jurassic or older age for the previously unconstrained Paleozoic to Mesozoic sandstone unit of the Goodnews terrane that it intrudes. A thick felsic tuff in the Gemuk Group in Taylor Mountains quadrangle yielded a U-Pb zircon age of 153.0±2.0 Ma, extending the age of magmatism in this part of the Togiak terrane farther back into the Late Jurassic. It is not known how these nearly coeval magmatic episodes relate to each other. Early Cretaceous The three Early Cretaceous plutonic rocks in our sample suite crop out in three different mapped terranes. The about 112-Ma age of the Granite Mountain pluton in the Koyukuk terrane is comparable to U-Pb zircon ages of 112 and 109 Ma that were previously reported from granitoids in this belt (Patton and others, 2009). The about 115-Ma age of the orthogneiss in the Ruby terrane is likewise consistent with the 118-113 Ma age range of plutons that intrude the main outcrop belt of Ruby terrane to the north (Roeske and others, 1998). The about 117-Ma gabbro is part of the Dishna River mafic-ultramafic complex, which has been regarded as part of the Innoko terrane (Miller and Bundtzen, 1994). The gabbro is unexpectedly young and shows that some igneous rocks in the Innoko terrane are coeval with the Early Cretaceous granitoids that intrude the Ruby terrane. Along strike to the southwest, a similar Early Cretaceous age of 117.8±0.7 Ma has been reported from the Nyac pluton (40Ar/39Ar hornblende; Wenz, 2005), which intrudes Jurassic arc volcanics of the Nyac terrane in Bethel quadrangle. A 118-112 Ma magmatic belt thus "stitches" the Koyukuk, Ruby, Innoko, and Nyac terranes. Mid-Cretaceous The Turonian (about 90 Ma) was an important time in the metallogenic history of south-central Alaska. This is the age of the Pebble deposit, a giant porphyry copper-gold system on the Alaska Peninsula (fig. 1) (Schrader and others, 2001; Goldfarb and others, 2013; Anderson and others, 2013). Approximately 90-Ma plutonic rocks crop out is a discontinuous belt that was previously known to extend from Kemuk Mountain (fig. 1) (Anderson and others, 2013) to the southernmost Talkeetna Mountains (Bleick and others, 2012) (barely east of the eastern edge of figure 1 at latitude 61.5° N). One intrusive rock in our sample suite is about this age—a small intermediate pluton (TA7) with a 40Ar/39Ar age of 87.7±1.7 Ma that intrudes the Togiak terrane in Taylor Mountains quadrangle. This new result shows that Pebble-age igneous centers are present across a strike-width of 300 km.

Implications for the Magmatic History of Western Alaska    33 At 90 Ma, the Sleetmute and Taylor Mountains quadrangles were the site of submarine fan sedimentation in the Kuskokwim basin. Situated on the inboard side of the Pebble igneous trend, this basin has been interpreted as a collisional foreland basin (Kalbas and others, 2007) or a retroarc foreland basin (Bradley and others, 2009). A tuff interbedded with Kuskokwim Group turbidites yielded a U-Pb zircon age of 88.3±1.0 Ma. This attests to explosive volcanism at this time, presumably from a source somewhere along the Pebble trend. Late Cretaceous Most of samples in the present study are Late Cretaceous to Paleocene in age. The regional pattern of magmatism is shown in a series of 2-m.y. time slices in figure 8A-N. The maps show a combination of ages obtained by various methods (for example, U-Pb zircon ages that essentially date magmatism versus K-Ar muscovite ages that date cooling through about 350 °C). This mix likely blurs some details. Only a smattering of magmatic activity is shown in the scenes covering 78 to 72 Ma (fig. 8A-C). A flare-up followed, which was most intense from about 72 to 68 Ma. Magmatism continued across the region from about 68 to 56 Ma, and then waned, particularly in the south. In general, changes from one scene to the next are gradual, but four exemplary time slices, shown with bold outlines, are sufficiently spaced to highlight the most significant changes in patterns of magmatism over a 24-m.y. interval: 78-76, 72-70, 64-62, and 56-54 Ma (figs. 8A, D, H, and L). The distribution of Late Cretaceous paleogeographic elements is an important constraint in deciphering the cause of the 72-68 Ma flare-up (fig. 9). This interval overlaps with the age range of a thick package of trench turbidites that comprise the Valdez Group and correlatives in the Chugach accretionary complex (V in fig. 9). Coeval turbidites of the Matanuska Formation record the existence and location of a subsiding forearc basin (Trop and Ridgway, 2007; M in fig. 9). In the central Alaska Range, the lower Cantwell Formation was deposited and deformed in a thrust-top, retroarc foreland basin setting (Trop and Ridgway, 2007; C in fig. 9). Ash beds in the lower Cantwell have yielded U-Pb zircon ages of 71.5±0.9 and 71.0±1.1 Ma (Tomsich and others, 2014). The inferred boundaries between accretionary wedge, forearc basin, orogenic belt, and retroarc foreland basin can be reconstructed on these grounds (fig. 9). Gold and mercury mineralization across much of southwestern Alaska dates from this same interval (Bundtzen and Miller, 1997; Gray and others, 1997). Most of the magmatic activity during this time appears to have been in the retroarc region. The cause of this wide, 72-68 Ma belt of magmatism along a continental-margin arc setting is not well understood. One possibility is low-angle subduction (Miller and others, 2007a). The inboard-to-outboard shift in the area of greatest magmatic activity between 72 and 68 Ma (see figs. 8D, E, and F) could perhaps have been produced by rollback of a subducted oceanic plate, as has been postulated for the Appenine orogen of Italy and its Ligurian Sea hinterland (Malinverno and Ryan, 1986). Melting, in this scenario, would be induced by decompression of asthenosphere that would have welled up into the resulting gap. Another possible mechanism for magma genesis at about 72 to 68 Ma is that southwestern Alaska was extended as it escaped westward toward a free face in the Bering Sea (Redfield and others, 2007). An analog would be the Neogene of Turkey, where westward escape of Anatolia along strike-slip faults produced a broad magmatic province not related to subduction (Pearce and others 1990). The shortcoming of this model is the lack of any structural evidence for extension; 70 Ma, instead, was a time of dextral strike-slip faulting and associated transpressional folding (Miller and others, 2002). Another conceivable way to generate both a magmatic flareup and regional mercury mineralization is a slab window resulting from ridge subduction. Northern California's mercury belt originated above a slab window (Smith and others, 2008). Ridge subduction has been well documented on Alaska's Pacific margin—but at 61 and 50 Ma (Bradley and others, 2000 and 2003a; see next section) it is too young. In any event, Neogene ridge subduction in the southern Andes has produced a magmatic lull along the arc (Forsythe and Prior, 1992; Gorring and others, 1997), not a flare-up. Two dated rock units from the 72-68 Ma interval provide improved age constraints on deformation events. The Chuilnuk pluton was emplaced just north of the Denali Fault in Sleetmute quadrangle (fig. 2). It truncates dextral en echelon folds in the Kuskokwim Group, which Miller and others (2002) attributed to pre-Chuilnuk dextral displacement along the Denali Fault. Previously, this event was bracketed between a conventional K-Ar biotite age on the Chuilnuk pluton of 68 Ma and about 85 Ma, which corresponds to the youngest nearby fossil ages from the Kuskokwim Group (Miller and others, 2002). This episode of dextral motion can be more narrowly constrained as older than the 70.2±1.0 Ma zircon age from the Chuilnuk, as reported herein, but younger than deposition of the youngest Kuskokwim Group strata at about 76 Ma, based on detrital zircons (Miller and others, 2007b). Paleocene and Early Eocene The case for ridge subduction along the Gulf of Alaska margin is based largely on a tightly dated belt of near-trench plutons in the Chugach accretionary complex (Bradley and others, 2000, 2003a; Haeussler and others, 2003). These 61 to 50 Ma plutons intruded into trench deposits, including some that had been accreted to the upper plate only a few million years before. Each pluton is interpreted to mark the place where an oceanic spreading center was subducted; the triple junction migrated west to east between 61 and 50 Ma. In the area of figure 8, the position of the triple junction is shown for four time slices. Tellingly, magmatism in western Alaska diminished or shut off entirely about when the triple junction was at the now-adjacent part of the accretionary complex. Analogous behavior is seen in Chile where the NazcaAntarctic spreading center is being subducted beneath South America. There, the present position of a slab window (which has as its center the downdip projection of the subducted ridge) corresponds to a magmatic gap along the arc (Forsythe and Prior,

34    Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages Trench Trench Trench Trench Trench Trench Trench Trench U-Pb K-Ar 40Ar/39Ar Seaward migration of magmatism from previous time slice 64-62 Ma 66-64 Ma 68-66 Ma 70-68 Ma 72-70 Ma 74-72 Ma 76-74 Ma 78-76 Ma 148° 152° 156° 160° 148° 152° 156° 160° 148° 152° 156° 160° 148° 152° 156° 160° 148° 152° 156° 160° 148° 152° 156° 160° 148° 152° 156° 160° 148° 152° 156° 160° 62° 58° 62° 58° 62° 58° 62° 58° 62° 58° 62° 58° 62° 58° 62° 58° A B D E F G H 200 KILOMETERS 100 MILES EXPLANATION Subduction zone—sawteeth on upper plate Figure 8.  Maps (A to N) showing the distribution of isotopic ages from igneous rocks in western Alaska in 2-million-year increments from 78 to 50 million years ago (Ma). The shaded relief was generated from the U.S. Geological Survey National Elevation Dataset (NED) 2-arc-second Digital Elevation Model (DEM). Conventional K-Ar ages are from the Alaska radiometric age database, accessible at https://mrdata.usgs.gov/akages/. Additional ages are plotted from Bradley and others (2000), Farris and others (2006), Goldfarb and others (2004), and Miller and others (2002). O, Index to 1:250,000-scale quadrangle maps; see caption to figure 1 for key to abbreviations. Orientation of the hypothesized Kula-Resurrection ridge is schematic.

Implications for the Magmatic History of Western Alaska    35 52-50 Ma 54-52 Ma 56-54 Ma Ridge at 55 Ma Approx. edge of slab window 58-56 Ma Ridge at 57 Ma Approx. edge of slab window 148° 152° 156° 160° 62° 58° 148° 152° 156° 160° 62° 58° 148° 152° 156° 160° 62° 58° 148° 152° 156° 160° 62° 58° 148° 152° 156° 160° 62° 58° 60-58 Ma Ridge at 59 Ma Approx. edge of slab window J K M N O Key to quadrangle maps LH MG ID MD MM KH SM TA TL TY GO BH RM HC 62-60 Ma Ridge at 61 Ma Approx. edge of slab window 148° 152° 156° 160° 62° 58° Trench Trench Trench Trench Figure 8.—Continued

36    Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages 152° 148° 156° 160° 62° 58° 72-68 Ma

M Hg-Sb mineralization

200 MILES 300 KILOMETERS Trench Accretionary w e d g e Forearc b asin Andean-t ype B order Range s Fa u t Or o g e n Retro-ar

f o re la n d

b a s n EXPLANATION U-Pb K-Ar 40Ar/39Ar Fault—dashed where approximate sawteeth on upper plate Thrust fault—dashed where approximate; sawteeth on upper plate Subduction zone—sawteeth on upper plate Figure 9.  Map showing Late Cretaceous paleogeographic elements and igneous rock samples with ages of 72-68 million years (Ma) in western Alaska. The shaded relief was generated from the U.S. Geological Survey National Elevation Dataset (NED) 2-arc-second Digital Elevation Model (DEM). 1992). We interpret the 55-43 Ma magmatic lull in southwestern Alaska as a time when a slab window lay at depth. An important related issue is the amount of coast-parallel strike-slip displacement between the Chugach terrane, which hosts the 61-50 Ma near-trench plutons, and its immediate backstop, the Peninsular terrane. The boundary between these two terranes is the Border Ranges Fault (fig. 9). If displacement is in the range of thousands of kilometers, as Cowan (2003) has suggested, our interpretation would be invalid. Provenance data and field relations, however, indicate that the Paleogene fill of the Matanuska forearc basin was derived from both outboard (Little, 1988) and inboard (Kortyna and others, 2013) flanks. For this sector of the margin, at least, this precludes large (terrane-scale) displacements since about 54 Ma between the Chugach terrane and its backstop. We therefore take the apparent paleogeographic relations at face value and in so doing find a ready explanation for what otherwise would be a problematic magmatic shutdown (Bradley and others, 2006). The slab window concept also bears on magma genesis immediately before the magmatic lull that began at about 55 Ma. The time leading up to the lull was one of widespread magmatism. For example, in the western Alaska Range (Tyonek quadrangle, fig. 1), Todd and others (2015) reported a magmatic flare-up from 63 to 56 Ma. In an overall setting of ridge subduction, subducting oceanic crust immediately adjacent to a slab window is only a few million years old, and its corresponding lithosphere is young, hot, and thin (Thorkelson, 1996). Where plate geometry is such that the slab window migrates, hot and thin oceanic lithosphere is subducted beneath a given location just before arrival of the slab window. If the slab window interpretation is valid in the present instance, these conditions would have existed in the western Alaska Range during the interval of a few million years leading up to about 55 Ma (Bradley and others, 2006), with subduction of young, hot, thin lithosphere along the edge of the slab window being responsible for widespread plutonism during this interval (figs. 8J-L). Mid-Eocene to Pliocene The present study has yielded only a few new dates that bear on younger events (Jones and others, 2014). During the entire mid-Eocene to Pliocene interval, the modern subduction geometry has been in place, with the Pacific Plate subducting to the northwest beneath the western Gulf of Alaska margin. Two newly dated Eocene rocks are likely related to this phase of subduction. One is the rhyolite dated at 49.1±0.3 Ma in Taylor Mountains quadrangle; the other is the ashfall tuff dated at 42.8±0.5 Ma in McGrath quadrangle. Two Oligocene plutons in the western Alaska Range, at 31.8±0.4 and 30.9±0.6 Ma, belong to a distinctly younger intrusive suite that ranges from gabbro to peralkaline granite. These plutons are located far inboard of the modern volcanic arc and the cause of magmatism is not clear. Finally, the 4.6±0.1-Ma basalt from a flow in Taylor Mountains quadrangle belongs to the Neogene basaltic province of western Alaska. These rocks were erupted in a distal retroarc setting; the cause of magmatism also remains elusive (Moll-Stalcup, 1994). The Flat Top flows were erupted onto a topographic surface that was already about as rugged and mountainous as today. The flows bury a straight valley along the Atsaksovluk strand of the Denali strike-slip fault. The volcanics themselves may be slightly disturbed by the fault (satellite images are equivocal on this point), but in any case the Atsaksovluk lineament clearly is older than the Pliocene volcanism.

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References Cited    41 Siwieck, B.R., and Till, A.B., 2006, Synthesis of geochronologic, geochemical, metamorphic, and paleontological data on rocks of oceanic origin in northern Alaska: Geological Society of America Abstracts with Programs, v. 38, no. 5, p. 83. Solie, D.N., Bundtzen, T.K., and Gilbert, W.G., 1991, K-Ar ages of igneous rocks in the McGrath quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Public Data File Report 160, 17 pages, one sheet, 1:250,000 scale. Smith, C.N., Kesler, S.E., Blum, J.D., and Rytuba, J.J., 2008, Isotope geochemistry of mercury in source rocks, mineral deposits and spring deposits of the California Coast Ranges, USA: Earth and Planetary Science Letters, v. 269, p. 399-407. Stacey, J.S., and Kramers, J.D., 1975, Approximation of terrestrial lead isotope evolution by a two-stage model: Earth and Planetary Science Letters, v. 26, p. 207-221. Steiger, R.H., and Jaeger, E., 1977, Subcommission on geochronology—Convention on the use of decay constants in geo and cosmochronology: Earth and Planet Science Letters, v. 36, p. 359-362. Thirlwall, M.F., 2000, Inter-laboratory and other errors in Pb isotope analyses investigated using a 207Pb-204Pb double spike: Chemical Geology, v. 163, p. 299-322. Thorkelson, D.J., 1996, Subduction of diverging plates and the principles of slab window formation: Tectonophysics, v. 255, p. 47-63. Till, A.B., Dumoulin, J.A., Phillips, J.D., Stanley, R.G., and Crews, Jesse, 2006, Generalized bedrock geologic map, Yukon Flats region, east-central Alaska: U.S. Geological Survey Open-File Report 2006-1304, scale 1:500,000, https://pubs.usgs.gov/ of/2006/1304/YF_text.pdf. Todd, E., Jones, J.V., III, Box, S.E., Saltus, R., Karl, S.M., and Haeussler, P.J., 2015, Western Alaska Range magmatic response to progressive accretion of the Wrangellia composite terrane and evolution of the southern Alaska margin: Geological Society of America Abstracts with Programs. v. 47, no. 4, p. 53. Tomsich, C.S., McCarthy, P.J., Fiorillo, A.R., Stone, D.B., Benowitz, J.A., and O'Sullivan, P.B., 2014, New zircon U-Pb ages for the lower Cantwell Formation—Implications for the Late Cretaceous paleoecology and paleoenvironment of the lower Cantwell Formation near Sable Mountain, Denali National Park and Preserve, central Alaska Range, USA, in Stone, D.B., Grikurov, G.E., Clough, J.G., Oakey, G.N., and Thurston, D.S., eds., Proceedings of the International Conference on Arctic Margins VI, Fairbanks, Alaska, May 2011: St. Petersburg, Russia, A.P. Karpinsky Russian Geological Research Institute (VSEGEI), p. 19-60. [Also available at http://www.vsegei.ru/ru/public/icam/icam-all_ vvs.pdf.] Trop, J.M., and Ridgway, K.D., 2007, Mesozoic and Cenozoic tectonic growth of southern Alaska—A sedimentary basin perspective, in Ridgway, K.D., Trop, J.M., Glen, J.M.G., and O'Neill, J.M., eds., Tectonic growth of a collisional continental margin, crustal evolution of southern Alaska: Geological Society of America Special Paper 431, p. 55-94,

Wallace, W.K., Hanks, C.L., and Rogers, J.F., 1989, The southern Kahiltna terrane; implications for the tectonic evolution of southwestern Alaska: Geological Society of America Bulletin, v. 101, p. 1389-1407. Wenz, Z.J., 2005, An investigation of the geology and gold mineralization of the Nyac district, southwest Alaska: U.S. Bureau of Land Management, Alaska State Office, Open-File Report 103, 156 p. Williams, I.S., 1998, U-Th-Pb geochronology by ion microprobe: Reviews in Economic Geology, v. 7, p. 1-35. Wilson, F.H., Dover, J.H., Bradley, D.C., Weber, F.R., Bundtzen, T.K., and Haeussler, P.J., 1998, Geologic map of central (interior) Alaska: U.S. Geological Survey Open-File Report 98-133, 64 p., 3 plates, scale 1:500,000. Wilson, F.H., Hults, C.P., Mohadjer, S., and Coonrad, W.L., compilers, 2013, Reconnaissance geologic map of the Kuskokwim Bay region, southwest Alaska, including the Bethel, Goodnews Bay, Nushagak Bay, Hagemeister Island, Baird Inlet, Cape Mendenhall, Kuskokwim Bay, Nunivak Island, Saint Matthew, and Pribilof Islands 1:250,000scale quadrangles: U.S. Geological Survey Scientific Investigations Map 3100, pamphlet 46 p., 1 sheet, scales 1:500,000, 1:300,000, 1:250,000, https://pubs.usgs.gov/ sim/3100/. Wilson, F.H., Hults, C.P., Mull, C.G, and Karl, S.M, compilers, 2015, Geologic map of Alaska: U.S. Geological Survey Scientific Investigations Map 3340, pamphlet 196 p., 2 sheets, scale 1:1,584,000, https://doi.org/10.3133/sim3340. Wilson, F.H., Shew, N., and DuBois, G.D., 1994, Map and tables showing isotopic age data, in The geology of North America: Geological Society of America DNAG Series, v. G-1, scale1:2,500,000. York, D., Hall, C.M., Yanase, Y., Hanes, J.A., and Kenyon, W.J., 1981, 40Ar/39Ar dating of terrestrial minerals with a continuous laser: Geophysical Research Letters, v. 8, 1136-1138. Zhang, M., Ewing, R.C., Boatner, L.A., Salje, E.K.H., Weber, W.J., Daniel, P., Zhang, Y., and Farnan, I., 2009, Pb* irradiation of synthetic zircon (ZrSiO4)—Infrared spectroscopic study—Reply: American Mineralogist, v. 94, p. 856-858.

Appendixes

44    Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages Appendix 1.—Analytical Methods U-Pb Zircon TIMS Analytical Methods, University of British Columbia Sample preparation, geochemical separations and thermal ionization mass-spectrometry (TIMS) were done at the Pacific Centre for Isotopic and Geochemical Research in the Department of Earth and Ocean Sciences, University of British Columbia. Zircon and other accessory phases were separated from samples using conventional crushing, grinding, and Wilfley table techniques, followed by final concentration using heavy liquids and magnetic separations. Mineral fractions for analysis were selected based on grain quality, size, magnetic susceptibility, and morphology. All zircon fractions were air abraded before dissolution to minimize the effects of post-crystallization Pb-loss, using the technique of Krogh (1982). Samples were dissolved in concentrated hydrofluoric (HF) and nitric (HNO3) acids in the presence of a mixed 233-235U-205Pb tracer for 40 hours at 240 C in polytetrafluoroethylene (PTFE) or alcohol) (PFA) microcapsules contained in high pressure vessels (Parr™ bombs). Sample solutions were then dried to salts at ~125 degrees Celsius (°C), rebombed and redissolved in 3.1 normal (N) hydrochloric acid (HCl) for 12 hours at 210 °C. Separation and purification of Pb and U employed ion exchange column techniques modified slightly from those described by Parrish and others (1987). Pb and U were sequentially eluted into a single beaker and loaded together on a single zone refined Re filament using a phosphoric acid-silica gel (SiCl4) emitter. Isotopic ratios were measured using a modified single collector VG-54R thermal ionization mass spectrometer equipped with an analogue Daly photomultiplier. Measurements were done in peak-switching mode on the Daly detector. U and Pb analytical blanks were in the range of 1 and 1-5 picograms (pg), respectively, during the course of this study. U fractionation was determined directly on individual runs using a 233-235U tracer, and Pb isotopic ratios were corrected for fractionation of 0.37 percent/ atomic mass unit, based on replicate analyses of the NBS-981 Pb standard and the values recommended by Thirlwall (2000). All analytical errors were numerically propagated through the entire age calculation using the technique of Roddick (1987). Weighted average 206Pb/238U ages and concordia ages were calculated using Isoplot (Ludwig, 2003). Concordia intercept ages and associated errors were calculated using a modified version the York-II regression model (wherein the York-II errors are multiplied by the MSWD) and the algorithm of Ludwig (1980). Unless otherwise noted, all errors are quoted at the 2-sigma level. U-Pb Zircon SHRIMP-RG Analytical Methods, USGS-Stanford University Zircon separations were done at the U.S. Geological Survey (USGS) in Anchorage using standard density and magnetic separation techniques. At the jointly operated USGS-Stanford University sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG) lab at Stanford University, zircons were hand-picked for final purity, mounted on double-stick tape on glass slides in 1×6-millimeter (mm) rows, cast in epoxy, ground and polished to a 1-micron (µm) finish on a disc 25 mm in diameter by 4 mm thick. All grains were imaged with transmitted light and reflected light (and incident light if needed) on a petrographic microscope, and with cathodoluminescence and back scattered electrons as needed on a JEOL 5600 scanning electron microscope to identify internal structure, inclusions, and physical defects. The mounted grains were washed with 1N HCl or ethylenediaminetetraacetic acid solution (if acid soluble) and distilled water, dried in a vacuum oven (to fully degas the sample), and coated with Au. Mounts typically sit in a loading chamber at high pressure (10-7 torr) for several hours before being moved into the source chamber of the SHRIMP-RG. Secondary ions are generated from the target spot with an O2-primary ion beam varying from 4 to 6 nano-ampere (nA). The primary ion beam typically produces a spot with a diameter of 20-40 µm and a depth of 1-2 µm for an analysis time of 9-12 minutes. Nine peaks are measured sequentially for zircons (the SHRIMP-RG is limited to a single collector, usually an EDP electron multiplier): 90Zr2 16O, 204Pb, Bgd (background, 0.050 mass units above 204Pb), 206Pb, 207Pb, 208Pb, 238U, 248Th16O, and 254U16O. Autocentering on selected peaks and guide peaks for low or variable abundance peaks (that is, 96Zr216O, 0.165 mass unit below 204Pb) are used to improve the reliability of locating peak centers. The number of scans through the mass sequence and counting times on each peak are varied according to sample age and U and Th concentrations to improve counting statistics and age precision. Measurements are made at mass resolutions of 6,000-8,000 (10 percent peak height), which eliminates all interfering atomic species. The SHRIMP-RG was designed to provide higher mass resolution than the standard forward geometry of the SHRIMP-RG I and II (Clement and Compston, 1994). This design also provides very clean backgrounds and, combined with the high mass resolution, the acid washing of the mount, and mastering the primary beam for 90-120 seconds over the area to be analyzed before data are collected, assures that any counts found at mass of 204Pb are actually Pb from the zircon and not surface contamination. In practice, greater than 95 percent of the spots analyzed have no common Pb. Concentration data for zircons are standardized against zircon standard SL-13 (238 parts per million, ppm, U) or CZ3 (550 ppm U) and age data against AS3 and AS57 zircons (1098 Ma, millions of years,) from the Duluth Gabbro (Paces and Miller, 1993), RG-6 (1440 Ma, granite of Oak Creek stock; Bickford and others, 1989), or R33 (419 Ma, quartz diorite of Braintree complex, Vermont; Black and others, 2004), which are analyzed repeatedly throughout the duration of the analytical session. Data reduction follows the methods described by Williams (1998) and Ireland and Williams (2003) and use the Squid and

Appendix 1 45 Isoplot programs of Ludwig (2001,  2003). For this study, the quoted U-Pb zircon ages are 206Pb/238U weighted-average ages calculated using Isoplot (Ludwig, 2003). 40Ar/39Ar Analytical Methods, University of Alaska, Fairbanks For 40Ar/39Ar analysis, samples were submitted to the Geochronology laboratory at the University of Alaska Fairbanks, where they were crushed, sieved, washed and handpicked for mineral phases or small phenocryst free whole-rock chips. The monitor mineral MMhb-1 (Samson and Alexander, 1987) with an age of 513.9 Ma (Lanphere and Dalrymple, 2000) was used to monitor neutron flux (and calculate the irradiation parameter, J). The samples and standards were wrapped in aluminum foil and loaded into aluminum cans of 2.5 centimeter (cm) diameter and 6 cm height. The samples were irradiated in position 5c of the uranium enriched research reactor of McMaster University in Hamilton, Ontario, Canada for 20 megawatt-hours (MWh). On their return from the reactor, the samples and monitors were loaded into 2 mm diameter holes in a copper tray that was then loaded in an ultra-high vacuum extraction line. The monitors were fused, and samples heated, using a 6-watt argon-ion laser following the technique described in York and others (1981), Layer and others (1987) and Layer (2000). Argon purification was achieved using a liquid nitrogen cold trap and a SAES Zr-Al getter at 400 °C. The samples were analyzed in a VG-3600 mass spectrometer at the Geophysical Institute, University of Alaska Fairbanks. The argon isotopes measured were corrected for system blank and mass discrimination, as well as calcium, potassium and chlorine interference reactions following procedures outlined in McDougall and Harrison (1999). System blanks generally were 2×10-16 mole (mol) 40Ar and 2×10-18 mol 36Ar, which are 10 to 50 times smaller than fraction volumes. Mass discrimination was monitored by running both calibrated air shots and a zero-age glass sample. These measurements were made on a weekly to monthly basis to check for changes in mass discrimination. A summary of all the 40Ar/39Ar results is given in table 1, with all ages quoted to the +/- 1-sigma level and calculated using the constants of Steiger and Jaeger (1977). The integrated age is the age given by the total gas measured and is equivalent to a potassium-argon (K-Ar) age. The spectrum provides a plateau age if three or more consecutive gas fractions represent at least 50 percent of the total gas release and are within two standard deviations of each other (Mean Square Weighted Deviation less than ~2.7). All samples were run three times to check on sample consistency. 40Ar/39Ar Analytical Methods, University of British Columbia These methods apply to a single sample, 01ADw07a, which was processed first for zircon, which also yielded a hornblende fraction. The hand-picked hornblende separate was washed in acetone, dried, wrapped in aluminum foil and stacked in an irradiation capsule with similar-aged samples and neutron flux monitors (Fish Canyon Tuff sanidine, 28.02 Ma; Renne and others, 1998). The sample was irradiated on February 15 through 17, 2006 at the McMaster Nuclear Reactor in Hamilton, Ontario, for 90  MWh, with a neutron flux of approximately 3×1,016 neutrons/cm2. Analyses (n=57) of 19 neutron flux monitor positions produced errors of <0.5 percent in the J value. The samples were analyzed on April 11 and 12, 2006, at the Noble Gas Laboratory, Pacific Centre for Isotopic and Geochemical Research, University of British Columbia, Vancouver, British Columbia, Canada. The mineral separates were step-heated at incrementally higher powers in the defocused beam of a 10 watt (W) CO2 laser (New Wave Research MIR10) until fused. The gas evolved from each step was analyzed by a VG5400 mass spectrometer equipped with an ion-counting electron multiplier. All measurements were corrected for total system blank, mass-spectrometer sensitivity, mass discrimination, radioactive decay during and subsequent to irradiation, as well as interfering Ar from atmospheric contamination and the irradiation of Ca, Cl, and K (isotope production ratios: Ca/

Details of the analyses, including plateau (spectrum) and inverse correlation plots, are presented in Excel spreadsheets. The plateau and correlation ages were calculated using Isoplot ver. 3.09 (Ludwig, 2003). Errors are quoted at the 2-sigma (95-percent confidence) level and are propagated from all sources except mass-spectrometer sensitivity and age of the flux monitor. The best statistically justified plateau and plateau age were picked based on the following criteria: Three or more contiguous steps comprising more than 50 percent of the 39Ar; Probability of fit of the weighted mean age greater than 5 percent; Slope of the error-weighted line through the plateau ages equals zero at 5 percent confidence; Ages of the two outermost steps on a plateau are not significantly different from the weighted-mean plateau age (at 1.8 sigma, six or more steps only); Outermost two steps on either side of a plateau must not have nonzero slopes with the same sign (at 1.8 sigma, nine or more steps only) U-Pb Zircon LA-ICP-MS Analytical Methods, Apatite to Zircon, Inc. Data were collected for the following isotopic masses: 202Hg, 204Hg+204Pb, 206Pb, 207Pb, 208Pb, 232Th, 235U, and 238U

46    Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages (250 data scans over 30 seconds) followed by 28Si and 91Zr (5 data scans over 4 seconds). The instruments used were a New Wave YAG 213 nm laser ablation (LA) system in line with a Finnigan Element2 magnetic sector, inductively coupled plasma, mass spectrometer (ICP-MS) at the Washington State University Geoanalytical Laboratory in Pullman, Washington (for example, Chang and others, 2006). All analyses were performed using a 20-µm spot. Following approximately 6 seconds of background data collection, laser ablation commenced and data were collected for the ablated material. Ablated material was transported to the plasma line using He; Ar was the plasma gas. Zircon standards for which independently accepted ages are published were designated as primary, secondary, and tertiary for purposes of U-Pb age calibration (appendix 2, table A4). Two primary and two secondary standard spots were analyzed before and following each group of ~25-30 tertiary standards and (or) unknown sample spots. Five spots of each tertiary standard were analyzed near the beginning and again near the end of the session. Data Modeling Previous LA-ICP-MS studies of U-Pb zircon dating deployed the so-called intercept method, which assumes that isotopic ratio varies linearly with scan number due solely to linearly varying isotopic fractionation (Chang and others, 2006; Gerhels and others, 2008). For the intercept method, a line is fitted to background-corrected isotopic ratio (for example, 206Pb/238U) versus data scan number and the intercept of the fitted line (corresponding to data scan number=0) is used as the isotopic ratio for age calculation and the error on the intercept is used for age error calculation. In this study, individual isotopes were modeled by fitting a sum of 10 Gaussian equations to the raw signal data (not background corrected) using chi-squared minimization. Two fitting passes were performed: after the first pass, all raw signal values greater than two standard deviations away from the sum of fitted Gaussians were designated outliers; the second pass fit the sum of Gaussians to the data excluding the outliers. The advantage of the present approach is that it avoids the assumption of linearly varying isotopic ratio with scan number, an assumption easily violated for zircons that may contain useful information (for example, a zircon for which the ablation pit variably penetrates two zones having different U-Pb ages). Measured background values for each isotope at each LA-ICP-MS spot were calculated as follows: (1) the final background scan was assigned as the scan closest to the global minima 232Th and 238U values; if no such global minima were found, the analysis was deemed a failure, (2) a line was fitted to the background values, outliers identified, and a line again fitted to the data excluding the outliers, (3) for a fitted line exhibiting a negative slope (indicative of a decaying background), the value of the line at the last background scan was assigned as the background value; for a fitted line exhibiting a zero or positive slope, the mean value of the data excluding the outliers was assigned as the background value, and (4) the error of the background value was set equal to the standard deviation of the all background values (excluding outliers) about their fitted line (negative slope) or mean (zero or positive slope). Session-wide fitted background values for each isotope were determined using all zircon standards and applied to all spots in the session. These steps were taken for each isotope—(1) measured background value versus spot number in the session was fitted to a 3rd-order polynomial, outliers identified, and the fitting repeated excluding the outliers, and (2) fitted background at each session spot was calculated using the 3rd-order polynomial. Session-wide fitted background error was set equal to the standard deviation of the measured background values (excluding outliers) about their respective fitted 3rd-order polynomial. For any spot (standard or unknown) where the measured background value exceeded the session-wide fitted value by more than 2 sigma, the background error was set equal to 1 sigma plus one half of the amount by which the measured background value exceeded the session-wide fitted value by 2 sigma. The sum of fitted Gaussians was used here primarily to identify outlier data and characterize signal noise. After the second fitting pass, the standard deviation of the nonoutlier data about their respective sum of fitted Gaussians was taken as the absolute signal error for each data scan. When N data scans contribute to a single isotopic signal value used for age calculation (only concordant scans when the number of concordant data scans is greater than zero; all data scans for common Pb-correction based on isotopic sums), the error of the single isotopic signal value was set equal to the product of (1) N1/2 and (2) the absolute signal error for each data scan. Pb/U Fractionation Factor Fractionation factors were determined for each data scan of each primary standard spot. For a particular isotopic ratio (for example, 206Pb/238U), the fractionation factor as used here equals the accepted isotopic ratio divided by the measured ratio. A two-dimensional grid (spot number, scan number) of fractionation factors for each isotopic ratio was constructed for the session as a whole by fitting a series of 4th-order polynomials (excluding outliers). Under the operating conditions of the LA-ICP-MS sessions in this study, fractionation factors were found to vary strongly with scan number, decreasing with increasing scan number (presumably due to increasing ablation pit depth and the effect this had on fractionation; for example, Paton and others, 2010). Fractionation factors were calculated using isotopic values based on the sum of fitted Gaussians. Ages, including when the standards were treated as unknowns, were calculated using raw isotopic signal values (excluding outliers) to avoid any bias due to artifacts of the fitting of the sum of Gaussians.

Appendix 1 47 Fractionation Factor Adjustment for Integrated alpha-damage Zircon is widely known to accumulate α-radiation damage (for example, Zhang and others, 2009 and references therein). It is assumed here that increased α-damage in a zircon leads to a decrease in the hardness of the zircon; this in turn leads to a faster rate of laser penetration into the zircon during ablation leading to shift in isotopic fractionation. Ages calculated for the primary, secondary, and tertiary zircon standards, when those standards were treated as unknowns, were used to construct a fractionation factor correction curve (exponential form). Much previous work has attempted to understand the chemical basis for why some standards work better with some zircons. The notion of matrix-matched standard and unknown zircons has been proposed largely on the basis of trace element chemistry (for example, Black and others, 2004). Here, time and crystallographic damage, parameters invisible to instruments used to characterize trace element chemistry, were introduced and applied in conjunction with U and Th chemistry. Common Pb Correction Common Pb was subtracted out using the Stacey and Kramers (1975) common Pb model for Earth. Ages and common Pb ratio were determined iteratively using a preset, session-wide minimum common Pb age value (default for each session was the age of the oldest age standard, which for both apatite and zircon was 1099 Ma FC-1 and (or) FC-5z). Preferred Age Uranium decay constants and the 238U/235U isotopic ratio reported in Steiger and Yäger (1977) were used in this study. 207Pb/235Uc (235Uc 137.88238U), 206Pb/238U, and 207Pb/206Pb ages were calculated for each data scan and checked for concordance; concordance here was defined as overlap of all three ages at the 1-sigma level (the use of 2-sigma level was found to skew the results to include scans with significant common Pb). The background-corrected isotopic sums of each isotope were calculated for all concordant scans. The precision of each isotopic ratio was calculated by using the background and signal errors for both isotopes. The fractionation factor for each data scan, corrected for the effect of accumulated α-damage, was weighted according to the 238U or 232Th signal value for that data scan; an overall weighted-mean fractionation factor for all concordant data scans was used for final age calculation. If the number of concordant data scans for a spot was greater than zero, then either the 206Pb/238U or 207Pb/206Pb age was chosen as the preferred age, whichever exhibited the lower relative error. If zero concordant data scans were observed, then the common Pb-corrected age based on isotopic sums of all acceptable scans was chosen as the preferred age. Common Pb was subtracted out using the Stacey and Kramers (1975) common Pb model for Earth. Ages and common Pb ratio were determined iteratively using a preset, sessionwide minimum common Pb age value (default for each session was the age of the oldest age standard which for both apatite and zircon was 1099 Ma FC-1 and (or) FC-5z).

48    Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages Appendix 2.—New Geochronology Data for Igneous-Rock Samples from Western Alaska This appendix of new geochronology data for igneous-rock samples from western Alaska is available online only as comma separated value (.csv) files and an Excel (.xlsx) file at https://doi. org/10.3133/pp1814D. The files contain the following tables: Table A1. Uranium-lead (U-Pb) thermal ionization mass-spectrometry (TIMS) data for zircon and monazite samples from western Alaska analyzed at the University of British Columbia. Table A2.  Uranium-lead (U-Pb) secondary ion mass-spectrometry (SIMS) data for igneous-rock samples from western Alaska analyzed at the U.S. Geological Survey-Stanford sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG) facility at Stanford University, California. Table A3.  Argon-40/argon-39 (40Ar/39Ar) data for igneous-rock samples from western Alaska analyzed at the University of Alaska Fairbanks. Table A4.  Argon-40/argon-39 (40Ar/39Ar) data for igneous-rock samples from western Alaska analyzed at the University of British Columbia. Table A5.  Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) data for zircon in igneous-rock samples from western Alaska analyzed by Apatite to Zircon, Inc., at Washington State University.

Menlo Park Publishing Service Center, California Manuscript approval date December 7, 2016 Edited by James W. Hendley II Design and layout by Cory Hurd

Bradley and others—Regional Patterns of Mesozoic-Cenozoic Magmatism in Western Alaska Revealed by New U-Pb and 40Ar/39Ar Ages—Professional Paper 1814-D ISSN 2330-7102 (online) https://doi.org/10.3133/pp1814D

Plates & figures from the original