Geochemical and petrographic data for intrusions peripheral to the Big Timber Stock, Crazy Mountains, Montana
The Paleocene Fort Union Formation hosts a compositionally diverse array of Eocene plugs, dikes, and sills arrayed around the Eocene Big Timber stock in the
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U.S. Department of the Interior U.S. Geological Survey Data Series 895 Geochemical and Petrographic Data for Intrusions Peripheral to the Big Timber Stock, Crazy Mountains, Montana
COVER. Prominent Eocene sill along the ridge crest, above rocks of the Fort Union Formation, 4 kilometers northeast of Gobblers Knob, Montana. (Photo by Anna B. Wilson, U.S. Geological Survey, 1992.)
Geochemical and Petrographic Data for Intrusions Peripheral to the Big Timber Stock, Crazy Mountains, Montana By Edward A. du Bray, Anna B. Wilson, and Bradley S. Van Gosen Data Series 895 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, Acting Director U.S. Geological Survey, Reston, Virginia: 2015 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 http://www.usgs.gov or call 1-888-ASK-USGS. For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprod To order this and other USGS information products, visit http://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: du Bray, E.A., Wilson, A.B., and Van Gosen, B.S., 2015, Geochemical and petrographic data for intrusions peripheral to the Big Timber stock, Crazy Mountains, Montana: U.S. Geological Survey Data Series 895, 19 p., http://dx.doi.org/10.3133/ds895. ISSN 2327-638X (online)
Contents Introduction 1 Analytical Methods 1 Data Fields 1 Geochemical Characteristics 3 Major Oxide Data 3 Trace-Element Data 8 Petrographic Characteristics 10 Synthesis 11 Acknowledgments 11 References Cited 11 Appendix 1. Status and Treatment of Samples of Sills and Plugs Peripheral to the Big Timber Stock, Crazy Mountains, Montana 14 Appendix 2. Geochemical and Petrographic Data for Samples of Sills and Plugs Peripheral to the Big Timber Stock, Crazy Mountains, Montana 15 Figures
1. Regional geologic setting for the Crazy Mountains, Montana, showing the Big Timber stock, associated radial dikes, and peripheral sills and plugs 2
2. Variation diagram showing relative alumina and alkali saturation of igneous rocks peripheral to the Big Timber stock, Crazy Mountains, Montana, as a function of molar major-oxide compositions 6
3. Variation diagram showing FeO/(FeO+MgO) values for igneous rocks peripheral to the Big Timber stock, Crazy Mountains, Montana, relative to boundaries between ferroan and magnesian rocks 7
4. Total alkali-silica variation diagram showing compositions of igneous rocks peripheral to the Big Timber stock, Crazy Mountains, Montana 7
5. Variation diagram showing Na2O+K2O-CaO values versus SiO2 content among samples of igneous rocks peripheral to the Big Timber stock, Crazy Mountains, Montana 8
6. Variation diagrams showing abundances of major oxides in igneous rocks peripheral to the Big Timber stock, Crazy Mountains, Montana 9
7. Trace-element, tectonic setting-discrimination variation diagram showing the composition of igneous rocks peripheral to the Big Timber stock, Crazy Mountains, Montana 10
Tables
1. Definition and characterization of data fields included in appendix 1 (status and treatment of samples) 3
2. Definition and characterization of data fields included in appendix 2 (geochemical and petrographic data) 4 Abbreviations Al2O3 aluminum oxide Am americium Ba barium CaO calcium oxide Cd cadmium Ce cerium FeO ferrous iron K potassium K2O potassium oxide La lanthanum MgO magnesium oxide MnO manganese oxide Na2O sodium oxide Nb niobium Nd neodymium P2O5 phosphorus pentoxide Rb rubidium SiO2 silicon dioxide Sr strontium TiO2 titanium dioxide Y yttrium Zr zirconium ppm parts per million
Geochemical and Petrographic Data for Intrusions Peripheral to the Big Timber Stock, Crazy Mountains, Montana By Edward A. du Bray, Anna B. Wilson, and Bradley S. Van Gosen Introduction In the Crazy Mountains of south-central Montana, the Paleocene Fort Union Formation hosts a compositionally diverse array of Eocene plugs, dikes, and sills arrayed around the Eocene Big Timber stock; all of these rocks are part of the Crazy Mountains magmatic system. As documented by du Bray and Harlan (1996) and Dudas (1990, 1991), Eocene igneous rocks in the Crazy Mountains include silicaundersaturated, strongly alkaline intrusive rocks and silicasaturated, alkaline to subalkaline intrusive rocks that are approximately coeval. Most of the strongly alkaline rocks are in the northern part of the Crazy Mountains and form sills, laccoliths, small stocks, and dikes. The strongly alkaline plug at Ibex Mountain (fig. 1) is located significantly farther south than other strongly alkaline rocks associated with the Crazy Mountains magmatic system. Compositionally less exotic alkaline to subalkaline rocks, including constituents of the Big Timber stock (du Bray and Harlan, 1996), form stocks and associated dike swarms that are mostly restricted to the southern part of the range (fig. 1). Other Eocene intrusions, including outcrops at Gobblers Knob, Raspberry Butte, and other nearby conformable intrusions form a set of sills that are satellitic to the Big Timber stock and constitute a third, relatively unstudied set of intrusions in the Crazy Mountains (fig. 1). The distribution of intrusive rocks in the Crazy Moun tains is portrayed on various geologic maps of the area, including those by Wolff (1938), Simms (1966), Tappe (1966), Starmer (1972), and du Bray and others (1993). The petrology and petrogenesis of the strongly alkaline rocks in particular were well documented by Dudas (1990, 1991). Subsequently, du Bray and Harlan (1996) conducted a detailed investigation concerning the petrogenesis of the Big Timber stock, and du Bray and others (2006) characterized the radial dike swarm associated with the Big Timber stock. None of these studies, however, have documented and synthesized the geochemis try and petrography of the sills that are peripheral to the Big Timber stock. The purpose of this report is (1) to present available geo chemical and petrographic data for several dozen igneous rock samples, which represent sills and plugs peripheral to the Big Timber stock, and (2) to provide a basic interpretive synthesis of these data. These samples were collected in 1992 during geologic mapping of the Big Timber stock by du Bray and others (1993). During field studies, 28 outcrop samples were collected for subsequent laboratory analysis. Petrographic data were acquired for 25 of these samples, and geochemical analyses were acquired for 26 of these samples. Five samples of strongly alkaline rock, four from the plug at Ibex Mountain and one from a small plug exposed along the Shields River northwest of the Big Timber stock, are petro graphically and geochemically distinct relative to all other samples described herein. Analytical Methods Petrographic characteristics of samples were determined using a standard petrographic microscope. All whole-rock major oxide chemical abundances were determined by wavelength-dispersive X-ray fluorescence spectrometry, using methods described by Taggart and others (1987), in analytical laboratories of the U.S. Geological Survey, Denver, Colorado. All iron abundances were converted to ferrous iron, and each major oxide analysis was recalculated to 100 percent on a volatile-free basis. Trace-element abundances were determined by energy-dispersive X-ray fluorescence spectroscopy (Elsass and du Bray, 1982; Yager and Quick, 1992) using 109Cd and 241Am radio-isotope excitation sources. All censored values were replaced by blank cells; for lower limits of determina tion, see Elsass and du Bray (1982). Data Fields Data were compiled using Microsoft Excel; they are presented here in two appendixes, found both at the back of this report and as Excel files (Appendix1.xlsx and Appen dix2.xlsx) linked from this document and from http://dx.doi. org/10.3133/ds895. The files can be accessed using software
2 Geochemical and Petrographic Data for Intrusions Peripheral to the Big Timber Stock, Crazy Mountains, Montana Figure 1. Regional geologic setting for the Crazy Mountains, Montana, showing the Big Timber stock, associated radial dikes, and peripheral sills and plugs. Collection sites for all samples described herein are labelled; complete sample numbers for samples ending in "V" or "W" include the suffix "92." Geologic features modified from Roberts (1972). (Click here to open fullsize, high-resolution image.) 20 KILOMETERS Base from Army Map Service, 1958, White Sulphur Springs and Bozeman, Mont., 1:250,000 Transverse Mercator projection, Zone 12 North American Datum of 1927 (NAD 27) 023W, 024W, 025W, 026W, 027W, 028W 022W D DD D DD D D D DD D D D D 021W D 004W, 005W, 006W, 007W, 008W D Gobblers Knob sill Ibex Mountain plug Loco Mountain stock Porcupine Butte sill Big Timber stock Raspberry Butte sill Billie Butte plug Alkaline to subalkaline dike Alkaline to subalkaline stock or sill Strongly alkaline plug or sill D Sample site and sample number MONTANA Map area Shields River EXPLANATION 011W, 012W 013W, 014W, 015W 002W 202338, 001V 018W 017W 016W 010W 003W 009W 020W 019W 010W 001W 10°45' 110°30' 110°15' 110°00' 46°15' 46°00' 10 MILES
Geochemical Characteristics 3 compatible with the .xlsx file format. Sample characteriza tion, geochemical, and petrographic data are presented in columns or sets of related columns. The contents of appendix 1 constitute basic sample information, including sample loca tion, sample treatment, and lithologic characterization for each sample; see table 1 for data field definitions. Appendix 2 con tains geochemical and petrographic observations; see table 2 for data field definitions. Geochemical data in some worksheet cells might appear to be more precise than displayed values, but the implied precision is a misleading artifact of computa tional processes used to create data-cell contents (for instance, recalculation to 100-percent volatile free). Blank cells in the appendix worksheets indicate either null values or that no data are available. In appendix 2, some blank cells reflect abundances that were reported as "less than the lower limit of determination for the analytical method used;" these values were replaced by blank cells to enable statistical analysis of the uncensored data. Geochemical Characteristics Major Oxide Data As is true of the Big Timber stock and its associated radial dike swarm (du Bray and Harlan, 1996; du Bray and others, 2006), major oxide characteristics of the sill-forming rocks peripheral to the Big Timber stock are consistent with a subduction-related petrogenesis. Compositions of three sam ples of the Ibex Mountain plug and one sample from a plug along the Shields River are quite distinct and are described separately. Relative to standard metrics (in cited sources), the sill-forming rocks are metaluminous (fig. 2) (Shand, 1951), magnesian (fig. 3) (Frost and others, 2001), and have composi tions (fig. 4) that straddle the alkaline-subalkaline dividing line of Irvine and Baragar (1971). In terms of the balance between abundances of CaO, Na2O, and K2O in these rocks, the sill compositions vary significantly across the entire calcic, calcalkalic, alkali-calcic, and alkalic spectrum (fig. 5), likely a misleading consequence of post-magmatic alteration and alkali mobility experienced by these rocks. Abundances of SiO2 in the sill-forming rocks range nearly continuously from about 46 to 63 weight percent (fig. 4), and their compositions range from basalt to dacite and their alkaline analogs. Concentrations of TiO2 and MgO vary considerably at lower SiO2 abundances but scatter less and decrease to lower values at higher SiO2 contents (fig. 6). Concentrations of FeO* (total iron, expressed in the ferrous state), MnO, and CaO (fig. 6) decrease in a linear fashion with increasing SiO2. Abundances of Na2O and P2O5 vary widely and yield no consistent variation relative to SiO2 content. Abundances of Al2O3 and K2O increase broadly with increasing SiO2 content; the K2O data array is dominated by transitional high-potassium to shoshonitic compositions (Gill, 1981). These trends and compositional ranges are similar to and overlap those of the Big Timber stock and its associated radial dike swarm (du Bray and Harlan, 1996; du Bray and others, 2006). Primary mafic magmas that assimilate crustal contami nants predictably evolve to more silicic compositions char acterized by progressively lower P2O5/K2O because crustal materials generally have P2O5/K2O less than 0.1 (Farmer and others, 2002). Among the sill-forming rocks, P2O5/K2O decreases with increasing SiO2 content and increases with increasing MgO content, which suggests that compositions of the magmas represented by these rocks evolved through variable contamination of primary mafic partial melts by crustally derived inputs. Similarly, Cousens and others (2008) suggested that decreasing CaO/Al2O3 with increasing SiO2 principally reflects crustal contamination. Among the sillforming rocks, CaO/Al2O3 decreases with increasing SiO2, which corroborates the influence of progressive crustal con tamination in the development of these rocks. Table 1. Definition and characterization of data fields included in appendix 1 (status and treatment of samples). FIELD_NAME FIELD_DESCRIPTION Field_ID Field-assigned sample identifier; Field_ID entries may link data in individual rows to the contents of particular rows in the other appendix or to the National Geochemical Database. Longitude In decimal degrees, relative to the North American Datum of 1927. Longitude is reported as a negative value (western hemisphere). Latitude In decimal degrees, relative to the North American Datum of 1927. Latitude is reported as a positive value (northern hemisphere). Chem "X" indicates chemical analysis for sample obtained (see appendix 2). TS "X" indicates thin section of sample prepared and examined using a petrographic microscope (see appendix 2). Locality_name Prominent geographic feature proximal to sample site(s). Lithology Sample composition according to the classification scheme of Le Maitre (2002). Igneous_form Form (sill, dike, or plug) of the igneous rock represented by each sample. Alteration Geochemical characteristics indicative of alteration; high LOI (loss on ignition) equates to values weight percent, and low K2O equates to values <0.5 weight percent.
4 Geochemical and Petrographic Data for Intrusions Peripheral to the Big Timber Stock, Crazy Mountains, Montana Table 2. Definition and characterization of data fields included in appendix 2 (geochemical and petrographic data). FIELD_NAME FIELD_DESCRIPTION Field_ID Field-assigned sample identifier; Field_ID entries may link data in individual rows to the contents of particu lar rows in the other appendix or to the National Geochemical Database. Locality_name Prominent geographic feature proximal to sample site(s). SiO2_pct Silicon, as silicon dioxide, in weight percent; based on major oxide data recalculated to 100 percent on a volatile-free basis. TiO2_pct Titanium, as titanium dioxide, in weight percent; based on major oxide data recalculated to 100 percent on a volatile-free basis. Al2O3_pct Aluminum, as aluminum trioxide, in weight percent; based on major oxide data recalculated to 100 percent on a volatile-free basis. Total iron, as ferrous oxide, in weight percent; based on major oxide data recalculated to 100 percent on a volatile-free basis. MnO_pct Manganese, as manganese oxide, in weight percent; based on major oxide data recalculated to 100 percent on a volatile-free basis. MgO_pct Magnesium, as magnesium oxide, in weight percent; based on major oxide data recalculated to 100 percent on a volatile-free basis. CaO_pct Calcium, as calcium oxide, in weight percent; based on major oxide data recalculated to 100 percent on a volatile-free basis. Na2O_pct Sodium, as sodium oxide, in weight percent; based on major oxide data recalculated to 100 percent on a volatile-free basis. K2O_pct Potassium, as potassium oxide, in weight percent; based on major oxide data recalculated to 100 percent on a volatile-free basis. P2O5_pct Phosphorus, as phosphorus pentoxide, in weight percent; based on major oxide data recalculated to 100 percent on a volatile-free basis. LOI_pct Volatile content lost on ignition, in weight percent. Initial, pre-recalculation sum of oxide abundances, in weight percent. Barium, in parts per million. Rubidium, in parts per million. Strontium, in parts per million. Yttrium, in parts per million. Zirconium, in parts per million. Niobium, in parts per million. Thorium, in parts per million. Gallium, in parts per million. Lanthanum, in parts per million. Cerium, in parts per million. Neodymium, in parts per million. Copper, in parts per million. Lead, in parts per million. Zinc, in parts per million. Tin, in parts per million. Tungsten, in parts per million. Arsenic, in parts per million. Antimony, in parts per million. Modal abundance of plagioclase phenocrysts relative to the whole rock, in volume percent. Abd_GrnAmph_phenos Modal abundance of green amphibole (magnesio-hornblende) phenocrysts relative to the whole rock, in volume percent.
Geochemical Characteristics 5 Table 2. Definition and characterization of data fields included in appendix 2 (geochemical and petrographic data).—Continued FIELD_NAME FIELD_DESCRIPTION Abd_BrnAmph_phenos Modal abundance of brown amphibole (magnesio-hastingsite) phenocrysts relative to the whole rock, in volume percent. Modal abundance of biotite phenocrysts relative to the whole rock, in volume percent—TR, trace amounts (<0.5 volume percent). Abd_Cpx_phenos Modal abundance of clinopyroxene phenocrysts relative to the whole rock, in volume percent. Modal abundance of olivine phenocrysts relative to the whole rock, in volume percent—TR, trace amounts (<0.5 volume percent). Abd_Opq Modal abundance of opaque iron-titanium oxide minerals relative to the whole rock, in volume percent. TotXtls Microscope-based estimate of total phenocryst content relative to the whole rock, in volume percent. ClrIndx Microscope-based estimate of color index (sum of the abundances of hornblende, biotite, pyroxene, olivine, and opaque iron-titanium oxide minerals), in volume percent. AgsPl Microscope-based estimate of average grain size of plagioclase phenocrysts, in millimeters. AgsGrnAmph Microscope-based estimate of average grain size of green amphibole (magnesio-hornblende) phenocrysts, in millimeters. AgsBrnAmph Microscope-based estimate of average grain size of brown amphibole (magnesio-hastingsite) phenocrysts, in millimeters. AgsBt Microscope-based estimate of average grain size of biotite phenocrysts, in millimeters. AgsCpx Microscope-based estimate of average grain size of clinopyroxene phenocrysts, in millimeters. AgsOl Microscope-based estimate of average grain size of olivine phenocrysts, in millimeters. AgsOpq Microscope-based estimate of average grain size of opaque iron-titanium oxide phenocrysts, in millimeters. MgsPl Microscope-based estimate of maximum grain size (length) of largest plagioclase phenocryst, in millimeters. MgsGrnAmph Microscope-based estimate of maximum grain size (length) of largest green amphibole (magnesiohornblende) phenocryst, in millimeters. MgsBrnAmph Microscope-based estimate of maximum grain size (length) of largest brown amphibole (magnesiohastingsite) phenocryst, in millimeters. MgsBt Microscope-based estimate of maximum grain size (length) of largest biotite phenocryst, in millimeters. MgsCpx Microscope-based estimate of maximum grain size (length) of largest clinopyroxene phenocryst, in millimeters. MgsOl Microscope-based estimate of maximum grain size (length) of largest olivine phenocryst, in millimeters. MgsOpq Microscope-based estimate of maximum grain size (length) of largest opaque iron-titanium oxide pheno cryst, in millimeters. Texture Characteristic petrographic textures as determined by microscopic observation—Aph, aphanitic; Hy, hyalophitic; Hc, holocrystalline; I, intersertal; E, equigranular; P, porphyritic; S, seriate; T, trachytic. Access_Mnrls Accessory minerals identified by microscopic observation; listed in order of decreasing abundance— Ap, apatite; Ttn, titanite. XlPl Microscope-based estimate of crystallinity of plagioclase phenocrysts—A, anhedral; S, subhedral; E, euhedral. XlGrnAmph Microscope-based estimate of crystallinity of green amphibole (magnesio-hornblende) phenocrysts— A, anhedral; S, subhedral; E, euhedral. XlBrnAmph Microscope-based estimate of crystallinity of brown amphibole (magnesio-hastingsite) phenocrysts— A, anhedral; S, subhedral; E, euhedral. XlBt Microscope-based estimate of crystallinity of biotite phenocrysts—A, anhedral; S, subhedral; E, euhedral. XlCpx Microscope-based estimate of crystallinity of clinopyroxene phenocrysts—A, anhedral; S, subhedral; E, euhedral. If more than one crystallinity type is present, the dominant form is listed first. XlOl Microscope-based estimate of crystallinity of olivine phenocrysts—A, anhedral; S, subhedral; E, euhedral. XlOpq Microscope-based estimate of crystallinity of opaque iron-titanium oxide phenocrysts—A, anhedral; S, subhedral; E, euhedral.
6 Geochemical and Petrographic Data for Intrusions Peripheral to the Big Timber Stock, Crazy Mountains, Montana Table 2. Definition and characterization of data fields included in appendix 2 (geochemical and petrographic data).—Continued FIELD_NAME FIELD_DESCRIPTION Petrog_Com Groundmass characteristics and otherwise noteworthy features. Groundmass minerals include plagioclase (Pl), hornblende (Hbl), clinopyroxene (Cpx), biotite (Bt), quartz (Qtz), and opaque iron-titanium minerals (Opq). The presence of secondary, alteration minerals, including chlorite (Chl), calcite (Cc), or epidote (Ep), is noted. [%, percent; mm, millimeters] HblClr Pleochroic colors of hornblende phenocrysts, if present. AltExtnt Microscope-based estimate of the extent of alteration—1 indicates a completely fresh sample, and 5 indicates a completely altered sample in which primary textures and minerals are not identifiable. Intermediate values of 2 through 4 identify progressively more altered samples. Figure 2. Variation diagram showing relative alumina and alkali saturation of igneous rocks peripheral to the Big Timber stock, Crazy Mountains, Montana, as a function of molar major-oxide compositions. Al2O3/(Na2O + K2O + CaO), molar Metaluminous Peraluminous Peralkaline Sill-forming sample Plug-forming sample EXPLANATION (Na2O + K2O)/Al2O3, molar
Geochemical Characteristics 7 Figure 4. Total alkali-silica variation diagram showing compositions of igneous rocks peripheral to the Big Timber stock, Crazy Mountains, Montana. Field boundaries from Le Maitre (2002). Alkalinesubalkaline dividing line from Irvine and Baragar (1971). Na2O+K2O, in weight percent Trachydacite Andesite Dacite Trachyandesite Basaltic trachyandesite Trachybasalt Basalt Basaltic andesite Phonotephrite Alkaline Subalkaline SiO2, in weight percent Sill-forming sample Plug-forming sample EXPLANATION Figure 3. Variation diagram showing FeO/(FeO+MgO) values for igneous rocks peripheral to the Big Timber stock, Crazy Mountains, Montana, relative to boundaries between ferroan and magnesian rocks. Ferroan-magnesian boundary from Frost and others (2001). SiO2, in weight percent Ferroan Magnesian Sill-forming sample Plug-forming sample EXPLANATION FeO*/(FeO*+MgO)
8 Geochemical and Petrographic Data for Intrusions Peripheral to the Big Timber Stock, Crazy Mountains, Montana Many geochemical features distinguish the compositions of the samples of the Ibex Mountain and plugs along the Shields River (fig. 1) from those of the sill-forming rocks. First, all four of these representative samples are composed of phonotephrite and have strongly alkaline compositions (fig. 4) relative to the alkaline-subalkaline dividing line of Irvine and Baragar (1971). Three of these four samples have agpaitic indices greater than 1 (fig. 2) and are therefore peralkaline; the fourth contains aegirine, which is likewise diagnostic of peralkaline magmas. The silica content of these rocks, about 48 to 49 weight percent, is significantly lower than that of most of the sill-forming rocks. Both the Na2O and K2O contents of these rocks are elevated, given their low silica contents, but they are especially sodic, with Na2O contents that range from about 5 to 8 weight percent (fig. 6). Like all other rocks in the Crazy Mountains, the strongly alkaline rocks are magnesian (fig. 3). Relative proportions of Na2O, K2O, and CaO in these rocks are consistent with alkalic to strongly alkali-calcic compositions (fig. 5). Relative to compositional trends depicted by all other alkaline to subalkaline rocks in the Crazy Mountains (fig. 6), the strongly alkaline rocks have low TiO2 and CaO abundances, high Na2O and K2O abundances, and remarkably high P2O5 abundances (fig. 6). Their Al2O3, FeO, MnO, and MgO abundances are approximately on trend with those of other alkaline to subalkaline rocks in the Crazy Mountains. Trace-Element Data Several aspects of trace-element data available for sillforming alkaline to subalkaline rocks of the Crazy Mountains are noteworthy. In particular, abundances of Ba (range, about 600-4,300 parts per million [ppm]; average, about 2,200) and Sr (range, about 400-1,400 ppm; average, about 900) in these rocks are especially elevated relative to the concentra tions of those elements in most igneous rocks (Turekian and Wedepohl, 1961). Rubidium (Rb) abundances in these rocks are relatively low, resulting in very low Rb/Sr values (average, 0.06). Abundances of Y, Zr, Nb, La, Ce, and Nd in these rocks are similar to those of other convergent-margin, broadly calcalkaline igneous rocks, such as those in the Andean, Kam chatka, and Central American volcanic arcs (Max Plank Insti tut für Chemie, 2010). Among the sill-forming rocks, Rb, Zr, La, and Ce abundances increase systematically with increasing silica content; Y abundances decrease; and Ba, Sr, Nb, and Nd abundances are uncorrelated with varying silica content. Most of the sill-forming rocks have relative abundances of Rb and Y+Nb that are consistent with a genesis in a volcanic arc setting (fig. 7). Most continental magmatic arc rocks have Ba/Nb greater than 15 (Gill, 1981). The sill-forming rocks have Ba/Nb values that average about 126, range upward to almost 240, and do not vary systematically with respect to silica content. Figure 5. Variation diagram showing Na2O+K2O-CaO values versus SiO2 content among samples of igneous rocks peripheral to the Big Timber stock, Crazy Mountains, Montana. Boundaries between various rock series from Frost and others (2001). Calcic Calc-alkalic Alkali-calcic Alkalic Na2O+K2O-CaO, in weight percent SiO2, in weight percent Sill-forming sample Plug-forming sample EXPLANATION
Geochemical Characteristics 9 Figure 6. Variation diagrams showing abundances of major oxides in igneous rocks peripheral to the Big Timber stock, Crazy Mountains, Montana. All abundances in weight percent, except zirconium (in parts per million). Field boundaries on K2O versus SiO2 diagram from Le Maitre (2002); high-K-shoshonitic dividing line from Ewart (1982). TiO2 FeO* MnO MgO CaO SiO2, in weight percent Abundance of oxide, in weight percent Abundance of oxide, in weight percent Abundance of zirconium, in parts per million SiO2, in weight percent Na20 K2O P2O5 Low-K Medium-K High-K Shoshonitic Zr Sill-forming sample Plug-forming sample EXPLANATION Al203
10 Geochemical and Petrographic Data for Intrusions Peripheral to the Big Timber Stock, Crazy Mountains, Montana Elevated Ba/Nb values have been associated with those mantle wedge magmas that derived subducted slab components through dehydration of the subducted-slab and attendant fluid flux-induced partial melting (Hawkesworth and others, 1995; Pearce and Peate, 1995; Cousens and others, 2008; Schmidt and others, 2008). Accordingly, highly elevated Ba/Nb ratios and noteworthy large-ion lithophile element (Ba and Sr) enrichments suggest significant involvement of a subducted-slab-derived fluid component in the petrogenesis of the magmas represented by the sill-forming rocks. Trace-element characteristics of the strongly alkaline rocks are highly distinctive. Although Ba and Sr abundances in the sill-forming rocks are elevated (averaging about 3,200 and 2,700 ppm, respectively), those for the strongly alkaline rocks are even greater. Rubidium abundances in the strongly alkaline rocks are higher, averaging about 71 ppm; however, their average Rb/Sr ratio is even lower (0.03) than that for the sill-forming rocks. Similarly, Y, Zr, and Nb abundances of the strongly alkaline rocks are significantly higher than those of the sill-forming rocks. The average Zr content of the strongly alkaline rocks (221 ppm) is lower than might be expected given their alkalinity and experimental work by Watson (1979), which demonstrated that the alkaline mag mas, such as those represented by these rocks, can contain significantly greater zirconium concentrations before zircon saturation is achieved and zirconium concentrations become buffered. Finally, the light rare earth element (La, Ce, and Nd) abundances of the strongly alkaline rocks are strikingly elevated relative to those characteristic of other igneous rocks (Turekian and Wedepohl, 1961). These trace-element charac teristics suggest that the petrogenetic history of these alkaline rocks was quite different from that responsible for magmas represented by the sill-forming rocks. A significant aspect of these distinctive characteristics is underscored by the fact that relative abundances of Rb and Y+Nb for the strongly alkaline rocks coincide with the within-plate field on the trace-element, tectonic setting-discrimination variation diagram (fig. 7), whereas those for the sill-forming rocks are equivalent to volcanic arc compositions. Petrographic Characteristics Most samples of the sill-forming rocks described herein have similar petrographic characteristics; in contrast, samples of the strongly alkaline plug at Ibex Mountain are petrographi cally distinct and thus are described separately. Many of the sill-forming rocks contain either green or brown amphibole. Electron microprobe analyses of green and brown amphibole contained in the nearby Big Timber stock indicate that they are composed of magnesio-hornblende and magnesio-hastingsite, respectively (du Bray and Harlan, 1996). Optical features of the green amphibole in sill and Big Timber stock samples are indistinguishable, which suggests that the green amphibole in the sills is also magnesio-hornblende. Similarly, brown amphibole in the sills and the stock are optically indistin guishable, which suggests that brown amphibole in the sills is magnesio-hastingsite. The sill rocks are variably porphyritic. Phenocryst abundances range from 0 to 65 percent, averaging about 20 percent. Color index ranges from 1 to 35 percent, averaging about 13 percent. Fine- to medium-grained pheno cryst assemblages include combinations of plagioclase, magnesio-hornblende, magnesio-hastingsite, clinopyroxene, and rare biotite in a fine-grained groundmass composed of combinations of plagioclase, magnesio-hornblende, magnesiohastingsite, clinopyroxene, opaque iron-titanium oxides, and variably devitrified glass. The groundmass minerals form intersertal intergrowths in most samples, although in several samples the groundmass consists of intergranular intergrowths. Among samples of dikes associated with the Big Timber stock, the composition of the mafic silicate minerals is strongly correlated with whole-rock composition (du Bray and others, 2006); basaltic trachyandesite dikes are dominated by clinopyroxene, low-silica trachyandesite dikes contain clinopyroxene and magnesio-hastingsite, high-silica trachy andesite to low-silica trachydacite dikes contain magnesiohastingsite, and high-silica trachydacite to rhyolite dikes contain biotite and magnesio-hornblende. The composition of the mafic silicate minerals contained in the sill-forming rocks correlate similarly, but less systematically, with whole-rock composition. Accessory minerals in the sill-forming rocks are Figure 7. Trace-element, tectonic settingdiscrimination variation diagram showing the composition of igneous rocks peripheral to the Big Timber stock, Crazy Mountains, Montana. Tectonic setting-composition boundaries from Pearce and others (1984). Rb, parts per million Y + Nb, in parts per million Volcanic arc Within plate Sill-forming sample Plug-forming sample EXPLANATION
References Cited 11 rare, though apatite was identified in several samples. The groundmass of many samples contains secondary calcite and (or) chlorite related to post magmatic alteration of many of these rocks. Similarly, many primary phenocrysts are variably altered and have been replaced by calcite and clay or sericite. The plug at Ibex Mountain is composed of rock that ranges from holocrystalline and equigranular (sample 202338) to porphyritic with a hyalophitic groundmass (sample 002W92). Pale green clinopyroxene is the dominant mineral in both of these samples; other mafic silicate minerals include tan to distinctly red-brown biotite and trace amounts of subhedral olivine. Clinopyroxene in sample 202338 is overgrown by distinctive emerald-green aegirine rims, and in both Ibex Mountain samples is distinguished by a well-developed sieve texture. These rocks lack quartz and feldspar but, befitting their strongly alkaline character, they contain nepheline, which forms anhedral, interstitial grains in sample 202338 and euhe dral phenocrysts in sample 002W92. Some nepheline is partly replaced by and (or) overgrown by acicular sprays of zeolite minerals. Sample 002W92 is further distinguished by the pres ence of cancrinite, which forms small discrete grains and over growths on nepheline crystals. The occurrence of cancrinite in strongly alkaline rocks, particularly in the northern Crazy Mountains, was highlighted by the detailed investigations of Simms (1966). Apatite is a characteristic accessory constituent of the Ibex Mountain plug. Synthesis The most significant finding derived from data presented herein is that the geochemical and petrographic characteristics of the sill-forming rocks are indistinguishable from those of the Big Timber stock and its radial dike swarm. Consequently, the central stock, associated radial dikes, and surrounding sills seem to be part of a single, coeval magmatic episode in the Crazy Mountains. By analogy, other still unsampled and undocumented sill-forming intrusions peripheral to and south and east of the Big Timber stock are probably also composed of rock related to magmatism responsible for formation of the Big Timber stock. In contrast, the small amount of data for the plug-forming rocks at Ibex Mountain and along the Shields River indicate that the geochemistry and petrography of these rocks are significantly different from those of the sill-forming rocks but remarkably similar to those of other strongly alkaline igneous rocks exposed north and west of the Big Timber stock. Characteristics of the sill-forming intrusions are in accord with their petrogenesis in a subduction-related setting, as is also true for the Big Timber stock and its radial dike swarm (du Bray and Harlan, 1996). Unusual trace-element abun dances, especially elevated concentrations of Ba and Sr, are consistent with significant crustal inputs through assimilation and (or) magmatism involving noteworthy inputs derived from fluids released during devolatilization of the downgoing, sub ducted slab. Finally, the geospatial and temporal coincidence of strongly alkaline and alkaline to subalkaline magmas in the Crazy Mountains remains something of a petrologic paradox. Acknowledgments Data compilation undertaken for this study was con ducted as part of the mineral resource assessment of Custer and Gallatin National Forests funded by the U.S. Geological Survey Mineral Resources Program. Constructive reviews by Ryan D. Taylor and Matthew Granitto are much appreciated and helped clarify data presentation. References Cited Cousens, B.L., Prytulak, Julie, Henry, C.D., Alcazar, Al, and Brownrigg, Tim, 2008, Geology, geochronology, and geo chemistry of the Miocene-Pliocene ancestral Cascades arc, northern Sierra Nevada, California and Nevada—The roles of the upper mantle, subducting slab, and the Sierra Nevada lithosphere: Geosphere, v. 4, p. 814-828. du Bray, E.A., Elliott, J.E., Wilson, A.B., Van Gosen, B.S., and Rosenberg, L.A., 1993, Geologic map of the Big Timber stock and vicinity, southern Crazy Mountains, Sweet Grass and Park Counties, south-central Montana: U.S. Geologi cal Survey Miscellaneous Field Investigations Series Map MF-2253, scale 1:24,000. du Bray, E.A., and Harlan, S.S., 1996, The Eocene Big Timber stock, south-central Montana—Development of extensive compositional variation in an arc-related intrusion by side-wall crystallization and cumulate glomerocryst remixing: Geological Society of America Bulletin, v. 108, p. 1404-1424. du Bray, E.A., Harlan, S.S., and Wilson, A.B., 2006, Petrol ogy of the Crazy Mountains dike swarm and geochronology of associated sills, south-central Montana: U.S. Geological Survey Professional Paper 1715, 21 p. Dudas, F.O., 1990, Petrogenesis and mantle source of igneous rocks in the Crazy Mountains, Montana: University Park, Pennsylvania State University, Ph.D. thesis, 442 p. Dudas, F.O., 1991, Geochemistry of igneous rocks from the Crazy Mountains, Montana, and tectonic models for the Montana alkalic province: Journal of Geophysical Research, v. 96, p. 13261-13277. Elsass, Françoise, and du Bray, E.A., 1982, Energy-dispersive X-ray fluorescence spectrometry with the Kevex 7000 system: Saudi Arabia, Deputy Ministry of Mineral Resources Open-File Report USGS-OF-02-52, 53 p.
12 Geochemical and Petrographic Data for Intrusions Peripheral to the Big Timber Stock, Crazy Mountains, Montana Ewart, Anthony, 1982, The mineralogy and petrology of Tertiary—Recent orogenic volcanic rocks with special reference to the andesitic-basaltic compositional range, in Thorpe, R.S., ed., Andesites: New York, John Wiley and Sons, p. 25-87. Farmer, G.L., Glazner, A.F., and Manley, C.R., 2002, Did lithospheric delamination trigger late Cenozoic potassic volcanism in the southern Sierra Nevada, California?: Geological Society of America Bulletin, v. 114, p. 754-768. Frost, B.R., Barnes, C.G., Collins, W.J., Arculus, R.J., Ellis, D.J., and Frost, C.D., 2001, A geochemical classification for granitic rocks: Journal of Petrology, v. 42, p. 2033-2048. Gill, J.B., 1981, Orogenic andesites and plate tectonics: New York, Springer-Verlag, 390 p. Hawkesworth, Chris; Turner, Simon; Gallagher, Kerry; Hunter, Arlene; Bradshaw, Tim; and Rogers, Nick, 1995, Calc-alkaline magmatism, lithospheric thinning and extension in the Basin and Range: Journal of Geophysical Research, v. 100, p. 10271-10286. Irvine, T.N., and Baragar, W.R.A., 1971, A guide to the chemi cal classification of the common volcanic rocks: Canadian Journal of Earth Sciences, v. 8, p. 523-548. Le Maitre, R.W., 2002, Igneous rocks—A classification and glossary of terms (2d ed.): Cambridge, United Kingdom, Cambridge University Press, 236 p. Max Plank Institut für Chemie, 2010, Geochemistry of rocks of the oceans and continents (GEOROC) [Query by geo logical setting—convergent margins]: Mainz, Germany, Max Plank Institut für Chemie, accessed June 10, 2014, at http://georoc.mpch-mainz.gwdg.de/georoc/. Pearce, J.A., Harris, N.B.W., and Tindle, A.G., 1984, Trace element discrimination diagrams for the tectonic interpreta tion of granitic rocks: Journal of Petrology, v. 25, p. 956-983. Pearce, J.A., and Peate, D.W., 1995, Tectonic implications of the composition of volcanic arc magmas: Annual Review of Earth and Planetary Sciences, v. 23, p. 251-285. Roberts, A.E., 1972, Cretaceous and early Tertiary deposi tional and tectonic history of the Livingston area, south western Montana: U.S. Geological Survey Professional Paper 526-C, 120 p. Schmidt, M.E., Grunder, A.L., and Rowe, M.C., 2008, Seg mentation of the Cascade arc as indicated by Sr and Nd isotopic variation among diverse primitive basalts: Earth and Planetary Science Letters, v. 266, p. 166-181. Shand, S.J., 1951, Eruptive rocks: New York, John Wiley, 488 p. Simms, F.E., 1966, The igneous petrology, geochemistry, and structural geology of part of the northwestern Crazy Mountains, Montana: Cincinnati, Ohio, University of Cincinnati, Ph.D. thesis, 339 p. Starmer, R.J., 1972, The distribution and geochemistry of the Big Timber dike swarm, Crazy Mountains, Montana: Cin cinnati, Ohio, University of Cincinnati, Ph.D. thesis, 90 p. Taggart, J.E., Lindsay, J.R., Scott, B.A., Vivit, D.V., Bartel, A.J., and Stewart, K.C., 1987, Analysis of geologic mate rials by X-ray fluorescence spectrometry, Chapter E of Baedecker, P.A., ed., Methods for geochemical analysis: U.S. Geological Survey Bulletin 1770, p. E1-E19. Tappe, John, 1966, The chemistry, petrology, and structure of the Big Timber igneous complex, Crazy Mountains, Montana: Cincinnati, Ohio, University of Cincinnati, Ph.D. thesis, 134 p. Turekian, K.K., and Wedepohl, K.H., 1961, Distribution of the elements in some major units of the Earth's crust: Geological Society of America Bulletin, v. 72, p. 175-192. Watson, E.B., 1979, Zircon saturation in felsic liquids— Experimental results and applications to trace element geochemistry: Contributions to Mineralogy and Petrology, v. 70, p. 407-419. Wolff, J.E., 1938, Igneous rocks of the Crazy Mountains, Montana: Geological Society of America Bulletin, v. 49, p. 1569-1626. Yager, D.B., and Quick, J.E., 1992, SUPERXAP manual: U.S. Geological Survey Open-File Report 92-13, 45 p. Publishing support provided by: Denver Publishing Service Center For more information concerning this publication, contact: Center Director, USGS Central Mineral and Environmental Resources Science Center Box 25046, Mail Stop 973 Denver, CO 80225 (303) 236-1562 Or visit the Central Mineral and Environmental Resources Science Center Web site at: http://minerals.cr.usgs.gov/
Appendixes
14 Geochemical and Petrographic Data for Intrusions Peripheral to the Big Timber Stock, Crazy Mountains, Montana Appendix 1. Status and treatment of samples of sills and plugs peripheral to the Big Timber stock, Crazy Mountains, Montana. [See table 1 for an explanation of data fields. Chem, chemical analysis; TS, thin section; LOI, loss on ignition; K2O, potassium oxide] Field_ID Longitude Latitude Chem TS Locality_name Lithology Ignous_form Alteration -110.448 Ibex Mountain Phonotephrite Plug -110.448 Ibex Mountain Phonotephrite Plug -110.461 Ibex Mountain Phonotephrite Plug -110.530 Shields River Road Phonotephrite Plug high LOI 001W92 -110.691 Gobblers Knob Trachyandesite Sill 002W92 -110.449
Ibex Mountain Plug 003W92 -110.153
Raspberry Butte Sill 005W92 -110.147 Raspberry Butte Trachydacite Sill 006W92 -110.147 Raspberry Butte Basaltic trachyandesite Sill 007W92 -110.147 Raspberry Butte Trachydacite Sill 008W92 -110.147 Raspberry Butte Trachyandesite Sill 009W92 -110.148 Raspberry Butte Trachyandesite Sill 010W92 -110.137 Raspberry Butte Trachyandesite Sill high LOI, low K2O 011W92 -110.366 Loco Mountain Basaltic trachyandesite Dike 012W92 -110.366 Loco Mountain Trachyandesite Dike high LOI 014W92 -110.362 Loco Mountain Trachyandesite Dike 015W92 -110.362 Loco Mountain Basalt Dike high LOI 016W92 -110.140 Amelong Creek Basaltic andesite Sill high LOI 017W92 -110.145 Amelong Creek Trachyandesite Sill high LOI 018W92 -110.159 Amelong Creek Trachyandesite Sill high LOI 019W92 -110.039 Grosfield Ranch Basalt Sill high LOI 020W92 -110.040 Grosfield Ranch Basaltic andesite Sill high LOI 021W92 -110.043 Grosfield Ranch Basaltic trachyandesite Sill high LOI 022W92 -110.044 Grosfield Ranch Trachybasalt Sill high LOI 024W92 -110.104 Grosfield Ranch Trachyandesite Sill high LOI 025W92 -110.104 Grosfield Ranch Basaltic andesite Sill high LOI 026W92 -110.104 Grosfield Ranch Trachyandesite Sill high LOI 027W92 -110.104 Grosfield Ranch Basaltic trachyandesite Sill high LOI Status and treatment of samples of sills and plugs peripheral to the Big Timber stock, Crazy Mountains, Montana. (Click here to open in Microsoft Excel.)
Appendixes 15 Appendix 2. Geochemical and petrographic data for samples of sills and plugs peripheral to the Big Timber stock, Crazy Mountains, Montana. (Click here to open in Microsoft Excel.) [See table 2 for an explanation of data fields. mm, millimeter; pct, percent; ppm, parts per million; LOI, loss on ignition; Abd, abundance; phenos, phenocrysts; Ags, average grain size; Mgs, maximum grain size; Xl, crystallinity. Texture: Aph, aphanitic; E, equigranular; Hc, holocrystalline; Hy, hyalophitic; I, intersertal; P, porphyritic; S, seriate; T, trachytic. Accessory minerals: Ap, apatite, Ttn, titanite. Crystallinity: A, anhedral; E, euhedral; S, subhedral] Field_ID Locality_name SiO2_pct TiO2_pct Al2O3_pct MnO_pct MgO_pct CaO_pct Na2O_pct K2O_pct Ibex Mountain Ibex Mountain Ibex Mountain Shields River Road 001W92 Gobblers Knob 002W92 Ibex Mountain 003W92 Raspberry Butte 005W92 Raspberry Butte 006W92 Raspberry Butte 007W92 Raspberry Butte 008W92 Raspberry Butte 009W92 Raspberry Butte 010W92 Raspberry Butte 011W92 Loco Mountain 012W92 Loco Mountain 014W92 Loco Mountain 015W92 Loco Mountain 016W92 Amelong Creek 017W92 Amelong Creek 018W92 Amelong Creek 019W92 Grosfield Ranch 020W92 Grosfield Ranch 021W92 Grosfield Ranch 022W92 Grosfield Ranch 024W92 Grosfield Ranch 025W92 Grosfield Ranch 026W92 Grosfield Ranch 027W92 Grosfield Ranch Field_ID Locality_name P2O5_pct LOI_pct Ibex Mountain 3,000 3,152 Ibex Mountain 3,164 2,289 Ibex Mountain 3,215 2,415 Shields River Road 3,371 2,916 001W92 Gobblers Knob 2,225 1,272 002W92 Ibex Mountain 003W92 Raspberry Butte 005W92 Raspberry Butte 2,014 006W92 Raspberry Butte 1,634 007W92 Raspberry Butte 2,405 008W92 Raspberry Butte 2,287 1,204 009W92 Raspberry Butte 1,701 010W92 Raspberry Butte 1,170 011W92 Loco Mountain 1,001 012W92 Loco Mountain 2,272 1,019 014W92 Loco Mountain 2,969 1,592 015W92 Loco Mountain 016W92 Amelong Creek 1,254 017W92 Amelong Creek 2,011 018W92 Amelong Creek 2,734 1,033 019W92 Grosfield Ranch 1,929 020W92 Grosfield Ranch 2,662 021W92 Grosfield Ranch 4,254 022W92 Grosfield Ranch 3,176 024W92 Grosfield Ranch 4,331 025W92 Grosfield Ranch 1,574 026W92 Grosfield Ranch 2,446 1,382 027W92 Grosfield Ranch 2,921
16 Geochemical and Petrographic Data for Intrusions Peripheral to the Big Timber Stock, Crazy Mountains, Montana Appendix 2. Geochemical and petrographic data for samples of sills and plugs peripheral to the Big Timber stock, Crazy Mountains, Montana.—Continued [See table 2 for an explanation of data fields. mm, millimeter; pct, percent; ppm, parts per million; LOI, loss on ignition; Abd, abundance; phenos, phenocrysts; Ags, average grain size; Mgs, maximum grain size; Xl, crystallinity. Texture: Aph, aphanitic; E, equigranular; Hc, holocrystalline; Hy, hyalophitic; I, intersertal; P, porphyritic; S, seriate; T, trachytic. Accessory minerals: Ap, apatite, Ttn, titanite. Crystallinity: A, anhedral; E, euhedral; S, subhedral] Field_ID Locality_name Ibex Mountain Ibex Mountain Ibex Mountain Shields River Road 001W92 Gobblers Knob 002W92 Ibex Mountain 003W92 Raspberry Butte 005W92 Raspberry Butte 006W92 Raspberry Butte 007W92 Raspberry Butte 008W92 Raspberry Butte 009W92 Raspberry Butte 010W92 Raspberry Butte 011W92 Loco Mountain 012W92 Loco Mountain 014W92 Loco Mountain 015W92 Loco Mountain 016W92 Amelong Creek 017W92 Amelong Creek 018W92 Amelong Creek 019W92 Grosfield Ranch 020W92 Grosfield Ranch 021W92 Grosfield Ranch 022W92 Grosfield Ranch 024W92 Grosfield Ranch 025W92 Grosfield Ranch 026W92 Grosfield Ranch 027W92 Grosfield Ranch Field_ID Locality_name Abd_GrnAmph_phenos Abd_BrnAmph_phenos Ibex Mountain Ibex Mountain Ibex Mountain Shields River Road 001W92 Gobblers Knob 002W92 Ibex Mountain 003W92 Raspberry Butte 005W92 Raspberry Butte 006W92 Raspberry Butte 007W92 Raspberry Butte 008W92 Raspberry Butte 009W92 Raspberry Butte 010W92 Raspberry Butte 011W92 Loco Mountain 012W92 Loco Mountain 014W92 Loco Mountain 015W92 Loco Mountain 016W92 Amelong Creek 017W92 Amelong Creek 018W92 Amelong Creek 019W92 Grosfield Ranch 020W92 Grosfield Ranch 021W92 Grosfield Ranch 022W92 Grosfield Ranch 024W92 Grosfield Ranch 025W92 Grosfield Ranch 026W92 Grosfield Ranch 027W92 Grosfield Ranch
Appendixes 17 Appendix 2. Geochemical and petrographic data for samples of sills and plugs peripheral to the Big Timber stock, Crazy Mountains, Montana.—Continued Field_ID Locality_name Abd_Cpx_phenos Abd_Opq TotXtls ClrIndx AgsPl AgsGrnAmph Ibex Mountain TR Ibex Mountain Ibex Mountain Shields River Road 001W92 Gobblers Knob 002W92 Ibex Mountain 003W92 Raspberry Butte TR 005W92 Raspberry Butte 006W92 Raspberry Butte 007W92 Raspberry Butte 008W92 Raspberry Butte 009W92 Raspberry Butte TR 010W92 Raspberry Butte 011W92 Loco Mountain 012W92 Loco Mountain 014W92 Loco Mountain 015W92 Loco Mountain 016W92 Amelong Creek 017W92 Amelong Creek 018W92 Amelong Creek 019W92 Grosfield Ranch 020W92 Grosfield Ranch 021W92 Grosfield Ranch 022W92 Grosfield Ranch 024W92 Grosfield Ranch 025W92 Grosfield Ranch 026W92 Grosfield Ranch 027W92 Grosfield Ranch Field_ID Locality_name AgsBrnAmph AgsBt AgsCpx AgsOl AgsOpq MgsPl MgsGrnAmph MgsBrnAmph MgsBt MgsCpx Ibex Mountain Ibex Mountain Ibex Mountain Shields River Road 001W92 Gobblers Knob 002W92 Ibex Mountain 003W92 Raspberry Butte 005W92 Raspberry Butte 006W92 Raspberry Butte 007W92 Raspberry Butte 008W92 Raspberry Butte 009W92 Raspberry Butte 010W92 Raspberry Butte 011W92 Loco Mountain 012W92 Loco Mountain 014W92 Loco Mountain 015W92 Loco Mountain 016W92 Amelong Creek 017W92 Amelong Creek 018W92 Amelong Creek 019W92 Grosfield Ranch 020W92 Grosfield Ranch 021W92 Grosfield Ranch 022W92 Grosfield Ranch 024W92 Grosfield Ranch 025W92 Grosfield Ranch 026W92 Grosfield Ranch 027W92 Grosfield Ranch
Appendix 2. Geochemical and petrographic data for samples of sills and plugs peripheral to the Big Timber stock, Crazy Mountains, Montana.—Continued [See table 2 for an explanation of data fields. mm, millimeter; pct, percent; ppm, parts per million; LOI, loss on ignition; Abd, abundance; phenos, phenocrysts; Ags, average grain size; Mgs, maximum grain size; Xl, crystallinity. Texture: Aph, aphanitic; E, equigranular; Hc, holocrystalline; Hy, hyalophitic; I, intersertal; P, porphyritic; S, seriate; T, trachytic. Accessory minerals: Ap, apatite, Ttn, titanite. Crystallinity: A, anhedral; E, euhedral; S, subhedral] Field_ID Locality_name MgsOl MgsOpq Texture Access_Mnrls XlPl XlGrnAmph XlBrnAmph XlBt XlCpx XlOl XlOpq Ibex Mountain Hc; E Ap S E S S Ibex Mountain Ibex Mountain Shields River Road 001W92 Gobblers Knob Hc; P Ap; Ttn S A A A; S S 002W92 Ibex Mountain Hy; P Ap E E; S A 003W92 Raspberry Butte Hy; P; T E E S S 005W92 Raspberry Butte Hy; P Ap S S A 006W92 Raspberry Butte P E S E A 007W92 Raspberry Butte P Ap, Ttn E S S S 008W92 Raspberry Butte P; I Ap; Ttn E S E A 009W92 Raspberry Butte P; Hy Ap E S S S S 010W92 Raspberry Butte P; I Ttn (secondary) S S A 011W92 Loco Mountain P; Hy Ap S A 012W92 Loco Mountain P; I Ap E S A 014W92 Loco Mountain Aph; S S S S 015W92 Loco Mountain P; I E E S 016W92 Amelong Creek P; I E E E 017W92 Amelong Creek P; Hc Ap S S 018W92 Amelong Creek P; Hc Ap E S 019W92 Grosfield Ranch P; I S S E 020W92 Grosfield Ranch P; I S S 021W92 Grosfield Ranch Aph; I S 022W92 Grosfield Ranch Aph; P E A 024W92 Grosfield Ranch P; I A S 025W92 Grosfield Ranch P; I S E A 026W92 Grosfield Ranch P; I S S S 027W92 Grosfield Ranch P; I Ap? S A Field_ID Locality_name Petrog_Com HblClr AltExtnt Ibex Mountain Includes about 47% anhedral, interstitial nepheline (0.5-4.5 mm, average 1.5 mm); variably altered to zeolite minerals. Cpx includes distinctive emerald-green aegirine overgrowths. 1+ Ibex Mountain Ibex Mountain Shields River Road 001W92 Gobblers Knob Groundmass: clay-altered Pl (0.5 mm), Bt (0.4 mm), Qtz (0.3 mm), Hbl (0.3 mm), Cpx (0.3 mm), Opq (0.2 mm). Fine to medium grained. Hbl completely altered to Chl. 2+ 002W92 Ibex Mountain Groundmass: turbid, moderately devitrified glass. Rock also contains 7% 0.2-1.2 (0.5) mm nepheline and 2% 0.5-1.2 (0.9) mm cancrinite (0.5-1.2 mm, average 0.9 mm). Cpx strongly serve textured. 003W92 Raspberry Butte Groundmass: moderately devitrified glass with Pl (0.1 mm), Hbl (0.05 mm), Opq (0.02 mm). Pale tan to tan 005W92 Raspberry Butte Groundmass: devitrified intergrowth of Pl (0.1 mm), Hbl (0.01 mm). Some secondary Chl. Pale tan to tan 006W92 Raspberry Butte Groundmass: intergranular intergrowth of Pl (0.1 mm), Cpx (0.01 mm), Opq (0.02 mm). Green amphibole completely altered to Chl. Pale tan to tan 007W92 Raspberry Butte Groundmass: intergranular intergrowth of Pl (0.2 mm), Opq (0.1 mm). Brown amphibole considerably altered to Chl. Green amphibole altered to Chl+Ep. Pale tan to tan 008W92 Raspberry Butte Groundmass: intersertal intergrowth of Pl (0.1 mm), Hbl (0.1 mm). Brown amphibole considerably altered to Chl. Green amphibole altered to Chl+Ep. Brown amphibole: Pale tan to tan Green amphibole: Yellow green to green 009W92 Raspberry Butte Groundmass: moderately devitrified glass with Pl (0.1 mm), Hbl (0.05 mm), Opq (0.02 mm). Green amphibole altered to Chl. Brown amphibole: Pale tan to tan Green amphibole: Pale green 2010W92 Raspberry Butte Groundmass: intersertal intergrowth of Pl (0.1 mm), altered/devitrified glass, Opq (0.03 mm). Pl almost completely replaced by Cc. Cpx, completely replaced by Cc + clay. 3+ 011W92 Loco Mountain Groundmass: intersertal intergrowth of altered (clay+Cc) Pl (0.2 mm), brown amphibole (0.05 mm), Chl, moderately devitrifed glass, Opq (0.05 mm). Cpx, moderately seived. 18 Geochemical and Petrographic Data for Intrusions Peripheral to the Big Timber Stock, Crazy Mountains, Montana
Appendix 2. Geochemical and petrographic data for samples of sills and plugs peripheral to the Big Timber stock, Crazy Mountains, Montana.—Continued [See table 2 for an explanation of data fields. mm, millimeter; pct, percent; ppm, parts per million; LOI, loss on ignition; Abd, abundance; phenos, phenocrysts; Ags, average grain size; Mgs, maximum grain size; Xl, crystallinity. Texture: Aph, aphanitic; E, equigranular; Hc, holocrystalline; Hy, hyalophitic; I, intersertal; P, porphyritic; S, seriate; T, trachytic. Accessory minerals: Ap, apatite, Ttn, titanite. Crystallinity: A, anhedral; E, euhedral; S, subhedral] Field_ID Locality_name Petrog_Com HblClr AltExtnt 012W92 Loco Mountain Groundmass: intersertal intergrowth of Pl (0.1 mm), brown amphibole (0.1 mm), Opq (0.04 mm). Abundant secondary Cc. Pale tan to tan 3+ 014W92 Loco Mountain Groundmass: intersertal intergrowth of Pl (0.1 mm), brown amphibole (0.1 mm), moderately devitrified glass, Opq (0.03 mm). Pale tan to tan 015W92 Loco Mountain Groundmass: intersertal intergrowth of Pl (0.1 mm), brown amphibole (0.05 mm), Opq (0.03 mm). Abundant secondary Cc; some secondary Chl. 016W92 Amelong Creek Groundmass: intersertal intergrowth of Pl (0.1 mm), brown amphibole (0.1 mm), Opq (0.03 mm). Abundant secondary Cc; some secondary Chl. 2+ 017W92 Amelong Creek Groundmass: intersertal intergrowth of Pl (0.2 mm), brown amphibole (0.1 mm), Opq (0.05 mm). Abundant secondary Cc and Chl. 018W92 Amelong Creek Groundmass: intersertal intergrowth of Pl (0.2 mm), Hbl (0.1 mm), Qtz (0.05 mm), Opq (0.02 mm). Abundant secondary Chl. 019W92 Grosfield Ranch Groundmass: intersertal intergrowth of Pl (0.1 mm), brown amphibole (0.05 mm), Opq (0.01 mm). Abundant secondary Chl and Qtz. 020W92 Grosfield Ranch Groundmass: intersertal intergrowth of Pl (0.1 mm), Hbl (0.05 mm), turbid glass, Opq (0.02 mm). Cpx completely altered to Chl+clay. Abundant secondary Chl and Qtz. 3+ 021W92 Grosfield Ranch Groundmass: intersertal intergrowth of Pl (0.2 mm), Hbl (0.1 mm), Opq (0.03 mm). Rock may have contained phenocrysts; if so, phenocrysts obliterated by alteration. 3+ 022W92 Grosfield Ranch Groundmass: aphanitic intergrowth of Pl (0.05 mm), Opq (0.01 mm), altered/devitrified glass. 024W92 Grosfield Ranch Groundmass: intersertal intergrowth of Pl (0.4 mm), Hbl (0.1 mm), Opq (0.05 mm). Abundant secondary Cc. 2+ 025W92 Grosfield Ranch Groundmass: intersertal intergrowth of Pl (0.2 mm), Hbl (0.2 mm), Opq (0.05 mm), Cpx (0.1 mm). 2026W92 Grosfield Ranch Groundmass: intersertal intergrowth of Pl (0.3 mm), Hbl (0.1 mm), Cpx (0.1 mm), Opq (0.04 mm). 027W92 Grosfield Ranch Groundmass: intersertal intergrowth of Pl (0.2 mm), Hbl (0.1 mm), Opq (0.04 mm), devitrified glass (?). Abundant secondary Cc. Hbl in ground mass altered to Chl. Appendixes 19
du Bray and others—Geochemical and Petrographic Data for Intrusions Peripheral to the Big Timber Stock, Crazy Mountains, Montana—Data Series 895 ISSN 2327-638X (online) http://dx.doi.org/10.3133/ds895
Plates & figures from the original
