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Variations in the Illite to Muscovite Transition Related to Metamorphic Conditions and Detrital Muscovite Content: Insight from the Paleozoic Passive Margin of the Southwestern United States Author(s): Charles Verdel, Nathan Niemi, and Ben A. van der Pluijm Source: The Journal of Geology, Vol. 119, No. 4 (July 2011), pp. 419-437 Published by: The University of Chicago Press Stable URL: http://www.jstor.org/stable/10.1086/660086 . Accessed: 21/10/2015 21:15

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This content downloaded from 23.235.32.0 on Wed, 21 Oct 2015 21:15:03 PM All use subject to JSTOR Terms and Conditions Variations in the Illite to Muscovite Transition Related to Metamorphic Conditions and Detrital Muscovite Content: Insight from the Paleozoic Passive Margin of the Southwestern United States

Charles Verdel,* Nathan Niemi, and Ben A. van der Pluijm

Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, U.S.A.

ABSTRACT X-ray diffraction (XRD) analysis of the illite to muscovite transition in pelitic rocks has been widely used in regional studies of low-grade metamorphism. Variations in detrital muscovite content of sediments are also measurable with XRD and may, in fact, be difficult to distinguish from metamorphic gradients. We utilize field transects to isolate detrital muscovite versus metamorphic reaction components of illite-muscovite variations in Paleozoic rocks of the western United States. One transect focuses on a single stratigraphic interval (Middle Cambrian pelites) and extends from Death Valley in the west to the Grand Canyon in the east. Detrital muscovite content is roughly constant along this 500-km-long transect, so illite-muscovite variations are ascribed to differences in low-grade metamorphism. In terms of both illite crystallinity and polytype composition, there is an overall increase in metamorphic grade to the west along the transect, from the diagenetic metapelitic zone at the Grand Canyon to epizone-low anchizone conditions near Death Valley. Most of the regional variation in illite parameters occurs between Frenchman Mountain and the southern Nopah Range, corresponding to the transition from cratonic to miogeoclinal facies and also to the leading edge of the Sevier thrust belt. In contrast to this “horizontal” transect that highlights metamorphic reactions, a vertical transect through the Paleozoic stratigraphy of the Grand Canyon targets temporal variations in the flux of detrital muscovite. In the Grand Canyon, illite crystallinity reaches a maximum at the base of the Pennsylvanian Supai Group before decreasing upsection. Deposition of the Supai Group was coeval with formation of basement- cored uplifts during the Ancestral Rocky Mountains orogeny, and we suggest that the upsection change in illite composition at the Grand Canyon reflects input of detrital muscovite eroded from these uplifts.

Online enhancement: appendix.

Introduction The transformation of illite to muscovite is the ba- tal structure of illite occur during subgreenschist sis for deﬁning low-grade metamorphism of pelites facies metamorphism and, given sufﬁcient time (e.g., Frey 1987; Merriman and Frey 1999) and has and temperature, result in crystallites that are in- been used to quantify metamorphic variations in distinguishable from muscovite. These crystallo- numerous areas (e.g., Weaver 1960; Duba and Wil- graphic variations, which are observable with both liams-Jones 1983; Hunziker et al. 1986; Robinson x-ray diffraction (XRD) and transmission electron and Bevins 1986; Awan and Woodcock 1991; Yang microscopy (TEM), track metamorphism from the and Hesse 1991; Warr and Rice 1994; Rahn et al. diagenetic zone to the epizone, or from roughly 1995; Jaboyedoff and The´lin 1996; Merriman and 100Њ to 300ЊC (Merriman and Frey 1999). Frey 1999; Merriman 2006; Ruiz et al. 2008; Hara However, direct inference of metamorphic grade and Kurihara 2010). Progressive changes in the crys- from illite-muscovite parameters is complicated by variations in detrital muscovite content. For ex- Manuscript received September 27, 2010; accepted March 9, ample, low-grade shales containing a large fraction 2011. of detrital muscovite may have similar XRD char- * Author for correspondence. Present address: Department of Geology, University of Kansas, Lawrence, Kansas 66045, acteristics as higher-grade slates that contain little U.S.A.; e-mail: [email protected]. detrital muscovite. Although detrital content var-

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This content downloaded from 23.235.32.0 on Wed, 21 Oct 2015 21:15:03 PM All use subject to JSTOR Terms and Conditions 420 C. VERDEL ET AL. iations may obscure metamorphic patterns, such Valley, correlative Paleozoic strata are 16 km thick variability may be scientifically useful in regional (e.g., McAllister 1956; Burchfiel 1969; Wright et al. analyses, for example, when detrital muscovite in- 1981). The regional thickness variation has been put to a sedimentary basin is tied to tectonism in ascribed to thermal subsidence accompanying the surrounding areas. Illite-muscovite studies hold po- Neoproterozoic breakup of Rodinia and the devel- tential, therefore, not only for identifying regional opment of a western Laurentian passive margin patterns of low-grade metamorphism but also for (e.g., Burchfiel and Davis 1972; Stewart 1972; Bond revealing variations in detrital muscovite content. et al. 1985; Burchfiel et al. 1992). Despite this potential, few natural experiments The craton-miogeocline hingeline generally co- have been conducted that attempt to isolate the incides with the front of the Cretaceous Sevier fold- detrital muscovite and metamorphic transforma- and-thrust belt (fig. 1; Burchfiel and Davis 1972; tion components of the illite-muscovite system. Burchfiel et al. 1992). The Sevier belt is the youn- Ideally, these experiments would be designed to gest and easternmost of several fold-and-thrust hold one variable (either metamorphic conditions belts located inboard of the Cordilleran magmatic or detrital input) constant and measure the re- arc that accounted for late Paleozoic through Cre- sponse of illite-muscovite parameters to changes in taceous shortening of the miogeocline and em- the other variable. placement of allochthonous strata on top of the In this article, we utilize field transects from the western margin of the miogeocline (fig. 1; e.g., western United States to examine the combined Armstrong 1968; Burchfiel et al. 1992; Miller et al. effects of metamorphism and detrital muscovite 1992; Wyld et al. 2003; DeCelles and Coogan 2006). content on measurements of the illite to muscovite West of the Sevier front, the Early to Middle Ju- transition. One 500-km-long transect is focused on rassic Luning-Fencemaker thrust belt places a thick a single stratigraphic interval that is progressively accumulation of Triassic back-arc, deep-marine fa- buried to greater depth toward the west, both from cies sediments on top of shelf facies (Oldow 1984; increases in the thickness of overlying strata and Wyld 2002; Martin et al. 2010). East of the Luning- from overriding thrust sheets. Because it is a single Fencemaker thrust belt, the Late Permian–Early formation, detrital muscovite content is presumed Triassic Golconda thrust carries Devonian through to be relatively constant along the transect, and Permian strata over the Roberts Mountain alloch- variations in illite-muscovite data are attributed thon, which itself was emplaced onto the mio- principally to metamorphic transformation. The geocline during the Late Devonian–Early Missis- other transects consist of stratigraphic profiles sippian Antler orogeny (e.g., Burchfiel et al. 1992). through low-grade Paleozoic sediments. Variations A common characteristic of these east-vergent in illite XRD data from these “vertical” transects, thrusts is the juxtaposition of deeply buried basinal which seemingly indicate inverted metamorphic sediments on top of shallower facies. In the Death gradients, are, in fact, unrelated to metamorphic Valley region, the miogeocline experienced Perm- conditions but instead reflect variations in detrital ian tectonic thickening along the Death Valley muscovite content. These results are among the thrust belt (fig. 1; Wernicke et al. 1988; Snow et al. first illite-muscovite data from the western United 1991; Snow 1992; Stevens and Stone 2005a, 2005b) States, and they illustrate that gradients in illite as well as Jurassic to Cretaceous thickening along parameters correspond with major tectonic bound- the east Sierran thrust system and the Sevier belt aries and demonstrate the utility for tracking de- (e.g., Carr 1980; Walker et al. 1995; DeCelles 2004). trital muscovite input. The present topography of the central Basin and Range province, which lies between the Cordille- ran magmatic arc to the west and the Colorado Background Plateau to the east, was largely formed during wide- Tectonic Setting. In the Cordilleran orogen of the spread mid-Tertiary E-W extension. As a result of western United States, relatively thin, cratonic fa- extension, Paleozoic-Cretaceous thrust faults and cies of Paleozoic strata are separated from thicker various stratigraphic levels of the Cordilleran mio- accumulations of Neoproterozoic-Paleozoic miogeocline are now exposed in widely separated geoclinal facies along the craton-miogeocline mountain ranges, providing useful markers for re- hingeline (e.g., Kay 1951; Picha and Gibson 1985; constructing the extensional history of the Basin Fedo and Cooper 2001). In cratonic locations such and Range (Snow and Wernicke 2000; McQuarrie as the Grand Canyon (fig. 1), the Paleozoic section and Wernicke 2005) as well as locations for deter- is approximately 1.5 km thick (e.g., Billingsley mining metamorphic conditions at various posi- 2000), whereas in distal locations such as Death tions within the original passive margin.

Figure 1. Tectonic setting of the Basin and Range and the Colorado Plateau, after Oldow (1984), Wernicke et al. (1988), and DeCelles and Coogan (2006). Abbreviations and sample locations: BM, Bare Mountain; EM, Eagle Mountain; FM, Frenchman Mountain; GC, Grand Canyon; GM, Grapevine Mountains; MP, Mesquite Pass; PR, Panamint Range; RS, Resting Spring Range; SN, southern Nopah Range. Details of the structural position of samples within the Death Valley thrust belt are shown in ﬁgure 4.

In contrast to the deformational history of the west (Blakey 1990) and seems to have been fed by miogeocline, Paleozoic strata of the Colorado Pla- ﬂuvial and eolian systems sourced from the An- teau have experienced little tectonic disturbance cestral Rocky Mountains (Blakey et al. 1988; Pe- since the time of their deposition. Regions to the terson 1988; Baars 2000). north and east of the Colorado Plateau were, how- Previous Quantiﬁcation of Low-Grade Metamorphism ever, affected by Pennsylvanian uplift of basement- in the Basin and Range. Conodont Alteration In- cored mountain ranges during creation of the An- dex. Stratigraphic burial, structural duplication, cestral Rocky Mountains (e.g., Miller et al. 1992). and magmatism have led to regional variations of Evidence for this orogeny comes primarily from low-grade metamorphism within the Cordilleran Late Mississippian–Early Pennsylvanian arkosic orogen. The most commonly used method for redbeds deposited in basins adjacent to the uplifts quantifying these variations has been the conodont (e.g., Mallory 1958, 1975; Kluth and Coney 1981; alteration index (CAI), a parameter based on pre- van de Kamp and Leake 1994; Sweet and Soreghan dictable color changes of conodont elements with 2010). Uplift of the Ancestral Rocky Mountains co- increasing temperature (Epstein et al. 1977; Reje- incides with coastal plain, shallow marine, and eo- bian et al. 1987). A large database of CAI measure- lian siliciclastic and carbonate sedimentation in ments from Nevada (Crafford 2007) highlights the the Late Mississippian–Permian Supai Group in the overall variability of metamorphic grade across the Grand Canyon region (e.g., McKee 1982; Blakey Great Basin. In general, CAI values in this database 1990; Rice and Loope 1991; Billingsley 2000), which are correlated with rock age, suggesting a regional grades laterally into the Callville Limestone to the metamorphic pattern dominated by burial meta-

This content downloaded from 23.235.32.0 on Wed, 21 Oct 2015 21:15:03 PM All use subject to JSTOR Terms and Conditions 422 C. VERDEL ET AL. morphism. As described below, a primary focus of et al. 1994; Kelley et al. 2001; Flowers et al. 2008). this study is metamorphic variations of Cambrian From these data it has been estimated that about pelites within the western United States, and we 4 km of Mesozoic strata were eroded from the cur- point out here that there is no obvious regional rent rim of the Grand Canyon during Late Creta- pattern to CAI results from Cambrian carbonates ceous–Tertiary time and that peak temperatures ex- in the Nevada database (fig. 2A). perienced by Permian strata on the Colorado Fission-Track and (U-Th)/He Thermochronology Plateau were approximately 90Њ–100ЊC (Dumitru et and Implications for Peak Temperatures of Cam- al. 1994). The Paleozoic section is approximately brian Strata. Low-temperature thermochronol- 1.5 km thick on the Colorado Plateau, implying ogy, another tool suitable for assessing sub- maximum burial depth of 5.5 km for Cambrian greenschist facies thermal histories of sedimentary strata at the Grand Canyon and peak temperatures rocks, has been used to investigate the erosional of roughly 125Њ–150ЊC, using reasonable geother- unroofing history of the Colorado Plateau, partic- mal gradients of approximately 25ЊCkmϪ1. The ularly at the Grand Canyon. Permian sandstones Cambrian interval was buried beneath progres- located just below the rim of the Grand Canyon sively greater accumulations of overlying sediment contain apatite grains with reset (U-Th)/He and fis- to the west, up to approximately 6 km of Paleozoic sion-track ages (Naeser et al. 1989, 2001; Dumitru strata in the Death Valley region as described

Figure 2. Low-grade metamorphic indicators in Nevada and the vicinity for Cambrian (A) and Triassic (B) sediments. Illite crystallinity (IC) from Cambrian pelites analyzed during this study and Triassic pelites along the Luning- Fencemaker fold-and-thrust belt (Wyld et al. 2003) are both standardized using the method of Warr and Rice (1994). Conodont alteration index data are from Crafford (2007). Abbreviations are as in ﬁgure 1.

This content downloaded from 23.235.32.0 on Wed, 21 Oct 2015 21:15:03 PM All use subject to JSTOR Terms and Conditions Journal of Geology V A R I A T I O N S I N I L L I T E – MUSCOVITE TRANSITION 423 above. Assuming that the Mesozoic section had a lously low IC, resulting in apparent IC variations constant thickness over the entire area, burial that do not reflect metamorphic conditions. depths of 10 km and peak temperatures of approx- IC is also influenced by the presence of smectite imately 250ЊC are implied for Cambrian sediments in interlayered illite/smectite (I/S). During prograde in miogeoclinal locations. As described below, this metamorphism, I/S containing significant interlay- predicted craton to miogeocline temperature range ered smectite is progressively converted to more for Cambrian strata corresponds quite closely with illite-rich compositions (e.g., Hower and Mowatt the range over which the illite to muscovite tran- 1966; Hower et al. 1976; Moore and Reynolds 1997). sition occurs (e.g., Merriman and Frey 1999). Tec- Expandable layers (i.e., smectite) will broaden the tonic thickening of the miogeocline during Paleo- 1-nm XRD peak, leading to larger values of IC (e.g., zoic-Cretaceous thrusting would increase peak Eberl and Velde 1989; Jaboyedoff et al. 2001; and temperatures in structurally lower thrust sheets. see fig. 3 in Jaboyedoff and The´lin 1996 for a sum- Low-Grade Metamorphism of Pelitic Rocks and Il- mary of clay mineral transformations and XRD lite Crystallinity. Illite Crystallinity and Poly- transitions during low-grade metamorphism). In types. Illite crystallinity (IC) is a commonly used short, both the I/S-to-illite and illite-to-muscovite parameter for describing low-grade metamorphism transitions are related to subgreenschist facies pro- of pelitic rocks (e.g., Kisch 1983; Frey 1987; Mer- grade metamorphism, and both involve a narrow- riman and Peacor 1999; Ku¨ bler and Jaboyedoff 2000; ing of the 1-nm XRD peak with progressively Jaboyedoff et al. 2001). Illite is a phyllosilicate con- higher metamorphic conditions. Because the XRD sisting of “packets” of 1-nm-thick sheets, which in response to both parameters is cumulative, IC val- turn are made up of tetrahedal and octahedral al- ues are typically used to define subgreenschist fa- uminosilicate layers separated by interlayer potas- cies metapelitic zones:IC 1 1Њ2v (thin illite packets sium cations (e.g., Moore and Reynolds 1997). IC that may contain significant interlayered smectite) refers to the width of the basal 1-nm illite XRD corresponds to the shallow diagenetic zone, IC val- peak at one-half of its height and is typically re- ues from 0.42Њ to 1Њ2v is the deep diagenetic zone, ported in units of DЊ2v, where Њ2v is the XRD angle from 0.30Њ to 0.42Њ2v is the low anchizone, from (Ku¨ bler 1967; Ku¨ bler and Jaboyedoff 2000). The 0.25Њ to 0.30Њ2v is the high anchizone, and IC ! width of the diffraction peak is fundamentally re- 0.25Њ2v (particularly, thick illite/muscovite packets lated to the c-axis parallel thickness of illite crys- with little if any interlayered smectite) is the epi- tallites and is also correlated with ordering between zone (e.g., Merriman and Peacor 1999; Kemp and neighboring packets. X-rays diffracted from thick Merriman 2009). These zones are broadly equiva- crystallites consisting of numerous 1-nm packets lent to the more typically used metamorphic facies destructively interfere, producing narrow peaks terminology: the diagenetic zone corresponds with centered at approximately 1 nm. Relatively thin zeolite facies, the anchizone corresponds with packets generate less destructive interference and prehnite-pumpellyite facies, and the epizone cor- produce broader peaks (Moore and Reynolds 1997). responds with greenschist facies. Above a certain Numerous detailed TEM studies that have focused crystallite thickness, IC values “saturate,” and fur- on pelites of various IC confirm that crystallite ther narrowing of the 1-nm peak is limited by op- packets thicken as IC decreases (Merriman et al. tical parameters of the x-ray diffractometer (Moore 1990, 1995; Nieto and Sanchez-Navas 1994; A´ rkai and Reynolds 1997). Saturation occurs at epizone et al. 1996; Dalla Torre et al. 1996; Jiang et al. 1997; (greenschist facies) conditions and precludes IC dis- Warr and Nieto 1998; Abad et al. 2001). Coarse- crimination of higher metamorphic grades. grained muscovite is characterized by thick crys- The crystallographic structure of illite also de- tallites and low IC, whereas authigenic illite crys- pends on the c-axis rotation of individual packets. tallites in very-low-grade sediments (e.g., zeolite Polytypes are structural classes defined by the facies) are thin and have large IC. The transition amount of rotation between neighboring packets from thin illite crystallites to thick muscovite crys- (Smith and Yoder 1956). Two illite polytypes are tals, as tracked by a decrease in IC, is controlled commonly recognized in nature: one is defined by primarily by temperature (Ji and Browne 2000) and two-packet monoclinic symmetry (2M1), and the can therefore reflect regional variations in low- other is characterized by disordered, one-packet grade metamorphism. Regional patterns can, how- monoclinic symmetry (1Md). The 2M1 polytype has ever, be disrupted by variations in the detrital mus- packets that are rotated 120Њ relative to adjacent covite content of different stratigraphic units. For layers, whereas the 1Md polytype does not have instance, very-low-grade shales that contain large regularly alternating rotations (Moore and Reyn- amounts of detrital muscovite will have anoma- olds 1997). Like IC, polytype compositions can be

This content downloaded from 23.235.32.0 on Wed, 21 Oct 2015 21:15:03 PM All use subject to JSTOR Terms and Conditions 424 C. VERDEL ET AL. estimated with XRD (e.g., Maxwell and Hower grade in these transects. Results that might suggest 1967; Grathoff and Moore 1996) and are sensitive inverted metamorphic gradients (i.e., upsection in- to environmental conditions and detrital musco- creases in metamorphic grade) are more easily ex- vite content (e.g., Yoder and Eugster 1955; Reynolds plained as variations in detrital muscovite content. 1963; Velde 1965; Hunziker et al. 1986). The pro- The vertical transects are designed to test the re- portion of the 2M1 polytype increases with increas- sponse to detrital muscovite fluxing, in particular, ing metamorphic grade and is highly correlated the possible effects of erosion of Pennsylvanian up- with IC (Yang and Hesse 1991; Hnat 2009). lifts of the Ancestral Rocky Mountains. Illite Crystallinity Studies in the Basin and Illite Crystallinity Analytical Methods. IC values Range. The only previous IC data from the Basin can be affected by several sample preparation fac- and Range come from Triassic pelites in the Lun- tors (e.g., Kisch and Frey 1987; Kisch 1991; Krumm ing-Fencemaker fold-and-thrust belt in northwest and Buggisch 1991; Warr and Rice 1994), so in stud- Nevada (fig. 2B; Wyld et al. 2003). The IC values ies that compare IC values it is important to prepare of these metasediments, normalized to the stan- samples uniformly. Samples were initially rinsed dards of Warr and Rice (1994), range from 0.21Њ to and dried and then disaggregated in a mortar and 0.39Њ2v, which corresponds to epizone to anchizone pestle. Approximately 100 g of each crushed sample conditions. Below we present the first Paleozoic were stirred into deionized water in a 1-L beaker illite-muscovite measurements from the Basin and and left to settle overnight. The supernatant was Range and include a comparison with the limited subsequently poured into a large glass dish and Triassic data. dried under a heat lamp. Once dry, material was removed from the dish and divided into four 50- mL plastic centrifuge tubes and again mixed with Sampling Transects and Methodology deionized water. The tubes were centrifuged at Transects. We collected samples along four tran- 1000 rpm for 165 s in a Centra CL2 centrifuge to sects. The first is a “horizontal” cross-margin tran- settle material 12 mm in equivalent spherical di- sect of the oldest pelitic interval that is preserved ameter. The supernatant (containing material !2 in both cratonic and miogeoclinal locations: illite- mm in diameter) was poured into a smaller glass muscovite-rich Middle Cambrian pelites referred to dish. Centrifugation was performed three times for as the Bright Angel Shale in eastern locations such each sample, resulting in 600 mL of !2-mm mate- as the Grand Canyon and as the Carrara Formation rial. This was again dried under a heat lamp. Ap- in locations to the west such as Death Valley (fig. proximately 40 mg of the !2-mm-material solution 1). The horizontal transect stretches from the was mixed with1gofwater and pipetted onto 27 Grand Canyon to Death Valley and crosses the cra- # 46-mm glass slides to produce oriented mounts. ton-miogeocline hingeline. Given that detrital Once dry, the mounts were scanned from 2Њ to 50Њ2v muscovite content is roughly constant within this with a CuKa Scintag X-1 x-ray powder diffracto- stratigraphic interval, we ascribe variations in ill- meter at the University of Michigan, using a ite-muscovite data along the transect to metamor- step size of 0.05Њ2v and an accelerating voltage of phic transitions. In other words, the transect is de- 40 kV. IC values were determined automatically, signed to test the responses of IC and polytype using the MacDiff software program (http:// composition to changes in low-grade metamor- www.ccp14.ac.uk/ccp/web-mirrors/krumm/html/ phism within the Cordilleran orogen. The Bright software/macdiff.html; Petschick et al. 1996). Angel/Carrara interval was chosen because (1) it Some samples were treated with ethylene glycol consists predominantly of illite-rich pelites, (2) out- (EG) to evaluate the presence of expandable clays, crops are widely accessible in both cratonic and but in general, EG treatment produced no measur- miogeoclinal locations, and (3) it is in the lower able effect on XRD results. part of the passive margin stratigraphy and was IC standards SW1–SW6 (Warr and Rice 1994) therefore particularly susceptible to regional meta- were prepared and analyzed, using the methods out- morphic variations arising from stratigraphic and lined above. A calibration curve was established by tectonic burial. fitting a straight line to the average IC values mea- The other three transects are “vertical” profiles sured at the University of Michigan (table A1, avail- of the Cambrian through Permian stratigraphy of able in the online edition or from the Journal of the Colorado Plateau. Variations in IC composi- Geology office) versus those given in Warr and Rice tions attributable to stratigraphic burial are mani- (1994). Normalized IC values of unknowns were fested as downsection increases in metamorphic determined with the equation for the calibration

This content downloaded from 23.235.32.0 on Wed, 21 Oct 2015 21:15:03 PM All use subject to JSTOR Terms and Conditions Journal of Geology V A R I A T I O N S I N I L L I T E – MUSCOVITE TRANSITION 425 curve: normalized value p 1.677 # measured value ϩ 0.011. R2 for this regression was 0.96. Illite Polytypes. Illite polytype compositions were determined for some of the samples. Random- powder mounts were prepared for !2-mm-size fractions that had been reacted with weak acetic acid to remove trace amounts of adhered carbonate. The mounts were scanned from 16Њ to 44Њ2v, and the resulting XRD patterns were compared with “end- member” patterns from two standards, the Cam- brian Silver Hill Formation from Montana, which contains 0% 2M1 and 100% 1Md illite, and Owl

Creek muscovite, which contains 100% 2M1 illite/ muscovite (Haines and van der Pluijm 2008), as well as synthetic intermediate patterns generated by linearly mixing the end-member patterns. Poly- type compositions were estimated by visually choosing the linearly mixed pattern that best matched the pattern of the unknown. Mixing Experiment. To investigate the effect on IC of mixing detrital muscovite with low-grade shales, we separated !2-mm-grain-size fractions from a high-IC Grand Canyon Pennsylvanian shale (sample GC11) and a low-IC, muscovite-rich Pro- terozoic leucogranite (sample HM17), using pro- cedures outlined above. These fractions were combined to create mixtures weighing about 40 mg that contained the following proportions of GC11: 11%, 42%, 79%, 87%, and 100%. Oriented mounts of these mixtures were prepared and analyzed as described above. XRD patterns from the mixing experiment are shown in figure 3. With an increasing proportion of coarse-grained muscovite, the 1-nm peak becomes progressively narrower and more in- tense. IC is 2.4Њ2v for 100% GC11 and 0.1Њ2v for the mixture containing 89% muscovite. Figure 3. X-ray diffraction results from mixing experiment. “Muscovite” refers to the !2-mm-size fraction of Results the muscovite-rich Proterozoic leucogranite of Greggs IC was measured on 31 Cambrian pelite samples Hideout (Howard 2003; Verdel et al. 2011), which was from nine locations (table A1; figs. 1, 2). Oriented mixed with high-illite-crystallinity clay from the lower Supai Group. XRD scans of representative samples are shown in figure A1, available in the online edition or from the Journal of Geology office. The samples come from the Grapevine Mountains is from the Marble from various tectonic levels within the Death Canyon thrust plate above the White Top backfold, Valley thrust belt (fig. 4). The Grand Canyon and while the Bare Mountain sample is from the Marble Frenchman Mountain samples are from the au- Canyon thrust plate below the White Top backfold. tochthon (craton). The Mesquite Pass sample is The Marble Canyon thrust plate is below the re- from the upper plate of the Bird Spring thrust and gionally extensive Last Chance thrust (e.g., Stewart the lower plate of the Keystone thrust. Samples et al. 1966). from Eagle Mountain and the southern Nopah, For four samples (one each from Frenchman Resting Spring, and Panamint Ranges are from the Mountain, Mesquite Pass, Bare Mountain, and the upper plate of the Wheeler Pass thrust and the Grapevine Mountains), reproducibility was lower plate of the Lemoigne thrust. The sample checked by preparing two or three separate oriented

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Figure 4. Locations and structural positions of Cambrian pelites analyzed as part of this study. A, Present-day locations; B, sample locations restored to 36 Ma (McQuarrie and Wernicke 2005); C, simplified cross-section of the Death Valley thrust belt; D, stratigraphic columns at each sample location are showing structural positions. Abbreviations are as in figure 1. Journal of Geology V A R I A T I O N S I N I L L I T E – MUSCOVITE TRANSITION 427 mounts from each sample. For these duplicates, dis- The average IC of the Cambrian pelitic interval crepancies range from 0.05Њ to 0.16Њ2v, from which at Frenchman Mountain is 1.05Њ2v, including one 0.1Њ2v, which sample that produced an anomalously low value ofע we estimate an analytical error of is consistent with previous estimates of IC preci- 0.71Њ2v. Average IC values progressively decrease sion (Robinson et al. 1990). along the transect in the next three locations to the All of the samples have XRD peaks at 1 nm, in- west: 0.69Њ2v (deep diagenetic zone) at Mesquite dicating the presence of illite-muscovite, but none Pass, 0.34Њ2v (low anchizone) in the southern No- of the samples contain coarse-grained muscovite pah Range, and 0.30Њ2v (also low anchizone) in the that is visible with a petrographic microscope. The Resting Spring Range (fig. 5). Average values from minerals most commonly present in the !2-mm-size the four locations to the NW, in the immediate fractions are illite-muscovite, quartz, calcite, feld- vicinity of Death Valley, range from 0.23Њ to spar, chlorite, and kaolinite. Particularly large IC 0.37Њ2v. The lowest IC sample analyzed during this values for some samples as described below may study is from the Carrara Formation at Bare Moun- indicate the presence of interlayered I/S. Peak tain. One individual IC measurement from this widths are not correlated with peak heights. The sample is 0.13Њ2v, which was the smallest mea- average IC of the Bright Angel Shale at the Grand sured in this study and is the same as pure HM17 Canyon is 1.0Њ2v (fig. 5; shallow to deep diagenetic muscovite, suggesting that the IC of the Carrara zone), and in individual samples it is as high as Formation at Bare Mountain is saturated or near 1.3Њ2v. In each of two Grand Canyon sampling tran- saturation. The Bare Mountain sample is from the sects, one sample from the middle part of the Bright footwall of the Bullfrog/Fluorspar Canyon detach- Angel Shale produced anomalously low values of ment but also from the hanging wall of the Gold 0.5Њ–0.6Њ2v, and these were included in the calcu- Ace fault, a major normal fault separating amphib- lation of the average value. olite facies rocks in its footwall from greenschist

Figure 5. Average illite crystallinity values from Cambrian pelites at nine locations along a transect from the craton, ע across the Cordilleran hingeline, and into the miogeocline. Locations are arranged from NW to SE. Error bars are standard deviation of individual measurements from each location. Metapelitic zone boundaries are from Merriman and Peacor (1999). Temperature estimates are from Merriman and Frey (1999).

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Figure 6. Stratigraphic columns and illite crystallinity at Frenchman Mountain and the Grand Canyon. Stratigraphy is from Billingsley (2000) and Castor et al. (2000). to subgreenschist rocks in the hanging wall (Hoisch tonic uplifts and IC of adjacent sedimentary basins. 2000). The total range of average values measured At the Grand Canyon, we analyzed 33 samples of along the transect is 0.23Њ (epizone, Bare Mountain) Paleozoic shales and sandstones from two closely to 1.05Њ2v (shallow diagenetic zone, Frenchman spaced stratigraphic sections along the Bright Angel Mountain). and South Kaibab trails (table A1). Samples were Illite polytype compositions of Cambrian pelites collected from the Cambrian Tapeats Sandstone also vary across the east-west transect. Polytype and the Bright Angel Shale, the Pennsylvanian Su- scans from representative samples are shown in fig- pai Group, and the Permian Hermit Formation and ure A2, available in the online edition or from the the overlying Coconino Sandstone. IC results from Journal of Geology office. The 100% 1Md standard the two transects are similar (fig. 6). The IC values used for these determinations is a sample of the are 0.9Њ–1.5Њ2v in the Tapeats Sandstone and gen- Silver Hill Formation (IC, 1.1Њ2v) from Montana, erally 1.0Њ–1.3Њ2v in the overlying Bright Angel which is itself a Middle Cambrian shale from a Shale. As mentioned above, along both transects, cratonic location (Hower and Mowatt 1966; Haines the middle part of the Bright Angel includes shales and van der Pluijm 2008). Illite polytype compo- with IC values of 0.5Њ–0.6Њ2v. The largest IC value sitions are roughly 90% 1Md at Frenchman Moun- we measured in this study (2.4Њ2v) comes from tain and Mesquite Pass, 65% 1Md at the southern shales at the base of the Supai Group along the

Nopah Range, and 0% 1Md (100% 2M1) at the Rest- Bright Angel trail transect. Above this, IC values ing Spring Range and Bare Mountain (ﬁg. A2). decrease to about 1.5Њ2v in the lower part of the Vertical Stratigraphic Transects through Paleozoic Supai Group and remain between this value and Sections. Grand Canyon. In addition to the 0.85Њ2v through the rest of the Grand Canyon cross-margin transect described above, we also section. measured IC stratigraphic proﬁles through Colo- Frenchman Mountain, Nevada. We collected rado Plateau Paleozoic strata to examine the po- samples of siliciclastic rocks along an additional tential link between detrital input from eroded tec- Paleozoic stratigraphic section at Frenchman

Mountain, a displaced block of Colorado Plateau amint ranges are all within the upper plate of the strata now adjacent to Las Vegas (fig. 1; Wernicke Wheeler Pass thrust and the lower plate of the Le- et al. 1988; Rowland et al. 1990; Fryxell and Due- moigne thrust (fig. 4). The pooled average of these bendorfer 2005; Castor et al. 2000). The IC value samples is 0.32Њ2v, with a standard error of 0.01Њ2v. of the Cambrian Tapeats Sandstone at the base of This is distinct from the Bare Mountain sample the sedimentary section is 0.98Њ2v (fig. 6). The over- (0.23Њ2v), which is in the lower plate of the Last lying Bright Angel Shale equivalent (the locally Chance thrust and below the White Top backfold. named Pioche and Chisholm shales; Castor et al. The Grapevine Mountains sample is above the 2000) has an average IC value of 1.05Њ2v, including White Top backfold (i.e., structurally higher than one sample with an anomalously low IC value of Bare Mountain) and has a slightly larger IC value, 0.71Њ2v. The Dunderburg Shale member of the 0.37Њ2v. It is conceivable, therefore, that with ad- Cambrian Nopah Formation has an IC value of ditional data it may be possible to discern meta- 1.07Њ2v. The next shale interval above this is at the morphic variations between individual thrust base of the Pennsylvanian Callville Limestone, and sheets using IC. it has an IC value of 0.64Њ2v, which is the lowest We find that the western U.S. Cambrian pelites value at Frenchman Mountain. Our stratigraphi- are one of only a few documented occurrences of cally highest sample from the Paleozoic section is the full range of low-grade metamorphism, span- from the Permian Hermit Formation, and it has an ning from the diagenetic zone to the epizone. In IC value of 0.72Њ2v. fact, the craton-miogeocline transect has a larger IC range than a classic Variscan location in south- west England, the region from where the IC stan- Discussion dards were collected (Warr and Rice 1994). We ob- Regional Pattern of Low-Grade Metamorphism. IC tained average IC values ranging from 0.15Њ to values of Cambrian pelites show an overall de- 0.36Њ2v (unstandardized) for the English standards crease from about 1Њ2v on the Colorado Plateau to and 0.13Њ to 0.65Њ2v (unstandardized) for the west- 0.3Њ–0.4Њ2v around Death Valley (figs. 2, 5). Most of ern U.S. Cambrian samples. The large range in sub- this variation occurs between Frenchman Moun- greenschist facies metamorphism suggests that tain (1.05Њ2v) and the southern Nopah and Resting Cambrian sediments in the western United States Spring ranges (0.30Њ–0.34Њ2v), locations that are on may be particularly useful natural laboratories for opposite sides of the Sevier thrust front and the future studies focused on low- and medium-tem- craton-miogeocline hingeline. Additionally, a low- perature metamorphic processes and thermo- 2v “shoulder” to the 8.8Њ2v (p 1 nm) illite peak, chronology. which probably indicates interlayered I/S, de- Source of Variation in Illite Polytypes. As with IC, creases in intensity as the illite peak becomes nar- illite polytype compositions also show systematic rower (fig. A1). We ascribe these differences to changes with location, ranging from 0% 2M1 in cra- metamorphic variations arising from two geologic tonic locations (Haines and van der Pluijm 2008) processes. First, Cambrian strata west of the hinge- to 100% 2M1 in miogeoclinal locations such as line are below at least 6 km of Paleozoic sediments Death Valley (fig. A2). It has previously been sug- and experienced higher temperatures as the result gested that the percentage of the 2M1 illite polytype of stratigraphic burial. Second, tectonic burial be- in pelites is related to and perhaps equal to the neath thrust sheets increased the contrast in meta- amount of detrital illite (e.g., Hower et al. 1963; morphic conditions between the craton and the Se- Grathoff and Moore 1996; Grathoff et al. 2001). vier hinterland (fig. 4). However, our findings suggest that for correlative From the southern Nopah Range and to the NW, Middle Cambrian pelites in the western United average IC values are 0.23Њ–0.37Њ2v (fig. 5), which States, in which the amount of detrital illite was is a range comparable to our estimate of analytical probably relatively uniform, major variations in ill- error. On the basis of this observation, it appears ite polytype composition are caused by differences that IC data from individual locations are insuffi- in metamorphic grade and not differences in the cient for resolving differences in metamorphic proportion of detrital illite. The correlation be- grade between thrust sheets within the Death tween IC and polytype compositions observed in Valley thrust belt. However, when the results from this study and previous studies (Yang and Hesse the same thrust sheet are combined, there is a sug- 1991; Hnat 2009) suggests that the illite to mus- gestion that thrust sheets may have statistically covite transition involves both crystallite thick- distinct IC values. Samples from Eagle Mountain ening and progressive ordering in the stacking sym- and the southern Nopah, Resting Spring, and Pan- metry of illite packets.

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Comparison of CAI and IC Data for Cambrian and measurements, which is greater for cratonic loca- Triassic Rocks of the Basin and Range. Our data also tions than at higher-grade locations in the mio- contribute to the comparison between IC and CAI geocline (fig. 5). This may partially be a result of measurements in the Basin and Range (fig. 2). CAI sampling bias, because we have more samples at values from Triassic carbonates are typically about cratonic sites than at miogeoclinal locations. How- 1–2 in eastern Nevada, and they increase rapidly ever, in cratonic versus miogeoclinal locations to 3–6 in western Nevada at approximately the lon- where a similar number of samples were analyzed gitude of the Luning-Fencemaker and Golconda (e.g., Frenchman Mountain vs. the Panamint thrusts (fig. 2B; Harris et al. 1980). The IC values Range), the cratonic site still displays greater var- of Triassic rocks west of the Luning-Fencemaker iability. Decreasing variability with decreasing IC thrust belt are 0.21Њ–0.39Њ2v (Wyld et al. 2003) and has been observed in other studies (Robinson et al. appear to correspond with a CAI of 4–5.5. For com- 1990; Yang and Hesse 1991; Jaboyedoff and The´lin parison, CAI values from lower Paleozoic rocks in 1996; Ruiz et al. 2008) and is probably related to the Death Valley region are generally 5–7 (Harris natural mixing of illites of different thicknesses. et al. 1980; Grow et al. 1994) and correspond with The IC value reflects the average c-axis perpendic- Cambrian IC values of 0.23Њ–0.37Њ2v. In the case of ular thickness of illite-muscovite crystallites in a both Triassic and Cambrian sediments in the Basin mixture containing a very large number of crystal- and Range, therefore, anchizone to epizone condi- lites that vary in thickness (e.g., Merriman et al. tions as determined from IC correspond with CAI 1990). Low-grade (e.g., diagenetic zone) mixtures values 14. consist of thick detrital muscovite crystals and thin At the lower end of the metamorphic range, com- authigenic illites (Aldega and Eberl 2005). As tem- parisons between IC and CAI are not as straight- perature increases, neoformed illites become forward. First, there are currently no Triassic IC thicker until they are eventually indistinguishable data available for eastern Nevada, precluding a di- in size from detrital muscovite. IC measurements rect comparison for low-grade Triassic sediments. of very low-grade shales are therefore highly sen- Second, CAI values of lower Paleozoic rocks from sitive to detrital muscovite content, leading to the craton are often 15 (Harris et al. 1980), although greater variability in samples of multiple beds from some Paleozoic carbonates near the Grand Canyon a single formation. As metamorphic grains ap- have CAI values !2 (Wardlaw and Harris 1984). In proach and possibly surpass the thickness of detri- other words, the metamorphic transition across the tal grains, variations in detrital muscovite content craton-miogeocline hingeline seems to be more of the protolith have little or no bearing on IC mea- clearly manifested in IC than in CAI. We suggest surements. As described below, the sensitivity of that for old and relatively low-grade rocks (i.e., low-grade shales to detrital muscovite content is those that have experienced a long history of mod- potentially useful in provenance studies. erate burial), CAI may not correspond well with A Pennsylvanian Flux of Detrital Muscovite and Up- metapelitic zones and IC. lift of the Ancestral Rocky Mountains. A complicat- IC values of Triassic rocks west of the Luning- ing factor in quantifying the illite to muscovite Fencemaker thrust belt (0.21Њ–0.39Њ2v; Wyld et al. transition in regional metamorphic studies is var- 2003) overlap remarkably closely with our mea- iation in detrital muscovite content of low-grade surements of Cambrian pelites from Death Valley sediments (e.g., Merriman 2005). For example, the (average values of 0.23Њ–0.37Њ2v). Both areas are erosion of muscovite-rich, basement-cored high- characterized by basinal strata that have undergone lands fluxes detrital muscovite into synorogenic significant stratigraphic and tectonic burial. In the sedimentary basins, an effect that is manifested as case of the Cambrian pelites, low-IC miogeoclinal an anomalously high muscovite content for very- facies are separated from high-IC cratonic facies low-grade sediments. Pennsylvanian strata of the along the Sevier thrust front. Although IC has not Colorado Plateau provide an opportunity to ex- been measured in eastern Nevada, we infer a sim- amine this process because they are coeval with ilar Triassic IC gradient across the Luning-Fence- formation of the Ancestral Rocky Mountains and maker thrust belt. These two examples illustrate may, in fact, have been sourced from them (Blakey large-magnitude illite-muscovite transitions re- et al. 1988; Peterson 1988; Baars 2000). lated to tectonic boundaries. IC values at both the Grand Canyon and French- Detrital Muscovite Influence on IC. In addition to man Mountain reach minimum values of 0.5Њ– the craton-miogeocline transition in average IC, 0.7Њ2v in the middle part of the Bright Angel Shale there is a change in the variability of individual IC (fig. 6). Values increase upsection to 1.1Њ2v in the

Dunderburg Shale Member of the Nopah Forma- southwestern Utah (fig. 7; Mallory 1975; McKee tion at Frenchman Mountain (which is not present 1982), though the source areas for Pennsylvanian in the Grand Canyon) and to 2.4Њ2v at the base of detritus may have been even farther north (Blakey the Supai Group in the Grand Canyon. In siltstones et al. 1988; Peterson 1988). at the base of the Callville Limestone at Frenchman Upsection decreasing IC has been recognized in Mountain, IC is 0.6Њ2v, and at the Grand Canyon, other synorogenic sedimentary sections. In the Al- values diminish upsection within the Supai Group pine Molasse Basin, an exhaustive data set of IC to approximately 1Њ2v. There is thus a Paleozoic measurements shows an upsection decrease (Mon- upsection decrease in IC at both locations, a trend nier 1982). IC data from the Arkoma Basin (Spo¨tl that is opposite to the one expected from burial et al. 1993), a Pennsylvanian foreland basin in the metamorphism. At Frenchman Mountain, the on- south-central United States (Houseknecht 1986), set of this upsection decrease is bracketed only be- are similar in many ways to those from Pennsyl- tween the Upper Cambrian Nopah Formation (IC, vanian strata of the Grand Canyon. In the Arkoma 1.1Њ2v) and the Lower Pennsylvanian Callville Basin, vitrinite reflectance steadily increases with Limestone (IC, 0.6Њ2v). However, the transition to depth in a number of drillholes through Pennsyl- lower IC values appears to be fully captured in silt- vanian marine sediments. In contrast, detailed IC stones at the base of the Pennsylvanian Supai profiles reveal low values (!0.5Њ2v) that show little Group along the Bright Angel trail in the Grand variation with depth and that are not correlated Canyon (fig. 6). with vitrinite reflectance. The discrepancy be- We interpret these IC profiles as reflecting two tween vitrinite reflectance and IC results, param- processes. First, upsection increases in IC, such as eters that are both presumably sensitive to tem- measured in some boreholes (from Teichmu¨lleret perature and variations in low-grade meta- al. 1979 as reproduced in Kisch 1987; Yang and morphism, suggests that illite in the basin is dom- Hesse 1991), result from burial metamorphism. inated by detrital illite eroded from the adjacent This is exemplified along the Bright Angel trail Ouchita Mountains (Spo¨ tl et al. 1993), the forma- transect by the overall increase in IC from the Ta- tion of which seems closely related to uplift of the peats Sandstone and Bright Angel Shale to the base Ancestral Rocky Mountains (Kluth and Coney of the Supai Group over a stratigraphic interval of 1981). roughly 400 m. Very large IC (0.24Њ2v) in the low- Comparison with Detrital Zircon U-Pb Ages. Are- ermost Supai Group, as well as the low-2v shoulder cent detrital zircon U-Pb geochronology study of (fig. A1), suggest that these sediments contain ap- the Paleozoic stratigraphy of the Grand Canyon preciable interlayered I/S, a characteristic of par- (Gehrels et al. 2011) is useful for comparison with ticularly low-grade shales. Second, upsection de- our IC results. Cambrian and Devonian strata of creases in IC, which appear to be inverted the Grand Canyon contain detrital zircon popula- metamorphic gradients, result from additions of de- tions with age peaks at 1.4 and 1.7 Ga. Starting at trital muscovite, as simulated by our mixing ex- the Mississippian Surprise Canyon Formation (rel- periment. This process is exemplified at the Grand atively thin, locally preserved paleochannels cut Canyon in the upper part of the Bright Angel trail into the underlying Mississippian Redwall Lime- transect, where IC rapidly decreases from 2.4Њ to stone), there is a major shift in detrital zircon pop- 1Њ2v at the base of the Supai Group and remains ulation that continues through the remainder of the between 0.85Њ and 1.2Њ2v through the Pennsylva- Paleozoic section (see fig. 5 in Gehrels et al. 2011). nian and Permian stratigraphy. In the Grand The primary difference is the appearance of detrital Canyon, the transition from upsection-increasing zircons with ages in the range of both ∼300–700 Ma IC to upsection-decreasing IC occurs in siltstones and 1–1.2 Ga in Late Mississippian to Permian sed- of Early Pennsylvanian age, coincident with uplift iments, that is, considerably younger ages than in of the basement-cored Ancestral Rocky Mountains. Cambrian and Devonian strata. The change in de- Our interpretation is that the Early Pennsylvanian trital ages is interpreted as a change in provenance, transition to upsection-decreasing IC marks the ar- from the derivation of zircons nearly exclusively rival of muscovite-rich sediment derived from from the crystalline basement of the southwestern these uplifts. Cross-stratification in the Pennsyl- United States during the Cambrian and Devonian, vanian sediments of the Grand Canyon generally to significant derivation of zircons from the Ap- indicates south-directed current directions (McKee palachian orogen, beginning no later than the Late 1982). The nearest Pennsylvanian uplift north of Mississippian. Basement uplifts of the Ancestral the Grand Canyon is the Piute positive element of Rocky Mountains are also a significant source of

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Figure 7. Paleogeography of the Ancestral Rocky Mountains. Arrows show current directions determined from cross- stratification in the Wescogame Formation (upper Supai Group; McKee 1982). Uplifts are from Marshak et al. (2000). zircons in Pennsylvanian and perhaps also Late However, the Ancestral Rocky Mountains seem to Mississippian strata of the Grand Canyon (Gehrels be the more likely detrital muscovite source on the et al. 2011). basis of two observations. First, as described above, We view the IC and zircon U/Pb detrital geo- the upward-decreasing IC pattern of the Pennsyl- chronology data as complementary. IC data can vanian Supai Group is similar to the IC pattern in provide insight into the proportion of detrital, clay- a Pennsylvanian foreland basin of the Ouchita size muscovite versus authigenic illite (as illus- Mountains (Spo¨ tl et al. 1993). Second, it is more trated in fig. 3), but of course they preserve no age probable that clay-size detrital muscovite could information. Detrital zircon U-Pb data preserve age withstand transport from the relatively close An- information and can be utilized to infer prove- cestral Rocky Mountains uplifts than from the nance, but they do not record changes in the total much more distant Appalachian orogen. We ac- flux of detrital minerals. In the case of the Grand knowledge that there is uncertainty in this inter- Canyon stratigraphy, a major change in IC coin- pretation, however. Detrital illite-muscovite 40Ar/ cides with a major change in detrital zircon age 39Ar data from the Grand Canyon section is one population, at least at the level of precision allowed potential avenue to better discriminate detrital by both data sets. As explained above, our inter- muscovite provenance (e.g., Verdel et al. 2011) and pretation is that the rapid decrease in IC within the could be the subject of a future study. lower Supai Group (fig. 6) signals a large flux of clay-size detrital muscovite. Unfortunately, we Conclusions have no data from the Surprise Canyon Formation to compare with detrital zircon data from Gehrels Illite crystallinity and illite polytype compositions et al. (2011). Since IC data have no age component, from Cambrian pelites in the southwestern United they cannot be used to distinguish between differ- States show a clear and systematic variation across ent possible sources (e.g., Appalachian or Ancestral the craton-miogeocline hingeline and correspond- Rocky mountains) on the basis of age differences. ing Sevier fold-and-thrust belt. We attribute the var-

This content downloaded from 23.235.32.0 on Wed, 21 Oct 2015 21:15:03 PM All use subject to JSTOR Terms and Conditions Journal of Geology V A R I A T I O N S I N I L L I T E – MUSCOVITE TRANSITION 433 iation to differences in subgreenschist facies meta- put of detrital muscovite eroded from basement- morphism, as opposed to detrital muscovite cored uplifts of the Ancestral Rocky Mountains. content. Average IC values are approximately 1Њ2v Figure 2, which shows all of the IC data in the in cratonic locations such as the Colorado Plateau, Basin and Range but only a small fraction of the decrease to approximately 0.7Њ2v immediately east CAI data, underscores the scarcity of IC measure- of the Sevier thrust front, and decrease further to ments in the western United States. Our data from 0.2Њ–0.4Њ2v in locations to the west within the Se- Cambrian pelites illustrate the existence of large vier hinterland. These values correspond to a tran- tectonic-related IC gradients, but there is clearly a sition from the shallow diagenetic zone of meta- need for additional spatial and stratigraphic cov- pelitic facies, through the deep diagenetic zone, and erage. As a relatively quick estimate of low-grade ultimately to the high anchizone and epizone. In metamorphic conditions, IC seems particularly one location within the hinterland (Bare Moun- useful in complementing low-temperature ther- tain), IC values are difﬁcult to discriminate from mochronology studies focused on subgreenschist pure muscovite. Illite polytype compositions also terranes. Our study also demonstrates the utility vary systematically, from 0% 2M1 on the craton to of this technique for tracking detrital input to sed-

100% 2M1 in the hinterland, and are also unrelated imentary basins. to detrital illite content. While the transect of Cambrian pelites isolates ACKNOWLEDGMENTS illite-muscovite variations due to metamorphic conditions, stratigraphic profiles through Paleozoic We thank R. Newton of the National Park Service sections of the Colorado Plateau highlight the ef- for scientific permits to the Grand Canyon, W. fect of detrital muscovite input on IC values. These Defliese for collecting some samples, and S. Janwa values reach a maximum at the base of the Penn- for assistance preparing and analyzing samples. We sylvanian Supai Group before decreasing upsection acknowledge constructive reviews from M. Jaboy- through the remainder of the Pennsylvanian and edoff and S. Kemp. S. Wyld provided many helpful Permian stratigraphy. This upsection decrease in comments to a previous version of the manuscript. IC, which is also observed in a Pennsylvanian fore- This research was supported by National Science land basin adjacent to the Ouachita Mountains Foundation grant EAR 0738435 and postdoctoral (Spo¨ tl et al. 1993), most likely results from the in- funds from the University of Michigan.

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