Construction of a Composite Pressure–Temperature Path: Revealing the Synorogenic Burial and Exhumation History of the Sevier Hinterland, USA

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J. metamorphic Geol., 2007 doi:10.1111/j.1525-1314.2007.00733.x

Construction of a composite pressure–temperature path: revealing the synorogenic burial and exhumation history of the Sevier hinterland, USA

C. R. HARRIS, 1 , 2 T. D. HOISCH 1 AND M. L. WELLS 3 1Department of Geology, Northern Arizona University, PO Box 4099, Flagstaff, AZ 86011, USA ([email protected]) 2Isotope Geochemistry and Mineral Resources, ETH Zentrum NW, 8092 Zu¨rich, Switzerland 3Department of Geosciences, University of Nevada, Las Vegas, Las Vegas, NV 89154, USA

ABSTRACT Metamorphic pressure–temperature (P–T) paths derived from 16 growth-zoned garnets, nine from this study and seven from a previous study, have been combined to construct a detailed composite path for an area in the hinterland of the Cretaceous to early Tertiary Sevier orogenic belt in southern Idaho and north-west Utah. Samples are from two Proterozoic units in the footwall of the Basin-Elba thrust: the schist of Mahogany Peaks in the central Albion Mountains, Idaho, and the schist of Stevens Spring in the Basin Creek area of the Grouse Creek Mountains, Utah, 40 km to the south. The simulated portions of the garnets analysed in this study grew from reactions involving the breakdown of chlorite in the upper greenschist to lower amphibolite facies. Multiple garnets were analysed from three samples. Overlapping segments of P–T paths from different garnets in the same sample correlate with respect to slope and garnet Mn concentration. The composite P–T path documents three episodes of sharply increasing pressures separated by two episodes of pressure decrease, all during progressively increasing temperatures. The path is interpreted to represent alternating episodes of synconvergent thrusting and extensional exhumation in the hinterland of the Sevier orogen. Burial was probably caused by the Basin- Elba fault, the only major thrust exposed in the region. Extensional exhumation may have occurred along the Mahogany Peaks or Emigrant Spring faults, or by extensional reactivation of the Basin-Elba fault. This method of correlating partial P–T paths to reveal a more complete composite path provides a powerful tool in unveiling orogenic histories in metamorphic terranes, where evidence of major structures responsible for burial and exhumation is commonly obscured by later events. Key words: garnet; Gibbs method; P–T path; Sevier; Utah.

Jamieson et al., 1998, 2002; Hoisch, 2005). Despite INTRODUCTION their great utility, the reconstruction of complete P–T Metamorphic rocks preserved in ancient mountain paths remains one of the foremost challenges in meta- belts represent one of the primary and potentially morphic petrology (Spear, 1993). enduring records of past orogenesis. The cycle of oro- The construction of complete P–T paths is hindered genic activity experienced by mountain belts commonly by a number of factors including: (i) lack of devel- culminates with profound extension and magmatism opment or preservation of mineral assemblages (Dewey, 1988) which can obscure earlier structures and appropriate for analysing P–T paths during all seg- fabrics formed during shortening. Consequently, the ments of the path, (ii) erasure of earlier histories reconstruction of changes in pressure and temperature through partial or complete diffusive re-equilibration from metamorphic rocks provides a view of past if temperatures exceed thresholds for cation diffusion, tectonism not always apparent from other geological and (iii) difﬁculty in correlating P–T paths from rocks observations (e.g. Selverstone, 1985; Spear et al., 1990). recording different or overlapping segments of paths Tectonic events including subduction, continental col- from the same locality. Although no single method lision, extension, lower lithospheric delamination and exists to accurately determine all parts of a P–T path arc magmatism impart characteristic changes in pres- in a given area, from the earliest burial to the ﬁnal sure and temperature to rocks during metamorphism; exhumation, methods which utilize growth zoning in these changes in pressure and temperature (P–T paths) garnet, such as the Gibbs method based on Duhem’s can be linked to tectonic processes through increasingly theorem (e.g. Spear, 1988), offer the greatest potential sophisticated thermal modelling (Oxburgh & Turcotte, to record the most complete P–T path. Metamorphic 1974; England & Thompson, 1984; Ruppel et al., 1988; garnet will record the portions of the prograde path Grasemann & Mancktelow, 1993; Platt & England, through which they grew, as long as temperatures do 1994; Ruppel & Hodges, 1994; Ketcham, 1996; not exceed thresholds for cation volume diffusion

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(600–650 °C, depending on cooling rate, e.g. Spear, migrating thrust front punctuated by local minor out- 1989). of-sequence deformation internal to the orogenic Orogenic P–T paths have classically been thought wedge (DeCelles, 1994; Lawton et al., 1997; Yonkee to have the form of a simple clockwise loop, et al., 1997; DeCelles, 2004). reflecting a basic concept of orogenesis starting with The kinematic history of the hinterland is less clear protracted shortening, followed by thermal relaxa- and includes a sparse record of Mid-Late Jurassic tion, and terminating with exhumation by combined shortening (Allmendinger et al., 1984; Elison, 1991; erosion and extension (e.g. England & Thompson, Miller & Allmendinger, 1991; Hudec, 1992) as well as 1984). In parallel with the recognition that plate Late Cretaceous shortening and extension (Miller boundary forces, gravitational potential energy, rock et al., 1988; Hodges & Walker, 1992; Camilleri & rheology and erosion dynamically interact to influ- Chamberlain, 1997; Wells, 1997; Wells et al., 1998; ence deformation kinematics (Davis et al., 1983; McGrew et al., 2000). Original stratigraphic juxtapo- Platt, 1986; Molnar & Lyon-Caen, 1988; England & sitions and deformation fabrics recording early short- Houseman, 1989; Royden, 1993; Rey et al., 2001), ening are commonly obscured by reactivation and orogenic histories involving synconvergent extension overprinting by normal faults and extensional flow and alternations in shortening and extension have fabrics (Camilleri & Chamberlain, 1997; Wells, 1997; been documented (Hodges et al., 1992b; Wallis et al., Lewis et al., 1999). The metamorphism provides the 1993; Wells, 1997; Ferranti & Oldow, 1999; Collins, strongest evidence for large magnitude shortening in 2002). Despite the theoretical predictions and field the hinterland of the orogen as: (i) systematic lateral examples of alternating shortening and extension, gradients in metamorphic pressure in stratigraphically P–T paths confirming such alternations are not equivalent and contiguous metamorphic rocks provide widely reported (e.g. Hoisch et al., 2002; Johnson & evidence for lateral gradients in thrust loading (Hoisch Brown, 2004). & Simpson, 1993; Camilleri & Chamberlain, 1997; In this study we present an unusually complete Lewis et al., 1999); (ii) Barrovian metamorphism of composite metamorphic P–T path determined from miogeoclinal metasedimentary rocks to pressures as exhumed mid-crustal pelitic schists from the Sevier high as 8–10 kbar indicates structural burial of two orogenic belt of the western United States. The garnets to three times that of stratigraphic depth (e.g. Hod- analysed in this study nucleated at different times and ges et al., 1992a; McGrew et al., 2000; Hoisch et al., thus captured different segments of the path. The P–T 2002). paths recorded in individual garnets overlap and were correlated to construct a composite P–T path. The composite path is much more complex than the simple GEOLOGY OF THE RAFT RIVER-ALBION-GROUSE P–T loops commonly assumed for tectonic burial and CREEK MOUNTAINS uplift, and is interpreted to represent alternating epi- In the Raft River, Albion and Grouse Creek moun- sodes of synconvergent thrusting and extensional tains of north-west Utah and southern Idaho (Fig. 1), exhumation. a succession of tectonically-thinned greenschist to upper amphibolite facies Mississippian to Proterozoic metasedimentary rocks, termed the Raft River REGIONAL TECTONIC HISTORY Mountains sequence by Miller (1983), unconformably The metamorphic rocks in this study are from the overlies Archean basement (Armstrong, 1968; Comp- hinterland of the Late Mesozoic to Early Cenozoic ton et al., 1977; Wells, 1997). This succession occurs Sevier orogen of the western United States. The Sevier over an area greater than 4000 km2 in all parts of the orogen represents a classic retroarc orogenic wedge, region except in the hangingwall of the Basin-Elba broadly similar in gross architecture to the modern thrust fault in the northern Albion Mountains (Miller, Andean orogen (Allmendinger, 1992; Burchfiel et al., 1980, 1983), which is composed of a different sequence 1992; Camilleri et al., 1997; DeCelles, 2004). The of rocks referred to by Miller (1983) as the Ôquartzite Sevier orogen is subdivided into an eastern foreland assemblageÕ, a >3500 m thick succession of over- fold-thrust belt and a western hinterland region that turned Late Cambrian and Neoproterozoic quartzite includes the isolated exposures of greenschist and and schist (Mt. Harrison sequence) overlain amphibolite facies Barrovian metamorphic rocks of tectonically by a sequence of upright quartzite and the Cordilleran metamorphic core complexes (Crit- schist of inferred Late Cambrian and Neoproterozoic tenden et al., 1980). Initial shortening in the Sevier age (Robinson Creek sequence). The Basin-Elba fault orogen progressed eastward from hinterland to fore- is apparently the only preserved thrust fault in the land from Middle Jurassic to Early Eocene time region as it places older strata over younger (Figs 1 (Armstrong & Oriel, 1965; Wiltschko & Dorr, 1983; & 2). Elison, 1991; Allmendinger, 1992; Taylor et al., 2000; Exhumation of the metamorphic rocks took place DeCelles, 2004). The progressive development of the during episodic Cenozoic extension along both west foreland fold-thrust belt from Early Cretaceous to and east vergent detachment fault systems (Compton Early Eocene is well understood, with an eastward et al., 1977; Malavieille, 1987; Wells et al., 2000b;

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Fig. 1. Generalized geological map of the Raft River-Albion-Grouse Creek Mountains, showing location of the Basin-Elba fault, Basin Creek area, and approximate sample locations. Bottom right inset shows location of metamorphic core complexes (solid areas) in the hinterland of the Sevier thrust belt and in the southern Basin and Range. The thick barbed line represents the leading edge of the thrust belt. RCS, Robinson Creek sequence; MHS, Mount Harrison sequence (Miller, 1983). Modiﬁed from Hoisch et al. (2002).

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Egger et al., 2003); synconvergent Mesozoic extension Albion Mountains (Figs 1 & 2a), and the Ôupper was also important (e.g. Wells et al., 1990, 1998; horizonÕ of the schist of Stevens Spring was sampled Hodges & Walker, 1992). The oldest and most widely from the Basin Creek area of the Grouse Creek distributed extensional fabric (Table 1) is a flat-lying Mountains, 40 km to the south (Figs 1 & 2b). As foliation and generally N-trending stretching lineation these two local ities lie within a contiguous exposure of interpreted to have developed during orogen-parallel the Raft River Mountains sequence and no bedding extensional flow at 105 ± 6 Ma (revised from Wells parallel faults are recognized between them, the two et al., 2000a). This fabric affects Archean to Permian units are considered to have shared a similar P–T rocks and accomplished variable but significant thin- history. ning of rock units. Additionally, low-angle, generally Hoisch et al. (2002) reported P–T paths from the bedding-parallel, top-to-west faults, including the Basin Creek area from two different horizons of the Mahogany Peaks and the Emigrant Spring faults schist of Stevens Spring, a Ôlower horizonÕ and an (Fig. 2c), accomplished significant attenuation of the Ôupper horizonÕ. The upper horizon garnets grew at rock units and are interpreted as extensional faults 475 °C and records one episode of approximately (Table 1) (Wells, 1997; Wells et al., 1998). The isothermal pressure increase, interpreted to represent Mahogany Peaks fault juxtaposes Ordovician marble rapid burial associated with thrusting. Garnet in the over the Proterozoic schist of Mahogany Peaks with an lower horizon grew at higher temperatures (600– estimated excision of 4–5 km, and its age is bracketed 630 °C) from a different reaction, and record a second by 40Ar/39Ar muscovite cooling ages of c. 90 and episode of burial following an episode of major 60 Ma from the hangingwall and footwall respectively decompression (Hoisch et al., 2002). In this study, five (Wells et al., 1998). The Emigrant Spring fault (Wells, new P–T paths are presented from two localities of the 1997) places Pennsylvanian(?) calcitic marble over schist of Mahogany Peaks, and four new P–T paths Ordovician and Silurian(?) dolomitic marble and re- are presented from the upper horizon of the schist of moves 5 km of stratigraphic section. Motion on the Stevens Spring. These are integrated with the seven Emigrant Spring fault has been previously interpreted paths previously determined from the schist of Stevens as prior to or synchronous with cooling recorded by an Spring (five from the upper horizon and two from the 40Ar/39Ar muscovite age of c. 89–88 Ma from a lower horizon) (Hoisch et al., 2002) (Fig. 2). greenschist facies calcitic marble mylonite (Wells et al., 1990; Wells, 1997); however, the cooling age may pre- date deformation as the temperatures of deformation ANALYTICAL METHODS may have been less than argon closure in muscovite. Mineral compositions were determined using the Kilometre-scale recumbent folds deform the Emigrant Cameca MBX electron microprobe at Northern Spring and Mahogany Peaks faults, leading to an Arizona University. A spot size of 1 lm and a beam interpretation of renewed contraction following current of 25 nA was used for garnet traverses. A 5 lm extension (Wells, 1997; D3 in Table 1). These recum- spot size and a beam current of 10 nA was used to bent folds are overprinted by an Eocene–Oligocene acquire matrix mineral data. The accelerating voltage extensional shear zone and detachment fault (D4 in in both cases was 15 kV. Line traverses across garnet Table 1). were run with a point spacing of 10–20 lm (Fig. 3). The P–T paths generated in this study are from two The garnet traverse data were processed to exclude any garnet-bearing units of Proterozoic schist in the foot- analysis that failed to produce a good stoichiometry, wall of the Basin-Elba fault. The schist of Mahogany within certain tolerances. Generally, excluded analyses Peaks was sampled close to the Basin-Elba fault in the represented inclusions or points that overlapped

Table 1. Sequence of Mesozoic to early Cenozoic deformation proposed in this and previous studies (see text) for the footwall to the Basin-Elba thrust fault, western Raft River, northern Grouse Creek and Albion Mountains.

Tectonic event Structure and kinematics Interpretation Timing Metamorphism

M1 First motion on Basin-Elba fault Contraction Pre-105 Ma, Metamorphism of Raft River assemblage; Jurassic or early isothermal P increasea,b Cretaceous D1 Top-to-north shearingd Orogen-parallel extensionc 105 ± 12 Mac D2 Emigrant Spring and Mahogany Peaks Fault Extensione,f c. 90–60 Maf Inferred decompression P–T pathb D3 Recumbent folding and second motion along Basin-Elba fault Contractione c. 60–45 Mag P increaseb D4A Top-to-WNW shearing along amphibolite facies MMSZ Extension Late Eoceneh aThis study. bHoisch et al. (2002). cWells et al. (in press). dMalavieille (1987). eWells (1997). fWells et al. (1998). gRevised from Hoisch & Wells (2004). hWells et al. (2000b).

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(a) (c)

42°17'30"

Mount Harrison Middle Detachment Fault sequence

Mcd Chainman Shale - Diamond Peak Formations

Emigrant Spring Fault

42°17'30"

42°15'

113°45' 113°42'30" 113°40' 113°37'30"

(b)

41°55'

Fig. 2. Geological maps of the sampled areas (see Fig. 1) and regional stratigraphy. (a) Map of the northern Albion Mountains (based on Miller, 1983 and R.L. Armstrong, unpublished map- ping) and locations for sample sites THAL4 and 41° THAL6. (b) Map of the Basin Creek area of the 42'30" 113°45' northern Grouse Creek Mountains (modiﬁed UH from Hoisch et al., 2002) and locations for sample sites UH and LH. All map units are keyed to the tectonostratigraphic column (c), with the 113°47'30" exception of Oligocene granite (black ﬁll).

Ó 2007 Blackwell Publishing Ltd 6 C . R . H A R R I S E T A L . inclusions. Multiple analyses of each matrix phase been entered. The calculation solves for all remaining were performed in each sample and averaged extensive and intensive parameters, including pressure (Table S1). Garnet element maps for Ca, Mg, Fe and and temperature, from which the P–T paths are Mn were generated by counting with wave dispersive constructed. For all simulations in this study, the spectrometers on a JEOL-733 electron microprobe ideal one-site mixing activity models pre-specified in at Rensselaer Polytechnic Institute and by counting the GIBBSGIBBS program were used for staurolite (Fe, Mg, with an energy dispersive system on a JEOL JSM- Mn), garnet (Fe, Mg, Ca, Mn), chlorite (Fe, Mg, 6480LV scanning electron microscope at Northern Mn), paragonite (Ca-K-Na) and plagioclase (Ca-Na), Arizona University (Fig. 4). Images were processed and pure end-members with activities of 1.0 were for with NIH IMAGE or IMAGE J to produce maximum assumed for muscovite and quartz. The thermody- contrast. namic data set used is contained in the Thermo.Dat file included with the GIBBGIBBSS program, which is described in Spear & Cheney (1989, their table 2) for NUMERICAL SIMULATION OF GARNET GROWTH the minerals of interest. USING THE GIBBS METHOD WITH DUHEM’S THEOREM Pressure–temperature paths were determined from Initial compositions and modes garnet growth zoning by numerically simulating garnet Of all the initial conditions required to be specified, growth based on the Gibbs method with Duhem’s only the initial composition of the garnet, corre- theorem (Spear, 1988, 1993). Garnet in this study dis- sponding to the core, is directly measurable. In most play bell-shaped Mn profiles, consistent with Rayleigh simulations, the actual (measured) compositions of fractionation during equilibrium growth (e.g. Hollister, the other minerals were used as initial values 1966). Zoning is generally concentric except for post- (Table S2) because the simulations predicted only growth modifications (Figs 3 & 4). The zoning profiles slight changes in composition and mode during gar- range from highly symmetric to slightly asymmetric. net growth. In all cases in this study, garnet was Garnet growth is accompanied by changes in inten- determined to have grown through chlorite-break- sive variables (composition of solid solution phases, down reactions (discussed later on for each case), and pressure and temperature) and extensive variables in all cases, chlorite was eliminated from the actual (amounts of phases). According to Duhem’s theorem, rock following garnet growth. Consequently, both the chemical systems possess two degrees of freedom. By initial mode and initial composition of chlorite had to specifying the changes in any two variables (the Ômon- be estimated. In general, the initial mode of chlorite itors parametersÕ), the changes in all the others can be in the simulation was assumed to be 1.5 times that solved. The program GIBBS (Spear et al., 1991), version of the actual garnet mode. The chlorite initial Fe/Mg Feb. 12, 1999, was used to perform the calculations. ratio was estimated by back calculating using Items required for setting up the calculation include the garnet-chlorite thermometer of Grambling specifying the nucleation density (number of garnet (1990), the assumed initial temperature for garnet nuclei per 100 cm3 of rock), the phases in equilibrium growth, and the measured garnet core composition with garnet during its growth, the initial compositions (Table S1). A chlorite stoichiometry typical of pelitic of the solid solution phases including garnet, the modal schist (R2+, Al, Si in the proportions 4.5:3:2.5) was abundances of the phases at the time garnet begins assumed, which is a pre-specified option in the GIBBGIBBSS growing, the temperature and pressure corresponding program. to initiation of garnet growth, the choice of monitors and the values of the monitors. Results obtained by this method are statistically robust (Kohn, 1993). Initial temperature and pressure For all garnet growth simulations in this study, a The initial temperature and pressure assumed in the model system appropriate to pelitic schist was models were determined through a variety of methods, assumed: K2O-Na2O-CaO-Al2O3-SiO2-MgO-FeO- as described later on for each sample. Mineral assem- MnO-H2O. The monitors were selected to be the blages in relation to known stability constraints and grossular content of garnet (Xgr) and the amount thermobarometry were useful in estimating initial 3 (mole) of garnet (MGar) in 100 cm of rock. The conditions, but in all cases in this study, the determi- grossular content of garnet was selected because it has nations carry large uncertainties. been shown to be the least susceptible of the eight- fold cations to diffusion (e.g. Spear, 1993). The Nucleation density changes in grossular content (DXgr) during garnet growth may be calculated from data contained in the The values assigned to nucleation density act upon the zoning profile, and the changes in the amount of calculation to divide the garnet grown into a specified garnet is determined through trial and error, that is, number of nuclei. Increasing the nucleation density when the simulation has produced garnet of the results in smaller garnet, all other things being equal. correct radius, then the correct value of DMGar has This gives the user a second way to adjust the garnet

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Fig. 3. Microprobe garnet traverses (shaded lines) and superimposed models (black lines). Segment boundaries utilized in the simulations are marked with vertical black lines. For the UH samples, calculated garnet-biotite thermometry temperatures (Holdaway, 2000) are plotted assuming 20% of total Fe in biotite is Fe3+ (Hoisch et al., 2002). Prograde re-equilibration features, interpreted from the thermometry, are labelled (c, crack; i, inclusion margin; r, rim). radius in the simulations, the other being altering the to be different than the mineral assemblage of the value of DMGar. However, the two methods are not actual rock. Thus, it was necessary to determine the equivalent, as altering the value of DMGar will modify full reaction history of each sample so that the correct the extent of reaction, whereas changing only the minerals could be associated with garnet growth in nucleation density will not. the simulations. Reaction histories are discussed for each sample in the subsequent sections. All garnets simulated in this study are interpreted to have Mineral assemblages grown from dehydration reactions, so it was neces- In all cases in this study, the mineral assemblage sary to include H2O ﬂuid in the list of phases in all present at the time of garnet growth was determined simulations.

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Fig. 4. Ca, Fe, Mg and Mn element maps for garnets. Lighter shades correspond to higher element concentrations. Lines correspond to the locations of microprobe line traverses. Ó ÔLÕ is the left end of the traverse and ÔRÕ is the right end, corresponding to Fig. 3. 2007 Blackwell Publishing Ltd P – T P A T H F R O M T H E S E V I E R H I N T E R L A N D 9

Determination of segment boundaries Thal4e To simulate garnet zoning profiles, each garnet was divided up into segments. Segment boundaries were Petrography and reactions located at changes in slope along the Ca profile. Points This sample is schist of Mahogany Peaks from the that display prograde reequilibration, as judged from Albion Range and contains garnet, staurolite, musco- garnet-biotite thermometry calculations (Fig. 3), were vite, paragonite, quartz and kyanite. Small grains avoided. Each segment is simulated in sequence from (5–10 lm) of graphite occur in the matrix, as sparse the core outward. To perform the calculations, values inclusions in garnet, and as abundant inclusions in of the monitor parameters DXgr and DMGar were staurolite. Ilmenite occurs in the matrix and as inclu- determined for each interval of growth (Table S3). sions in garnet and staurolite. Accessory rutile and tourmaline occur sparsely throughout. Chlorite occurs only along the rims of garnet as a secondary alteration Obtaining a good fit product. The garnet is porphyroblastic, up to 8 mm in The fit of the simulated profile to the analysed profile diameter and corroded. Embayments are commonly was judged by visual comparison. Only two parame- filled by staurolite (Fig. 5), suggesting a reaction in ters, Xgr and the garnet radius (controlled by DMGar), which staurolite grew at the expense of garnet. The are guaranteed to fit. In some cases, good fits to Xalm, staurolite is porphyroblastic (2–7 mm) and corroded. Xpy and Xsp were obtained through the trial and error Small (20–50 lm) kyanite grains are localized along adjustment of three parameters: the nucleation density, the margins of corroded staurolite, suggesting a reac- the initial value of XMn-Chl and DMGar (adjusted for tion that consumed staurolite and produced kyanite. each segment). The initial value of XMn-Chl makes little Muscovite, quartz and paragonite make up the fine- difference to the calculated P–T path, but is important grained (<10 lm) matrix. Minor deformation because it controls the size of the Mn reservoir, and related to flattening and/or shear strains is apparent therefore the rate at which Rayleigh fractionation (Fig. 5). causes the garnet Mn concentration to drop towards The sample underwent a complex reaction history the rim. In the simulations, only slight changes in the that was determined through the interpretation of compositions accompanied garnet growth for most textural evidence and AFM plots (Fig. 7). The bulk minerals (Tables S2 & S4). Adjustments were made composition of this sample (Table S1) plots above the to the initial compositions to account for changes in garnet-chlorite join on an AFM plot, and is thus a those cases where the changes appeared significant high-alumina variety of pelitic schist (Fig. 7). The bulk (Table S2). Adjustments to modes were also made in composition plots approximately in the same location some cases (Table S5). as chloritoid would plot, although no chloritoid was An important complication to consider when mak- found in this sample. In the pure KFMASH system, ing adjustments to initial modes and initial composi- chloritoid would be expected to stabilize at some point; tions is that all garnets in this study are partially however, minor amounts of Ca and Mn in garnet can resorbed because of prograde reactions (and in the displace chloritoid stability to conditions outside the THAL samples, also because of minor chlorite alter- realm of ordinary crustal metamorphism (Spear & ation). The resorption was accompanied by changes to Cheney, 1989; Wang & Spear, 1991; Mahar et al., the modes and compositions of the matrix phases, and 1998). Apart from the effect of Ca and Mn compo- in some cases, changes to the mineral assemblage. nents on chloritoid stability, the KFMASH system is Because only the early portion of the history corre- considered to be a good approximation for the pur- sponding to the growth of the garnet, from the core to poses of interpreting petrogenesis. Interpreting from the resorbed rim, could be simulated, it is not valid the work of Spear & Cheney (1989) in the KFMASH to judge the quality of fit by detailed comparisons system using AFM plots and the estimated bulk between the final simulated mineral compositions and composition (Table S1), garnet grew inside the modes to the actual values from the rock. Our expe- assemblage chlorite + staurolite ( + paragonite + rience is that simulations are very sensitive to changes quartz + muscovite) mainly from the breakdown in the monitor parameters, but not very sensitive to of chlorite as the three-phase triangle chlorite + small changes in the values input for initial matrix staurolite + garnet swept to the left (Fig. 7). With compositions or modes, so it is only necessary to know further progradation, the reaction garnet + chlo- those values approximately. rite staurolite + biotite was crossed, partially con- Simulations using measured values as initial values sumin¼g garnet. As a result of this reaction, many sometimes failed because of a phase or phase compo- embayments along garnet rims became filled with nent being completely consumed prior to the comple- staurolite (Fig. 5). The reaction ceased when all chlo- tion of garnet growth. This was the result of an rite was consumed. With further progradation, the insufficient reservoir of a component needed for garnet biotite + staurolite + garnet triangle migrated to the growth. When this occurred and how it was dealt with right to encompass the bulk composition, ultimately is discussed below for the individual cases. eliminating biotite from the assemblage while growing

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Ky regression of measured mineral compositions in the rock (Table S1, using the rim composition for G3):

0:23pg 0:53st 1:61qtz 0:64grt 0:08ms St þ þ ¼ þ St 4:26ky 0:07ilm H2O: St St þ þ þ Chl The presence of kyanite grains localized along the margins of corroded staurolite grains is consistent with Ky this interpretation. Grt The above discussion predicts that garnet growth St was followed by partial consumption, then by two Ky further episodes of growth. A hiatus in the garnet zoning profiles is plainly seen in all three of the garnets St analysed from this sample. Petrographically, the hiatus THAL4E is represented by concentrations of graphite inclusions that align with discontinuities in the compositional profiles (Fig. 6). A portion of the rim of one garnet was mapped for Y, and displays an annulus consisting of a spike in the Y content just beyond the discontinuity. Yttrium-rich annuli in garnet have been interpreted to result from consumption followed by regrowth when consumption reaction involves the breakdown of minerals that host significant reservoirs of Y (Pyle & Spear, 1999). In this case, the garnet-consuming Grt reaction also consumed muscovite, which is a major host for Y in pelitic schist (e.g. Grauch, 1989). When muscovite was consumed, Y was released into the area along the consuming garnet rim, and then incorpo- Bt rated into the garnet when the garnet regrew. The garnet-consuming reaction releases H2O, and it may be speculated that the fluid dissolved small amounts of UH-3 graphite from within the matrix and precipitated it into the area along the garnet rim. When the garnet regrew, the graphite that had been deposited along Bt the corroded rim was occluded, forming the graphite Chl concentrations that mark the discontinuity in the profile. Grt

Thermobarometry Well-calibrated thermometers such as garnet-biotite and garnet-chlorite could not be applied to this sample St St due to the lack of appropriate mineral assemblages. For the same reason, well-calibrated barometers, such as GASP or MBPG, could not be applied. The only reliable constraint on pressure and temperature is the THAL6B presence of kyanite and the inferred discontinuous reaction garnet + chlorite staurolite + biotite Fig. 5. Plane light photomicrographs showing textural charac- (Spear & Cheney, 1989), which¼ was crossed during teristics of the different bulk compositions. Scale bars are 1 mm. Mineral abbreviations deﬁned in Fig. 7. See text for description progradation. The garnet growth segments to be and interpretation of textures. modelled grew at temperatures below the discontinuous reaction. Consistent with these constraints, additional staurolite and garnet (Fig. 7). At this stage, conditions of 500 °C and 5 kbar were assumed for the staurolite should have been idioblastic, as no reaction initiation of garnet growth. to this point has consumed it. With further progradation, the kyanite + garnet + staurolite triangle swept to the right to encompass the bulk composition, Simulation of garnet growth zoning causing kyanite and garnet to grow at the expense of Three garnet from THAL4E were simulated (designated staurolite (Fig. 7). The reaction is conﬁrmed by G1, G2 and G3), all from the same polished section. Of

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UH-5 G1 UH-3 G1 THAL4E G1

R L L

R L

UH-3 G2 THAL6B G1 UH-5 G2 L

R R

R L THAL4E G3 THAL6C G1 THAL4E G3 L

L R L R

Fig. 6. Plane light photomicrographs of garnets analysed in this study. Lines across garnets mark the locations of analytical microprobe traverses; ÔLÕ is the left end of the traverse and ÔRÕ is the right end, corresponding to Fig. 3. Scales bars are 1 mm. the three stages of garnet growth, only the first stage segments, and G3 was well simulated in seven seg- of growth, which took place within the assemblage ments. All three garnet display a high degree of sym- chlorite + staurolite + muscovite + paragonite + metry. G3 provided the most complete path of any quartz, could be simulated numerically to determine single garnet modelled in this study. P–T paths. Although the beginning of the second stage In this rock, the reservoir for Ca needed to make the of growth, following the hiatus, could theoretically be grossular component of garnet is contained entirely in simulated, there is no feature that would allow one to paragonite as the margarite component. When the interpret where the change from the second stage to the model was run using measured values as initial values, third stage occurs. Because the mineral assemblage the simulations ran out of margarite component before changed between the second and third stages of garnet garnet growth was completed. In order to provide growth, and because it is essential to associate the cor- sufficient Ca for garnet growth, the initial values of rect portion of the garnet profile with the correct both the paragonite composition and modal abun- assemblage in the simulations, neither could be simu- dance were adjusted to higher values than determined lated. Thus, garnet from this sample was numerically for the actual rock (Tables S2 & S5). simulated from the cores to the hiatuses. G3 contains a faint irregularly shaped cluster of graphite inclusions near the centre, which corresponds to a distinct area of Thal6b and Thal6c increased Ca concentration (Figs 3 & 4). This feature is interpreted as a resorbed older garnet, or possibly a Petrography and reactions relict of an overgrown phase (e.g. Hirsch et al., 2003), These two samples are schist of Mahogany Peaks from and so was avoided in the modelling. the Albion Range (Fig. 2a). Both contain garnet, The profile for G1 was well simulated in a single staurolite, muscovite, biotite, paragonite, plagioclase segment of growth, G2 was well simulated in two and quartz. Small grains (<2 lm) of graphite occur in

Ó 2007 Blackwell Publishing Ltd muscovite; resulted demonstrate garnet staurolite reaction garnet migration compositions. (g) Fig. 1 F (d) F 2 (a) e e Gr Gr Labelled C 7. . t t occurred rim R AFM . from grt H in St St of consumption, Ky, A + THAL4E bulk slight R three-phase projections using Bt Arrows Bt R chl kyanite; I K due K S y compositions, y E offset ¼ T small to st A show as L prograde + Pg, . from triangles the growth (Thompson, spot b paragonite; t migration Chl Chl (g) bt across assumed size + Mg Mg composition of migration responsible grt during clear F (c) e 1957) +

of Fe-St biotite and Pl, Chlor rims s three-phase t analysis, of F plagioclase; triangle (e) F (b) showing

(d). Fe-Grt + Ky ±Pl ±Pg +H +Ms +Qz e ﬁeld e for itoid ky on Gr Gr compositi 2 Further Gar O t + staurolite, initial t for necessary migrated grt 102-3 reactions St triangles normal St 102-5 Qtz, + garnet Bt ons. progradation 4E Bt Bt s K K 6B t and 6C quartz; y to Assumed due y triangle pelitic St K responsible in growth. y the the direction to right chloritoid St, Bt complete small 6B (e, 6C resulted biotite In staurolite. Chl Chl f). (e, for samples grain of Mg Mg Biotite f). net garnet consumption compositions (grey In in size. dehydration. THAL4E, consumption THAL4E, analyses TIE LINES MINERAL UH-5 UH-3 THAL4E THAL6C COMPOSITIONS THAL4B B shaded growth. U Bt, F L (c) F (f) K e Mg e

biotite; Gr Gr of area) compensate plotted texturally t t Bulk

THAL6B chlorite (a–f) of St Chl, St and biotite compositions on Prograde Bt chlorite; precisely K late resulted K triangles Bt and y Ó for y and 2007 kyanite low THAL6C, the sequence; Grt, Blackwell from plotted in alkali are corrosion and (g) Chl garnet; Chl shown discontinuous are totals Mg Mg re-growth mineral Publishing prograde (a–c) shown Ms, as of

that Grt + Chl stars. Ltd to of St + Bt P – T P A T H F R O M T H E S E V I E R H I N T E R L A N D 1 3 the matrix, as sparse inclusions in garnet and as these samples. The assumed starting conditions for abundant inclusions in staurolite. Ilmenite occurs in garnet growth (475 °C for THAL6B-G1 and 490 °C the matrix and as inclusions in garnet and staurolite. for THAL6B-G2 and 5 kbar for both) are consistent Chlorite occurs only along the rims of garnets as a with those constraints. secondary alteration product. The garnets is porphyroblastic (up to 8 mm in diameter) and embayed. Embayments are commonly filled by staurolite (Figs 5 Simulation of garnet growth zoning & 6), suggesting a reaction in which staurolite grew at Garnet growth was simulated within the assemblage the expense of garnet. The staurolite is porphyroblastic chlorite + staurolite + muscovite + paragonite + and subidioblastic. Fine-grained (<10 lm) muscovite, plagioclase + quartz. For garnet 3, the initial garnet quartz, plagioclase and paragonite make up the matrix. composition was taken to be the average composition at Minor deformation related to flattening and/or shear the margins of the high Ca-feature (shaded area in strains resulted in micro-scale shears of low displace- Fig. 4) and a non-zero starting radius was assumed. ment (Fig. 5). THAL6C-G1 was well simulated with a single step These samples are similar to THAL4E in that they (Fig. 3). For THAL6B-G1, only 80% of the radius of are a high-alumina variety of pelitic schist, but differ in the garnet profile was successfully simulated (Fig. 3). that the bulk composition is significantly more mag- The profile is notably asymmetric, so only the left half nesium-rich (Fig. 7). Using the estimated bulk com- was simulated, although corresponding points along position (Table S1) and interpreting the AFM plots of both halves were averaged to determine compositions at Spear & Cheney (1989), garnet growth is predicted to the segment boundaries. The first 80% of the radius was have occurred via the same reaction as THAL4E, from simulated in two segments of growth, the first yielding a the breakdown of chlorite as the three-phase triangle good fit and the second fitting less well, particularly with chlorite + staurolite + garnet swept to the right respect to Fe. Many attempts to simulate the remainder (Fig. 7). Garnet growth ended when the reaction of the profile failed due to the tendency of Fe to drop garnet + chlorite staurolite + biotite was crossed, towards the rim, unlike the actual profile. Although the partially consumin¼g garnet. As in THAL4E, the reason for the inability to simulate this portion of the resultant embayments along the garnet rims are com- profile could not be uniquely determined, one possible monly filled by staurolite. With further progradation, explanation is open system behaviour with respect to Fe, migration of the biotite + staurolite + garnet trian- leading to a bulk increase in the amount of Fe in the rock gle to the right partially consumed biotite and grew corresponding in time to the growth of the last 20% of additional staurolite and garnet; however, unlike the garnet radius. THAL4E, biotite is preserved. The additional growth In these samples, the reservoir for Ca needed to of staurolite is reflected as clear narrow rims on many make the grossular component of garnet is contained staurolite grains, overgrowing the inclusion-rich core. in both paragonite as the margarite component and in The preservation of biotite in these samples is due to plagioclase. The garnet growth reaction involved pro- the more Mg-rich bulk composition and also to the ducing paragonite at the expense of plagioclase. In discontinuous reaction being displaced to higher tem- order to prevent the simulation from running out of peratures than in THAL4E due to the higher concen- paragonite, it was necessary to adjust the initial pla- trations of Ca plus Mn in the garnet cores (Table S1). gioclase mode to a higher value than estimated for the This interpretation requires that garnet growth began rock (Table S5). later in these samples than in THAL4E, which is useful information for linking up P–T path segments derived from the different samples (discussed later on). UH-3 and UH-5 Moreover, unlike THAL4E, staurolite grains are not corroded, because within this bulk composition the Petrography and reactions prograde staurolite breakdown reaction that affected These two samples are pieces of one 20 cm boulder of THAL4E did not occur (Fig. 5). schist of Stevens Spring from the Basin Creek area in the northern Grouse Creek Mountains (Fig. 2b). They are from the upper horizon of the schist of Stevens Thermobarometry Spring, as described in Hoisch et al. (2002). The Although samples THAL6B and THAL6C contain samples contain the mineral assemblage quartz + both garnet and biotite, the biotite grew during a muscovite + biotite + plagioclase + garnet, with garnet-consuming reaction, and so is not in equilib- accessory ilmenite, allanite (mainly as inclusions in rium with exposed garnet rims. Consequently, garnet- garnet), monazite (only in the matrix), apatite and biotite thermometry calculations would be invalid and zircon. Ilmenite occurs in the matrix and as inclusions so were not made. Because the outcrop for this sample in garnet. There is no chlorite in these samples, neither is nearby the outcrop of THAL4E (Fig. 2a), it is rea- primary nor secondary. Garnet is porphyroblastic and sonable to assume a shared history, and that the con- up to 6 mm in size. Matrix silicate phases are generally straints available to THAL4E would also apply to 0.2–0.8 mm. Garnets commonly possesses spiral

Ó 2007 Blackwell Publishing Ltd 1 4 C . R . H A R R I S E T A L . inclusion trails defined by quartz and ilmenite and are result in temperatures being underestimated by no corroded along rims (Fig. 5). The corroded rims more than 18 °C. However, associating garnet cores truncate the interior zoning patterns, except for fea- with matrix plagioclases in barometry calculations tures related to post-consumption prograde diffusive did not produce good results in this study, so reequilibration, which caused Fe-enrichment along barometry calculations were made using garnet rims rims, cracks and some inclusion margins (Fig. 4). The (Fig. 8). consumed portions of the garnet rims are notably Considering these complicating factors, the results devoid of muscovite, suggesting a muscovite-consum- should be considered to possess large uncertainties, but ing reaction. Unlike the THAL samples, these contain garnet rims yielded generally consistent temperatures of no paragonite and the matrix is dominated by plagio- 450–460 °C and consistent pressures for the intersec- clase. There is also less late shear deformation, even tions of the barometry equilibria of 4.5–5.5 kbar though the rocks occur within a well-developed (Fig. 8). The determined conditions are appropriate for Tertiary shear zone. The resistance to shearing may the growth of garnet via chlorite breakdown in pelitic be attributable to the abundance of plagioclase in rocks (Spear & Cheney, 1989). For the simulations, the the sample, which may have provided greater resis- initial pressure was assumed to be 5 kbar and the initial tance to deformation overprints relative to more mica temperature of garnet growth was taken to be the cal- and quartz-rich adjacent lithologies (e.g. Passchier & culated temperatures using the garnet cores at 5 kbar Trouw, 2005). and average matrix biotite assuming 20% of Fe to be The estimated bulk composition plots below the Fe3+ (Hoisch et al., 2002). garnet-chlorite join on an AFM plot (Table S1), indi- cating a low-alumina (common) variety of pelitic schist. Using the estimated bulk composition and Simulation of garnet growth zoning interpreting from the AFM plots of Spear & Cheney Garnet growth in these samples was simulated within (1989), garnet growth is predicted to have occurred via the assemblage quartz + muscovite + biotite + essentially the same reaction as the THAL samples, chlorite + plagioclase. Two garnets from each of the that is, by the breakdown of chlorite. Garnet corrosion two samples were analysed and simulated. Garnet- is interpreted to have occurred via a fluid-absent biotite thermometry, plotted for each point along reaction resulting from a major drop in pressure during each traverse in Fig. 3, was used as a guide for progradation. evaluating whether analysed points were acceptable options for segment boundaries. Points near rims, along cracks, and along inclusion margins display Thermobarometry prograde reequilibration, and so could not be used as These samples contain mineral assemblages appropri- segment boundaries. Two of the garnets possess ate for the application of well-calibrated thermo- profiles that are slightly asymmetric (UH-3-G1 and barometry methods, specifically, garnet-biotite UH-3-G2), so one half of each garnet profile was thermometry and MBPG barometry. Unlike the chosen for simulation. The profiles of the other two THAL samples, garnet is interpreted to have grown in garnets (UH-5-G1 & UH-5-G2) are highly symmetric, the presence of biotite. For these samples, garnet-bio- and the half with the most complete record was tite thermometry (Hodges & Spear, 1982; Holdaway, chosen for simulation. UH-5-G2 and UH-3-G1 were 2000) and MBPG barometry (Hoisch, 1991) were cal- each simulated in two segments of growth, while culated using garnet compositions as described below UH-3-G2 required three segments and UH-5-G1 and average matrix compositions for biotite and pla- required four segments. gioclase, in an effort to determine the P–T conditions of initiation of garnet growth (Fig. 8). The results of the calculations must be interpreted CREATING THE COMPOSITE P–T PATH bearing in mind a number of complicating factors. In this study, the simulated portions of all garnets grew Garnet consumption causes the Fe-content of matrix from very similar reactions involving the breakdown of biotite to increase, causing the calculated temperature chlorite. However, because of differences in the bulk to artificially increase (Kohn & Spear, 2000). Garnet chemistry of the rocks and possibly other factors, each consumption also releases Ca from the consumed garnet initiated growth at a different point along the garnet rim, which becomes incorporated into P–T path. Because the initial P–T conditions assumed plagioclase, thus increasing the anorthite content for the initiation of the growth of each garnet could beyond what it was at the end of garnet growth. only be approximated, the generated paths are unlikely Because matrix compositions change during garnet to be in their correct relative positions in P–T space. growth, combining matrix compositions with garnet However, the determined P–T paths may be shifted core compositions introduces error in thermobarom- slightly in P–T space without introducing significant etry calculations. For rocks from this outcrop, errors because the path calculations are not sensitive to Hoisch et al. (2002) estimated that changes in biotite small changes in the assumed initial pressure and composition accompanying garnet growth could temperature.

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UH-3 G1 UH-3 G2 10 10

9 9

8 8

7 7

6 6 (kbar) P 5 5

4 4 MBPG geobarometry: involving 3 3 Tschermak components 2 2 not involving 400 500 600 700 400 500 600 700 Tschermak T (°C) T (°C) components UH-5 G1 UH-5 G2 Garnet-biotite 10 10 geothermometry: 9 9 H&S 10% Fe3+ 8 8 20% Fe3+ 7 7

6 6 (kbar) P 5 5

4 4

3 3

2 2 400 500 600 700 400 500 600 700 T (°C) T (°C)

Fig. 8. Geothermobarometry calculated from analysed mineral compositions. MBPG geobarometry (Hoisch, 1991) and garnet-biotite geothermometry were calculated using garnet rims as follows (see Fig. 3): UH-3-G1 and G2 used point 3, UH-5-G1 used point 4, UH-5-G2 used point 2. Geothermometry (Holdaway, 2000) shown for calculations assuming 10% and 20% of total Fe in biotite is Fe3+. Geothermometry labelled H&S (Hodges & Spear, 1982) assumes all Fe in biotite is Fe2+.

Several approaches were used to determine the rel- reservoir during garnets growth, such as might occur if ative positions of the P–T paths in P–T space. Garnets subtle Mn depletion halos developed. Similarly, paths analysed within the same polished section grew in close may be correlated based on Mn content for the two proximity to each other, and therefore drew from the garnets analysed from UH-3 and the two garnets same Mn reservoir during growth. This allows Mn analysed from UH-5. Each pair of garnets substan- concentration along garnets profiles to be used to tially overlap in their Mn contents, and therefore, so temporally correlate P–T path segments from different should their P–T paths. garnets (e.g. Spear & Daniel, 2003). The correlation of A second approach involves correlating path seg- the paths of the three garnets analysed in THAL4E is ments with similar slopes. The repositioning of the shown in Fig. 9. The correlation is consistent with three paths from THAL4E based on Mn content de- garnets growth initiating at different times along the scribed above places into alignment similarly sloping same P–T path, and with growth of the garnets over- segments (Fig. 9). The same is true of the two garnets lapping to varying degrees. The approach appears to from UH-3 and the two garnets from UH-5 (Fig. 10). work with the exception of the first segment of the path In addition, the slope of the first segment of the P–T generated for G2, which is slightly out of place with path from THAL4E-G3 (schist of Mahogany peaks respect to Mn composition compared with the simi- from the Albion range) is in good agreement with larly-sloping final segment of G3. This could be due to similar segments of P–T paths from the upper horizon the development of slight heterogeneities in the Mn of the schist of Stevens Spring from the northern

Ó 2007 Blackwell Publishing Ltd 1 6 C . R . H A R R I S E T A L .

(a) 9 (b) Inferred G3 G1 G2 time step X step X step X Least recent Sp Sp Sp 8 X = Sp 0.008 core 0.0739 T, P 1 0.0719 0.068 2 0.0680 7 0.051 Sp T, P 3 0.0619 X T, P 4 0.0508 core 0.0391 6 core 0.0374 (kbar) 0.012 (G3) P 0.015 (G1) Decreasing T, P 1 0.0210 0.021 (G2) 5 0.0190 5 0.074 1 0.0151 6 0.0123 G1 T, P 2 0.0080 G2 4 G3 Most recent

450 500 550 600 T (°C)

Fig. 9. Correlation of XSp values among garnet from THAL4E. (a) Pressure–temperature plot of P–T paths with Xsp values labelled for selected segment boundaries. (b) Xsp values at segment boundaries v inferred time.

Grouse Creek Mountains (Hoisch et al., 2002; and the Using all these approaches, the individual paths two UH-5 garnets in this study). The paths of the two were combined into a single composite path in Fig. 10 garnets from UH-3 were found to be similar to those by shifting each calculated path as specified in from the same outcrop reported in Hoisch et al. (2002), Table S6. The distance between samples sites in the in that they possess a similar characteristic inflection northern Grouse Creek Mountains (UH samples, where temperatures change from increasing to schist of Stevens Spring) and sample sites in the Albion decreasing. To align them with those in Hoisch et al. Mountains (THAL samples, schist of Mahogany (2002), the paths in the current study were shifted as Peaks) is 40 km (Fig. 1). The distance between these indicated in Table S6. When the starting pressure for localities and the uncertainties in the determined the P–T paths for UH-5-G1 and G2 is shifted down pressures of metamorphism allows for significant dif- from 5 to 4 kbar, the paths align with the slope and ferences of structural level. Although similar P–T path location of the first path segments of the UH-3 paths, segments for the two rock units most probably record and thus the UH-5 paths appear to represent a cooler, the same tectonic events, the absolute pressures may be earlier portion of the P–T history. somewhat offset from the depiction in Fig. 10. How- A third approach involves considerations of mineral ever, major structures that would vertically separate equilibria. Qualitatively, the interpretation of reactions rocks between the two areas have not been identified. based on AFM plots indicates that garnet growth in Synorogenic low-angle normal faults have been iden- THAL6B and THAL6C should have begun after tified in the region (Wells et al., 1998), but the areas of THAL4E (Fig. 7). In addition, the repositioning of the interest lie in the footwalls of these faults and so could four UH paths, described above, is supported by the not be offset by them. calculated rim temperatures for all four garnets, as all the four paths appear to have ended at similar temperatures (Fig. 8). Conceivably, one could use the TECTONIC IMPLICATIONS OF THE COMPOSITE Gibbs method of Spear & Selverstone (1983) to P–T PATH determine the relative displacement of paths from The composite P–T path overlaps and extends the rocks that possess the same mineral sub-assemblage previously published P–T paths from Basin Creek representing a thermodynamic variance of three or reported in Hoisch et al. (2002). Some general state- less, which would allow the calculation to be run on ments regarding tentative correlations between path the basis of the differences in garnet core compositions. segments and geological structures and the tectonic However, this was not case for the samples in this significance of these paths can be made, although study. correlations with mapped geological structures and

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9 from Hoisch et al. (2002) (a) Schist of Stevens Spring, Basin Creek (b) Schist of Mahogany Peaks, Albion Range (c) All UH-3 G1 8 rims exhum UH-3 G2

7 rims ation - UH-5 G1 UH-5 G2 6 D

(kbar) 2

P upper horizon THAL4E G1 M1 5 lower rims THAL4E G2 Ky cores Ky Ky Sil horizon Sil Sil 3 And And And D THAL4E G3 4 cores cores THAL6B G1 3 400 450 500 550 600 450 500 550 600 450 500 550 600 650 THAL6C G1 T (°C) T (°C) T (°C)

Fig. 10. P–T paths generated from garnet growth simulations and shifted as indicated in Table S6. Al2SiO5 stability ﬁelds from Pattison (1992); And, andalusite; Ky, kyanite; Sil, sillimanite). (a) Paths from schist of Stevens Spring samples in Basin Creek. (b) Paths from schist of Mahogany Peaks samples, Albion Mountains. (c) Paths from all samples. Tentative correlations of path segments to tectonic events (Table 1) are shown.

deformation fabrics in the region (e.g. Wells, 1997) greenschist-lower amphibolite facies Ordovician strata. and with dated events in the fold-thrust belt to the east The Ordovician rocks lie in the hangingwall of the remain speculative. Mahogany Peaks fault, a west-directed low-angle Emplacement of the hangingwall rocks of the Basin- normal fault that post-dates initial motion on the Elba fault most probably caused the burial events Basin-Elba fault and juxtaposes a hangingwall flat in recorded in the P–T paths as well as widespread upper Ordovican rocks against a footwall flat in Neoprote- greenschist to amphibolite facies metamorphism of the rozoic rocks. The simplest interpretation of these footwall across the exposure of the Raft River Moun- relationships is that the Ordovician metasedimentary tains sequence (M1 in Table 1). The relationship be- rocks were derived from an up-dip segment of the tween the thrust at the base of the Robinson Creek west-tilted footwall of the Basin-Elba fault and dis- sequence and the Basin-Elba fault may indicate that the placed down-dip, minimally cutting out the Cambrian Basin-Elba fault represents the floor thrust to a duplex section, to rest on the Neoproterozoic schist of fault zone of tectonically thickened Late Cambrian and Mahogany Peaks. The foreland thrusts sole into a Neoproterozoic rocks, or that the Robinson Creek basal decollement in Late Proterozoic siliciclastic rocks sequence represents a faulted upright limb to a fold to the west, and are interpreted to eventually cut down nappe, with the Mount Harrison sequence representing westward into crystalline basement (Allmendinger, the lower overturned limb (Miller, 1980, 1983). The 1992; Yonkee, 1992; Camilleri et al., 1997). Thus, it is Basin-Elba fault accomplished significant crustal unlikely that the basal decollement to the foreland thickening as the hangingwall sheet had an estimated fold-thrust belt buried rocks as young as Ordovician in stratigraphic thickness greater than 12 km. the study area, requiring a western thrust (e.g. the By inference, initial motion on the Basin-Elba fault Basin-Elba fault), now internal to the orogenic wedge, is interpreted to be older than the 105 (±12) Ma to have buried the metamorphic rocks of the Raft orogen-parallel extensional fabric, as this fabric is River sequence. The Basin-Elba fault may have fed slip prograde and the metamorphic conditions require into the Manning Canyon detachment, a hinterland significant tectonic burial (Table 1). The Willard thrust thrust, thought to be Late Jurassic, which forms a sheet, and other Ômegathrust sheetsÕ that carry thick bedding-parallel fault over a broad region of north- Proterozoic–Lower Cambrian clastic successions at western Utah and places Pennsylvanian over Missis- their bases, including the Canyon Range and Sheep sippian strata (Allmendinger & Jordan, 1981). Rock, were initially translated in Early Cretaceous Field geological relationships provide evidence for at time (144–110 Ma) (DeCelles, 2004). As the Basin- least one episode of renewed thrust motion on the Elba fault lies in a more western position within the Basin-Elba fault following an episode of extension. orogenic wedge than the Willard thrust (Camilleri The extensional Mahogany Peaks fault is folded into et al., 1997), and because it is unlikely that the Basin- an east-vergent, tight to isoclinal, overturned syncline Elba fault fed slip into the Willard thrust (see below), in the immediate footwall of the Basin-Elba fault we infer that its age of initial motion is older than (Miller, 1980, 1983), suggesting a kinematic link initial motion on the Willard thrust. between east-vergent folding and motion on the fault It is unlikely that the Willard thrust, and other (D3 in Table 1). We tentatively correlate the youngest thrusts of the foreland fold-thrust belt, received slip segment of the composite P–T path, a pressure from the Basin-Elba fault. The metamorphic rocks in increase, with this renewed thrusting event. The age the footwall of the Basin-Elba fault include upper constraints of renewed thrusting, c. 60–45 Ma, deter-

Ó 2007 Blackwell Publishing Ltd 1 8 C . R . H A R R I S E T A L . mined by Th-Pb dating of monazite inclusions in Armstrong, F. C. & Oriel, S. S., 1965. Tectonic development of garnet (revised from Hoisch and Wells 2004), are Idaho-Wyoming thrust belt. American Association of Petro- leum Geologists Bulletin, 49, 1847–1866. consistent with garnet growth following motion on the Burchfiel, B. C., Cowan, D. S. & Davis, G. A., 1992. Tectonic Mahogany Peaks fault that is bracketed between c. 90 overview of the Cordilleran orogen in the western United and 60 Ma (Table 1). States. In: The Cordilleran Orogen; Conterminous U.S., G-3 The exhumation events recorded in the composite (eds Burchfiel, B.C. Lipman, P.W. & Zoback, M.L.), pp. 407– P–T path may have occurred by a combination of 479. Geological Society of America, Boulder, CO. Camilleri, P. A. & Chamberlain, K. R., 1997. Mesozoic tectonics extensional reactivation of the Basin-Elba fault (Hod- and metamorphism in the Pequop Mountains and Wood Hills ges & Walker, 1992), motion along the extensional region, northeast Nevada: implications for the architecture Mahogany Peaks and Emigrant Spring faults (Wells, and evolution of the Sevier orogen. Geological Society of 1997; Wells et al., 1998), extensional flow associated America Bulletin, 109, 74–94. Camilleri, P., Yonkee, W. A., Coogan, J. C., DeCelles, P. G., with mid-Cretaceous orogen-parallel extension or McGrew, A. & Wells, M., 1997. Hinterland to foreland along structures not yet recognized. We tentatively transect through the Sevier orogen, NE Nevada to SW correlate the two major decompressional segments of Wyoming: structural style, metamorphism, and kinematic the path with motion along the Mahogany Peaks and history of a large contractional orogenic wedge. In: Protero- Emigrant Spring extensional faults, because the Pro- zoic to Recent Stratigraphy, Tectonics, and Volcanology, Utah, Nevada, Southern Idaho and Central Mexico, Vol. 42 (eds terozoic pelitic schists lie in the footwall of both and Link, P.K. & Kowallis, B.J.), pp. 297–309. Brigham Young this is permitted by the available geochronology. The University Geology Studies, Provo, Utah. temperature increase of 50 °C that accompanied the Collins, W. J., 2002. Hot orogens, tectonic switching, and crea- first decompression is attribut able to thermal relaxa- tion of continental crust. Geology, 30, 535–538. Compton, R. R., Todd, V. R., Zartman, R. E. & Naeser, C. W., tion associated with thrusting along the Basin-Elba 1977. Oligocene and Miocene metamorphism, folding, and fault (Hoisch et al., 2002); however, heating following low-angle faulting in northwestern Utah. Geological Society of the second decompression may require an additional America Bulletin, 88, 1237–1250. heat source, such as might result from lower litho- Crittenden, M. J., Coney, P. J., & Davis, G. H. (eds), 1980. spheric delamination. Cordilleran Metamorphic Core Complexes, Geological Society of America Memoir 153. Geological Society of America, Boulder, CO. Davis, D., Suppe, J. & Dahlen, F. A., 1983. Mechanics of fol- ACKNOWLEDGEMENTS d-and-thrust belts and accretionary wedges. Journal of Geo- Reviews by A. Martin, J. Cheney and an anonymous physical Research, 88, 1153–1172. DeCelles, P. G., 1994. Late Cretaceous-Paleocene synorogenic reviewer greatly improved the paper. We thank sedimentation and kinematic history of the Sevier thrust belt, J. Wittke for assistance with microprobe analysis at Northeast Utah and Southwest Wyoming. Geological Society NAU, K. Becker at RPI for generating some of the of America Bulletin, 106, 32–56. garnet element maps and E. Kelly for drafting the DeCelles, P. G., 2004. Late Jurassic to Eocene evolution of the geological map in Fig. 1. This work was supported by Cordilleran thrust belt and foreland basin system, western USA. American Journal of Science, 304, 105–168. NSF grants EAR-9805076 and EAR-0061048 to TDH, Dewey, J. F., 1988. Extensional collapse of orogens. Tectonics, 7, EAR-9805007 and EAR-00610089 to M.L.W., and 1123–1139. grants from the Colorado Scientific Society and Egger, A. E., Dumitru, T. A., Miller, E. L., Savage, C. F. I. & Geological Society of America to C.R.H. NSF grant Wooden, J. L., 2003. Timing and nature of Tertiary plutonism and extension in the Grouse Creek Mountains, Utah. Inter- EAR-0421452 supported the acquisition of the JEOL national Geology Review, 45, 497–532. JSM-6480LV used to produce some of the element Elison, M. W., 1991. Intracontinental contraction in western maps in this study. North America: continuity and episodicity. 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Rela- SUPPLEMENTARY MATERIAL tions between hinterland and foreland shortening: Sevier orogeny, central North American Cordillera. Tectonics, 19, The following supplementary material is available for 1124–1143. this article online http://www.blackwell-synergy.com: Thompson, J. B., 1957. The graphical analysis of mineral assemblages in pelitic schists. American Mineralogist, 42, 842– Table S1. Mineral and calculated bulk composi- 858. tions. Wallis, S. R., Platt, J. P. & Knott, S. D., 1993. Recognition of Table S2. Parameters used in garnet growth simu- syn-convergence extension in accretionary wedges with lations. examples from the Calabrian Arc and the eastern Alps. Table S3. Monitor parameters, X and M , American Journal of Science, 293, 463–495. D gr D gar Wang, P. & Spear, F. S., 1991. A ﬁeld and theoretical analysis of used in garnet growth simulations. garnet + chlorite + chloritoid + biotite assemblages from Table S4. Measured v simulated mineral composi- the tri-state (MA, CT, NY) area, USA. Contributions to tions. Mineralogy and Petrology, 106, 217–235. Table S5. Initial and ﬁnal mineral modes from gar- Wells, M. L., 1997. Alternating contraction and extension in the hinterlands of orogenic belts; an example from the Raft River net growth simulations, compared with visually esti- Mountains, Utah. Geological Society of America Bulletin, 109, mated modes. 107–126. Table S6. P–T path displacements.

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