Abstract

The western Idaho shear zone is a major, lithospheric-scale structure separating accreted terranes of the Blue Mountains from continental North America. We document the occurrence of the western Idaho shear zone in West Mountain, west-central Idaho. Rocks deformed by the western Idaho shear zone at West Mountain are dominantly orthogneisses, although exposures on West Mountain containing screens of metamorphosed sedimentary rocks are also present. Steeply E-dipping, N-NNE–oriented foliations and downdip lineations characterize the fabric in the orthogneisses, consistent with dextral transpressional kinematics. The foliation orientation changes from 005° to 024° from the northern to the southern part of the field area, and this is interpreted to reflect a primary along-strike variation in the orientation of the western Idaho shear zone. The westernmost unit in West Mountain (Four Bit Creek tonalite) has a U-Pb zircon age of 101 ± 3.0 Ma, yet it is only weakly deformed. We interpret this unit to have been emplaced pretectonically, thus constraining the initiation of the western Idaho shear zone. The youngest unit at West Mountain is the undeformed Rat Creek granite (88.2 ± 3.3 Ma). U-Pb analyses of zircons from orthogneisses at West Mountain span ages of 111–91 Ma, indicating both precursory and continuous magmatism coeval with western Idaho shear zone deformation. Two Lu-Hf garnet isochron ages, 97.3 ± 0.7 Ma and 99.5 ± 1.4 Ma, are interpreted to indicate peak metamorphism during western Idaho shear zone deformation. Geochemical analyses suggest that the westernmost exposed orthogneiss units are dominantly derived from continental material in West Mountain, and yet there is also evidence for a component of accreted terrane rocks at depth east of the western Idaho shear zone.

INTRODUCTION

The Idaho-Oregon EarthScope project aimed to investigate the growth and modification of continental margins, as dominated by active plate-boundary processes including subduction, terrane accretion and translation, deformation, and voluminous magmatism. Western Idaho exposes a superb example of the abrupt juxtaposition of accreted oceanic terranes of the Blue Mountains to the Precambrian North American continent, known as the arc-continent boundary (e.g., Hamilton, 1963, 1969; Armstrong et al., 1977; Fleck and Criss, 1985; Manduca et al., 1993; McClelland et al., 2000; Tikoff et al., 2001; Giorgis et al., 2008; Blake et al., 2009; Gray et al., 2012). The Late Cretaceous western Idaho shear zone currently demarcates this boundary, and, consequently, this structure was the focus of our investigation. The Idaho-Oregon EarthScope active seismic line (Davenport et al., 2015) crossed the western Idaho shear zone at West Mountain.

West Mountain contains critical features that illustrate the nature of the overall arc-continent boundary. Here, we show that the western Idaho shear zone at West Mountain is characterized by an ∼20° bend in orientation, interpreted as having existed during shear zone deformation. This change in orientation is consistent with fabrics to the north being oriented ∼000° (Manduca et al., 1993; Giorgis et al., 2008) and fabrics to the south being oriented ∼020° (Benford et al., 2010). We also provide the first quantitative constraints on deformation conditions within the western Idaho shear zone, indicating pressures of ∼4.4 kbar and temperatures of ∼730 °C. The western Idaho shear zone was initiated after ca. 104 Ma, constrained by weakly deformed granites that we interpret as cooling prior to shear zone deformation. Geochronology from the Sage Hen area suggests that deformation was active at ca. 98 Ma and ceased by ca. 88 Ma, consistent with a ca. 90 Ma cessation estimate obtained from further north in the western Idaho shear zone (Giorgis et al., 2008). The geochronology also indicates that magmatism occurred simultaneously with deformation. The peak metamorphism occurred at 98–97 Ma, constrained by Lu-Hf analysis of garnets. A third Lu-Hf date on garnet of 110 ± 1.0 Ma was obtained from a mafic gneiss within the migmatite complex, and it is interpreted to result from an earlier migmatite-forming event (see Montz and Kruckenberg, 2016). The geochemical analyses suggest that a component of the Blue Mountains terrane occurs inboard at depth east of the western Idaho shear zone adjacent to the field area. Together, these data suggest that an earlier terrane accretion episode—the formation of the Salmon River suture zone—was nearly completely overprinted by deformation on the younger western Idaho shear zone. Further, these data constrain tectonic development of western Idaho and help interpret the Idaho-Oregon EarthScope geophysical data.

GEOLOGIC BACKGROUND

The tectonic boundary between accreted terranes and Precambrian North America in the Idaho-Oregon-Washington region is unique in the North American Cordillera (Fig. 1). It is ∼5 km wide at the surface and is marked by an abrupt change in both age and rock composition (e.g., Hamilton, 1963, 1969; Lund and Snee, 1988), as well as sharp Sr, Nd, and O isotopic gradients (e.g., Armstrong et al., 1977; Fleck and Criss, 1985, 2004; Manduca et al., 1993). The current demarcation of the boundary is the western Idaho shear zone, which is spatially coincident with sharp isotopic and compositional gradients (e.g., Tikoff et al., 2001). The western Idaho shear zone modifies the Late Jurassic–Early Cretaceous accretionary boundary known as the Salmon River suture zone (Walker, 1986; Selverstone et al., 1992; Snee et al., 1995; McClelland et al., 2000).

The Salmon River belt, a zone of highly deformed and metamorphosed rocks, outcrops on the western margin of the western Idaho shear zone. Deformation recorded within the Salmon River belt occurred in the Early Cretaceous (144–128 Ma), as constrained by garnet geochronology (Getty et al., 1993; McKay, 2011; Wilford, 2012). How the timing of the accretion of the Blue Mountain province to North America relates to the garnet geochronology is presently unresolved. There are two dominant models: (1) Accretion occurred ca. 144–128 Ma, resulting in tectonites of the Salmon River belt (e.g., Vallier, 1995; Gray and Oldow, 2005; Schwartz et al., 2014); or (2) accretion occurred prior to ca. 160 Ma, with subsequent 144–128 Ma deformation occurring during “normal” subduction (e.g., LaMaskin et al., 2015; LaMaskin and Dorsey, 2016). If the former model is correct (e.g., deformation in the Salmon River belt was associated with Blue Mountain accretion), there is also debate as to whether deformation in the western Idaho shear zone was a separate and overprinting event (e.g., McClelland et al., 2000) or whether it was the youngest part of the progressive deformation associated with accretion (e.g., Gray et al., 2012).

Magmatism in the region of the western Idaho shear zone initiated in the mid-Cretaceous, after collision of the Blue Mountains terranes with North America (Manduca et al., 1993; Giorgis et al., 2008). In the area surrounding McCall, Idaho (Fig. 1), the Hazard Creek and the Little Goose Creek complexes (Manduca et al., 1992; Giorgis et al., 2008) intrude into the Salmon River suture zone and obscure the Early Cretaceous structures. The fabric of the western Idaho shear zone is characterized by a N-S–striking foliation that dips steeply to the east with a downdip lineation. Removal of the tilt caused by Miocene extensional deformation results in a foliation that is subvertical and N-S striking with a subvertical lineation (Tikoff et al., 2001). The majority of the western Idaho shear zone is contained within the Little Goose Creek complex, predominantly a mylonitic potassium feldspar megacrystic orthogneiss (Manduca et al., 1993). The subvertical foliations and lineations observed in the mylonites of the western Idaho shear zone are most consistent with transpressional deformation (e.g., Tikoff and Greene, 1997). Rare shear sense indicators on the lineation normal face, such as winged porphyroclasts and offset mafic layers, indicate a dextral sense of shear (McClelland et al., 2000; Giorgis et al., 2008, 2016). Finite strain analysis suggests that the E-W contractional component of deformation resulted in ∼80–90 km of shortening (Giorgis and Tikoff, 2004; Giorgis et al., 2005). These kinematics and finite strain estimates suggest the western Idaho shear zone records significant shortening across the arc-continent boundary, in addition to a dextral transcurrent component of displacement.

The timing of deformation is constrained by U-Pb zircon dating of igneous units. The Payette River tonalite is interpreted as syntectonic (Manduca et al., 1993; Benford et al., 2010), and it has U-Pb zircon age constraints of ca. 91 Ma in several localities (Giorgis et al., 2008; Unruh et al., 2008; Benford et al., 2010). Giorgis et al. (2008) also reported a crosscutting 90 Ma (U-Pb on zircon) pegmatitic dike in the Little Goose Creek complex, constraining the cessation of western Idaho shear zone deformation to be ca. 90 Ma.

There is a sharp isotopic boundary between the plutons on either side of the western Idaho shear zone. The western plutons, which intruded accreted island-arc crust, have more mantle-like Sr, Nd, and O isotope compositions (87Sr/86Sr ratios <0.7045; εNd >0; whole-rock δ18O values of +7‰–8‰), whereas those to the east, which intruded older North American crust, have more of an evolved crustal signature (87Sr/86Sr ratios of >0.707; εNd <0; whole-rock δ18O values of >+10‰; Fleck and Criss, 1985, 2004; Criss and Fleck, 1987; Manduca et al., 1992; King et al., 2007; Unruh et al., 2008). These geochemical trends follow the western Idaho shear zone and indicate that the change in lithosphere from island arc to continental crust occurs over a short distance (∼10 km), suggesting a subvertical boundary (Manduca et al., 1992). The discontinuity becomes even sharper (<10 km) when the effects of Miocene extension are removed (Tikoff et al., 2001).

STRUCTURAL GEOLOGY

Geological Mapping

West Mountain is a N-trending range located in west-central Idaho, ∼90 km north of Boise (Fig. 1). Previous reconnaissance geological mapping of West Mountain is collated in the 1° × 2° Baker quadrangle of Mitchell and Bennett (1979) and the Payette National Forest map of Lund (2004), and detailed geologic mapping in the northern section was reported by Bonnichsen (1987). Pre-Tertiary basement is predominantly exposed on the crest and eastern flank of the mountain range. On the western flank, Columbia River basalts cover the ridges, while basement exposures can be found in isolated stream cuts. Our geological mapping was conducted between the latitudes of 44°15′N and 44°30′Ν and the longitudes of 116°2′30″W and 116°17′W (Fig. 2). Detailed rock and microstructural descriptions are given in Appendix 1 and in Braudy (2013). All units, except the Payette River Tonalite (e.g., Manduca et al., 1993), are informally named.

The central part of West Mountain contains compositionally distinct orthogneisses based on modal mineralogy (Fig. 2). From W to E, these orthogneiss units are the Mesa Creek, Muir Creek, Rammage Meadow, and Sage Hen orthogneisses (Appendix 1). Individual orthogneisses are elongate N-S and vary in width from 1.0 to 2.5 km. These rocks contain a N- to NNE-striking foliation dipping steeply to the east, containing a downdip lineation. This fabric is a result of western Idaho shear zone deformation, which was traced southward from the McCall area by Bonnichsen (1987) and Giorgis et al. (2008). The gneissic complexes are similar to those described by Manduca (1988; also Manduca et al., 1993), consisting dominantly of orthogneisses, but also locally paragneisses. On West Mountain, there appears to be a larger paragneiss component relative to the orthogneisses observed in the western Idaho shear zone to the north or the south.

To the west of the orthogneiss belt—and mostly covered by basalt flows—is the Four Bit Creek tonalite, which contains a significantly weaker foliation development than the adjacent gneisses. The Payette River Tonalite occurs immediately east of the orthogneiss belt; its western contact exhibits solid-state deformation, while the central and eastern parts show parallel magmatic deformation. Similar fabric development in the Payette River Tonalite was described by Manduca et al. (1993) in the McCall area and Benford et al. (2010) in the Owyhee Mountains. The Rat Creek granite is exposed on the east side of West Mountain, east of the Payette River Tonalite, and it displays only a weak magmatic foliation (Appendix 1).

Wall-Rock Screen

The western Idaho shear zone in the vicinity of West Mountain also includes an elongate (0.5 km × 7 km) body of metamorphosed sedimentary rocks, termed herein the Sage Hen wall-rock screen. The screen is composed of three different supracrustal rock types: quartzite, pelitic schist, and calc-silicate paragneiss (Fig. 3). The quartzite unit comprises the majority of the wall-rock screen and contains quartz + biotite + diopside ± muscovite ± wollastonite. The pelitic schist is found exclusively on the eastern margin of wall-rock screen and contains quartz + biotite + sillimanite ± garnet ± potassium feldspar ± cordierite ± plagioclase ± wollastonite. The calc-silicate paragneiss is found solely on the southern end of the wall-rock screen. It contains quartz, biotite, calcite, diopside, and wollastonite. The presence of this sedimentary enclave is unusual, because the western Idaho shear zone almost exclusively occurs in orthogneiss elsewhere along its N-S extent. Fabrics within the paragneisses are generally parallel to the overall western Idaho shear zone fabrics, although they generally show more variation than the adjacent orthogneisses.

Migmatite Complex

A migmatite complex is found along the eastern edge of the western Idaho shear zone (Fig. 2). This complex was identified by Braudy (2013), and fully documented by Montz and Kruckenberg (2016). The migmatite complex consists of nonplanar zones defined by foliated granites and schists, which correspond to diatexites and metatexites, respectively. In addition, there are small pods of mafic material typically included in the diatexites. The granitic rocks contain quartz, plagioclase, potassium feldspar, and variable amounts of biotite. The schists contain quartz, plagioclase, potassium feldspar, biotite, and locally garnet. They are typically banded, with alternating leucocratic and melonocratic compositional bands defined primarily by biotite content. Foliation is defined by compositional banding along with elongation of quartz, plagioclase, and potassium-feldspar crystals. Folding is occasionally observed in the schists. In general, the fabric in both of these units is parallel to the western Idaho shear zone fabric: N-S–striking, steeply E-dipping foliation and a downdip lineation.

Mafic pods occur throughout these units, typically smaller than 1 m but occurring up to 3 m in diameter. The mineralogy is hornblende, orthopyroxene, biotite, quartz, plagioclase, potassium feldspar, and garnet. Elongated hornblende and orthopyroxene define the foliation and the lineation. The foliation is parallel to the overall western Idaho shear zone foliation, but the lineation is typically shallowly N-pitching (∼20°; Scheuermann, 2011). These mafic pods are interpreted as disaggregated mafic dikes. The mineralogy of this unit in particular indicates high-grade (uppermost amphibolite to granulite facies) conditions.

Structural Analysis

Field Fabrics

The orientation of fabrics is broadly consistent throughout the study area. All units contain a well-developed solid-state foliation, defined primarily by ribboned quartz and biotite stringers. Gneissic banding is locally present as compositional bands and/or grain-size reduction bands. Gneissic layering is best developed in areas with large amounts of wall-rock assimilation, as denoted by an increased mica content and the presence of garnet and sillimanite. Within the foliation plane, a prominent downdip stretching lineation is defined by biotite and aligned hornblende porphyroclasts where present.

Foliations generally strike N in the northern section of the field area and show a systematic change in orientation to a NNE orientation toward the south in the shear zone. West Mountain has been resolved into three sections in order to illustrate this change in orientation (Fig. 4). The orientation of these three sections is as follows: The northern section shows an average orientation of 005/80E, the central section shows an average orientation of 016/76E, and the southern section shows an average orientation of 024/78E.

Primary Bend of the Shear Zone

The orientations of foliations within West Mountain show a clockwise sense of curvature moving N to S. In the northern section of the field area, the foliations strike ∼005°; in the southern section, the foliations strike ∼024°. This change in foliation orientation could be a result of post–western Idaho shear zone deformation along normal faults. Normal faulting—and local vertical axis rotation (Giorgis et al., 2006)—is documented within West Mountain (Bonnichsen, 1987).

We present four lines of evidence for the structural bend being primary, and therefore existing during movement on the western Idaho shear zone. First, folding of foliation shows a systematic change in orientation of the lineation, which produces a linear trend in plots of lineation pitch versus foliation strike (e.g., Duebendorfer, 2003). The data from West Mountain instead show that lineations cluster around the strike azimuths (Fig. 5), indicating a lack of overprinting deformation. Second, we were able to map fault blocks within West Mountain. The foliation appears to bend within individual fault blocks, and thus it is not related to post–western Idaho shear zone normal faulting. Third, the igneous bodies located immediately east of the western Idaho shear zone (e.g., Rat Creek granite) contain only magmatic fabrics. Yet, these fabrics are oriented N-S throughout the field area, regardless of whether they are juxtaposed with N-S– or NNE-oriented western Idaho shear zone foliations. Given that these fabrics must have formed shortly after western Idaho shear zone cessation (see Geochronology section), they require that the bend in the western Idaho shear zone fabrics existed by that point. Finally, the foliation orientations are consistent with regional trends. The foliation in the western Idaho shear zone near McCall, to the north of the field area, strikes N-S. The orientation of foliations in the southern section of West Mountain is parallel (∼024°) to the foliation in the western Idaho shear zone in the Owyhee Mountains (Benford et al., 2010), an area located ∼150 km SSW of the field area. The similarities in foliation orientation and some rock units (e.g., Payette River Tonalite) suggest that the western Idaho shear zone was continuous from the Owyhee Mountains to West Mountain, although it is almost completely buried by younger volcanic and sedimentary rocks.

Kinematics

Western Idaho shear zone fabrics throughout West Mountain are consistent with dextral transpressional kinematics, similar to other studies of the western Idaho shear zone (e.g., McClelland et al., 2000; Giorgis et al., 2008). Poorly developed or domainal shear sense indicators are observed on planes normal to the foliation and parallel to lineation. In contrast, consistent dextral shear sense indicators are found throughout West Mountain, in both outcrops and thin section, on planes normal to both foliation and lineation (Fig. 6). The results are consistent with kinematic modeling of transpression (e.g., Fossen and Tikoff, 1993). Furthermore, chocolate-tablet boudinage of felsic and mafic dikes—indicating two directions of elongation and overall flattening strain—is locally observed. These structures require flattening fabrics, which are also predicted by kinematic modeling of transpression (e.g., Fossen and Tikoff, 1993). In addition, asymmetric boudinage of the dikes—in the plane perpendicular to lineation and foliation—is consistent with dextral kinematics. In contrast, symmetric boudinage of dikes is observed locally in the vertical plane, normal to foliation and parallel to lineation.

More recently, Michels et al. (2015) used a new technique, crystallographic vorticity analysis, to show that the vorticity axis in the quartzite in the Sage Hen metamorphic wall-rock screen is parallel to lineation. This treatment was confirmed and expanded upon by Giorgis et al. (2016). Taken together, the data are consistent with an interpretation of dextral transpressional kinematics characterizing the western Idaho shear zone in this location.

GEOTHERMOBAROMETRY

Analytical Methods

Of the >20 samples collected in the Sage Hen screen, only pelitic schist sample SH13–8 contained the appropriate low-variance mineral assemblage for geothermobarometry: quartz + biotite + sillimanite + garnet + cordierite + plagioclase. Garnet is anhedral and typically poikilitic. Prismatic sillimanite helps to define the foliation but also occurs as fibrolite mats, and cordierite is a matrix phase that appears to be part of the peak metamorphic assemblage (Fig. 7). Representative mineral analyses for garnet, cordierite, biotite, and plagioclase from sample SH13–8 (Table 1) were collected on a JEOL JXA-8900R electron microprobe at the Department of Earth Sciences, University of Minnesota. Operating conditions were 40° takeoff angle, and a beam energy of 15 keV. The beam current was 20 nA, and the beam diameter was 10 µm for biotite and 1 µm for garnet, plagioclase, and cordierite. X-ray maps and line scans of garnet show little to no zoning, with the exception of a slight increase in the Fe/(Fe + Mg) ratio near garnet rims, suggesting partial reequilibration during cooling (Fig. 7). Therefore, the garnet core composition (Table 1) was used in our geothermobarometry calculations. Mineral equilibria were computed using winTWQ version 1.02 (Berman et al., 2007), which uses the Jun92.GSC database with activities models for garnet (Berman, 1990), biotite (McMullin et al., 1991), and plagioclase (Furhman and Lindsley, 1988). This version was chosen because the Jun92.GSC database contains thermodynamic data for anhydrous cordierite, assumes ideal mixing in cordierite, and yields a better-constrained result than the DEC06.DAT database, which uses a hydration model in cordierite (Fig. 8; see Berman et al., 2007). Calculations assumed all iron as FeO.

Results

Figure 8 plots eight equilibria from metapelite sample SH13–08 between garnet, biotite, plagioclase, cordierite, quartz, and sillimanite, where three of the equilibria are independent (see Berman, 1991). All the equilibria intersect in a relatively tight cluster in pressure-temperature (P-T) space, suggesting the minerals in SH13–08 retained equilibrium compositions (Berman, 1991). Two of the independent reactions include the well-calibrated and widely used garnet–Al silicate–plagioclase (GASP) geobarometer (Ghent, 1976) and garnet-biotite exchange thermometer (Ferry and Spear, 1978). The intersection of these reactions is at 4.4 kbar and 731 °C.

GEOCHRONOLOGY

Analytical Methods

We conducted U-Pb zircon and Lu-Hf garnet geochronology on the distinct intrusive suites recognized at West Mountain. Zircons were dated by laser ablation–inductively coupled plasma–mass spectrometer (LA-ICP-MS) U-Pb methods closely following Gaschnig et al. (2010). U-Pb results were plotted and final crystallization ages were calculated using Isoplot (Ludwig, 2003). Crystallization ages for igneous samples were calculated by taking a weighted mean age of the clustered 206Pb/238U ages. For these calculations, only internal measurement errors were used. The calibration error, based on the average reproducibility of the zircon standards during the relevant analytical blocks, was quadratically added to the weighted mean error in order to determine the total uncertainty. In the case of the detrital zircon sample 10NB257, each spot analysis must be considered a stand-alone age, so the calibration error was quadratically added to the internal error of each analysis.

The details of the Lu-Hf garnet dating methodology are given in Zirakparvar et al. (2011) and Wilford (2012); a brief summary is provided in Appendix 2. Individual garnets from each sample were also analyzed for elemental compositions using the JEOL 8500F field emission electron microprobe at Washington State University. High-resolution X-ray mapping was performed on two garnets per sample, with two points per garnet (core and rim) included in each analysis.

U-Pb Zircon Geochronology

Representative cathodoluminescence (CL) photos of zircons are shown in Figure 9, sample locations are given in Figures 2 and 10, and U-Pb age plots are shown in Figures 1113. Outcrop GPS coordinates are given in Table DR11. A summary of age results is given in Table 2, and full data are reported in Table DR2.

Results—Metamorphosed Sedimentary Sample within the Western Idaho Shear Zone

Sample 10NB257 is a quartz-rich paragneiss from within the Muir Creek orthogneiss, and it was collected from near the location of orthogneiss sample 10NB376. The zircons it contains are interpreted to be detrital. Of 75 analyses, only about one quarter yielded ages less than 15% discordant (Fig. 11). Another quarter of the zircon grains were more strongly discordant, and the remaining half showed disrupted Pb/U but constant 207Pb/206Pb ratios during analysis. Although use of the intercept method to correct for time-dependent (down-hole) fractionation renders the 206Pb/238U and 207Pb/235U ages of the latter group meaningless, their 207Pb/206Pb ages may retain geological significance. The overall ages shown by the different categories of analyses are generally similar, with a range of Mesoproterozoic to Paleoproterozoic ages, a peak around 1.85 Ga, and a smaller number of Neoarchean ages. A sole apparently Jurassic grain is also present, but this is one of the highly discordant analyses. Many grains contain narrow rims that appear dark in CL images. We were able to analyze three such rims, and these yielded Early Cretaceous ages, with two overlapping around 114 Ma and one at 126 Ma. These rims have very low Th/U ratios (<0.02), which is sometimes used as an indicator of metamorphic zircon growth (e.g., Hoskin and Black, 2000; Rubatto, 2002). Given the highly deformed nature of the paragneiss, we interpret these ages to represent metamorphic growth.

Results—Metamorphosed Igneous Sample within the Western Idaho Shear Zone

Zircon samples from the originally igneous units within the western Idaho shear zone in the West Mountain area are characterized by complex U-Pb systematics. CL images (Fig. 9) commonly show discrete cores and rims in many grains and a variety of zoning textures amongst different grains from the same sample.

Sample 10RMG010 is a garnet-bearing biotite tonalite from the Mesa Creek orthogneiss. Zircons show complex zoning patterns, with oscillatory-zoned cores and often similarly zoned mantles, surrounded by dark bands and narrow bright rims (Fig. 9A). Two cores yielded Late Triassic ages, and three yielded Mesoproterozoic ages. The remaining analyses range from 97 to 114 Ma (Fig. 12A). No systematic patterns between Th/U and age occur. If the cores and mantles with oscillatory zoning and rims with similar appearances are combined, they yield a weighted mean of 111 ± 4 Ma. We interpret these data to record the age of magmatism, whereas the younger bright rims reflect metamorphic growth associated with the western Idaho shear zone.

Sample 10NB376 is a granodioritic orthogneiss from a complex outcrop of the Muir Creek orthogneiss containing both meta-igneous and sillimanite-bearing metasedimentary lithologies. Zircons commonly have dark rims with convoluted zoning or no zoning at all surrounded by brighter and weakly zoned rims (Fig. 9B). Three Triassic inherited cores were observed, and the other ages of both cores and rims range from 110 to 100 Ma (Fig. 12B). No systematic patterns between Th/U and age occur, and there is no clear older-younger relationship between the cores and rims. If all Cretaceous analyses are pooled, an age of 106 ± 3 Ma is obtained, but this age has a relatively high mean square of weighted deviates (MSWD) of 7.3. Our preferred interpretation of the U-Pb systematics is that the high end of the age range (ca. 110 Ma) reflects an igneous age, which was followed by younger metamorphism, leading to both Pb loss and new zircon growth.

Sample 10RMG009 is a biotite granodiorite from Four Bit Creek tonalite. This is the westernmost of the samples collected from the West Mountain area. The majority of zircons show discrete oscillatory-zoned cores with slightly brighter rims, which generally show less distinct zoning in CL (Fig. 9C). Cores are commonly separated from rims by narrow nonluminescent bands. Two grains contained distinct cores, mantles, and rims, with the cores giving ages of 152 and 180 Ma. The remaining analyses of both cores and rims yield ages between 91 and 108 Ma (Fig. 12C). The cores have ages consistently older than 98 Ma and Th/U ratios greater than 0.1. The rims partially overlap in age and Th/U but extend to younger ages and lower Th/U ratios. The majority of cores yield ages between 99 and 105 Ma and a weighted mean of 101 ± 3 Ma. We interpret this age to reflect the age of magmatism for this unit, whereas two slightly older ages (107 and 108 Ma) reflect minor inheritance, and the younger rims reflect later metamorphic growth related to the western Idaho shear zone.

Sample 10RMG011 is a biotite-hornblende tonalite. In CL, individual zircons show a variety of zoning textures (Fig. 9D). There are examples of cores with oscillatory zoning surrounded by narrow, bright rims with limited zoning, as is the case with 10RMG009 and 10RMG010, but other grains contain bright cores with dark, zoned rims. No pre-Cretaceous xenocrysts were observed. Ages form a continuous range from 102 to 91 Ma (Fig. 12D). The majority of the rims and cores occupy the lower and higher portions of the age range, respectively, and it is possible to calculate a rim weighted mean of 93.0 ± 3.4 Ma, whereas the cores are too scattered to yield a single meaningful weighted mean. It is difficult to ascertain whether this younger age is igneous or metamorphic. Since the cores have more variable ages, we prefer the explanation where the rims provide an igneous age and the cores reflect variable inheritance of older intrusive units.

Sample 10RMG012 is a biotite-garnet tonalitic orthogneiss from the Sage Hen orthogneiss, collected from the north shore of Sage Hen Reservoir, ∼0.8 km east of sample 10RMG011. Sample 10RMG012 yields a continuum of ages from 102 to 91 Ma (Fig. 12E) (along with one Mississippian xenocryst), but there is no correlation between age and core/rim position or Th/U. Thus, it is not possible to calculate a meaningful weighted mean age for this sample. In light of Lu-Hf garnet geochronology results for this sample (reported later herein) indicating an age of 97.1 ± 0.8 Ma, it is likely that the older U-Pb ages (ca. 100 Ma) represent the igneous protolith crystallization age, whereas the smear of younger U-Pb ages reflects a combination of metamorphic growth, recrystallization, and/or Pb loss.

Results—Igneous Rocks East of the Western Idaho Shear Zone

Several samples dated in this study come from the Rat Creek granite and its probable southern correlatives (Figs. 2 and 10). These samples are similar in that their zircons show oscillatory zoning (Fig. 9E) and have simple U-Pb systematics, with a scarcity of inherited cores. We also dated two samples from an adjacent portion of the Atlanta peraluminous suite, which represents the main component of the Atlanta lobe of the Idaho batholith.

Sample 07RMG47 is a sphene-rich hornblende-biotite granodiorite from Banks and is notable for containing large crystals of sphene. A few zircon grains show the effects of common Pb, and another yielded an age slightly younger than the others, but the majority yield a weighted mean of 88.4 ± 2.2 Ma (Fig. 13A).

Sample 10RMG020 is a biotite granodiorite from a few kilometers east of Banks. This sample shows greater age complexity than the other Rat Creek granite samples. Three inherited cores with ages of 1777, 159, and 101 Ma were identified, and the main cluster of rim ages yields a weighted mean age of 88.9 ± 1.3 Ma (Fig. 13B).

Sample 10NB22 is a biotite granodiorite from Tripod Summit in the southeastern part of the West Mountain field area. This sample shows relatively simple U-Pb systematics, with a weighted mean age of 88.2 ± 3.0 Ma (Fig. 13C).

Sample 10RMG022 is a biotite granodiorite collected from near Horseshoe Bend. Although a few analyses show the effects of common Pb, U-Pb systematics of 10RMG022 are otherwise simple, yielding a weighted mean of 84.4 ± 1.7 Ma (Fig. 13D). This sample represents the westernmost exposure of the Atlanta lobe but is still east of the projected trace of the western Idaho shear zone, which is here covered by Miocene and younger volcanic and sedimentary rocks. The sample lacks hornblende, but it is richer in biotite, sphene, and oxides than the main phases of the Atlanta peraluminous suite and may be broadly correlative to the Rat Creek granite.

Sample 10RMG044 is a biotite granodiorite collected from near Skunk Creek Summit, on the east side of Long Valley, and it represents the westernmost exposure of the Atlanta peraluminous suite at this latitude. This sample shows Early Cretaceous inherited components along with the effects of common Pb and probable Pb loss. The main cluster of rim ages yields a weighted mean of 83.0 ± 2.3 Ma (Fig. 13E).

Sample 10RMG042 is a biotite granodiorite from Peace Valley, ∼5 km east of sample 10RMG044. This sample shows Mesozoic inheritance similar to 10RMG044, but it shows greater scatter in rim ages. A weighted mean age calculated from these rim ages has a very high MSWD, but it is likely that the sample crystallized around 80 Ma (Fig. 13F).

Lu-Hf Garnet Geochronology

We used garnet Lu-Hf geochronology to constrain the timing of metamorphism in garnet-bearing lithologies within and associated with the western Idaho shear zone. Three garnet-bearing samples were collected for Lu-Hf analysis from West Mountain: two from the Potter Pond migmatite complex and one from the Sage Hen orthogneiss (near Sage Hen reservoir). The samples from the migmatite complex—both the biotite gneiss (10NB377) and the amphibolite gneiss (10NB379)—plot near the almandine end member, but the sample (10RMG012) from the garnet-bearing tonalite in the Sage Hen unit was not analyzed for garnet chemistry. Lu-Hf isotopic data are given in Table 3. Major-element X-ray maps of all garnets used in the study indicate single-stage growth with no zonation (Fig. 14).

The first sample (10NB377) from the Granite Peak complex comes from a garnet-bearing, biotite quartzofeldspathic gneiss. The individual garnet fractions (five garnet and three whole rocks) yield a weighted average date of 110.0 ± 1.0 Ma with a MSWD 2.0 (Fig. 15A). The initial 176Hf/177Hf value for the isochron (excluding fraction G3) is 0.282840 ± 0.000035, which corresponds to an initial εHf value (calculated at 110 Ma) of about +4. This positive initial εHf value indicates the protolith for this sample was composed of fairly juvenile, recently mantle-derived material. The second sample (10NB379) is from the Granite Peak complex and comes from a boudin of garnet-bearing amphibolite, less than 300 m from the first sample. Four garnet and two whole-rock fractions yield a Lu-Hf date of 99.5 ± 1.4 Ma with a MSWD of 0.2 (Fig. 15B). The initial 176Hf/177Hf value for the isochron is 0.282480 ± 0.000027, which corresponds to an initial εHf value (calculated at 99.5 Ma) of about −9. The significant difference in both the ages and the εHf values between these two samples suggests both different timing and protolith. Montz and Kruckenberg (2016) also observed different phases of migmatite development, with the youngest major peak at ca. 100 Ma. Hence, we infer that the older 110.0 ± 1.0 Ma ages suggest garnet growth during migmatite formation prior to deformation in the western Idaho shear zone.

The last sample (10RMG012), also chosen for U-Pb zircon geochronology, was taken from a garnet-bearing tonalite orthogneiss in the Sage Hen orthogneiss. The individual garnet fractions (three garnet and one whole rock) yield a weighted average date of 97.1 ± 0.8 Ma with a MSWD of 0.1 (Fig. 15C). Moreover, these garnets all have initial εHf values (calculated at 97.3 Ma) of −6 (Table 3). This negative initial εHf value indicates the protolith for this sample was composed of old, continental material.

GEOCHEMISTRY

Analytical Methods

All rock samples from which zircons were dated were also characterized geochemically for: (1) major- and trace-element abundances; (2) Sr, Nd, and Pb isotopes in whole-rock powders; and (3) Hf isotopes in zircon zones adjacent to those analyzed for U-Pb geochronology. We also analyzed previously dated zircons for Hf isotopes (with U-Pb results provided in McClelland and Oldow, 2007; Giorgis et al., 2008; Gray et al., 2012) from exposures of the Hazard Creek, Little Goose Creek, and Payette River Tonalite complexes to the north in order to provide points of comparison for the Hf data from the West Mountain zircons.

Major- and trace-element, and Sr, Nd, and Pb isotope analyses were conducted at Washington State University following the procedures of Gaschnig et al. (2011). Major- and trace-element analyses were conducted by X-ray fluorescence (XRF) and ICP-MS. In situ multicollector (MC) LA-ICP-MS Hf isotopic analysis of zircon was conducted following the methods of Gaschnig et al. (2013) at Washington State University utilizing the same laser system used for U-Pb analysis coupled to a Thermo-Finnigan Neptune MC-ICP-MS. A summary of results is given in Table 4, and full data are reported in Tables DR3 and DR4.

Isobaric interference of 176Yb and 176Lu on 176Hf was corrected for by monitoring interference-free isotopes of Lu and Yb and using the natural isotopic composition of these elements to determine the contribution of 176Yb and 176Lu to the mass 176 signal. The Yb mass bias was determined by comparing the measured 173Yb/171Yb ratio to the natural ratio suggested by Chu et al. (2002) in each measurement.

Analyses of the zircon standards Mud Tank, FC-1, and R-33 were used to correct unknown analyses for external fractionation. Hf isotopic data are presented in epsilon notation, calculated using chondritic uniform reservoir (CHUR) values of 176Hf/177Hf = 0.282785 and 176Lu/177Hf = 0.0336 (Bouvier et al., 2008) and a decay constant of 1.867 × 10−11 yr−1 (Scherer et al., 2001; Söderlund et al., 2004).

Results

Samples of the dated intrusive units tend to occupy a relatively narrow intermediate to felsic compositional range and are mostly peraluminous (i.e., aluminum saturation index >1; Fig. 16A). Comparison to, and correlation with, the established units of Manduca et al. (1992) exposed around McCall is not straightforward because those three complexes show significant internal variability and external compositional overlap, but we find that the proportions of CaO, Na2O, and K2O tend to distinguish the Hazard Creek, Little Goose Creek, and Payette River Tonalite complexes (Figs. 1, 16B, and 16C). The Four Bit Creek tonalite generally matches the composition of the porphyritic facies of the Little Goose Creek complex. In contrast, the Mesa Creek and Muir Creek units partially overlap with the Hazard Creek complex, but the isotope data presented in the following sections, combined with age constraints, argue against this correlation. The Sage Hen units—on which the western Idaho shear zone is centered—also generally matches the composition of the porphyritic facies of the Little Goose Creek complex. The Rat Creek granite and correlative units exposed around Banks and Horseshoe Bend generally match the compositional range of the Payette River Tonalite.

Trace-element characteristics of the major intrusive units show broadly similar characteristics (Fig. DR1), including the depletion of Nb and Ta relative to the large ion lithophile elements (LILEs) and light rare earth elements (LREEs), which is characteristic of arc rocks around the world (e.g., Pearce and Peate, 1995). Certain aspects of the trace-element geochemistry of the Four Bit Creek tonalite and Mesa Creek orthogneiss (both west of the 0.706 Sr line) are distinct from the Sage Hen and Muir Creek units to the east, including relative enrichment of Sr over Nd (considered another indicator of arc magmas) seen in the former units and greater fractionation of the REEs seen in the latter units. The undeformed Rat Creek unit is distinguished from the other older units by the distinctly smaller enrichment of Nb and Ta over the LILEs and LREEs. This lack of strong depletion in Nb and Ta is also a characteristic feature of the Atlanta peraluminous suite (Gaschnig et al., 2011).

Whereas the differences in major-element composition of the different intrusive units do not correlate clearly with geographic and structural position or age, 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, and 207Pb/204Pb isotope ratios tend to increase in the case of Sr and Pb and decrease in the case of Nd going from west to east and older to younger units across West Mountain (Figs. 17 and 18). The Four Bit Creek and Mesa Creek units, which are the westernmost intrusive units exposed in the West Mountain, have significantly less radiogenic Sr and Pb isotope compositions and more radiogenic Nd compositions than all of the other intrusive units. It is important to note, however, that these units do not reach the primitive isotope compositions of the Hazard Creek complex or older Blue Mountains island-arc basement. In fact, these units are more isotopically consistent with the Little Goose Creek complex. The Sage Hen and Muir Creek units, which show the greatest amount of western Idaho shear zone deformation, contrast greatly with the Four Bit Creek and Mesa Creek units by exhibiting comparatively continental isotopic signatures, with partial overlap with both the eastern Little Goose Creek complex and Payette River Tonalite. The Rat Creek samples occupy this same range (Fig. 17).

In addition to measuring Sr, Nd, and Pb isotope compositions on whole-rock powders, we also conducted in situ Hf analyses of the same zircons that were dated (Figs. 18A and 18C). The advantage of the in situ Hf analysis is that isotope geochemistry can be paired directly with age information. The similar behavior of the Sm-Nd and Lu-Hf systems leads to the expectation that Nd and Hf isotopic ratios should closely correlate. The intrusions at West Mountain broadly follow this rule, exhibiting the same decrease in epsilon values with decreasing age and west-to-east geographic position. However, some decoupling between Hf and Nd is apparent in the Four Bit Creek and Mesa Creek units, which show more radiogenic Hf values than might be expected based on their Nd compositions (Fig. 18). Specifically, most analyses of zircons from the Four Bit Creek unit, and all analyses of the Mesa Creek zircons, closely match the Hf isotope composition of the Hazard Creek complex, whereas the whole-rock Nd isotope compositions are considerably less radiogenic and similar to the Little Goose Creek complex. In addition to this decoupling, significant Hf isotope heterogeneity is present in some samples, particularly 10RMG012 of the Sage Hen orthogneiss. This sample shows a bimodal distribution with epsilon Hf values around −2 and −10, with no relationship to age or core-rim position (Fig. 18A). A few of the Early Cretaceous through Triassic inherited zircon cores found in the samples east of the 0.706 Sr line were analyzed for Hf. Of these zircon cores, all but one exhibit radiogenic Hf compositions characteristic of the plutons west of the 0.706 Sr line (Fig. 18C).

DISCUSSION

Magmatism and Zircon Inheritance

Intrusive igneous rocks in the West Mountain segment of the western Idaho shear zone were emplaced into a substrate of metamorphosed sedimentary material with lithologies suggestive of continental derivation. Metamorphosed sedimentary rocks with lithologies suggestive of island-arc and/or deep basinal lithologies, such as the Riggins Group to the north along the western Idaho shear zone, are absent. It is likely that some, if not all, of the screens in the West Mountain segment are Precambrian. Mesoproterozoic Belt Supergroup and Neoproterozoic Windermere Supergroup rocks are common east of the 0.706 Sr line in central and northern Idaho (e.g., Lund, 2004; Lewis et al., 2007). Most of these rocks are dominated by detrital zircon age populations between 1.4 and 1.9 Ga and 2.7 and 2.5 Ga, with small numbers of 1.0–1.3 and ca. 0.7 Ga grains distinguishing the younger Windermere rocks from the older Belt rocks. Detrital zircons from the quartzitic paragneiss at West Mountain (sample 10NB257 from within the Muir Creek orthogneiss) contain Mesoproterozoic, Paleoproterozoic, and Neoarchean age populations (Fig. 11) characteristic of units of both the Belt and Windermere rocks. The presence of a concordant 1056 Ma age, however, excludes correlation with the older Belt Supergroup and suggests correlation with the Neoproterozoic Windermere Group (Lund et al., 2003; Fanning and Link, 2004). Although Precambrian zircons are also common in certain Mesozoic sedimentary rocks of the Blue Mountains Province (LaMaskin et al., 2011), the highly unradiogenic Nd isotope composition of the paragneiss argues against correlation with Blue Mountains rocks.

The Early Cretaceous rim ages found in the paragneiss are interpreted to be metamorphic in origin. The three rim ages from the zircons (114, 114, and 126 Ma) correlate well with metamorphic events that have been identified in other parts of the Salmon River belt to the north (Selverstone et al., 1992; Getty et al., 1993; Snee et al., 1995, 2007; McKay, 2011). These new age dates do not resolve whether metamorphism likely resulted from Blue Mountains accretion (Salmon River suture zone; e.g., Vallier, 1995; Gray et al., 2012; Schwartz et al., 2011a, 2011b) or contractional deformation associated with normal subduction (LaMaskin et al., 2015).

All of the intrusive rocks in West Mountain predating the Payette River Tonalite show complex histories of magmatism and probable metamorphism. Zircons of all these units show either multiple discreet mid-Cretaceous age components or significant scatter, making the assignment of “single” crystallization ages difficult. While zircons from all of these samples show distinct cores and rims in CL, the younger and older age components in a given sample often do not correlate strictly with core and rim position (so that the core of one zircon may have the same age as the rim of another). Similarly complex U-Pb systematics with a similar age range were observed by Giorgis et al. (2008) and Unruh et al. (2008) in western Idaho shear zone–related rocks to the north of the study area. In detail, the complex systematics in individual samples can be explained by three different processes, which may have all been in operation during the ca. 114–92 Ma interval. The first possibility is that the oldest ages in a sample reflect the age of magmatism, and the younger ages represent variable degrees of Pb loss due to the persistence of high-temperature subsolidus conditions through ca. 92 Ma. The second possibility is that the older components in a sample reflect the magmatic age of an earlier intrusive unit that was remelted or assimilated in a new magma, which then crystallized new zircon crystals and new zircon layers around old crystals. The third possibility is that the ages in a single sample are a mixture of magmatic and metamorphic ages. We prefer the latter option for several samples (as described earlier herein), which differs from the first option in that it invokes new zircon growth during subsolidus metamorphic conditions rather than loss of Pb from existing crystals.

The Payette River Tonalite is considered to be the main component of the border zone suite of the Idaho batholith and is followed by the formation of the Atlanta peraluminous suite (Gaschnig et al., 2010). The Atlanta peraluminous suite began at 83 Ma, but primarily intruded between 80 and 70 Ma (Gaschnig et al., 2010, 2011). The new dates presented here for the Rat Creek granite and its probable correlative intrusive units at Banks and Horseshoe Bend, and an 85 Ma date for a lithologically similar sample adjacent to the 0.706 Sr line north of Riggins (Slate Creek; Gaschnig et al., 2010), provide evidence for a previously unrecognized period of transitional magmatism adjacent to the WISZ between 92 and 83 Ma. The presence of a petrographically similar unit on the east side of the Payette River Tonalite in the McCall area (Kuntz, 2007) suggests that, similar to the Payette River Tonalite, the Rat Creek granite or equivalent rocks may parallel the western Idaho shear zone along much of its entire length.

Preservation of the Salmon River Suture Zone?

In addition to providing a complex record of magmatism and metamorphism, zircons from most of the units associated with the western Idaho shear zone, along with one sample of the Rat Creek granite and both from the Atlanta peraluminous suite, contain Jurassic, Triassic, Mississippian, and Proterozoic inherited components. These data and the isotopic tracer data reported here provide information on the preexisting crustal architecture of the West Mountain area. The inherited Mesozoic zircon components match igneous ages found in the Blue Mountains (Schwartz et al., 2011a, 2011b; Kurz et al., 2012, 2016), and the radiogenic Hf isotopic compositions of the majority of the Mesozoic cores support derivation from Blue Mountains island-arc crust. In contrast, Mississippian and Proterozoic ages are not known in the igneous record of the Blue Mountains but are present as detrital zircon ages in Mesozoic sedimentary rocks of the Blue Mountains (LaMaskin et al., 2011). Zircon inheritance therefore suggests incorporation of Blue Mountains igneous and/or sedimentary material in the West Mountain magmas on both sides of the 0.706 Sr line. On the east side of the 0.706 Sr line, this Blue Mountains crustal material probably is present at depth, since wall rocks at the present level of exposure are of continental affinity, and the magmas on this side are mixtures of Blue Mountains and North American crustal material. Although Sr and Nd isotopic data show that the units on the west side of the 0.706 Sr line are not isotopically primitive enough to represent pure melts of Blue Mountains basement (and/or new juvenile mantle melts), the Nd isotopic compositions of these rocks overlap with those of Jurassic and Triassic Blue Mountains mudrocks (LaMaskin et al., 2013). The mismatch between Nd and Hf zircon isotopic systematics seen in these westernmost West Mountain units may be a reflection of the singular dominance of zircon in the Hf budget, whereas Nd is shared amongst a greater number of phases and is more likely to provide an “average” composition during a mixing process.

If this interpretation is correct, then the Salmon River suture zone must have locally extended eastward to the cratonic side of the western Idaho shear zone. This geometry is consistent with other geological structures in Idaho. The Woodrat shear zone in the Syringa embayment (Lewis et al., 2014; Schmidt et al., 2016) is interpreted as a remnant of the Salmon River suture zone that currently exists inboard of the ca. 95 Ma Ahsahka shear zone (a structural continuation of the western Idaho shear zone; Schmidt et al., 2016). Therefore, we interpret that a portion of the Salmon River suture zone once extended east of the 0.706 Sr line in the West Mountain area. This Salmon River suture zone, however, was both subsequently subjected to significant magmatism and offset from its westward continuation by western Idaho shear zone deformation.

Constraints on the Timing of the Western Idaho Shear Zone

Detailed geologic mapping, high-precision geochronology, and geothermobarometry constrain the timing of initiation, peak metamorphism, and the cessation of deformation in the western Idaho shear zone (Fig. 19). Our new geothermobarometry results from the Sage Hill screen of 4.4 kbar and 731 °C are similar to those found regionally, although this is the first direct constraint on conditions during western Idaho shear zone deformation. To the north of the Sage Hen area, Weston (1992) showed that metasedimentary rocks from the Granite Lake and Fisher Creek Saddle pods in a northern section of the Payette River Tonalite experienced minimum conditions of 5.8 kbar and 640 °C. The presence of magmatic epidote in the Hazard Creek complex and Payette River Tonalite suggests the presence of >7 kbar (or >6 kbar for hydrous conditions) rocks on both sides of the western Idaho shear zone (Zen and Hammarstrom, 1984; Manduca et al., 1993). The Potters Pond migmatite complex occurs right at the change in orientation in the western Idaho shear zone. Scheuermann (2011) noted the presence of orthopyroxene in the complex and estimated peak metamorphic conditions of ∼700 °C and ∼6.5 kbar, although these conditions may have existed prior to western Idaho shear zone deformation (Montz and Kruckenburg, 2016). In the South Mountain area of the Owyhee Mountains, Freeman (1982) noted that kyanite (rather than sillimanite) and staurolite are present in a series of pelitic schists exposed along Wallace Creek. From this assemblage, he estimated conditions of ∼550 °C and ∼4.5 kbar. This lower pressure estimate is consistent with our data from the Sage Hen wall-rock screen. Both of the studies with lower pressure estimates occurred south of the bend in the western Idaho shear zone in West Mountain, where the western Idaho shear zone fabric is oriented 024°. The along-strike difference in exhumation—with less exhumation occurring south of the Sage Hen “bend”—is consistent with a structural model of increased exhumation where the western Idaho shear zone is oriented N-S relative to where it is oriented at NNE (Benford et al., 2010).

We can constrain the initiation of the western Idaho shear zone using data from the 101 ± 3 Ma (U-Pb zircon age) Four Bit Creek tonalite. This unit is relatively undeformed compared to other orthogneisses of West Mountain (Appendix 1). If this unit was clearly spatially associated with the shear zone, it could not have been a magma when deformation was occurring. Deformation of a molten rock would impart a well-developed tectonic foliation or a strong magmatic foliation. Therefore, we propose that deformation associated with the western Idaho shear zone must have initiated after emplacement of the Four Bit Creek tonalite. There are orthogneisses within West Mountain that are older than the Four Bit Creek tonalite exhibiting a strong tectonic fabric, such as the Mesa Creek orthogneiss (111 ± 4 Ma) and the Muir Creek orthogneiss (106 ± 3 Ma). The structural position and petrology of the Mesa Creek orthogneiss and the Muir Creek orthogneiss explain the well-developed foliations noted throughout these units, despite their age. These units were emplaced in the center of the western Idaho shear zone, leading to the accommodation of more strain than the Four Bit Creek tonalite. Additionally, the higher biotite content of Muir Creek orthogneisses would cause this unit to be mechanically weaker. A similar relationship between strong solid-state fabrics associated with deformation along the western Idaho shear zone was also found in the McCall area (i.e., the Hazard Creek complex; Manduca et al., 1993).

The difficulty with this interpretation is twofold. First, the Four Bit Creek tonalite is largely covered by Tertiary basalts, making characterization of the deformation patterns difficult. Second, the Four Bit Creek tonalite is on the western margin of the western Idaho shear zone. This marginal location is consistent with extrapolating the western edge of the shear zone in the better-exposed outcrops further northward on West Mountain (Bonnichsen, 1987). A far better constraint on the initiation of shearing could be placed if there was an undeformed pluton within the western Idaho shear zone in an area of continuous exposure. The Crevice pluton, exposed along the main Salmon River, provides just such an opportunity (Blake et al., 2009; Gray et al., 2012). The Crevice pluton is dated at 105 ± 3 Ma and 103.9 ± 2.7 Ma from U-Pb dates on zircons in the Riggins area (Gray et al., 2012). The Crevice pluton exhibits a variation of fabrics from isotropic and magmatic in the center to strong solid-state foliations near its margin. Yet, the pluton is located within the western Idaho shear zone, surrounded by strongly deformed rocks of the Kelly Creek schist (e.g., Blake et al., 2009). The absence of a strong, penetrative tectonic fabric indicates that western Idaho shear zone deformation was not occurring during emplacement of the Crevice pluton: Any intruding magma would be significantly weaker (less viscous) than the surrounding schists, and hence it would be highly deformed. Consequently, the Crevice pluton must have intruded during a time of tectonic quiescence, and it was later deformed after cooling. A similar interpretation for an undeformed granitic rock surrounded by deformed schist was given by Paterson et al. (1989). We note that this interpretation differs from that of Gray et al. (2012), who suggested that the overprinting of western Idaho shear zone fabrics on the pluton margins indicates a gradual change from suturing to transpressional deformation.

Lu-Hf analyses on garnets collected from two different outcrops within West Mountain give ages of 99.5 ± 1.4 Ma and 97.3 ± 0.7 Ma. Based on field and microstructural relationships (i.e., garnets grew within the foliation plane) and petrography (i.e., garnets contain inclusion trails consistent with dextral motion), garnet growth was concurrent with deformation associated with the western Idaho shear zone. Thus, peak metamorphism associated with the western Idaho shear zone occurred at ca. 98 Ma (Fig. 19).

The orthogneiss units in West Mountain, east of the Four Bit Creek tonalite, record a spectrum of emplacement ages. Ages range from as early as 111 ± 4 Ma to 91 Ma. Although the Sage Hen orthogneiss is dated in multiple locations, only one sample provides a resolvable age of 93 ± 3.4 Ma. Another sample ∼0.5 km to the east shows an unresolvable continuous age spectrum of 102–91 Ma. The interpretation of these data is that there was roughly continuous plutonism coeval with the western Idaho shear zone. This interpretation is strengthened by the observation that all orthogneisses contain a strong tectonic foliation, consistent with a partially molten state during deformation.

The age of the Payette River Tonalite, the age of the Rat Creek granite, and crosscutting dikes within the Little Goose Creek complex provide evidence for the cessation of the western Idaho shear zone. Waning deformation within the western Idaho shear zone was synchronous with the emplacement of the Payette River Tonalite, which formed on or near the eastern margin of the western Idaho shear zone at ca. 91 Ma (Manduca et al., 1993; Giorgis et al., 2008). In the McCall area, Giorgis et al. (2008) dated an undeformed, crosscutting pegmatite dike within the Little Goose Creek complex at 90 Ma. This constraint is consistent with the undeformed Rat Creek granite, dated at 88.2 ± 3 Ma, which appears as post-tectonic. Therefore, we interpret the age of the Rat Creek granite to indicate cessation of the western Idaho shear zone in West Mountain.

CONCLUSIONS

New data from West Mountain, west-central Idaho, allow us to constrain deformation and timing aspects of the western Idaho shear zone and hypothesize about regional tectonics.

  • (1) Steeply E-dipping, N-NNE–oriented foliations and vertical lineations characterize the fabric in the orthogneisses, consistent with dextral transpression.

  • (2) The foliation declination changes from 005° to 024° from the northern to the southern part of the field area, and this is interpreted to reflect a primary along-strike variation in the western Idaho shear zone. This bend is consistent with 000°–striking foliation to the north near McCall, Idaho (e.g., Manduca et al., 1993), and 020°–striking foliation in the Owyhee Mountains (e.g., Benford et al., 2010).

  • (3) The western Idaho shear zone was active at 4.4 kbar and ∼730 °C, as constrained by thermobarometric analysis of the metamorphosed sedimentary wall rocks.

  • (4) Peak metamorphism in the western Idaho shear zone occurred from ca. 99 (99.5 ± 1.4) Ma to 97 (97.3 ± 0.7) Ma, as constrained by Lu-Hf dates on garnets.

  • (5) U-Pb analyses on zircons from orthogneisses within West Mountain span the ages 114–91 Ma, indicating both precursory and continuous magmatism coeval with western Idaho shear zone deformation.

  • (6) Zircon inheritance in the West Mountain intrusive units suggests that Blue Mountains crustal material extends east of the western Idaho shear zone at depth.

  • (7) The youngest unit at West Mountain is the undeformed Rat Creek granite (88.2 ± 3 Ma), which is consistent with cessation of western Idaho shear zone deformation at ca. 90 Ma.

  • (8) Western Idaho shear zone deformation started after ca. 104 Ma (age of Crevice pluton; Gray et al., 2012) and probably after 100.9 ± 3 Ma (age of Four Bit Creek tonalite). These units are within (Crevice pluton along main Salmon River) or immediately adjacent to (Four Bit Creek tonalite) the western Idaho shear zone, yet they are locally undeformed (Crevice pluton) or very weakly deformed (Four Bit Creek tonalite).

We thank a variety of field assistants for their help: Amanda Montgomery, Tina Porter, Peter Scheuermann, and Will Montz. We thank W. Montz for his willingness to share his geological map of the Porters Pond migmatite domain. This work was supported by National Science Foundation grants EAR-0844260 and EAR-1251877 to Tikoff, and grants EAR-0537913 and EAR-0844149 to Vervoort.

APPENDIX 1: ROCK UNITS AND MICROSTRUCTURAL DESCRIPTIONS

Four Bit Creek Tonalite

The Four Bit Creek tonalite is a coarse-grained unit, containing plagioclase feldspar, quartz, biotite, and muscovite (secondary), with trace amounts of sphene, zircon, apatite, potassium feldspar, allanite, and opaque oxides. Locally, the rock contains up to 10% hornblende. Sericite occurs as a secondary phase, mantling plagioclase grains and lining microcracks. Chlorite associated with biotite is interpreted to result from alteration.

On the outcrop scale, this unit exhibits a weak foliation defined by aligned biotite. Quartz grains are slightly elongate in hand sample. Biotite content varies from 3% to 10%. Two isolated northern exposures, including at Four Bit Creek, are finer grained and have higher biotite content than other exposures, which typically results in the development of a stronger foliation. We interpret the local increases in biotite content to result from increased sedimentary wall-rock assimilation, an effect that is more obvious in other units.

Microstructurally, the Four Bit Creek tonalite is only weakly deformed relative to adjacent units. Large (>500 µm) quartz grains show strong undulatory extinction and are slightly elongated but do not form quartz ribbons. There are pervasive, thin (<50 µm) zones of fine-grained recrystallized minerals, dominantly quartz-rich, which appear to have accommodated localized deformation. This pattern of partitioned deformation is distinctly different from rocks affected by the western Idaho shear zone exposed immediately to the east. Feldspars exhibit deformation twins, and myrmekitic intergrowths are noted on plagioclase grain boundaries forming parallel to the trace of the foliation plane. Feldspar microstructures are consistent with deformation occurring above 600 °C (e.g., Tullis and Yund, 1987; Simpson and Wintsch, 1989; Kruse and Stünitz, 1999; Altenberger and Wilhelm, 2000).

Mesa Creek Orthogneiss

This rock contains plagioclase feldspar, quartz, and biotite, with minor amounts of potassium feldspar, apatite, and zircon. The assemblage of sillimanite, muscovite, and garnet is generally present. Sericite is found mantling plagioclase feldspar grains and lining microcracks. The Mesa Creek orthogneiss has a tonalitic composition overall, although the presence of sillimanite, muscovite, and garnet is interpreted to indicate extreme assimilation of metasedimentary wall-rock material during intrusion.

In outcrop, this unit exhibits a strong solid-state foliation defined by quartz ribbons and biotite stringers. Additionally, the rock exhibits a compositional foliation defined by alternation of biotite-rich bands. Sillimanite, muscovite, and biotite define the lineation. Clusters of garnet porphyroclasts are locally noted, but they typically lack inclusions in thin section. This unit does not outcrop well, and outcrops occur primarily in road cuts. The microstructures for this unit and all other units affected by deformation associated with the western Idaho shear zone are generally similar and are discussed together at the end of this section.

Muir Creek Orthogneiss

The Muir Creek orthogneiss is coarse grained and locally leucocratic. Primary phases include plagioclase feldspar, quartz, and biotite. Traces of potassium feldspar, apatite, and zircon are noted. Garnet occurs locally as an accessory phase. Pervasive sericite mantles plagioclase feldspar grains and lines microcracks. The composition of this unit is highly variable, ranging from granitic to quartz dioritic in composition.

At the mesoscale, this unit contains a strong solid-state foliation defined by quartz ribbons, discontinuous biotite stringers, and a weak compositional banding. Lineation is defined by stretched and aligned biotite grains. Garnets are present as localized clusters of dodecahedral porphyroclasts. Deformed aplite dikes crosscut this unit.

Rammage Meadow Orthogneiss

Rammage Meadows is the type locality of this coarse-grained, felsic, and locally melanocratic orthogneiss. The Rammage Meadow orthogneiss contains plagioclase feldspar, quartz, and biotite, with hornblende occurring locally. Traces of apatite, sphene, epidote, potassium feldspar, opaque oxides, and zircon are found. Sericite mantles plagioclase and lines microcracks. Secondary biotite is present as an alteration product of hornblende, and primary biotite is locally altered to chlorite. The distinction between primary and secondary biotite is based on petrographic relationships. Secondary biotite appears as mantles and tails along hornblende grains, whereas primary biotite shows no textural connection to hornblende. The protolith of this gneiss is a hornblende-bearing tonalite.

This unit contains a strong solid-state foliation, defined by a well-developed compositional banding, grain-size reduction bands, quartz ribbons, and biotite stringers. Within fine-grained bands, the mineralogy remains consistent but grains size is reduced in planar domains paralleling the foliation plane. The lineation is defined by both hornblende and biotite. Locally, hornblende varies from 1 mm to up to 1 cm in size. Deformed pegmatite and aplite dikes crosscut this unit. Large-scale (tens of meters), moderately deformed hornblende-bearing diorite and garnet-bearing tonalite dikes are locally observed.

Sage Hen Orthogneiss

This unit is coarse grained, megacrystic, and felsic. Primary phases of plagioclase feldspar, quartz, potassium feldspar, and biotite are present, with trace amounts of sphene, zircon, allanite, and apatite. Locally, garnet is present. Sericite is noted mantling plagioclase feldspar grains. The igneous protolith to this gneiss was a porphyritic granodiorite.

This unit contains a well-developed solid-state foliation defined by quartz ribbons and biotite stringers, with stretched biotite grains defining the lineation. Poikilitic porphyroclasts of potassium feldspar contain inclusions of biotite. Garnets occur in rust-colored porphyroclasts and are found in clusters. Local increases in biotite content commonly coincide with the appearance of garnets, interpreted to indicate zones of increased contamination by sedimentary wall rocks. Both pegmatite and aplite dikes cut through exposures of this rock unit. Locally, shear bands oblique to the foliation are noted. Coincident with the development of shear bands, this rock unit becomes migmatitic, with melt segregations anastomosing around boudins of mafic gneiss.

Microstructures

Despite petrologic variations, the Mesa Creek orthogneiss, Muir Creek orthogneiss, Rammage Meadow orthogneiss, and Sage Hen orthogneiss all show similar microstructures. Within orthogneisses of West Mountain, a foliation is defined in thin section by the elongation of quartz ribbons, alignment of biotite, parallelism of elongate feldspars, and localized grain-size reduction bands. In thin section, the lineation is defined by stretching of biotite and/or hornblende. Recrystallized quartz grains average ∼100 μm and show foam texture consistent with high-temperature quartz plasticity (Schmid et al., 1980, 1987; Schmid and Casey, 1986; Hirth and Tullis, 1992; Fitz Gerald and Stünitz, 1993). Quartz grains locally display chessboard extinction, which is also consistent with high-temperature deformation (>650 °C; Kruhl and Huntemann, 1991; Masberg et al., 1992; Kruhl, 1996, 1998; Lee et al., 2012). Where chessboard extinction is not present, there is a strong undulatory extinction within elongated quartz grains. Large relict grains show core and mantle structures. Quartz-quartz grain boundaries exhibit pronounced grain-boundary bulging, indicative of grain-boundary migration recrystallization. These mechanisms are consistent with high-temperature deformation (>500 °C; Kruhl, 2001; Kruhl and Paternell, 2002; Lee et al., 2012) and amphibolite facies metamorphism (Schmid et al., 1980, 1987; Schmid and Casey, 1986; Fitz Gerald and Stünitz, 1993). Feldspar grains contain deformation-induced twinning consistent with high-temperature deformation (Vernon, 2004, and references therein).

Payette River Tonalite

The Payette River Tonalite has been described in previous work (Manduca et al., 1992, 1993; Giorgis et al., 2008) and is correlative to the Whisky Ridge tonalite in the Owyhee Mountains in SE Idaho (Benford et al., 2010). The rock is a coarse-grained, hornblende-bearing tonalite and has been dated at 91.5 ± 1.1 Ma (Giorgis et al., 2008) and 90.43 ± 0.03 Ma (Benford et al., 2010) using U-Pb on zircon. Primary phases include plagioclase feldspar, quartz, hornblende, biotite, and potassium feldspar, with trace amounts of sphene, epidote, zircon, opaque oxides, and apatite. The rock has a distinctive “salt and pepper” look, resulting from the presence of large (up to 1 cm), euhedral hornblende crystals, and it contains mafic enclaves. However, there are local variations in the grain size and amount of hornblende. Sericite occurs as alteration mantles of plagioclase grains as well as along microcracks. Secondary biotite is associated with the breakdown of hornblende. Chlorite is noted as an alteration feature of primary biotite.

West to east variations in the development of a tectonic foliation occur in this unit. Western outcrops of the Payette River Tonalite contain a well-developed solid-state foliation, defined by quartz ribbons and biotite stringers, with oriented hornblende and biotite grains defining the downdip lineation. Moving east, the solid-state foliation grades into a parallel magmatic foliation defined by biotite schlieren. Sheared mafic enclaves are commonly present, although locally absent, and are flattened in foliation and elongated parallel to the lineation. Undeformed to weakly deformed pegmatite and aplite dikes crosscut this unit.

The transition in foliation development noted through field observations is consistent with microstructural observations. At the western margin of this unit, quartz grains are predominantly recrystallized, exhibiting a foam texture, whereas relict grains show bulging sutures. Biotite is partitioned into bands parallel to the trend of the solid-state foliation. Plagioclase feldspars show abundant deformation twins. Moving eastward in the Payette River Tonalite, the solid-state microstructure grades into igneous textures. In the central portion of the intrusion, frequent myrmekite is found forming at euhedral plagioclase-feldspar grain boundaries. Biotite-rich bands become less frequent, with biotite forming weakly aligned euhedral grains in the easternmost portions of the unit.

Rat Creek Granite

This phaneritic, two-mica granite is named after exposures in Rat Creek, located on the eastern flank of West Mountain. Primary mineral phases include plagioclase feldspar, quartz, potassium feldspar, and biotite. Trace mineralogy includes sphene, muscovite, oxides, and zircon. Sericite is noted as alteration mantles around feldspars, and biotite is altered to chlorite.

The Rat Creek granite is undeformed, but it contains a minor magmatic fabric defined by biotite schlieren. Potassium feldspar is present as oikocrysts with biotite inclusions. This unit contains exclusively magmatic microstructures. Euhedral grains of potassium and plagioclase feldspar meet at 120° angles. Quartz grains are not recrystallized and exhibit a foam texture. Euhedral biotite grains are weakly aligned, which forms the basis for measuring a magmatic fabric.

Metamorphosed Sedimentary Rocks

Throughout West Mountain, localized lenses of metasedimentary wall-rock material are exposed but are too small to be individually mapped. However, within the Payette River Tonalite, a continuous relict screen of metamorphosed sedimentary wall rock parallels the contact between the Payette River Tonalite and the Sage Hen orthogneiss (Figs. 2 and 3). This screen is 0.5 km in width and 7 km in length.

This screen contains quartzites, calc-silicate gneisses, and pelites, each with its own characteristic texture and mineralogy. The quartzite makes up ∼85% of the screen. Quartzites contain quartz, biotite, diopside, wollastonite, and trace amounts of muscovite, cordierite, zoisite, and zircon. Calc-silicate gneisses, found at the southern edge of the screen, contain quartz, calcite, biotite, diopside, and wollastonite. Traces of graphite, sphene, apatite, and rutile are found. The metamorphic mineral assemblage found in calc-silicate rocks of quartz, calcite, diopside, and wollastonite is consistent with amphibolite facies metamorphism (e.g., Spear, 1995). The schist is pelitic in composition. It contains quartz, biotite, sillimanite, and garnet with trace amounts of cordierite, zircon, plagioclase, orthoclase, and graphite.

A well-developed foliation is present in all parts of the screen. The foliation is defined by ribboned quartz, local biotite stringers, and compositional banding. Biotite and locally sillimanite define the lineation. Where compositional bands are present, folding is observed. Folds are all subvertical, tight to isoclinal, with fold hinges parallel to the lineation. Where metamorphosed sedimentary rocks are in contact with plutonic rock, the plutonic rock commonly has a bleached appearance. Deformed pegmatite dikes locally cut through the metamorphosed sedimentary materials. Microstructurally, the quartzites record a significantly larger grain size (up to ∼3 mm) than the pelites or calc-silicates. Quartz grains are elongate (defining the foliation), exhibit undulatory extinction, and are filled with inclusions of wollastonite, diopside, and zircon. Quartz-quartz grain boundaries exhibit pronounced grain-boundary bulging, indicative of grain-boundary migration recrystallization.

The age of the metamorphosed screen is unknown, but it may be a sliver of Neoproterozoic-aged strata deformed by the shear zone, based on the age of detrital or inherited zircons (e.g., sample 10NB257; see section on geochronology for further details).

APPENDIX 2: Lu-Hf GARNET DATING METHODS

For each sample, garnets were handpicked into 0.25 g fractions, crushed, and dissolved in HF-HNO3 on a hot plate. Two whole-rock powder fractions per sample were also prepared, with one dissolved on a hot plate with the garnets and the other dissolved in high-pressure Teflon bombs. Hf and Lu were separated and purified using cation exchange columns and analyzed using a ThermoFinnigan Neptune MC-ICP-MS at Washington State University. Hf isotope analyses were corrected for fractionation using the exponential law and 179Hf/177Hf = 0.7325, and they were normalized to the JMC 475 Hf standard. Lu samples were measured relative to a series of mixed Lu-Yb solutions in order to constrain Yb interference and mass bias corrections (Vervoort et al., 2004). Hf isochron ages were calculated using the program Isoplot (Ludwig, 2003) and a decay constant of 1.867 × 10–11 yr–1 (Scherer et al., 2001; Söderlund et al., 2004).

A typical Lu-Hf isochron consists of four garnet and two whole-rock fractions. Points omitted from isochrons in our study are due to isobaric interferences of 176Yb and 176Lu on 176Hf (Table 3). One potential problem with dating garnets, however, is that a portion of radiogenic isotopes measured in garnet fractions may be located in inclusions rather than in the garnet lattice itself (Scherer et al., 2000). Trace mineral inclusions with high Lu content such as apatite could potentially lead to erroneous Lu-Hf ages if they have a different isotopic composition, or if they have different Lu-Hf closure history, from the enclosing garnet.

1GSA Data Repository Item 2016323, Figure DR1: Trace-element characteristics of the (A) intrusive units west of and within the western Idaho shear zone and (B) the Rat Creek granodiorite, normalized to chondrite; Table DR1: Geochronology and geochemistry sample information; Table DR2: U-Pb zircon geochronology; Table DR3: Whole-rock major- and trace-element geochemistry; Table DR4: In situ zircon hf isotope geochemistry, is available at www.geosociety.org/pubs/ft2016.htm, or on request from editing@geosociety.org.