TECTONICS, VOL. 27, TC2009, doi: /2006tc002071, 2008

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1 TECTONICS, VOL. 27,, doi: /2006tc002071, 2008 Coolwater culmination: Sensitive high-resolution ion microprobe (SHRIMP) U- and isotopic evidence for continental delamination in the Syringa Embayment, Salmon River suture, Idaho Karen Lund, 1 J. N. Aleinikoff, 1 E. Y. Yacob, 1 D. M. Unruh, 1 and C. M. Fanning 2 Received 7 November 2006; revised 17 November 2007; accepted 7 January 2008; published 30 April [1] During dextral oblique translation along Laurentia in western Idaho, the Blue Mountains superterrane underwent clockwise rotation and impinged into the Syringa embayment at the northern end of the Salmon River suture. Along the suture, the superterrane is juxtaposed directly against western Laurentia, making this central Cordilleran accretionary-margin segment unusually attenuated. In the embayment, limited orthogonal contraction produced a crustal wedge of oceanic rocks that delaminated Laurentian crust. The wedge is exposed through Laurentian crust in the Coolwater culmination as documented by mapping and by sensitive high-resolution ion microprobe U-, Sr i, and e Nd data for gneisses that lie inboard of the suture. The predominant country rock is Mesoproterozoic paragneiss overlying Laurentian basement. An overlying Neoproterozoic (or younger) paragneiss belt in the Syringa embayment establishes the form of the Cordilleran miogeocline and that the embayment is a relict of Rodinia rifting. An underlying Cretaceous paragneiss was derived from arc terranes and suture-zone orogenic welt but also from Laurentia. The Cretaceous paragneiss and an 86- orthogneiss that intruded it formed the wedge of oceanic rocks that were inserted into the Laurentian margin between 98 and 73, splitting supracrustal Laurentian rocks from their basement. Crustal thickening, melting and intrusion within the wedge, and folding to form the Coolwater culmination continued until 61. The embayment formed a restraining bend at the end of the dextral transpressional suture. Clockwise rotation of the impinging superterrane and overthrusting of Laurentia that produced the crustal wedge in the Coolwater culmination are predicted by oblique collision into the Syringa embayment. Citation: Lund, K., J. N. Aleinikoff, E. Y. Yacob, D. M. Unruh, and C. M. Fanning 1 U.S. Geological Survey, Denver, Colorado, USA. 2 Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia. Copyright 2008 by the American Geophysical Union /08/2006TC (2008), Coolwater culmination: Sensitive high-resolution ion microprobe (SHRIMP) U- and isotopic evidence for continental delamination in the Syringa Embayment, Salmon River suture, Idaho, Tectonics, 27,, doi: / 2006TC Introduction [2] During the Early Cretaceous, relative plate motion between various Pacific plates and Laurentia changed from orthogonal to right-lateral oblique [Engebretson et al., 1985]. Locally, collisional vectors along segments of the western Laurentian margin ranged from orthogonal through near parallel, resulting in various accretion trajectories and complex syn- and post-accretion dextral movement of terranes along the continental margin [Davis et al., 1978; Monger, 1979; Gabrielse, 1985; Cowan et al., 1997; Wyld and Wright, 2001]. Allochthonous terranes and entrained continental margin fragments were translated along primary accretionary structures within the allochthonous collage and along secondary intracontinental faults [Saleeby, 1983; Saleeby et al., 1992; McClelland et al., 2000]. During the Cretaceous, and according to latitude along the Canadian Cordillera, variation in the obliquity of plate motions [Engebretson et al., 1985] resulted in major along-strike changes from discrete dextral strike-slip faulting on the Tintina-northern Rocky Mountain trench fault zone in the north to lesser strike-slip and increased dextral oblique compression in the foreland fold and thrust belt to the south near the Canada-U.S. border [Price and Carmichael, 1986]. In the southern U.S. Cordillera, a broad zone of accreted island arc terranes and intervening pericratonic terranes were accreted by alternating orthogonal and transcurrent interactions [Oldow et al., 1984; Wyld and Wright, 2001; Umhoefer, 2003]. [3] In west-central Idaho, the Salmon River suture (Figure 1) formed as part of the late Mesozoic terrane accretion and translation belt. However, the Salmon River suture, related structures, and associated allochthonous terranes form an isolated, inboard-stepped, segment of the accretionary belt. The Salmon River suture is also an unusual Cordilleran terrane boundary because it: (1) juxtaposes Permian to Jurassic island-arc rocks directly against Proterozoic Laurentian rocks with no intervening transitional terranes [Lund, 1984, 1988; Lund and Snee, 1988], (2) is well-defined by change of initial 87 Sr/ 86 Sr ratios in nearby 1of32

2 Figure 1. Location map showing accretionary structures from southern British Columbia to northern Nevada and extent of Salmon River suture (modified from Wyld and Wright [2001]). Purple, Laurentian rocks, undivided; light blue, Neoproterozoic to early Paleozoic miogeoclinal rocks of northern U.S. Cordillera; pink, Mesoproterozoic to Tertiary plutonic rocks; teal, transitional continental slope rocks; green, accreted Paleozoic to Mesozoic rocks; sz, shear zone; f, fault; LC line, Lewis and Clark line; Cz, Clearwater zone. Initial Sr isotopic signatures for intrusive rocks from Armstrong et al. [1977] and Kistler and Peterman [1978] are and The 60 clockwise rotation of Blue Mountains superterrane is from Housen and Dorsey [2005]. plutons from <0.704 (oceanic) on the west to >0.706 (continental) on the east across a remarkably narrow (1 to 10 km) zone [Armstrong et al., 1977; Fleck and Criss, 1985; Criss and Fleck, 1987], (3) has a corresponding abrupt change in do 18 values in the plutons [Fleck and Criss, 1985; nduca et al., 1992; King et al., 2002], and (4) has an abrupt 90-degree change in trend from northnortheast striking through most of western Idaho to weststriking from Idaho to central Washington [Armstrong et al., 1977; Fleck and Criss, 2004; Thiessen et al., 1992; Mohl and Thiessen, 1995]. Although initial Sr isotopic data trace the change in orientation of the boundary [Fleck and Criss, 2004] and the age and kinematics of several shear zones near the corner are available [Strayer et al., 1989; Davidson, 1990; Payne and McClelland, 2002], proposed origins for the change are conflicting. Suggested origins include Mesozoic rifting [Davis et al., 1978], oroclinal bending [Carey, 1955; Schmidt et al., 2003], northeast-directed subduction [Strayer et al., 1989], or offsetting sinistral faulting [Payne and McClelland, 2002; McClelland and Oldow, 2004, 2007]. [4] As part of an oblique accretionary margin involving dextral translation of terranes and transpressional deformation along the Cordillera, the original shape and subsequent modification of this corner of the Salmon River suture (herein referred to as the Syringa embayment) [e.g., Rankin, 1976; rshak, 2004] are critical to integrating tectonic models for the Cordillera to north and south. We present geologic mapping and structural analysis, sensitive highresolution ion microprobe (SHRIMP) U- data for igneous and metasedimentary rocks, and Sr and Nd isotopic data for igneous rocks that provide constraints on tectonic geometry and evolution. Our data, presenting the embayment as part of an originally irregular continental margin, provide insights into processes occurring in a restraining bend along a continental margin undergoing oblique collision. The interpretations have implications for transport and truncation of terranes along oblique collisional boundaries and for three-dimensional geometry of local orthogonal contraction within a larger transpressional system. Further, the interpretations provide insights into the time span and style of dextral translation along the Cordillera. [5] The name Salmon River suture refers to the boundary between the Blue Mountains superterrane and Laurentian basement. At the latitude of Idaho, the superterrane and continent shared no common history prior to Cretaceous time when the Blue Mountains terranes began to impinge, and eventually lodged, against Laurentia. In the rock record near the suture, a complex, prolonged history of accretion was dominated by Cretaceous dextral transpression [Lund and Snee, 1988; McClelland et al., 2000; Giorgis et al., 2005] that was accompanied by a broad zone of bilateral metamorphism, deformation, and plutonism [Lund and Snee, 1988]. However, nomenclature for the tectonic features is confused in the literature. The names western Idaho suture zone and western Idaho shear zone have been used interchangeably for steeply dipping strain fabrics that characterize plutonic rocks near the suture and for the underlying basement structure whereas Salmon River suture has been used inclusively for the structures related to juxtaposition of the two crustal plates. To prevent confusion, we use the following terminology: western Idaho shear zone refers to the discrete shear zone superposed on plutonic rocks and high-grade metamorphic rocks in the central zone of the suture [McClelland et al., 2000; Giorgis et al., 2005] and constrained by intrusive units to be about ; Salmon River suture refers to the regional boundary between Blue Mountains and Laurentian plates, includes the related structures, and is named for the geographic feature near which a juxtaposition of rocks of very different origin was first noted by Lindgren [1904]. Thus, the long and complex evolution along the Salmon 2of32

3 River suture included formation of the discrete western Idaho shear zone. 2. Setting [6] The Blue Mountains superterrane is a structurally telescoped complex of Permian to Jurassic island arc, ocean crust, and forearc fragments: the Wallowa, Olds Ferry, Baker, and Izee terranes of Silberling et al. [1992] and Vallier [1995] that formed distant from North America [Hillhouse et al., 1982] and (or) from their present latitude [Housen and Dorsey, 2005] perhaps partly with influence from Laurentia [Schwartz et al., 2006]. These terranes were amalgamated along east-vergent thrust faults and shear zones that are now steepened [nn, 1991]. Prior to and during amalgamation, the terranes underwent local lowgrade metamorphism and ductile deformation. Age data document late Middle Jurassic to Early Cretaceous amalgamation based on U- and 40 Ar/ 39 Ar dating of stitching plutons at [Walker, 1986; Snee et al., 1995; Vallier, 1995] and Sm-Nd dating of garnet core zones that permit an early metamorphic event at [Getty et al., 1993]. After amalgamation, Jurassic and Cretaceous overstepping sediments were deposited across several of the terranes [Dickinson et al., 1979; White et al., 1992; Dorsey and Lenegan, 2007]. The Blue Mountains superterrane underwent about 60 clockwise rotation after about 145 [Wilson and Cox, 1980], 37 of which were post- Albian [Housen and Dorsey, 2005], and 16 of which were post-eocene [Gromme et al., 1986]. [7] The western edge of Laurentia in Idaho is poorly known, having been inundated by Late Cretaceous plutons of the Idaho batholith and little studied. Country rocks were generally thought to be Mesoproterozoic [Bond, 1978] or older [Harrison, 1972; Lewis et al., 1998, 2001]. Recently, a broad northwest-striking zone of Neoproterozoic strata was documented [Lund et al., 2003a] but the age of most of the gneisses remains undetermined. An abundance of inherited zircon in Late Cretaceous Idaho batholith rocks frustrated early attempts to determine crystallization ages. Across the region, the metamorphism in Laurentian rocks is commonly concluded to be a function of magmatic heating during emplacement of the Idaho batholith [Hietanen, 1962; Lang and Rice, 1985; Barton et al., 1988; Criss and Fleck, 1990; Carey et al., 1992; Grover et al., 1992; House et al., 1997] although crosscutting relationships document postmetamorphic intrusion [Reid, 1959; Lund, 2004] North-Northeast Striking Segment of the Salmon River Suture [8] The first documentation of the Salmon River suture as an important tectonic feature was through determination of initial Sr isotopic data that documented oceanic values to the west and continental values to the east; this ultimately identified the edge of Laurentia and the extent of accreted terranes in the Cordillera [Armstrong et al., 1977]. Detailed Sr and O isotopic studies in Idaho and southeastern Washington [Fleck and Criss, 1985; Criss and Fleck, 1987] and along several segments of the Salmon River suture [nduca et al., 1992; King et al., 2002; Fleck and Criss, 2004] document that an abrupt change in isotopic character of plutons from oceanic to continental sources occurs across a zone as narrow as 1 to 10 km (in comparison to hundreds of kilometers of intermediate values in Nevada and Canada [Kistler and Peterman, 1978]). [9] Early studies across the Salmon River suture attributed the deformation to intrusion of the Late Cretaceous Idaho batholith on the east [Hamilton, 1963; Myers, 1968, 1982]. However, interpretation of field mapping conducted in areas that include substantial metamorphosed country rock and on both sides of what is now known to be the Salmon River suture indicates that the suture is the axis of symmetry toward which metamorphic and deformational intensity increase [Myers, 1982; Lund, 1984, 2004; Hoover, 1986; Lund and Snee, 1988]. Dating of inverted-metamorphic, thrust-bounded plates in the eastern edge of the Blue Mountains superterrane document pulses of thrust faulting and cooling of structural plates from about 130 to 85 that young toward the suture ( 40 Ar/ 39 Ar cooling ages of biotite and hornblende [Lund and Snee, 1988]; Sm-Nd ages of garnet [Getty et al., 1993; Snee et al., 1995]). Near the suture, plutons are elongated parallel to the suture and contain vertical to steep east-dipping foliations and downdip lineations [Hamilton, 1963; Myers, 1982; Onasch, 1987; Lund and Snee, 1988; Lund et al., 1993; nduca et al., 1993; Lund, 1995; Tikoff et al., 2001] as well as discrete mylonitic zones indicating dextral motion [McClelland et al., 2000; Giorgis and Tikoff, 2004; Giorgis et al., 2005]. Along its 200-km length, tectonic transport was directed upward and outward from the axis of the structure [Lund and Snee, 1988; nduca, 1988; Selverstone et al., 1992; nduca et al., 1993; Lund, 2004] defining the geometry of a diverging, flower structure across the Salmon River suture interpreted to have formed during dextral transpression [Lund and Snee, 1988; Lund, 2004]. From mapping and geochronologic data, it was interpreted that the structural system narrowed into a central, discrete, vertical structure during its evolution [Lund, 1984, 2004; Lund and Snee, 1988]. Alternatively, detailed kinematic data from plutonic rocks in the center of the structure are interpreted as the result of extreme shortening of an 85- to 100-km-wide magmatic belt and isotopic transition during dextral transpression [Giorgis et al., 2005]. [10] Ductile events were followed by normal-fault uplift, dated at by 40 Ar/ 39 Ar cooling studies [Lund and Snee, 1988]. Uplift continued until after the Miocene, as shown by deposition and preservation of flows of the Miocene Columbia River Basalt Group only west of the suture (except for minor, thin occurrences such as in the southwestern corner of Figure 3) Syringa Embayment [11] The change in trend of the Salmon River suture to westward in the Syringa embayment is based on Sr isotopic ratios of exposed plutons (Figure 3) [Criss and Fleck, 1987; Fleck and Criss, 2004] and on map units [Hietanen, 1962]. The steeply northeast-dipping Woodrat Mountain fault (Figure 3) is the suture structure that separates oceanic units 3of32

4 Figure 2. Location map showing generalized extent of oceanic terranes in eastern Oregon and western Idaho, general extent of Cretaceous overstepping deposits, location of Salmon River suture, and general extent of Neoproterozoic-Paleozoic and Mesoproterozoic Laurentian rocks (extent of terranes of Blue Mountains superterrane from Vallier [1995]; extent of Neoproterozoic-Paleozoic rocks from Lund et al. [2003a], Lund [2004], this study, and Lewis et al. [2007]). Gray overlay pattern, extent of Orofino shear zone [from McClelland and Oldow, 2007] showing overlap of structures described in this paper. to the southwest from continental rocks to the northeast [Lewis et al., 1992, 1998; Lewis and Stanford, 2002]. [12] Southwest of the embayment, isolated exposures of Mesozoic island arc-derived foliated tonalite and lesser hornblende gneiss, calcareous phyllite, and marble were correlated with Permian to Triassic volcanic rocks of the Seven Devils and Riggins Groups [Hietanen, 1962; Lewis et al., 1992], both exposed near Riggins, Idaho (Figure 2). East and northeast of the embayment, continental rocks were correlated with the lower three units of the Mesoproterozoic Belt Supergroup [Hietanen, 1962; Van Noy et al., 1970; Greenwood and Morrison, 1973; Toth, 1983; Lewis et al., 1998, 2001] and the locally named Paleoproterozoic Syringa Sequence, which was considered to be basement to the Belt Supergroup [Lewis et al., 1998, 2001]. [13] jor plutonic units identified by previous mapping studies include granodiorite-tonalite that intruded oceanic rocks west of the Salmon River suture at about 113 (U- dating [Lee et al., 2004]). Granite, granodiorite, and tonalite plutons intruded continental rocks [Greenwood and Morrison, 1973; Lewis et al., 1992] but remain largely undated because previous U- dating of plutonic rocks [Reid et al., 1973; Toth and Stacey, 1992] was difficult to interpret owing to limitations of early techniques with respect to the presence of xenocrystic cores and metamorphic overgrowths of zircon grains. [14] Metamorphic grade and amount of deformation increase toward the suture in the Syringa embayment (similar to along the main segment of the suture to the south). Structural fabrics are steeply east-northeast dipping [Hietanen, 1962; Davidson, 1990]. Isotopic and mapping studies indicate that the high-grade rocks include oceanic, continental, and perhaps transitional rocks [Armstrong et al., 1977; Criss and Fleck, 1987; Davidson, 1990; Schmidt et al., 2003; Fleck and Criss, 2004]. 40Ar/39Ar cooling dates step down from about 88 near the crustal boundary to about 60 to the northeast [Snee et al., 1987; Davidson et al., 1988; Payne et al., 2001; Fleck and Criss, 2004]. 4 of 32

5 [15] A previously identified pre-miocene structure on Figure 3 is the northwest-striking, Late Cretaceous(?) Glade Creek fault [Lewis et al., 1992, 1998]. This fault was recently interpreted as a young local manifestation of a regionally significant aeromagnetic anomaly named the Clearwater zone (Figure 2) and suggested to be a multiply reactivated, originally Mesoproterozoic (about 1500 ) structure in Laurentian basement [Sims et al., 2005]. [16] The Glade Creek fault and other structures identified as part of the Clearwater zone by Sims et al. [2005] were recently described as the northern boundary of the Orofino shear zone [Payne et al., 2001; Payne and McClelland, 2002; McClelland and Oldow, 2004, 2007]. The southern boundary of this 20-km-wide zone of shearing was described as the west-striking segment of the suture. This zone is interpreted to be a Late Cretaceous top-to-southwest zone of sinistral thrust faulting that offset the Salmon River suture and that extended well into the continental interior linking Laramide structures in Wyoming to the Cretaceous accretionary margin [Payne et al., 2001; Payne and McClelland, 2002; McClelland and Oldow, 2004, 2007]. This model infers a single structure, which overlaps and kinematically combines all the structural elements that are shown on Figures 3a and 3b and that are separately studied and dated in the present study. 3. Rock Units [17] Our new structural and isotopic data show that previously unrecognized tectonic domains and rock units lie in the Syringa embayment (Figure 2). Between the Salmon River suture and Clearwater zone, six rock packages are exposed in progressively lower structural levels toward the core of a teardrop-shaped structural culmination, herein named the Coolwater culmination. Informal rock units presented in this study (Figure 3) are defined herein for the first time except for the Coolwater Ridge orthogneiss of Greenwood and Morrison [1973]. Units are described in order from oldest to youngest, paragneisses first and orthogneisses following Gneiss of Elk City [18] The gneiss of Elk City as used in this study is a composite unit dominated by migmatitic muscovite-biotitefeldspar-quartz gneiss (Figure 4a). Less abundant lithologic units are not shown on Figure 3 but include granite augen gneiss dated at 1370 (TIMS U- age of 1370 ± 100 [Evans and Fischer, 1986]). Structurally below mapped thrust faults (south-central Figure 3), 1 10 cm sillimanitemuscovite-quartz intergrowths form porphryoblasts that may be pseudomorphs after kyanite. Near the structural contact with underlying gneiss of Swiftwater Creek, the gneiss of Elk City bears garnets as large as 10 cm. Leucosomes in migmatite are parallel to compositional layering. The layering probably mimics alternating shale and siltstone in the protolith but layering is transposed as evidenced by intrafolial folds and refolding of foliation. Thickness of the gneiss of Elk City is unknown owing to intense folding and thrust faulting although it may be several kilometers thick. [19] Previously, these rocks were correlated with the oldest formations of the Mesoproterozoic Belt Supergroup [Reid et al., 1973; Lewis et al., 1998]. Earlier structural descriptions and interpretations of these rocks [Reid, 1959; Zimmermann, 1982; Carlson, 1981; Lewis et al., 1990, 1998] are mostly not used owing to previous combining of units of different age, origin, and (or) structural domain Gneiss of Syringa [20] The gneiss of Syringa (Figure 3) is dominated by massive, rusty weathering, pyritiferous, garnet- and kyanitebearing, muscovite-biotite-quartz-feldspar gneiss. Relict arkosic or quartz-pebble conglomerate zones (Figure 4b), 200-m-thick quartzite units, and minor calc-silicate-bearing layers are locally present. This unit probably originated dominantly as immature feldspar-pebble graywacke interbedded with more mature feldspar-bearing sandstone. No primary sedimentary structures have been recognized. Thickness estimates are probably compromised by unrecognized deformation but the unit may be about 1000 m thick. [21] Along the Woodrat Mountain and Lowell faults, the abundance and grain size of garnet, sillimanite, and (or) kyanite increase. Kyanite porphyroblasts as large as 10 cm are commonly intergrown with biotite and may form as much as 50 percent of the rock. Garnet porphyroblasts as large as 8 cm are associated with the kyanite, commonly have sillimanite rims, and form as much as 15 percent of the rock [Van Noy et al., 1970]. Quartzite near the Lowell thrust fault is sillimanite- and garnet-bearing. The characterization of metamorphic conditions with respect to this upgrading awaits further study. [22] The gneiss of Syringa was first correlated with the Mesoproterozoic Ravalli Group of the lower Belt Supergroup [Reid et al., 1973] and more recently as part of the possibly Paleoproterozoic Syringa sequence [Lewis et al., 1992, 1998, 2001, 2004]. The gneiss of Syringa is structurally continuous with the Neoproterozoic to Cambrian(?) Umbrella Butte Formation [Lund et al., 2003a], exposed in the southwestern corner of Figure 3. These may be correlative units based on physical proximity and structural continuity but the relative abundances of quartzite and schist differ Gneiss of Swiftwater Creek [23] The gneiss of Swiftwater Creek is predominantly massive epidote-hornblende-biotite-quartz-feldspar gneiss. Felsic conglomerate (Figure 4c) and feldspar-quartz gneiss form lesser zones. Aluminosilicate minerals were not observed, making observation and comparison of metamorphic grade difficult. Other than local preservation of conglomerate, primary sedimentary features are not preserved. This unit probably originated as volcanogenic, calcareous graywacke, and silty feldspathic sandstone. Thickness of the gneiss of Swiftwater Creek is unknown, as upper and lower contacts are structural or intrusive, but as much as 0.5 km thickness is preserved. 5of32

6 Figure 3. Generalized geologic map, cross section, and structural data for the Coolwater culmination. (a) Geologic map of Coolwater culmination and vicinity. Geology compiled from this study and Lewis et al. [1992]. BRf, Brushy Gulch Red River fault; Sf, Syringa fault; WMf, Woodrat Mountain fault. (b) Cross section for A-A 0 showing geometry of Coolwater culmination. (c) Contoured equal area plot of poles to foliation for Coolwater culmination, data from northeastern quarter of geologic map. Orientation of fold axis for antiformal Coolwater culmination (36, 108) shown in red. (d) Equal area plot of mineral lineations (triangles) and minor fold axes (diamonds), from northeastern quarter of geologic map. Orientation of gentle refold axis (3, 188) shown in red. 6of32

7 Figure 3. (continued) [24] These rocks were first mapped as the Wallace Formation in the middle of the Mesoproterozoic Belt Supergroup [Greenwood and Morrison, 1973; Reid et al., 1973] and more recently included in the possibly Paleoproterozoic Syringa sequence [Lewis et al., 1992, 1998] Augen Gneiss of Apgar Creek [25] The augen gneiss of Apgar Creek (Figure 3) originated as rapakivi porphyritic biotite granodiorite (Figure 5a). Rapikivi potassium feldspar phenocrysts are as large as 4 cm. Quartz is undulose and ribboned in thin section. The augen gneiss is cut by 2- to 10-cm-thick aplitic veinlets. Both the augen gneiss and veinlets were deformed by discrete mylonitic shear zones, which are parallel to the Clearwater zone. The pluton was intruded by younger, undeformed granite and granodiorite that obscure the original size and shape of the augen gneiss as well as its relations with country rock structures. However, because the augen gneiss is only present along the Clearwater zone, its emplacement probably was structurally controlled by the zone Coolwater Ridge Orthogneiss [26] The Coolwater Ridge orthogneiss [Greenwood and Morrison, 1973] originated as fine- to medium-grained, epidote- and apatite-bearing, biotite trondhjemite (Figure 5b). Thin garnet-bearing leucosome melt fractions are common. The orthogneiss is strongly foliated, generally not lineated, and exhibits weak compositional layering parallel to foliation and leucosomes. It forms a teardrop-shaped body about 17 km long and 8 km wide and is the structurally lowest exposed unit. 7of32

8 was dated at 94 ± 1.4 [Toth and Stacey, 1992]; however, this multigrain TIMS date seemed controversial because the age of inheritance was markedly younger than that of nearby intrusive rocks Orthogneiss of Andys Hump [28] The orthogneiss of Andys Hump is garnet-sillimanite porphyritic granodiorite orthogneiss. This orthogneiss contains several percent garnet that is 3 to 6 cm in diameter. Sillimanite also forms several percent of the rock, some in porphyroblasts as large as 3 cm long. The orthogneiss of Andys Hump lies structurally above or along the flank of the Coolwater Ridge orthogneiss (Figure 3) but intrusive relations are obscured by deformation. Figure 4. Photographs of paragneisses. (a) Migmatitic muscovite-biotite-quartz-feldspar gneiss of Elk City at location 01KL112 (Figure 2), with hammer for scale. (b) Feldspathic-grit schist in gneiss of Syringa from 2 km west of location 01KL107 (Figure 3), with handlens for scale. (c) Gneiss of Swiftwater Creek from 1 km northwest of location 03KL016 (Figure 3), with hammer for scale. [27] This trondhjemite orthogneiss is unusually finegrained, non-hornblende-bearing, and leucocratic compared to other tonalitic plutons in the region [e.g., Lewis et al., 1998]. A sample from the northwestern part of this pluton Figure 5. Photographs of orthogneisses. (a) Rapikivi augen gneiss of Apgar Creek from location 01KL107 (Figure 3), with hammer for scale. (b) Garnet-biotite trondhjemitic Coolwater orthogneiss from about 3 km southeast of location 01KL113 (Figure 3). Note leucosomes and tight to isoclinal folding within foliation. Hammer shown for scale. 8of32

9 3.7. Ultramafites [29] Isolated bodies of ultramafic and lesser mafic rocks include pyroxenite, dunite, gabbro, [Lewis et al., 1992], and garnet peridotite. Pyroxenite and dunite are exposed in lumpy podiform outcrops as much as 100 m across. Garnet peridotite contains as much as 75 percent garnet in an amphibolitic matrix and is found as pods related to quartzite in the gneiss of Syringa. Exposures are near to and structurally below the southwestern extent of the Lowell thrust fault. Several more ultramafic bodies lie 2 20 km northwest of the study boundary in an area not mapped in detail [Anderson, 1934]. On the basis of the characteristics and setting of these bodies between the gneisses of Syringa and Elk City and those of Swiftwater and Coolwater, they are structural slivers. 4. Geochronology 4.1. Methods [30] Zircons were extracted from rock samples (about 5 10 kg) by standard mineral separation techniques, including crushing, grinding, Wilfley table, magnetic separator, and heavy liquids. Zircons from metasedimentary rocks were randomly sprinkled onto double-stick tape prior to mounting. Zircons from igneous rocks were hand-picked onto doublestick tape, mounted in epoxy, ground to approximately halfthickness, and polished with 6 mm and1mm diamond suspension. All grains were imaged in transmitted and reflected light on a petrographic microscope, and in cathodoluminescence (CL) on a scanning electron microscope. [31] Zircons were dated using SHRIMP II and SHRIMP-RG at the Research School of Earth Sciences, Australian National University and the USGS/Stanford SHRIMP-RG at Stanford University. Analytical procedures follow the methods described by Williams [1998]. For zircon from igneous rock samples, SHRIMP U- analysis consisted of 6 cycles through the mass stations, whereas detrital zircons were measured for 4 cycles. A typical analytical spot, excavated by the primary oxygen beam operated at 3 6 na, is about mm in diameter and mm in depth. Raw data were reduced and plotted using the Squid 1 and Isoplot 3 programs of Ludwig [2001] and Ludwig [2003], respectively. / 238 U measured ratios were referenced against zircon standard R33 (419 ± 1 ) [Black et al., 2004]. Ages of samples (cited at 95% confidence limits) are weighted averages of selected / 238 U ages. For relative probability plots, we use 207 / ages for grains older than 1550 and / 238 U ages for younger grains. Only data that are less than 10% discordant are used for relative probability plots Metasedimentary Rock Samples Gneiss of Elk City [32] Zircons in the gneiss of Elk City (01KL112, Figure 3) were derived entirely from Proterozoic sources ranging in age from about 1.4 to 1.8 Ga (Table 1 and Figure 6a). Five grains are younger than Of these five, the weighted average of four of the grains is 1427 ± 17 (MSWD = 2.1). The slightly high MSWD suggests that the four ages may not comprise a coherent age group. [33] The time of deposition of the gneiss of Elk City is constrained to be between about 1425 (age of the youngest group of detrital zircons) and 1370 (age of granite augen gneiss that intruded the gneiss of Elk City). These data indicate that this unit is significantly younger than the to Prichard Formation [Anderson and Davis, 1995; Sears et al., 1998] with which it has previously been correlated. The Paleoproterozoic and Archean detrital zircons reflect provenance from Laurentian basement such as that presently exposed in southwestern Montana Gneiss of Syringa [34] Zircons from a quartzite unit in the gneiss of Syringa (03KL017, Figure 3) are primarily about , plus three grains are about 2.6 Ga and one grain is about 2.97 Ga (Table 1 and Figure 6b). ny analyses are discordant and were excluded from the relative probability plot. Additionally, three metamorphic rims yielded ages of about (Table 1). [35] These new geochronologic data show that the gneiss of Syringa is significantly younger than previously thought. Five analyses with / 238 U ages between about 775 and 900 indicate that the maximum age for this rock is middle Neoproterozoic; the only direct minimum age for these rocks is provided by Late Cretaceous crosscutting plutons. Ages of metamorphic overgrowths indicate that this rock was metamorphosed in the Late Cretaceous. [36] The older detrital zircon grains ( ) ultimately were derived from Laurentian (Proterozoic and Archean) basement rocks but may have been recycled from regionally extensive Mesoproterozoic strata. The peak at 1450 represents grains coming from Mesoproterozoic strata. The peaks at 1050 and 1185 define sources from rocks bearing late Mesoproterozoic zircons, possibly from rocks metamorphosed at 1.1 Ga about 80 km north [Sha et al., 2004; Vervoort et al., 2005]. Significantly, similar zircon populations are common in Neoproterozoic-Cambrian miogeoclinal (Rodinia rift margin) rocks of the Canadian Cordillera [Ross et al., 2005] Gneiss of Swiftwater Creek [37] Zircons from the gneiss of Swiftwater Creek (03KL016, Figure 3) were derived primarily from Phanerozoic sources, mostly dated at 100 to 225 (Table 1 and Figure 6c). The few Neoproterozoic and Mesoproterozoic grains indicate lesser derivation from Laurentian sources. ny zircons from the quartzite contain large dark (in CL), unzoned metamorphic overgrowths (Figure 6c, inset). / 238 U ages of 13 overgrowths yield ages between about 68 and 78 ; one overgrowth is about 90. Age data for several analyses of overgrowths are relatively imprecise because they contain low U (see large error ellipses in Figure 6c). Using the Unmix routine of Isoplot [Ludwig, 2003], the ages of metamorphic overgrowths can be divided into two populations, 70.3 ± 1.2 and 77.6 ± 2.9. [38] Our data show that the gneiss of Swiftwater Creek (previously correlated with the more than Wallace Formation) is a Late Cretaceous unit. The time of deposition 9of32

10 Table 1. SHRIMP U-Th- Data for Detrital Zircon From Metasedimentary Rocks, Western Idaho Sample a Measured 204 Measured 207 % Common U, ppm Th/U b 238 U, 207 b, % Discordant d 03KL017 (Gneiss of Syringa) R R R R R of b U % 207 b %

11 Table 1. (continued) Sample a Measured 204 Measured 207 % Common U, ppm Th/U b 238 U, 207 b, % Discordant d KL112A (Gneiss of Elk City) b U % 207 b % 11 of 32

12 Table 1. (continued) Sample a Measured 204 Measured 207 % Common U, ppm Th/U b 238 U, 207 b, % Discordant d KL016 (Gneiss of Swiftwater Creek) a Analytical sessions: 03KL017 analyzed on SHRIMP I, Research School of Earth Sciences, Australian National University (RSES)-4/03; 03KL016 analyzed on SHRIMP-RG, USGS/Stanford-10/03; 01KL112 analyzed on SHRIMP-RG, RSES-4/03; R (rim). b Corrected for common. The / 238 U ages corrected for common using the 207 -correction method; 207 / and 238 U/ corrected for common using the 204 -correction method. Only 207 / ages >1000 are listed. c A 1-sigma error. 238 b U % 207 b % 12 of 32

13 Figure 6. Tera-Wasserburg plots of SHRIMP U- isotopic data for metamorphic rocks. (a) Gneiss of Elk City, (b) gneiss of Syringa, and (c) gneiss of Swiftwater Creek. Unfilled ellipses in Figures 6a and 6b are grains determined to be discordant. 13 of 32

14 Figure 7. Relative probability plot for detrital zircons from samples of metasedimentary rocks showing distinctly different age distributions, reflecting differences in provenance and depositional age. of the gneiss is constrained to be between about 98 (age of the youngest detrital zircon) and 86 (age of the crosscutting Coolwater Ridge orthogneiss, see below). Additionally, the detrital zircon data indicate that the provenance of the gneiss was predominantly Late Triassic to Early Cretaceous island-arc rocks with minor contributions from Proterozoic Laurentian sources Summary of Metasedimentary Rocks [39] In summary, detrital zircons from three metasedimentary units in western Idaho have distinctly different age distributions (Figure 7), reflecting significant differences in provenance and depositional age. The gneiss of Syringa probably was deposited after 775 (possibly equivalent to the younger part of the Windermere Supergroup), primarily contains Ga (plus minor early Proterozoic and Archean) zircons, and was derived from Laurentian sources. The gneiss of Elk City was deposited at about 1.4 Ga, primarily contains Ga zircons (plus a minor earliest Paleoproterozoic component), and was derived from Laurentian sources. The gneiss of Swiftwater Creek was deposited in the Late Cretaceous (between about 86 and 98 ), primarily contains 100- to 225- grains (plus minor Ga components). It was derived mostly from oceanic terrane sources currently west of the Salmon River suture but also contains minor input from Laurentian sources Intrusive Rock Samples Augen Gneiss of Apgar Creek [40] The augen gneiss of Apgar Creek is an elongated plutonic suite located about 20 km east of the Salmon River suture (01KL107, Figure 3). Toth and Stacey [1992] obtained highly discordant (20 50%) multigrain thermal ionization mass spectrometry (TIMS) data that yielded an upper intercept age of 1751 ± 63 (MSWD = 53), interpreted as the time of crystallization. The large MSWD was considered to be indicative of multiple Phanerozoic disturbances to the isotopic systematics, causing the lower intercept age of 24 ± 140 to be geologically meaningless. The augen gneiss was interpreted as part of the Paleoproterozoic basement to the Idaho batholith [Toth and Stacey, 1992]. [41] Zircons from augen gneiss of Apgar Creek are euhedral, colorless, and contain fine concentric, oscillatory zoning in CL (Figure 8a). Some grains contain small, partially resorbed inherited cores. Twelve (of thirteen) analyses yield an age of 94 ± 1 (Figure 8b). One analysis is slightly younger, probably owing to minor loss (Table 2). Two inherited cores yield ages of about Ga, indicative of derivation from Laurentian sources. [42] Our SHRIMP data indicate that the protolith of the gneiss of Apgar Creek crystallized in the Late Cretaceous, and therefore is an early phase of the Idaho batholith. The occurrence of Paleoproterozoic cores within zircons from the augen gneiss (now documented by our SHRIMP analyses) resulted in mixed (i.e., older) TIMS ages Coolwater Ridge Orthogneiss [43] The Coolwater Ridge trondhjemite orthogneiss, located about 15 km east of the Salmon River suture, was previously dated by multigrain TIMS U- analyses, resulting in an age of 94 ± 1.4 [Toth and Stacey, 1992]. Those data show an inheritance pattern that can be interpreted either that (1) this pluton has a much lower proportion of inheritance from Mesoproterozoic sources than other nearby plutons (as concluded by Toth and Stacey) or (2) the age of inheritance is younger than Mesoproterozoic and, thus, possibly derived from different sources than other nearby plutons. During the course of our study, zircons from three samples of the orthogneiss were analyzed: (1) trondhjemitic orthogneiss containing leucosomes (01KL113, Figure 3), (2) trondhjemitic orthogneiss devoid of macroscopic leucosomes (sample 05KL095), and (3) leucosome material (06KL078). [44] Zircons from the Coolwater Ridge orthogneiss are euhedral and prismatic (Figure 9a). Most zircons show concentric oscillatory zoning in CL; some grains have parallel oscillatory zoning. Nearly all grains have overgrowths of various shades (light to dark gray in CL) and size (thick to thin). We primarily analyzed core zones to determine the crystallization age of the orthogneiss protolith, plus overgrowths to provide constraints on the timing of metamorphism. [45] A total of six SHRIMP analytical sessions (on three samples) were conducted to obtain a very large data set (130 analyses total) for the Coolwater Ridge orthogneiss (Table 2). Originally, coarse zircons from 01KL113 were analyzed during three sessions (72 analyses; 66 cores, 6 overgrowths). Although the cores generally show seemingly simple oscillatory zoning, the ages range through a continuum from about 84 to 263 (Figure 9b). It is possible that this age range is due to a combination of factors including inheritance, magmatic crystallization, and 14 of 32

15 Figure 8. Tera-Wasserburg plots of SHRIMP U- isotopic data and cathodoluminescence (CL) images of representative zircon from intrusive rocks. (a) CL images of zircon from augen gneiss of Apgar Creek, (b) Tera-Wasserburg plot for augen gneiss of Apgar Creek, (c) CL images of zircon from orthogneiss of Andys Hump, and (d) Tera-Wasserburg plot for orthogneiss of Andys Hump. metamorphic overgrowths. Because no obvious age groupings are evident from this data set, we dated coarse zircons (21 analyses) from leucosome-free orthogneiss (sample 05KL095), attempting to avoid ages younger than the crystallization age of the protolith of the orthogneiss. Most analyses of these zircons are in the range of about 85 to 110, similar to data from sample 01KL113, again with no apparent age groupings (Figure 9b). Assuming that inheritance is more likely to occur in coarser grains (igneous zircon forms around xenocrystic seeds crystals), we dated a population of fine-grained zircons (17 analyses) from orthogneiss sample 01KL113. Six analyses of areas composed of fine, concentric, oscillatory zoning (Figure 9c) yield a weighted average age of 86.2 ± 0.8 (Figure 9d). Other zones have older ages (about 90 to 200 ). [46] Two possible interpretations of age data from the fine-grained zircons are that the 86- age represents: (1) the time of crystallization of the orthogneiss protolith, or (2) the time of formation of the leucosomes (i.e., postcrystallization metamorphism). To determine the validity of either hypothesis, we obtained zircons from light-colored leucosomal material carefully separated in the field and lab from darker trondhjemitic orthogneiss (Figure 5b). Zircons from the leucosomes are characterized by a partially resorbed core, dark (in CL), irregular, thin inner overgrowth, and light (in CL) broad outer overgrowth that forms the external euhedral morphology (Figure 9e). Seven ages from cores are about , similar to coarser grains from the Coolwater orthogneiss. Three analyses of inner overgrowths (see example in Figure 9e) are 76.1 ± 1.2, whereas of 32

16 Table 2. SHRIMP U-Th- Data for Zircon From Intrusive Rocks, Western Idaho Sample a Measured 204 Measured 207 % Common U, ppm Th/U b 238 U, 01KL107 (Augen Gneiss of Apgar Creek) KL046 (Orthogneiss of Andys Hump) KL113 (Coolwater Ridge Orthogneiss) session R R session of b, 238 y U % 207 b %

17 Table 2. (continued) Sample a Measured 204 Measured 207 % Common U, ppm Th/U b 238 U, 19.2R R R session session 4 (fine-grained zircons) of b, 238 y U % 207 b %

18 Table 2. (continued) Sample a Measured 204 Measured 207 % Common U, ppm Th/U b 238 U, KL095 (Coolwater Ridge Orthogneiss) R R R R R R R R R KL078 (Coolwater Ridge Orthogneiss Leucosome) 1.1OR OR OR IR? IR OR OR OR OR OR OR OR IR OR OR a Analytical sessions: 03KL046 and 06KL078 analyzed on SHRIMP-RG, USGS/Stanford (11/03 and 8/06, respectively); 01KL107 and 01KL113 analyzed on SHRIMP II and SHRIMP-RG, Research School of Earth Sciences, Australian National University (4/03); R (rim), IR (inner rim), OR (outer rim). b Corrected for common (except samples labeled R). The / 238 U ages corrected for common using the 207 -correction method; 207 / and 238 U/ corrected for common using the 204 -correction method. Only 207 / ages >1000 are listed. c A 1-sigma error. 207 b, 238 y U % 207 b % analyses of outer overgrowths are 64.1 ± 1.2 (Figure 9f). Although it is possible that the 76- ages are actually mixtures of age components from orthogneiss protolith (86 ) and overgrowths (64 ), CL imaging enabled precise location of the primary ion beam on zones that appear to be homogeneous. [47] In summary, the trondhjemitic protolith of the Coolwater orthogneiss crystallized at 86.2 ± 0.8. The 18 of 32

19 Figure 9 19 of 32