Detrital-Zircon Geochronology of the Sawtooth Metamorphic Complex, Idaho: Evidence for Metamorphosed Lower Paleozoic Shelf GEOSPHERE; V

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Research Paper THEMED ISSUE: Active Margins in Transition—Magmatism and Tectonics through Time: An Issue in Honor of Arthur W. Snoke

GEOSPHERE Detrital-zircon geochronology of the Sawtooth metamorphic complex, Idaho: Evidence for metamorphosed lower Paleozoic shelf GEOSPHERE; v. 12, no. 4 strata within the Idaho batholith doi:10.1130/GES01201.1 Chong Ma1,*, Philip Bergeron2, David A. Foster1, Barbara L. Dutrow2, Paul A. Mueller1, and Chrissy Allen1 11 figures; 1 table; 1 supplemental file 1Department of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA 2Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, USA

CORRESPONDENCE: macmachong@gmail.com ABSTRACT interpretations for why lower Paleozoic shelf strata had not been deposited or CITATION: Ma, Chong, Bergeron, P., Foster, D.A., present in the Idaho batholith region (e.g., Yates, 1968; Burchfiel et al., 1992; Dutrow, B.L., Mueller, P.A., and Allen, C., 2016, Detrital-zircon geochronology of the Sawtooth meta U-Pb ages of detrital zircons from metasedimentary rocks of the Sawtooth Pope and Sears, 1997; Dickinson, 2004, 2006, 2009). More recent mapping morphic complex, Idaho: Evidence for metamor- metamorphic complex (SMC) are key for identifying the stratigraphic age of and detrital-zircon analyses of isolated exposures within and surrounding the phosed lower Paleozoic shelf strata within the Idaho metamorphosed strata within the Idaho batholith region of the Cordilleran Idaho batholith have shown that some metasedimentary rocks thought to be batholith: Geosphere, v. 12, no. 4, p. 1136–1153, doi:10.1130/GES01201.1. orogen. The SMC is an exposure of medium- to high-grade metasedimentary Mesoproterozoic or older are actually Neoproterozoic to possibly early Paleo rocks surrounded by the Idaho and Sawtooth batholiths. U-Pb ages of detrital zoic (e.g., Lund et al., 2003; Lewis et al., 2012, 2014), suggesting that Paleozoic Received 19 May 2015 zircons from SMC quartzites and quartzofeldspathic gneisses yield two dis- shelf deposits might be continuous across the region. Revision received 29 April 2016 tinctive age spectra consisting of primary 2900–2510 Ma and 1990–1760 Ma The metasedimentary rocks of the Sawtooth metamorphic complex (SMC), Accepted 27 May 2016 zircons for one group and 1870–1670 Ma, 1490–1330 Ma, and 1220–1020 Ma Idaho, are upper amphibolite to granulite-facies rocks exposed at high eleva Final version published online 12 July 2016 zircons for the other group. The suite of samples also yields a small number tions within the Atlanta lobe of the Idaho batholith (Fig. 1B; e.g., Reid, 1963; of zircons with concordant Cambrian and Neoproterozoic ages. Statistical and Dutrow et al., 1995). Recent U-Pb analyses of detrital zircon within the SMC visual comparisons of age spectra with detrital-zircon data from Proterozoic metapsammites revealed Grenville-aged zircons (Bergeron, 2012), opening the and Paleozoic strata deposited on the western passive margin of Laurentia possibility that the protoliths were Neoproterozoic or younger. In this study, we suggest that the SMC rocks were deposited in the Cambrian and Middle Ordo- present U-Pb data from detrital zircons that help define the stratigraphic age vician. The identification of lower Paleozoic shelf strata in the SMC, along with and provenance of metapsammites in the SMC, which suggest the presence of similar sections recently identified in the Stibnite and Gospel Peaks inliers lower Paleozoic shelf strata in the Idaho batholith region. of the Idaho batholith, suggest that the Cordilleran passive margin sequence was continuous along the western margin of Laurentia, including the area occupied by the Idaho batholith. PRE-CRETACEOUS COUNTRY ROCKS IN THE IDAHO BATHOLITH REGION

INTRODUCTION The country rocks to the Idaho batholith were initially assumed to be pre- Belt Supergroup (>1500 Ma) basement (Armstrong, 1975) or Mesoproterozoic The original distribution and preservation of Neoproterozoic to Paleozoic (Belt) metasedimentary rocks (e.g., Bond et al., 1978; Fisher et al., 2001). More shelf strata along the Cordilleran passive margin in the region now occupied recent studies document a northwest-trending belt of Neoproterozoic Winder by the Idaho batholith (Fig. 1A) remain major questions with important impli mere Supergroup rocks exposed as roof pendants in Cretaceous and younger cations for the Neoproterozoic rift geometries of western Laurentia and Cor granitoids of central Idaho (Lund et al., 2003; Lund, 2004; Lewis et al., 2012). dilleran margin segmentation, as well as the role of basement highs in early Lewis et al. (2010) also showed that quartzites (<2% feldspar) of the Syringa Paleozoic depositional systems (e.g., Lund, 2008; Dickinson, 2009; Yonkee metamorphic succession in north-central Idaho are likely Neoproterozoic on et al., 2014). It had been proposed that metasedimentary rocks within the the basis of detrital-zircon ages. These metamorphosed Neoproterozoic rocks Idaho batholith were metamorphosed equivalents of the Proterozoic Belt in (north-) central Idaho are generally composed of a wide range of lithologies, Supergroup (e.g., Bond et al., 1978; Fisher et al., 2001). This led to a variety of including garnet- and kyanite-bearing, micaceous, quartzofeldspathic gneiss; For permission to copy, contact Copyright amphibolite; schist; calc-silicate gneiss; marble; quartzite; diamictite; and bi Permissions, GSA, or [email protected]. *Current address: Department of Geosciences, Auburn University, Auburn, Alabama 36849, USA modal volcanic strata (e.g., Lund et al., 2003; Lund et al., 2008). The quartzites

W120° W112° Montana Sawtooth B A batholithbatholith normal fault B e 6 l Stanley t D Idaho batholith B Z Sawtooth a metamorphic complex Idaho s W in R batholith Eocene R W dikes Washington K Fig. 3 N45° WISZ Blue Mountains Idaho batholitbatholithh 5 D Quaternary Figure 1. (A) General geologic map of the n N Cordilleran orogenic belt in the western Q deposits United States showing the outcrop ex- Sawtooth batholith Fig.1B J tent of Cambrian to Ordovician strata, Idaho H 5 Km batholith modified from DeCelles (2004), Foster et al. (2007), and the U.S. Geological Sur- Snake River Plai Oregon Sr=0.706 vey digital geologic maps of the United i Idaho States. Note that some of the Cambrian– California Key for A: Ordovician outcrops include undifferenti- Nevada ated Neoproterozoic and Paleozoic strata. N41° Cambrian to Ordovician strata (B) Generalized geologic map of the Saw- tooth metamorphic complex and vicinity Cretaceous-Eocene batholith Sevier fold-thrust belt modified from Fisher et al. (2001); see the box in Figure 1A for location. DV—Death Valley; MD—Mojave Desert; WI—White- Antler/Sonoma allochthon Cordilleran hinterland Inyo Range; WISZ—western Idaho shear zone. Metamorphic core complex Major thrust

Accreted terrane Major synform

Sierra Utah Neoproterozoic to Lower Cambrian sample locations including Nevada Caddy Canyon Quartzite, Mutual Formation, Prospect Mountain Arizona batholith WI Quartzite, Geertsen Canyon Quartzite, Camelback Mountain Quartzite, Wood Canyon Formation, and Campito Formation N (Andrews Mountain member), see references in Figure 10. DV Middle Ordovician sample locations including Eureka Quartzite 200 km and Kinnikinic Quartzite, see references in Figure 10. MD

are typically meta-quartz arenites intercalated with quartzite-conglomerate SAWTOOTH METAMORPHIC COMPLEX (Lund et al., 2003; Lewis et al., 2010). In contrast, documented occurrences METASEDIMENTARY ROCKS of Cambrian–Ordovician (meta)sedimentary rocks in the Idaho batholith re gion are rare; the nearest ones to the study area are those in northern and The Sawtooth metamorphic complex (SMC) is located in the northern por east-central Idaho (Fig. 2) (Ross, 1947; Bush, 1989). Lund et al. (2003), however, tion of the Sawtooth Range, Idaho (Fig. 1B). Although glacial moraines cover suggested that the gray marble and calc-silicate gneisses of the Gospel Peaks the eastern margin, it is presumably bounded by the Sawtooth normal fault to and Stibnite sections (Fig. 2) in the Idaho batholith are Cambrian, based on the east (e.g., Reid, 1963; Thackray et al., 2013), the Cretaceous Idaho batholith their stratigraphic position above Neoproterozoic rocks. This study provides to the south and west, and the Eocene Sawtooth batholith to the north. The geochronological evidence supporting the presence of Cambrian–Ordovician SMC section was first mapped by Reid (1963) as undifferentiated Precambrian strata in the Idaho batholith region. “Thompson Peak metamorphic rocks.” Lewis et al. (2012) assigned it to the

Neoproterozoic Windermere by dikes and pods of Cretaceous and Eocene igneous rocks (Metz, 2010; Ma, Supergroup and equivalent rocks 2015). The structures within the SMC include north-south–striking domains of Colville Neoproterozoic-Cambrian(?) strata isoclinal and closed folds alternating with shear zones typical of wrench-domi Sandpoint Cambrian and Ordovician shelf strata nated transpression (Ma, 2015). Outcrops that include metapsammite in the SMC are characterized by Lakeview Cambrian to Ordovician syenite, granite, and gabbro north-south–striking compositional layers with predominantly steep dips. Photographs of typical outcrops are shown in Figure 4. Overall, the SMC Cretaceous and Paleogene batholith metapsammites contain extensively recrystallized quartz and feldspar (Metz, Cambrian and Ordovician quartzites 2010; Bergeron, 2012) with three lithotypes distinguished in this study. Type A as in Figure 1A I is the most abundant and is a quartzofeldspathic gneiss consisting of quartz (75%–85%), feldspar (10%–15%), and micas (5%–10% biotite, with minor Idaho muscovite) (e.g., Fig. 5A). Mylonitic fabrics occur locally in meter-scale shear Kgd N Gospel batholith zones within the type I metapsammites (e.g., Fig. 5B). Type II is a white, foli Table DR 1A. Sample location and lithology Washington Sample name LatitudeLongitude Lithology Peaks MC13ST-6 44°08'48.4"N 115°00'6.8"W brown quartzite (BitterrootA lobe) ated quartzite consisting of 98% quartz with <2% biotite and feldspar (e.g., Fig. MC13ST-19 44°08'56.7"N 114°59'26.3"Wmylonitic quartz-rich paragneiss MC14ST-11 44°08'41.2"N 115°01'07.9"W quartz-rich paragneiss section MC14ST-15 44°09'24.2"N 115°02'22.8"Wmylonitic quartz-rich paragneiss Oregon 5C; Metz, 2010). This type crops out as 3–10-m-thick layers within calc-silicate MC14ST-16 44°08'56.8"N 115°01'59.7"W quartz-rich paragneiss CA13ST-1 44°08’46.3”N 115°00’17.4”W white quartzite and biotite-rich quartzofeldspathic gneisses. Type III is a brown, strongly re CA13ST-4 44°08’45.9”N 115°00’17.6”W white quartzite CA13ST-5 44°08’43.3”N 115°00’25.4”W quartz-rich paragneiss 100 km crystallized quartzite with <1% biotite and occurs as large boudins (tens of CA13ST-8 44°08’53.1”N 114°59’11.1”W quartz-rich paragneiss ST11-08 44°08’46.3”N 115°00’16.9”W white quartzite ST11-02 44°08’43.9”N 115°00’42.3”W quartz-rich paragneiss meters scale) within biotite-rich quartzofeldspathic gneisses (e.g., Fig. 5D). In MB11-6144°09'29.5"N 114°59’19.0”W quartz-rich paragneiss Stibnite section MB11-4044°09'28.8"N 115°00’32.4”W quartz-rich paragneiss this study, 14 samples of the SMC metapsammites (Supplemental Table 1A)1, ST09-01 44°09'01.4"N 114°59’11.6”W quartz-rich paragneiss Montana Idaho including ten samples of the type I quartzofeldspathic gneiss (two of which are 1Supplemental Tables. Tabulations of sample loca batholith mylonitic), three of the type II white quartzite, and one of the type III brown tions and the complete U-Pb isotopic data. Please visit (Atlanta lobe) quartzite, were analyzed to determine the detrital-zircon U-Pb ages. http://dx .doi .org /10 .1130/GES01201 .S1 or the full-text article on www.gsapubs.org to view the Supplemental A SMC in Tables. la P

METHODS r Idaho Falls e v i Samples were trimmed free of veins and alteration, crushed, milled, and R

e Pocatello sieved prior to zircon separation using traditional magnetic and density-based ak Sn methods, followed by handpicking under a binocular microscope. A careful cleaning of crushing and separation facilities with high-pressure air and/or Idaho alcohol was performed prior to processing each sample to preclude cross con tamination. The zircon grains were mounted in epoxy, polished, and imaged

Figure 2. Distribution of Cambrian to Ordovician shelf strata and plutons in Idaho and Wash- using cathodoluminescence (CL) and backscattered electrons (BSE) to identify ington, modified from U.S. Geological Survey digital geologic maps of the U.S. Neoproterozoic internal structures (e.g., cores and rims) and compositional variations. U-Pb Windermere Supergroup, Syringa metasedimentary rocks, Gospel Peaks, and Stibnite sections isotopic analyses were performed on a Nu-Plasma multi-collector inductively are after Lewis et al. (2010) and Lund et al. (2003). SMC—Sawtooth metamorphic complex. coupled plasma mass spectrometer integrated with a New Wave 213 nm ultra violet Nd:YAG laser ablation system at the University of Florida following methods of Mueller et al. (2008). FC-1 zircon from the Duluth Complex with Neoproterozoic–Cambrian Windermere Supergroup on the basis of lithologic an age of 1099.0 ± 0.6 Ma (Paces and Miller, 1993; Black et al., 2003; Mattin similarity to the Gospel Peaks section (Fig. 2). The SMC lithologies present son, 2010) was used as the primary age reference for data calibration and drift include aluminous, quartzofeldspathic, calc-silicate, mafic, and migmatitic correction. R33 zircons from the Braintree Complex with an age of 419.26 ± gneisses, along with mica schist, marble, and metapsammite (Fig. 3; Dutrow 0.39 Ma (Black et al., 2004) were used as the secondary standard (Supplemen et al., 1995; Metz, 2010; Bergeron, 2012; Fukai, 2013; Ma, 2015). Mineral as tal Table 1B; see footnote 1). Two FC-1 zircons were analyzed after every ten un semblages in aluminous (para)gneisses indicate metamorphic conditions knowns. Both standards and unknowns were ablated for 30 seconds (300 laser consistent with transitional granulite facies (e.g., Dutrow et al., 1995; Metz, shots) with identical laser and mass spectrometer parameters. Drift corrections 2010; Dutrow et al., 2013). Most SMC paragneisses have been intensely and were made by linear bracketing with the FC-1 standards. Data were reduced pervasively deformed by folding, thrusting, and dextral shearing and intruded using an in-house Excel spreadsheet and plotted using Isoplot (Ludwig, 2012).

Quaternary biotite-rich sample Q bg mb marble qg quartzofeldspathic 115°02′30″ cover quartzofeldspathic gneiss location 115°00′ AL F ORE ST gneiss TI ON leucogranitic ms mica schist 0 0 li strike-slip fault 0 0 9 intrusion mst metasiltstone 4 4 20 0 9 9 mps Idaho batholith ag amphibole gneiss thrust fault, teeth mps metapsammite granitoid on hanging wall 9966 0000 gi gabbroic intrusion cs calc-silicate gneiss gbg garnet-biotite gneiss magmatically foliated Q Q 900 granitoid mb

0 Marshal 8 940 8

92 0 0

2 00 2

bg 0 0 gi 0 88qg 0 Lak 8 e

0 cs 0 0 66

0 0 4 0 9 9 00 0 00 0 N44°09′30″

N44°09′ 0 88 9 bg 0 bg bg 00 (A) cs 0 MB11-61 0 MB11-40 MC14ST-15 0 mps li 9 cs

mps 9

30 ″ mps

0 ms 0 qg ms gbg Q mst Q cs bg Q 0 mps 1 bg 0 Q 0 ms 0 2ms qg 0 2 8 0 9 0 800 4 ms 9 1 0 0 0 0 Q 8 6 0 mps 4 li 9 0 0 9 qg 0 4 0 00 2 W i ams bg 9 Williams Peak cs bg ST09-01 0 cs 9 Pe k 00 ms 6 bg 9000 8 0 bg ag Idaho batholith mps 0 bg Q Q C mps mb cs 00 ms bg gi H 94 A ag 00 li MC14ST-16 L 98 L bg ag (C) CA13ST-8 I qg m S MC13ST-18 qg ps 98 MC13ST-19 li N 00 CA13ST-1 bg AMC14ST-12A qg gbg Q li T N44°08′45″ I qg cs O (F) N44°08′45″ N MC13ST-6 N qg CA13ST-5 0 A 0 ST11-080 bg ST11-02 0 0 qg qg L 2 MC14ST-11 ms 9 9 mb F qg qg CA13ST-4 O 0 Q R Q cs E mps S 90 0 200 0 00 T mpson 8 0 cs 80

2 8

Idaho batholith 0 8

9 8 6

Meters 6

qg 0 0

0 0 0

ag bg 0 bg cs0 mps

0 0 1 00 0 Thompson Peak 0 44 0 0 0 0 bg 0 9 0 0 8 0 0 mb 0 0 0 2 8 0 2 2 9 9 115°02′30″ 9 0 115°00′ 1

Figure 3. Simplified geologic map of the Sawtooth metamorphic complex showing sample locations, see the box in Figure 1B for map location. Intrusive dikes of the Challis volcanics and plutons of the Eocene Sawtooth batholith are not shown. Italic (A), (C), and (F) represent locations of the photos (A), (C), and (F) in Figure 4. The base map is from the 7.5 min quadrangle series produced by the U.S. Geological Survey; the geologic mapping was conducted by Chong Ma and assistants (Sutie Xu and Chrissy Allen) on the basis of previous mapping (Metz, 2010; Bergeron, 2012).

Age distributions are displayed as kernel density estimations (KDEs) utilizing applied to sample sets with less than 300 entries, regardless of the method used. DensityPlotter (Vermeesch, 2012). Tabulations of sample locations and the In this study, none of the SMC samples yielded >300 analyses with ages <5% complete U-Pb isotopic data are in the Supplemental Tables (see footnote 1). (206Pb/238U ages) or <10% (207Pb/206Pb ages) discordance. With this caveat, we Quantitative comparisons of detrital-zircon age spectra can be done in sev provide results of three methods of quantitative comparison. The Kolmogorov- eral ways, but the success of these tests that are commonly used for comparing Smirnov (K-S) test provides a test of the possibility that two sample sets were detrital zircons (e.g., Gehrels, 2012; Saylor and Sundell, 2016) is dependent on derived from the same population based on the total spectrum of ages and the sizes of the respective data sets in a nonlinear way. Saylor and Sundell (2016) errors from each sample by comparing cumulative probability curves (e.g., suggested that the reliability of such comparisons is difficult to constrain when Press et al., 1986; DeGraaff-Surpless et al., 2003; Sircombe and Hazelton, 2004).

A B

bt-qfgs

mps (type II) mps (type I)

bt-qfg

C mps (type II) D

mps (type III)

mps (type I)

E F

mylonitic mps (type I) mps (type I)

Figure 4. Outcrops of typical metapsammites in the Sawtooth metamorphic complex. The outcrop in (B) is the locality for sample CA13ST-4; (D) is for MC13ST-6; and (E) is for MC13ST-19. Locations of the outcrops in (A), (C), and (F) are marked in Figure 3. Type I—quartzofeldspathic gneiss; type II—white quartzite; type III—brown quartzite; bt-qfg—biotite-rich quartzofeldspathic gneiss; mps—metapsammite. Person, trekking pole, notebook, pen, and hammer for scales. The front outcrop in (A) is ~20 m wide.

A B

5 mm Groups A, B 5 mm Groups A, B Figure 5. Photomicrographs of lithotypes of the Sawtooth metamorphic com- C D plex quartzites and quartzofeldspathic gneisses, including (A) quartzofeldspathic gneiss (sample MC14ST-16); (B) quartzo feldspathic gneiss with mylonitic fabrics (MC13ST-19); (C) white, foliated quartzite (CA13ST-1); and (D) brown quartzite (MC13ST-6). (E) Cathodoluminescence images of typical detrital zircons contain- ing cores and overgrowths from multiple samples. Groups A and B represent clas- sifications of detrital-zircon age spectra as in Figure 6.

Groups A, B 5 mm Group A 5 mm MC13ST-6 100 µm 100 µm MC13ST-19 E

1811 1858 2662 1065 2148 2659

The primary output of the K-S test is the probability, P value, which indicates test for the SMC samples are presented in Table 1, and those for comparisons the likelihood that two suites of detrital zircons were drawn randomly from of the SMC sample age spectra to selected detrital-zircon data from non-SMC the same parent population, interpreted here as provenance, at a specific con localities in the Cordillera are shown in Supplemental Table 1C (see footnote fidence level. A P value >0.05 suggests that the observed age spectra of two 1). It is important to note, however, that the magnitude of the P value is not di groups of zircons could have been derived from random sampling of the same agnostic of relative probability, i.e., a P value of 0.07 does not indicate a higher parent population (provenance) at the 95% confidence level. Results of the K-S probability of the test being successful than a value of 0.06 because the K-S

TABLE 1. STATISTICAL K-S, OVERLAP, AND SIMILARITY COMPARISONS OF DETRITAL ZIRCON AGES BETWEEN THE SAWTOOTH METAMORPHIC COMPLEX METAPSAMMITE SAMPLES Samples CA13ST-5 ST11-08 MC13ST-6 MC14ST-15 MC13ST-19 CA13ST-1 MB11-40 ST11-02 MB11-61 ST09-01 MC14ST-11 K-S test P values ST11-08 0.233 MC13ST-6 0.538 0.148 MC14ST-15 0.889 0.164 0.337 MC13ST-19 0.000 0.000 0.000 0.000 CA13ST-1 0.000 0.000 0.000 0.000 0.989 MB11-40 0.000 0.000 0.000 0.000 0.3020.583 ST11-02 0.000 0.000 0.000 0.0000.001 0.0090.025 MB11-61 0.000 0.000 0.000 0.0000.010 0.0110.034 0.976 ST09-01 0.000 0.000 0.000 0.000 0.0600.062 0.0550.775 0.853 MC14ST-11 0.000 0.000 0.000 0.0000.029 0.0530.160 0.5580.736 0.732 MC14ST-16 0.000 0.000 0.000 0.0000.041 0.044 0.0620.339 0.7610.806 0.580 Overlap ST11-08 0.488 MC13ST-6 0.464 0.594 MC14ST-15 0.472 0.361 0.356 MC13ST-19 0.281 0.190 0.136 0.340 CA13ST-1 0.251 0.157 0.129 0.2230.535 MB11-40 0.139 0.035 0.000 0.1780.364 0.433 ST11-02 0.284 0.216 0.179 0.3050.591 0.4410.277 MB11-61 0.299 0.168 0.151 0.4660.630 0.5680.408 0.512 ST09-01 0.241 0.137 0.081 0.2420.624 0.4680.468 0.4810.559 MC14ST-11 0.372 0.283 0.191 0.3310.630 0.4640.302 0.4780.519 0.545 MC14ST-16 0.298 0.191 0.166 0.3330.627 0.4790.297 0.5620.539 0.4800.718 Similarity ST11-08 0.562 MC13ST-6 0.542 0.646 MC14ST-15 0.525 0.455 0.499 MC13ST-19 0.231 0.130 0.124 0.156 CA13ST-1 0.239 0.188 0.127 0.2170.698 MB11-40 0.101 0.036 0.000 0.0640.539 0.578 ST11-02 0.345 0.252 0.239 0.3660.660 0.5370.372 MB11-61 0.270 0.144 0.140 0.3510.670 0.5390.462 0.562 ST09-01 0.207 0.148 0.070 0.2100.574 0.4380.451 0.5610.550 MC14ST-11 0.341 0.345 0.233 0.3050.683 0.6140.520 0.5930.603 0.603 MC14ST-16 0.260 0.259 0.161 0.2810.700 0.6570.452 0.6990.618 0.5930.730 Note: In the Kolmogorov-Smirnov (K-S) test P values section, bold type values indicate sample pairs passed the K-S test (P values >0.05), and normal type values indicate those not passed. The Excel macros of the Arizona LaserChron Center were utilized to generate the table.

test is a form of null hypothesis test. As a consequence, the absolute value of P is not 20% more likely to indicate similarity than a value of 0.5. Results of the cannot be used to indicate that any proposed relationship is more or less likely overlap and similarity tests are presented in Table 1 for the SMC samples and to be correct than the probability assigned to the test initially. We also utilized Supplemental Table 1C (see footnote 1) for comparisons of the SMC and those the overlap and similarity functions described by Gehrels (2000). Although not non-SMC samples in the Cordillera. null-hypothesis–based tests, the values generated (from 0 to 1, no match and For the presumed metamorphic overgrowths on detrital zircons analyzed perfect match, respectively) are also not surrogates for the relative probability in this study, the TuffZirc algorithm of Isoplot (Ludwig, 2012) was utilized to of the samples being from the same parent population or provenance, i.e., 0.6 calculate the growth ages. The TuffZirc algorithm calculates a median age of

the largest coherent (statistically within analytical error, 2s) group of zircons dant, respectively, in sample MC13ST-19, and 632 ± 9 Ma, 2s, 1.5% discordant and interprets it as the true age of the population with an asymmetric uncer in sample MC14ST-16). Two samples also contain Tonian zircons (742 ± 21 Ma, tainty derived from the 95% confidence errors of the median206 Pb/238U ages 2s, 0.1% discordant in sample ST11-02 and 892 ± 13 Ma, 933 ± 13 Ma, 999 ± (Ludwig, 2012). 66 Ma, 2s, 3.2%, 2.0%, –1.9% discordant, respectively, in sample MC14ST-11). The combined zircons from Group B samples show major age concentrations of 1870–1670 Ma (cumulative 27%), 1490–1330 Ma (cumulative 25%), and RESULTS 1220–1020 Ma (cumulative 37%), along with nine Neoproterozoic–Cambrian zircons (Fig. 8B). Detrital Zircons Overall, the results show that Group A comprises dominantly Meso-Neo archean (ca. 2900–2510 Ma) and Paleoproterozoic (ca. 1990–1760 Ma) ages Detrital zircons display both euhedral and anhedral external morpholo (Fig. 8A), and Group B is dominated by Paleoproterozoic (ca. 1870–1670 Ma), gies. Most zircon interiors are rounded and show faint oscillatory zoning (e.g., early Mesoproterozoic (ca. 1490–1330 Ma), and late Mesoproterozoic (ca. Fig. 5E). Interiors of 1601 zircons were analyzed, resulting in 552 analyses 1220–1020 Ma) ages (Fig. 8B). Two samples, CA13ST-4 (type II lithology) and that are less than 10% or 5% discordant based on 206Pb/238U versus 207Pb/206Pb CA13ST-8 (type I), yield high proportions of discordant grains so are not as ages for grains older than 1000 Ma and 206Pb/238U versus 207Pb/235U ages for signed to groups or plotted. Only two (578 ± 11 Ma, 3002 ± 14 Ma, 2s) out of grains younger than 1000 Ma, respectively. Analyses that show greater than 100 and three (1863 ± 8 Ma, 2698 ± 6 Ma, 2768 ± 7 Ma, 2s) out of 137 zircons 10% or 5% discordance are omitted from the following discussion and are not from those two samples, respectively, meet the concordance criteria (<10% for shown in the plots. Ten out of the 552 least discordant analyses give ages ages >1000 Ma or <5% for ages <1000 Ma). There is not a significant relation younger than 1000 Ma (Supplemental Table 1D [see footnote 1]). Six of the ship between the two groups of age spectra and lithology. Quartzofeldspathic 14 metapsammitic samples analyzed contain at least one grain younger than gneisses (type I lithology) and white quartzites (type II lithology) are repre 1000 Ma (Fig. 6). sented in both groups. Samples CA13ST-5 (type I lithology), ST11-08 (type II), MC13ST-6 (type III), and MC14ST-15 (mylonitic type I) all contain a major concentration of ages at Detrital-Zircon Overgrowths ca. 1800 Ma and a secondary concentration at ca. 2700 Ma (Fig. 6A). K-S evalu ations of age spectra of detrital zircons from all SMC samples show that these Cathodoluminescence and BSE images show subrounded cores with meta four samples are similar to each other but distinct from the rest, as shown by morphic overgrowths up to ~100 mm wide for many detrital zircons (e.g., Fig. the cumulative age-probability curves (Fig. 7). Results of the K-S test suggest 5E). For example, >30% of the zircons analyzed from samples MC13ST-6 and that the sources of these four samples are indistinguishable at the 95% confi MC13ST-19 contain overgrowths wider than 30 mm. In CL images, the over dence level, because the P values are all larger than 0.05 (0.148–0.889; Table 1). growths are commonly zoned and generally darker than their corresponding These four spectra (Fig. 6A), therefore, are classified as Group A. Results of the cores (Figs. 9A and 9B). Fifty-five zircon overgrowths from samples MC13ST-6 overlap and similarity tests (Table 1) also support this classification. The com and MC13ST-19 yield 206Pb/238U ages with less than 5% discordance based on bined results from Group A samples yield age concentrations of 2900–2510 Ma 206Pb/238U versus 207Pb/235U ages. (cumulative 29%) and 1990–1760 Ma (cumulative 56%), with a subordinate Sample MC13ST-19 is from a strongly sheared metapsammite that contains group at 2120–2040 Ma (cumulative 6%) (Fig. 8A). winged feldspar porphyroclasts and S-C fabrics indicative of dextral strike-slip Zircons from eight samples, MB11-40, ST11-02, MB11-61, ST09-01, MC14ST-11, shearing. Quartz, feldspar, and biotite are highly recrystallized and/or sheared. MC14ST-16 (type I lithology), along with MC13ST-19 (mylonitic type I) and Twenty-six overgrowths yield 206Pb/238U ages of ca. 85.3–117.0 Ma, and 13 of CA13ST-1 (type II), are all dominated by ages clustered near ca. 1800 Ma, ca. those give a TuffZirc age of 89.2 +1.5/–0.8 Ma and a concordia intercept age of 1400 Ma, and ca. 1100 Ma (Fig. 6B). These age spectra produce similar, but not 89.5 ± 0.7 Ma (mean square of weighted deviates [MSWD] = 0.52) (Fig. 9A). Sam identical, cumulative age-probability curves (Fig. 7) and therefore are classi ple MC13ST-6 is from a tens of meters–scale boudin of metapsammite within fied as Group B. The slight variations in the age-probability curves are likely biotite-rich quartzofeldspathic gneiss. Quartz in this sample is strongly recrys caused by the sampling statistics of different numbers of grains available for tallized. Twenty-nine overgrowths give 206Pb/238U ages of ca. 79.7–98.4 Ma, and individual samples due to discordance. Results of the overlap and similarity 15 of those yield a TuffZirc age of 91.8 +1.1/–0.4 Ma and a concordia intercept tests (Table 1) support this classification as well. The Group B samples are the age of 91.8 ± 0.5 Ma (MSWD = 1.3) (Fig. 9B). As shown in the concordia plots, only ones to have Paleozoic or Neoproterozoic zircons. Two samples contain minimal inheritance of older material and minor Pb loss might have occurred Cambrian zircons (496 ± 10 Ma, 2s, 0.8% discordant in sample MB11-40 and for some of the zircon overgrowths in samples MC13ST-6 and MC13ST-19 re 529 ± 18 Ma, 2s, 1.8% discordant in sample ST11-02). Two samples contain spectively. Therefore, the TuffZirc ages that have excluded some grains likely Ediacaran zircons (542 ± 12 Ma and 605 ± 14 Ma, 2s, –1.4% and 4.5% discor involved inheritance or Pb loss are better estimations of the true ages.

A Group A B Group B

10 ST11-02 15 7

s (n=61/149) CA13ST-5 5 litho-type I MB11-40 11 (n=50/101) (n=17/48) 2 litho-type I 0 litho-type I 7 0.4 0.7 1.01.3 1.61.9 2.22.5 2.83.1 1.11.4 1.8 3

Number of analyse 6 MB11-61 0 4 6 (n=32/69) 2 1.0 1.3 1.6 1.9 2.2 2.5 2.8 3.1 4 0 litho-type I 1.8 2.7 2 14 0.4 0.7 1.0 1.3 1.6 1.92.2 2.52.8 0 1.1 1.4 1.8 0.4 0.7 1.01.3 1.61.9 2.22.5 2.83.1 1.11.4 1.8 10 ST11-08 (n=54/158) Figure 6. Kernel density estimation (KDE) plots of detrital-zircon ages for the Saw- 7 litho-type II MC13ST-19 tooth metamorphic complex metapsam- 17 (n=61/149) mites classified into Groups A (A) and B 3 (B). Only 207Pb/206Pb ages <10% discordant litho-type I, 206 238 ST09-01 and Pb/ U ages <5% discordant are 0 11 mylonitic plotted; discordance based on 206Pb/238U (n=21/61) versus 207Pb/235U for ages <1000 Ma and 1.0 1.3 1.6 1.9 2.2 2.5 2.8 3.1 litho-type I 206 238 207 206 6 Pb/ U versus Pb/ Pb for ages 1.8 2.7 5 4 >1000 Ma. Circles under each KDE plot are 2 the distribution of ages without errors. 0 Each KDE curve is normalized so that MC13ST-6 0 every curve contains the same area; verti- 12 0.4 0.7 1.01.3 1.61.9 2.22.5 2.83.1 cal scale is also normalized based on histo- (n=38/140) 0.4 0.7 1.0 1.3 1.6 1.92.2 2.52.8 1.11.4 1.8 litho-type III grams (same for the KDE plots in Fig. 10). 9 1.1 1.4 1.8 16 n = number of <10% or 5% discordant analyses out of the total analyzed. 6 12 MC14ST-11 (n=81/140) 3 8 litho-type I 0 CA13ST-1 (n=33/113) 4 1.0 1.3 1.6 1.9 2.2 2.5 2.8 3.1 11 litho-type II 1.8 2.7 0

7 0.4 0.7 1.01.3 1.61.9 2.22.5 2.83.1 1.11.4 1.8 MC14ST-15 3 14 (n=22/125) 0 litho-type I, 10 MC14ST-16 6 mylonitic 0.4 0.7 1.0 1.3 1.6 1.92.2 2.52.8 7 (n=77/139) 4 1.1 1.4 1.8 litho-type I 3 2 Age (Ga) 0 0 0.4 0.7 1.01.3 1.61.9 2.22.5 2.83.1 1.0 1.3 1.6 1.9 2.2 2.5 2.8 3.1 1.11.4 1.8 1.8 2.7 Age (Ga) Age (Ga)

1.0 Samples: CA13ST-5 0.8 ST11-08 Group A MC13ST-6 MC14ST-15 0.6 CA13ST-1 Age Probability MB11-40 Figure 7. Cumulative age-probability plots 0.4 MC13ST-19 derived from detrital-zircon ages of the ST11-02 Sawtooth metamorphic complex samples. Group B MB11-61 0.2 ST09-01 Cumulative MC14ST-11 MC14ST-16 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Age (Ga)

DISCUSSION the detritus and presence of zircon derived from igneous rocks formed and exposed near the time of deposition (e.g., Fedo et al., 2003; Andersen, 2005; Discordance of the U-Pb Ages Dickinson and Gehrels, 2009; Cawood et al., 2012). Four of the SMC samples analyzed contain zircons with concordant Cambrian or latest Neoproterozoic More than half of the detrital zircons analyzed from the SMC samples are ages (ca. 496 Ma, ca. 529 Ma, ca. 542 Ma, and ca. 578 Ma) (Supplemental Table discordant at the 5% (<1000 Ma) or 10% (>1000 Ma) levels, respectively. U-Pb 1D [see footnote 1]), indicating that they were deposited no earlier than the data from the overgrowths on detrital zircons from two samples give Cre Cambrian or the Neoproterozoic–Cambrian boundary. Six of the samples have taceous ages. The overgrowths are generally dark in CL images suggesting maximum depositional ages less than 1100 Ma, while results from three other that they are relatively high in uranium (Rubatto and Gebauer, 2000). Oscilla samples indicate deposition after 1800 Ma. tory zoning and euhedral forms observed in CL images for many of the wide Lower Cambrian and Middle Ordovician quartzites from the passive mar (>30 mm) overgrowths (e.g., Figs. 9A and 9B) indicate a fluid-rich environment gin of western Laurentia in Nevada, Utah, and California commonly show (Corfu et al., 2003). Such environment could facilitate growth of euhedral crys age spectra dominated (>99%) by Mesoproterozoic and older detrital zircons tals, and the fluid(s) can have variable uranium contents, which may lead to (Gehrels and Dickinson, 1995; Stewart et al., 2001; Baar, 2009; Lawton et al., development of oscillatory zoning at the crystal-fluid interface (Rubatto and 2010; Workman, 2012; Gehrels and Pecha, 2014; Yonkee et al., 2014; Chapman Gebauer, 2000). This suggests that the overgrowths grew during a hydro et al., 2015). The youngest detrital zircons in some of these Cordilleran Cam thermal or metamorphic event(s); such growth may well have resulted in Pb brian–Ordovician strata are hundreds of millions of years older than the in loss for some detrital zircons. The overgrowths with ages between 91.8 +1.1/ ferred depositional age (e.g., Figs. 10B and 10C), so that the lack of Cambrian –0.4 Ma and 89.2 +1.5/–0.8 Ma suggest that the metamorphic and/or hydrother zircons in some of the SMC samples does not preclude them being part of the mal event(s) were associated with intrusion of the early metaluminous plutons same depositional succession as those samples that do have Cambrian zircons. (ca. 100–85 Ma) before emplacement of the dominant peraluminous plutons Another approach to constraining depositional age is to compare the age (ca. 80–67 Ma) of the Atlanta lobe of the Idaho batholith (Gaschnig et al., 2010). spectra of detrital zircons from the unknown samples to those from other, better constrained, stratigraphic successions (e.g., Fedo et al., 2003; Gehrels, Depositional Ages of SMC Paragneisses 2012, 2014). The strata must have an established detrital-zircon age spectrum that is referred to as a detrital-zircon fingerprint (Ross and Parrish, 1991), ref The following discussion is based only on the 552 (34%) detrital zircons that erence (Gehrels et al., 1995), barcode (Sircombe, 2000; Link et al., 2005), or yield data meeting the concordance criteria stated above. chronofacies (Lawton et al., 2010). This approach of correlating detrital-zircon The ages of the youngest detrital zircons constrain the depositional age age spectra is commonly used in supercontinent reconstructions (e.g., Ireland of a sedimentary stratum, the quality of which depends upon the provenance of et al., 1998; Rainbird et al., 1998; Berry et al., 2001).

1.8 Ga Middle Pennsylvanian–Lower Permian strata (sandstones of the Wood River A Formation) in south-central Idaho (Link et al., 2014). The sample locations for these data are shown in Figure 1A. s Group A Although similarities exist between the SMC age spectra and those of the Combination of lower Paleozoic sections to the south and east based on a visual comparison of zircons in Fig. 6A age spectra, a more quantitative comparison of these spectra utilizing the K-S test may provide a more robust statistical test for similarities of sources among 2.7 Ga 37 samples, largely dependent on sample sizes (e.g., Berry et al., 2001; Saylor and Sundell, 2016). The combined detrital-zircon age data (Fig. 8A) of the Group A 18 (n=164) samples (Fig. 6A) are characterized by an age concentration between 1990 and

Number of analyse 1760 Ma (56%) and a subordinate concentration between 2900 and 2510 Ma 0 (29%), accompanied by a smaller group at 2120–2040 Ma (6%). Each of the first 0.7 1.0 1.3 1.6 1.92.2 2.52.8 3.1 two age concentrations is present in all the Group A samples, while the last one, 2120–2040 Ma zircons, has not been detected in sample MC14ST-15 due to the small number of concordant analyses (n = 22/125, Fig. 6A). The aggregate age spectrum (Fig. 8A) remarkably matches those of the Middle Ordovician 1.1 Ga Group B Eureka Quartzite (Fig. 10B) from southern and east-central Nevada (Gehrels Combination of B and Dickinson, 1995; Workman, 2012; Gehrels and Pecha, 2014) and the Kinniki 90 zircons in Fig. 6B nic Quartzite from east-central Idaho (Baar, 2009). A K-S test of the combined SMC Group A detrital zircons (Fig. 8A) versus published data from the Middle 67 1.8 Ga Ordovician quartzites (Fig. 10B) gives a P value of 0.089 (Supplemental Table 1.4 Ga 1C [see footnote 1]), indicating that there is a better than 95% chance they had 45 a common provenance, i.e., that the zircons in these samples were derived (n=383) from the same population. Comparisons between the age spectra of the aggre 22 gate Group A detrital zircons and those from other Paleozoic–Proterozoic strata in the U.S. Cordillera available for comparison (Fig. 10), however, did not pass 0 the K-S test at the 95% confidence level (P values <0.003; Supplemental Table 1C [see footnote 1]). Therefore, the SMC Group A metapsammites are possible 0.4 0.7 1.0 1.3 1.6 1.92.2 2.52.8 3.1 equivalents to some of the Middle Ordovician quartzites from the Cordilleran Age (Ga) passive margin successions, which is also supported by the results of the simi larity test (Supplemental Table 1C [see footnote 1]). Spectra of some other Figure 8. Age spectra of detrital zircons from combined Sawtooth metamorphic complex samples of Groups A (A) and B (B) as in Figure 6. n = number of analyses. Ordovician quartzites, perhaps those that are older than the Middle Ordovician in the Great Basin, do not match the SMC Group A spectrum, as shown by the detrital-zircon data from the lower Vinini Formation that is upper Lower to Detrital-zircon age spectra from the following Proterozoic–Paleozoic sedi lower Middle Ordovician in age (Gehrels and Pecha, 2014; Linde et al., 2016). mentary successions in the North American Cordillera are displayed in Fig It is noted that the available detrital-zircon age spectrum of the Neoprotero ure 10 for comparison with those of the SMC samples: (1) The Missoula and zoic Windermere Supergroup contains a large number of ages that exist in the Lemhi Groups of the Mesoproterozoic Belt Supergroup (the lower Belt is not spectrum of the SMC Group A samples (Ross and Parrish, 1991; Gehrels and included because it contains 1490–1610 Ma detrital zircons that are absent in Ross, 1998). The ca. 2700 Ma peak and the 2120–2040 Ma ages, however, are the SMC samples) (Ross and Villeneuve, 2003; Stewart et al., 2010); (2) the missing in the Windermere spectrum (Fig. 10G). The detrital-zircon age spec Neoprotero zoic Windermere Supergroup (Ross and Parrish, 1991; Gehrels and trum of the Cambrian Gold Creek Quartzite from Lakeview of northern Idaho Ross, 1998); (3) the Syringa metasedimentary rocks (Lewis et al., 2007, 2010); (Fig. 2) also contains abundant ages that occur in the SMC Group A spectrum (4) the Neoproterozoic to Ordovician Cordilleran passive margin strata (quartz (Lewis et al., 2010), but it has a much smaller Neoarchean peak, and the 2120– ites of Caddy Canyon, Mutual Formation, Prospect Mountain, Eureka, and 2040 Ma ages are largely missing in the Gold Creek Quartzite. Comparisons their equivalents in California, Nevada, Utah, and Idaho; Gehrels and Dickin of the age spectra of the Windermere Supergroup and Gold Creek Quartzite son, 1995; Stewart et al., 2001; Baar, 2009; Lawton et al., 2010; Workman, 2012; with the SMC Group A spectrum failed the K-S test at the 95% confidence level Gehrels and Pecha, 2014; Yonkee et al., 2014; Chapman et al., 2015); and (5) the (Supplemental Table 1C [see footnote 1]).

0.054 MC13ST-19 Interceptsat A 91 93 98 89.49±0.68 [0.69] Ma TuffZirc Age = 89.17 + 1.48 / – 0.81 Ma 120 0.052 MSWD=0.52 (97.8% conf, from coherent group of 13) Figure 9. U-Pb data of detrital-zircon overgrowths and representative cathodolumi- 0.050 nescence images of detrital zircons from

110 Pb

20 6 samples MC13ST-19 (A) and MC13ST-6 /

Age (Ma ) 89 (B). Circles on zircons represent spots for Pb 0.048

9 88 20 7 100 8 91 laser ablation, and the number next to 238 / each is the 206Pb/238U age without error.

Pb 0.046 For each sample, all the analyses with 20 6 90 <5% and >–3% discordance (based on 206Pb/238U versus 207Pb/235U) are displayed 0.044 66 68 70 72 74 76 on a TuffZirc age plot (Ludwig, 2012); the 238 206 100 µm 80 U/ Pb coherent group of zircons used for calcu- lating the TuffZirc age are then shown on MC13ST-6 Intercepts at 91.76±0.46 [0.48] & a Tera-Wasserburg plot. In each TuffZirc B 0.054 93 93 92 3579±1700 Ma age plot, the horizontal green band shows MSWD=1.3 the inferred age and uncertainty of the 100 0.052 syngenetic zircons; red boxes are the error bars for the inferred syngenetic zircons; 96

Pb 0.050 blue boxes are the error bars for the in- Age (Ma) 20 6 92 / terpreted inherited zircons and those that U

Pb 93 likely experienced Pb loss; the single gray 238 0.048 /

4 92 20 7

9 box indicates the ignored analysis with an 88 9 Pb anomalously high error. All error bars and 20 6 84 0.046 ellipses are shown at the 2σ level.

80 TuffZirc Age = 91.79 + 1.06 / – 0.40 Ma 66 68 70 72 74 (96.5% conf, from coherent group of 15) 100 µm 76 238U/206Pb 94

The combined age spectrum from Group B samples shows concentra ton et al., 2010; Yonkee et al., 2014), the Geertsen Canyon Quartzite from north tions at: 1870–1670 Ma (27%), 1490–1330 Ma (25%), and 1220–1020 Ma (37%) ern Utah (Gehrels and Pecha, 2014; Yonkee et al., 2014), and the Camelback (Fig. 8B). Age spectra of zircons extracted from Cambrian and Neoproterozoic Mountain Quartzite from southeastern Idaho (Yonkee et al., 2014). The large quartzites from the Laurentian passive margin also show primary peaks at ca. component of Grenville-age zircons in the SMC samples is observed in the age 1800 Ma, 1400 Ma, and 1100 Ma, and most also have a subordinate group of spectrum of the Cambrian Campito Formation (Stewart et al., 2001; Chapman ca. 2700 Ma zircons (Figs. 10C–10E). K-S comparisons between the combined et al., 2015) and that of the Osgood Mountain Quartzites formed from latest Group B spectrum (Fig. 8B) and those of the Cambrian–Neoproterozoic strata Neoproterozoic through earliest Cambrian time (Linde et al., 2014). This cor of the Laurentian passive margin (Figs. 10C–10E) failed at the 95% confidence relation of the SMC Group B metapsammites with those Cordilleran Cambrian level, presumably because of the much smaller 2700 Ma peak and more pro quartzites is supported by the results of the similarity test (Supplemental Table nounced 1100 Ma peak in the SMC spectrum (Fig. 10C), as well as the addi 1C [see footnote 1]) and is consistent with the presence of three Cambrian tional 1490–1610 Ma ages and much smaller 1800 Ma peaks in the spectra (496 ± 10 Ma, 529 ± 18 Ma, 542 ± 12 Ma, 2s) and several Neoproterozoic zircons of the Neoproterozoic Mutual Formation (Fig. 10D; Gehrels and Pecha, 2014; in these samples. Yonkee et al., 2014) and Caddy Canyon Quartzite (Fig. 10E; Lawton et al., 2010; The Neoproterozoic Syringa metasedimentary rocks give a somewhat simi Gehrels and Pecha, 2014; Yonkee et al., 2014). The lack of abundant Archean zir lar age spectrum to the SMC Group B samples (Fig. 10F; Lewis et al., 2007, 2010), cons in the SMC rocks may be related to the extensive degree of discordance, but the 1400 Ma peak is largely missing in the Syringa spectrum, which is prob which would generally affect the older, more metamict zircons. ably why it failed the K-S test at the 95% confidence level (Supplemental Table The aggregate age spectrum of the Group B samples (Fig. 8B) is most simi 1C [see footnote 1]). The detrital-zircon age distribution in the Pennsylvanian– lar to the spectra of some Cambrian quartzites (Fig. 10C) from the western Permian sandstones (Fig. 10A; Link et al., 2014), Neoproterozoic Windermere margin of Laurentia, including the Wood Canyon Formation (Stewart et al., Supergroup (Fig. 10G; Ross and Parrish, 1991; Gehrels and Ross, 1998), and 2001; Wooden et al., 2013) and Campito Formation (Chapman et al., 2015) from the Mesoproterozoic Belt Supergroup (Fig. 10H; Ross and Villeneuve, 2003; southern California, the Prospect Mountain Quartzite from central Utah (Law Stewart et al., 2010) also failed the K-S test at the 95% confidence level when

GEOSPHERE | Volume 12 | Number 4 Ma et al. | Detrital-zircon geochronology of the Sawtooth metamorphic complex Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/4/1136/4178109/1136.pdf 1147 by guest on 30 September 2021 on 30 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/4/1136/4178109/1136.pdf Research Paper 37 127 191 104 139 0 36 73 89 44 69 34 63 136 16 36 0 Number of analyses 0 0 0 0 45 0 91 0 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 Ga A B C E D G F H 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.1 Ga (n=1000) 1.1 Ga 1.1 Ga 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.1 Ga 1.1 Ga 1.4 Ga 1.8 Ga 1.4 Ga 1.6 1.6 Age (Ga) 1.6 1.6 1.6 1.6 1.6 1.4 Ga 1.8 Ga 1.8 Ga 1.8 Ga 1.8 Ga 1.7 Ga 1.9 1.9 1.9 1.9 1.9 1.9 1.8 Ga 1.8 Ga 1.8 Ga 2.1 Ga SMC Group B 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 (this study) SMC Group A 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 (n=142) (this study) 2.7 Ga 2.7 Ga 2.7 Ga (n=851) 2.8 2.8 2.8 2.8 2.8 2.8 2.8 2.8 (n=51) (n=326) (n=930 (n=509) (n=467) 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 )

3. 4 3. 4 3. 4 3. 4 3. 4 3. 4 3. 4 3. 4

Windermere Supergroup Neoproterozoic (Ross and Parrish, 1991; Gehrels Ross, 1998) Belt Supergroup Mesoproterozoic (Stewart et al., 2010; Ross and Villeneuve, 2003) Missoula and Lemhi Groups sandstones Pennsylvanian - Permian Yonkee et al., 2014) (Lawton et al., 2010; Gehrels and Pecha, 2014; southeastern Idaho, northern & central Utah (Gehrels and Pecha, 2014; Yonkee et al., 2014) southeastern Idaho, northern & central Utah Mutual Fm Neoproterozoic Caddy Canyon Quartzit Neoproterozoic (Link et al., 2014) Wood River Formation: south-central Idaho shelf strata Middle Ordovician (Stewart et al., 2001; Chapman 2015) White-Inyo Range Campito Fm (Andrews Mountain member): Death Valley area & Mojave Desert region Wood Canyon Fm: shelf strata Lower Cambrian Workman, 2012; Gehrels and Pecha, 2014) (Gehrels and Dickinson, 1995; Baar, 2009; Kinnikinic Qtz: east-central Idaho Eureka Qtz: southern & east-central Nevada metasedimentary rocks Neoproterozoic Syringa Yonkee et al., 2014) (Lawton et al., 2010; Gehrels and Pecha, 2014; Camelback Mountain Qtz: southeastern Idaho Geertsen Canyon Qtz: northern Utah Prospect Mountain Qtz: central Utah (Lewis et al., 2007, 2010)

Formation; n = number of analyses. literature data. Qtz—Quartzite; Fm— are the distribution of ages from the parison. Circles under each KDE plot in (B) and (C), respectively, for com - Groups A and B of Figure 8 are shown tively. Combined age spectra of the ages >1000 Ma and <1000 Ma, respec - compile data from the literature for and <5% discordances were used to Cordillera. Screening criteria of <10% to Permian from the North American strata ranging from Mesoproterozoic (KDE) plots of detrital-zircon ages of Figure 10. Kernel density estimation

GEOSPHERE | Volume 12 | Number 4 Ma et al. | Detrital-zircon geochronology of the Sawtooth metamorphic complex 1148 Research Paper

compared to the SMC Group B spectrum (Supplemental Table 1C [see foot 1330 Ma grains were likely derived from the Granite-Rhyolite province and/or note 1]). Visual inspection of these data shows that their age spectra either lack a variety of possible granitoids of western Laurentia including those within the one or more age populations (e.g., the Windermere and Belt Supergroups) Belt basin (e.g., Goodge and Vervoort, 2006; Bickford et al., 2015). The 1990– or have additional populations (e.g., the Pennsylvanian–Permian sandstones) 1670 Ma zircons are consistent with sources from the Yavapai-Mazatzal-Mojave compared to that of the SMC Group B samples. provinces, Trans-Hudson orogen, Great Falls tectonic zone, and/or recycled The depositional ages for the SMC Group A samples, which include types from parts of the upper Belt Supergroup or Neoproterozoic sequences (e.g., I, II, and III lithologies, are interpreted to be Middle Ordovician on the basis of Hoffman, 1988; Mueller et al., 2002, 2007; Ross and Villeneuve, 2003; Ansdell, comparisons of detrital-zircon age spectra with a Middle Ordovician reference 2005; Foster et al., 2006; Amato et al., 2008; Bickford et al., 2008; Wooden et al., derived from the literature (e.g., Gehrels et al., 1995). The Group B samples, 2013; Gifford et al., 2014; Yonkee et al., 2014). The Archean and early Paleo including types I and II lithologies, are most similar to some Cambrian quartz proterozoic zircons (3000–2000 Ma) have potential basement sources in the ites from the western Laurentian margin, which is consistent with the ages Wyoming and Mojave provinces as well as the Grouse Creek block and the of the youngest detrital zircons analyzed from three samples of this group. In Farmington zone, and/or recycled from the Proterozoic sedimentary rocks in addition, the mix of protoliths for the SMC paragneisses may include quartz- western Laurentia (e.g., Mueller and Frost, 2006; Mueller et al., 2007; Mueller rich sandstone, feldspathic sandstone, carbonaceous siltstone, mudstone, et al., 2011; Wooden et al., 2013; Yonkee et al., 2014). and carbonate (Metz, 2010; Fukai, 2013); this mix is similar to the unmetamor The Neoproterozoic–Cambrian detrital zircons (ca. 742 Ma, ca. 632 Ma, ca. phosed Cambrian–Ordovician succession in southeastern Idaho that contains 605 Ma, ca. 578 Ma, ca. 542 Ma, ca. 529 Ma, and ca. 496 Ma) may have origi quartzite, argillite, carbonate, shale, and arkosic quartzite (e.g., Armstrong and nated from local rift-related igneous rocks on the western margin of Lauren Oriel, 1965), further supporting correlation of the SMC metapsammites with tia (Lund et al., 2003 and references therein). Reports of rift-related magma the Cambrian–Ordovician strata. The identification of highly metamorphosed tism from east-central Idaho (Fig. 2) include at least two discrete magmatic lower Paleozoic sedimentary rocks in the SMC adds to the area in which meta pulses represented by syenite-diorite suites and tuffaceous diamictite at ca. morphosed Neoproterozoic rocks of the Laurentian margin have been identi 665–650 Ma and ca.500–485 Ma (Lund et al., 2010). fied in the Idaho batholith region (e.g., Lund et al., 2003; Lewis et al., 2007, 2010) and expands the age range of these strata to Middle Ordovician. Regional Implications

Implications for the SMC Lithostratigraphy The presence of early Paleozoic shelf strata in the Idaho batholith region (SMC) combined with their likely presence farther north in the Stibnite and Strong deformation and high-grade metamorphism have obfuscated the Gospel Peaks inliers (Fig. 2) (Lund et al., 2003) suggests that the early Paleo primary sedimentary structures of the SMC metasedimentary rocks. The only zoic Cordilleran passive margin shelf succession was once continuous from possible way to reveal the stratigraphic relationship for the lithologic units is the Great Basin through northern Idaho into Canada. This also indicates that placing the inferred Cambrian and Middle Ordovician rocks into the structural the Cambrian–Ordovician shelf strata probably extend from east-central Idaho context of the SMC, making an assumption about the younging direction in westward into the Idaho batholith. The results of this study suggest that there synforms and antiforms that repeat the units across strike. The amount of may be other not identified Cambrian–Ordovician shelf strata in the Idaho ba transposition within the units, however, makes any reconstruction of a strati tholith region such as those in the SMC, which could be the metamorphosed graphic column depicting the relative ages of the quartzites and quartzofeld and erosional remnants of the original shelf succession in this region. This spathic gneisses with the adjacent strata, including calc-silicate gneisses, mica assumes that the SMC rocks are not an allochthonous slice of the Cordilleran schists, and quartzofeldspathic gneisses, extremely poorly constrained. margin translated by Mesozoic dextral strike-slip faults from the Mojave or Great Basin, which seems unlikely because of their position well inboard of Provenance of the SMC Metapsammites the edge of western Laurentia represented by the location of the western Idaho shear zone (Fig. 1A). Both of the detrital-zircon age groups identified in the SMC rocks (Groups A and B) are consistent with known sources in Laurentia, either from base ment exposed in the early Paleozoic or recycled from Proterozoic sedimentary CONCLUSIONS rocks (Fig. 11). The 1200–1000 Ma zircons were likely derived from the Gren ville orogen of eastern or southern Laurentia and coeval igneous rocks such U-Pb geochronology of detrital zircons from quartzites and quartzofeld as the Pikes Peak batholith of Colorado, as suggested for other early Paleo spathic gneisses in the SMC reveals two distinctive groups of age spectra. zoic and Neoproterozoic sequences in western Laurentia (e.g., Rainbird et al., Group A exhibits primary concentrations of ages at 2900–2510 Ma and 1990– 1992; Stewart et al., 2001; Mueller et al., 2007; Howard et al., 2015). The 1490– 1760 Ma. Group B is characterized by 1870–1670 Ma, 1490–1330 Ma, and 1220–

<0.78 Ga Windermere Supergroup 1.80–1.68 Ga Yavapai province 1.3–0.95 Ga Grenville province 1.84–1.80 Ga Trans-Hudson orogen 1.47–1.40 Ga Belt Supergroup 2.0–1.8 Ga juvenile orogens/arcs 1.55–1.35 Ga Granite-Rhyolite province e >2.5 Ga Archean crust 1.69–1.60 Ga Mazatzal province Continental rift boundary Hearn Canada

Trans-Hudso Superior province U.S.A. Medicined Belt Hat Figure 11. Basement of Precambrian United GFTZ States, modified from Whitmeyer and Karl- strom (2007). SMC—Sawtooth metamor- SMC phic complex; GFTZ—Great Falls tectonic WyomingW n Grenville zone. province Grouse Greek Yavapai e Mojave Mazatzal N Granite-Rhyolit

500 km

1020 Ma grains. Group B also contains three detrital zircons dated at 542 ± collecting and backpacking out samples possible. We are grateful to David Fluetsch and Lieze Dean of the Sawtooth National Recreation Area (National Forest Service) for facilitating the permit for 12 Ma, 529 ± 18 Ma, and 496 ± 10 Ma along with several Neoproterozoic grains. collecting samples. Dr. Ann Heatherington and Dr. George Kamenov are acknowledged for assis The ages of these detrital zircons are consistent with Laurentian sources. The tance with the zircon analyses, and we thank Celina Will and Xiaogang Xie for CL/BSE imaging detrital-zircon age spectra from the SMC Groups A and B metapsammites are assistance. This research was supported by National Science Foundation grants EAR-1145073 to similar to those from the Middle Ordovician and Cambrian shelf strata of west Dutrow and EAR-1145212 to Mueller and Foster. ern Laurentian margin, respectively. REFERENCES CITED The presence of early Paleozoic shelf strata in the Idaho batholith region Amato, J.M., Boullion, A.O., Serna, A.M., Sanders, A.E., Farmer, G.L., Gehrels, G.E., and Wooden, represented by the SMC metapsammites and their likely presence in north- J.L., 2008, Evolution of the Mazatzal province and the timing of the Mazatzal orogeny: In central Idaho (Lund et al., 2003) suggest that the Cordilleran passive margin sights from U-Pb geochronology and geochemistry of igneous and metasedimentary rocks shelf successions were continuous from the Great Basin through the Idaho in southern New Mexico: Geological Society of America Bulletin, v. 120, no. 3–4, p. 328–346, doi:10.1130/B26200.1. batholith region into northern Idaho and western Washington. Andersen, T., 2005, Detrital zircons as tracers of sedimentary provenance: Limiting conditions from statistics and numerical simulation: Chemical Geology, v. 216, no. 3, p. 249–270, doi:10 .1016/j.chemgeo.2004.11.013. ACKNOWLEDGMENTS Ansdell, K.M., 2005, Tectonic evolution of the Manitoba-Saskatchewan segment of the Paleo Reviewer Karen Lund, one anonymous reviewer, the Guest Associate Editor Joshua Schwartz, and proterozoic Trans-Hudson Orogen, Canada: Canadian Journal of Earth Sciences, v. 42, no. 4, the Science Editor Shanaka de Silva are thanked for their helpful revisions, comments, and sug p. 741–759, doi:10.1139/e05-035. gestions (on the earlier version of the manuscript), which greatly improved the presentation and Armstrong, F.C., and Oriel, S.S., 1965, Tectonic development of Idaho-Wyoming thrust belt: discussion. We thank the Colorado Scientific Society and the Geological Society of America for American Association of Petroleum Geologists Bulletin, v. 49, no. 11, p. 1847–1866. financial support of Ma’s fieldwork and the Belt Association, the New Orleans Geological Society, Armstrong, R.L., 1975, Precambrian (1500 m.y. old) rocks of central Idaho—The Salmon River Marathon Oil, and the Department of Geology and Geophysics at Louisiana State University for Arch and its role in Cordilleran sedimentation and tectonics: American Journal of Science, support of Bergeron’s study. The field assistance of Sutie Xu, Andy Whitty, and Denis Norton made v. 275, p. 437–467.

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