Downloaded from geology.gsapubs.org on October 2, 2010 Geology Paleogeographic implications of non−North American sediment in the Mesoproterozoic upper Belt Supergroup and Lemhi Group, Idaho and Montana, USA Eric D. Stewart, Paul K. Link, C. Mark Fanning, Carol D. Frost and Michael McCurry Geology 2010;38;927-930 doi: 10.1130/G31194.1 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geology Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. 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Notes © 2010 Geological Society of America Downloaded from geology.gsapubs.org on October 2, 2010 Paleogeographic implications of non–North American sediment in the Mesoproterozoic upper Belt Supergroup and Lemhi Group, Idaho and Montana, USA Eric D. Stewart1*, Paul K. Link1, C. Mark Fanning2, Carol D. Frost3, and Michael McCurry1 1Department of Geosciences, Idaho State University, Pocatello, Idaho 83209, USA 2Research School of Earth Sciences, Australian National University, Canberra, ACT 5000, Australia 3Department of Geology, University of Wyoming, Laramie, Wyoming 82071, USA ABSTRACT ington and Idaho (Doughty et al., 1998), but A non–North American provenance for the lower Belt Supergroup of North America has is otherwise absent in western Laurentia (Ross been used to support various pre-Rodinian paleogeographic reconstructions. Unlike the lower and Villeneuve, 2003). The presence of signifi - Belt Supergroup, most upper Belt Supergroup provenance studies have inferred Lau- cant 1570–1590 Ma detrital zircons in the Belt rentian sediment sources. We test this hypothesis by analyzing U-Pb and Lu-Hf isotopes on Supergroup, combined with an inferred west- detrital zircons, and whole-rock Nd isotopes from the Missoula (upper Belt Supergroup) and ern provenance from paleocurrents (Cressman, Lemhi Groups, and comparing to possible Laurentian sources. Detrital zircons from 11 sand- 1989), suggests that the continent off Lauren- stones analyzed show dominant ages between 1680 and 1820 Ma. These zircons are predomi- tia’s western margin included crystalline rocks nantly magmatic in paragenesis. Belt Supergroup–aged (1400–1470 Ma) and 2400–2700 Ma of that age. The 1570–1590 Ma magmatic populations represent minor components. Lu-Hf isotopic analyses for 1675–1780 Ma Missoula rocks in South Australia (Goodge et al., 2008) Group and Lemhi Group detrital zircons range from εHf(i) +9 to –12 and +8 and –7, respec- and north Queensland (Sears and Price, 2003; tively. Belt Supergroup–aged grains from the Bonner Formation, Missoula Group, have εHf(i) Blewett et al., 1998) support various Lauren- values between +5 and –9, exceeding coeval ranges from the Mojave and Yavapai terranes tian connections with Australia. [εHf(i) between +5 and 0]. Whole-rock Nd isotopes from Lemhi Group argillites yield a range The bulk of the Missoula Group was depos- in εNd(1400) between +1.1 and –5.9. Immature feldspathic sediment, nearly unimodal detrital ited between 1454 ± 9 and 1401 ± 6 Ma (Evans zircon spectra, and dissimilar Belt Supergroup–aged zircon Hf signatures suggest that distal et al., 2000). Previous provenance studies using portions of the Yavapai and Mojave terranes intruded by A-type magmas were not the source detrital zircon and monazite found dominate for the Missoula and Lemhi Groups. Instead, a slightly modifi ed Mesoproterozoic proto- ages between 1640 and 1860 Ma (Ross et al., SWEAT (southwestern United States and East Antarctica) model can best account for the 1991; Ross and Villeneuve, 2003). These ages sedimentologic and isotopic characteristics of the Missoula and Lemhi Groups. An alternative are common in the Mojave and Yavapai prov- model with a source from southeastern Siberia and the Okhotsk Massif is less preferred. inces of the southwestern United States. Prov- enance ages combined with northeast-directed INTRODUCTION 1470 Ma (Sears et al., 1998; Evans et al., 2000), paleofl ow (Winston, 1986) led most workers to Reconstructions of the early Neoproterozoic contain 1570–1590 Ma detrital zircon popula- infer a southwest Laurentian source for the Mis- supercontinent Rodinia remain contentious tions (Ross and Villeneuve, 2003; Link et al., soula Group, as contrasted with a non-Lauren- (e.g., see Li et al., 2008). Models involving 2007). This age is found in felsic intrusive tian, 1570–1590 Ma source for the Lower Belt western Laurentia commonly invoke the cre- rocks of the Priest River Complex of Wash- (Ross and Villeneuve, 2003). ation of a late Paleoproterozoic proto-Rodinia that persisted until Rodinia breakup (e.g., Missoula Group Australia Figure 1. Paleogeographic Sears and Price, 2003; Goodge et al., 2001) 5 km Garnet Range 2a reconstruction, modifi ed to explain the lack of Grenville-aged belts in McNamara Hearne Province from Goodge et al. (2008). (>2.5 Ga) southwestern Laurentia. Different reconstruc- Bonner (1b) Tie points: 1a—Proposed Mt. Shields (1b, 1c) Gawler Antarctic and Lauren- Shepard Craton 2b tions place East Antarctica (Moores, 1991; Snowslip (1b) tian ca. 1.7 Ga crystalline 0 km Goodge et al., 2008), Australia (Blewett et Priest River Medicine Hat sources for Missoula (1b, Lemhi Group Terre Complex (2.6–3.3 Ga) 1c) and Lemhi Group (1c, al., 1998), south China (Li et al., 2008), and Adelie >2.5 Ga–1.58 Ga 15 km Swauger (1d) 1d, 1e, 1f) samples. 2a— Siberia (Sears and Price, 2003) off the western s King George Great Fall V Land 1b c Zone 1590 Ma Gawler Range Vol- margin of Laurentia during deposition of the Tectoni Gunsight (1c, 1d) 1e 1c canics. 2b—1580 Ma felsic Belt Supergroup. 10 km 1f 1d augen gneiss (Doughty et Apple Creek The Mesoproterozoic Belt Supergroup (Pur- 1a al., 1998) from Priest River cell Supergroup in Canada) is a 15–20-km-thick 5 km Complex. Figure is compiled Big Creek (1d, 1e)** Grouse Wyoming C from Goodge et al. (2008), package of largely siliciclastic strata extending Block k. Craton West Fork (1d) 1a Inyo Creek (1d, 1f)* (>2.5 Ga) (>2.5 Ga) Winston (1986), Foster et al. from southwest Montana (United States) to 0 km Victoria (2006), Bennett and DePaolo Northern boundary East Antarctica Land southern British Columbia (Canada) (Fig. 1; 1a (1987), and Fanning et al. of 1.47–1.37 Ga Santaquin Complex Harrison et al., 1974). The Belt Supergroup, Laurentian A-type (1988). *1f is from Yellow- composed of the Lower Belt, Ravalli, Piegan, granites jacket Formation, correlated Belt to Inyo Creek Formation, and Missoula Groups, has played an important Yavapai Supergroup Mojave and West Fork Formation (1.84–1.7 Ga) (1.8–1.7 Ga) part in the proto-Rodinia debate. The lowest Lemhi Group and (Winston et al., 1999). **1e is correlatives three groups, deposited between ca. 1454 and km 00 from Hoodoo Quartzite, cor- 1.59–1.58 Ga 4 magmatism Mazatzal related to Big Creek Forma- (1.6–1.7 Ga) *E-mail: [email protected]. tion (Winston et al., 1999). © 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, October October 2010; 2010 v. 38; no. 10; p. 927–930; doi: 10.1130/G31194.1; 5 fi gures; Data Repository item 2010257. 927 Downloaded from geology.gsapubs.org on October 2, 2010 The Lemhi Group is an ~15-km-thick pack- (2008). For the Gunsight and Mount Shields Hf isotopes from Missoula Group detrital zir- age of largely siliciclastic Mesoproterozoic samples, new grain mounts were prepared from cons dated between 1675 and 1780 Ma yield a strata found in east-central Idaho (Fig. 1; Rup- another aliquot of the same zircon separate ana- wide range in initial ratios, εHf(i) between +9 pel, 1975). Detrital zircon age spectra from the lyzed previously in Arizona. Additional detri- and −12 (Fig. 3). The εHf(i) values for 1675– Lemhi Group match those of the Missoula Group tal zircon SHRIMP U-Pb ages and Hf isotope 1780 Ma detrital zircons from the Lemhi Group (Link et al., 2007). Detrital zircons (Link et al., ratios were determined for the Bonner Forma- are similar, ranging between +8 and −7. An 2007) and crosscutting intrusions (Doughty and tion and an additional sample of the Mount isotopically heterogeneous source is inferred, Chamberlain, 1996) bracket Lemhi Group depo- Shields Formation (both location 1b in Fig. 1). ranging from Paleoproterozoic depleted mantle sition between 1454 ± 9 Ma and 1370 Ma. Cathodoluminescence (CL) images were used (highest values) to recycled Archean basement It is widely recognized that Lu-Hf isotope to identify magmatically zoned zircon grains (lowest values), and/or mixtures thereof. ratios on dated detrital zircons can be used as a between 1675 and 1780 Ma and younger than Initial Hf isotope values for Belt Super- geochemical tracer in provenance studies (e.g., 1470 Ma for the Lu-Hf analysis.