To Synglacial Rift-Related Volcanism in the Neoproterozoic (Cryogenian) Pocatello Formation, SE Idaho: New SHRIMP and CA-ID-TIMS Constraints
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Pre- to synglacial rift-related volcanism in the Neoproterozoic (Cryogenian) Pocatello Formation, SE Idaho: New SHRIMP and CA-ID-TIMS constraints Joshua A. Keeley1, Paul K. Link1, C. Mark Fanning2, and Mark D. Schmitz3 1DEPARTMENT OF GEOSCIENCES, IDAHO STATE UNIVERSITY, POCATELLO, IDAHO 83209, USA 2RESEARCH SCHOOL OF EARTH SCIENCES, AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, ACT 0200, AUSTRALIA 3DEPARTMENT OF GEOSCIENCES, BOISE STATE UNIVERSITY, BOISE, IDAHO 83725, USA ABSTRACT Volcanic and diamictite-bearing strata of the Neoproterozoic Pocatello Formation record middle Cryogenian glaciation and alkaline to subal- kaline within-plate magmatism during Rodinia rifting. New mapping along the Oxford Ridge segment of the southern Bannock Range in SE Idaho has resolved stratigraphic relationships between the Scout Mountain and underlying Bannock volcanic members of the Pocatello For- mation. Bannock Volcanic Member metabasalt has an upper gradational contact with over 250 m of Scout Mountain Member that includes extrabasinal and volcaniclastic diamictite, in turn overlain by a volcaniclastic unit (the Oxford Mountain tuffi te). Previous attempts to date the tuffi te include three sets of analyses of the original sample (06PL00) and one resample (04JK09) that yielded sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon concordia ages of ca. 709, 702, and 686 Ma and one isotope dilution–thermal ionization mass spectrom- etry (ID-TIMS) age of 687.4 ± 1.3 Ma. Several new samples of plagioclase-phyric volcanic sandstone and the tuffi te, dated via high-precision (~0.1%) chemical abrasion (CA) ID-TIMS, have multimodal zircon populations with single-crystal ages ranging from as old as 709 Ma to as young as 685 Ma, confi rming the epiclastic nature of the deposit. The majority of grains in one sample yielded a 206Pb/238U weighted mean age of 685.5 ± 0.4 Ma, which provides a robust maximum age of deposition. From the type section of the lower Scout Mountain Member, Pocatello Formation at Portneuf Narrows, we report four new SHRIMP maxi- mum depositional ages between 705 ± 5 Ma and 682 ± 6 Ma. A 691 ± 4 Ma (SHRIMP) volcanic clast from the cobble conglomerate member provides a maximum depositional age, and provides a geochronologic correlation with the Oxford Mountain tuffi te. The data are interpreted to support a lithostratigraphic correlation between the diamictite on Oxford Mountain and the lower diamictite at Portneuf Narrows and to show that the upper glaciogenic diamictite in the Portneuf Narrows section is younger than 685 Ma. This 685 Ma age from rift-related rocks that underlie the Brigham Group passive-margin succession provides a maximum age for onset of rift subsidence. ε − Lu-Hf analyses of 685–730 Ma igneous zircons yield enriched initial Hf values in the range +2 to 17, indicating that they crystallized from magma that incorporated depleted Paleoproterozoic to Archean crustal components of the underlying Farmington Canyon Complex and Wyoming craton. LITHOSPHERE; v. 5; no. 1; p. 128–150; GSA Data Repository Item 2013034 | Published online 14 November 2012 doi: 10.1130/L226.1 INTRODUCTION zoic glacial phases are the Middle Cryogenian siderably younger bound is the U-Pb sensitive “Sturtian,” the Late Cryogenian “Marinoan,” high-resolution ion microprobe (SHRIMP) age Volcanic, siliciclastic, and minor carbonate and the Ediacaran “Gaskiers” (Hoffman and Li, of 660 ± 6 Ma from a tuff in a dropstone-bearing strata of the Neoproterozoic Pocatello Formation 2009). The Sturtian, Middle Cryogenian gla- sandstone within the Sturtian glacial succes- in southeast Idaho record rifting of the supercon- ciations derive their name from exposures of sion near Copley in South Australia (Fanning tinent Rodinia during widespread regional gla- diamictite in the “gorge of the River Sturt,” south and Link, 2006, 2008). In South Australia, the ciation (Fig. 1) (Ludlum, 1942; Crittenden et al., of Adelaide, South Australia (Howchin, 1901; postglacial Tapley Hill Formation is composed 1983; Link, 1983; Link et al., 1994; Smith et al., Preiss, 2000; Preiss et al., 2011). This name has of thick transgressive shale that yielded a Re-Os 1994; Lorentz et al., 2004; Corsetti et al., 2007). been applied worldwide, and current age con- age of 643 ± 2.4 Ma from the basal Tindelpina These strata bear on paleogeographic, Snow- straints on the glaciation suggest it was diachro- Shale Member (Kendall et al., 2006). ball Earth, and Rodinia rift models that seek to nous between ca. 716 Ma and ca. 660 Ma. The The chronostratigraphic term “Marinoan” explain drastic tectonic and climatic fl uctuations older bound is the chemical abrasion–isotope comes from the Marino Arkose Member of the near the onset of complex life (Hoffman, 1991; dilution–thermal ionization mass spectrometry Wilmington Formation, which underlies the Hoffman et al., 1998; Li et al., 2008; Hoffman (CA-ID-TIMS) U-Pb zircon age of 716.47 ± glaciogenic Elatina Formation (Williams et al., and Li, 2009). 0.24 Ma from diamictites of the Mount Harper 2008). The Elatina glaciation of Marinoan age in Neoproterozoic glacial successions are rec- Group in the Yukon Territory representing the Australia has been interpreted to have occurred ognized worldwide, but the number, timing, and Rapitan glaciation (Macdonald et al., 2010). The at 635 Ma on the basis of chemostratigraphic duration of glacial episodes remain controver- Ghubrah Formation in Oman is slightly younger, correlation with dated glacials in South China sial. Three generally agreed upon Neoprotero- at 711.5 ± 0.3 Ma (Bowring et al., 2007). A con- and Namibia (Hoffman and Schrag, 2002). The 128 For permission to copy, contact [email protected] | |Volume © 2012 5 Geological| Number Society1 | LITHOSPHERE of America Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/5/1/128/3723042/128.pdf by guest on 25 September 2021 Pre- to synglacial rift-related volcanism in the Cryogenian Pocatello Formation | RESEARCH W112°30’ W112°00’ W111°30’ and ice-distal debris-fl ow tillite. Rare striated quartzite clasts are present. AB Regional equivalents of the Pocatello For- N 43°00’ N 43°00’ mation include the informal formation of China SNAKE RIVERPeak PLAIN Perry Canyon in the northern Wasatch Range, the Mineral Fork Formation in the central to Pocatello NPR southern Wasatch Range, and the Otts Canyon OR MT and overlying Dutch Peak Formations in the Portneuf Edwardsburg Bannock R Sheeprock Mountains, Utah (Fig. 3; Crittenden Fm. Narrows samples BRR et al., 1971, 1983; Link et al., 1993; Link and Pocatello Fm. - 64JK09 Christie-Blick, 2011). Glacial dropstones that - 15PL08 ID pierce underlying laminations are common in - 63JK09 the Mineral Fork Formation in northern Utah NV UT - 62JK09 (Christie-Blick, 1983). Dropstones have also ID - Idaho O SPR MT - Montana Oxford Peak been found in the upper Otts Canyon Forma- ange NV - Nevada Area of B tion immediately underlying the ~1.5-km-thick OR - Oregon N 42°00’ IDAHO stratifi ed and massive diamictite of the Dutch UT - Utah UTAH CV PT Neoproterozoic exposures Peak Formation (Christie-Blick, 1983; Link and Idaho Neoproterozoic exposures Christie-Blick, 2011). The formation of Perry WF L Abbreviations Wasatch Range Canyon displays two horizons of thick diamic- BRR - Bear River Range tite separated by several hundred meters of gla- CV - Cache Valley ciomarine strata. The lower of these diamictites L - Logan Perry LM - Little Mountain Canyon has been interpreted to bear dropstones. The NPR - Northern Portneuf Range Great two diamictite units here have been interpreted O - Oxford Salt to represent two stades in one glaciation (Crit- PT - Paris Thrust Lake tenden et al., 1983). Overlying the Pocatello SLC - Salt Lake City LM SPR - Southern Portneuf Range WT Formation, in the Brigham Group, incised val- WF - Wasatch Fault leys between the Caddy Canyon Quartzite and WT - Willard Thrust Fremont Is. the Inkom Formation (Fig. 3) are interpreted Thrust fault, dotted where N 41°00’ as indirect evidence for eustatic changes due to concealed late Cryogenian glaciations (Link and Christie- Active normal fault, dashed where concealed Blick, 2011). Town/City SLC The timing and duration of rifting of the Neo- Lower Cambrian exposure proterozoic to Paleozoic western North American N passive margin also remain poorly constrained Neoproterozoic exposure Cottonwood area (Moores, 1991; Karlstrom et al., 1999). How- 0 10 20 mi ever, recent age constraints have signifi cantly 0 25 50 improved our understanding. Radiometric ages km along the western Laurentian Cordillera begin with the ca. 780 Ma Gunbarrel mafi c dike swarm, Figure 1. Regional maps. (A) Map showing the location of Neoproterozoic diamictite-bearing suc- cessions along the Cordillera, modifi ed from Fanning and Link (2004). (B) Location map showing interpreted to suggest crustal extension induced the study areas of the Portneuf Narrows and Oxford Mountain in the Bannock Range, SE Idaho. by a mantle plume beneath Rodinia (Harlan et al., Also shown are locations of correlative Neoproterozoic sections: formation of Perry Canyon in Perry 2003). Mechanical rifting along the Cordillera is Canyon and the Mineral Fork Formation in the Cottonwood area east of Salt Lake City. then recorded by thick volcanic- and diamictite- bearing successions from which volcanic rocks provide ages of 716–711 Ma (Macdonald et al., Marinoan, late Cryogenian glaciation is brack- ada (Bowring et al., 2003). It was suggested by 2010), ca. 688 Ma (Ferri et al., 1999), and ca. eted between 655 Ma (Zhang et al., 2008) and Hoffman and Li (2009) that the time constraints 685 Ma (Lund et al., 2003). Thin volcanic rocks 632 Ma (Condon et al., 2005), with equiva- for the Gaskiers glaciation are too brief for a in the overlying Brigham Group and correla- lent 635 Ma U-Pb zircon ages recorded from global glaciation. tives yield ages of 580–570 Ma (Fig. 3; Christie- Namibia (Hoffmann et al., 2004), Oman (Bow- The diamictites that crop out throughout Blick and Levy, 1989; Colpron et al., 2002).