Geochemistry of the Neoproterozoic Johnnie Formation and Stirling Quartzite, southern Nopah Range, California: Deciphering the Roles of Climate, Tectonics, and Sedimentary Process in Reconstructing the Early Evolution of a Rifted Continental Margin Open Access
The Neoproterozoic Stirling Quartzite and Johnnie Formation in the southern Nopah Range, southeastern California, comprise a thick sequence of predominantly siliciclastic sediment that is exposed along the Cordilleran margin. Located above the syn-rift Kingston Peak Formation, they mark the early deposits of passive margin sedimentation during the breakup of the Rodinia supercontinent. Disagreement exists between field-based observations and subsidence modeling as to whether these units represent rift or passive margin deposition. In this study, major-, trace-, and rare earth-element (REE) geochemistry, and U-Pb detrital zircon geochronology are used to determine their provenance, paleoclimatic information, and, consequently their paleotectonic setting.In southeastern California, the Neoproterozoic Stirling Quartzite comprises an ~700 m thick succession of fluvial subarkose and quartzarenite (lower member) overlain by tidal mudstone with dolomite intercalations (middle member), and capped with more alluvial arenites (upper member) perhaps all deposited as a result of the fragmentation of Rodinia. This study utilizes bulk-rock geochemistry coupled with detrital zircon U-Pb geochronology to determine the provenance, paleoclimate, and tectonic setting of deposition. Chemical Index of Alteration (CIA) values for mudstones (corrected for metasomatism) range from 70 to 82, and possibly higher, which is indicative of moderate to intense weathering and is consistent with the mineralogic maturity of adjacent arenites. Stirling Quartzite mudstones possess fractionated overall rare earth element (REE) patterns (avg. (La/Yb)N = 10.46) with a consistent negative europium anomaly (avg. Eu/Eu* = 0.68). The LREE are fractionated (avg. (La/Sm)N = 4.24), whereas the HREE are relatively flat (avg. (Gd/Yb)N = 1.49). All of these characteristics are typical of a continental crust source. Mixing calculations that balance REE profiles and Th/Sc suggest a proportion of 88 % granodiorite and 12 % high-K granite for source rocks. A detrital zircon age profile [grains analyzed by sensitive high resolution ion microprobe (SHRIMP)] from a lower Stirling Quartzite sample has peaks at 1452 Ma, 1358 Ma, 1236 Ma, and 1054 Ma, and a number of peaks in the Archean. A sample from the upper member has a different profile with major peaks at 1432 Ma, 1703 Ma, 1782 Ma; there are also grains of Archean age similar to the lower member sample. Integration of the geochemical, isotopic, and geochronologic data reveals that the Stirling Quartzite was derived from a mixture of local and distal plutonic sources, of dominantly continental crustal composition, under moderate to intense weathering. Geochemical comparison of Stirling Quartzite sediments with those derived from rifting (Oronto Group) show significant deviations, consistent with deposition in a passive margin setting, and recent sequence stratigraphic interpretations.Geochemical and petrologic evidence confirm that Johnnie Formation mudstones and sandstones were the initial siliciclastic deposits laid along the Cordilleran Laurentian margin following the Neoproterozoic break-up of Rodinia. Johnnie Formation sediment has corrected CIA values between 63 and 83, and likely higher, which suggests moderate to intense weathering of the source. Modeling suggests the unweathered source likely possessed a composition of a 90 % granodiorite + 10 % high-K granite. This mixture balances petrographic observations, major element geochemistry, and the REE: (La/Sm)N = 4.19 ± 1.26, (Gd/Yb)N = 1.34 ± 0.38, Eu/Eu* = 0.63 ± 0.09 and (La/Yb)N = 9.55 ± 2.27. The hypothesis of a primary tectonic control on sediment composition (i.e. rift-basin deposition) is rejected because Johnnie Formation sediments largely lack lithic fragments that are indicative of derivation from a source area with rugged topography. Feldspars are distributed unevenly in finer grained sediments. Observed fluctuations in feldspar content of sediments from the lower to upper Johnnie Formation are attributed to increased abrasion and hydrodynamic sorting, which differentially segregated feldspars into finer grained sediments. A total of 104 detrital zircon grains from two stratigraphically distinct samples of the Neoproterozoic Johnnie Formation in southeastern California were analyzed by SHRIMP. Samples were taken from quartz arenites in the lower and middle Johnnie Formation, which overlay sediments of rift-basin origin in the Kingston Peak Formation, to ascertain the position within the succession of the rift-to-drift transition. A 207Pb/206Pb age profile of detrital zircons from the lower Johnnie Formation has major peaks at 1749 Ma, and 1658 Ma, a subordinate peak at 1461 Ma, a lesser peak at 1239 Ma, and a few older Paleoproterozoic and Archean grains. A sample from the middle member has peaks at 1428 Ma, 1319 Ma, and 1074 Ma; a number of Paleoproterozoic peaks, and a number of peaks of Archean age similar to the Stirling Quartzite. The middle Johnnie Formation has a greater proportion of late Grenville age detritus, lesser amounts of older ~1400 Ma Mesoproterozoic grains than the either the lower Johnnie Formation or Stirling Quartzite. When combined with detrital zircon data from the overlying strata, these data indicate a general increase upsection of late Grenville age detritus from ~30 % in the middle Johnnie Formation to ~5 % in the upper Stirling Quartzite to ~60 % in the Wood Canyon Formation in response to erosion of source areas, which temporarily shifted the influx of sediments from distal to local sources. These data support a Laurentian provenance for Johnnie Formation sediments consistent with contributions from both distal sources in the cratonic interior and local basement sources in the rift shoulder. Thus the Johnnie Formation-middle Wood Canyon Formation succession formed an early passive margin partly constrained by the continental edge consistent with recent sequence stratigraphic interpretations. A statistical comparison of Johnnie Formation and Stirling Quartzite detrital age distributions to those from Mesoproterozoic successions in the western United States and northwestern Mexico enable linkage of Johnnie Formation and Stirling Quartzite strata to late Meso- to early Neoproterozoic epicontinental basins in Sonora, Mexico (El Alamo Formation, Las Viboras Group; Sierra Chiltepin Formation, Cerro Las Bolas Group) fringing Rodinia's margin, and Neoproterozoic miogeoclinal basins that formed in response to Rodinia's breakup (Rancho Curiel unit, Caddy Canyon Quartzite, Mutual Formation) with stream systems that tapped late Grenville gneisses and intrusives in the core of the orogen.
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