THE REAL MCCOY: STRATIGRAPHY, GEOCHRONOLOGY, GEOCHEMISTRY, AND A NEW AGE MODEL OF THE MCCOY CREEK GROUP IN EAST-CENTRAL NEVADA

The timing and nature of Neoproterozoic rifting of the western margin of Laurentia remains poorly constrained. The McCoy Creek Group (MCG) of east-central Nevada provides a distal record of Ediacaran rifting on the western margin of North America. This study presents detrital zircon geochronology and stable isotope geochemistry from measured sections of the MCG. Detrital zircon from the lower MCG show a cosmopolitan age spectrum with source rock ages from 1.0-1.8 Ga. Detrital zircon from the uppermost member of the MCG, the Osceola Argillite, produce a spectrum with ages from 1.1-2.9 Ga and a peak at 1.8 Ga. Stable isotope geochemistry of MCG limestones show enriched to moderately depleted δ¹³C values near its base and extremely depleted values (~ -12‰) in the Osceola Argillite. The cosmopolitan detrital zircon age spectrum from the lower MCG is consistent with a broad, distal source area. The age spectrum defined by the Osceola Argillite is consistent with a more local source area, possibly the Transcontinental Arch or Mesoproterozoic rocks of the Belt Supergroup, and suggests that significant topography still existed during its deposition. The extremely depleted δ¹³C values within the Osceola Argillite are consistent with values found in rocks that record the Neoproterozoic Shuram excursion. Previous studies show positive δ¹³C values in lower MCG strata, between +7 and +10‰, which are similar to those found in the ~580-570 Ma Nadaleen Formation of NW Canada. These data facilitate correlations between the Death Valley region, the Nevada-Utah corridor, and through to NW Canada and are consistent with margin-wide Late Ediacaran rifting of the western margin of North America.