A HEMATITE (U-TH)/HE MINIMUM AGE FOR CRYOGENIAN TAVA SANDSTONE, COLORADO, AND VARIATIONS IN DETRITAL ZIRCON PROVENANCE THAT ILLUMINATE THE PALEOGEOGRAPHY OF THE REGION (Invited Presentation)

A cryptic record of Cryogenian paleogeography is preserved in Colorado in sparse yet widespread outcrops of Tava sandstone, a deep red to purple sandstone and quartzite which forms large fault-bounded panels and injected dikes within Proterozoic crystalline rock. Tava sandstone consists of granule- to cobble-sized clasts of dominant polymict quartz and subsidiary granite suspended in a non- to poorly-sorted sand matrix, and may be aptly termed diamictite. At 3 locations, specularite veins cut injectites.

A stratigraphic context is lacking, but detrital zircon (DZ) U-Pb age data reveal contrasting provenance for injected versus in-faulted diamictite. All 13 Tava DZ samples display a 0.97 to 1.33 Ga age distribution attributable to the Grenville orogen. Injectites are distinguished by ca. 1.4 and 1.7 Ga DZs indicative of local plutonic sources. The DZ age distinctions suggest an integrated, well-homogenized sediment during formation of the major Tava sandstone phase, contrasting with emergent bedrock highs and segmented sediment transport at the time of sandstone liquefaction and injectition.

To provide independent constraints on a younger age limit for Tava sandstone, we obtained hematite (U-Th)/He dates on coarse but variable grain size specularite. We compare these results to zircon (U-Th)/He data from one Tava sample, and both zircon and apatite (U-Th)/He data from the Boulder Creek Granodiorite host. Individual hematite He dates for four polycrystalline samples range from ~610-145 Ma. Samples characterized by distributions of thick hematite plates appear to yield the oldest dates (610-512 Ma and 600-318 Ma), with additional work underway to test the date-grain size relationship. Apatite He dates from host rock are 111-59 Ma. Individual Tava sandstone zircon He dates of 331-51 Ma exhibit an inverse date-eU correlation, likely from radiation damage accumulation due to prolonged residence of Tava zircons at near surface conditions. The zircon He dates are younger than individual host granite results of 465-415 Ma for the same eU range, implying potential U-Th zoning effects or He loss due to fluid circulation during Ancestral Rockies and/or Laramide tectonism. We interpret the oldest hematite He dates of 610 and 600 Ma from two samples to provide a younger age limit upon the time of injection of Tava sandstone.