ISOTOPIC CONSTRAINTS ON THE TIME OF DEPOSITION OF THE THUNDERHEAD SANDSTONE, OCOEE SUPERGROUP: U-PB GEOCHRONOLOGY OF MONAZITE AND XENOTIME OVERGROWTHS

The Thunderhead Sandstone is in the lower part of the Great Smoky Group (GSG), in the western Blue Ridge of TN/NC. Rocks of the Snowbird Group are structurally below the GSG (juxtaposed along the Greenbriar fault) and are structurally above rocks of the Walden Creek Group (WCG; juxtaposed along the Dunn Creek fault). Collectively, these three Groups comprise the Ocoee Supergroup which lies unconformably above Mesoproterozoic basement gneiss. Elsewhere, rocks of the WCG and GSG probably are stratigraphically above rocks of the Snowbird Group. The WCG is overlain by Early Cambrian rocks of the Chilhowee Group; the contact is both transitional and locally unconformable. Traditionally, rocks of the Ocoee Supergroup are interpreted as Neoproterozoic in age, but reported fossils suggest a controversial Paleozoic age for rocks of the WCG. This geochronologic study attempts to determine the age of deposition of the Thunderhead Sandstone by dating overgrowths of xenotime (YPO4) and monazite (CePO4) using SHRIMP U-Pb microanalysis.

Thin (<20 microns), delicate, discontinuous overgrowths of xenotime (formed epitaxially on isostructural detrital zircon) and monazite (overgrowing detrital monazite) are interpreted as post-depositional in origin (i.e. diagenetic and(or) metamorphic). Both minerals are dated in situ from polished thin sections, preserving textural information. Because the overgrowths can form during multiple events, the oldest post-depositional overgrowths represent the minimum age of deposition of the sediment. Conversely, the youngest detrital zircon and monazite ages are the maximum age of deposition. Ideally, there is a small time span between minimum and maximum ages.

Chlorite-grade Thunderhead Sandstone was sampled along the Little River, opposite Meigs Falls, TN. Monazite overgrowths yield an age of 670 ± 21 Ma (9 analyses on 5 grains); xenotime overgrowths yield an age of 560 ± 16 Ma (11 analyses on 7 grains). Detrital zircon and monazite range in age from 990-1200 Ma and 1030-1080 Ma, respectively. These data indicate that the Thunderhead Sandstone was deposited between about 670 and 990 Ma. It is unclear if the monazite age records the time of diagenesis during shallow burial or whether the ages of both monazite and xenotime overgrowths record subsequent low grade metamorphic events.