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