GEOCHRONOLOGIC CONSTRAINTS ON THE TIMING OF DEPOSITION AND METAMORPHISM OF THE OCOEE SUPERGROUP: SHRIMP U-PB DATING OF XENOTIME AND MONAZITE OVERGROWTHS ON DETRITAL MINERALS

The Ocoee Supergroup, western Blue Ridge of TN/NC, is a sequence of clastic (coarse to fine), mainly arkosic, rift-facies metasedimentary rocks. It is subdivided into the Snowbird, Great Smoky (GSG), and Walden Creek (WCG) Groups, which are separated by major thrust faults. The Ocoee Supergroup has been interpreted as Neoproterozoic because it was deposited on Mesoproterozoic basement rocks, and WCG rocks are in conformable contact with the overlying Early Cambrian Chilhowee Group. However, reported Paleozoic fossils suggest that the WCG may be younger than Cambrian. Using SHRIMP, we have dated diagenetic and/or metamorphic overgrowths of xenotime (on detrital zircon) and monazite (on detrital monazite) to determine constraints on the times of deposition of four units from the Ocoee Supergroup: the Thunderhead and Cades Sandstones of the GSG, and the Shields and Sandsuck Formations of the WCG. Ages of detrital monazite and zircon from all samples are primarily Mesoproterozoic (1.1 to 1.0 Ga); the youngest age populations (~950 Ma but possibly 870 Ma) represent the maximum age of deposition. In contrast, the oldest post-depositional overgrowths represent the best estimate of the minimum age of deposition. Xenotime overgrowths on detrital zircon from the Thunderhead, Cades, and Shields samples are about 570 Ma; xenotime from the Sandsuck is about 400 Ma. Textural evidence indicates that all of these xenotime dates are post-depositional metamorphic ages. Well-preserved monazite overgrowths with metamorphic textures from the Thunderhead formed during several events (685 ± 14, 638 ± 9, and 580 ± 9 Ma). Older monazite overgrowth ages were obtained from the Cades (781 ± 49 Ma; n=3) and Shields (853 ± 21 Ma; n=2). However, these ages are from separated grains, and textural evidence to discriminate between pre- and post-deposition overgrowth is lacking. We conclude that the Ocoee Supergroup was deposited between about 930 and 700 Ma (possibly at about 850 Ma) and was metamorphosed at lower greenschist facies several times in the Neoproterozoic. The ~ 570 Ma thermal event corresponds to the period of Catoctin volcanism in central Virginia to Pennsylvania. However, the older thermal events of ~855, ~780, ~685, and ~640 Ma are not known in Laurentian Neoproterozoic rocks.