EVIDENCE FOR LATE NEOPROTEROZOIC DEPOSITION OF THE OCOEE SUPERGROUP: SHRIMP U-PB AND TRACE ELEMENT ANALYSIS OF DIAGENETIC XENOTIME AND MONAZITE

The Ocoee Supergroup, western Blue Ridge of TN/NC, is a sequence of clastic, 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 previously been interpreted as either Neoproterozoic or Paleozoic.

SHRIMP U-Pb geochronology and trace element analysis were used to determine the time of deposition of Ocoee units. Ages of detrital zircon and monazite from 7 samples are mostly 1.0-1.2 Ga; the youngest age populations (~950 Ma) represent the maximum age of deposition. Xenotime overgrowths on detrital zircons yield ages of: 562 ± 16 Ma (Thunderhead Sandstone) and 581 ± 140 Ma (Cades Sandstone) for the GSG, and 551 ± 26 Ma (Shields Fm.), and 407 ± 35 Ma (Sandsuck Fm.) for the WCG. Monazite overgrowths on detrital monazites from the Thunderhead formed at 685 ± 14, 638 ± 9, and 580 ± 9 Ma, whereas only early Neoproterozoic monazite overgrowth ages were obtained from the Cades and Shields rocks.

Rare earth element (REE) concentrations of xenotime of known origin (igneous, metamorphic, diagenetic, and hydrothermal) from clastic rocks of the Belt Supergroup in Idaho and Montana, yield diagnostic patterns. When compared to REE data from xenotime of the Ocoee Supergroup, these patterns suggest that ~570 Ma xenotime from 3 units formed during diagenesis and thus approximate the time of deposition of the GSG and WCG. REE patterns for xenotime from the Sandsuck Fm. suggest that ca. 400 Ma overgrowths formed during metamorphism. The Thunderhead monazite age of 580 ± 9 Ma indicates that these overgrowths formed synchronously with the diagenetic xenotime. All older ages of monazite overgrowths in Ocoee rocks are interpreted to have formed in provenances prior to erosion and deposition, i.e. are detrital. Xenotime overgrowths on detrital zircons are fragile and do not survive sedimentary transport, whereas monazite overgrowths are more robust and can be either pre- or post-depositional origin. Thus, caution is advised when interpreting ages of monazite overgrowths.

We conclude that the GSG and WCG were deposited during the late Neoproterozoic, approximately synchronous with clastic sedimentation and Catoctin volcanism (~571-563 Ma) in central Virginia to Pennsylvania during rifting of Rodinia.