Paper No. 3
Presentation Time: 1:45 PM
AGE OF THE WALDEN CREEK GROUP REVISITED, REHASHED AND FINALLY RESOLVED? EVIDENCE FROM AGES OF DETRITAL ZIRCON, DETRITAL MONAZITE, AND METAMORPHIC/DIAGENETIC MONAZITE FROM THE WHILHITE, SANDSUCK, AND SHIELDS FORMATIONS, WESTERN GREAT SMOKY MOUNTAINS
Originally interpreted to be Precambrian in age (King 1964), subsequent discovery of calcareous microfossils (Unrug et al. 2000) and conodonts (Repetski et al. 2006) place the upper Ocoee Walden Creek Gp. (WCG) in a younger depositional framework (Silurian or younger). A Paleozoic age requires an origin as an exotic unit emplaced during a later tectonic event or post-Taconic successor basin. Detrital zircon (DZc) U-Pb geochronology (LA-ICP-MS), detrital monazite (DMnz) Th-Pb geochronology (SIMS), feldspar compositions (EPMA), zircon-tourmaline-rutile (ZTR) indices, and framework mineral modes were used to assess WCG provenance. Similar analysis of lower (Snowbird Gp.: SG) and middle (Great Smoky Gp.: GSG) Ocoee units (Chakraborty et al. 2012) provides a basis for comparison. Feldspar compositions in the WCG are similar to the SG and GSG. The WCG heavy mineral suite is dominated by Tour and Ru, rather than Zc, Ap, and Ep, as in SG and GSG, but is similar to the SG and GSG in its low modal abundance of Mnz. DZc and DMnz U-Pb ages were obtained for three formations of the WCG (four Wilhite Fm., two Shields Fm., one Sandsuck Fm.; n = 620 grains). DZc grains were imaged by CL/BSE to ensure LA analysis of single Zc growth zones. DZc ages of all units in the WCG show similar distributions: dominant age modes are ca. 1000 and 1150 Ma, with minor modes at 1450 and 650 Ma. The youngest age (605+/- 5 Ma), is from the Wilhite Fm. DZc age distributions closely match the underlying Ocoee and its local Mesoproterozoic basement sources. If the WCG units analyzed here are indeed Paleozoic, then it is remarkable that not a single Paleozoic-aged DZc was found. Petrography, EDS, BSE, and X-ray mapping of DMnz grains separated from indurated, Chl-grade metasandstones revealed the presence of DMnz grains and metamorphic/diagenetic Mnz grains. Ages of rounded, inclusion free DMnz are 920 to 1210 Ma. Ages of xenoblastic, inclusion-rich Mnz are 430 to 540 Ma. Textures and the age range for the latter are interpreted to correspond to time of Mnz growth during Cambrian diagenesis and Ordovician (Taconian) metamorphism; i.e., the WCG sandstones were deposited before Taconian orogenesis. Final resolution of the age of the WCG will require DZc and DMnz analysis in the specific samples in which Paleozoic fossils were documented.