SHRIMP U-PB AGES OF DETRITAL ZIRCONS FROM THE CARVERS GAP GRANULITE GNEISS: NEW EVIDENCE FOR A MISSING RECORD OF SEDIMENTATION AND METAMORPHISM IN THE APPALACHIAN BLUE RIDGE

The Carvers Gap Granulite Gneiss (CG) is a granulite-facies metamorphic rock previously interpreted as a 1.8-Ga orthogneiss. The CG and the metasedimentary Cloudland Gneiss (CLG) constitute the northernmost part of the Mars Hill terrane in NC and TN. Two samples collected about 1.6 km apart near the summit of Roan Mountain, TN-NC yielded zircons that were dated by SHRIMP. Zircons from CG are rounded, pitted, light pink to dark brown, show truncated oscillatory zoning in cathodoluminescence (CL), and lack significant metamorphic rims. In contrast, all zircons from CLG have very large, dark (in CL) metamorphic overgrowths; most of the grains have small, remnant oscillatory-zoned cores. Zircons from both samples are detrital, with varying amounts of metamorphic overprinting. Age populations indicate different sources for the two samples. CG cores have major peaks at 1.68, 1.35-1.18, and 1.02-0.97 Ga. Most cores and rims in CLG are between 1.2 and 1.0 Ga. A maximum age of deposition for both samples is 1.0 Ga. A minimum age constraint for deposition and metamorphism is provided by the cross-cutting Neoproterozoic (0.76 Ga) Bakersville mafic dikes. Our new data indicate that the CG is not 1.8 Ga, and thus should not be correlated with Paleoproterozoic rocks of Amazonia.

CG and CLG are part of a belt of small (~0.25-1 km in length) paragneiss bodies that occur in the Blue Ridge from northern VA to TN-NC. Seven previously analyzed samples of graphitic paragneisses from Virginia consist of alternating layers of quartz-plagioclase schist, garnet-biotite schist, and quartzite that were retrograded to greenschist-facies in the Carboniferous. These samples contain detrital zircons with ages between 1.2-1.03 Ga, which matches the ages of local metagranitoids. Two samples of rocks mapped as “Carolina Gneiss” from western NC also contain detrital zircons that indicate depositional ages of <1.0 Ga. The results of this study suggest that the sedimentary protoliths of the paragneisses were derived from, and unconformably deposited on, Grenvillian basement about or slightly after 1.0 Ga, and metamorphosed at amphibolite- to granulite-facies conditions. These rocks represent a “missing record” of material that was eroded from above the currently exposed basement; as yet, the tectonic setting is not entirely understood.