INVESTIGATING THE ORIGIN OF GRANITE CLASTS IN CONGLOMERATES OF THE MOUNT ROGERS FORMATION, SW VA

In the Blue Ridge of the Appalachian mountain chain in SW Virginia, Mesoproterozoic basement (~1.3 to 1.0 Ga) is overlain noncomformably by the Neoproterozoic Mount Rogers Formation (MRF), which represents volcanism and sedimentation in an initial stage of continental rifting of the eastern Laurentian margin of Rodinia. The MRF volcanic and sedimentary deposits are regionally separated into an upper volcanic sequence consisting of rhyolitic lavas and pyroclastic flows, and a more diverse lower sequence of bimodal volcanic rocks and alluvial fan conglomerates and arkoses. The conglomerates are dominated by rhyolite and granitoid clasts, which generally are interpreted to have been sourced from the MRF and the basement granitoids, respectively. Rhyolites in the MRF have been dated at ~750-759 Ma. In the region, no exposed bedrock has been dated within the age gap between ~1.0 and 0.76 Ga. Recent U-Pb zircon dating of rhyolite clasts in the MRF conglomerate, however, has yielded three age populations of clasts at 780-770 Ma, 764-756 Ma, and 748-755 Ma. In addition, a pink granite clast yielded a U-Pb zircon age of 779.5 +/- 11 Ma. The conglomerates contain various plutonic igneous clasts, but the dominant type in all outcrops is the pink granite variety.

Our preliminary study focuses on investigating the granite clasts in the conglomerates using petrographic comparison of the pink granite clasts in the MRF conglomerates with our existing basement granitoid samples. With the recent dating of the pink granite clast as ~780 Ma, our study tests the previous interpretation that the pink granite clasts in the MRF conglomerates originate from the basement. Using the IUGS classification, our basement granitoids plot as tonalite and granodiorite, whereas granite clasts plot as granite and alkali feldspar granite. This is an early step in gaining insight as to whether the clasts are related to the Mesoproterozoic basement or another event that took place around the 780 Ma timeline. Our ongoing study will focus on systematically sampling basement granites for comparison with MRF conglomerate clasts, as well as those in the overlying glaciogenic deposits of the Konnarock Fm.