GEOCHEMISTRY OF RHYOLITE CLASTS FROM THE LOWER MOUNT ROGERS FORMATION, SW VIRGINIA

The Mount Rogers Formation (MRF) is located in southwestern Virginia in the Blue Ridge region of the Appalachian mountain chain. The MRF represents volcanism and sedimentation in an initial stage of continental rifting of the eastern Laurentian margin of Rodinia. The volcanic and sedimentary deposits are regionally separated into upper and lower divisions of the MRF. The upper MRF consists of three rhyolitic units: an older, less evolved Buzzard Rock Rhyolite (755.0 +/- 6.6 Ma); a middle unit dominated by lava flows, the Whitetop Rhyolite (753.3 +/- 2.0 Ma); and a younger, dominantly pyroclastic Wilburn Rhyolite (749.7 +/- 3.1 Ma) [U-Pb zircon ages from Tollo et al., 2012]. The lower MRF contains conglomerates and bimodal volcanic rocks, including the Fees Rhyolite (753.1 +/- 2.7 Ma) along with alluvial fan conglomerates and arkoses. Framework grains in the conglomerates are dominated by cobble to boulder-sized clasts of rhyolite. Our study focuses on investigating the provenance of the rhyolite clasts by comparing their geochemistry with the known MRF rhyolites.

Our recent U-Pb zircon dating of clasts from MRF conglomerate shows that rhyolite clasts fall into three distinctive age groups: ~753 Ma, ~760 Ma, and ~770-780 Ma. The ~753 Ma and ~760 Ma clasts appear geochemically similar to each other, but are distinct from all the MRF rhyolites with an exception of the Buzzard Rock Rhyolite. Clasts in the ~770-780 range show more geochemical variation. All clast samples have a smaller negative Eu anomaly (Eu/Eu* = 0.022-0.029) than the known MRF rhyolites (Eu/Eu* = 0.001-0.007), except for the Buzzard Rock Rhyolite (Eu/Eu* = 0.056, based on one analysis). The clasts appear to be slightly depleted in HREEs in relation to the other MRF rhyolites and they display a less dramatic negative Eu anomaly than the Whitetop and Wilburn Rhyolites. The geochemistry and ages of the clasts are evidence that a broader range of volcanic events occurred during intracontinental rifting of Rodinia than previously recognized.