ERUPTIVE SEQUENCE AND PROCESSES IN A NEOPROTEROZOIC INTRACONTINENTAL RIFT: THE MOUNT ROGERS FORMATION, SW VA

Convergence of supercontinents, and their subsequent rifting and breakup, is a major theme in the theory of plate tectonics. Two end-member models for rifted margins have been proposed: the active rift model, in which a rising plume of magma causes tension and rifting in the overlying crust, vs. the passive model in which stretching in the crust is caused by far-field stresses related to plate movements. In ancient rifts, the relative timing, sequence, and style of volcanic eruptions can help discriminate between these models. The supercontinent of Rodinia was assembled between 1.3-1.0 Ga. The eastern Laurentian margin of Rodinia began to extend at ca. 760-780 Ma, but did not completely break up until ca. 550 Ma, with the opening of the Iapetus ocean. In SW Virginia, the Neoproterozoic Mount Rogers Formation (MRF) records the eruption of bimodal volcanics during early stages of rifting, ~760-750 Ma. Geochemical analyses of basalts and rhyolites from the MRF show a plume ‘geochemical signature’ for the magmas, suggesting that the active rift model may be applicable. It is important to determine if the eruptive sequence and field relationships are consistent with this model.

Along with clastic sedimentary rocks and basalt, the MRF includes several rhyolite members: the Fees, Buzzard Rock, Whitetop, and Wilburn Rhyolites (Rankin, 1993). Each of these is distinguished by its phenocryst assemblage and distinctive textures that indicate emplacement by either lava flows or pyroclastic processes. Previous mapping by our research group has recognized an additional rhyolite body, informally named the Bearpen rhyolite. Based on field mapping, the Bearpen locally appears to occur stratigraphically between the Fees Rhyolite in the lower MRF and the Whitetop Rhyolite in the upper MRF, and is distinguished by clusters of reddish alkali feldspar phenocrysts, flow banding, and presence of fiamme. However, its U-Pb zircon age date of 756 Ma +/- 3.1 Ma overlaps with ages of the other rhyolite members (as reported by Tollo et al., 2012), except for the younger Wilburn Rhyolite. This study describes the petrographic and microscopic characteristics of the Bearpen, in order to understand its formation processes and place in the eruptive sequence.