BREAKING UP RODINIA: MAGMATIC AND SEDIMENTOLOGICAL EVIDENCE FOR A MANTLE PLUME TRIGGER

During Neoproterozoic breakup of Rodinia, the Eastern North American (Laurentian) margin experienced an ~200 m.y. history of rifting that occurred in two major pulses. An initial phase of intracontinental rifting between ~ 780-680 Ma was followed by a hiatus in activity before outpouring of basalt at ~ 570 Ma, during relatively rapid extension that led to continental breakup and opening of the Iapetus Ocean. The first magmatic episode was characterized by bimodal volcanism and intrusion of mafic dikes and A-type granitoid plutons. Remnants of Cryogenian anorogenic magmatism are exposed over a distance of >700 km in the French Broad and Shenadoah massifs of the Appalachian Blue Ridge province, and show a general pattern of age progression decreasing from southwest to northeast. The Neoproterozoic (~760-750 Ma) Mount Rogers Formation (MRF) represents the largest preserved eruptive facies of the bimodal, dominantly silicic large igneous province that formed during this early stage of intracontinental rifting. Volcanic rocks in the MRF share geochemical similarity with recent volcanic rocks from the East African Rift and Yellowstone-Snake River Plain, and thus are consistent with mantle plume activity during this phase.

Identifying evidence for plumes in the geologic past is complicated by subsequent erosion and, commonly, deformation associated with later tectonic events including continental collision. Plume-related features that may be preserved in the geologic record, however, include significant domal uplift, that precedes rifting. Uplift can lead to a widespread sedimentary hiatus between basement rocks and plume-related volcanics. In the region there is no rock record between ~1.0 to 0.76 Ga, although recent U-Pb dating of rhyolite clasts in conglomerate associated with the MRF volcanics has yielded ages of clasts as old as 780 Ma, demonstrating the existence of a larger volcanic field of longer duration that was uplifted and rapidly eroded. A major unconformity between the Mesoproterozoic basement and the MRF and related rocks is consistent with pre-rift doming and uplift. Together, the field and geochemical data are best explained by a mantle plume that impacted Rodinia, triggering the regional extension and intracontinental rifting that preceded breakup of the Laurentian margin.