THE CRYOGENIAN INTRA-CONTINENTAL RIFTING OF RODINIA: EVIDENCE FROM THE LAURENTIAN MARGIN IN EASTERN NORTH AMERICA

Global cycles of continental collision and break-up have been recognized since the proposal of the plate tectonics model in the 60’s. Although this model is widely accepted today, it is still not clear what processes are responsible for those cycles, especially the break-up of supercontinents. The geologic history of the Eastern North American (Laurentian) margin encompasses two complete Wilson cycles that brought about the assembly and subsequent disaggregation of two supercontinents, Rodinia and Pangea. In the southern and central Appalachian region, Mesoproterozoic basement rocks were affected by two episodes of crustal extension separated by >100 m.y.; a Cryogenian phase spanning the interval 765-700 Ma, and an Ediacaran event at ca. 565 Ma. During the Cryogenian phase, the continental crust was intruded by numerous A-type felsic plutons and extensional mafic dikes. At ca. 760-750 Ma a bimodal volcanic sequence erupted onto the uplifted and eroded basement. This sequence, known as the Mount Rogers Formation (MRF), comprises a bimodal basalt-rhyolite lower section, and an upper section of dominantly peralkaline rhyolitic sheets As few constraints exist on the processes that trigger supercontinental break-up, we provide new field and geochemical evidence from the well-preserved volcanic rocks of the Cryogenian lower MRF, with the goal of elucidating the process that induced the initial stage of the break-up of Rodinia and how this affected the evolution of the Eastern Laurentian margin. The geochemical compositions of the Cryogenian lavas are remarkably similar to modern continental intra-plate settings. Geochemical, geophysical and tectonic evidence suggest that the common denominator controlling the melting processes in these intraplate settings (e.g., East African Rift, Yellowstone-Snake River Plain) is deep mantle plume activity. Evidence from the MRF suggests that the initial phase of extension of the Laurentian margin at ~760 was triggered by mantle plume activity. As suggested by numerical modeling in plume-lithosphere interaction scenarios, it is possible that lithospheric weakness caused by a mantle plume that impacted Rodinia triggered the regional extension and produced the intra-continental rifting of what became the Laurentian margin.