PLUME INDUCED NEOPROTEROZOIC MAGMATISM IN EASTERN LAURENTIA AND ITS BEARING ON THE BREAKUP OF RODINIA

The assembly and breakout of the supercontinent Rodinia is the subject of considerable debate. Although the exact spatial relationship of continental masses within Rodinia is not well constrained, there is considerable agreement that during the late Precambrian the supercontinent was in an extensional phase of the Wilson Cycle. The proposed mechanisms for extensional tectonics leading to rifting within eastern Laurentia range from (1) gravitational collapse of crustal regions overthickened by Grenville orogenesis, (2) flanks of an active encratonic rift zone similar to the Red Sea region and (3) a failed rift associated with a triple junction. In the central Appalachian orogen (Virginia and North Carolina), nearly thirty Neoproterozoic alkaline to metaluminous volcanic/plutonic complexes have been recognized. Although granites and rhyolites dominate these igneous complexes, compositional variations range from gabbro through diorite to syenite. The common occurrence of modal fluorite, hypersolvus alkali feldspar, zoned allanite, biotite and sodic amphibole in the granites, coupled with high SiO2, Na2O+K2O, FeO/MgO, Zr, Nb, Ga and Y, suggest affinity with A-type granites. The mineralogy and chemical composition of these rocks coupled with field data, including micro structural emphasis suggests emplacement of these igneous complexes along extensional fault zones. The distribution of these complexes over a distance of nearly 600km requires the development of a crustal scale thermal anomaly. We utilize the concept of plume head arrival ages, i.e. earliest episode of magmatism in a given region reflecting the initial mantle/crust interaction, to suggest that the pattern of progressive decrease in ages along strike is most likely related to an eastern Laurentian plume. Plume head arrival ages of 765 to 754 Ma in the southern part of the region are measurably older than 730 Ma observed in the north and yield a plate motion rate of 2 cm/year. We suggest that recognizing plume tracks during the Neoproterozoic, and their spatial relationship to the development of rifted continental margins is the challenge in geodynamic modeling of the breakup of supercontinents.