EASTERN LAURENTIA MESO- TO NEOPROTEROZOIC CRUSTAL EVOLUTION: JUVENILE, REWORKED, EXOTIC, RIFTED AND ALMOST-RIFTED COMPONENTS, BURIED OR BARELY EXPOSED (Invited Presentation)

The eastern margin of Laurentia expanded from 1.5 to 0.3 Ga via addition of juvenile crust (Granite-Rhyolite province), recycling of older crustal material (Laurentian Grenville and Paleozoic Appalachian terranes), and/or transfer of exotic crust (the south-central Appalachian “Amazonian” Grenville). Most of the eastern Laurentian basement in the U.S. is covered and only interpretable via basement well data and geophysics. The focus here will be on petrology, geochronology, and isotope geochemistry. Current interpretation includes: (1) Pb isotope compositions of south-central Appalachian Grenville-aged crust support the exotic Amazonia transfer hypothesis; (2) the exotic crust records the complete Grenville orogenic cycle (~1.35-0.95 Ga) as observed in the Adirondacks; (3) based on Nd-Hf isotopes and U-Pb zircon crystallization ages, Grenville orogenesis (Shawinigan and Ottawan phases) profoundly reworked (penetrative deformation under high-grade metamorphic conditions) extensive tracts of juvenile Archean through Mesoproterozoic crust and added minor new juvenile material. Questions remain: (1) To what extent do typical Grenville crustal components extend southward into the eastern U.S. beneath Paleozoic cover? (2) Where was the Mesoproterozoic Laurentian margin and how is it manifested (distinct or cryptic)? (3) What was the precise timing and mechanism of transfer? (4) How does formation of the East Continent Rift System fit within the overall long-lived collisional setting of eastern Laurentia in the Mesoproterozoic? (5) Does the Grenville Front “exist” where it is projected to intersect the ECRS? Grenville rocks are encountered in the subsurface of Ohio (Geons 16, 14, 12, 11, 10) and Kentucky east of the Grenville Front (Geon 14 granite overprinted by Ottawan granulite-facies metamorphism). In the Blue Ridge basement of western NC/TN, the complete Grenville orogenic cycle is preserved in the exotic component implying a complimentary history to the native component. 1.6-1.9 Ga detrital zircons preserved in high grade paragneisses having the same T_DM ages indicate the reworked crustal component was sedimentary. Pb isotopes for oldest basement components (1.34-1.37 Ga) overlap exotic Amazonian and native Laurentian values implying crustal mixing at depth during Rodinian assembly.