THE GRENVILLIAN EVOLUTION OF SOUTHEASTERN LAURENTIA AND POTENTIAL TECTONIC LINKS TO WESTERN AMAZONIA

A growing body of evidence supports the hypothesis that southeastern Laurentia, typified by south-central Appalachian basement (SCAB), is an exotic piece of western Amazonian crust accreted during the Grenville orogeny and orphaned during Neoproterozoic continental rifting. Similarities in protolith ages and Pb isotopes point to the Paraguá terrane of Amazonia (southwestern Brazil and eastern Bolivia) as the likely source for SCAB. The pre-Grenvillian southeastern Laurentian margin consisted of Southern (1.38-1.35 Ga) and Eastern (1.50-1.40 Ga) Granite-Rhyolite crust; the latter is present in the subsurface of eastern Kentucky and Tennessee. The oldest phase of the Grenville Orogenic Cycle in SCAB is represented by crystalline basement of the Great Smoky Mountains Basement Complex (GSMBC), which consists of 1.35-1.30 Ga (pre-Elzevirian) granitic orthogneiss and mafic xenoliths that represent some of the oldest crust in Appalachian Grenville massifs. These rocks correlate with Geon 13 pre-Elzevirian crust in VT, NY, ONT, and QUE. Shawinigan (Geon 11) and Ottawan (Geon 10) magmatism dominate in SCAB and GSMBC but Elzevirian crust (Geon 12) is minor. Two suites of migmatitic paragneiss in the GSMBC are resolvable based on DZ ages: (1) pre-Ottawan, which exhibits major DZ age modes at 2.0-1.6 and 1.4-1.3 Ga, requiring a component of older (exotic) Proterozoic crust in the sediment source, and has Ottawan metamorphic overgrowths; and (2) post-Ottawan paragneiss with Neoproterozoic depositional ages and Taconic metamorphic ages. Neodymium T_DM model ages for granitic orthogneiss, xenoliths, and older paragneiss range from 1.8-1.6 Ga, indicating that the rocks were derived from recycling of Proterozoic crust (i.e., they are not juvenile), consistent with the 2.0-1.6 Ga detrital zircon grains in pre-Ottawan paragneiss and with 1.8-1.7 Ga inherited zircon in some 1.33 Ga orthogneiss and xenoliths. Whole rock Pb isotope compositions of GSMBC rocks overlap the field of compositions characteristic of western Amazonian terranes and of other basement rocks from SCAB. Initial Paraguá-Laurentia collision occurred ca. 1.35-1.32 Ga with final transfer of SCAB to Laurentia occurring after 1.20 Ga within the sinistral oblique collision zone between Laurentia and Amazonia inferred by previous workers.