UNRAVELING THE PROTRACTED HISTORY (1800-450 MA) OF SOUTHEASTERN LAURENTIA RECORDED IN METAMORPHIC ROCKS OF THE GREAT SMOKY MOUNTAINS BASEMENT COMPLEX (GSMBC)

Lead isotope compositions for eastern Laurentia (Grenville of Adirondacks/Texas, Appalachian Inliers, Granite-Rhyolite provinces) and Amazonian Grenville-aged crust demonstrated that southern Appalachian Grenville basement is largely exotic and was transferred to Laurentia from Amazonia during Rodinia-forming collision. The Mars Hill terrane (MHT), a third distinct crustal component, was distinguished by Pb isotope compositions and putative ca. 1.8 Ga meta-igneous rocks. The latter have since been shown to be meta-sedimentary rocks (Aleinikoff et al. 2013) with a distinct detrital zircon (DZ) age mode at 1.6-1.8 Ga that requires an unusually old source for the MHT. Few other details of the history of the putative exotic Amazonian Grenville rocks and the Laurentian-Amazonian collision zone have been elucidated. New Pb-Nd isotope analysis and zircon U-Pb geochronology of ortho- and paragneisses in the GSMBC reveal the following results: (1) the oldest rocks are ca. 1.35 Ga granitic orthogneiss and mafic xenoliths that have native Laurentian Pb isotope affinities and were tectonically mixed with the regionally more extensive Amazonian crustal component; they are the same age as the oldest crust throughout the Appalachians and Adirondacks. (2) Pre-Grenvillian paragneisses (sedimentary protolith inferred from heterogeneous DZ zoning and multiple DZ age modes) contain major DZ age modes at 1.6-1.8 and 1.4-1.3 Ga with a major Ottawan-Rigolet (1050-980 Ma) metamorphic zircon age mode proving a pre-Grenvillian depositional age; (3) Shawinigan orthogneisses (ca. 1150 Ma) must have intruded the older basement complex of 1.35 orthogneisses and paragneisses; (4) post-Grenvillian paragneisses dominated by the “Grenville age doublet” (Ottawan and Shawinigan age modes) derived from the now fully exhumed Grenville orogen, with minor older age modes recycled from pre-Grenvillian paragneisses. Depositional age of the latter was ca. 600 Ma based on youngest DZs. Taconian metamorphism overprints almost all original lithologic features in the GSMBC resulting in widespread zircon U-Pb discordance that reduces the number of useful DZ ages, which can be corrected for using the known time of discordance. The pre-Grenvillian paragneisses are similar to MHT meta-sedimentary rocks in having the 1.6-1.8 Ga DZ age mode.