U-PB GEOCHRONOLOGIC AND PB ISOTOPIC EVIDENCE IN SUPPORT OF THE EXOTIC MARS HILL TERRANE-TRANSFER HYPOTHESIS: COMPELLING LINKS BETWEEN SOUTHEASTERN LAURENTIA AND THE PARAGUÁ CRATON OF SOUTHWESTERN AMAZONIA

Although primarily a Paleozoic collisional orogen, the southern Appalachian Blue Ridge exposed in the Great Smoky Mountains Basement Complex (GSMBC) of western NC contains crustal age components as old as 1.9 Ga. The GSMBC consists primarily of: 1.35 – 1.30 Ga (pre-Elzevirian phase of Grenville orogenic cycle) orthogneiss and mafic xenoliths that represent some of the oldest crust in Appalachian basement massifs; ca. 1.15 (Shawinigan) and 1.05 Ga (Ottawan) granitic orthogneiss; and migmatitic paragneiss derived from clastic protoliths with either pre-Ottawan (late Mesoproterozoic) or post-Ottawan (Neoproterozoic) depositional ages. Pre-Ottawan paragneiss exhibits multiple detrital zircon age modes at 2.0 to 1.5 Ga that require a component of Proterozoic crust in the sediment source region that is much older than any other basement rocks in southeastern Laurentia. Neodymium T_DM model ages for granodioritic orthogneiss and paragneiss range from 1.8-1.6 Ga, indicating that the ~1.33 Ga crustal component was derived from recycling of Proterozoic crust (i.e., it is not juvenile), and consistent with the 1.9-1.6 Ga detrital zircon grains in pre-Ottawan paragneiss and with 1.8-1.7 Ga inherited zircon in orthogneisses. Whole rock Pb isotope compositions of GSMBC rocks overlap the field of compositions characteristic of Amazonian crust and of other basement rocks from the south-central Appalachians. The Pb isotopes and geochronology in orthogneiss, mafic xenoliths, and pre-Ottawan paragneiss are consistent with a correlation of the GSMBC with the Mars Hill terrane (MHT) of the greater Grenvillian southern Appalachian basement that is considered exotic to Laurentia and transferred during Rodinian assembly prior to ca. 1.2 Ga. Similarities in protolith ages, detrital zircon ages, and Pb isotopes point to the Paraguá terrane of Amazonia (southwestern Brazil and eastern Bolivia) as a likely crustal match from which the MHT was derived. Initial Amazonia-Laurentia collision occurred at 1.35-1.32 Ga with final transfer of MHT to Laurentia occurring after 1.20 Ga within the sinistral oblique collision zone between Laurentia and Amazonia. Late Neoproterozoic Rodinian breakup terminated a ~600 m.y. shared history for SE Laurentia and SW Amazonian.