THE NATURE AND EXTENT OF EXOTIC CRUSTAL COMPONENTS IN SOUTHERN LAURENTIA: NEW AGE AND ND-PB ISOTOPE EVIDENCE FROM GRENVILLE BASEMENT GNEISSES, EASTERN GREAT SMOKY MOUNTAINS

The recognition of an old (1.8-1.6 Ga), cryptic crustal component within southern Appalachian (Grenvillian) Blue Ridge basement with age and Nd-Pb isotopic systematics distinct from Laurentia has led to the proposal that much of that basement is exotic to Laurentia and ultimately of Amazonian origin (Tohver et al. 2004). Another previously unrecognized cryptic age component was recently identified within the Mesoproterozoic basement complex of the Great Smoky Mtns. (GSM; Anderson & Moecher 2008; Loughry & Moecher 2010). Granitic to mafic orthogneisses originally mapped as Cherokee gneiss were discovered to be considerably older than the majority of Grenville metaplutonic basement orthogneisses (1.05-1.15 Ga) comprising the basement complex: single spot U-Pb ion probe ages ranging from 1.25 to 1.35 Ga were obtained for granitic and mafic orthogneisses. Such ages correspond to the oldest age component usually found among Grenville rocks in eastern Laurentia, which are proposed to be juvenile and the ultimate source of more abundant 1.0 to 1.2 Ga Grenville magmas. Detailed zircon U-Pb geochronology and Nd-Pb isotope analysis currently in progress will attempt to refine age relations and test whether these older Grenville components and more typical 1.15 Ga granitic orthogneisses are: (1) petrogenetically related (source and extracted magma), (2) tectonically juxtaposed, and (3) are of Laurentian or Amazonian affinity.

A concordant SHRIMP II U-Pb crystallization age of 1331+8 Ma (1σ) was obtained for oscillatory zoned zircon from an equigranular granitic orthogneiss that contains mafic to intermediate xenoliths. Nearby granitic orthogneisses similar to other Grenville basement rocks yields a SHRIMP II U-Pb crystallization age for oscillatory zoned zircon of 1159+4 Ma (1σ). Nd isotopic analysis of these and related rocks are currently in progress and will shed light on source characteristics. Whole-rock and K-feldspar Pb isotopes are being measured to compare the Pb composition of these units with published data for Grenville-aged units in eastern Laurentia, Mexico, and Amazonia.