ND-PB ISOTOPE SYSTEMATICS AND ZIRCON U-PB GEOCHRONOLOGY SUPPORT A NATIVE CRUSTAL COMPONENT WITHIN GRENVILLE BASEMENT, EASTERN GREAT SMOKY MOUNTAINS

Grenville basement in the central and southern Appalachians (SCAB) has long been recognized as potentially being exotic relative to Grenville basement elsewhere in eastern Laurentia (Adirondacks, Texas, Ontario: ATO) and of Amazonian affinity. An anomalously old crustal component (ca. 1.8 Ga) has also been detected within the SCAB in western North Carolina. Basement lithologies in the Dellwood area include migmatitic biotite paragneiss, a variety of granitic orthogneisses exhibiting varying states of deformation, and hand-sample to map-scale masses of amphibolite. Orthogneiss consists of rocks of typical Grenville age (1.13-1.18 Ga: U-Pb zircon SHRIMP and LA-ICP-MS crystallization ages), but older (1.33 Ga: U-Pb zircon SHRIMP) orthogneiss is present, some containing more mafic xenoliths that are as old as 1.38 Ma (LA-ICP-MS). Detrital zircon age spectra (LA-ICP-MS) for three samples of Bt-paragneiss (#grains = 59-100/sample, all imaged by BSE/CL) are strikingly distinct from one another and include major age modes at ca.1.05, 1.15, 1.35 and 1.75 Ga with an age gap between 1.7 to 1.4 Ga. The youngest age is 0.82 Ga; the oldest age is 1.97 Ga. One sample is similar to the Ocoee Supergroup composite detrital zircon age spectrum, and one lacks the 1.15 age mode that defines the Grenville event in eastern Laurentia. T_DM ages for orthogneiss and xenoliths are 1.50 to 1.75 Ga. K-feldspar Pb isotopic compositions for Dellwood samples define a field that overlaps fields defining “juvenile Laurentia” and Amazonia/SCAB (however, the latter field is based on whole-rock Pb isotopic values). The 1.38 Ga xenolith plots in the middle of the juvenile field, which is defined by feldspar Pb compositions from rocks in the ATO and Granite-Rhyolite province. In contrast to previous studies of southeastern U.S. Blue Ridge basement rocks, the Pb data support the presence of a native Laurentian component within SCAB. The 1.33-1.38 Ga aged orthogneisses are similar in age to the oldest crustal component found in other Grenville massifs in eastern Laurentia and Australia (1.30-1.35 Ga), and overlap in age and T_DM with southern Granite-Rhyolite province rocks. These gneisses potentially represent arc magmatism related to initial convergence associated with the assembly of Rodinia.