EVOLUTION OF THE BLUE RIDGE BASEMENT COMPLEX IN THE EASTERN GREAT SMOKY MOUNTAINS: EVIDENCE FROM ZIRCON U-PB GEOCHRONOLOGY AND ND-PB ISOTOPE GEOCHEMISTRY OF BASEMENT GNEISSES

The Great Smoky Mtns. basement complex consists primarily of crustal components that include: (1) ~1330-1350 Ma orthogneiss and entrained mafic xenoliths that represent some of the oldest crust in Appalachian Grenville massifs (similar to pre-Elziverian rocks in the Adirondack, Green Mtn., Hudson Highlands and French Broad massifs); (2) ca. 1150 Ma augen and granitic orthogneiss correlating with the Shawinigan phase of Grenville magmatism; (3) paragneisses with inferred sedimentary protoliths and metaclastic rocks of demonstrated sedimentary origin (cover rocks) that have either pre- to syn-Grenville (i.e., Mesoproterozoic) vs. post-Grenville (Neoproterozoic) depositional ages, and that experienced Taconian migmatization. Mesoproterozoic paragneisses lack the “Grenville age doublet” typical of all post-Grenville clastic sequences and contain major detrital zircon (DZ) age modes (1.6-1.8 Ga) that require a component of Proterozoic crust in the source region. Youngest DZ ages are consistent with a max. depositional age of ca. 1000 Ma. The Neoproterozoic paragneisses exhibit the Grenville age doublet in detrital zircon age distributions that match the age distribution of Ottawan and Shawinigan magmatic/metamorphic events in eastern Laurentia. These rocks also contain scattered 600-750 Ma DZs that constrain their max. depositional age to late Neoproterozoic. Most zircon U-Pb age systematics exhibit variable lead loss interpreted to result from high-grade Taconian (ca. 450 Ma) regional metamorphism and migmatization. Nd T_DM ages for all gneises are 1.6-1.8 Ga, indicating that all rocks were derived from recycling of Proterozoic crust (i.e., they are not juvenile), which is consistent with 1.6-1.8 Ga DZ ages in pre- to syn-Grenville paragneisses. Pb isotope compositions confirm the presence of an exotic (Amazonian) crustal component in the source region for the protoliths of the pre-Elziverian orthogneisses and xenoliths; this exotic component was incorporated to varying degree in the evolution of the basement complex. This oldest age component may represent an Amazonian pre-Grenville analog to the ca. 1.35 Ga native Laurentian crust present in Adirondack and northern Appalachian basement massifs that all formed on opposing sides of a pre-Rodinian ocean basin closed during Grenville collision.