Paper No. 10-4
Presentation Time: 2:00 PM
GEOCHEMICAL AND GEOCHRONOLOGICAL EVIDENCE FOR THE MESOPROTEROZOIC TO EARLY PALEOZOIC EVOLUTION OF THE EASTERN LAURENTIAN MARGIN, GREAT SMOKY MOUNTAINS/BLUE RIDGE BASEMENT COMPLEX
Mapping by the USGS defined the bedrock units of the Great Smoky Mountains basement complex as exposed in the Dellwood-Hazelwood-Sylva region of western NC. Widespread upper amphibolite facies migmatization, complex folding, and reconstitution of mineral assemblages obscure identification of protoliths and tectonic affinity of many units. A decade of petrologic, geochemical, isotopic and geochronologic analysis provides evidence for a protracted history of crustal accretion and reworking. The oldest basement component is a ~1.33 Ga felsic orthogneiss containing mafic xenoliths of ~1.38 Ga age (U-Pb SHRIMP). The other definitively igneous ages are for ~1.15 augen orthogneiss. Ky-Sil grade metasedimentary cover units (Copper Hill/Great Smoky/Otto) have detrital zircon (DZ) U-Pb LA-ICP-MS age distributions that define the “Grenville doublet” (dominant age modes at ~1.05 and 1.15 Ga) with minor modes at ~1.25, 1.35 and scattered Neo- and Paleoproterozoic grains, similar to DZ age patterns for the Ocoee Sgp., indicating a Laurentian margin source and Neoproterozoic depositional age. Much of the region consists of heterogenous, variably migmatitic gray Bt-Plag paragneiss. Analysis of ~2500 DZ grains from paragneisses and metasandstones yields age distributions that are multi-modal and mirror the Ocoee age distributions, consistent with a sedimentary protolith and Laurentian margin provenance. Paragneisses also exhibit a ca. 460 age mode of variable amplitude that correlates with degree of migmatization, consistent with Taconian regional metamorphism. Many DZ ages in paragneiss are discordant and define an array consistent with variable Pb loss at ca. 450 Ma, supporting the inference that regional metamorphism was Taconic. In contrast, two samples of migmatitic paragneiss in the Dellwood quadrangle yield age distributions that lack one of the Grenville age modes and/or contain a major mode at 1.7-1.8 Ga. The protoliths of these paragneisses could represent syn-Grenville sedimentary units or exotic crustal components. Feldspar Pb isotope analysis and whole rock Nd isotope analysis of several of the dated lithologies confirm the presence of a non-Laurentian crustal component (Amazonia?) and mostly derivation (crustal recycling) from older (non-juvenile) crustal sources of 1.6 to 1.8 Ga age.