GEOCHRONOLOGY AND GEOCHEMISTRY OF BASEMENT ROCKS, DELLWOOD AREA, EASTERN GREAT SMOKY MOUNTAINS (NC)

Precambrian basement for Ocoee rocks in the eastern Great Smoky Mts. region is exposed in the Dellwood and Clyde quads. Basement lithologies were mapped (Hadley and Goldsmith, 1963) as (1) ‘Plutonic rocks' (pCga: augen gneiss; pCg: flaser and granitoid gneiss) in lithologic contact with (2) ‘Carolina gneiss' consisting of interfolded layered biotite and muscovite gneiss (pCc) and layered hornblende and biotite gneisses (pCch) with rare amphibolite (pCa) and minor schist. Basement and Ocoee cover define late, open to tight map scale folds. Major and trace element geochemical analysis, petrography, structural patterns, and zircon U-Pb ion microprobe geochronology provide further insight into the origin of the basement complex. Plutonic rocks are generally leucocratic granitic to qtz-dioritic orthogneiss (Bt-Ep-Ttn-Pl-Qtz-Mc+Hbl), locally porphyritic with varying degrees of foliation development and late protomylonitization. Most pCc is migmatite Bt-Ep-Ttn-Pl gneiss with distinct granitic leucosomes (forming refolded isoclinal folds); bulk compositions are broadly felsic. Zircon from all pCg and pCga orthogneiss exhibits CL zoning and age patterns typical of Blue Ridge basement of Grenvillian affinity: zoned magmatic cores yielding ages of 1050-1250 Ma in most orthogneiss samples, some with dull (metamorphic) rims of ca. 1000 Ma; and discreet grains at ca. 460 Ma in leucosome (time of melting). Zircon in migmatitic pCc occurs as course-grained Grenvillian magmatic zircon (1200 Ma); Grenville metamorphic (?) zircon (1000 Ma); rare older grains (1530, 1700 Ma); and small zircons in leucosomes (460 Ma). Overall, the abundance of demonstrably igneous rock compositions and ages point to a Grenville basement complex composed of plutonic rocks intruded into older meta-igneous gneisses. Taconic Ky/Sil-grade metamorphism and migmatization, and ‘Neo-Acadian' folding and biotite-grade ductile deformation overprint and obscure the nature of pCg – pCc contacts. However, the former require an existing infrastructure for intrusion, and the older zircon component support such a role for the latter. The new ages agree with basement ages published by U.S.G.S. researchers and the time of peak Taconic regional metamorphism based on monazite chemical ages for Great Smoky Ky/Sil-grade metapelites.