ORIGIN OF BLUE RIDGE BASEMENT ROCKS, DELLWOOD QUAD, WESTERN NC: NEW EVIDENCE FROM U-PB ZIRCON GEOCHRONOLOGY AND WHOLE ROCK GEOCHEMISTRY

Terrane discrimination in polycyclic continental basement rocks is challenging due to high-grade metamorphism and intense deformation. Based on early USGS mapping the Blue Ridge basement in the Dellwood quadrangle of the eastern Great Smoky Mountains was proposed to consist of augen orthogneisses of Laurentian (Grenvillian) affinity interfolded with migmatitic hornblende and biotite paragneisses (“Carolina Gneiss”) and amphibolites of uncertain affinity. The contact between the augen orthogneisses and Carolina paragneiss was subsequently interpreted as the premetamorphic Hayesville fault (folded boundary between the western and eastern Blue Ridge). However, detailed study reveals that the hornblende gneiss of Hadley and Goldsmith (1963; H&G) is a heterogeneous map unit consisting of (1) metaplutonic rocks (non-migmatitic, unfoliated, xenolith-bearing, Cpx-bearing granites) similar to granulitic gneisses of the western NC basement to the northeast; (2) variably foliated and folded felsic orthogneisses; (3) strongly migmatitic, folded Hbl+Bt-bearing gneisses; (4) foliated and lineated garnet amphibolites more similar to amphibolites originally mapped by H&G, and similar to amphibolites in Clyde quadrangle to the east. Migmatization in Dellwood quadrangle has been dated as Taconian. Field relations, petrology, and geochemistry suggest that felsic orthogneisses are related to metaplutonic rocks via (post-Taconian) progressive deformation and reconstitution. Whole rock XRF geochemistry reveals likely protoliths of felsic gneisses to be geochemically distinct from the enveloping migmatitic Bt±Hbl paragneisses. Preliminary age data on samples of Hbl-bearing rocks (1230 ± 20 Ma zircon U-Pb ion microprobe geochronology; 1304 ± 13 Ma zircon U-Pb LA-ICP-MS) and field relationships suggest that the metaplutonic gneisses and their intensely deformed equivalents are components of the Mesoproterozoic Grenville basement, and not part of a Neoproterozoic cover sequence deposited on the rifted margin of Laurentia, all subsequently metamorphosed during the Taconian orogeny. The age and lithologic relations of Mesoproterozoic rocks and paragneisses in the Dellwood area are similar to those of the Trimont Ridge complex along strike to the southwest in the Franklin area.