THE NATURE AND ORIGIN OF INNER PIEDMONT TACONIAN AND NEOACADIAN MAGMATISM: SYNTHESIS OF RECENT AND NEW GEOCHEMICAL DATA

The Brindle Creek fault (BCF) separates Laurentian-affinity, early Paleozoic Tallulah Falls and Poor Mountain Formations of the eastern Tugaloo terrane (western Inner Piedmont, WIP) intruded by Ordovician-Silurian granitoids from mixed Laurentian and Peri-Gondwanan affinity, Silurian-Devonian metasedimentary units of the Cat Square terrane (eastern Inner Piedmont, EIP) intruded by Devonian-Mississippian granitoids. Ordovician to Silurian volcanic arc granitoids of the WIP are characterized by strong depletion of incompatible elements (<100 sample/chondrite); high K/Na, Ti, HFSE, LIL and REE; low Sr (50-305 ppm) and initial ε_Nd values (-2 to 2); and negative Eu anomalies. These felsic-intermediate plutons are contemporaneous and spatially associated with Poor Mountain Amphibolite-metatuff, which has a MORB or back-arc origin. Tectonic discrimination plots, ε_Nd, ⁸⁷Sr/⁸⁶Sr, and δ¹⁸O indicate the younger, Devonian-Mississippian granitoids of the EIP are intraplate, anatectic melts, probably derived from melting local metasedimentary or basement rocks.

New major and trace element geochemical analyses, in addition to its location in the BCF, supports a WIP tectonic setting for the Silurian Brooks Crossroads Granodiorite. The EIP Rocky Face pluton that occurs in the southern portion of the Gilreath 7.5-minute quadrangle, NC is a moderate to well-foliated, medium-grained, leucocratic ± epd + gar + musc + bt granite containing biotite gneiss and amphibolite xenoliths. The Rocky Face pluton may be the northeast equivalent of the Toluca Granite because of petrographic similarities and characteristic low TiO₂ (0.63-0.286 wt.%) and U/Th (0.018-0.090 ppm) values. A map-scale body of amphibolite, uncommon in the EIP, borders the southern contact of the Rocky Face pluton (Hiddenite quadrangle). The Hiddenite amphibolite has a flat REE pattern and plots as N-MORB on tectonic discriminant diagrams suggesting that it may represent the relict ocean floor on which EIP sediments were deposited.