Insights into the tectonostratigraphic setting of the Southern Appalachians during the Blountian tectophase from an integrated geochemical analysis of magmatic phenocrysts in the Ordovician Deicke K-bentonite

We evaluate competing hypotheses regarding tectonic models of the early Taconic Orogeny in the Southern Appalachians during the Blountian tectophase with new geochemical data obtained from analyses of apatite and zircon phenocrysts, and melt and mineral inclusions therein, from the Ordovician Deicke K-bentonite (453.35 ± 0.10 Ma). Apatite geochemistry confirms that samples from different locations represent the same eruptive event, and zircon geochemistry and UPb geochronology (including several Grenville-age inherited detrital cores) confirm an evolved magma of continental-arc composition, with the trace-metal geochemistry of magmatic Taconic-age rims on Deicke zircons pointing to a provenance of continental crustal igneous rock melts, specifically a granitic melt with a high level of differentiation, rather than a melt that was mantle-derived, or strongly mantle-influenced. Melt inclusions within Deicke phenocrysts are dacitic (apatite) to rhyolitic (zircon), differences that imply an evolving Deicke magma during its eruption.