Geochemical Systematics Among Amphibolitic Rocks in the Central Blue Ridge Province of southwestern North Carolina

The Central Blue Ridge sub-province of the southern Appalachian Mountains preserves an unique and complex geologic history. The Cartoogechaye terrane is the westernmost terrane of the Central Blue Ridge sub-province, and is characterized by extensive olistostromal sequences, including mafic-ultramafic massifs, isolated mafic units, and block-in-matrix structures of varying scales. This study investigates the genetic and tectonic relationships, and regional chemical and metamorphic trends of the amphibolitic rocks entrained within units of the Cartoogechaye and nearby terranes, toward constraining the origins of these regional sequences, and examining the rationale for the current Blue Ridge terrane designations. A distinct compositional variation exists between the northern and southern portions of the Cartoogechaye terrane, evident in the mafic rocks of the terrane. The amphibolite blocks and mega-blocks of the Willets-Addie mafic unit, in the northeastern portion of the Cartoogechaye terrane, indicate igneous rock protoliths of a calc-alkaline composition that are different from the mafic-origin amphibolitic massifs of the southwestern Cartoogechaye terrane (Ryan et al., 2005). Amphibolitic blocks of the Tathams Creek/Sylva area, immediately southwest of the Willets-Addie study site, show rare earth element systematics indistinguishable from the more mafic rocks in the Willets-Addie area, albeit with some chemical variation related most likely to variable migmatization of the rocks regionally. Mafic rocks in the adjoining Mars Hill terrane to the northwest show similar chemical trends, even though the Mars Hill terrane is recognized as different from the Cartoogechaye terrane, based on dating results from enclosing granitiods and migmatitic segregations. In the southwestern Cartoogechaye terrane, the Carroll Knob mafic complex preserves chemical signatures suggestive of ocean crustal origins, similar to the Buck Creek mafic-ultramafic suite (Berger et al. 2001, Peterson et al., 2009). However, the amphibolites in the Carroll Knob complex indicate pyroxene-rich cumulate and gabbroic protoliths consistent with an active oceanic magma system undergoing continuous magmatic replenishment and crystallization. West of the Carroll Knob complex, the Kimsey Bald mafic body includes amphibolites with protoliths comparable to the MORB-like, high-Ti amphibolites of the Buck Creek suite. The few amphibolite samples from the Lake Chatuge complex examined in this study also shows ocean crustal affinities, similar to those in the Buck Creek, Kimsey Bald, and Carroll Knob complexes. The chemical distinctions among these amphibolite suites, and the differences in the inferred crustal ages among their enclosing crustal units point to a possible boundary between the northern and southern regions of the Cartoogechaye terrane, one related either to likely crustal protoliths, or to a change in tectonic environment. The varied types of blocks comprising the Tathams Creek and associated Cartoogechaye units may indicate a transitional zone between the upper plate-derived accretionary sequences observed to the northeast and dominantly lower oceanic plate lithologies exposed in the southwestern extent of the terrane