HELPING DEFINE BLUE RIDGE TERRANES USING AMPHIBOLITE CHEMISTRIES – AN EXPERIMENT IN THE CENTRAL BLUE RIDGE OF WESTERN NORTH CAROLINA

Advances in the application of geochronologic analysis over the past decade have resulted in a significant re-alignment and re-definition of lithotectonic terranes within the Blue Ridge province, and have also raised questions about the affinities of lithologically diverse rock units within terranes, as well as markedly similar units in different terranes. Challenges in terrane analysis approaches in the Blue Ridge, including complex deformation, high grade metamorphism with variable migmatization, lack of fossils, and the prevalence of olistostromes, interpreted by some as accretionary block-in-matrix sequences, that complicate stratigraphic correlations, point to the need for further tools for use in establishing regional lithologic and structural relationships. Mafic rock geochemistry has historically been an important means of distinguishing tectonic setting in modern environments, so we seek to test this approach, examining associated amphibolite rock units as a means of distinguishing among terranes in the Central Blue Ridge (CBR).

We have analyzed bulk compositions and rare earth element (REE) abundances of amphibolites mapped within the Cartoogechaye and Cowrock terranes (CT; CR), Dahlonega Gold Belt (DGB) and Mars Hill terrane (MHT) of the CBR, and compared our results to published data for other Blue Ridge amphibolites and the Bakersville metagabbro dikes. Our mafic rock data display two fairly distinct geochemical “fingerprints”. Rocks from the southern CT and CR terranes south of the Trimont Ridge complex, display tholeiitic fractionation trends, high V/Ti, and light-REE depleted rare earth patterns consistent with formation in a mid-ocean ridge setting. Major element chemistries suggest cumulate mafic protoliths, and many preserve field and petrologic characteristics of cumulates. MHT and northern CT amphibolites follow calc-alkaline fractionation trends, show low V/Ti and preserved REE patterns more typical of arc settings, and preserve major element signatures more consistent with lavas or non-cumulate intrusive rocks. Our amphibolite data points to the possibility of a boundary or lithologic discontinuity within the CT near Trimont Ridge, suggesting there may be some utility in using geochemistry to resolve more finely among BR terranes.