MAFIC ROCK GEOCHEMISTRY AS A TOOL TO CONSTRAIN CENTRAL BLUE RIDGE TERRANES IN WESTERN NORTH CAROLINA

Terrane analysis provides an important framework for tectonic interpretations of the southern Appalachian Blue Ridge (BR). Lithotectonic terranes within the Western (WBR), Central (CBR), and Eastern Blue Ridge (EBR) provinces have been defined using litho-stratigraphic mapping and structural analysis; age determinations of plutonic rocks and detrital grains in metasediments; and metamorphic ages. Challenges in terrane analysis, particularly in the CBR, include complex deformation and amphibolite-granulite grade metamorphism with variable migmatization; lack of fossils; and olistostromes, interpreted by some as accretionary block-in-matrix sequences, that complicate stratigraphic correlations. In modern environments, mafic rock geochemistry is a critical means of distinguishing tectonic setting. We suggest that mafic rock geochemistry can also be a powerful tool for distinguishing among CBR terranes.

We have conducted major, trace, and rare earth element (REE) analyses of amphibolites and other mafic rocks from the Cartoogechaye terrane (CT) and Mars Hill terrane (MHT) of the CBR. We compare our data to published results for amphibolitic rocks from across the BR, and from the Bakersville metagabbro dikes. WBR mafic rocks are geochemically quite uniform, while EBR mafic rocks vary widely. CBR mafic rocks display two fairly distinct geochemical “fingerprints”. Rocks from the southern CT and northern Cowrock (CR) terrane in SW NC, south of the Trimont Ridge complex, display tholeiitic trends, relatively high V/Ti, and LREE depleted rare earth patterns consistent with formation in a mid-ocean ridge setting On an MgO vs. Al₂O₃ plot most fall within or near a delineated “cumulate triangle,” suggesting cumulate mafic protoliths, and many preserve field and petrologic characteristics of cumulates. MHT and northern CT amphibolites follow calc-alkaline trajectories, have low V/Ti and REE patterns more typical of arc settings, and cluster tightly outside the “cumulate triangle” in MgO vs. Al₂O₃ space. Our data from entrained mafic assemblages points to the possibility of a boundary within the CT near Trimont Ridge, and the potential utility of using geochemistry to distinguish more finely among BR terranes.