GEOCHEMISTRY AND PETROGENESIS OF THE CARROLL KNOB MAFIC/ULTRAMAFIC COMPLEX, MACON COUNTY, NORTH CAROLINA

The Carroll Knob (CK) mafic/ultramafic complex in the SW Blue Ridge of NC includes extensive outcrops of amphibolites and meta-gabbroic rocks, along with uncommon exposures of altered dunite and magnesian schists, and rare metatroctolites. Samples of representative lithologies were collected from three areas of CK that were mapped and surveyed geophysically during the 2000 REU Summer program. Major element and trace element compositions of samples were obtained via DC plasma emission spectrometry.

Volatile-free elemental abundances in CK rocks (MgO: 7.69-38 wt%; Al₂O₃:5.69-19.8 wt%; Fe₂O₃: 6.9-19.3 wt%; TiO₂: 0.05-0.98 wt%; Cr: 36-390 ppm; Ni: 29-1100 ppm; Zr: 12-35 ppm; Sr: 14-180 ppm; Y: 6-15 ppm) show similarities to modern oceanic gabbros and ultramafic cumulates. On major element covariation diagrams, CK amphibolite compositions are consistent with cumulates of plagioclase + pyroxene ± olivine, a contention supported by relict orthocumulate textures observed in some outcrops. CK amphibolites plot with calc-alkaline basalts and low-K tholeiites on Pearce-type discrimination diagrams, but very low concentrations of Zr, Y, and Ti in CK rocks point to pyroxenite/gabbroic protoliths.

Our data for CK amphibolites are in good agreement with the earlier results of Walter (1990). While the immobile elements Zr, Y, and Ti are lower overall in amphibolites from CK than in Buck Creek (BC) amphibolites, or in rocks from the Kimsey Bald mafic complex to the west, CK amphibolite compositions are similar to "type 2" Buck Creek amphibolites (Berger et al. in press), which are believed to represent gabbroic to troctolitic cumulate protoliths. While we agree with the contention of Walter (1990) that BC, CK, and Kimsey Bald mafic rocks largely reflect cumulate sequences, the available data leaves unclear the tectonic environments in which these units developed.