CLINOPYROXENE-BEARING ROCKS: A RARE COMPONENT IN BLUE RIDGE METAULTRAMAFIC ROCK BODIES

Ophiolites from different tectonic settings are characterized by different assemblages of rocks, including basalt/gabbro and differentiates, dunite, and various types of peridotites and pyroxenites. Recognition of the relative abundances and combinations of mafic and ultramafic rocks in alpine orogens provides clues to the tectonic setting in which these rocks formed.

Metaultramafic rocks of the Blue Ridge Belt of western North Carolina and northern Georgia are mainly olivine-rich metadunite/metaharzburgite or hydrated amphibole-chlorite-talc schists. Metaorthopyroxenite is present in a few metadunite bodies, but clinopyroxene(Cpx)-bearing metaultramafic rocks are rare. Based on optical properties, various workers identified Cpx-bearing rocks in several metaultramafic bodies (e.g. Frank, NC), but the presence of Cpx was not confirmed in our studies.

The only confirmed Cpx-bearing metaultramafic rocks occur in the Webster-Addie Complex (WAC) of western North Carolina. Discontinuous bodies of metadunite in the WAC host lenses of metawebsterite, metaclinopyroxenite, and metaorthopyroxenite ranging in thickness from a few millimeters to tens of meters. Both orthopyroxene (Opx) and Cpx show extensive exsolution related to recrystallization. Compositions of coexisting host-lamellae pairs yield recrystallization temperatures on the order of 800 to 900˚ C, similar to temperatures recorded in the enclosing metamorphic rocks. Opx is relatively unaltered in WAC rocks, showing only minor replacement by talc. Cpx, however, is commonly partially replaced by tremolite/edenite. This pattern is consistent with the high grade (upper amphibolite to granulite facies) of metamorphism in the area. The recrystallization of Cpx to amphibole observed in WAC rocks may explain the relative rarity of Cpx-bearing rocks in other Blue Ridge metaultramafic bodies.