DISMEMBERED OPHIOLITES OF THE ASHE METAMORPHIC SUITE OF NW NORTH CAROLINA, USA

Bodies of metaultramafic rock (mu), widely distributed within the Ashe Metamorphic Suite (AMS) of the western North Carolina Blue Ridge Belt, include traditional Mg-amphibole-bearing "amphibolites" with quartz ± feldspar + Mg-amphibole (QF-amphibolites) plus quartz-deficient, locally talc-, chlorite-, and Mg-amphibole-bearing rocks (TC-amphibolites). The mu bodies range from equant, upper amphibolite/ eclogite facies masses in the southwest to highly elongate, chlorite- or amphibole-dominated TC-amphibolite bodies metamorphosed to lower amphibolite to greenschist facies in the northeast. Mineralogical and textural data from mu bodies reveal three to five metamorphic assemblages that show decreasing grade over time. The mu protoliths range from dunite-peridotite-pyroxenites to metabasites.

Our analyses support the idea that a few mu TC-amphibolite rocks had basic (basaltic/gabbroic) protoliths, while contravening arguments support a metasomatic hypothesis of origin for all TC-amphibolites. We establish (1) that some metaultramafic bodies are petrologically heterolithic; (2) that bodies of TC-amphibolite are in contact with all types of country rocks rather than primarily with QF-amphibolites; (3) that although basic in chemistry, most so-called Todd-type rocks with high Cr-, Ni-, and Mg, had olivine or pyroxene-dominated protoliths; (4) that the mineralogy and textures of TC-amphibolites exhibit up to three or more periods of metamorphism; and (5) that contacts thought to be metasomatic can be shown, in well exposed locales, to be structural. MgO-CaO-Al₂O₃ and trace element chemistries of many of the TC-amphibolites match most closely the compositions of plagioclase peridotites. These data suggest that mu and associated metamafic rocks of the AMS represent fragments of dismembered ophiolites. Different metaultramafic bodies have different chemistries that were largely inherited from the protoliths and different mineral assemblages that reflect both chemical composition and metamorphic history.