The Ashe Metamorphic Suite, Northwest North Carolina; metamorphism and observations on geologic history

Abstract

The Ashe Metamorphic Suite (AMS) consists of amphibolite schist and gneiss (metabasalts), pelitic schist and gneiss (metasediments), and ultramafic rocks. Evidence for four metamorphic events (M1, M2, M3, M4) exists. M1 affected only the ultramafic rocks in the suite. The peak metamorphism, M2, was an upper amphibolite facies event that affected all lithologies in the AMS during the Ordovician Taconic Orogeny. Three M2 zones of the amphibolite facies are recognizable in the pelitic rocks. Conditions during M2 are estimated to have been P > 7 kb and T = 700 degrees C. The effects of M3 and M4 are minor in the pelitic and mafic rocks and identified with the growth of chlorite upon partial hydration of these rocks. There are at least four possible plate tectonic sites for the origin of the AMS: (1) on a passive, ensialic margin, (2) on a passive, ensimatic or transitional margin, (3) on an active, ensimatic margin, and (4) in an ensialic or ensimatic, back-arc basin.--Modified journal abstract.