MICROSTRUCTURES OF MULTIPLE DEFORMATION EVENTS IN A BLUE RIDGE MOUNTAINS DISMEMBERED OPHIOLITE

Many of the ultramafic bodies of the Eastern Blue Ridge consist of complexly layered mafic and ultramafic protoliths, which underwent multiple metamorphic events (e.g. Raymond et al., 2016 in IGR). The Edmonds body, found in the Ashe Metamorphic Suite (AMS) along the boundary with the Alligator Back formation, is the largest coherent block of mostly ultramafic material in this unit. It is mineralogically zoned from NE to SW: 1) mostly talc and chlorite, 2) coarse olivine with talc, tremolite, and serpentine, 3) fine olivine with tremolite, serpentine, and talc, 4) mostly chlorite and amphibole at the southern tip and in layers adjacent to the main body.

A detailed study of the microstructural relationships between these minerals shows consistent patterns in crystallization history. In the olivine-bearing central zones, coarse olivine porphyroclasts are the earliest assemblage. Fine-grained olivine with a strong shape fabric formed by dynamic recrystallization of coarse olivine. Fabric-forming tremolite and talc formed after olivine under high-strain conditions. These grains are cross-cut by late tremolite and talc. Olivine and tremolite are cross-cut by late serpentine. Oxides (usually Cr-bearing magnetite) appear in layers parallel to foliation, associated with abundant chlorite and serpentine. In the chlorite-tremolite-rich zones, chlorite and Mg-hornblende form the fabric. Some samples show cm-scale crenulations with polygonal arc grain-growth textures.

Our findings support for the following deformational and metamorphic events: 1) deformation of original olivine, likely as part of a shear zone at temperatures >700 °C 2) growth of tremolite, talc, and chlorite (less than 650 °C) during large-scale folding and stretching of the AMS to form a NE-SW striking fabric, 3) continued growth of tremolite and talc after deformation to produce cross-cutting crystals, 4) serpentinization and continued chlorite growth during retrograde metamorphism. This metamorphic record includes the early eclogite and late lower amphibolite to greenschist metamorphic events observed by others, but lacks the upper amphibolite metamorphism observed closer to the Grandfather Mountain Window. The Edmonds body and its surrounding rocks likely reached lower peak temperatures during the deformation of the AMS.