MICROSTRUCTURAL STUDY OF A ZONED ULTRAMAFIC BODY IN THE EASTERN BLUE RIDGE MOUNTAINS

The Ashe Metamorphic Suite (AMS) in the Eastern Blue Ridge Mountains contains metamorphic rocks formed and reworked during the formation of the Appalachian Mountain range. The AMS includes multiple ultramafic bodies with varying mineralogy and texture. One such body is the Edmonds body of Virginia and North Carolina. The Edmonds body is mineralogically and texturally zoned. In the most northeastern section of the body, the rocks are dominated by talc, with very small amounts of olivine. In the center of the body, olivine grains are larger and more abundant, with tremolite, talc, anthophyllite, and chlorite. Nearing the southwestern portion of the body, olivine grains are smaller and less abundant, as is talc, and contains greater amounts of tremolite and chlorite. The minerals and textures in the Edmonds could represent a gradient of varying metamorphic reaction: The northeastern to interior zone quenched a higher temperature assemblage with lower fluid content while the southwestern zone, in contact with cooler and more fluid-rich rock, was altered by retrograde metamorphism.

The Blue Ridge ultramafic bodies are separated by Scotford and Williams (1983) into two principle petrographic groups: Todd-type and Edmonds-type. Todd-type ultramafics are generally characterized by high concentrations of tremolite and chlorite with lesser talc and minor amounts of olivine. Edmonds-type are characterized by higher concentrations of olivine, antigorite, anthophyllite, and talc, with lesser amounts of tremolite and chlorite. Typically, an ultramafic body in the AMS is determined to be one of the two petrographic groups. However, the Edmonds body contains both Todd-type and Edmonds-type rocks. The presence of both could imply that individual bodies are not distinctly different in regards to original chemistry but rather signify different zones of metamorphic reaction. The Edmonds body, being the largest of the AMS ultramafic bodies, may preserve both zones while the smaller bodies preserve only one of the zones