A Petrofabric Study of the Day Book Dunite, Western North Carolina

Abstract

The Day Book dunite is one of many ultramafic bodies within the Ashe Formation (Precambrian) of Yancey County, North Carolina that has experienced post-emplacement deformations. This olivine-rich (Fo92-95) body contains minor orthopyroxene, chromite, serpentine, and talc. Olivine occurs as polygonal, equant, recrystallized grains with triple-point junctions, and also as larger porphyroclasts. A petrofabric analysis of olivine crystallographic axes yields differing results. Olivine from the center of the dunite body does not exhibit a preferred orientation. This is interpreted to be the result of recrystallization during the Taconian Orogeny. Olivine of the dunite margin exhibits a preferred orientation, interpreted to be the result of a later, less intense deformational event. Numerous ultramafic rocks, including those in the Southern Appalachians, exhibit various olivine fabrics, suggesting variable deformational histories. The Day Book dunite body is in Yancey County, North Carolina, approximately 4.8 kilometers north of Burnsville, North Carolina, along State Highway 197 (Fig. 1). This paper reports the relationship of the olivine fabric of the Day Book dunite to the tectonic history of that body. GENERAL GEOLOGY The Day Book dunite body is enclosed by mica gneisses with interlayered amphibolites of the Ashe Formation (Neuhauser, 1982; Swanson, 1981). Rankin (1970) described the Upper Precambrian Ashe Formation as metamorphosed mafic volcanics and metamorphosed sulfidic sandstones and quartz-feldspar rich shales. Granitic pegmatites intrude the dunite body along joints and fractures and along its contact with the country rock (Neuhauser, 1982; Kulp et al., 1951). No age dates have been established for those pegmatites that cut the Day Book dunite, although Odom and Fullagar (1973) indicated dates of 430 m.y. for the Spruce Pine pegmatites that cut the Ashe Formation. Swanson (1981) described the contact between the Day This content downloaded from 157.55.39.244 on Fri, 17 Jun 2016 06:31:37 UTC All use subject to http://about.jstor.org/terms VOLUME 91, NUMBERS 3-4 147