THE NATURE AND ORIGIN OF ORTHOPYROXENITE LENSES AND LAYERS IN DUNITES FROM THE CHESTNUT GAP QUARRY, SYLVA SOUTH QUADRANGLE, JACKSON COUNTY, NC

Uncharacterized orthopyroxenites occur as lenses of uncertain origin and variable size associated with metamorphosed dunites of the Chestnut Gap Quarry. Host dunites contain minor orthopyroxene as ephemeral layers and accessory chromite. Together these rocks are part of the Webster-Addie ultramafic ring complex, which is surrounded locally by amphibolite and biotite gneiss of the Ashe metamorphic suite of the Eastern Blue Ridge province. Detailed field work in the Chestnut Gap Quarry shows orthopyroxenite occurs both as lenses within dunite and as pods separated from dunite by a stockwork of hydrothermal veins. The orthopyroxenite lenses are parallel to orthopyroxene layering where locally present in the dunite. The size of the lenses ranges from 3 to 30 cm in long dimension. The mineralogy of the lenses is dominated by orthopyroxene, but may include minor amounts of clinopyroxene. The texture of the orthopyroxenites is phaneritic, subhedral granular, and massive with the constituent orthopyroxenes ranging from 1-30 mm. Also exposed in the quarry are thick (25-40 cm) hydrothermal veins, which cut through the dunite and produce vein assemblages zoned from the outside inward with: 1) tremolite/anthophyllite +/- talc; and, 2) vermiculite, which locally contains granitic pegmatite. Research in progress on mineral chemistry will investigate whether the orthopyroxenite lenses and layering are primary igneous features of the dunite, resulting from crystal settling, or whether they are of metamorphic origin.