MICROSTRUCTURE AND PETROLOGY OF A FOLDED, METAMORPHOSED ULTRAMAFIC BODY IN THE ASHE METAMORPHIC SUITE OF NORTHWESTERN NORTH CAROLINA

The Ashe Metamorphic Suite of the Eastern Blue Ridge province, a package of meta-sedimentary and meta-igneous rocks of Taconic age, contains small (many less than 1 km² exposure) bodies of meta-ultramafic rock that have varying mineral assemblages and deformational fabrics. A body exposed near Nathan’s Creek and Shatley Springs, North Carolina with a clearly folded map pattern has been sampled and studied in detail. Careful mapping accompanied by optical and electron microscope petrographic study helps to clarify the conditions and relative timing of metamorphic and deformational events in this body.

The foliation, defined by tremolite needles, platy chlorite, and stretched olivine porphyroclasts, is parallel to lithologic contacts, indicating it formed prior to the broad folding that created the current map pattern. This last stage of folding is usually attributed to the Alleghenian orogeny. Centimeter to meter-scale parasitic folds are also present, yielding hinge lines that trend toward the southwest.

A few samples contain relict olivine porphyroclasts surrounded by chlorite and ilmenite in a matrix of chlorite, tremolite, and anthophyllite. Some samples contain minor talc. Populations of anthophyllite needles in most samples are both parallel and cross-cutting foliation and do not display a strong lineation. Chlorite forms lenses and layers defining the foliation and surround and dissect the stretched olivine porphyroclasts.

These mineral assemblages represent several phases in the emplacement and modification of the ultramafic body. The olivine likely represents the original mantle peridotite, while the chlorite, tremolite, and ilmenite represent a later breakdown. The late anthophyllite indicates the rocks remained in the amphibolite facies post-fabric-forming-deformation, long enough to grow the unoriented needles. The Alleghenian folding event did not reset these internal fabrics.