B-TYPE OLIVINE FABRICS WITHIN THE BUCK CREEK ULTRAMAFIC BODY EMPLACED DURING SOUTHERN APPALACHIAN TACONIC SUBDUCTION

We document the predominance of B-type olivine fabrics preserved in relatively pristine dunites within the Buck Creek ultramafic complex, emplaced into Laurentian continental crust during Taconic subduction. The Buck Creek complex has been previously characterized as a fragment of oceanic cumulate emplaced under conditions of ~850°C and 1.0-1.4 GPa. Metamorphic assemblages and microstructures indicate nearly anhydrous conditions at peak metamorphism and the activity of dislocation creep with some evidence of grain boundary sliding. Grain size piezometry indicates stress conditions of ~25-41 MPa. Analysis of crystallographic preferred orientations and intracrystalline misorientations indicate the primary activity of the [001](010) slip system, leading to the development of B-type olivine fabrics. We suggest that the Buck Creek dunites formed as ocean crust cumulates, and weak B-type fabrics may have formed early in the cumulate setting. However, well-developed CPO maxima and moderate fabric strengths (with J-indices from ~2-5; M-indices from ~0.5-0.25) suggest the B-type fabrics formed during partial subduction beneath the Laurentian continental crust at deformation conditions similar to those in the shallow mantle wedge. We document that deformation at Buck Creek occurred at lower differential stress conditions and likely lower water content than typically associated with B-type CPOs, broadening the range of conditions under which these fabrics may form.