B-TYPE OLIVINE CRYSTALLOGRAPHIC PREFERRED ORIENTATION (CPO) FABRICS AND TEXTURAL ANALYSIS OF CUMULATE DUNITES FROM THE BUCK CREEK COMPLEX, NORTH CAROLINA

Thin section textures, paleopiezometry, and Electron Backscatter Diffraction (EBSD) data provide constraints on the conditions of formation and deformation of dunites in the Buck Creek complex (BCC), one of the largest ultramafic bodies in the southern Appalachians. Previous work has determined that the complex is an emplaced fragment of partially subducted cumulate ocean crust that experienced anhydrous prograde metamorphic conditions to about 800˚C and 1.0 GPa. Thirteen olivine-rich dunite/troctolite samples from the BCC are the focus of this study.

Textural and grain-size observations, consistent with differential stress ranging from ~ 27 – 43 MPa (~ 33 MPa avg.), can be related to the deformation mechanisms of dislocation creep and/or grain boundary sliding and temperatures of 800-900^oC at typical mantle strain rates. Some olivine grains preserve non-oxide micro-inclusions that could represent crystallized melt inclusions.

The BCC dunite samples preserve strong olivine crystallographic preferred orientation (CPO) patterns; however, the strain reference frame has been difficult to establish due to a lack of unambiguous fabric at the hand-sample scale. Strong [100] maxima and girdle patterns in [010] and [001] were initially interpreted as D-type fabrics. However, new Crystallographic Vorticity Axis (CVA) analyses from internally-deformed olivine grains establish a kinematically-significant frame to analyze the crystallographic data. The vorticity axis identified through CVA analysis lies parallel to the olivine [100] axis, consistent with a B-type olivine fabric ((010)[001]slip system). Misorientation analysis across subgrain boundaries also shows slip consistent with B-type fabrics.

B-type fabrics have been associated with fore-arc ex situ peridotites emplaced at relatively high stress, low water conditions. For example, B-type fabrics in dunites from the Songshugou ultramafic massif, China, have been interpreted to have formed by melt-assisted grain boundary sliding accommodated by diffusion creep in a cumulate setting. The textural and CPO observations, cumulate setting, and presence of possible melt inclusions in the BCC dunites suggest similar conditions of deformation.