MICROSTRUCTURE AND PALEOPIEZOMETRIC CONSTRAINTS ON DEFORMATION CONDITIONS OF OLIVINE FROM THE SOUTHERN APPALACHIAN BUCK CREEK ULTRAMAFIC COMPLEX

Microstructural and paleopiezometric analyses help constrain conditions and mechanisms of deformation of the Buck Creek ultramafic complex in southwestern North Carolina. Previous work has determined that the complex is an emplaced fragment of partially subducted ocean crust that experienced anhydrous prograde metamorphic conditions to about 800˚C and 1.0 GPa. Twelve olivine-rich dunite/troctolite samples from the Buck Creek ultramafic complex are the focus of this study. Previous work with electron backscatter diffraction (EBSD) shows that the olivine crystallographic preferred orientation (CPO) patterns are generally consistent with axial-[100] or D-type fabric formed under moderate to high stresses with low water content. Axial fabrics may also result from non-plane strain conditions (e.g. constriction or flattening).

Textures and recrystallized grain size were determined using the petrographic microscope and deformation conditions were interpreted by comparison with experimental studies. The dominant olivine grain sizes range from 1.5 – 2.6 mm. Recrystallization textures include kink bands and subgrains, undulatory extinction, core-mantle structures, bulge-recrystallization, and encapsulated grains. Micro-inclusions of mineral grains (too small for microscopic ID) in some olivine locally appear to be oriented by the olivine crystal structure or along microcracks. One micro-inclusion appears to be twinned.

Average recrystallized grain and subgrain diameters (127 – 166 μm) are relatively consistent among the samples. Based on experimental work, recrystallized grain size can serve as a paleopiezometer and be used to estimate deformation mechanisms. The range of sizes are consistent with differential stress ranging from ~ 27 – 43 MPa (~ 33 MPa avg.). This can be related to deformation mechanisms of diffusion creep, dislocation creep, and/or grain boundary sliding, consistent with relatively high temperature deformation conditions. At 33 MPa, experimental work suggests that D-type fabrics form at ~ 1400 – 1500˚C. Integration of field and textural observations, paleopiezometry, and EBSD data may point to preservation of olivine deformation textures formed prior to crustal emplacement and deformation.