Paper No. 20-8
Presentation Time: 8:00 AM-12:00 PM
USING OLIVINE LPO AND MISORIENTATION ANALYSIS TO CONSTRAIN DEFORMATION AND EMPLACEMENT CONDITIONS OF THE BUCK CREEK ULTRAMAFIC COMPLEX, BLUE RIDGE PROVINCE, NORTH CAROLINA
New studies using Electron Backscatter Diffraction (EBSD) to measure olivine lattice preferred orientation (LPO) data from the Buck Creek (BC) ultramafic complex, in southwestern North Carolina may constrain conditions and mechanisms of olivine deformation and provide information on emplacement conditions of this large southern Blue Ridge ultramafic body. Earlier work indicates that the BC complex is a fragment of cumulate ocean crust that remained anhydrous along the prograde P-T path to lower crustal conditions (~ 800o
C/1.0 GPa) and experienced localized hydration at peak metamorphic conditions. Olivine is well preserved in most BC dunites, despite localized hydration. Preliminary EBSD data collected for several oriented dunite samples indicates preservation of strong olivine LPO fabrics. Interpreting these fabrics with respect to local strain axes requires determination of olivine shape fabrics using 3-directional cuts of the rocks.
Pole figures showing crystallographic axes plotted with respect to strain axes reveal a strong LPO with  axes parallel to the shear direction and  axes perpendicular to foliation. The pattern is consistent with experimentally determined E-Type olivine fabrics related to activity of the (001) slip system. E-Type fabrics imply moderate H2O activity and are found in arc settings and in peridotites emplaced into the crust, possibly associated with infiltration of aqueous fluids. Post-peak hydration of the BC complex could explain E-type fabrics. Misorientation analysis of EBSD data evaluates the crystallographic orientation of rotation axes across subgrain boundaries that can be linked to active slip systems and deformation mechanisms, independent of pole figure LPO. Preliminary work shows rotation axes parallel to  or in an orientation between  and . Misorientation analysis of individual grains with distinct subgrains points to micro-scale variations in subgrain rotation axes. Variations could reflect differences in active slip systems and/or relative activity of edge vs. twist dislocations. Work is ongoing to better constrain olivine LPO and micro-scale misorientation analysis and interpret them in the context of conditions of crustal deformation and macro-scale structures.
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