OLIVINE EBSD AND EQUILIBRIUM ASSEMBLAGE CONSTRAINTS ON CONDITIONS OF FORMATION AND EMPLACEMENT OF THE BUCK CREEK ULTRAMAFIC COMPLEX, NORTH CAROLINA

New studies integrating interpretation of olivine lattice preferred orientation (LPO) data and metamorphic equilibrium phase assemblages may help place constraints on the emplacement history of the Buck Creek (BC) ultramafic complex, among the largest in the southern Appalachians. The BC complex is dominated by dunite, interlayered at map scale with rocks of troctolite protolith and enclosed by mafic rocks (gabbroic protolith). Earlier work on the BC complex indicates that 1) it is a fragment of cumulate ocean crust; 2) the rocks remained anhydrous along the prograde P-T path; 3) the metamorphic coronal assemblages between primary olivine-plagioclase in the meta-troctolites formed at > 800^oC/1.0 GPa; and 4) localized hydration initiated at peak metamorphic conditions.

Current efforts aim to further constrain the evolution of P-T conditions for the BC complex using equilibrium assemblage diagrams for rocks of troctolite protolith. Initial results indicate that peak assemblages developed at > 800^oC/0.8 GPa, similar to earlier results from thermobarometry. These conditions are consistent with those published for granulite facies rocks at Winding Stair Gap (~10 km NE) and recent P-T estimates during movement of the Chunky Gal Mountain Fault that forms part of the boundary of the BC complex, possibly suggesting emplacement of the complex into the base of the continental crust.

Despite localized hydration/serpentinization, olivine is well preserved in most BC dunites. Preliminary EBSD analysis of olivine in dunite reveals a strong [100] maximum parallel to the shear direction and (001) axes perpendicular to foliation, consistent with Type E pole figures ((001)[100] slip system). The most common olivine LPO fabrics (Type A – (010)[100] slip system) form under relatively dry low/moderate stress conditions. Experimental work suggests that the change in active slip plane from Type A to E results from increased hydration. Type A fabrics are most commonly observed in ophiolites; Type E fabrics have been observed in some collision zones. One explanation for this fabric type might be formation during peak/post-peak metamorphic hydration and emplacement of the complex into the lower crust.