Growth, behavior, and textural sector zoning of biotite porphyroblasts during regional metamorphism and the implications for interpretation of inclusion trails: Insights from the Pequop Mountains and Wood Hills, Nevada, USA

Metapelites in the Pequop Mountains and Wood Hills, Nevada, contain biotite porphyroblasts that are part of a Barrovian metamorphic sequence that formed in response to tectonic burial. Inclusion trails and patterns in these biotite porphyroblasts provide a remarkable record of their growth and behavior in this environment. Accompanied by a strong component of coaxial strain, the porphyroblasts underwent a constructive phase that involved growth characterized by textural sector zoning followed by a destructive phase involving fracturing, rotation, and minor residual growth. Textural sector zoning is the result of uninhibited syntectonic growth in all directions. Growth along porphyroblast margins that parallel foliation involved incorporation of inclusions, whereas growth along margins perpendicular to foliation involved syntaxial precipitation of biotite in dilating strain shadows, which generally precluded development of inclusions. This growth mechanism partially accommodated strain and produced porphyroblasts with a characteristic hourglass-shaped included core bounded by zones of relatively unincluded biotite. Cessation of growth of biotite triggered onset of the destructive phase and ultimately resulted in the transference of some strain to the porphyroblasts and the filling of strain shadows with mostly quartz instead of biotite. Residual growth of biotite in the destructive phase was largely restricted to strain shadows and extension fractures. Progression through the constructive and destructive phases results in production of inclusion trails with a diversity of dip angles, dip directions, and trail geometries and patterns. Therefore, caution must be used when inferring strain histories on the basis of inclusion trails. Furthermore, although textural sector zoning has been reported in a variety of other porphyroblast species, where it is thought to develop in a state of hydrostatic stress in pretectonic or intertectonic porphyroblasts, zoning in biotite is significant in that it is strain induced and hence an indicator of syntectonic growth.