Ultra high-temperature and subsolidus shear zones: examples from the Poe Mountain anorthosite, Wyoming

Ultra high-temperature shear zones formed with interstitial melts present during the emplacement of the Poe Mountain anorthosite, Wyoming. The shear zones are characterized by coarse-grained plagioclase with flat extinction and rare deformation microstructures. Dissected grain microstructures suggest that plagioclase underwent recrystallization by ‘fast’ grain boundary migration. Orthopyroxene crystallized from interstitial melts as strain-free, foliation-parallel tablet grains. clase has a magmatic petrofabric, characterized by poles to (010) normal to the shear foliation, and [100] parallel to the foliation and movement direction. Poles to (001) have a broad distribution with a point maximum roughly parallel to the foliation and normal to the movement direction. Orthopyroxene petrofabrics result from oriented growth parallel to the shear foliation. [100] defines a point maximum perpendicular to the shear foliation, and [010] and [001] have great circle distributions approximately parallel to the shear foliation. tructures of the ultra high-temperature shear zones contrast with those of granulite-grade shear zones cutting across the intrusion. The granulite-grade shear zones initiated as fractures, which evolved into shear zones by dynamic recrystallization. Deformed primary plagioclase and orthopyroxene recrystallized by grain boundary rotation into smaller polygonal grains.