ULTRA HIGH-TEMPERATURE AND SUBSOLIDUS SHEAR ZONES - EXAMPLES FROM THEPOE MOUNTAIN ANORTHOSITE, WYOMING

Ultra high-temperature shear zones formed with interstitial melts pres ent during the emplacement of the Poe Mountain anorthosite,Wyoming. Th e shear zones are characterized by coarse-grained plagioclase with fla t extinction and rare deformation microstructures. Dissected grain mic rostructures suggest that plagioclase underwent recrystallization by ' fast' grain boundary migration. Orthopyroxene crystallized from inters titial melts as strain-free,foliation-parallel tablet grains. Plagiocl ase has a magmatic petrofabric, characterized by poles to (010) normal to the shear foliation, and [100] parallel to the foliation and movem ent direction. Poles to (001) have a broad distribution with a point m aximum roughly parallel to the foliation and normal to the movement di rection. Orthopyroxene petrofabrics result from oriented growth parall el to the shear foliation. [100] defines a point maximum perpendicular to the shear foliation- and [010] and [001] have great circle distrib utions approximately parallel to the shear foliation. Microstructures of the ultra high-temperature shear zones contrast with those of granu lite-grade shear zones cutting across the intrusion. The granulite-gra de shear zones initiated as fractures. Which evolved into shear zones by dynamic recrystallization. Deformed primary plagioclase and orthopy roxene recrystallized by grain boundary rotation into smaller polygona l grains. (C) 1998 Elsevier Science Ltd. All rights reserved.