CHARACTERIZATION OF A SMALL MYLONITIC SHEAR ZONE, WESTERN ADIRONDACKS

Field observation and thin section analysis are being used to characterize a small mylonitic shear zone in the western Adirondacks. The shear zone is located in exposures on the banks of the Moose River ~25.3 km east of Lyons Falls, NY, at latitude of 43° 36’08” and longitude of 75° 12’35” (McKeever 15 minute USGS Quadrangle). The shear zone strikes 163° and dips 55° E, contains a similarly oriented foliation and a lineation with plunge of 3° toward 346° . Thickness of the shear zone is approximately 15 centimeters. Lithology in this area is granitic gneiss, amphibolite and metasedimentary rock (unit CG of Whitney et al., 2002). The shear zone is within a pegmatite in the granitic gneiss and cuts across the gneissic foliation of 046° /23° NW. From a thin section cut parallel to the lineation and perpendicular to the foliation the minerals quartz, feldspar, biotite, hornblende, apatite and zircon were all observed. Equant quartz and feldspar grains approximately 120-160 microns in diameter dominate the matrix. The feldspar was non-perthitic microcline and plagioclase. Biotite occurs as flakes and in thin bands running throughout the matrix. There were also perthitic feldspar augens, amphibole augens and quartz ribbons. The augens range in size from 1.6 mm to 6.9 mm. Field observations of asymmetric tails on augens of feldspar and amphibole indicate top to the north/northwest movement. Quartz ribbons are about 3.4mm long with a length to width ratio of 9:1. Petrographic analysis indicates that the hornblende is being replaced by an unknown mafic phase and calcite. The quartz grains contain planes of fluid inclusions normal to the foliation that cross subgrain boundaries. These subgrain boundaries indicate the quartz underwent dislocation creep. Grains of both feldspar and quartz are believed to have undergone grain boundary migration recrystallization as well. With further study the crystallographic orientations of the quartz grains and their relationship to the foliation and lineation will be determined along with the approximate temperature at which this shear zone developed.