DEFORMATION SPANNING HIGH- TO LOW-GRADE IN THE PISCEO LAKE SHEAR ZONE, SPECULATOR, NEW YORK

The Piseco Lake shear zone (PLSZ) is a major ductile structure that extends east-west across the southern Adirondacks and well know for ribbon lineations. The northern boundary of the shear zone is delineated by fabric transition in the area of Speculator, New York. Outside the shear zone, rocks of granitic composition are dominated by gneissic foliation that is defined by aggregates of polygonal feldspars, quartz and accessory high-grade minerals such as hypersthene. Inside the shear zone, rocks of similar composition contain an intense mylonitic foliation and mineral lineation defined by mica streaks and by ribbons of quartz and feldpsars. Recent mapping in the area of Speculator Mountain, and within the PLSZ revealed rocks of high- and low-grade defining deformation fabrics. Minor granite pods both cross cut and are parallel to the shear zone foliation. Although the leucosomes cross cut the foliation, portions of the leucosomes define linear and planar structures parallel to the mineral lineations and folations in the shear zone. This relation is interpreted to represent the occurrence of partial melt during deformation, and most likely represents high-temperature conditions during deformation. However, near by, there are intensely deformed charnockitic (mylonitic) rocks that contain fabric-forming biotite and chlorite. Biotite and chlorite aggregates form both mylonitic foliation and mineral lineation. This metamorphic mineral assemblage is typical of greenschist facies conditions and suggests the shear zone deformation extended through low-T. Local granitic and gabbroic rocks also show evidence for the same low-T metamorphism associated with PLSZ deformation. These field and petrologic relations suggest that the PLSZ initiated at high-T conditions, but was long-lived and last active during low-T conditions are preserved in the greenschist facies deformation assemblage.