GREENSCHIST FACIES METAMORPHISM ASSOCIATED WITH THE PISECO LAKE SHEAR ZONE, CENTRAL ADIRONDACKS, NEW YORK

The Piseco Lake shear zone is a 15-20 km wide ductile shear zone that transects E-W across the Grenville basement of the southern Adirondacks. The PLSZ exhibits spectacular deformation features, and is a classic structure for ribbon lineations. Heterogeneous strain is apparent, with rocks exhibiting L-S, L>S and L fabric variation over distances of 10’s of meters. The bulk deformation was interpreted previously to be associated with regional granulite facies metamorphism of Ottawan age.

Oriented thin sections, from outcrops along West Canada Creek, were analyzed for the relationship between macro- and microscopic rock fabrics, and metamorphic index minerals. These thin sections revealed abundant chlorite, biotite, and minor muscovite, in addition to recrystallized plagioclase, K-feldspar and quartz. Biotite and chlorite occur as isolated blade-shaped grains, intergrowths of both minerals, and aligned aggregates. The orientations of the metamorphic index minerals were quantified in thin sections. Chlorite and biotite blades show strong alignment and define mineral lineations in sections cut parallel to foliation planes. Blades of chlorite and biotite also show strong alignment in sections perpendicular to foliation and parallel to macroscopic lineation. Sections orthogonal to the macroscopic lineation contain chlorite and biotite grains with low aspect ratios and radial distribution. These orientations clearly demonstrate that greenschist facies minerals are geometrically related to the bulk fabrics in the rock defined by recrystallized feldspars and quartz ribbons, and not a late static retrograde effect. Numerous locations along the length and width of the PLSZ have been studied as well, and the only evidence for granulite facies metamorphism is the presence of rare relict crystals of hypersthene and augite. But, these minerals do not define a deformation fabric. Only greenschist facies assemblage described here is directly linked to the deformation fabrics in the PLSZ, therefore, suggesting a significant greenschist facies metamorphic episode in the southern Adirondack Mountains. The Piseco Lake shear zone is overlapped by unmetamorphosed Paleozoic sedimentary rocks on the western side of the Adirondack massif, therefore, the age of the greenschist facies deformation must be older. However, the absolute age remains to be quantified.