IT’S A STRETCH: HIGH TEMPERATURE DEFORMATION IN THE PISECO LAKE SHEAR ZONE

A thermodynamic, chemical, and textural analysis of Grenville-aged felsic- and mafic-L-S and L tectonites from the Piseco Lake Shear Zone (PLSZ) has been undertaken to determine the pressure-temperature conditions of attending deformation. The Piseco Lake Shear Zone (PLSZ) is a kilometric (10-20km) width sinistral shear zone striking E-W across the southern Adirondack Highlands. The mineralogic assemblage in the mafic tectonites is problematic containing the assemblage OPX + CPX + GAR +PLAG +HBL. This 5 phase assemblage violates the phase rule as either HBL or GAR should fail via reaction HBL + GAR = OPX + PLAG+CPX+PLAG +H₂O. Textural relationships, however, show clean contacts between all phases in the “metastable” assemblage. The assemblage lacks quartz and the HBL is F rich. The equilibrium assemblage in the felsic gneiss consists of QTZ +KSPAR +PLAG +HBL. Chlorite is common but clearly occurs as a post-deformation alteration product of original HBL. Both the mafic and felsic gneiss samples show post-deformation textural annealing with no evidence of subsequent deformation. Temperatures were modelled for the mafic gneiss using garnet-pyroxene geothermometers and HBL-PLAG geothermometer for the felsic gneisses. Results of these analyses yield consistent temperatures between 700-750°C for both the felsic and mafic gneiss. These samples record deformation within the PLSZ at granulite facies conditions which is consistent with previously determined peak temperature conditions for the Adirondack Highlands as well as the intensity of fabric development in and the 10-20km width of the shear zone. There is no doubt that the PLSZ represents an important tectonic element in the Adirondack Highlands during the Ottawan-phase of the Grenville Orogen.