EVIDENCE FOR RETROGRESSION DURING PROGRESSIVE DEFORMATION IN THE PISECO LAKE TECTONITE ZONE, SOUTHERN ADIRONDACKS, NEW YORK

The Piseco Lake tectonite zone (PLz) is defined in the southern Adirondacks by a zone of E-W trending spectacular L>>S and L-S mylonitic fabrics developed in meta-igneous rocks during syntectonic recrystallization (Ottawan orogeny). Mapping of these fabrics illustrates the PLz is ~15 km wide, >100km long, and locally overlaps the Piseco antiform. Consistent with previous work on ribbon lineations in the Piseco antiform, mineral elongation lineations are sub-horizontal, defined by dynamically recystallized K-feldspar, plagioclase and quartz, and trend consistently 100°-110°. L vs. S fabric intensities define a core zone delimited on the north and south by boundary zones. The core is defined by rocks with intense lineations. Foliation is variable in intensity and attitude to illustrate a dominantly L>>S fabric. The relatively weak foliation in the core zone is moderate to gently N- and S-dipping. In contrast, rocks in the northern and southern boundary zones have the same intensely developed lineations, but foliation is equally intense, illustrating L-S fabrics. Asymmetric structures in rock of the boundary zone show mostly sinistral sense of shear. In the core zone tectonite, the L>>S fabric is defined locally by aggregates of chlorite, biotite and muscovite, apparently overprinted upon a higher grade assemblage. Some of these rocks contain ribbons of quartz, recrystallized feldspar lineations, and chlorite and fibrous amphibole that locally contain relict fragments of clinopyroxene. Within the northern boundary zone, mafic phases defining the L-S fabric are mostly hornblende, clinopyroxene and minor biotite, suggesting amphibolite facies conditions. But, at some locations in the northern boundary zone hornblende and cpx was observed with extensive replacement by fibrous chlorite and biotite suggesting retrograde overprint. As the core zone is approached from the south, tectonite fabrics show progressively intense mylonitization into the L>>S fabrics. These fabric relationships suggest localization of deformation into the core zone. We conclude that the formation of the L>>S fabrics in a relatively narrow zone (~5 km) that was last active at greenschist facies conditions, localized within a much wider zone that was active initially at higher metamorphic grade.