MULTIPLE PROTEROZOIC METAMORPHIC EVENTS IN THE GRENVILLE PROVINCE, NORTHWESTERN NEW YORK STATE: EVIDENCE FROM ION MICROPROBE (SHRIMP) ANALYSIS

The Grenville Province in northwestern New York and Canada is characterized by lithotectonically distinct belts of rock separated by ductile shear zones. In northwestern New York, the Carthage-Colton shear zone (CCSZ), a ductile northwest-dipping mylonite zone, separates the western Adirondack Lowlands from the Adirondack Highlands to the east. High-grade rocks in these lithotectonic domains are considered to have had distinct metamorphic and cooling histories, suggesting that the CCSZ represents a major regional tectonic boundary. Previous TIMS analyses yielded ages for metamorphic sphenes in the Lowlands of ca. 1130-1150 Ma (representing metamorphism and deformation during the Elzevirian orogeny) whereas, directly across the CCSZ, metamorphic sphenes in the Adirondack Highlands have previously reported ages that are consistently 1030-1050 Ma (the Ottawan orogeny). This age difference, combined with an absence of Ottawan ages in the Lowlands, led to proposals of multiple episodes of motion along the CCSZ, with an early period of transpression and a later period of extension. However, new U-Th-Pb SHRIMP analyses of metamorphic sphenes and zircons in two rocks from the eastern Lowlands and one rock from the western Highlands all show evidence of both orogenies. In the eastern Lowlands, sphene and zircon ages cluster between 1120-1160 Ma (Elzevirian orogeny), but almost all sphene grains show evidence of the younger Ottawan metamorphism. In the western Highlands, sphenes primarily yield ca. 1000-1040 Ma ages related to pervasive granulite-facies metamorphism associated with the Ottawan orogeny; however, both sphenes and zircons also preserve some ages indicative of earlier metamorphism during the Elzevirian orogeny. These data indicate that, although the Highlands may have been metamorphosed to higher grades than the Lowlands during the Ottawan orogeny, both lithotectonic domains appear to have been affected by Elzevirian and Ottawan metamorphic events. Proposed late extensional motion along the CCSZ at 945 Ma may then represent the final juxtaposition of these domains, which appear to have had similar metamorphic histories during the late Proterozoic.