NEW CONSTRAINTS ON FORMATION OF ORTHOGNEISS IN SOUTHERN ADIRONDACKS USING INTEGRATED PETROLOGY, ZIRCON U-PB GEOCHRONOLOGY, HF-IN-ZIRCON, AND WHOLE ROCK GEOCHEMISTRY

The Adirondack Mountains (ADK) form the southern part of the Grenville Province, a poly-deformed orogenic complex formed in the Mesoproterozoic during the formation of Rodinia. The ADK is subdivided into two domains, separated by the Carthage-Colton Mylonite Zone. The Lowlands are characterized by upper amphibolite-facies metasedimentary rocks, including marbles, evaporites, and volcanic units, whereas the Highlands are formed by granulite-facies meta igneous rocks and anorthosite-mangerite-charnockite-granite magmatic complex. A large suite of granulite-facies orthogneisses exposed in the Southern Highlands yields the oldest (>1.3 Ga) ages, however, those ages come from a limited number of outcrops. We present new petrological, geochemical, and geochronological results from several quartzofeldspatic orthogneiss units to better document the timing and tectonic setting of these rocks. All samples are mainly composed of quartz, felspar, biotite, pyroxene, and opaque minerals. Foliation and lineation are defined by elongated quartz ribbons and feldspar grains, or biotite laminae. High-temperature microstructures, such as flame perthites in feldspars or checkerboard extinction in quartz are commonly observed in thin sections. Major element analyses show that samples have calc-alkaline affinities, suggesting an arc environment. U-Pb zircon geochronology is in progress and will provide new insights into the earlier geologic evolution of the ADK and its relation to other terranes in the Grenville Province.