U-PB SHRIMP GEOCHRONOLOGY OF SYNKINEMATIC LEUCOGRANITES IN THE CARTHAGE-COLTON SHEAR ZONE, NORTHWEST ADIRONDACK HIGHLANDS: IMPLICATIONS FOR TECTONIC COLLAPSE OF THE GRENVILLE OROGEN

U-Pb SHRIMP geochronological analyses of zircon from intrusive granites within the Carthage-Colton Shear Zone (CCSZ) establish crystallization ages in the range 1046 ± 7 Ma to 1039 ± 10 Ma. The CCSZ is a major crustal detachment that forms the northwest boundary of the Adirondack Highlands (Central Granulite Terrane) portion of the Grenville Province in New York State. Field relationships unequivocally demonstrate that leucogranites of the Lyon Mountain Granite (LMG) suite were emplaced synchronously with the development of mylonites and ultramylonites during northwest-southeast extension. Locally, the granite contains high-temperature hydrothermal quartz-sillimanite segregations that formed during emplacement of the granitic magma. The q-s segregations acted as strain markers during top down-to-the northwest transport. The U-Pb zircon dates from these granites tightly constrain an early phase of crustal extension on the CCSZ. The position and age of the leucogranites along the CCSZ indicates that granite melt generation and crustal extension were coeval and related to regional uplift and the initial stages of collapse of the Adirondack portion of the Grenville orogen as crustal thickening during the Ottawan collisional event reached its maximum. Production of leucogranite melts was initiated by decompression of the hotter granulite-facies Adirondack Highlands which acted as the lower plate along which extension occurred. These melts acted to weaken the formerly strong, dehydrated rocks of the granulite facies Adirondack Highlands ‘core’ and facilitated more rapid crustal extension and unroofing. Associated hydrothermal activity that is documented in nearby rocks of the Adirondack lowlands, which acted as the upper plate of the CCSZ, was driven by magmatic fluids from a broad belt of LMG plutons which lie along the SE margin of the CCSZ. These fluids also acted to weaken rock and promote more rapid extensional collapse of the orogen.