U-TH-PB GEOCHRONOLOGY OF MONAZITE IN A QUARTZ-SILLIMANITE NODULAR LEUCOGRANITE FROM THE CARTHAGE-COLTON MYLONITE ZONE (CCMZ), NORTHWESTERN ADIRONDACK HIGHLANDS, NEW YORK

The Carthage-Colton Mylonite Zone (CCMZ) is a prominent NE-trending and NW-dipping feature that represents the juxtaposition boundary between the Adirondack Lowlands and Highlands. U-Th-Pb dating of polygenetic monazite ([LREE, Th]PO4) in a high-grade, quartz-sillimanite nodular, leucogranite from within the CCMZ has revealed Ottawan (~1055 Ma) and Rigolet (~994 Ma) orogenic signatures. The leucogranite has local mylonitic textures with a northwest dipping foliation. The quartz-sillimanite elongated segregations have been previously interpreted by McLelland et al. (2002) as magmatic-hydrothermal in origin with their formation taking place during syntectonic emplacement of the leucogranite. Electron microprobe analysis conducted on 155 spots in 3 monazite grains define apparent age populations of 1055 ± 5 Ma and 994 ± 8 Ma (weighted means reported with 95% confidence). The older age population of ~1055 Ma is interpreted as growth during emplacement of the leucogranite and synchronous tectonism along the CCMZ during the Ottawan orogeny, as the supercontinent Rodinia was being assembled. Published work by Selleck et al. (2005) and McLelland et al. (2002)provide U-Pb ion microprobe (SHRIMP) results for nine spots obtained from well developed, oscillatory zoned zircon rims yield a weighted mean age of 1046 ± 7 Ma (2-sigma error) and U-Pb TIMS analysis for six zircons yield an upper intercept age of 1035 ± 3.8 Ma,respectively. These zircon ages, determined by two separate isotopic techniques, closely coincide with the older monazite age population obtained using chemical dating via electron microprobe analysis, further validating this dating technique. However, because monazite can crystallize/recrystallize over a wider range of metamorphic conditions, the younger age population documents an additional event, interpreted as occurring during the Rigolet orogeny at ~994 Ma (perhaps extensional normal faulting as proposed by others). Additionally, small clusters of younger age populations suggest that the CCMZ may have been subsequently reactivated; however, further monazite analysis is required to obtain a larger and more statistically sound dataset for populations < 994 Ma.