TESTING PETROGENETIC MODELS OF ANORTHOSITE MAGMATISM II: ZIRCON U-PB GEOCHRONOLOGY OF CROSS-CUTTING LITHOLOGIES OF THE MARCY MASSIF, ADIRONDACK HIGHLANDS, GRENVILLE PROVINCE

Proterozoic anorthosite magmatism is a classic problem in igneous petrology. Most models agree that anorthosite forms as a result of plagioclase floatation from melts that ponded at the base of the continental crust. Such large-scale floatation of plagioclase would require large melt volumes, but the emerging consensus that magmatic systems are emplaced incrementally calls this petrogenetic model into question. Field relationships in the Marcy Massif of the Adirondack Highlands, NY, have long demonstrated that anorthosite magmatism was incremental, though current geochronologic constraints cannot resolve the timescales of individual magmatic pulses. Here, we examine cross-cutting lithologies of genetically related phases of the Marcy Massif to investigate the timescale(s) of anorthosite magmatism. At Woolen Mill, Elizabethtown, NY, blocks of anorthosite are intruded by leucogabbro and gabbro, thought to represent residual melt after anorthosite fractionation. We sampled anorthosite that comprises blocks, as well as the leucogabbro that invades the anorthosite. Preliminary LA-ICP-MS data support field observations; the anorthosite yields a weighted mean age of 1153 ± 5 Ma, and cross-cutting leucogabbro yields a weighted mean age of 1137 ± 6 Ma. However, zircon U-Pb dates from these samples show signatures of Pb loss, so future work will apply the CA-ID-TIMS technique to mitigate the effects of Pb loss and further refine our emplacement ages.