TESTING PETROGENETIC MODELS OF ANORTHOSITE MAGMATISM I: ZIRCON U-PB GEOCHRONOLOGY OF SPATIALLY DISTRIBUTED SAMPLES OF THE MARCY MASSIF, ADIRONDACK HIGHLANDS, GRENVILLE PROVINCE

Anorthosite is found globally but has mystified geologists for over one hundred years. Most petrogenic models use plagioclase flotation and accumulation at the top of magma chambers in the lower crust. However, in upper crustal analogues, very high melt fluxes are required to sustain the high melt fraction necessary for intrusion scale fractionation. This suggests that massif-type anorthosite formation may be the result of discrete episodes of high magma flux. Previous investigation of the Marcy Massif in the Adirondack Highlands NY analyzed 13 spatially distributed samples via SHRIMP U-Pb zircon geochronology and obtained ages ranging from 1165 to 1140 Ma (McLelland). All ages overlapped within their analytical uncertainty, yielding a mean age of 1154 +/- 6 Ma. Thus, it remains unclear whether the Marcy Massif formed during a single event or incrementally. Here, we apply tandem LA-ICP-MS and CA-ID-TIMS dating to spatially distant anorthosite from the Marcy Massif to test our hypothesis that anorthosite cumulates formed in a single episode of high-flux magmatism. Our two samples come from the St. Regis-Marcy lobe of the Marcy Massif, one from the northwestern margin along Rt. 3, and another from Roaring Brook on Giant Mountain. Both samples yielded a range of ages from ca. 1200-1000 Ma, consistent with previous work and reflecting igneous emplacement at ca. 1150 Ma followed by metamorphism during the ca. 1050-1030 Ma Ottawan orogeny. Multiple analyses were performed on each grain, and only grains with internally consistent zircon ages were used in our weighted mean calculations. Weighted means of 1150 土7 Ma and 1162土5 Ma were found and interpreted as the magmatic ages. One sample from Roaring Brook contains a population of complex zircon that provides clear visual evidence for a multi-stage magmatic history. Oscillatory zoned and high uranium cores are surrounded by oscillatory zoned and low uranium rims, though their ages are indistinguishable through LA-ICP-MS. We aim to resolve these differences with in-progress CA-ID-TIMS dating of microsampled zircon domains. Using CA-ID-TIMS dating allows us to interpret precise dates and determine if our hypothesis that the Marcy Massif was emplaced all at once is correct. Our results will improve the understanding of anorthosite cumulates globally.