TESTING MODELS OF ANORTHOSITE PETROGENESIS WITH TANDEM LA-ICP-MS AND CA-ID-TIMS U-Pb ZIRCON GEOCHRONOLOGY AND GEOCHEMISTRY OF THE MARCY MASSIF, ADIRONDACK HIGHLANDS, GRENVILLE PROVINCE, NY

Proterozoic massif-type anorthosite magmatism is an enduring enigma of igneous petrology, though broadly accepted petrogenetic models invoke fractionation and buoyancy-driven accumulation of plagioclase. Such models are difficult to reconcile with a growing consensus bolstered by high-precision geochronology that most magmatic systems are crystal rich, emplaced incrementally, and that anorthosite massifs represent particularly long-lived magmatic systems. Previous SHRIMP U-Pb analysis of zircon from 13 samples from the Marcy massif Adirondack Highlands, Grenville Province, NY could not resolve the composite history evident from field relationships and yielded a single mean age of 1154 ± 6 Ma. Here, we present the pilot results of tandem LA-ICP-MS and CA-ID-TIMS dating of anorthosite, leucogabbro, and ferrodiorite from the Marcy Massif intended to test petrogenetic models and resolve the tempo of magmatism within the Marcy massif. Zircon from these samples display varied oscillatory, sector, homogenous, and complex zoning patterns. LA-ICP-MS geochronologic results show a continuum of ages from ca. 1200-1050 Ma consistent with previous SHRIMP analyses and reflecting igneous crystallization of anorthosite at ca. 1155 Ma followed by ca. 1050-1035 Ma granulite grade metamorphism during the Ottawan orogeny. In general, grains with growth zoning record older ages than those with homogenous or complex zoning, though there is substantial overlap between ages owing to the relatively low precision of initial LA-ICP-MS analyses. TREE patterns from all three lithologies are typical of zircon, with chondrite normalized values climbing smoothly from <1 to >1000 from La to Lu, excepting positive Ce and negative Eu anomalies. Clear lithologic variation is evident in the TREE data; the ferrodiorite has on average higher normalized values than anorthosite and leucogabbro. Further, zircon from the ferrodiorite show lower Ti/Hf ratios consistent with crystallization from a lower temperature, more fractionated melt. We interpret the ferrodiorite to be residual, interstitial melt that was extracted from anorthosite crystal mush. In progress CA-ID-TIMS dating of these samples aims to reveal the timescale(s) of this magmatic process.