Investigating the Marcy Anorthosite: Trace Element Zoning in Garnet Despite Compositional Homogeneity

The geology of the Adirondacks is old and complex, with many metamorphic facies belonging to meta-igneous suites and metasedimentary belts. The Marcy Anorthosite is a mafic anorthosite massif complex situated in the High Peaks of the eastern Adirondacks. The history of the Adirondacks is characterized by several severe periods of magmatism and metamorphism throughout the Mesoproterozoic Period (1600 Ma - to 1000 Ma). In the Highlands, intrusion of the Marcy Anorthosite during the Shawinigan (1190 - 1140 Ma), and intense metamorphism centralized on the eastern portion of the Adirondacks during the Ottawan Orogeny (1090 - 1020 Ma) created the rock formations we observe today. The existence of orthopyroxene (opx) in meta-gabbros and anorthosites categorize these rocks as granulite-facies, a very high-grade of metamorphism. Metamorphic PT conditions in the Highlands were 750º to 800º C and 7.5 to 8.0 kilobars. This study aims to investigate the geochemical differences within a seemingly homogenous meta-anorthosite. Samples were collected from the Orebed Brook near Saddleback Mountain in Keene Valley, NY. The anorthosite is gabbroic to ferrodioritic, with clear contacts ranging from clear to more diffuse. A highlighter-texture, where bright-white plagioclase surrounds darker minerals is common, and feldspars with exsolution lamellae are easily visible. Garnet, where visible, rims pyroxene crystals regardless of overall composition. Samples used in this study represent the range of compositions seen at the Orebed Brook. Trace element zoning in garnet and pyroxene crystals in three representative samples were analyzed via LA-ICPMS. These garnets form a necklace-like texture around larger clinopyroxene (cpx) crystals within the samples. The garnets show no major element zoning and are 60% almandine in composition with sub equal amounts of Ca, Mg, and minor Mn. However, the garnets are zoned in heavy rare earth elements (HREE), with enriched cores and depleted rims in a bell-shaped curve. Cpx is diopside ~(Ca0.8Mg0.6Fe0.4Al0.2) and zoned from core to rim in REE. Diopside crystals from more mafic anorthosites have Mg-rich exsolution textures. The most mafic sample has opx with very low abundances of REE compared to cpx within the same sample. The meta-anorthosites from the Orebed appear to contain relatively simple compositions of granulite facies mineral assemblages, but the trace element zoning clearly indicates a more complicated metamorphic history.