PETROLOGY AND GEOCHEMISTRY OF A TRANSECT ALONG THE MARCY ANORTHOSITE MASSIF AND METAIGNEOUS CONTACT ZONE, NORTHEASTERN ADIRONDACK HIGHLANDS OF NEW YORK

Massif-type anorthosites worldwide are mineralogically and compositionally diverse, and have been petrologic enigmas for decades. Numerous samples of anorthositic rocks have been collected and analyzed from the Adirondack Highlands of New York, but the geochemical data is scattered throughout the region. When samples are dispersed throughout the largest park in the contiguous United States, surface to subsurface correlations become debatable. This study examines the petrology and geochemistry of the Marcy anorthosite massif, and neighboring metaigneous lithologies along transects across the margin of the massif. A total of 34 samples were collected and anaylzed using inductively coupled plasma-mass spectroscopy and X-ray fluorescences. These geochemical data will be compared to over 200 samples of the AMCG suite from the literature. Gabbroic anorthosite with a gneissic foliation and metaluminous mineral assemblage show an overall theoliitic trend. Plots of P₂O₅ vs MgO, SiO₂ vs MgO, SiO₂ vs Cr, MgO vs TiO₂, Cr vs Sm(ppm) show two distinct trends and one outlier. The variation in trends and the outlier appear to be caused by different varieties of anorthosite and associated gabbroic anorthosites. Samples ploted on a TAS diagram (SiO₂ vs Na₂O+K₂O) range from basanite to trachyandesite . Upper crust-normalized REE diagrams exhibit significant depletion in Nb, Pb, and Zr. The goal of this study is to gain insight into how the Marcy anorthosite was emplaced, evolved through time, and what are the massif's contact relationships with adjacent rocks. Current data suggests that the rim zone may provide key insight into the evolution of the Marcy massif and provide insight into the age relations between the core and marginal zones. In addition, some of the variations within the marginal zone may be due to igneous layering.