INVESTIGATING THE ROLES OF FRACTIONAL CRYSTALLIZATION, CRUSTAL CONTAMINATION, AND SOURCE HETEROGENEITY OF THE CROTON FALLS AND PEACH LAKE PLUTONS

The Croton Falls and Peach Lake plutons are often associated with the alkali-rich Cortlandt-Beemerville trend in southeastern New York as they are found at its easternmost edge. Recent dating of the Stony Point, Rosetown, and Cortlandt plutons indicates they intruded during the late Ordovician at 447.1 +/- 4.5 Ma based on U-Pb zircon geochronological data (Bailey et al., 2023). Plutons in the Cortlandt Complex consist of gabbro, norite, diorite, cortlandtite, hornblendite, peridotite, clinopyroxenite, and websterite that are likely derived from mantle-derived alkali basalt parental magmas with varying degrees of crustal contamination (e.g. Bender et al., 1984). However, other authors have argued that the diversity can be explained solely by fractional crystallization (Temples, 2012). This project is designed to look at the isotopic composition of these plutons to understand the roles of fractional crystallization, crustal contamination, and source heterogeneity while also determining their relationship to the Cortlandt Complex as well as carefully mapping the different lithologies, flow structures, and layering to decipher how the plutons were emplaced. New York state geologic mapping by Prucha et al., (1968) identified the rocks as mafic (gabbro and diorite) with small amounts of ultramafic rocks (olivine hornblende pyroxenite and minor hornblendite). Initial field mapping identified different regions of gabbro, diorite, hornblendite, and rare pyroxenite. Additionally, preferred orientations of feldspars were found throughout the area with strong magmatic foliations and flow present in some dioritic samples, and even some gabbroic samples. These foliations were trending approximately west/southwest with a shallow plunge. Complex intrusive relationships between the different magmatic phases were also identified. Sr-Nd-Pb isotope analysis will be completed using a multi-collector inductively coupled plasma-mass spectrometer (MC-ICP-MS) coupled with bulk rock geochemistry. The latter will build on data collected previously by Del Conte et al., 2023, as they found diverse lithologies with distinct geochemical differences based on a small sample set with some similarities between these two plutons and the Cortlandt complex but differences between the more evolved members in particular.