THE MANHATTAN PROJECT: CONSTRAINING PEAK METAMORPHIC PRESSURE-TEMPERATURE CONDITIONS OF THE MANHATTAN PRONG

The Manhattan Prong has a complex geological and tectonic history which includes multiple sedimentary sources, several mountain-building episodes and extensive glaciation. As a result of these processes, the broader New York City has a complicated history that can be difficult to unravel. City structures, such as buildings and roads covering most of this region have created an obstacle in producing an accurate geologic map, as few contact boundaries are exposed or clear. The Manhattan Prong bedrock is primarily composed of two different types of schist, Manhattan Schist and Hartland Schist. The purpose of this study is to constrain the peak metamorphic pressure-temperature conditions of the Manhattan Prong in order to differentiate the two types of schist, as well as to improve the methodology for creating geologic maps under similar conditions. The samples used were taken specifically from Morningside Park in Upper Manhattan and results were compared with other Manhattan locations. High-grade metamorphic rocks produced from prograde metamorphism indicate that there was a peak pressure-temperature condition during metamorphism. Using the resulting mineral assemblages as geothermometers and geobarometers, metamorphic pressure-temperature conditions and paths can be inferred from equilibrium constants for mineral reactions. Using quartz-in-garnet inclusion barometry (i.e., QUIG), pressure-temperature conditions of garnet growth have been determined, as well as crystallization temperatures during metamorphism using garnet-biotite and garnet-muscovite-plagioclase pairs, to be remarkably similar to rocks from other Manhattan locations thus indicating that the bedrock at Morningside Park is Manhattan Schist.