LOWEST GRADES OF METAMORPHISM IN AUTOCHTHONOUS SHALES OF NEW YORK'S MID-HUDSON VALLEY

Metamorphism above the biotite isograd has been extensively studied in New York’s Mid-Hudson Valley since 1930. It peaks at anatexis at the New York/Connecticut border and regularly decreases to the west to the biotite isograd. Metamorphism below this isograd, however, has been not studied extensively, although understanding sub-biotite metamorphism will give a more complete picture of the region. This study used XRD analysis to study metamorphism occurring to parautochthonous rocks deposited in the black shale basin created to the west of the Taconic Orogeny. The rocks only experienced one metamorphic event, that of the Taconic Orogeny. The rocks studied consist of several distinct units containing turbidites, shales, phyllites, and mélange. We collected 28 samples spanning over 45 km perpendicular to the biotite isograd. Samples were analyzed in two grain sizes (<2μm, < 0.5μm) to ensured removal of detrital material. The lithology consists mainly chlorite and illite, detrital quartz, and negligible smectite. However, the lack of swelling in samples treated with ethylene glycol does not rule out minor interstratification. Using the Kübler and Árkai indices samples were characterized in the late diagenetic zone, anchizone, or epizone. We used the b₀ of dioctahedral micas as a geobarometer to determine the pressures the formation experienced. The Kübler index gave more consistent results with higher R² values than the Árkai index. Results indicate late diagenesis in the westernmost part of the transect and anchizone conditions throughout the rest of the basin except near the biotite isograd where it shifts to epizone. The mean b₀ parameter of 9.042 Å is consistent with relatively high-pressure metamorphism. The lithology, b₀ parameter, and conodont alteration index indicate that this material was metamorphosed in an accretionary collision, characterized by low geothermal gradients (15-25 °C/km). This region, however, may have experienced gradients that are more consistent with that of a back-arc basin (35-45 °C/km), perhaps because of its proximity to the core of the Taconic Orogen.