MINERALOGY AND STABLE ISOTOPES OF DUTCHESS AND LITCHFIELD COUNTY METASEDIMENTARY ROCKS

Regional metamorphic conditions in Dutchess County during the Taconic Orogeny are well constrained by the presence of a complete Barrovian sequence in pelitic metasedimentary rocks, spanning from unmetamorphosed rocks near the Hudson River to sillimanite + K-feldspar grade near the Connecticut border. However, little research has been conducted on other sedimentary bulk compositions in the same region. The purpose of this study is to characterize the mineralogical and isotopic compositions of metamorphosed Ordovician rocks with carbonate and sandstone protoliths in Dutchess County, NY and Litchenfield County, CT.

Barth (BSA Bull 1936) reported rare glaucophane in quartz-rich quartz+ chlorite+ garnet schists in the garnet zone of Clove Valley. Nine samples of this lithology were examined using transmitted light microscopy and backscattered electrons (BSE). Samples contain quartz+ chlorite+ garnet± biotite± tournaline. No glaucophane was observed.

The second part of this study focuses on the Wappinger and Stockbridge marbles from Dutchess and Litchfield counties, and the Inwood marble from Westchester County. Mineralogy was determined using microscopy and X-ray diffraction, and stable isotope data was collected using mass spectrometry. Samples are dominated by dolomite, though calcite is also frequently observed. Minor minerals in most samples include muscovite and quartz, while some samples also contain pyrite, apatite, rutile, zircon, and hematite. Isotope analysis showed that the Wappinger and Stockbridge samples have δ¹³C(carbonate) values between -1.3 and 0.9‰ (PDB) and δ¹⁸O (carbonate) values that range from 20.8 to 24.1‰ (SMOW). Trace reduced carbon has δ¹³C that ranges from -22.3 to -1.1‰ (PDB), which is not in high-temperature equilibrium with host carbonate (and is likely post-metamorphic).