GEOCHEMISTRY OF THE BANKER POND METABASALTS, HOOSICK FALLS, NEW YORK

Banker Pond is one of a few isolated localities of metabasaltic rocks exposed within the Rensselaer Plateau slice of Taconic Allochton in eastern New York. These metabasaltic rocks are intermixed with greywacke of the Nassau Formation, age of which is uncertain. Formation of metabasalts was previously explained by rifting at the North American craton margin during opening of Iapetus Ocean in late Proterozoic. The above hypothesis was supported by geochemistry of basalts from all the localities with exception of the Banker Pond (Ratcliff, 1987). The purpose of this study is to get a better understanding of the tectonic environment of the formation of the Banker Pond metabasalts. We obtained new representative major and trace element data by X-ray fluorescence analysis from various locations within the Banker Pond outcrop, and compared it to geochemical data from modern plate tectonic settings from the GEOROC database using discrimination diagrams. We used the Ca/((Al-Na-K)/2) vs. Mg/((Al-Na-K)/2) diagram to evaluate degree of chemical change of basalts due to hydrothermal alteration by sea water, calcite precipitation, or weathering. To minimize effects of metamorphism we used the immobile trace and minor elements (Zr, V, Y, Cr, Ti, Ce, Nb) in discrimination diagrams. Banker Pond basalts fall in Mid-Oceanic Ridge Basalt (MORB) or Island-Arc Tholeiite (IAT) fields. On most diagrams there is a certain overlap between the two. However, Banker Pond has more definite IAT signature than metabasalts from other localities within Rensselaer Plateau, which contradicts original hypothesis of their formation at extensional environment. We discuss other possibilities to explain this contradiction. It is known that basalts that are transitional between MORB and IAT can form in back arc basins. We also compare our results to volcanics of Stonyford Volcanic Complex, Northern California to suggest a forearc environment as an option.