MIDDLE PROTEROZOIC A-TYPE GRANITE PLUTONISM IN THE WESTERN HUDSON HIGHLANDS, NEW YORK

The geochemistry of 30 samples of Middle Proterozoic granites and granitic gneisses from the western Hudson Highlands, southeastern New York were investigated in order to understand their petrogenesis in relation to deformation and tectonic events within this part of the Grenville orogenic belt. These rocks are interpreted as syntectonic, A-type granites that were emplaced and metamorphosed to upper amphibolite to granulite grade metamorphism during the Ottawan phase (1.1 to 1.0 Ga) of the Grenville Orogeny. The samples consist of moderately foliated, medium- to coarse-grained hornblende granite and leucogranite sheets to strongly foliated, fine- to medium-grained hornblende-biotite granitic gneisses. Western Hudson Highlands granites and granitic gneisses are metaluminous to peraluminous granites (ASI=0.9 to 1.1) with almost all having a A-type chemical characteristics defined by high K/Na (1-3), Ba/Sr (2-10), Fe/(Fe+Mg) (0.82-0.90), Ba (500-1500 ppm), Zr (350-900 ppm), Y (75-150 ppm), and low Sr, (100-200 ppm), MgO (<0.8%), CaO (<2%), Cr and Ni (<5 ppm). Only two of the leucogranite sheets have I-type characteristics (K/Na <1). Systematic chemical differences between granites and leucogranites are the result of fractional crystallization (feldspars, zircon, monazite) and/or variations in crustal source chemistry. Given the abundance of felsic gneisses in the Hudson Highlands, tonalitic to granodioritic source rocks are the likely anatectic source for the granites. Based on relatively high K, Ba, and Y, and low Sr and Ca in the less fractionated samples indicates that anatexis probably occurred at mid-crustal levels with residual plagioclase in the source rock (no garnet in the source). In this case, A-type granitic magmatism does not necessarily reflect anorogenic tectonism, but rather the availability of felsic source rocks in the crust of Hudson Highlands at the time of anatexis. Future U-Pb zircon SHRIMP ages will ultimately shed light on the timing and tectonic processes which may have caused the major crustal melting event(s) that generated these distinctive A-type granites.