SIGNIFICANCE OF THE ANTWERP-ROSSIE GRANITOIDS, ADIRONDACK LOWLANDS, NORTHERN NEW YORK

The Adirondack Lowlands in northern New York are part of the Central Metasedimentary Belt of the Grenville Province. The area is widely known for exposures of highly deformed metasedimentary-hosted zinc deposits and several distinct suites of plutonic rocks. The geochemistry and isotopic systematics of these igneous rocks can be used to place additional constraints on the tectonic history of the region. Exposures of the Antwerp-Rossie Granitoids (ARG) are found only between the Carthage-Colton Mylonite Zone and Black Lake Shear Zone. They have been dated by U-Pb zircon methods at 1210+/-10 Ma (Wastneys et al, 1999) and are the oldest granitic rocks currently recognized in the Lowlands. Variably deformed, they contain xenoliths and extensively intrude the base of the supracrustal sequence including marbles, pelitic gneisses, and amphibolites. They range from granite to gabbro with silica contents between 51.7 to 72.67%. Petrographic study shows relict pyroxene and zoned plagioclase in the more mafic members and myremikite, perthite and quartz with undulatory extinction in felsic samples. Accessory minerals include zircon, apatite, and a small amount of monazite. The ARG are peraluminous to metaaluminous and show strong linear trends on Harker diagrams, especially when Si is plotted against Mn, Fe, Mg, Ti, and Ca. On AFM diagrams, the ARG show a well-defined calc-alkaline trend. On tectonic discrimination diagrams, they plot in the volcanic arc granite field (Carl and Delorraine, 1997). Their geochemical and textural characteristics differ from other younger Lowlands granitoids including the Hermon Granites (1182+/- 8 Ma; Heumann et al., 2006) and the Rockport Granites and domal alaskitic leucogranites (1172+/-5; Wasteneys et al., 1999). These results suggest that the ARG formed from early subduction related melts emplaced into the basal portions of the Lowlands metasedimentary sequence. Their limited spatial distribution and volume may indicate that only small amounts of oceanic crust were subducted prior to collision, perhaps during the collapse of a small back-arc basin in which the metasedimentary sequence was deposited. Several samples will be investigated for Sm-Nd isotopic systematics to determine the mantle separation age and possible origin of the crust they were derived from.