GEOCHEMISTRY OF MIDDLE PROTEROZOIC CANADA HILL GRANITE AND ASSOCIATED POST-OTTAWAN DEFORMATION

Twenty-four samples were collected of the Canada Hill Granite (Helenek and Mose, 1984; Ratcliffe, 1992) from the Hudson Highlands, NY in the West Point-Bear Mountain area, new major- and trace-element data have been obtained. The Canada Hill Granite is a Middle Proterozoic rock with a U-Pb zircon age of 1010 ± 6 Ma (Aleinikoff and Grauch, 1990). The Canada Hill Granite occurs in the cores of late folds comprising three small plutons as several 5-10 m sheets and lenses within migmatitic metapelitic gneisses. The rock is coarse-grained, weakly deformed leucogranite and is composed of large quartz, white to gray potassium feldspar, white plagioclase feldspar, and biotite. Within some sheets, small, localized inclusions of restitic accessory garnet, biotite, and sillimanite schollen are present. Initial geochemistry by Helenek and Mose (1984) indicated that the granite is a S-type granite with a high initial ⁸⁷Sr/⁸⁶Sr (0.7186). Recent geochemistry has revealed that the Canada Hill Granite is metaluminous to slightly peraluminous (A/CNK=0.98-1.09) and is characterized by high SiO₂ (69-77 wt%), low CaO (0.74-1.71), high K₂O/Na₂O (1.11-6.70), and plot as syn-collosionial and/or volcanic arc granitoids on tectonic discrimination diagrams. Trace element analysis has revealed three broad classes of REE patterns within in the granite. Some lenses have positive europium anamolies, where Eu/Eu* ranges from 2.39-11.58 and HREE depletion (1x chondrite) indicates the presence of garnet during melting. Some of the granites exhibit a negative europium anomaly (Eu/Eu*=0.11-0.26) owing to feldspar fractionation. Some lenses of Canada Hill Granite have large Eu/Eu* (2.29-11.70) and HREE enriched patterns (10-40x chondrite) due to undigested metapelitic garnets. The differing geochemistry is due to heterogeneous melt and extraction processes between granitic lenses and migmatitic metapelites. Based on chemistry and the association with other granitoid suites of similar age in the Hudson Highlands; the Canada Hill Granite is interpreted to have formed by late dextral shearing from late to post-Ottawan deformation and related tectonic escape.