GEOCHEMISTRY OF THE LATE MESOPROTEROZOIC MOUNT EVE GRANITE: IMPLICATIONS FOR LATE- TO POST-OTTAWAN TECTONICS IN THE NJ/HUDSON HIGHLANDS

The Mount Eve Granite is a post-orogenic, A-type granitoid that consists of several small plutonic bodies found in the northwestern margin of the NJ/Hudson Highlands. Mount Eve Granite has a U-Pb zircon crystallization age of 1020 ±4 Ma (Drake et al., 1991) and is relatively undeformed. Thus, it is important for constraining tectonomagmatic events that affected this region during the late stages of the Ottawan Orogeny. Characteristic field relations include intrusive contacts that crosscut the dominant regional foliation in surrounding Mesoproterozoic gneisses, xenoliths of gneissic country rock, and a weak deformational fabric. Representative samples from the type-locality at Mount Eve near Pine Island, NY and Pochuck Mountain, Owens, NJ are equigranular, medium- to coarse-grained syenogranite to quartz monzonite consisting of quartz, microperthite, oligoclase, with minor hornblende, fayalite, biotite, and accessory zircon, monazite, allanite, and magnetite. Whole-rock analyses indicate Mt. Eve Granite is metaluminous to slightly peraluminous (A/CNK=0.6 to 1.2) and has an A-type geochemistry defined by high K/Na (1-5), Ba/Sr (3-12), Fe/(Fe+Mg) (0.8-0.98), Ba (400-3000 ppm), Zr (200-1000 ppm), Y (30-130 ppm), Ta (2.5-6 ppm), total REE (300-1000 ppm), and low MgO (<1%), Cr and Ni (<5 ppm), and relatively low Sr (200-700 ppm). Large negative Eu anomalies (Eu/Eu*=0.1-0.6) and low Sr, Ba, Y, and Zr in fractionated samples suggest crystallization of feldspars ± zircon ± monazite ± allanite. The least fractionated samples have negative Eu anomalies and high HREE contents indicating a garnet-free crustal source with residual feldspar. Possible modes of origin include dry melting of charnokitic gneisses or Fe-rich mafic to intermediate diorites within the Mesoproterozoic basement. Two possible tectonic mechanisms for generating Mount Eve Granite include: (1) residual thermal input from a major lithospheric delamination event during or immediately after peak Ottawan orogenesis (~1080-1030 Ma?) or (2) broad orogenic relaxation between peak Ottawan and a late (~1010-1000 Ma) high-grade, right-lateral transpressional event recently documented further east in the NJ/Hudson Highlands (e.g., Gates et al., 2001; Valentino et al., 2001, Ratcliffe, 2001).