GEOCHEMISTRY OF MIDDLE PROTEROZOIC MOUNT EVE GRANITES FROM THE NJ/HUDSON HIGHLANDS: A-TYPE GRANITES RELATED TO POST-OROGENIC COLLAPSE?

The Mount Eve Granite is a post-tectonic Middle Proterozoic rock with a U-Pb crystallization age of 1020 ±4 Ma (Drake et al., 1991) and strong A-type geochemical characteristics. Sixteen samples were collected from three different localities including the type-locality near Mount Eve and Adam near Pine Island, NY; Pochuck Mountain, Owens, NJ, and southeast of Lake Mohawk in Sparta, NJ. The samples are equigranular, medium- to coarse-grained rocks consisting of quartz, microperthite, oligoclase, with minor hornblende and biotite, and accessory zircon, allanite, and magnetite. Major and trace element analysis were obtained by ICP-OES and INAA. Samples from Mt. Eve/Adam are strongly metaluminous (A/CNK=0.6 to 1.0) whereas Pochuck Mt. and Lake Mohawk samples are slightly peraluminous (A/CNK=1.0 to 1.2). Samples from all localities have strong 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). Major and trace element data support the interpretation that some of the granites are related by crystal fractionation processes. For example, large negative Eu anomalies (Eu/Eu*=0.1-0.6) and low Sr, Ba, Y, and Zr in fractionated samples suggest crystallization of feldspars and accessory phases (zircon, monazite, allanite). Parental granites also have negative Eu anomalies and high HREE contents indicating a garnet-free, mid-crustal source with residual feldspar. Regionally, the Middle Proterozoic basement gneisses are mostly tonalitic, trondhjemitic, granodioritic in composition, which on partial melting could yield A-type granites. The Mount Eve Granites are post-Ottawan (1.1-1.0 Ga) deformation which suggesting that they represent a crustal melting event possibly due to mantle delamination and associated orogenic collapse during the late stages of the Ottawan phase of the Grenville Orogeny.