CONTINENTAL ARC ORIGIN AND SUBSEQUENT PARTIAL MELTING OF MIDDLE PROTEROZOIC TONALITE PROTOLITHS, LOSEE METAMORPHIC SUITE, NEW JERSEY AND HUDSON HIGHLANDS

The Losee Metamorphic Suite is thought to be the basement of the New Jersey/Hudson Highlands (Volkert and Drake, 1999). The Losee consists of layered and foliated plagioclase and quartz rich gneisses, poorly foliated granofels, massive granitoids, amphibolites, and charnockites. Previous studies indicate a tonalitic/dacitic protolith for the gneisses, which are thought to have developed in a continental arc setting. The granofels and granitic phases, which plot in the trondhjemite field, are thought to represent partial melts of the layered tonalitic gneisses that formed during Grenville metamorphism. Recent studies of the Losee suite in the eastern and western New Jersey and New York Highlands has been performed to develop a more complete history pertaining to the origin and subsequent metamorphism of these rocks. Results from recent analyses indicate that the tonalite protolith contains relatively high SiO2 (~65 wt%) and Al2O3 (17-20 wt%) content with Na2O/K2O ratios ranging from 3 to 5. The tonalites display LREE enrichment (La/Yb=35 to 173) and HFSE depletion (La/Ta=8 to 25) characteristic of continental arc volcanics. Negligible Eu anomalies and HREE depletion suggest the partial melting of a garnet bearing basaltic source, leaving behind an eclogite residue. Trondhjemites representing partial melts of the protolith during Grenville metamorphism contain higher SiO2 (69-74 wt%) and lower Al2O3 (12-16 wt%) content than the tonalite source. Residues left behind as a result of partial melting were discovered in the western New Jersey Highlands. These HREE enriched residues contain relatively low SiO2 (~55 wt%), and high FeO (~10 wt%) and MgO (3-5 wt%) content. REE enriched trondhjemites were collected on the eastern edge of the eastern Highlands where there is an absence of the tonalite protolith. This may be attributed to the close proximity of a major shear zone located east of the eastern Highlands. This shear zone is associated with the Ramapo Fault that separates the eastern Highlands with the Mesozoic rocks of the Newark basin.