GEOCHEMISTRY AND TECTONIC INTERPRETATION OF MAFIC GNEISSES IN THE WESTERN HUDSON HIGHLANDS, NY

The western Hudson Highlands has undergone extensive metamorphism of Middle Proterozoic Grenvillian age (1.3 to 1.0 Ga) and is composed of mostly upper amphibolite to granulite grade gneisses and granites. Mafic gneisses (amphibolites, meta-diorites/gabbros) within this area can be used to indicate whether the petrogenesis of these rocks is of meta-sedimentary or meta-volcanic origin and the tectonic setting in which they were formed. About 20 samples were collected from a wide area within Harriman and Sterling Forest State Parks for sample diversity. Protolith determination based on the major and trace element chemistry is indicative of metamorphosed mafic volcanic rocks and in the case of samples of the Lake Tiorati diorite, the result of the metamorphism of mafic plutonic rocks. The determination of a mafic metavolcanic rock type was evidenced by the relatively low SiO2 (48-52%) and relatively high MgO (5-10%) and FeO (8-13%) in the amphibolites. Most of the samples have the classic HFSE depletions (low Ti, Ta, and Hf) and high LREE/HFSE ratios (ie. La/Ta > 25) characteristic of island arc volcanic rocks and were generated at relatively shallow depths in the mantle (<60 km) from lack of HREE and Y depletions. Many of the samples are geochemically similar to alkaline mafic volcanics from the modern Andes. The Lake Tiorati diorite has a coarse grained texture with variable foliation development, more indicative of a metaplutonic rock than metavolcanic amphibolite. U-Pb zircon dating methods have yielded an age of 1008 Ma ± 4 Ma for the crystallization of the Lake Tiorati diorite. The diorite is a mildly alkaline (Na20=3.5%, K2O=1%), mafic plutonic rock with SiO2 ~51%, Al2O3 ~15%, FeO ~9%, MgO ~7%. As with the amphibolites, the diorite has low TiO2 (~0.5%), suggestive of an arc-type plutonic rock. Lake Tiorati diorite represents arc-contaminated, mantle-derived magmas that may be related to mantle delamination and orogenic collapse at the end of the Ottawan orogenic event (~1.01 Ga).