MAGMATIC HISTORY AND GEOCHEMISTRY OF META-IGNEOUS ROCKS FROM THE SW HUDSON HIGHLANDS, NY: TECTONIC IMPLICATIONS AND REGIONAL CORRELATIONS

Recent geochemical, structural, and geochronological investigations in the southwestern Hudson Highlands, NY has identified at least four discrete tectonomagmatic events. The earliest igneous events are represented by a sequence of amphibolites, qtz-plag gneisses (metavolcanic), and quartzofeldspathic gneisses (metavolcanoclastic). The amphibolites and qtz-plag gneisses are variably HFSE depleted and LREE-enriched volcanic rocks and are interpreted to have erupted in a continental arc or back-arc setting. The quartzofeldspathic unit has SHRIMP zircon core ages of 1230 to 1160 Ma indicating a Late Elzevirian to early AMCG age and has a mild A-type chemistry. This reflects eruption of A-type felsic volcanics and subsequent transport into sedimentary basins during syn- and post-Elzevirian lithospheric delamination and orogenic collapse. The second event is represented by a metaplutonic, hornblende granite with A-type chemistry that is correlated with the Byram Intrusive Suite (~1095 Ma, Drake et al., 1991; ~1100 Ma, Volkert et al., 2000). It is interpreted to represent the late stages of AMCG-type, delamination-related crustal heating that immediately preceded major Ottawan crustal shortening. The third magmatic event generated a suite of syn- to late-orogenic alaskite sheets that have syn-COLG granite signatures and depleted HREE contents indicative of deep crustal melting in a thickened crust. No ages are currently available for this unit; however, based on field and textural relations, we tentatively assign a syn- to late Ottawan age (~1050 Ma). The fourth magmatic event in this region is the post-Ottawan emplacement of small, mafic diorite plutons that have SHRIMP zircon crystallization ages of 1008±4 Ma that are essentially synchronous with a major right-lateral, ductile shearing event. These rocks have strong depletions in HFSE and high HREE and Y contents indicating shallow partial melting (<65 km) of asthenospheric or arc-modified lithospheric mantle with subsequent crustal contamination. Regional correlation of rock types and events defined here are strikingly similar to those recognized elsewhere in the Canadian Grenville Province, the Adirondacks, and in the northern Blue Ridge and strongly suggests a similar tectonomagmatic history for the Hudson Highlands.