POST-OTTAWAN MAGMATIC HISTORY AND GEOCHEMISTRY OF INTRUSIVE ROCKS FROM SOUTHWESTERN HUDSON HIGHLANDS, NY: PLUTONISM RELATED TO DUCTILE TRANSPRESSIONAL DEFORMATION?

At least three discrete intrusive igneous rock suites of post-Ottawan age (<1030 Ma) occur within the northernmost New Jersey Highlands and the Hudson Highlands of New York. These are, in order of age: (1) the Mount Eve Granite (1020±4 Ma; Drake et al., 1991); (2) the Canada Hill Granite (1010±6 Ma; Aleinikoff and Grauch, 1990), and (3) the Lake Tiorati Diorite (1008±4 Ma). These rocks consist of small, dispersed plutonic bodies that form a chemically diverse suite of igneous rocks that range from A- and S-type granites to calc-alkaline, I-type diorite. All three suites crosscut the dominant regional gneissosity of the surrounding ortho- and paragneisses and, in general, are only weakly deformed themselves. The Mount Eve Granite consists of small stocks of medium to course-grained syenogranite to quartz monzonite with essentially no deformational fabric and occurs only in a small area in the vicinity of Pine Island, NY and on Pochuck Mountain, NJ. Mt. Eve Granite is metaluminous to slightly peraluminous and has strong A-type geochemical signatures (e.g., high Fe, Ba, Zr, Y, HFSE, REE). The Canada Hill Granite (Helenek and Mose, 1984; Ratcliffe, 1992) consists of small plutons of coarse-grained, weakly deformed leucogranite that occurs primarily in the cores of late, upright folds in the West Point-Bear Mountain, NY area. It is a strongly peraluminous granite with strong S-type chemical characteristics (high Al, high initial 87Sr/86Sr, accessory garnet-sillimanite). The Lake Tiorati Diorite consists of small plutons of coarse to very coarse-grained, diorite in the Popolopen Lake and Sloatsburg quadrangles. These rocks have strong I-type, calc-alkaline, volcanic arc geochemical signatures (e.g., low Ti, HFSE). The Lake Tiorati Diorite is locally cut by northeast-trending dextral transpressional system of ductile shear zones. The relatively restricted and localized areal extent, the small volumes, the synchronicity, and the extreme chemical diversity shown by these plutons places severe constraints on possible tectonic models for their origin. An attractive mechanism for the generation of this suite of magmas is localized melting of variable crustal and mantle sources due to the onset of the post-Ottawan, high-grade, dextral transpressional event that occurred in this region between ~1010 to 915 Ma.