GEOCHEMISTRY OF THE LAKE TIORATI METAGABBRO, HUDSON HIGHLANDS, NY

The Lake Tiorati Metagabbro is a small mafic meta-plutonic rock unit exposed in the Hudson Highland, NY in Harriman State Park. It has been mapped in the Sloatsburg and Popolopen Lake quadrangles (Dodd, 1965; Valentino et al., 2001). A total of 24 samples were collected from various locations along the 3 exposed bodies in Harriman State Park and Sterling Forest for whole-rock major and trace element analysis. The largest exposed body is on the west side of Lake Tiorati and is approximately 5 km long and 0.3 km wide. The mineralogy of the metagabbro is dominated by large crystals of hornblende, plagioclase, clinopyroxene, and biotite. The clinopyroxene displays prominent hornblende coronas in most samples. The meta-gabbro is coarse grained and the texture varies from granoblastic to highly foliated and is cut by right lateral ductile shear zones. SHRIMP U-Pb zircon dating has yielded a crystallization age for the Lake Tiorati Metagabbro of ~1008±4 Ma (Gates et al., 2001) and constrains ductile shearing to post-Ottawan. The metagabbro is a calc-alkaline, metaluminous rock with ranges of SiO₂ (47-51 wt%), Al₂O₃ (12-22%), Fe₂O₃ (5.3-11.3%), MgO (3.6-11%), and generally low TiO₂ (<1%). The trace element data obtained shows high La/Nb (2-20), LREE enrichment (La/Yb ~3-20), and a variable europium anomaly (Eu/Eu* ~0.8-1.2). The major and trace element geochemistry suggest that the metagabbro represents a mantle-derived magma with continental arc signatures. These arc signatures originated in the continental lithospheric mantle, which was affected by prior arc processes. The melt was generated as a result of lithospheric thinning during a post-Ottawan high-grade ductile shearing event.