Petrologic and Fluid Inclusion Constraints on the Tectonic Evolution of the Manhattan Prong, Southeastern New York

Abstract

PETROLOGIC AND FLUID INCLUSION CONSTRAINTS ON THE TECTONIC EVOLUTION OF THE MANHATTAN PRONG, SOUTHEASTERN NEW YORK by Adam Henry Robert J. Tracy, Chairman Department of Geological Sciences (ABSTRACT) The results of a combined mineral equilibria and fluid inclusion study show that the Manhattan Prong, southeastern New York, has experienced multiple metamorphic events. Two episodes of intrusion, separated by approximately 100 million years, have superimposed contact aureoles on the Taconic regional metamorphic gradient in the northeastern Manhattan Prong and have modified the regional assemblage to different degrees. The assemblage Sil-Bt-Grt-Qtz-Pl+Ksp+Ms in regionally metamorphosed Manhattan Schist records P-T conditions of 4-5 kbar and 650-700 oC. Garnet porphyroblasts, homogenous with respect to major elements but zoned with respect to P and Y, contain ubiquitous, primary, CO2-rich fluid inclusions which have a Th = 10-24 oC. Manhattan Schist collected adjacent to the Croton Falls and Peach Lake mafic complexes, intrusions thought to be related to the Late Ordovician Cortlandt Complex, record P-T estimates of 4 kbar and 700 oC and 4.2 kbar and 550-600 oC respectively. The lack of fluid inclusions in garnet porphyroblasts indicates that the regional metamorphic assemblage has been completely modified by the contact effects of the mafic intrusions. However, the presence of Ky+Sta along with the slight compositional zoning of garnets in Peach Lake samples suggests that the contact assemblage may have been modified by a later metamorphism. Manhattan Schist collected adjacent to ~350 Ma granites (Brock, 1993) has been partially modified by contact metamorphism and shearing but vestiges of the regional metamorphic assemblage remain. Garnet porphyroblasts contain abundant CO2-rich fluid inclusions and P-T estimates using Bt and Pl inclusions and garnet core compositions are similar to estimates of regional metamorphic conditions. Evidence of modification includes garnet overgrowths that are elevated in Ca and depleted in Mn, Y and Sc, and CO2-rich fluid inclusions that have reequilibrated to higher density (Th = 2-18 oC). Rim compositions of porphyroblasts yield P-T estimates of 5-6 kbar and 550-600 oC. The elevated Ca content of the overgrowths along with the presence of Ky in the matrix suggests that the reaction An = Ky + Grs + Qtz may have been active during the overprinting metamorphism. The increase in pressure recorded in the granite aureoles in the Manhattan Prong is inconsistent with the results of P-T studies of the Rowe-Hawley belt, approximately 20 km to the east across Cameron's Line. This suggests that these two terranes may have been separated in the Devonian.