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.
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