Title page for ETD etd-62197-163013


Type of Document Master's Thesis
Author Henry, Adam T.
Author's Email Address adhenry@vt.edu
URN etd-62197-163013
Title Petrologic and Fluid Inclusion Constraints on the Tectonic Evolution of the Manhattan Prong, Southeastern New York
Degree Master of Science
Department Geological Sciences
Advisory Committee
Advisor Name Title
Bodnar, Robert J.
Szabo, Csaba
Tracy, Robert J. Committee Chair
Keywords
  • Metamorphic Petrology
  • Garnet Zoning
  • Manhattan Prong
  • Fluid Inclusions
  • New England
Date of Defense 1997-05-13
Availability unrestricted
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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