Date of Award

5-2014

Document Type

Thesis

Degree Name

Master of Science (MS)

College/School

College of Science and Mathematics

Department/Program

Earth and Environmental Studies

Thesis Sponsor/Dissertation Chair/Project Chair

Matthew Gorring

Committee Member

Stefanie Brachfeld

Committee Member

Gregory Pope

Abstract

Various P-T determinations have been reported on metasedimentary rock suites in the New Jersey and Hudson Highlands using different geothermobarometers, but all have yielded relatively large P-T ranges and none have used cordierite in a quantitative way. The presence of variable cordierite and its narrow stability field in metapelitic gneisses provides an excellent opportunity to better constrain the peak P-T conditions. Calcium depleted, clay-rich, arkosic sedimentary protolith was deposited during the Mesoproterozoic in a continentaloceanic arc basin during the Elzevirian orogeny (~1300-1200 Ma). The Ottawan metamorphic event (~1090-1020 Ma) transformed this protolith into the metasedimentary unit exposed in the Hudson Highlands, southeastern NY (Gates et al., 2001). Twenty samples from outcrops with inlerlayered metapelites and metapsammitic gneisses were sampled from four localities within Harriman State Park, NY in the western Hudson Highlands. The determination of the metamorphic conditions based on the prograde phases of the biotite-cordierite, biotite-cordieritegarnet, and biotite-garnet metapelitic gneisses is the focal point of interest in the current study.

Whole-rock major elements analysis of the composite suite of samples defines the differentiation of the metapsammitic and metapelitic gneisses as: metapelites = weight % K2O/ CaO > 1; metapsammitic gneiss = weight % K2O/ CaO <1. Chemical composition and weight % oxide data was collected from five samples that displayed co-existing garnet, biotite, and locally occurring plagioclase and cordierite, to perform geothermobarometry. Garnet-cordierite and garnet-biotite thermometers and grt-sil-crd-qtz, garnet-aluminasilicate-plagioclase (GASP), and garnet-biotite-plagioclase-quartz (GBPQ) barometers were used. The P-T determinations for five samples with appropriate mineralogy are as follows:

QB-7: 588 °C (±38) at 4.4 kilobars (± 2)

SB-5: 593 °C (± 19) at 5.3 kilobars (± 0.4)

QB-1: 654 °C (±18) at 5.1 kilobars (± 0.1)

TB-3: 745 °C (± 18) at 6.0 kilobars (± 0.2)

SB-7: 730 °C (± 19) at 6.1 kilobars (± 0.1)

The P-T reported is indicative of a transition from middle-amphibolite to lower-granulite facies metamorphism. The appearance and disappearance of cordierite with increasing metamorphic grade is expressed as biotite-cordierite gneiss->biotite-cordierite-garnet gneiss->garnet-biotite gneiss. The presence of cordierite in the metapelites of Hudson Highlands is not as imperative as previously thought in regards to determining peak metamorphism conditions. However, its appearance and disappearance provides a unique opportunity to determine the rates of metamorphic processes for the metasedimentary unit exposed in the Hudson Highlands.

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