Document Type

Thesis

Date of Degree Completion

Summer 2025

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Dr. Christopher Mattinson

Second Committee Member

Dr. Hannah Shamloo

Third Committee Member

Dr. Carey Gazis

Abstract

High pressure (HP) granulites form within collisional and subduction zone tectonic environments. Due to high temperature and presence of melt, it is difficult to recover their pressure and temperature (P-T) history by conventional methods. An integration of multi-method P-T estimations provides insight on these deep crustal processes.

We apply petrographic analysis, Zr-in-Rt thermometry, phase equilibria modeling, and conventional thermobarometric calculations to constrain metamorphic conditions of Early Paleozoic HP granulite samples formed during late stage continental subduction/collision from the North Qaidam terrane.

HP granulites contain the mineral assemblage Grt + Pl ± Cpx + Qtz + Ky + Rt ± Czo. Compositional banding is common throughout samples and is interpreted to record migmatization. All samples exhibit various degrees of amphibolite facies overprinting. Whole rock compositions range from basaltic (dark bands) to rhyolitic (light bands).

Phase equilibria models combined with Zr-in-rutile thermometry of two granulite samples reproduce the peak observed mineral assemblages + melt at 20-22 kbar and 800-825°C, which we interpret to be the most robust record of peak conditions in these samples. The peak P-T conditions suggest that these HP granulite samples originated from a high-temperature part of the subducted slab rather than from the overlying plate. Thermobarometric calculations using the Ky-Cpx-Grt-Qtz, Ky-Grt-Pl-Qtz, Grt-Pl-Cpx-Qtz, and Ab-Jd-Qtz systems of mafic bands yield pressure estimates of 13-17 kbar (at 775°C), which we interpret to reflect compositional resetting during retrogression, recording the conditions when melt was last stable in the given assemblage. Thermobarometric calculations within compositionally felsic bands are not coherent.

We interpret the P-T discrepancy within felsic bands to stem from an inaccurate bulk composition as a result of melt loss during metamorphism. We conclude that the alteration of bulk composition and mineral chemistry through deep crustal migmatization and exhumation respectively obscure the results of single-method thermobarometry, but an integration of thermobarometric methods provides a segment of the P-T path showing high-T, melt-present decompression late in the continental subduction/collision process.

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