Dissertation/Thesis Abstract

A metamorphic pressure-temperature time path from the Wood Hills, Elko County, eastern Nevada
by Wills, Marci A., M.S., Northern Arizona University, 2014, 88; 1556632
Abstract (Summary)

The Wood Hills, located in eastern Nevada in the hinterland of the Sevier orogenic belt, includes sedimentary rocks of Cambrian to Devonian age that underwent burial, folding and metamorphism associated with Sevier orogenesis. I studied the metamorphosed Cambrian Dunderberg Shale, a pelitic schist with the mineral assemblage garnet-staurolite-kyanite-plagioclase-muscovite-biotite-quartz, to determine the age of garnet growth, the conditions of metamorphism and the pressure-temperature path. The garnet crystals are generally <2 mm in diameter and comprise about 1% of the rock's volume. P-T conditions were evaluated using isochemical plots (pseudosections) calculated using DOMINO software and the thermodynamic dataset of Holland and Powell (2000), as well as GASP and garnet-biotite thermobarometry (calibrations of Holdaway, 2001). Thermobarometry indicated P-T conditions of 6.8-7.1 kbar and 610-630 °C. The pressures determined are about 1 kbar higher than estimated by plotting the isopleths for the garnet core compositions (Xpy, Xgr, Xalm, and Xsps) on isochemical plots. Conditions of 625 °C and 6.6 kbar, consistent with thermobarometry within uncertainties, fall inside the rock's mineral assemblage field, and were used as starting conditions for garnet growth simulations to determine P-T paths. Using the program GIBBS, which performs calculations based on Duhem's theorem, and selecting as monitors the changes in mole fraction of grossular (ΔXgr) and moles garnet (ΔMgar), it was determined that garnets grew from core-to-rim during 10°C of temperature increase while pressure decreased 500 bars then increased 400 bars. This path suggests that garnets grew during a transition from exhumation to burial. The timing of garnet growth was determined using the Lu-Hf method, which yielded an age of 82.8 ± 1.1 Ma. Approximately 130 km to the northeast, in the Basin Creek area of the Grouse Creek Mountains, previously studied garnets in Neoproterozoic metasedimentary rocks (schist of Steven's Spring) preserve a P-T path indicative of prograde burial and yielded a similar age of 85.5 ± 1.9 Ma. The similar age supports the hypothesis from previous studies that both areas experienced burial by the Windermere thrust at 82-86 Ma.

Indexing (document details)
Advisor: Hoisch, Thomas
Commitee: Duebendorfer, Ernest, Ort, Michael, Wells, Michael
School: Northern Arizona University
Department: School of Earth Sciences and Environmental Sustainability:
School Location: United States -- Arizona
Source: MAI 53/01M(E), Masters Abstracts International
Subjects: Geology
Keywords: Core complex, Metamorphic, Path, Pressure, Temperature, Wood hills
Publication Number: 1556632
ISBN: 9781303921506
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