Year of Award

2021

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Geosciences

Department or School/College

Geosciences

Committee Chair

Julia Baldwin

Committee Co-chair

Rebecca Bendick

Commitee Members

Julia Baldwin, Rebecca Bendick, Christopher Palmer

Keywords

petrology, wyoming province, petrogenesis, geology, precambrian

Publisher

University of Montana

Subject Categories

Geochemistry | Geology | Tectonics and Structure

Abstract

Studying cordierite orthoamphibole gneisses (COG) from five different mountain ranges across the Montana Metasedimentary Terrane (MMT) of the Wyoming Province offers a unique opportunity to elucidate the enigmatic origins and petrogenesis of the lithology in addition to gaining a further understanding of Precambrian crustal assembly processes. Geochemical analyses suggest that COG originates from a basalt that underwent metasomatic alteration, likely via seawater, prior to metamorphism. Moreover, COG is considered to represent oceanic crust that was part of the epicontinental sea adjacent to the Wyoming Province before collision with the Medicine Hat Block. Field observations of associated lithologies such as marbles, quartzites, and amphibolites provide context for the interpretation of the geologic environment which coincides with the proposed petrogenetic model. Petrography and phase equilibria models show that COG retains a robust geologic record that includes ocean basin closure prior to collision, the tectonics of the Big Sky Orogeny (ca. 1.78-1.72 Ga), and the regional tectonic unroofing and orogenic collapse that followed. Phase equilibria models from across the MMT depict an overall trend of thermotectonic conditions in the Big Sky Orogeny being the highest in the Tobacco Root Mountains and decreasing in grade as you increase in proximity to Giletti’s Line. However, the Highland Mountains are considered the exception to this trend, as they are interpreted to be island arc terrane that was accreted onto the margin of the Province during basin closure.

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© Copyright 2021 Brianna K. Crenshaw ms