Granulites are the dense, strong metamorphic rocks that are produced during high- (HT) to ultrahigh-temperature metamorphism (UHT) and partial melting of Earth's crust. Granulites are ubiquitous in exhumed Archean cratons and are thought to comprise much of Earth's stable lower crust. Understanding the mechanisms responsible for crustal heating in Archean terranes is thus paramount to understanding the stabilisation of early continental crust, and whether such mechanisms resemble modern tectonic processes. It is therefore important to quantify the pressure–temperature–time (P–T–t) paths of Archean granulites, as such paths can be diagnostic of heating mechanism. This dissertation explores: 1) novel approaches to reconstructing the P–T–t paths of granulites, and 2) what the deciphered P–T–t paths of rocks from two Archean granulite terranes reveal about Archean crustal heating. The first chapter shows how petrologic modelling at multiple scales from a texturally heterogeneous granulite can provide "snapshots" of the P–T path, which would be difficult to reconstruct otherwise. The remaining chapters are focused on reconstructing the P–T–t paths of two Archean granulite terranes: the Beartooth Mountains, and the Pikwitonei granulite domain (PGD). The second and third chapters present evidence for cryptic HT metamorphism of the Beartooth granulites at ~2.7 Ga characterized by rapid (< 1 Ma) exhumation at HT and fast cooling (~10-100 C/Ma) in the middle crust. This suggests advective/conductive heating over short length-scales. In the fourth chapter, thermobarometric data suggest the western PGD experienced UHT decompression followed by cooling in the lower crust. High-precision zircon and monazite dates reveal apparently episodic crystallization over at least ~24 Ma. This episodicity could reflect multiple thermal cycles or the control of local reactions on zircon/monazite crystallization during cooling. High-spatial resolution petrochronology provides temporal constraints on prograde metamorphism. These data suggest metamorphism in the PGD was driven by a long-lived heat source over large length-scales near the base of the lithosphere. Disparities in the timescales, length-scales, and the depth and amount of heating between the terranes may suggest different crustal heating mechanisms in each, and that the late Archean Earth may have been tectonically diverse. / Ph. D. / Granulites are the dense and strong rocks that are produced during extreme heating, high- (HT) to ultrahigh-temperature (UHT) metamorphism, and partial melting of Earth’s continental crust. Granulites are common in exhumed fragments of Earth’s ancient, Archean (> 2.5 billion yrs old) crust and are thought to comprise much of Earth’s stable lower crust. Understanding the mechanisms responsible for extreme heating of the crust is thus crucial to understanding how Earth’s early continents were stabilised, and how plate tectonic processes have evolved through Earth history. Metamorphic rocks preserve a commonly cryptic record of the path(s) they took through the crust, which are controlled in part by the heating mechanism(s) and tectonic settings involved in their genesis. It is therefore important to quantify the pressure–temperature–time (<i>P– T–t</i>) paths of Archean granulites, as such paths may be diagnostic of particular crustal heating mechanisms. This dissertation explores: 1) novel approaches to reconstructing the pressure– temperature–time (<i>P–T–t</i>) paths of granulites, and 2) what the deciphered <i>P–T–t</i> paths of rocks from two Archean granulite terranes reveal about Archean crustal heating. The first chapter presents a novel way to reconstruct the <i>P–T</i> path of a texturally complex and heterogeneous rock, which reveals a more detailed history than conventional methods would. The second, third, and fourth chapters are focused on reconstructing the <i>P–T–t</i> paths of granulites from two Archean terranes that record HT/UHT metamorphism ~2.7 billion years ago: the Beartooth Mountains, and the Pikwitonei granulite domain (PGD). These studies show that crustal heating of the Beartooth granulites was brief (< 1 million years duration) and seems to have occurred over short length-scales in the middle crust, while crustal heating of the PGD was long-lived (> 24 million years duration) and occurred over large length-scales near the base of the crust. The disparities in the timescales, length-scales, and the depth and amount of heating between the two terranes may suggest different crustal heating mechanisms operated in each, and that the Earth may have been tectonically diverse ~2.7 billion years ago.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/77915 |
| Date | 05 June 2017 |
| Creators | Guevara, Victor Emmanuel |
| Contributors | Geosciences, Caddick, Mark J., Beard, James S., Tracy, Robert J., Spotila, James A., Law, Richard D., Brown, Michael |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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