How Hot, How Deep, How Long: Constraints on the Tectono-Metamorphic Evolution of Granulite Terranes

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Virginia Tech


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.



Metamorphism, Granulite, Archean, UHT