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Professor Ali Polar

2024 Howard Street Robinson Medalist听

University of Windsor听

Friday, November 29听

11- 12pm听

FDA 232

The Mechanism of Archean Continental Growth: Evidence from Field Relationships, and Volcanic Rock and TTG Geochemistry

Abstract:

Archean terrains consist predominantly (>90%) of contemporaneous tonalite-trondhjemitegranodiorite suites (TTGs) and greenstone belts, representing relict fragments of early continental crust. One of the most contentious areas of Earth Science today is the tectonic mechanism by which Archean continental crust was generated. Although studies on Archean terrains over the past four decades have enhanced our understanding of how these terrains formed, the nature of large-scale tectonic processes that produced Archean continental crust still remains highly controversial. This controversy and ongoing substantial debate have centered around opposing uniformitarian and non-uniformitarian tectonic models. The uniformitarian models propose that modern-style plate tectonic processes (e.g., subduction, accretion, collision) operated in the Archean, whereas the non-uniformitarian models postulate that vertical tectonics or other unknown processes (e.g., sagduction, catalytic delamination, heat pipe, mantle upwelling, stagnant lid) operated in the Archean. The question is which of these models is supported by the Archean rock record?

A review of the main lithological, structural, and geochemical data from 4.0 to 2.5 Ga Archean terrains reveals that they were generated at convergent plate boundaries by accretion of oceanic arcs, continental arcs, oceanic crust, and trench turbidites. Extensive geochemical data (>8,200 samples) from Eoarchean to Neoarchean volcanic rocks plot mostly in the oceanic arc, continental arc, and back-arc fields in tectonic setting discrimination diagrams, which suggest that subduction-related geodynamic processes operated throughout the Archean. Similarly, a comparison of large trace element data (>3500 samples) from Eoarchean to Neoarchean TTGs with those from Phanerozoic arc TTGs (4100 samples) suggests that most Archean TTGs also formed in arc environments. The positive Pb and negative Nb anomalies of most Archean volcanic rocks and TTGs are fully consistent with subduction-related settings. The temporal variations in the incompatible trace element ratios of Archean volcanic rocks and TTGs, coupled with their lithological associations, demonstrate that intra-oceanic arc magmatism was prominent in the Eoarchean. The trace element systematics of Paleoarchean volcanic rocks and TTGs signify the beginning of Andeanstyle continental arc magmatism between 3.5 and 3.2 Ga. From ~ 3.2 Ga there was a gradual transition in intra-oceanic arc magmatism to more abundant Andean-type continental margin magmatism in the form of voluminous TTGs and sanukitoids. From 3.2 to 2.5 Ga juvenile oceanic crust and arcs continued to form, accompanied by more active continental margin magmatism until 2.7-2.5 Ga, by which time there were sufficient crustal rocks to amalgamate into incipient large continents, the fragmentation of which started the first complete classical Wilson Cycle Plate Tectonics of breaking apart and re-assembling large continental masses. Field evidence for uniformitarian tectonic processes in the Archean rock record is exuberant, whereas field evidence to support the non-uniformitarian models is lacking. 2024 Howard Street Rob

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