Archaean Processes, Crustal Growth and the Onset of Plate Tectonics

ABSTRACT In this contribution, we address how plate tectonics can be recognised from the geological record. We explore evidence for when the continental crust became rigid enough to facilitate plate tectonics, through the onset of dyke swarms and large sedimentary basins. Metamorphic regimes are sensitive to tectonic setting, but blueschists are largely restricted to terrains < 700 Ma old. Older terrains tend to be characterised by medium T/P (Barrovian) and high T/P (Buchan) metamorphism, which tend to be linked to crustal thickening and extension. Granulite facies metamorphism provides evidence for crustal thickening regionally by the late Archaean. Minor and trace element ratios of magmatic rocks are widely used to distinguish magmas generated in subduction and non-subduction-related settings. Both features are observed in mafic rocks from 3.8-2.7 Ga, and encouragingly, there appear to be links between geochemistry and tectonic style, in those rocks with non-subduction features, they tend to be associated with dome and basin architecture. Granite-greenstone terrains are characterised by bimodal magmatism, and while this is a feature of intraplate magmatism in younger terrains, it remains unclear how readily bimodal magmatism may be generated by subduction-related processes in the Archaean. Archaean cratons evolve through a period of mafic/ultramafic-TTG magmatism to more granitic phases with sanukitoid magmas as the cratons stabilise and sedimentary basins develop, and different cratons stabilised at different times from 3.1–2.5 Ga. Globally, there is a marked change in the estimated composition of juvenile continental crust from mafic to more intermediate compositions at ca. 3 Ga. It is argued that crustal evolution may be better constrained through crustal archives, rather than the development of depleted mantle, and by an apparent reduction in the rate of crustal growth at ca. 3 Ga. This is attributed to an increase in the rates at which continental crust was destroyed, and that, coupled with the other changes at the end of the Archaean, is taken to reflect the onset of plate tectonics as the dominant global regime.