Abstract

The work of Sutton & Watson (1951) was one of the cornerstones in the interpretation of the evolution of early Precambrian (Archaean and early Proterozoic) gneiss complexes and in the documentation of mafic dyke swarms that intruded recently accreted Archaean cratons. Since then, the concept of geological terranes and terrane accretion has been extended back to the Archaean, and different events can be distinguished by increasingly precise isotopic dating techniques. There is growing evidence that, with certain clear provisos, the principle of uniformitarianism can be applied as equally to the early Precambrian as it can through the rest of the geological record. In terms of mafic magmatism, there are similarities between some ancient and modern suites, both intrusive and extrusive, but there are also some major differences, such as the presence of komatiites in the Archaean and the formation of large-scale layered intrusions and extensive dyke swarms at the end of the late Archaean and in the early Proterozoic. The mantle has evolved continuously with the progressive extraction of mafic magmas but, allowing for this, continental tholeiites appear to have changed little with time. Ancient mid-ocean ridge basalts are elusive because of their poor preservation record. The geochemistry of mafic volcanics occurring in many greenstone belts commonly suggests a continental contamination component and that these volcanic rocks are most closely akin to those from present-day arc systems. The relative abundance of early Precambrian intrusive mafic suites parallels the crustal growth curve, reflecting the global crustal accretion–differentiation superevent towards the close of the Archaean. Bimodal tholeiitic–noritic mafic magmatism associated with many of these intrusive suites stems from this rapid turnover in the mantle–mafic crust–continental crust system. This, in turn, gives a possible link between the respective evolutions of Archaean to Recent komatiitic, noritic and boninitic suites.

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