Space and time in granite plutonism

Granitic suites formed in the last 2500 Ma are either calc-alkaline or, less commonly, alkalic. The space-time trends of granitoid rocks from modern compressional arcs include variations towards more silicic and potassic products. But even the most ‘mature’ magmas of these arcs have Fe/Mg ratios near to 1, in contrast with the strongly iron-enriched alkalic suites that characterize zones of crustal extension. Both calc-alkaline and alkalic suites are mainly mantle-derived; they evolved, respectively, from tholeiitic and alkali basalt parental magmas which have been subject, in most examples, to a two-stage fractionation history. Other factors that may influence the evolution of these parental magmas include, first, the addition of variable, but minor, amounts of crustal melt and, secondly, the persistence of refractory mineral phases in the zone of melting. Contributions from subducted continental detritus are thought to be of minor significance. A long-term trend, over the entire history of the crust from more sodic to more potassic calc-alkaline magmatism may reflect the declining importance of subducted ocean lithosphere in magma generation and the increasing role of the overlying volatile-fluxed mantle wedge. This trend and, in particular, the widespread development of alkalic granite suites in mid-Proterozoic times, may be linked to the declining vigour of tectonic and associated magmatic processes during the Earth’s history. These alkalic suites may indicate a period when the continental lithosphere had become stable enough to resist fragmentation, leading to intracontinental ‘rift and swell’ zones of magmatism. In some cases, new plate cycles, with more normal calcalkaline igneous rocks, seem to have occurred in the same zones. Although much of the evidence favours the operation of plate tectonic and subduction processes in magma generation during Archaean and early Proterozoic times, this link cannot be taken as proved except for the last ca . 1000 Ma.