Geodynamic significance of S-type granites in circum-Pacific orogens

In Phanerozoic circum-Pacific orogenic belts, most "post-collisional" S-type granites and associated high-temperature, low-pressure metamorphic complexes formed during early are extension. The granites are part of a tripartite association consisting of (1) inboard S-type granite, (2) outboard oceanic arc, and (3) intervening, turbidite-filled backarc basin. S-type granites herald the formation of new outboard oceanic are and extensional backarc systems, but thickening of a preexisting, sediment-dominated backarc basin is a prerequisite for their generation. In these environments, S-type plutonism is triggered by renewal of arc magmatism following thickening, when hot basaltic magmas are intruded into the thickened backarc crust once slab retreat is reestablished. With ongoing extension during retreat, the crust becomes progressively thinned, the sedimentary contribution is diminished, and the granites lose their S-type character. Such tripartite associations involving S-type granite are probably diagnostic of repeated slab-retreat episodes, and the Jurassic U.S. cordillera might be an example.