Phanerozoic plutonism in the Cordilleran Interior, U.S.A.

Plutonism was widespread from mid-Mesozoic through Paleogene time in the Cordilleran Interior of the United States (CI, defined as the region underlain by broadly autochthonous ancient crust inland from the Sierra Nevada batholith). Intrusive activity here was broadly synchronous with intrusion of the coastal Sierra Nevada and Peninsular Ranges batholiths, but in detail, timing in the CI differed from that nearer the coast: Triassic plutons are absent in the CI, the Jurassic intrusive peak is less pronounced than in the Sierra Nevada, and the most intense plutonic activity occurred later than in the Sierra Nevada and Peninsular Ranges. Granitoid rocks of the CI, especially the younger ones, have clear isotopic signatures of ancient crustal source components and are commonly strongly peraluminous; both of these characteristics represent major contrasts with the coastal batholiths. The North American craton, which forms the basement of the CI and from which the plutons were to a considerable extent derived, underwent intense orogeny and high-grade metamorphism during the Early Proterozoic. For the next 1.5 b.y. the continent remained intact and free of orogenic modification. Compressional tectonics, manifested both by thin-skinned thrusting and ductile nappe formation, as well as plutonism, characterized Mesozoic reactivation. Emplacement of the dominant Cretaceous CI plutons was roughly synchronous with the major deformation. CI plutonism was distinctive in several respects: (1) although no continental collision and no apparent extensional tectonism were involved, it extended far inland (800 km at present; probably >400 km at the time of intrusion) in old, previously stable crust; (2) intermediate metaluminous (<65 percent SiO 2) as well as felsic peraluminous granitoids show clear isotopic evidence for major crustal input; (3) strongly peraluminous granitoids have moderate concentrations of large-ion lithophile elements and fairly high Sr contents, not the high LIL and low Sr that characterize such rocks in other belts in ancient crust. Although CI magmatism extended remarkably far inland and evidence for a major subcrustal component is in many cases absent, it almost certainly was directly related to a subduction-induced thermal regime. Crustal thickening by sediment accumulation or overthrusting was probably of little importance in inducing magmatism. The thermal regime necessary for extensive Cretaceous–early Paleogene anatexis was probably brought about by increased mantle heat flux, perhaps resulting from lithospheric erosion, and/or by emplacement of subcrustal magmas at deep levels.