Proterozoic crustal provinces in the southwestern United States decrease in age south from the Archean Wyoming province. Supracrustal rocks define three major age provinces from north to south: 1.72 to 1.80, 1.65 to 1.72, and 1.1 to 1.2 b.y. Successions within each province are similar and define two assemblages: a bimodal volcanic assemblage commonly overlain by a quartzite-shale assemblage. Precambrian granitic plutons intrude all three provinces. Geochemical and Sr-isotope data indicate a variably depleted upper mantle source for basalts and a short-lived (<100 m.y.) lower-crustal source for granitic and felsic volcanic magmas.

These observations can be explained by a plate-tectonics model involving successive marginal basin closures and Andean-type orogenies associated with southward-migrating arcs. The net result is to accrete ≥ 1,300 km of continental crust to the southwestern margin of North America between 1.8 and 1.1 b.y. ago.

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