Geochemistry and origin of early Proterozoic supracrustal rocks, Dos Cabezas Mountains, southeastern Arizona

Early Proterozoic supracrustal rocks in the Dos Cabezas Mts include volcanic and hypabyssal rocks, amphibolite (metabasalt), and various felsic metasediments. Felsic and basaltic igneous rocks define a bimodal suite with the former divided into three populations: rhyodacites, quartz latites and rhyolites. Volcanics share major- and trace-element distributions in common with modern volcanics from continental rift systems but few in common with arc or oceanic volcanic rocks. Dos Cabezas basalts can be produced by approx 20% melting of an incompatible-element-enriched lherzolite source in the mantle followed by approx 40% fractional crystallization of olivine. Rhyodacites can be produced by partial melting of an undepleted granulite source in the middle crust, and quartz latites and rhyolites by fractional crystallization of rhyodacite magma. The Proterozoic succession in the Dos Cabezas Mts appears to have formed in a continental rift system similar to or part of an early Proterozoic rift system in central and southern New Mexico and perhaps in central Arizona.-L.di H.