Early Proterozoic supracrustal rocks in the Dos Cabezas Mountains in southeastern Arizona include felsic volcanic and hypabyssal rocks, amphibolite, feldspathic quartzite, arkose and conglomerate, quartzite, and phyllite. Felsic volcanics are chiefly water-laid tuff and associated volcaniclastic sediments. Feldspathic quartzites, arkoses, and conglomerates are derived from granitic and felsic volcanic sources.
Felsic and basaltic igneous rocks define a bimodal suite, and felsic rocks can be 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 and few in common with arc or oceanic volcanics. Dos Cabezas basalts can be produced by ∼20% melting of an incompatible-element-enriched Iherzolite source in the mantle followed by as much as 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 Mountains appears to have formed in a continental rift system similar to or part of early Proterozoic rift systems in central and southern New Mexico and, perhaps, in central Arizona.