Geochemistry and Tectonic Setting of Early Proterozoic Supracrustal Rocks in the Southwestern United States

Abstract

A revision of previous models of Proterozoic crustal accretion in the southwestern U.S. is presented, based on the more extensive data now available. Five Early Proterozoic predominantly volcanic supracrustal terranes are recognized, with ages of 1760-1800 Ma, 1730-1740 Ma, 1720 Ma, 1680-1700 Ma, and 1650 Ma. In some areas they are overlain by a sixth, comprised chiefly of quartzite-pelite. In the oldest three terranes mafic volcanic rocks of largely submarine origin are more abundant than felsic. In the 1680-1700 Ma and 1650 Ma terranes felsic rocks exceed mafics, volcanics are mixed submarine and subaerial, and non-volcanogenic sediments are important. Least mobile incompatible element distributions in basalts from the 1760-1800 Ma and 1720 Ma terranes are similar to those in basalts from evolved oceanic arcs and associated back-arc basins. Basalts from the other terranes resemble those from continental-margin arcs and associated back-arc basins at early stages of opening. Incompatible element ratios in...