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 the most primitive basalts suggest the existence of at least three mantle sources. Andesites, which are important at only two localities, have geochemical characteristics of either evolved oceanic arcs (Yavapai Supergroup) or continental-margin arcs (Pedernal Hills). Felsic volcanics from the 1760-1800 Ma and 1720 Ma terranes have low HFS and heavy REE contents and are compositionally similar to those from modern arc systems and were probably produced by fractionation from associated basalts. Felsic units from the other volcanic terranes have relatively high HFS and heavy REE contents and resemble volcanics from continental rifts or back-arc basins in or near continental crust. Clastic sediments from the 1680-1700 Ma and 1650 Ma terranes are compositionally similar to sediments from continental rifts or near-continent back-arc basins. The 1760-1800 Ma terrane appears to reflect a succession of oceanic arcs rapidly accreted to the Archean Wyoming province from the southeast, while the 1730-1740 Ma terrane is a back-arc basin formed within the 1760-1800 Ma terrane. The 1720 Ma terrane is interpreted as an evolved back-arc basin in which depleted mantle is tapped. The 1680-1700 Ma and 1650 Ma terranes are remnants of an extensive continental-margin arc system and associated back-arc basins. Bimodal volcanism, characteristic of most of the Early Proterozoic terranes, may reflect differences in preservation of arc and back-arc successions.