Abstract

Two lithologic assemblages are identified in the early Proterozoic (∼1700 Ma) supracrustal rocks of the Pinal Schist in southeastern Arizona. A western assemblage is composed largely of quartz wacke turbidites, and an eastern assemblage includes bimodal volcanics and variable amounts of sediment, chiefly feldspathic quartzite and conglomerate. Basalts from the eastern assemblage have incompatible-element distributions similar to those found in basalts from continental-margin arcs or evolved oceanic arcs, whereas those from the western assemblage have distributions similar to within-plate or back-arc basin basalts. Felsic volcanics range in composition from rhyodacite to rhyolite and exhibit incompatible-element distributions similar to those found in felsic volcanics from continental rifts and back-arc basins near continents. Provenance studies of sediments from both assemblages indicate sources rich in granite and felsic volcanics. The quartz wackes are similar in composition to sediments from continental-margin arcs and associated back-arc basins. An increase in mafic components in these rocks from north to south in the Pinal basin may reflect compositional polarity in an arc system.

The eastern assemblage may represent remnants of a major northeast-trending continental-margin arc system, and the western assemblage may have been deposited in an intra-arc basin or a northwest-trending aulacogen which opened on the southwest.

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