The Pinal Schist: an early Proterozoic quartz wacke association in southeastern Arizona

At the type locality near Globe, Arizona the Pinal Schist of early Proterozoic age is comprised largely of quartz wacke turbidites which appear to have been deposited as a submarine fan. Sand grains are principally monocrystalline and polycrystalline quartz, derived chiefly from plutonic sources with a minor contribution from low-grade metamorphic sources. The subrounded to rounded shapes of quartz sand and the sparsity of detrital feldspar indicate a significant pre-turbidite erosional history. Felsic volcanic fragments also comprise variable proportions of the clasts. QFL modal parameters indicate a source for Pinal sediments comprised chiefly of granites and felsic volcanics, representing a recycled terrane. Major and trace element contents of Pinal sediments also suggest that they are largely quartz wackes and that none are graywackes. REE elements exhibit typical Phanerozoic shale patterns and reflect the importance of granite or rhyolite with negative Eu anomalies in the source area. High U, Th, Rb, and Hf contents and high Rb/Sr, La/Th and K/Na ratios also favor a source of dominantly granite and rhyolite. An arc setting is clearly unacceptable for the Pinal sediments. Geochemical and petrographic results favor either a continental ridt or collisional orogen. A model is proposed in which Pinal turbidites accumulate in an inundated continental rift. Granite and felsic volcanic sources lay to the east and represent older uplifted segments of the rift. Intense erosion in an intertidal zone may have rounded quartz grains and selectively removed feldspars prior to turbidity-current transport.