Abstract

The Ouachita orogeny was the result of plate convergence at the southern margin of the North American continent, although the nature of the converging southern plate and the direction of subduction remain uncertain. The presence of areally extensive tuff layers interbedded with shale in the Mississippian Stanley Group of the Ouachita Mountains, Oklahoma and Arkansas, provides the potential to define the tectonic environment of volcanism from the geochemistry of the tuffs and thereby delimit the subduction configuration. The tuffs contain relic primary magmatic quartz, plagioclase, and alkali feldspar and range from crystal- to vitric-rich. Mineralogical sorting and diagenetic effects have caused chemical variability within individual tuff units, but overall the tuffs have retained their primary igneous geochemical characteristics. The Beavers Bend and Hatton tuffs are geochemically very similar and are more evolved (higher SiO2, Rb, Th, REE; lower Sr, Ba) than the Mud Creek tuff. The stratigraphically equivalent Sabine Rhyolite is geochemically distinct from the tuffs, having less fractionated rare earth element (REE) patterns and different trace element ratios. Both the Ouachita tuffs and Sabine Rhyolite have the geochemical characteristics of subduction-related magmas (for example, strong depletion of Nb and Ta relative to other incompatible trace elements). Consideration of trace element systematics in the tuffs compared to those of modern high-silica volcanic rocks from different subduction-related tectonic settings suggests a continental arc origin and implies southward subduction beneath a southern continent during Carboniferous ocean basin closure.

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