GEOCHEMICAL EVOLUTION OF THE PARADISE MOUNTAIN CALDERA COMPLEX, DAVIS MOUNTAINS, TRANS-PECOS TEXAS

The Paradise Mountain Caldera (~20x14 km), located west of Fort Davis, produced two major ignimbrite/lava complexes at ~35.6 and 35.3 Ma. The older unit (Barrel Springs Fm.) is composed of rheomorphic tuff and associated lava; it spread into the eastern Davis Mountains and northeastward into the Barilla Mountains from a hypothesized source area north of the present-day Point of Rocks intrusion. The younger unit (Wild Cherry Tuff) formed thick intracaldera deposits and an outflow sheet, which spread southward to the margin of the Paisano Volcano, northeastward into the Barilla Mountains (only 2 m thick), northward into the central Davis Mountains and westward an unknown distance. Locally, the two units are separated by trachytic lava of the Mount Locke Formation; near the caldera, the Wild Cherry Tuff is overlain by chemically similar rhyolite lava (Casket Mountain Fm.). Intracaldera tuff was intruded by the syenitic Point of Rocks intrusion and locally intensively silicified and kaolinized.

Collectively, the above units form a series ranging from trachyte to alkali rhyolite. Variation between Mount Locke trachyte and Barrel Springs rhyolite can be modelled by fractionation of a mineral assemblage of pc, cpx, mt, and ap; that within Barrel Springs units by fractionation of af, cpx, mt, and ap. Barrel Springs rhyolite is less evolved than Wild Cherry Tuff. Alkali rhyolite of the Wild Cherry Tuff and Casket Mountain units, although similar in major element chemistry, show wide variation in incompatible trace elements, suggesting that evolution in terms of major element chemistry had reached an impasse.