Evolution of the Early Oligocene Bonanza Caldera, northeast San Juan Volcanic Field, Colorado

Abstract

The 36‐Ma‐old Bonanza caldera developed within a slightly older sequence of intermediate composition volcanics (Rawley Andesite), equivalent to the Conejos Formation of the greater San Juan volcanic field. Vent facies lava flows, monolithic breccias, and numerous intrusions dominate the early sequence within and near the caldera. Distal lava flows and abundant lahars of Rawley Andesite dip radially outward, suggesting that the Bonanza caldera formed near the summit of a large composite volcano. The caldera collapsed in a trapdoor fashion when less than 50 km3 of dacite and rhyolite ignimbrite of the Bonanza Tuff erupted from a major vent area near the north/northwest caldera margin. Listric normal faulting along a western ring fault system dropped basal ignimbrites down to the east with a maximum displacement of nearly 1 km. Northwest trending basement structures may have influenced the orientation of the ring fault system. Andesite and dacite lava flows overlie the Bonanza Tuff, marking a return to passive volcanism following explosive eruptions. Slightly younger silicic stocks and exogenous rhyolite domes were emplaced along the caldera margins and were cut by ring fracture faults, suggesting a protracted history of caldera collapse. Rawley Andesite, lower Bonanza Tuff, and late lava flows are chemically similar, high‐K andesites and dacites. Initial 87Sr/86Sr ratios (0.70529–0.70688) of the intermediate suite preclude major contamination with 87Sr‐rich Precambrian upper crust. The compositional homogeneity of these rocks and the absence of Plinian airfall components to the ignimbrites suggest that subvolcanic magma chambers at Bonanza did not become strongly compositionally zoned. Rhyolite ignimbrite of the upper Bonanza Tuff (initial 87Sr/86Sr = 0.70850) may be consanguineous with late silicic stocks, rather than with the volcano‐building andesitic suite. The Bonanza caldera appears to be closely related to two Oligocene calderas to the north (Mount Aetna and Grizzly Peak), which share a common position along the western edge of the northern Rio Grande rift system. Early Oligocene silicic magmatism and caldera formation along this trend may mark the initial phases of rift evolution, perhaps heralding a change to a tensile stress regime prior to physiographic development of the rift valley.