USING GEOCHEMISTRY TO ESTABLISH THE CONNECTION BETWEEN DACITES AND RHYOLITES OF THE BONANZA TUFF, SAN JUAN VOLCANIC FIELD, CO

The 33 Ma Bonanza Tuff (1,000 km³) erupted from the Bonanza Caldera in the San Juan Volcanic field in Colorado as a single ignimbrite sheet that is compositionally zoned vertically and laterally. The Bonanza Tuff alternates from dacite to rhyolite (62-76% SiO₂) several times. Both compositions contain crystal-rich dacitic and crystal-poor rhyolitic pumice. Unique to the Bonanza Tuff is the eruption of dominantly dacite deposits to the west and rhyolite deposits to the east. The current hypothesis suggests that both rhyolites and dacites may have been derived from the same, but different levels of a stratified magma reservoir. Alternatively, the dacites and rhyolites were tapped from two exclusive, but nearby reservoirs. Investigating the geochemistry of such a unique pulsating system and testing these models allows us to understand the configuration of the near-surface magma plumbing system that led to the Bonanza Tuff supereruption.

We present detailed petrography, whole rock and mineral scale geochemistry to examine the geochemical relationships between the Bonanza rhyolite and dacite tuffs and included felsic and mafic pumices, i.e., if they are connected through fractional crystallization or mixing.

Petrographic observations show that plagioclase, hornblende and sanidine concentrations vary in both rhyolites and dacites across the thickness of the tuff, whereas biotite content remains fairly constant. XRF major oxides and trace element analyses indicate that dacites and rhyolites are related through crystal fractionation. Analyses of felsic pumice clasts from both ignimbrite types reveal a similar overall composition as the matrix of rhyolite tuffs, suggesting that both dacites and rhyolites likely formed in and were derived from the same magma reservoir. We are currently also analyzing mafic pumices and testing this hypothesis further by analyzing mineral compositions in different alternations of the two Bonanza tuffs by electron microprobe. If the crystals in the different tuffs are similarly zoned and indicate patterns of fractional crystallization, we conclude that they likely grew in the same (stratified) reservoir. Patterns compatible with mixing will necessitate separate sub-caldera reservoirs.