GEOCHEMISTRY OF VOLCANIC UNITS OF THE BONANZA CALDERA (CENTRAL COLORADO): IMPLICATIONS FOR MAGMA SOURCES AND VENT LOCATIONS

The ~38-33 Ma Bonanza volcanic center is located north of the Rio Grande Rift and along a NNW alignment with the Mt. Aetna and Grizzly Peak calderas to the north. Bonanza has been considered part of the San Juan volcanic field, but the relatively early timing of caldera collapse and onset of silicic activity distinguishes it from calderas of the western and central parts of the San Juan field. Prior to caldera formation, Bonanza was a composite cone built of high-potassium andesite and dacite lavas. Explosive eruptions of <50 cubic kilometers of felsic magma led to trapdoor-style caldera collapse. Pyroclastic units include the dacitic lower Bonanza Tuff (LBT) and rhyolitic upper Bonanza Tuff (UBT). Activity resumed to andesitic and dacitic lavas, followed by intrusion of silicic stocks along the caldera margin.

Petrologic and new geochemical data exhibit similarities between the LBT and the andesitic/dacitic lavas that occur above and below the pyroclastic units, whereas the UBT is geochemically distinct from other Bonanza volcanic units. Major and trace element compositions are inconsistent with derivation of UBT rhyolitic magma by fractionation involving intermediate parental magmas represented by the LBT and Bonanza lavas. The UBT is interpreted as a crustal melt, involving a source distinct from those that supplied intermediate magmas.

The distinctive nature of the UBT relative to other Bonanza volcanic units raises the question whether UBT magma was erupted from local vents or from vents distal from the Bonanza caldera. However, the UBT shares geochemical characteristics with late silicic stocks emplaced at vents along the caldera margin, suggesting that UBT rhyolitic magma was erupted from those (or other) vents proximal to the caldera.