FIELD AND GEOCHEMICAL STUDY OF THE BALD MOUNTAIN VOLCANIC CENTER, CENTRAL OREGON: CONSTRAINTS ON THE EVOLUTION OF A SILICIC MAGMA SYSTEM IN A BACK-ARC SETTING

Ash-flow tuffs associated with the Bald Mountain volcanic center (BMVC) provide insight into silicic magmatism in a back-arc setting relative to the early High Cascades. Located in the NW Basin and Range, ~52 km east of the Cascades, the BMVC is the probable source for the Peyerl Tuff (PT), which records several eruptions during the Pliocene from an obscured caldera delineated based on the semi-circular distribution of rhyolite domes (MacLeod and Sherrod, 1992).

The PT consists of multiple flow units, which display a range of characteristics, including welding, thickness, lithic content, and pumice clasts, that vary from proximal to distal localities across the distribution area. This study focuses on two distinctive non-welded, pumice-rich ash-flow tuffs identified on the basis of pumice clast composition and inferred stratigraphic relationships. The older of the two units contains a homogeneous pumice population consisting of dark gray to black clasts. The younger of the two contains a more heterogeneous population consisting of light to medium gray pumices with less common dark gray-black clasts. XRF and ICP-MS analyses of single pumices from these units provide a direct sampling of the magma at the time of eruption. Pumices from the homogeneous non-welded tuff are andesitic (59.3-59.5 wt. % SiO₂), while those from the heterogeneous tuff are nearly all rhyolitic (70.6-71.8 wt. % SiO₂). For most major and trace elements, the samples plot in tight clusters with minimal variability between individual pumices within a given compositional group. Notably, the sparse dark gray-black pumices in the heterogeneous tuff are compositionally similar to the andesitic variety in the homogeneous tuff, suggesting a petrologic connection between the two units.

The preliminary stratigraphic relationships and geochemical data indicate a working model for the evolution of the BMVC. The non-welded tuff with homogeneous pumices records an early eruption of andesitic magma, and the heterogeneous non-welded tuff represents a later rhyolitic eruptive phase. The data also suggest a likely petrogenetic link between the earlier andesitic and later rhyolitic eruptions. More broadly, this study provides a framework for interpreting the influence of extension and subduction related processes on silicic magmatism in a back-arc setting.