PETROLOGIC ANALYSIS OF THE TEPEE DRAW TUFF, NEWBERRY VOLCANO, OREGON: PRELIMINARY CONSTRAINTS ON COMPOSITIONAL ZONING IN THE PRE-ERUPTIVE MAGMA CHAMBER

The Tuff of Tepee Draw (Qtp) is the oldest known ash-flow deposit related to Newberry Volcano (0.50 Ma). With an estimated volume of 10 km³, Qtp has been inferred to represent the earliest caldera-forming eruption (MacLeod et al., 1981). The deposit, which is well exposed on the east flanks of Newberry, varies in degree of welding and lithic content with distance from the caldera. At proximal localities (11-12 km from caldera), Qtp grades from moderately welded tuff upward into densely welded tuff. In distal outcrops (18-19 km from caldera), the base consists of non-welded ash-flow tuff containing abundant (20-25 %) pumices up to 25 cm in diameter, becoming moderately welded higher in the section. In hand sample, pumices are characteristically homogeneous and range from light gray to dark gray. Some, however, display conspicuous banding. Petrographically, pumices are crystal poor, with plagioclase and quartz as the dominant phenocrysts.

XRF and ICP-MS whole-rock chemical data were obtained from both single pumice clasts and bulk-tuff samples. As a group, Qtp samples range from 68.2 to 72.4 wt. % SiO₂ (normalized to 100% volatile-free). The homogeneous pumices are rhyolite (70.5-72.4 wt. % SiO₂), and the banded pumice is dacite (68.2 wt. % SiO₂). The bulk-tuff samples have a more limited compositional range (69.4-70.6 wt. % SiO₂), falling between the dacite and rhyolite pumices. On Harker variation diagrams, major oxides display systematic trends with increasing SiO₂. Trace element variation diagrams show that with respect to SiO₂, Rb, Zr, Ba, La, Yb, Hf, and Ta contents increase, and Sr and Eu decrease. On REE diagrams, the samples form a tight group, show pronounced Eu anomalies, display LREE enrichment, and have flat to slightly concave upward HREE trends.

A distinct compositional gap is evident between the dacite and rhyolite pumice samples. Select trace element diagrams (Hf and Ta) show that the rhyolite pumices may be further subdivided into at least two compositional groups. The available data suggest that Qtp represents eruption of a zoned magma chamber that was possibly arranged in discrete compositional layers. This study provides a framework for continued petrologic analyses of Qtp, the goal of which is to constrain the configuration of the pre-eruptive chamber in the context of the evolving Newberry magma system.