CHARACTERIZATION OF THE DESERT SPRINGS TUFF IN COMPARISON TO LATER ERUPTIVE DEPOSITS IN THE TUMALO VOLCANIC COMPLEX, BEND, OR

The area around Bend Oregon has five rhyodacitic explosive deposits (Desert Springs Tuff, Tumalo Tuff, Bend Pumice, Columbia Canal, and Shevlin Park Tuff), collectively known as the Tumalo Volcanic Complex (TVC), ranging in age from 180- 670 ka. This study focuses on the oldest unit, Desert Springs. In outcrop, samples were lightly welded and have a pinkish orange ashy matrix and contain basaltic lithic fragments and pumice clasts up to 6 cm, varying in color from dark brown to light pink. All TVC deposits have relatively similar mineral assemblages of plag + opx + cpx + ox, but Desert Springs has a higher overall crystallinity of ~11%, as opposed to the younger crystal-poor (~<1%) deposits. Plagioclase is the dominant mineral phase (~75%) and appears to be comprised of multiple populations of crystals, from oscillatory zoned tabular crystals with rounded calcic cores to elongate, unzoned euhedral crystals. Some plagioclase has abundant inclusions (ap + il + mt). The matrix oxides (~13%) are predominantly titano-magnetite with exsolution lamellae. The pyroxenes occur in sub-equal proportions (~6%) and are mostly euhedral in shape. Although most phenocrysts are isolated, several feature complex intergrowths and occasionally occurr as glomerocrysts.

Previous whole-rock major and trace element chemistry work on the Tumalo Tuff/Bend Pumice and Shevlin Park Tuff suggest that the former may have formed in a more reducing, extensional environment (Zr/Sr >2.5; FeO*/MgO >6), whereas the latter, younger Shevlin Park Tuff has chemistry akin to classical subduction zone environments (Zr/Sr 0.3-0.9; FeO*/MgO 2.7-4.7). The Desert Springs Tuff ranges from 64-68 wt% SiO2, which is a higher silica content than Shevlin Park, but lower than Tumalo/Bend. Similarly, the Desert Springs samples plot transitionally (Zr/Sr 1-2; FeO*/MgO 3.1-5.7) with respect to extension vs subduction chemical fingerprinting. REE data shows a fairly linear pattern with a La/ Lu value of 4 and slight upward concavity in MREE for all Desert Springs samples. Compared to the Tumalo/Bend samples, it has a slightly lower La/Lu ratio and less pronounced Eu anomaly, but steeper slope and anomaly relative to Shevlin Park. Future work will explore the transition and potential relationship between the Desert Spring and subsequent eruptions.