EXPLOSIVE FLARE-UP FOLLOWING THE LATE NEOGENE INITIATION OF THE CENTRAL OREGON HIGH CASCADE ARC

The Deschutes Formation of Central Oregon (~7.4- 4.0 Ma) preserves a remarkable stratigraphy that records the initial stages of the High Cascade arc following an eastward shift in volcanism ~7.5 Ma. Over 100 airfall tuffs and 50 laterally extensive ignimbrite units are contained within the formation, suggesting that the arc may have been much more magmatically productive and explosive during this phase than at any other time within the last 17 Ma. This study aims to evaluate this history of explosive volcanism by establishing a comprehensive record of the ages, volumes, composition, and petrogenesis of each explosive deposit in the within the Deschutes Fm.

Initial estimates for some of the larger ignimbrite units suggest volumes ranging up to 10 km³ DRE, with sources near the Three Sisters and Mt. Jefferson regions. Furthermore, new ⁴⁰Ar-³⁹Ar ages of plagioclase contained in pumice indicate that most of this large volume of silicic material was erupted within a relatively short period of time, between 5.38 ±0.06 Ma and 6.05 ±0.13 Ma (1σ). Thus, the early phases of the High Cascades in this region were marked by a significant silicic flare-up event. We suggest that extension, expressed locally by the Cascade graben, may have contributed to the formation and eruption of large volumes of silicic magma.

Glass compositions are broadly bimodal and single eruptions can contain multiple populations of pumice which range between 52 and 72 wt.% SiO_2. This and the widespread existence of banded pumice, suggests that many eruptions involved multiple magma types. Trace element data (e.g. Nb) demonstrates differing trends between pumice sourced from Mt. Jefferson and those sourced from the Three Sisters area, indicating that each source sampled compositionally different crust and/or mantle sources.

Analyses of pumice glass also indicate that Deschutes Fm. ignimbrites are generally much higher in FeO* for a given CaO or SiO₂ than Quaternary Cascade products, and instead are more similar to volcanics from the High Lava Plains. This suggests hotter and drier melting conditions during rift-related mantle upwelling and partial melting of mafic crust. Furthermore, the eastward shift of magmatic activity to the High Cascades, and subsequent anatexis of previously un-melted crust, could have helped to produce large volumes of silicic magma.