Probing magma storage conditions and the controls of silicic magma formation: Tumalo Volcanic Center, central Oregon

Abstract:

Silicic magmas (>60-65 wt. % SiO₂) play an important role in the formation, stabilization, and evolution of continental crust in arc settings. High silica magmas are also associated with hazards related to explosive volcanic eruptions. Despite the well understood implications of silicic magma formation, the processes that generate silicic magmas in convergent margins are still incompletely known. The Tumalo Volcanic Center (TVC) of central Oregon represents an ideal study location for probing the processes related to silicic melt formation within the Cascade Arc, where rhyolitic eruptive products are relatively rare. The TVC is a major but highly understudied ~700 km² region located west of Bend, OR, that consists of mid-to-late Pleistocene pyroclastic flows, tephra fallout, and domes and lava flows. Three large intermediate-to-silicic pyroclastic flows deposits are included in the TVC: the Desert Springs Tuff, Tumalo Tuff, and Shevlin Park Tuff. The TVC potentially represents the transition from high-FeO*, “hot-dry-reducing” silicic magmas produced in the earliest High Cascade stages, as preserved in the ~5 Ma Deschutes Formation, and lower-FeO*, “cold-wet-oxidizing” magmas documented at late Pleistocene and Holocene eruptions at the Three Sisters Volcanic Center. Both low FeO* and high FeO* eruptive products have been documented in the TVC. Results from a global assessment of rhyolite compositions and tectonic environment suggest that although the low and high FeO* rhyolites occur in subduction zone settings, the high FeO* magmas are also similar to those that occur in intraplate and other non-subduction settings, consistent with greater contributions from extended fractionation of basaltic magmas. Further planned work includes using mineral (pl ± opx and cpx) and melt chemistry to deduce pre-eruptive magma storage conditions (i.e., pressure, temperature, and fO₂) for both types of rhyolites. Results of this study will help identify controls on silicic magma formation in a continental arc that exhibits low overall amounts of rhyolite production.