THE ERUPTIVE CHRONOLOGY OF THE TUMALO VOLCANIC CENTER, CENTRAL OREGON

The Tumalo Volcanic Center (TVC) of central Oregon represents a locus of silicic volcanic activity that occurred during the mid to late Pleistocene. This ~700 km² region is composed of silicic to intermediate pyroclastic flows, cinder cones, tephra fallout deposits, domes, and lava flows, totaling at least 30-50 km³ of erupted material over ~500 kyr. The TVC is one of a limited number of places where rhyolite erupts along the modern Cascade Volcanic Arc, making it an ideal locality to study the processes of silicic melt generation.

Although the TVC has long been an area of interest, it has also remained relatively understudied. One area of uncertainty is the chronology of eruptive activity. Many TVC units lack ages or were dated using less reliable methods (e.g. K-Ar). A more precise and accurate chronology will enable more detailed studies of magma evolution, volcanic hazards, and evaluation of the relationships between units – particularly the relationship between major ignimbrites (i.e., the Tumalo Tuff, Shevlin Park Tuff, and the Desert Springs Tuff) and potential source vent locations.

We are conducting a thorough evaluation of the timing of TVC eruptions using ⁴⁰Ar/³⁹Ar, and zircon U-Pb and U-Th techniques. Each TVC eruptive unit will be analyzed by several methods. For ⁴⁰Ar/³⁹Ar analyses, approximately 0.1 grams of plagioclase and glass or groundmass, depending on whether the sample was a pumice, tuff, or lava, will be analyzed via incremental heating procedure. In addition, for the Tumalo Tuff, Shevlin Park Tuff, and the Desert Springs Tuff, multi-grain plagioclase aliquots will be analyzed via total fusion. These ages will be combined with zircon U-Pb and U-Th interior and surface ages.

Our new chronology for the TVC will provide important context for how these eruptions fit with other regional volcanic activity, such as the High Lava Plains and other Cascade Arc eruptions and will elucidate how magma genesis and volcanic behavior varied across the TVC through time and space. Additionally, by analyzing TVC samples using different approaches, such as groundmass vs. phenocryst, and total fusion vs. incremental heating for ⁴⁰Ar-³⁹Ar, and zircon U-Th and U-Pb surface and interior ages, this study will help assess the validity of different methods and techniques for relatively young, low- to moderate-K volcanic products.