The Evolution of the Tumalo Volcanic Center in Central Oregon as Recorded in Zircon Ages and Trace Elements

The Tumalo Volcanic Center (TVC) lies between the city of Bend and the Three Sisters Volcanic Complex in central Oregon. This cluster of volcanic vents and deposits represent a period of heightened explosive volcanism in the area. There are three major (~1-10 km3) ignimbrite deposits (Desert Spring Tuff, Tumalo Tuff-Bend Pumice, Shevlin Park Tuff) as well as numerous smaller explosive deposits, lava domes (Melvin Butte, Bearwallow Butte) and cinder cones (Triangle Hill). Compositionally, this cluster spans from andesitic to rhyolitic and likely erupted across the last 700,000 years.

One of the most important questions to ask about the units of the TVC is whether they represent a singular, evolving magmatic system or are discrete units that are not directly connected magmatically. Additionally, identifying source vents for the major ignimbrites has been problematic at best. Zircon can be a powerful tool to both unravel the potential petrologic and eruptive relationships between units in a volcanic center such as the TVC.

We have sampled all the major explosive and effusive units across the TVC to extract zircon. SHRIMP-RG for U-Pb and/or U-Th ages and trace element compositions will be combined with concurrent Ar/Ar dating for these units to deconvolve the magmatic history of the TVC as well as attribute tuffs to specific domes or cones in the area. Preliminary SHRIMP-RG U-Pb data (378±44 ka, 423±19 ka, 603±181 ka) for the TVC suggest that units such as the Tumalo Tuff have a long history of zircon-saturated conditions recorded.

Our initial data suggests that there may be a genetic relationship between magmas erupted across the history of the TVC, possibly driven by rejuvenation of magmatic mushes by new inputs of basalt into the lower crust. Our new data will resolve this problem and add essential chronological constraints to these important explosive silicic deposits that sit underneath a region that over 100,000 people call home.