EVOLUTION OF SILICIC MAGMATISM IN THE TUMALO VOLCANIC CENTER AND THREE SISTERS REGION OF THE CENTRAL OREGON CASCADES DOCUMENTED THROUGH ZIRCON PETROCHRONOLOGY

The Tumalo Volcanic Center (TVC) in the central Oregon Cascades is a group of explosive and effusive volcanic deposits that have been loosely grouped in the literature for more than half a century. Previous workers identified three large (5->10 km3) tuffs in the TVC: Desert Spring Tuff (DST), Tumalo Tuff & Bend Pumice (TTBP; a single eruptive event) and Shevlin Park Tuff (SPT). We present new zircon petrochronology and Ar/Ar geochronology suggesting that the TVC should be redefined into multiple systems that share crystal cargo.

Our new Ar/Ar eruption ages define four groups: (1) the DST at ~640 ka; (2) the Tumalo Dome Field (TDF) that includes four domes that erupted from ~442 to ~391 ka and the TTBP (~440 ka); (3) SPT and Pumice of Columbia Canal (253-261 ka) and (4) Todd Lake Volcano (TLV; ~187 ka).

We analyzed U-Pb ages and trace element compositions in 150+ zircon via SHRIMP-RG. Zircon ages range from ~200 ka to 7 Ma, with a majority from 300 to 500 ka. We make the following observations: (1) TDF units share zircon age spectra (~350-500 ka); (2) the SPT and TLV zircon overlap with the TDF with a small component of zircon <350 ka; (3) no zircon were identified in the DST. Zircon range from 6,000-10,000 ppm Hf, 15-30 Yb/Gd, and < 0.6 Th/U. Eu/Eu* is dominantly <0.2.

These zircon are also compositionally similar to nearby Rock Mesa (RM) and Devils Hills (DH), rhyolite domes erupted at 2-3 ka. Previous work (Stelten and Cooper, 2012) showed that some zircon from RM and DH are in U-Th radioactive equilibrium (>300 ka). This indicates that TDF zircon are being recycled in recent rhyolitic magmatism under Three Sisters, suggesting the underlying crust was reworked by silicic magmatism during the peak of TDF zircon crystallization (300-500 ka).

The TDF is best compared to California’s Lassen Domefield (LDF). In both cases, numerous small (<2 km³) intermediate to silicic domes distributed over ~100 km² and are marked by recycling of zircon crystals. They are spatially associated with mafic volcanism: the Central Oregon Cascades for the TDF and the Poison Lake Chain for the LDF. Dy/Yb vs La/Sm for zircon suggest that the TDF magma is derived from a drier magma, with less evidence of amphibole fractionation. These data imply that TDF and post-TDF may be more closely related to “hot and dry” High Lava Plains magmatism rather than “cold and wet” Cascades magmatism.