CORRELATING THE ERUPTIVE PRODUCTS OF THE TUMALO VOLCANIC FIELD IN THE OREGON CASCADES AND EXAMINING THE ZIRCON FROM THE TUMALO TUFF

The Tumalo Volcanic Field (TVF) is a ~700 km² volcanic region located within the Cascade arc between Bend, Oregon and Three Sisters. The TVF is part of a region of silicic magmatism and could be related to the late Pleistocene/Holocene rhyolites of the nearby Three Sisters/Newberry. The TVF has produced several 1-10 km³ explosive eruptions: the Desert Spring Tuff (~650 ka), the Tumalo Tuff (TT)/Bend Pumice (both ~300-440 ka) and the Shevlin Park Tuff (<260 ka). The smaller Pumice of Columbia Canal (~170-300 ka) along with lava of Three Creek Butte (?) and Triangle Hill (~260 ka) have also been identified, with the latter two postulated as potential sources of the larger tuffs.

New XRF major/trace element analyses were performed on 13 samples from the TVF to compositionally correlate deposits. The Bend Pumice, Tumalo Tuff and the rhyolite of Three Creek Butte (TBT) plot between 73 and 74.5 wt% SiO₂ and cluster in nearly all elements. The Desert Spring Tuff, Shevlin Park Tuff and the pumice of Columbia Canal (DSC) are broadly correlative (66.5-68 wt% SiO₂) although do not cluster as tightly as the TBT samples. The DPS samples are higher in Fe₂O₃, TiO₂ and CaO. The andesite of Triangle Hill (TH) is separated from all other samples with the lowest silica content (58.9 wt%).

Three U-Pb dates were collected via SHRIMP-RG for zircon from the Tumalo Tuff: 378 ± 44 ka; 423 ± 19 ka and 603 ± 181 ka. Trace element analyses of these zircon reveal that they have low Eu/Eu* (<0.2) relative to zircon from other Cascade volcanic systems. These zircon plot within the field of central Oregon Cascades in Th/U versus Yb/Gd. Melt inclusions analyzed via electron microprobe from two TT zircon are 73.6-74.3 wt% SiO₂, 4.8-4.9 wt% Na2O and 0.16-0.19 wt% Cl. This is identical to the TT pumice analyzed, meaning these zircon grew from a magma body that maintained this composition for potentially 10⁴ years.

These data suggest two models: (1) two overlapping magmatic systems occupying the same space or; (2) a single system tapping different parts of a long-lived magmatic system. We support model (2) as the zircon from the TT predate the eruption significantly supporting crystal recycling. The DSC system is more mafic and bookends the history of the TVF, while the TBT is a high silica eruption that could be generated from remelting existing mush in the TVF from previous activity.