PETROLOGY AND ORIGIN OF CRUSTAL XENOLITHS FROM TUMALO CINDER CONE, CENTRAL OREGON CASCADES

Tumalo Cinder Cone, a Quaternary High Cascades volcanic center located about 8 km southwest of Bend, OR, is well-known for its crustal xenoliths. The goal of this study is to characterize the petrology of these xenoliths and gain insight into the nature of the crust beneath central Oregon in an area near the inferred edge of the Columbia Embayment. Specifically, we seek to determine whether these xenoliths were derived from: (1) pre-Tertiary rocks at the western edge of the continent, (2) Oligocene-Miocene intrusions associated with Western Cascades magmatism, or (3) Pliocene or younger plutons associated with High Cascades volcanism.

Approximately 40 xenoliths have been collected; they range between 3 cm x 2 cm and 11 cm x 7 cm size and weigh between 4 and 510 grams each. All appear to be broadly granitic in composition (quartz diorite – granodiorite) and many appear to have undergone partial melting during ascent as evidenced by their vesicular textures and the presence of mixing textures at the margins. Chemical analysis by XRF and ICP-MS indicate the xenoliths range from 63-79 wt.% SiO₂ and are moderately LREE enriched (La_N = 80-90; La/Yb_N = 4.4 – 5.0; Eu/Eu* = 0.4 – 0.2). Spidergrams indicate the xenoliths originated in a subduction zone setting. Relative to data from Miocene granitic rocks in the Western Oregon Cascades, the Tumalo xenoliths have systemically lower CaO and higher Na₂O at equivalent SiO₂. A trend toward lower CaO/Na₂O with decreasing age has been observed among data from Cascade plutonic and volcanic rock, suggesting that these xenoliths are younger than Miocene. Zircons separated from one xenolith are being dated by LA-ICP-MS to further test this conclusion.