AN OLDER NEWBERRY? THE PLEISTOCENE DOMES AND FLOWS OF THE CENTRAL OREGON EASTERN HIGH CASCADES

The Pleistocene domes and flows of an area we are calling the central Oregon eastern High Cascades (COEHC) include a series of silicic products distributed on a spatial and temporal scale seen in other magmatic environments associated with extensional tectonics. The COEHC is the arc-parallel region west of the dominantly Holocene rocks of the Newberry Volcanic Center (NVC) and east of the Three Sisters – Mount Bachelor chains. The latter is largely evolved calc-alkaline products typical of arc volcanism, the former has tholeiitic materials linked to extension-associated sources.

Compositions range from basalt to trachydacite. Samples generally contain plag + TiFe ox + cpx ± ol ± opx, with rare amphibole, phases consistent with thermodynamic modeling. Accessory minerals include zrn, bdy, and ap. Geothermobarometric calculations on a limited number of equilibrium pairs point to mid-crustal initial crystallization at 400 to 600 MPa between 800 (trachydacite) and 1050 °C (basalt). Oxygen fugacity values are near FMQ, and titania activity is 0.4 to 0.54.

Most of these lavas deviate from arc-like (Cascadian) geochemistry. Fluorapatite grains have Cl between 0.25 and 0.5 wt.%, depleted values in comparison to published convergent-margin apatite data. Whole-rock compositions are more strongly ferroan (tholeiitic) and alkaline than those of typical Oregon High Cascades. The Fe enrichment and alkalinity, however, are nearly identical to NVC trends, suggesting that the extensional influence seen at the NVC may hold for these materials to its west.