PLEISTOCENE SILICIC DOMES AND THEIR MINERAL POPULATIONS, EASTERN HIGH CASCADES, CENTRAL OREGON

The High Cascades exhibit an abundance of mafic to intermediate volcanism with diminutive (and arguably anomalous) silicic production. A number of Pleistocene silicic vents, domes, flows, and coulees occur west of Bend, central Oregon, in an area confined by the Three Sisters Wilderness Area on the west, the Deschutes River to the east, Melvin Butte to the north, and Eaton Butte to the south. Previous workers grouped together a portion of these silicic flows, defining them as products of the Tumalo Volcanic Center; the interrelationship of the remaining silicic units is less constrained.

We report ongoing progress in defining 22 previously mapped silicic features. Eight samples have been analyzed through petrographic and EDS analyses, seven from rhyolite or rhyodacite flows, and one from dacite. Preliminary results exhibit sample-to-sample similarities including a groundmass content of quartz, feldspar, and lesser glass (the dacite is a hypohyaline exception). Feldspar microstructure ranges from disoriented to trachytic. Typical phenocrysts include plagioclase, olivine, ortho- and clinopyroxene, ilmenite, and magnetite.

Olivine typically, and some pyroxene, and plagioclase, exhibit disequilibrium features such as resorbed margins, sieve texture, and alteration rims; many are glomerocrystic. Micron-scale apatite is a common accessory grain. Some samples contain the zirconium-bearing minerals baddeleyite and zircon. Preliminary analysis suggests that the phenocryst assemblage is partially xenocrystic. The broad similarities of the current sample set indicate that Pleistocene silicic volcanism undergoes similar processes throughout the study area.