PETROLOGY OF RHYOLITES IN THE CENTRAL WESTERN CASCADE RANGE, OREGON

The early magmatic history of the Cascade Arc is recorded in the Oligocene and early Miocene volcanics of the Western Cascade province. These rocks consist chiefly of silicic tuffs, tholeiitic basalts and basaltic andesites. Although the mafic end of this system has been well studied, the silicic rocks are poorly documented owing to difficulties inherent in dating and chemically analyzing hydrothermally altered pyroclastics. We have addressed this problem by analyzing relatively unaltered samples from a complex of silicic domes and lava flows exposed within the Menagerie Wilderness, north of the South Santiam River. The study area is on the northern edge of a series of rhyolite exposures that define a rough circle approximately 15 km in diameter, and which may represent an eroded caldera complex. Silicic lavas in the Menagerie area range in composition from medium K rhyolite to trachydacite (high K) and dacite (low K). Three main silicic centers were discovered within the study area and were distinguished mainly through petrographic characteristics. Phenocryst assemblages include plagioclase-hornblende-magnetite (Rooster Rock Rhyolite), plagioclase-quartz-magnetite (Soda Fork Rhyolite) and quartz-plagioclase-biotite-hornblende-magnetite (Moose Mountain Rhyolite). Chondrite normalized REE plots have moderate Eu anomalies and flat MREE-HREE patterns, consistent with fractionation of plagioclase and hornblende. However, trace element modeling indicates that Menagerie area rhyolites can not be related to any likely parent composition by crystal fractionation alone, and instead were crustal melts that have been modified by crystal fractionation. We are currently using Ar40-Ar39 dating techniques on hornblendes from the study area in the hopes of better constraining the age of the system.