Tholeiitic and alkalic basalts of the Eocene Siletz River Volcanics, Oregon Coast Range

Abstract

These rocks consist of a thick lower unit of tholeiitic basalt anda thin upper unit of alkalic basalt and associated porphyries. Petrochemicalstudies indicate that older basaltic pillow lavas of the lower unithave the most primitive composition. The abundance of porphyriesin the upper unit suggests that crystal fractionation was importantin production of the aphyric alkalic rocks, although some volatiletransfer may have occurred. Plots of analyzed basalts show similaritiesto those of the Hawaiian tholeiitic and alkalic suites. The marinevolcanic accumulation is typical of flows in the Tertiary eugeosynclineof the Oregon-Washington coast in early to middle Eocene, thoughthe rocks of the upper unit are more localized. The lowermost basaltsappear to be typical of basalt from the adjacent Pacific Basin whereasthe upper part of the lower unit and the upper unit are similar toHawaiian basalts.