Petrology of Cascade Head Basalt, Oregon Coast Range, USA.

Abstract

Cascade Head Basalt is one of three Tertiary volcanic sequences located in the Oregon Coastal Range in the forearc of the Cascade volcanic arc; these sequences are a sea floor series represented by the Siletz River Basalt, a tholeiitic series represented by Yachats Basalt, and an alkalic series represented by Cascade Head. Cascade Head rocks formed in a marine environment, and include submarine clastic sedimentary rocks and subaerial lava flows, ranging in composition from picrobasalt through trachyte, with alkaline basalt being most common. Major and trace element modeling indicates that variation within the Cascade Head series could have been accomplished through fractionation of observed phenocrysts. Comparison with Hawaiian basalts of Loihi and Mauna Kea volcanoes indicates that Cascade Head Basalt was most likely erupted during the initial phase of shield magmatism, with the companion Yachats Basalt representing the more voluminous, middle phase of tholeiitic shield volcanism.