Petrology of Late Eocene basaltic lavas at Cascade Head, Oregon Coast Range

Abstract

Cascade Head is one of three prominent Middle to Late Eocene volcanic centers in the Oregon Coast Range that erupted alkalic basalt. At Cascade Head, 300–600 m of submarine to subaerial volcanic rocks are interbedded with thin-bedded, tuffaceous, brackish-water marine siltstone of the Nestucca Formation. The intercalated basalts in the Nestucca Formation are informally named Yachats Basalt of Cascade Head. This volcanic sequence consists of basal submarine basaltic breccia and lapilli tuff, ankaramitic basalt, aphyric submarine to subaerial alkali basalt lavas (which account for 75% of the volcanic pile), hornblende trachyandesite lavas and dikes, and an uppermost basaltic sandstone unit. The Nestucca Formation also contains andesitic and more silicic ash deposits. Major- and trace-element trends and mass-balance modeling of the Cascade Head lavas are compatible with fractional crystallization of the observed phenocryst phases: olivine, clinopyroxene, plagioclase + apatite ± ilmenite. The suite is characterized by enrichments in high field strength elements (HFSE) and by steeply negatively sloping rare earth element (REE) patterns ((La/Lu) n = 15–20). Transition metal contents are low (Ni, 5–36 ppm; Cr, 3–87 ppm; Sc, 1–22 ppm), indicating that none of the lavas are primitive. Elemental abundance diagrams are typical of continental alkalic basalt in that they show K depletion, and Nb and Ta enrichment. Element abundance diagrams of interbedded andesitic ash are distinct from those of the basalts, are depleted in Nb, Ta, and Ti, and indicate an arc source. The geochemical signature of the Yachats Basalt of Cascade Head is consistent with a fore-arc tectonic setting that was undergoing extension during late Eocene time. The basalts may be related to nearby hot spot activity but they could also have erupted through a “slab window” formed during plate reorganization during a decrease in the rate of convergence between the Farallon and North American plates.