Alkali Basalts at Saddleblanket Mountain, Central Oregon Cascade Range

Abstract

Nepheline-normative alkali olivine basalts, including some with groundmass biotite, occur at Saddleblanket Mountain, an early High Cascade volcano located in the Western Cascades of central Oregon. These basalts are enriched in incompatible elements, especially Ba, compared with older subalkaline volcanic rocks in this area and with many arc rocks in other areas. Trace element data, including large Ba/Zr ratios, are believed to reflect metasomatism in the upper mantle source region, probably by a fluid derived from the subducted slab. The alkali basalt magma was derived from smallpercentage partial melting of metasomatized peridotite. Relatively rapid rise of this small-volume magma was facilitated by the development of an extensional tectonic environment during early High Cascade volcanism, and only minor olivine fractionation occurred. The extensional environment may have developed as a result of relaxed compression because of slowed, more oblique subduction. The Cascade Range is a continental volcanic arc extending from northern California, through Oregon and Washington, and into southern British Columbia. The geologic evolution of the Cascades has been summarized by Hammond (1979), McBirney and White (1982), and Priest et al. (1983). Traditionally, the Cascade Range has been divided into the Tertiary Western Cascades and the mostly Quaternary High Cascades (e.g., Peck et al., 1964). These terms have been used both in the time sense and as physiographic provinces. In this study the framework of volcanic episodes suggested by Priest et al. (1983) for the central Oregon Cascades is followed. Reconnaissance field work in the area of Winberry Creek in the central Oregon Western Cascades province led to the discovery of unusual alkali basalts at Saddleblanket Mountain. This paper describes some aspects of these alkali basalts and compares them to other lavas in the area. The Winberry Creek-Saddleblanket Mountain study area is located east of Fall Creek Reservoir in Lane Co., Oregon (Fig. 1). Volcaniclastic rocks This content downloaded from 157.55.39.112 on Wed, 07 Sep 2016 04:53:00 UTC All use subject to http://about.jstor.org/terms 34 TRANSACTIONS OF THE KANSAS ACADEMY OF SCIENCE