THOLEIITE TREND OBSERVED IN THE SPRINGFIELD QUADRANGLE

The Little Butte Volcanics of the Western Cascade Group have long been recognized as being a calc-alkaline series. However, in the summer of 2003 a tholeiite trend was observed during a field mapping project near Springfield, Oregon. End members of the tholeiitic trend include mostly tholeiites that are found in the highlands of Coburg Hills, Camp Creek Ridge, Mt. Pisgah and the Wallace Creek area near Jasper while the silicic end members are represented by a large deposit of rhyolite found on the southern end of Coburg Hills and a smaller deposit on the southwestern edge of Camp Creek Ridge. Ages of the sequence in the area range from around 30.88 Ma at the base of the section near Spores Point to ~ 25 Ma at the top of the section in the Wallace Creek area. On Camp Creek Ridge, aphanitic tholeiites are found low in the section with a gradual increase in size and quantity of phenocrysts upsection. A similar relation exists elsewhere in the highland areas of the Quadrangle suggesting a sudden outpouring of primitive aphanitic tholeiites and, for the phenocryst-rich tholeiites, some residence time to allow crystallization of the prolific ortho- and clinopyroxenes.