Geology, Petrology, and Geochemistry of a Section of the High Cascades south of Mt. McLoughlin, Oregon

Ben Schiffer

The 1994 Keck Geologic Consortium project in the Southern Oregon Cascades was conducted in an area south of the Mt. McLoughlin composite volcano under the guidance of Professor Stan Mertzman. This study involves a 18 km2 area located 20 km south-southwest of the volcano on the southern boundary of the Winema National Forest. Field mapping was followed by petrographic and geochemical analysis of 9 distinct volcanic units ranging from basalt to andesite. Age dating using K/Ar techniques was also employed for some units in the area. The result of this study indicates the presence of two distinct volcanic series. Seven of the units mapped conform to a calc-alkaline series, while two units are high alumina olivine tholeiites from Burton Butte.

The calc-alkaline suite is best explained as a series of magmas that evolved from a more primitive parent in one or more shallow magma chambers. Geochemical trends indicate fractionation of olivine, pyroxenes, amphiboles and plagioclase. Chondrite normalized spider diagrams of the calc-alkaline series displays negative anomalies at Th and Nb with peaks at K and Sr, which indicates genesis at an active continental margin, with possible crustal contamination or assimilation.

The tholeiites from Burton Butte do not match the trends exhibited by the calc-alkaline units and do not have characteristics common to calc-alkaline type volcanics. The isolation, age difference, and peculiar lithologies exhibited by the tholeiites indicates possible extension related volcanism. High alumina olivine tholeiites may be derived from upper-mantle sources that have undergone significant depletion. This scenario of oblique horizontal compressive forces at the active margin, creating extension in the back-arc area, providing conduits for fast rising mafic magmas. The proximity of the mapping area to the Klamath Basin, the northwestern most extent of Basin and Range extension, supports this hypothesis.