ORIGIN OF GEOCHEMICAL DIVERSITY WITHIN AND BETWEEN ERUPTIVE SYSTEMS, SOUTHERN CASCADE VOLCANIC RANGE, CALIFORNIA

The Cascade volcanic range extends for ~1,250 km from Lassen Peak in northern California to Meager Mountain in British Columbia and is comprised of over 4,000 individual vents. Within the southern portion of the arc Mount Shasta and Medicine Lake Volcano (MLV) form two spatially and temporally associated volcanic systems. Mount Shasta, located along the main Cascade arc axis, preserves evidence of magmatism directly associated with ongoing subduction processes. Medicine Lake Volcano lies farther to the east and its magmatism is driven by subduction-related processes and processes linked to Basin and Range extensional tectonomagmatism. Much of the previous work in this portion of Cascadia has focused on these two large eruptive systems yet important aspects of the region’s magmatic history are recorded in smaller systems that are not well characterized.

The focus of this study is a volcanic field situated NE of Mount Shasta and adjacent to the northwest margin of MLV. This field, hereafter referred to as the BST (Bray-Sharp-Tennant field), pre-dates activity at Shasta and MLV. The BST consists of small shield and cinder cone volcanoes and eruptive fissures all clearly influenced by E-W extension. Mapping, petrographic observations, and major/trace element geochemistry identify the main magmas erupting from the larger shield vents and associated cones as olivine basalt through two-pyroxene andesite, with high-Mg andesite emanating from a single tephra cone and low-K, high-Al olivine tholeiite (HAOT) from numerous eruptive fissures. The most primitive magmas identified are HAOT and Mg-rich calc-alkaline basalt. The HAOTs are interpreted as near primary partial melts of relatively dry mantle wedge material. The primitive calc-alkaline basalts have elevated Sr/Y and Sr/P suggesting an important role for slab derived fluids. The larger shield systems display within and between vent compositional heterogeneity related to differing primary magma inputs followed by fractional crystallization and/or open system processes. For example, correlated increases in K/P and Rb/Nb with degree of differentiation suggest a role for contamination by felsic crust. Further geochemical and isotopic analyses coupled with detailed petrographic observations will provide the basis for modeling these complex crustal processes.