BASIN AND RANGE EXTENSION IN A CONTINENTAL ARC SETTING: MAJOR AND TRACE ELEMENT EVIDENCE FOR HYBRID LAVAS IN THE POISON LAKE VOLCANIC CHAIN, CALIFORNIA

Lassen Volcano in northern California is the southernmost Cenozoic volcanic center in the Cascade Arc that spans the Pacific Northwest and is the result of subduction of the Gorda plate beneath North America. The Lassen area is also influenced by Basin and Range crustal extension, with NW-trending normal faults. Lassen and its attendant volcanic centers have erupted mostly calc-alkaline lavas typical of continental arcs. Basin and Range extension and crustal thinning also produces tholeiitic lavas in the region, a result of anhydrous melting at relatively shallow depths. Lavas produced by these two processes have distinctive chemical properties, particularly in their ratios of fluid-mobile large ion lithophile elements (LILE) to immobile high-field strength elements (HFSE). East of Lassen Volcano in the back-arc region lies the Poison Lake Chain, a 25-km long volcanic field of flows and cones that erupted 100-110 ka (Muffler, et al., 2011). The Poison Lake Chain is the ideal location to search for potential mixing of calc-alkaline and tholeiitic lava types. We collected basalt samples from 30 separate flows in the Poison Lake chain, and have undertaken a comprehensive geochemical analysis to determine their mixing proportions. The samples were prepared in the College of Charleston’s High Temperature Geochemistry Lab. Whole rock major and trace elements were analyzed via XRF and ICP-MS at Washington State University. Crystal separates of olivine, clinopyroxene, and plagioclase from each sample were analyzed at USGS Menlo Park via electron microprobe. The results show variability in trace element compositions and ratios that indicate mixing of magmas and hybridized compositions, with significant variability within the small volcanic field. Additional work with isotope analysis further supports variability within the flows among Hf, Pb, Sr, and Nd ratios. Modeling using Magma Chamber Simulator reveals that the lavas cannot be related via fractional crystallization alone, and must involve variable contributions from both arc-related calc-alkaline and Basin and Range tholeiitic magmas.