BASIN AND RANGE EXTENSION IN A CONTINENTAL ARC SETTING: A SR, ND, PB, AND HF ISOTOPIC ANALYSIS OF HYBRID LAVAS IN THE POISON LAKE VOLCANIC CHAIN, CALIFORNIA

The Poison Lake Volcanic Chain is a 25-km long chain of small, monogenetic cones and flows in the back-arc region of the Lassen Volcanic field in Northern California that erupted between 100-110 ka (Muffler, et al., 2011). Lassen is part of the Cascade Arc in the Pacific Northwest and is a product of subduction of the Gorda plate beneath North America, producing calc-alkaline lavas typical of continental arcs. Basin and Range extension has expanded westward into the Lassen area, with NNW-trending normal faults and crustal thinning. The Poison Lake Chain is aligned along the trend of these faults, indicating the extensional faulting has controlled magma ascent to the surface. Regional Basin and Range extension and crustal thinning typically produce tholeiitic lavas resulting from low-pressure melting of shallower mantle in relatively anhydrous conditions. We hypothesize that magmas produced by both processes are generated at different depths and are variably mixing in the Lassen back-arc to create hybrid magmas prior to eruption in the Poison Lake Chain. Lavas produced by these two processes have distinctive Sr, Nd, Pb, and Hf isotopic characteristics which can reveal the proportions of these two sources of magma in the lavas. We collected basalt samples from 30 of the 39 mapped flow units in the Poison Lake Chain and have undertaken a comprehensive geochemical analysis of the lavas. The rocks were prepared for analysis at the College of Charleston. The Sr, Nd, Hf, and Pb isotopes were collected via TIMS and MC-ICP-MS at Savannah River National Laboratory. In addition, whole rock major and trace elements were analyzed via XRF and ICP-MS at Washington State University. Olivine, clinopyroxene, and plagioclase mineral separates were also analyzed for major element compositions at USGS Menlo Park using an electron microprobe. The isotopic results show variable extents of magma mixing. Magma chamber simulator modeling reveals that all of the sampled flow units are not related via fractional crystallization alone, and likely reflect contributions from subduction and Basin and Range magmas variably mixing at depth leading to hybrid lava eruption at the Poison Lake Chain.