PRIMITIVE BASALTS IN THE POISON LAKE CHAIN, LASSEN REGION, DEMONSTRATE THE SMALL SCALE OF MANTLE HETEROGENEITIES BENEATH THE SOUTHERN CASCADES

We present Sr, Nd, and Pb isotopic data and trace element models for mantle-derived lavas erupted in the Poison Lake chain (PLC), a small area (30km²) located in the Lassen region in the Southern Cascades. The PLC includes 39 contemporaneous arc-related basaltic volcanoes and lava flows that have been subdivided into nine groups defined by mineralogy and major element compositions (Muffler et al., 2011). Previous major-element work defined six of the groups as primitive (>6 wt.% MgO) and concluded that PLC primitive basalts show no evidence of significant crustal interaction (Muffler et al., 2011). Therefore, any variation in geochemical composition must be derived from the composition of the mantle. We present new isotopic data that confirm distinct groups of mantle-derived PLC lavas that experienced little interaction with the crust. Models of trace element compositions suggest that the mantle in this small area has been variably modified by subduction inputs (e.g., sediment, fluid, slab, etc.). Pb and Sr isotopic ratios are enriched relative to MORB mantle (²⁰⁶Pb/²⁰⁴Pb: 18.89-19.05; ²⁰⁷Pb/²⁰⁴Pb: 15.60-15.67; ⁸⁷Sr/⁸⁶Sr:0.7037-0.7042), reflecting ancient sediment/fluid contribution to the mantle. Nd isotopes (εNd=2.93-7.32) and trace element ratios (e.g., Nb/Zr, Nd/Sr, La/Yb) that reflect source heterogeneity and subduction input reveal distinct differences between these six groups. Trace element compositions from all groups reflect mantle compositions with sediment contributions. In addition, mantle compositions of one group have trace elements that signal fluid contamination, and other groups have trace elements indicators of signal slab influence. Because isotope ratios and trace element compositions for lavas erupted in the PLC demonstrate little crustal input but a range of subduction input indicators, lavas in the PLC can be used to establish the small scale at which mantle heterogeneity exists in the southern Cascade region and other continental arcs.