POISON LAKE CHAIN PRIMITIVE BASALTS AS PROXIES FOR MANTLE COMPOSITIONS BENEATH THE SOUTHERN CASCADES

The Poison Lake chain, east of Lassen Volcanic National Park in northern California, encompasses 43 vents that comprise 9 stratigraphically and petrographically defined groups of chemically distinct primitive calc-alkaline basalts. Eight of the groups are temporally associated, ranging in age from ~100 to ~120 ka, and the ninth is ~200 ka. Because of its limited area (5 km east-west and 20 km north-south), the Poison Lake chain represents an ideal location to study the mantle beneath the southern Cascades, where its proximity to Basin and Range extension provides conduits for magma to reach the surface efficiently with little contamination. We present new trace-element and Sr-isotope data for four of the geochemical groups in the Poison Lake chain. The four groups analyzed, the basalts of Robbers Spring (~202 ka), Pittville Road (~117 ka), old railroad grade (~102 ka) and Bogard Buttes (~100 ka) were chosen based on their spatial distribution and age relationships. Trace-elements and Sr-isotopes confirm the distinct geochemical groups, previously recognized with major-element data. Sr-isotope ratios for all four groups are primitive and mantle-like (0.7037-0.7042), suggesting little to no crustal contamination as these basalts ascended through the crust. Some groups exhibit slight major- and trace-element compositional variation that, in combination with the Sr-isotopic data, suggest that individual group compositional arrays are produced by one of the following processes: minimal fractional crystallization, variation in degree of partial melting, or mixing with similar mafic magmas. Despite their proximity to one another, petrography and unique trace-element and isotopic compositions among the four groups preclude direct genetic relationships or a common source for these basalts. The results of this study indicate that each group erupted in the Poison Lake chain represents an independent, small batch of mantle melt that traversed the crust with only a short residence time. Furthermore, the small volume erupted and close proximity of vents in the Poison Lake chain require that these basalts are derived from small source domains; thus, their chemical and isotopic diversity reveal small-scale heterogeneity in the mantle beneath this part of the Cascades.