COMPLEX MAGMA EVOLUTION ILLUSTRATED BY BASALT CRYSTAL POPULATIONS IN THE POISON LAKE CHAIN, LASSEN REGION, SOUTHERNMOST CASCADES

We present petrography, whole-rock, and mineral chemistry of four lithologic groups of Poison Lake chain (PLC) basalts to characterize the wide variety of crustal processes that basaltic andesite, basalts and primitive basalts can experience on their traverse through arc crust. The PLC, located ~30 km ENE of Lassen in the southernmost Cascades, encompasses a series of ~100ka calc-alkaline basaltic cones and flows that are dominated by compositions with Mg#>60. The contemporaneous basalts erupted in this small area (<30 km²) include a variety of major and trace-element compositions; some basalts are highly primitive (MgO>8 wt%) whereas others reflect varying degrees of crustal modification. Because of the geochemical variability, the 39 basaltic cones and flows erupted in the PLC were subdivided into 9 groups based on geochemistry, (Muffler et al., 2011). Recent work identified three groups that likely traversed the crust with little to no modification. However, other basalt groups carry crystal cargo that show evidence of a variety crustal modification processes.

The basalts in this study include whole rock compositions that range from relatively primitive (MgO>6wt %; Cr>200 ppm) to evolved (MgO<6wt%; Cr<50 ppm). The petrography of modified PLC basalts reveals groundmass of plagioclase microlites + olivine/clinopyroxene and larger crystals (>0.5mm) of olivine, plagioclase and clinopyroxene. Modified PLC basalts of this study include an assortment of crystal cargo types (phenocrysts, glomerocrysts, crystal clusters, xenocrysts, antecrysts, etc.) with a range of textures (simple zoning, complex zoning, resorption, etc.). Using the compositions, type(s) of crystal cargo, and textures, we characterize the range of crustal modification exhibited in PLC basalts, including crystal fractionation, thermal and compositional mixing, cannibalization, and remobilization, and present models for more complex magma evolution within and among individual groups in the small area of the PLC. This study sheds light on the variety of crustal processes that can modify basalts in continental arcs and has implications for how we understand the generation of crust in arcs.