Cinder cone volcanism is the Lassen region: A window into the southern Cascade Arc crust

Abstract:

Regional volcanism in the vicinity of the Lassen Volcanic Center is characterized by mafic calc-alkaline magmatism manifested as small cinder cones and larger steep-sided cones or shields, and less abundant low-potassium olivine tholeiites that erupt from fissures and produce valley-filling lava flows. Previous studies of these mafic magmas have provided insight on the role of volatiles, slab melting (Walowski et al., 2016, EPSL, 446, 100-112), and oxidation processes (Muth et al., 2019, AGUFM, V31F-0185) in “warm” endmember subduction zones. Here, we aim to better constrain the depths of crustal storage, eruption triggering mechanisms, and ascent processes and timescales of cinder cone magmas in the Lassen region. We compare olivine-phyric calc-alkaline basalts and CPX-bearing high-Mg basaltic andesites and employ whole rock geochemistry, a in situ mineral geochemistry, melt inclusion chemistry, thermo-barometric estimates, and trace element diffusion chronometry in olivine and CPX. Unsurprisingly, each volcano tells its own story. Clinopyroxene and olivine are often found with broad, homogeneous cores and normally-zoned rims, but some cones display phenocryst assemblages with complex sector-zoned augite or abundant glomerocrysts. Melt inclusions, corrected for CO₂ loss to vapor bubbles, provide minimum estimate of crystallization from the mid- to lower crust, and in some cases, shed light on the timing and complexity of crustal assimilation. Despite variability in the magmatic histories of individual volcanoes, we hypothesize that most of these mafic magmas reside and crystallize primarily in the lower crust and ascend rapidly to the surface (in months to years) prior to eruption. Ultimately, building an inventory of magma storage locations and ascent timescales for the mafic magmas in this region can improve constraints on arc magma petrogenesis, differentiation in the arc crust, and the processes that drive eruptive style and hazard potential of cinder cones.