PETROGENESIS OF PLIOCENE-MIOCENE LAVAS OF SUSANVILLE, CALIFORNIA

Recent mapping (1:24000), sampling and geochemical analyses of Miocene volcanic rocks in northern California have produced more detailed descriptions of the lithologic variety of lavas and their compositions, potential magmatic sources, and petrogenetic relationships. The field area is centered around Susanville California in Lassen County, on the southern edge of the Modoc Plateau and the western edge of the Great Basin. Areas of study include Bald Mountain, Diamond Mountain, Fredonyer Butte, Little Fredonyer, and the Bizz Johnson National Recreation Trail area. This work has focused on more detailed classification of Miocene lavas by geochemical analysis of individual lavas, which primarily range from andesite to basalt. Thin section analyses indicate that lavas have typical intermediate-mafic phase assemblages, dominated by plagioclase, olivine, and clinopyroxene phenocrysts with varying groundmass phase assemblages. Basalts are calk-alkaline basalts with high Ba similar to the Cascade lavas, but are dissimilar from arc lavas REE ratios. Geochemical analyses will further constrain lava compositions and petrogenesis. We have also begun correlating Susanville–area lavas with other regional volcanic provinces, including the Steens Basalt, Lovejoy Basalt, and mafic lavas of the Basin and Range and Modoc Plateau using petrography and geochemical analyses. This, and ongoing work characterizes lavas previously mapped on 1:250,000 in order to improve our understanding of volcanism that preceded eruptive products of modern Cascades volcanoes and of the Modoc Plateau.