GEOCHEMISTRY AND PETROGENESIS OF MID-MIOCENE VOLCANOES FROM THE ANCESTRAL CASCADES OF NORTHEASTERN CALIFORNIA

Several well-preserved, mid- to late Miocene volcanoes in northeastern California could well have an origin associated with the late Tertiary development of the ancestral Cascades volcanic arc. To assess this possibility, we have analyzed seventeen samples from the Spanish Springs Peak volcano and ten additional samples from four nearby volcanoes for major- and trace-elements using a Philips Magix Pro X-ray spectrometer. The analytical results demonstrate that these volcanoes are predominately composed of basaltic trachyandesite lavas that exhibit mildly alkali to calc-alkaline differentiation trends. The analyses also fall in the basic high potassium andesite field of Gill (1981). However, selected analyses with similar silica contents from the late Tertiary Western Cascades (duBray et al., 2006) do not share these same properties, but instead they are basaltic andesites that fall along subalkaline (tholeitic and calc-alkaline) trends and plot in the basic medium to low-potassium andesite series of Gill (1981). These differences demonstrate that the volcanoes in northeastern California contain lavas that are chemically distinct from the majority of lavas associated with the Western Cascades. Furthermore, the calculated K₅₅ values (i.e., K₂O normalized to a silica content of 55%) for both lava suites are distinct, with the northeastern California lavas having a K₅₅ value of 1.97%, which is more than double that of the Western Cascades lavas at 0.69%. As noted by Dickinson and Hatherton (1975), higher K₅₅ values in arc volcanic suites typically correlate with greater depths to the seismic zone of the underlying subducting slab. This is consistent with the sampled volcanoes lying further inland from the oceanic trench, presumably above a deeper part of the subducting Juan de Fuca plate. An alternative hypothesis is that the higher alkalinity of these volcanoes is due to a greater degree of mantle metasomatism that may or may not be related to mid- to late Miocene subduction.