Geochemical characterization of the San Francisco Volcanic Field source

The San Francisco Volcanic Field (SFVF) is an active clustering of basalt flows and evolved silicic complexes in Arizona, USA, within the transition zone of crustal thickening from the Basin and Range Province (BRP) to the Colorado Plateau (CP). Although the field is not associated with typical volcanic geodynamic/tectonic regimes (e.g. subduction zones or mantle plumes), peak magma production was sufficient to produce a central stratovolcano. A subcontinental lithospheric mantle source has been inferred for transition zone volcanics, possibly related to sub-Moho or sub-lithosphere topography through processes like edge-driven convection. The central stratovolcano and coeval surrounding lower-volume, monogenetic basalt eruptions suggest there could be systematic spatial variability in chemistry and magma production rates within the SFVF source; on a much larger scale, variability has been observed in dynamic plume-driven volcanic provinces, such as Iceland and the Hawaiian Islands. In order to constrain the source chemistry and geometry, and investigate magmatic evolution of the SFVF, we have measured major and trace element abundances, and Nd, Sr, and Pb isotope ratios in Brunhes-aged samples that cover the eastern SFVF (including the stratovolcano and surrounding basalts). Stratovolcano mafic samples span a compositionally equivalent or narrower isotopic range than that observed in the surrounding monogenetic basalts; stratovolcano samples have ɛNd from -6 to -0.5, while surrounding samples range from -11 to +3. Sr isotopes in stratovolcano and more distal samples are comparatively unradiogenic, and range from 0.7028 to 0.7042. Pb isotopes in all samples are enriched beyond depleted mantle and above the NHRL. Some non-proximal samples have nearly identical isotopic values; for instance, several samples (in some cases samples from the stratovolcano and nearby cones) may cluster together in Sr-Nd space. When we filter data to exclude samples with <5 wt% MgO, the remaining display sub-MORB/OIB Nb/U ratios that range from 37 to 16. Despite this range, none of these samples have low Nb/Ta consistent with crustal contamination; rather, most show Nb/Ta ratios that range from MORB/OIB (Nb/Ta = 16) to superchondritic (>20). The non-mantle Nd isotopes, the comparatively unradiogenic Sr isotopes, and the modest enrichment in Pb isotopes, along with the high Nb/Ta ratios in the more primitive samples, is consistent with a source for SFVF magmas in metasomatized subcontinental lithospheric mantle. The most evolved stratovolcano sample (a rhyolite) has higher ⁸⁷Sr/⁸⁶Sr and has potentially assimilated more crust, but isotope and trace element chemistry in the mafic stratovolcano samples suggests a common source and largely common plumbing with some surrounding monogenetic basalts. The chemistry of the stratovolcano can be explained by mixing and homogenization of magmas derived from the same sources as the surrounding basalts, with some assimilation of shallower crust in the most evolved samples. A chemically distinct source is not required, and the high melt production in the center of the field relative to surroundings is a rate phenomenon, but not necessarily compositionally driven.