Isotopic evidence for the origin of Cenozoic volcanic rocks in the Pinacate volcanic field, northwestern Mexico
Six volcanic rocks, reconnaissance samples representing most of the temporal and compositional variation in the Pinacate volcanic field of Sonora and Arizona, are characterized for major element and NdSr isotopic compositions. The samples consist of basanite through trachyte of an early shield volcano, and alkali basalts and a tholeiite from later craters and cinder cones. With the exception of the trachyte sample, which has increased 87Sr/86Sr due to crustal effects, all 87Sr/86Sr values fall between 0.70312 and 0.70342, while ϵNd values are all between + 5.0 and + 5.7. Clinopyroxene in a rare spinel-lherzolite nodule derived from the uppermost mantle beneath the field has 87Sr/86Sr of 0.70320 but ϵNd of + 8.8, three ϵNd units higher than the volcanic rocks. Both the volcanic rocks and the nodule record the presence of asthenospheric, rather than enriched lithospheric mantle beneath Pinacate. This is consistent with one or both of (a) proximity of Pinacate to the Gulf of California spreading center and (b) presence of similar asthenospheric mantle signatures in volcanic rocks over a wide contiguous area of the southwestern USA. We consider the comparison to other southwestern USA magma sources as the more relevant alternative, although a definite conclusion is not possible at this stage.