Pb Isotopes and Trace Elements of the Pinacate Volcanic Field, Northwestern Sonora, Mexico: A Basin and Range Mini-Plume near the EPR Spreading Center

The Pinacate Volcanic Field (PVF; 31° 46′ N, 113° 30′ W) is located in the southern Basin and Range of NW Mexico, ca. 100 km NE of the Wagner Basin in the northern Gulf of California. It contains two series of Quaternary lavas. The older Santa Clara series (> 1.7 - 1.1 Ma) ranges from alkali basalt to trachyte and forms a shield volcano, Volcán Santa Clara. This shield is mantled with and surrounded by lavas of the 1500 km² Pinacate series (≥ 1.2 Ma - 12 ka), which comprises tholeiites and transitional to mildly alkaline basalts, hawaiites and mugearites forming hundreds of cinder cones and commonly bearing cognate megacrysts. PVF lavas have incompatible element ratios indicating an OIB source with little or no crustal contamination (P/K > 0.3). Pb isotopic ratios were measured on 18 samples representing the temporal, spatial, and compositional range of the two series. Pinacate Pb lies on or almost on the NHRL and is notably radiogenic (²⁰⁶Pb/²⁰⁴Pb up to 19.56). Santa Clara series basalts and trachytes exhibit relatively homogeneous Pb isotopic ratios that generally lie within the field defined by the more isotopically heterogeneous Pinacate series lavas. Most of the Pb isotopic ratios do not indicate significant crustal contamination and are all more radiogenic than those reported for southern Gulf of California (Alarcon Rise) and EPR MORB. Furthermore, Pinacate Pb isotopes are distinct from those of block-faulted early to middle Miocene lavas found just to the east. Together with existing ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios, this isotopic shift can be attributed to melting of deeper, garnet-bearing asthenosphere, possibly as a mini-plume welled up near, but distinct from, the spreading center in the adjacent Gulf of California.