The La olivina pyroxenite suite and the isotopic compositions of mantle basalts parental to the Mid‐Cenozoic arc volcanism of northern Mexico

Abstract

Two chemically and isotopically distinct groups of pyroxenite xenoliths occur in ∼2 Ma basanites at the La Olivina xenolith locality within the Camargo basalt field, Chihuahua, Mexico. Both groups occur in composite mantle‐peridotite/pyroxenite xenoliths, indicating that they formed in the upper mantle. One group, which has igneouslike textures and chemical and isotopic compositions similar to the host basanite, is assumed to have formed from the Camargo magmas and is thus late Cenozoic in age. The second group has equigranular, recrystallized textures and is unlike the Camargo basanites in isotopic and trace element characteristics. This group appears to have formed in a manner analogous to the igneous‐textured group but from magmas related to mid‐Cenozoic arc‐related basalts that occur in the La Olivina vicinity. The equigranular pyroxenites have Sr‐Nd‐Pb isotopic compositions similar to these mid‐Cenozoic basalts, and magmas calculated to be in equilibrium with the pyroxenites have trace element concentrations similar to these basalts. The recrystallized textures of the equigranular pyroxenites reflect their longer mantle residence time. The equigranular pyroxenites provide a unique opportunity to assess the relative contributions of crust and mantle components to the mid‐Cenozoic continental arc‐related basalts of northern Mexico. They provide evidence that the isotopic compositions of the mid‐Cenozoic basalts (e.g., 87Sr/86Sr = ∼0.7043; ϵNd = ∼ + 1; 206Pb/204Pb = ∼18.7) are subcrustal in origin. It appears, therefore, that MASH processes are not responsible for the Sr‐Nd isotopic uniformity that is observed in the basalts, nor for their “bulk earth” Sr‐Nd isotopic values. The pyroxenites and basalts have elevated 207Pb/204Pb with respect to 206Pb/204Pb compared to oceanic basalts. The subcrustal origin of this signature suggests that it is the result of incorporation of a sedimentary component from the subducted slab. The Sr‐Nd‐Pb isotopic compositions of the equigranular pyroxenites are similar to those present in “enriched” lithospheric mantle in southwestern North America. This enrichment has been observed both directly in metasomatized peridotite xenoliths and indirectly in late Cenozoic basalts of the Rio Grande Rift and Southern Transition Zone of the Colorado Plateau. For the latter, the enrichment has been thought to be Proterozoic in age, but the evidence from the equigranular pyroxenites suggests that the event responsible for the observed isotopic compositions may have instead occurred during the mid‐Cenozoic.