Hybrid origin of Rio Grande rift hawaiites

Weakly alkaline lavas erupted in the Pliocene Cerros del Rio and El Alto vo lcanic fields in the Espanola basin of the Rio Grande rift have the unusual trace element signature of approximately chondritic K/Nb combined with hig h La/Nb and Th/Nb, These lavas have previously been interpreted as the prod ucts of modified are-source-like mantle, ultimately related to subduction o f the Farallon plate beneath North America during the early Cenozoic, We sh ow that the chemical signature can be produced by contamination of strongly silica undersaturated K-depleted magmas by continental crust, and that the re is no need to invoke a subduction component in Rio Grande rift magma gen esis. The same signature appears among voluminous Miocene lavas of the Jeme z Mountains volcanic field. Hence, while nephelinite and basanite magmas re sembling oceanic island basalts have been recognized as trace components in Rio Grande rift magmatism, our results may require a drastic upward revisi on of their volumetric significance in the northern rift through late Mioce ne-Pliocene time.