Weakly alkaline lavas erupted in the Pliocene Cerros del Rio and El Alto volcanic fields in the Española basin of the Rio Grande rift have the unusual trace element signature of approximately chondritic K/Nb combined with high La/Nb and Th/Nb. These lavas have previously been interpreted as the products of modified arc-source–like mantle, ultimately related to subduction of the Farallon plate beneath North America during the early Cenozoic. We show that the chemical signature can be produced by contamination of strongly silica undersaturated K-depleted magmas by continental crust, and that there is no need to invoke a subduction component in Rio Grande rift magma genesis. The same signature appears among voluminous Miocene lavas of the Jemez Mountains volcanic field. Hence, while nephelinite and basanite magmas resembling oceanic island basalts have been recognized as trace components in Rio Grande rift magmatism, our results may require a drastic upward revision of their volumetric significance in the northern rift through late Miocene-Pliocene time.

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