BASIC MAGMATISM ASSOCIATED WITH LATE CENOZOIC EXTENSION IN THE WESTERN UNITED-STATES - COMPOSITIONAL VARIATIONS IN SPACE AND TIME

Widespread basic magmatism across much of the western United States in the late Cenozoic followed the cessation of subduction along the Pacific coast. This volcanism accompanied lithospheric extension, block faulting and regional uplift. In an attempt to assess the relative contribution of asthenosphere and mantle lithosphere to magmas across the western United States we have analyzed, for major and trace elements, a suite of 750 basic (MgO > 4%) lava samples from all the major volcanic fields in the region. The data were divided into seven sets representing the main tectonomagmatic provinces: Basin and Range (BR), Western Great Basin, Transition Zone (TZ), Colorado Plateau, Snake River Plain, Southern Rocky Mountains, and Great Plains. It was further divided into relatively recent (< 5 Ma) and older (> 5 Ma) subsets on the basis of field relations and K-Ar data. The < 5 Ma subset shows striking chemical differences between the provinces. All the BR lavas and some of the TZ lavas are indistinguishable from ocean island basalt (OIB) and therefore appear to have a source within the asthenosphere. In contrast, lavas from the other provinces generally show enrichments in Ba and depletions in Nb and Ti compared to OIB. This is accompanied by higher Sr-87/Sr-86 and lower Nd-143/Nd-144 than in the BR lavas. The older (> 5 Ma) subset shows no great differences between the BR and the other tectonomagmatic provinces; all have high La/Nb and Ba/Nb. Crustal contamination alone cannot be responsible for these variations. We conclude that many of the magmas have inherited their chemical and isotopic characteristics from a lithospheric mantle source enriched by fluids expelled from a subducted slab. Pelagic sediment, returned to the mantle by subduction, is a possible agent for fluids rich in Ba, radiogenic Sr and unradiogenic Nd, but very poor in Nb. At least some of this enrichment must have accompanied the formation of the Proterozoic crust. It appears that subduction-enriched lithospheric mantle was involved in the generation of all extension-related basic magmas across the western United States until relatively recently. Only in the younger BR and parts of the TZ have asthenosphere-derived magmas, uncontaminated by lithosphere, reached the surface. These observations conflict with models in which uplift and extension are caused by the replacement of mantle lithosphere by asthenosphere. They are best explained by the progressive erosion of the lithospheric mantle over a plume currently located beneath the Southern Rocky Mountains.