The Role of Lithospheric Mantle in the Generation of Late Cenozoic Basic Magmas in the Western United States

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Abstract
Late Cenozoic (143Nd/144Nd is interpreted as mixing between asthenosphere-derived magmas (BR) and magmas derived from the lithospheric mantle (restricted to the TZ and SP). The metasomatism of the lithospheric mantle responsible for the high La/Nb may have been superimposed on a much older enrichment in light rare-earth elements responsible for the low 143Nd/144Nd. Involvement of lithospheric mantle in late Cenozoic magma genesis has been significant only around the margins of the BR where the greatest volumes of magma were erupted. This is consistent with extension models (Keen, 1985) which show that when lithosphere is stretched over a horizontal distance which is large in comparison to its thickness (as in the western U.S.A.), maximum thinning will occur at the edges of the zone of extension. The involvement of lithospheric mantle in magma genesis is likely to be greatest in regions of maximum extension.

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