Abstract

More than 800 Sr isotopic analyses of Cenozoic volcanic rocks have been compiled for the western United States and parts of northern Mexico. These data indicate significant variations in 87Sr/86Sr that reflect changes in tectonic evolution and differences in magmatic style (that is, origin and evolution of igneous rocks) for this region as a function of time and space.

Late Cenozic volcanic rocks (mainly basalts) represent magmatism associated with essentially extensional tectonism over much of the western United States. In general, 87Sr/86Sr ratios in these basalts are lower than in early Cenozic rocks but display systematic variations in space. For the most part, isotopic compositions of these rocks are considered representative of their respective mantle source regions. If this is the case, the mantle source regions are isotopically heterogeneous and seemingly reflect a compositional provinciality that is correlated with the age of crustal rocks in this region. Specifically, ancient subcontinental lithospheric mantle may be characterized by higher 87Sr/86Sr ratios than the mantle beneath younger crustal terranes.

Early Cenozoic volcanic rocks (mainly intermediate to silicic calc-alkalic in composition) formed while the western United States was undergoing regional compression associated with subduction along the Pacific margin. Sr isotopic ratios in these rocks are generally higher and more variable than in the younger extensional-regime magmas, and they are highest in areas of the eastern Cordillera where old sialic crust is present. The isotopic data are in accord with the involvement of crustal material in the genesis and evolution of these “calc-alkalic” magmas. Crustal contamination of primitive subduction-related magmas and crustal anatexis due to advective heat transfer associated with their ascent are considered as dominant factors in early Cenozic magmatism. Variables in the subduction process itself, in the nature and age of crust and mantle wall rocks, and in the amount and provenance of subducted sediments likely contribute to the diversity of subduction-related magmatism.

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