Geochemical evidence for flat-slab subduction in Cenozoic western North America
Academic Affiliation: Senior, University of Arizona, Geosciences
Science Research Mentor: G. Lang Farmer – University of Colorado at Boulder
Writing and Communication Mentor: Freddy Blume – UNAVCO
Voluminous intermediate to silicic composition magmatism occurred in the Cenozoic throughout much of western North America and is generally attributed to melting in the upper mantle triggered by shallowing and resteepening of oceanic lithosphere subducting along the western continental margin during this time. If so, then not only should there be systematic variations in the position of the magmatic activity through time, but these magmas should also should carry geochemical signatures characteristic of arc magmatism, including enrichments in the abundances of large-ion lithophiles (LIL) relative to high-field-strength elements. With this issue in mind, a re-examination of space-time-composition patterns of Cenozoic magmatism in the western United States and northern Mexico is presented here, using igneous rock data compiled in the on-line western North American Volcanic and Intrusive Rock Database (NAVDAT).
An investigation of 5,625 volcanic rocks from the Rocky Mountain region in the western United States revealed latitudinal variations in the age of the mid-Tertiary magmatic pulse, and longitudinal variations in chemical composition, with alkalic character increasing to the east, and the highest Sr/Nb concentrations found in the central Rocky Mountain region. Isotopically, there was little to no variation in initial 87Sr/86Sr ratios, while there were marked differences in initial ENd values, with values increasing to both the east and west of the central Rockies. Space-time patterns from 6,788 volcanic and plutonic rock samples from northern Mexico indicate that while there is evidence for periods of magmatic migration from 120 Mya to the present, these migrations are confined to the northernmost latitudes (36° N–28° N).
Taken together, these observations suggest that (1) variations in the mantle source region and the intensity of metasomatism existed during the mid-Tertiary in the western United States, and (2) flat-slab subduction did not extend into the southernmost latitudes (28° N–16° N) of northern Mexico.