Temporal and spatial magmatic evolution of the Rio Grande rift
Nancy J. McMillan
Three mantle source regions are recognized in the geochemistry of mafic Rio Grande rift basalts: asthenospheric mantle, subduction-modified lithosphere, and GIB (ocean island basalt)-modified lithosphere. The most common basalt type has low and variable εNd (+2 to -8), moderate 87Sr/86Sr ratios (0.704 to 0.712), and low Nb/Ba and Ta/Ba ratios. The combination of enriched isotopic compositions and arc-like trace element signatures indicates that these basalts are partial melts of subcontinental lithosphere to which subduction-derived fluids have been added in the distant past. The second most common basalt type has high εNd (+4 to +8), low 87Sr/86Sr (< 0.704), and high Nb/Ba and Ta/Ba ratios, similar to young Basin and Range basalts and ocean island basalts. The OIB-like trace element and isotopic signatures suggest that these basalts are partial melts of upwelling, decompressing asthenosphere. The least common basalt type has high Nb/Ba and Ta/Ba ratios similar to the asthenosphere-derived basalts, but lower εNd (+2 to -7) and slightly higher 87Sr/86Sr (0.704 to 0.705). These basalts are interpreted to be melts of subcontinental lithosphere to which small degree partial melts of convecting asthenosphere have been added, again in the distant past. Basalts from lithospheric sources are found rift-wide; the lithosphere is the only source region that produced mafic magmas north of the Jemez lineament. In contrast, the southern half of the rift records a transition from lithospheric to asthenospheric source regions by 10 Ma in the Las Cruces and Socorro areas, at about 4 Ma in the Lucero volcanic field near Albuquerque, and in the Pleistocene–Holocene Zuni-Bandera volcanic field.
- McMillan, Nancy J., 1998, Temporal and spatial magmatic evolution of the Rio Grande rift, in: Las Cruces Country II, Mack, G. H.; Austin, G. S.; Barker, J. M., New Mexico Geological Society, Guidebook, 49th Field Conference, pp. 107-116.