During the Pleistocene and Holocene, alkaline and tholeiitic magmas were erupted in the Zuni-Bandera volcanic field (ZBVF) on the western flanks of the Rio Grande Rift, New Mexico. While most of the alkaline basalts are magnesian (i.e.,>8.0% MgO), the tholeiitic basalts show evidence of fractionation of olivine and clinopyroxene. The ZBVF lavas have intraplate chemistry similar to that of alkaline and tholeiitic basalts in other continental and oceanic environments, and Sr and Nd isotopes range from depleted compositions similar to mid-ocean ridge basalts to enriched compositions similar to ocean islands like Kerguelen. Slightly higher Th/Ta and Ba/Nb ratios in some of the ZBVF tholeiitic basalts correlate with isotopic ratios, and this may indicate minor involvement of a Th rich component (i.e., crust). Alternatively, such characteristics may merely reflect intraplate processes if one considers that the isotopic variation observed in the ZBVF is not that different from what is observed in Hawaiian basalts.

Systematic changes in elemental and isotopic ratios with the degree of partial melting (Zr/Y) reveal that the geochemistry of the ZBVF magmas is dominated by two components: (1) a depleted mantle component which produced and enriched alkaline magma as a small degree melt (e.g., Ta/Yb=6.0; {La/Yb}N=60; Zr/Y=19; 87Sr/86Sr=0.703); and (2) and enriched mantle component which produced tholeiitic magmas as a larger degree melt (e.g., Ta/Yb<0.5;{La/Yb}N<15.0; Zr/Y=3--4; 87Sr/86Sr=0.706). While the depleted component is interpreted to be MORB asthenosphere the enriched component is more problematical and may reside in plume-contaminated asthenosphere or stratified lithosphere. The lower lithosphere benath the Proterozoic crust of the western USA appears to be chemically stratified and comprises a depleted periodotite protolith stabilized in the Proterozoic that may have been subsequently enriched due to interaction with fluids related to (1) suprasubduction processes during Proterozoic-Phanerozoic accretion, (2) upwelling plumes (Rayton-Clayton) in the Phanerozoic, and (3) asthenospheric upwelling in the late Phanerozoic. Alternatively plume-contaminated asthenosphere provides a possible source for the enriched intraplate aspect of the ZBVF lavas. ¿American Geophysical Union 1991