New Mexico Geological Society Annual Spring Meeting — Abstracts

The elemental and isotopid signatures of both asthenospheric and lithospheric portions ofthe mantle (acknowledged to be source reservoirs during continental extension-related magmatism) are well documented for the Rio Grande rift and adjacent provinces. Major and trace element and stable and radiogenic isptope data have been acquired on Pliocene-age alkaline basaIts from the Rio Puerco Valley (RPV), located just east of Mount Taylor volcano, in the transition zone between the southeastern Colorado Plateau and Rio Grande rift tectonic provinces. This setting offers an unique opportunity to monitor the evolution of mantle source regions (asthenosphere vs. lithosphere) associated with continental rifting.

Over 40 volcanic necks occur in the RPV as small volume plugs, flows, and dikes. Rio Puerco basalts are ne-normative and have low SiO₂ (42-49 wt.%), high Ni, Cr, and Mg #, and REE and spiderdiagram trends similar to OIB. Nd and Sr isotopic data defines a homogeneous group; E_Nd =+3.5 to +4.8; ⁸⁷Sr/⁸⁶Sr = 0.70361 -0.70407. Pb isotopes are fairly radiogenic (²⁰⁶Pb/²⁰⁴ = 18.84 -19.26; ²⁰⁷Pb/²⁰⁴Pb = 15.56 -15.61; ²⁰⁸Pb/²⁰⁴Pb = 38.64 -39.00) and plot ahead ofthe geochron and above the NHRL. Petrogenetic modeling suggests most Rio Puerco basalts can be generated by ≤2% partial melting of garnet peridotite followed by ≤10% fractional crystallization of olivine and clinopyroxene. However, this model breaks down when applied to basalts from the southern portion of the field indicating either a different parental source or increased fractional crystallization. There is no geochemical or isotopic evidence consistent with crustal assimilation processes.

Alkaline basalts from highly extended portions of the southern Rio Grande rift and Basin and Range provinces, believed to have originated in the asthenosphere, typically have low ⁸⁷Sr/⁸⁶Sr (≤0.7035) and high E_Nd (≥+5), close to MORB or a depleted (asthenospheric) mantle end-member. Less extended portions of the northern Rio Grande rift (e.g. Taos Plateau and Cerros del Rio in New Mexico; Yampa province and Elkhead Mts in NW Colorado) tend towards enriched (lithospheric) mantle sources (e.g. ⁸⁷Sr/⁸⁶Sr ≥0.7035, E_Nd ≤+2). By comparison, ⁸⁷Sr/⁸⁶Sr and E_Nd values for Rio Puerco basalts lie between depleted and enriched mantles, consistent with a source contribution from both reservoirs. According to oceanic mantle, endmember reservoirs (Zindler and Hart, 1986), Rio Puerco basalts are a mixture of DMM and EM2 and, high ²⁰⁶Pb/²⁰⁴Pb ratios require a third HIMU component. Pb isotopes further confirm that the enriched component is old Proterozoic (1739 Ma), subcontinental lithosphere. When placed in a tectonomagmatic evolutionary framework the Rio Puerco basalts represent a lithospheric source (EM2 + HIMU),that has been modified by the asthenosphere (DMM) during Late Cenozoic asthenospheric upwelling brought on by continental rifting and resulting in the thermal, mechanical, and chemical transformation of the lithosphere (consistent with Perry et aI., 1987). Preexisting structure is also a factor and it is believed Laramide-age crustal and sub-crustal (?) structures facilitated the transport of alkaline magmas by channeling melts along crustal weaknesses.