This study reports major- and trace-element compositions of late Cenozoic (5 Ma and younger) mafic volcanic rocks from southern New Mexico. Stratigraphic sampling is reported for two of the volcanic fields, the Potrillo and the Jornado del Muerto volcanic fields; reconnaissance sampling, for an additional seven fields. These are among the first chemical analyses for some of these volcanic fields, and they provide insight into the dynamics of melt generation in this continental rift zone.

The lavas fall into two groups: (1) an alkaline suite of basanite, alkali basalt, and trachybasalt with low SiO2 and high abundances of incompatible elements (TiO2, Nb, Rb, and Sr); (2) a subalkaline suite of subalkali basalts with high SiO2 and low abundances of the incompatible elements. Compositions of volcanic rocks from the Geronimo volcanic field, the Taos Plateau volcanic field, and the transition zone in central New Mexico are similar to those reported in this study, confirming that these are general characteristics of lavas throughout the region. For many of the large fields, the volcanic rocks belong exclusively to one compositional group or the other. Furthermore, there appear to be no patterns to the distribution of these compositional groups; for example, the Potrillo volcanic field, which is composed exclusively of alkaline lavas, and the Jornado del Muerto volcanic field, which is dominantly subalkaline, both occur in the axis of the Rio Grande rift. We see no evidence for evolution in melt chemistry during the past 5 m.y.

In the Potrillo volcanic field, for which we have the best documentation of stratigraphic changes in composition, incompatible elements increase upsection and are correlated with decreasing Mg number, suggesting that magmas underwent differentiation before their final ascent. Phenocrystic olivine and plagioclase, which are liquidus phases at low pressure, imply a shallow depth for this crystallization event.

Processes responsible for chemical divergence between the alkaline and subalkaline groups occurred at an early stage of magmatic evolution. Correlation between incompatible-element enrichment and silica undersaturation suggests that variable degrees of partial melting played a significant role in the generation of the two lava types.

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