The Pleistocene Cat Hills volcanic field of the central Rio Grande rift consists of relatively homogeneous high-alkali olivine tholeiite, typical of late Cenozoic rift volcanism. Phenocrysts of olivine (Fo64–84) (with abundant inclusions of Cr-spinel) and plagioclase (An48–70) and partially resorbed xenocrysts of slightly more sodic plagioclase (An55–60) characterize these rocks. Chondrite-normalized rare earth data show high LREE/HREE (La/Yb ≅ 5.5) and a small positive europium anomaly. Modeling indicates that these high-alkali tholeiitic lavas evolved from a parental composition with 100 × Mg/(Mg + Fe2+) = 61, (Na2O + K2O) = 4.3%, and SiO2 = 49.2% (equivalent to the most mafic Cat Hills lava) by fractional crystallization of 5% to 6% of olivine and a trace of Cr-spinel, and by addition of 3% to 6% of Na-plagioclase. We suggest that the plagioclase xenocrysts are high-pressure phenocrysts which became out of equilibrium with the melt at shallow crustal conditions. Evolution of the lavas may have proceeded in isolated apophyses of a rising dike by sinking of olivine and included spinel and by flotation of plagioclase.
The Cat Hills parent lava may have been derived from a primary (that is, unmodified mantle-derived) melt by crystallization and removal of 7.5% olivine or 17% augite, or a mixture of both, and by the simultaneous addition of 10% plagioclase. This inferred primary magma had a LREE-enriched composition very similar to that of the Cat Hills parent lava. Modeling of possible mantle compositions indicates that the source material for the Cat Hills magma, even if garnet was a residual phase, was also LREE-enriched.