The late Oligocene Cieneguilla basanites, Santa Fe County: Records of early Rio Grande rift magmatism
— Jennifer Lindline, Michael Petronis, Rachell Pitrucha, and Salvador Sena

We conducted petrogenetic studies of 25-26 Ma Cieneguilla basanites of the Cieneguilla volcanic field in the southern Española Basin to improve our understanding of early Rio Grande rift magmatism. We sampled 3-4 lava flows at two different locations north of La Cienega: an unnamed knoll (LCB1) and Cerro Seguro (LCB2). All sampled Cieneguilla basanites are olivine porphyritic, magnetic, slightly vesicular, and relatively unweathered. They contain mildly to moderately altered olivine phenocrysts in a very fine-grained framework of intergranular clinopyroxene-magnetite and interstitial nepheline. The estimated titanium content of the Fe-Ti oxide phase ranges from 0.106-0.210 at LCB1 and from 0.019-0.115 at LCB2. The Cieneguilla basanites are mildly alkaline and fall within the sodic series of the alkali olivine basalt scheme. They have low silica values (42.40-44.10 wt %) and high MgO values (11.50-13.50 wt %) and are some of the most primitive eruptive products in the central Rio Grande rift. The rocks display a narrow range of CaO (10.68-11.45 wt %), FeO+Fe₂O₃ (12.37-13.25 wt %), K₂O (0.61-1.05 wt %) and TiO₂ (2.16-2.37 wt %). Compatible trace elements, like Sr (758-1031 ppm), Cr (457-553 ppm), Ni (263-288 ppm), and Co (61.1-67.6 ppm) show a wider range of concentrations. The major and trace element values are sufficiently distinct between LCB1 and LCB2 to suggest that they are separate batches of magma, yet similar trace element patterns suggest that they share a similar source. The basanites are enriched in LREE relative to HREE ((La/Yb)_N=18-26). They have (La/Nb)_N values between 1.2-1.5 and moderately high Nb/Ba and Ta/Ba ratios consistent with an origin from an ocean island basalt-modified lithosphere source region. These data suggest that the Cieneguilla basanites have both asthenospheric and lithospheric mantle components and that the Rio Grande rift contained mixed mantle source regions during its early history.