TIMING AND GEOCHEMICAL EVOLUTION OF SERVILLETA BASALT ALONG THE RIO GRANDE GORGE, COLORADO AND NEW MEXICO

Subalkaline basalt to basaltic andesite lava flows formally known as Servilleta Basalt (SB) are the volumetrically dominant and most widely distributed rock type of the Pliocene Taos Plateau volcanic field. Pleistocene incision by the Rio Grande into the bedrock-floored plateau has resulted in spectacular exposures of sometimes thick (~240 m) accumulations of Servilleta Basalt in the walls of the Rio Grande gorge. SB within and along the length of the Rio Grande gorge has ⁴⁰Ar/³⁹Ar ages between ~5.3 Ma and ~3.3 Ma but SB older than 4 Ma is restricted to some lava flows exposed between La Junta point, at the confluence of the Red River and Rio Grande, and the Gorge Bridge crossing northwest of Taos, NM. Vertical sampling of SB through thick flow sequences within the gorge yields precise emplacement histories and also reveals small but systematic major and minor element concentration variations (including Cu, Zn, Rb, Sr, and SiO₂) throughout the sequence. ⁴⁰Ar/³⁹Ar data show that these trends developed over short (0-250 ka) timescales, and probably relate to partial assimilation of crust, possibly at multiple depths. Combined field, geochemical, and ⁴⁰Ar/³⁹Ar data consequently record short-lived changes in tholeiitic melt compositions in response to regional extension and development of the Rio Grande rift. To some extent, the age, lateral extent, and thickness of exposed SB reflects the paleotopographic surface of the southern San Luis Basin prior to onset of Pliocene Taos Plateau volcanic field magmatism; paleotopographic highs diverted some flows while topographic lows were areas of infilling and accumulation. The topography of the basin was defined by contemporaneous or precursory andesitic to dacitic volcanoes, extensional faulting and subsidence of sub-basins within the San Luis Basin, and prograding alluvial fan deposits that originated in the Sangre de Cristo and Picuris Mountains. The combined data indicate that SB flowed onto the southern San Luis Valley beginning ~ 5.3 Ma, filling preexisting paleotopographic lows, and progressively filling the basin until the present-day plateau-like landscape developed ~3.3 Ma.