Middle Miocene Magmatism on the Eastern Flank of the Rio Grande Rift, Northeastern New Mexico

Abstract

The Las Vegas Igneous Field (LVIF) is a volumetrically small but regionally significant collection of mid-Miocene mafic dikes, plugs, and stocks that represent a previously undocumented episode of igneous intrusive activity on the east flank of the Rio Grande rift to the east of the Sangre De Cristo Mountain Front Range. The intrusions are all comprised of hornblende + plagioclase + augite ± olivine with variable amounts of hydrothermal alteration. The dikes (n=14) intrude the Carlile Shale and Niobrora Group, strike NNE, and range in size from meters to decimeters in width and meters to kilometers in length. One of the dikes, the 5 km long Buena Vista intrusion, is a composite intrusion consisting primarily of gabbro with a differentiated plagioclase-rich central portion. The smaller intrusions include the 55 m-wide Reed Ranch plug, and the Milton Ranch stocks, each measuring approximately 25 m-wide. The Reed Ranch plug and the Milton Ranch stocks intrude the Graneros Shale and are located just east of Las Vegas and north of the Las Vegas National Wildlife Refuge. Compositionally, they are similar to the Buena Vista intrusion, but contain significantly more olivine forming large glomerocrysts and fractionating to augite. The LVIF basalts are alkaline and fall within the sodic series of the alkali olivine basalt scheme. They have low SiO 2 values (43.59-48.59 wt.%) and moderate MgO values (3.61-6.41 wt.%). They are sufficiently mafic that their incompatible trace element compositions are controlled mainly by their mantle source region(s). The rocks are enriched in the light rare earth elements relative to the heavy rare earth elements (La/Yb) N =29-37. MORB-normalized trace element patterns show selective enrichments in the incompatible elements Ba, Th, K, Nb, and Ta, which are characteristic of melts originating in the subcontinental lithosphere. The LVIF basalts have moderate Ta/Ba and Nb/Ba ratios which are also signatures of the lithospheric mantle, modified by subduction zone fluids. We recognize the LVIF as a mid-Miocene magmatic episode that predates, and is petrographically distinct from, the 8.0-0.8 Ma Mora-Ocate volcanic field. The chemistry of LVIF rocks indicate they originated from a fertile fluid-modified lithospheric mantle during a mid-Miocene period of rapid extension associated with Rio Grande rifting, but focused east of the Sangre de Cristo Mountains. The origin of magmatism associated with the LVIF remains enigmatic; we postulate strain associated with rifting to the west was partitioned eastward beyond the Sangre de Cristo Mountains into the transition zone between the Rocky Mountain Front and the Great Plains.