New Mexico Geological Society Annual Spring Meeting — Abstracts

Middle Miocene magmatic rocks in the Las Vegas region represent a volumetrically small but regionally significant collection of mafic dikes, plugs, and stocks that represent an 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 consist of hornblende + plagioclase + augite ± olivine with variable amounts of hydrothermal alteration. The dikes (n=14) 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 25 m-wide Milton Ranch stocks. We propose that the eastern shoulder of the Rio Grande rift in the Las Vegas region is underlain by fertile subcontinental lithosphere influenced by subduction related processes, from the accretion of Proterozoic terrains to the subduction of the Farallon plate. The hydrous nature of the parent mafic melt, implied by the high proportion of hornblende, as well as the presence of Boron (a large-ion lithophile element), indicated by accessory tourmaline, are hallmarks of a dehydrating slab fluid-enriched system. An alternative hypothesis proposes that the high volatile content of the Las Vegas region intrusions is an inherent feature of asthenosphere melt generated by foundering of the Farallon plate. To distinguish mantle source regions, we conducted a geochemical study of the Las Vegas mafic suite, including trace element and isotope makeup, to compare to known mantle source regions of subcontinental lithosphere with upper- and lower-crustal contamination and asthenosphere with insignificant crustal contamination. Ten samples were taken from the regional intrusions, including 6 from the Buena Vista intrusion. The samples were crushed and powdered at New Mexico Highlands University and taken to the Czech Academy of Sciences (Prague) for isotope separation and Thermal Ionization Mass Spectrometer (TIMS) analysis. ⁸⁷Sr/⁸⁶Sr values for 9 of the samples with SiO₂ weight percent less than 48.60 range from 0.70395-0.70430 (average 0.70412). One sample, taken from the plagioclase-rich core of the Buena Vista intrusion (SiO₂ weight percent = 60.48) shows an elevated ⁸⁷Sr/⁸⁶Sr value of 0.70608. The Sr isotopic data for the majority of the Las Vegas intrusions suggest derivation from an enriched mantle source or an asthenospheric mantle source with crustal contamination. An enriched mantle source is more likely, as the rocks also show enrichment in the light rare earth elements relative to the heavy rare earth elements (La/Yb)_N=29-37 and selective enrichments in the incompatible elements Ba, Th, K, Nb, and Ta, which are characteristic of melts originating in the subcontinental lithosphere enriched by arc fluids or hydrous mafic magmas. Thus, mafic magmatism in the Las Vegas area originated from a fertile fluid-modified lithospheric mantle during a mid-Miocene period of extension focused east of the Sangre de Cristo Mountains.