MANTLE SOURCE IDENTIFICATION FOR MIDDLE MIOCENE MAGMATISM ON THE EASTERN FLANK OF THE RIO GRANDE RIFT, NORTHEASTERN NEW MEXICO

A collection of mafic dikes, plugs, and stocks in the Las Vegas region represents an episode of Mid-Miocene igneous activity on the east flank of the Rio Grande rift to the east of the Sangre de Cristo Mountains. The intrusions consist of Hbl + Pl + Aug ± Ol with accessory Tur. The dikes (n=14) range in size from ms to dms in width and ms to kms in length. The 5 km long Buena Vista intrusion is a composite intrusion consisting primarily of gabbro with a differentiated Pl-rich center. 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. The hydrous nature of the parent mafic melt (high proportion of Hbl and accessory Tur) is a hallmark of a dehydrating slab fluid-enriched system. An alternative hypothesis proposes that the high volatiles are an inherent feature of an asthenosphere melt. To distinguish mantle source regions, we obtained trace element and isotope data for 10 samples to compare values to known mantle source regions of subcontinental lithosphere with upper- and lower-crustal contamination and asthenosphere with insignificant crustal contamination. The samples were processed at New Mexico Highlands University and taken to the Czech Academy of Sciences for isotope separation and TIMS analysis. 87Sr/86Sr values for 9 of the samples with SiO2 wt. % 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 (60.48 SiO2 wt. %) 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 REEs relative to the heavy REEs (La/Yb)N=29-37 and selective enrichments in the incompatible elements Ba, Th, K, Nb, and Ta. These enrichments are characteristic of subcontinental lithosphere melts 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.