New Mexico Geological Society Annual Spring Meeting — Abstracts

We report preliminary petrologic results on mafic and felsic orthogneisses of the Hermit’s Peak batholith, a Proterozoic plutonic-metamorphic complex in the southern Sangre de Cristo Mountains northwest of Las Vegas, New Mexico. We studied rocks that outcrop along County Road 65 in the Gallinas Canyon, which dissects a portion of the batholith. Major rock types include quartzofeldspathic gneisses and laminated amphibolites. The felsic gneisses contain microcline + albite + quartz ± biotite in a medium- to coarse-grained anhedral granular texture. Muscovite is rare and often secondary in origin. The felsic gneisses commonly display quartz ribbons and microcline porphyroclasts and are interpreted as metagranites. They have Rb- (Y+Nb) and Nb-Y variations indicative of volcanic-arc-granites. The mafic gneisses contain hornblende + plagioclase + quartz + titanite ± epidote and display a fine to medium-grained subhedral granular to idioblastic texture. They show igneous differentiation trends on Niggli variation diagrams and are interpreted as metabasalts. They plot as island arc tholeiites and ocean island arc basalts on tectonic discriminant diagrams. The granite gneisses and amphibolite gneisses are interpreted as part of an arc system that was accreted to North America during the assembly history of the continental lithosphere. The bimodal nature of igneous activity suggests a magmatic rift may have been operative during their formation. We continue to analyze our data to see if differentiation of the proposed rift setting (juvenile or continental) is possible and test whether our data are consistent with an arc accretion model or if an expanded model including crustal extension is required.