PETROLOGIC COMPARISON OF PRECAMBRIAN ROCKS IN THE LAS VEGAS AND SANTA FE AREAS, SOUTHERN SANGRE DE CRISTO MOUNTAINS, NEW MEXICO

Precambrian rocks of north-central New Mexico are Proterozoic in age and broadly divided into Early Proterozoic metasedimentary and metavolcanic rocks that formed during an episode of continental lithosphere assembly and Middle Proterozoic granites that intruded during pervasive within-plate tectonism. We conducted preliminary field, petrographic, and whole rock geochemical studies of Precambrian rocks in the Las Vegas and Santa Fe, NM, areas to compare their geological characteristics and examine any lateral differences across this segment of the Proterozoic orogen. Both the Las Vegas and Santa Fe outcrops include appreciable amounts of amphibolite gneiss and hornblende-biotite schist that are interpreted as metabasalts. The amphibolites and schists display equigranular textures, penetrative foliations, and local isoclinal folding. The mineralogy includes major Albite-twinned plagioclase, hornblende, biotite and quartz and accessory epidote and titanite. On MORB-normalized multi-element diagrams, the amphibolites and schists from both localities show high Rb relative to the other large ion lithophile elements Sr and Ba, and decreasing amounts of Th, Nb and Zr. These geochemical characteristics are indicative of formation in an island arc setting. We interpret the metamorphic rocks to have originated as oceanic lithosphere that was accreted to North America during the early episode of continental assembly. Both the Las Vegas and Santa Fe metamorphic terranes are intruded by significant amount of granite (up to 60% of the outcrop). The Santa Fe granite is an augen gneiss and plots as a volcanic arc granite on granitoid discrimination diagrams. The deformation texture and volcanic arc geochemical signature suggest that the Santa Fe granite was emplaced during the early Proterozoic episode of plate collision and accompanying subduction. The Las Vegas granite (Hermit’s Peak) has an undeformed texture and a within-plate geochemical signature, suggesting it was emplaced during the later Proterozoic period of anorogenesis. Geochronology data and additional structural and petrologic data are warranted to better integrate these rocks into the region’s overall tectonomagmatic framework.