CHARACTERIZING GRANULITE FACIES METAMORPHISM OF THE PICURIS OROGENY, TAOS RANGE, NEW MEXICO

High grade metamorphic rocks exposed in the northern Taos Range of New Mexico contain mineral assemblages and chemical zoning with implications that granulite facies metamorphism occurred alongside crustal thickening during regional Proterozoic tectonism. Published U-Pb monazite and Lu-Hf garnet ages suggest that granulite facies metamorphism occurred during the ~1.45–1.4 Ga Picuris orogeny. Previous studies of the Taos Range mostly interpret regional pressure-temperature conditions recording upper amphibolite facies with hints of granulite facies metamorphism. Titanite grains from two metabasites collected near Rio Costilla exhibit prominent discrete compositional zoning with Zr content of 170–190 ppm in rims and 67–94 ppm core concentrations. Zr in titanite thermometry from the two samples provides temperature constraints of 724–739 °C and 726–735 °C using rim concentrations, and 690–706 °C and 681–700 °C using core concentrations at 7 kbars. Additionally, a garnet-bearing amphibolite from Jaroso Canyon contains zoned garnet, with Xgrs ranging from 0.11 to 0.14. Garnet-hornblende Fe-Mg exchange thermometry yields temperatures 632–645 °C and garnet-plagioclase-hornblende barometry yields pressures 6–8.6 kbars, reflecting variation in Ca content of garnet, hornblende, and plagioclase. Variably preserved resorption textures, such as increases in Xsps toward garnet rims, suggest these rocks underwent retrograde metamorphism, overprinting granulite facies mineral assemblages with better preserved amphibolite facies characteristics and mineralogy. Compared to nearby Proterozoic rocks to the south, the northern Taos Range represents part of the highest metamorphic grade belt of the Picuris orogenic zone, which extends north into southern Colorado.