CRYSTALLOGRAPHICALLY ORIENTED RUTILE NEEDLES IN GARNET FROM THE TAOS RANGE OF NORTHERN NEW MEXICO SUGGEST (ULTRA) HIGH TEMPERATURE PROTEROZOIC METAMORPHISM

Uplifts in northern New Mexico and southern Colorado expose high grade metamorphic rocks that represent significant crustal thickening during Proterozoic tectonism. Metamorphic garnet from the northern Taos Range, New Mexico contains two types of rutile inclusions: rounded coarse rutile and crystallographically oriented rutile needles. The several micrometer wide rutile needles only occur within garnet core domains in two pelitic schist samples. The samples include the assemblage garnet + biotite + quartz + K–feldspar + muscovite + plagioclase + sillimanite ± tourmaline + accessory minerals. Garnet cores are almandine-rich (~0.85) with low pyrope (near 0.07) and spessartine (0.03–0.07), plus very low grossular (~0.01–0.03). All garnets show shifts in composition near the rims, consistent with the disappearance of the rutile needle inclusions. Ti solubility in garnet is known to increase with temperature. Similar oriented rutile crystals in metamorphic garnet in other orogens are interpreted to represent exsolution/precipitation of TiO₂ from high Ti garnet in response to a decrease in Ti solubility in garnet due to cooling. Garnet with Ti contents leading to rutile exsolution/precipitation during cooling are estimated to represent metamorphic temperatures over 900ºC in other localities, in many cases representing extreme UHT/UHP metamorphic conditions. The timing of granulite facies or UHT metamorphism in these samples is not directly constrained but based on U-Pb monazite ages from nearby samples likely took place during the Mesoproterozoic Picuris Orogeny.