MULTIPLE GENERATIONS OF RUTILE INCLUSIONS IN METAMORPHIC GARNET FROM THE TAOS RANGE, NM: EVIDENCE FOR HIGH TEMPERATURE GRANULITE FACIES REGIONAL METAMORPHISM

Granulite facies metamorphism is recorded in Proterozoic quartzite and metapelite in the structurally lowest parts of the Taos Range in northern New Mexico. Metamorphic garnet containing rutile inclusions is found in multiple samples of different bulk composition from two localities within the footwall block of the Cordova Creek normal fault. Zr in rutile thermometry for the larger inclusions may be used to further constrain the conditions of garnet growth. Rutile occurs as both rounded coarse grains and as fine grains that are crystallographically oriented with respect to the garnet. Needles are roughly 1 micron across and up to 100 microns in length, and are limited to the relatively high Ca cores of garnet. Electron Probe X-ray mapping and microanalysis may be used in order to identify micron-scale variation in garnet zoning. Ti solubility in garnet is known to increase with temperature. The rutile needles are interpreted to represent TiO2 exsolution or precipitation during cooling of high Ti garnet that formed at granulite or Ultra High Temperature metamorphic conditions. Such crystallographically oriented rutile needles have been observed in other origins, interpreted to represent metamorphic temperatures over 900 degrees C. Recently determined metamorphic zircon and monazite ages (Baker et al. 2018, Stotter et al. 2015) from contiguous rocks in nearby Cedro Canyon suggest regional metamorphism occurred during the ~1450–1370 Ma Picuris orogeny. Evidence for regional Granulite facies metamorphism in the early Mesoproterozoic provides evidence that Proterozoic plate tectonics continued through this time and was not episodic or hindered by supercontinent cycles.