ACTIVE ULTRA-HIGH TEMPERATURE METAMORPHISM IN THE LOWER CRUST OF THE RIO GRANDE RIFT, NM

Granulites and ultra-high temperature (UHT) metamorphic rocks record the chemical differentiation of continental crust, but determining the geodynamic mechanisms that drive such high-T metamorphism is difficult because granulite terranes are modified during their exhumation. Furthermore, constraints on granulite formation in active tectonic environments are limited to inferences from near-surface observations. To establish a direct link between active tectonic setting and UHT metamorphism, we conducted a systematic U-Pb petrochronological and thermobarometric investigation of <20 ka lower crustal xenoliths from the southern Rio Grande Rift (RGR), New Mexico. In metapelite xenoliths, zircon with Cenozoic U-Pb ages contain 30-70 ppm Ti, indicative of HT-UHT conditions (861-969°C), and rutile exhibits zero-age U-Pb dates, despite U concentrations between 3 and 30 ppm. Zircon from mafic granulite xenoliths exhibit similar Ti concentrations and have REE compositions that vary systematically with age: domains with U-Pb ages between 20 and 40 Ma are characterized by flat chondrite-normalized HREE slopes, whereas zones with younger U-Pb ages (5-20 Ma) have steep HREE slopes. Zircon Ti and rutile U-Pb data imply HT-UHT conditions in the RGR lower crust since the onset of regional extension ~30 Ma to the present day_. Changes in zircon HREE concentrations record a transition from shortening to extension; older zircon grew within thickened Laramide crust at higher pressure, garnet-stable conditions, while younger zircon growth occurred at pressures lower than those required for garnet stability. Diagnostic indicators of HT-UHT in the lower crust are contiguous across the US-Mexico Basin and Range tectonic province, suggesting that these conditions represent an area rivalling the size of the largest exhumed granulite terranes. Thermal-kinematic models demonstrate that the RGR likely attained HT-UHT conditions in response to mantle thinning following gravitational collapse of orogenically-thickened lithosphere. We speculate that this tectonic process may explain the heat source for UHT metamorphism in exhumed terranes with similar clockwise PTt paths and modern settings where gravitational collapse and loss of a lithospheric root has been recognized (e.g. the Tibetan Plateau, and the Betic-Rif Arc).