LITHOSPHERE AND ASTHENOSPHERE MAGMA SOURCES IN THE RIO GRANDE RIFT - INSIGHTS FROM GEODYNAMIC MODELS

The greater Rio Grande Rift (RGR) is a modern rift system extending from northern Colorado into northern Mexico. Extension decreases northward from 50-100% in southern New Mexico to <10% in central Colorado. The crust thickness decreases from 45-50 km beneath the bordering Great Plains and Colorado Plateau to ~40 km beneath the rift. Lithosphere thickness decreases from 150-180 km beneath the Great Plains and 100-120 km beneath the Colorado Plateau to 45-60 km beneath the rift. Syn-rift magmatism is sparse and mostly concentrated along older NE trending structural fabrics that offset rift segments in New Mexico and Colorado, and within a few relatively small volcanic centers. Trace element and isotopic characteristics of mafic rocks erupted within the RGR and on its flanks indicate that early syn-rift magmas are dominantly sourced from a metasomatized lithosphere that likely resulted from the long prior history of subduction beneath the western U.S. Later magmas are dominated by melts derived from the asthenosphere. We use geodynamic models to test whether the timing of magmatism in the RGR is consistent with these petrogenic models. The geodynamic models show that fertile mafic metasomes in the lithosphere (pyroxenite, eclogite, hydrated lherzolite) begin melting at the onset of extension under a range of likely RGR geotherms. Melt production rates from pyroxenite and wet lherzolite in the lithosphere mantle peak between 70-80% extension. Melt production from eclogite in the lithosphere mantle peaks between 60-75% extension but decreases rapidly afterward as this composition is consumed. Depending on initial lithosphere thickness, fertile components in the asthenosphere may also begin melting at the onset of extension or there may be a delay between the onset of melting in the lithosphere and asthenosphere. Peak asthenosphere melt production rates are reached after 50-60% extension for both eclogite and wet lherzolite, and after 90-100% extension for pyroxenite. For comparison, peak melt production from dry lherzolite mantle is not reached until after >90% extension. The geodynamic models support suggestions that early syn-rift magmatism in the RGR resulted primarily from melting metasomatised lithosphere, with asthenosphere melts appearing later in the more highly extended southern parts of the rift.