CONTRIBUTIONS OF LITHOSPHERIC AND ASTHENOSPHERIC SOURCES TO RIO GRANDE RIFT MAGMATISM - INSIGHTS FROM GEODYNAMIC MODELS

The Rio Grande Rift (RGR) has been active since ca. 28 Ma, extending southward from central Colorado into northern Mexico. Extension decreases northward from 50-100% in southern New Mexico to <10% at the northern tip of the rift. The crust is ~37 km thick beneath the rift, and 45-50 km beneath the bordering Great plains and Colorado Plateau. Lithosphere thickness is 45-60 km beneath the rift and 150-180 km beneath the Great Plains. The rift is largely amagmatic, with most magmatism concentrated along old NE trending structural fabrics in New Mexico and Colorado and in 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 magmatism is dominated by melts sourced from a metasomatized lithosphere. This has been attributed to the long prior history of subduction beneath the western U.S. Younger volcanic rocks show a transition from early magmas sourced within the metasomatized lithosphere to later magmas dominated by asthenosphere melts. We use geodynamic models to test whether the timing of magmatism in the RGRis consistent with hypothesized metasomatic models. The models show that fertile mafic metasomes in the lithosphere (pyroxenite, eclogite, hydrated lherzolite) begin melting at the onset of extension under a range of probable geotherms. Peak melt production from the lithosphere occurs between 70-80% extension for pyroxenite and wet lherzolite. Peak melt production from eclogite occurs at 60-75% extension, but falls off rapidly afterward as this composition becomes quickly depleted. 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 lithosphere melting and asthenosphere melting. Peak melting in the asthenosphere occurs at 50-60% extension for both eclogite and wet lherzolite, and at 90-100% extension for pyroxenite. For comparison, peak melting of dry lherzolite mantle (lithosphere or asthenosphere) does not occur until >90% extension. The models support suggestions that early syn-rift magmatism in the RGR is primarily a result of melting metasomatised lithosphere, with asthenosphere melts appearing later in the more highly extended southern parts of the rift.