SYN-RIFT MAGMATISM IN THE RIO GRANDE RIFT AND SEQUENTIAL MELTING OF A CHEMICALLY HETEROGENEOUS MANTLE

The North American Rio Grande Rift (RGR) consists of a series of tectonically linked basins and magmatic centers spanning from northern Mexico to northern Colorado. Tectonism was most rapid during the Miocene and has continued at a slower pace since. Extension decreases northward, from 50-100% in southern New Mexico to <10% in central Colorado. Syn-rift magmatism is sparse, with the largest magmatic centers concentrated along reactivated transverse structural trends. Smaller volcanic centers are scattered along the length of the rift. Trace element and isotopic characteristics indicate that most syn-rift magmas were sourced from relatively fusible compositions in the uppermost mantle, which have been attributed to intrusion and hydration during the prior period of subduction beneath the western U.S. In contrast, geochemical attributes of the youngest magmas in the highly extended southern part of the rift are typical of melts derived from depleted asthenosphere lherzolite. We infer that the geotherm beneath the RGR lay close to the solidi of relatively fusible rocks in the upper mantle prior to rifting (mafic and ultramafic intrusions and “damp” areas), and that these rocks began to undergo decompression melting at the onset of extension. Along-strike variations in the volume of these early syn-rift eruptions reflect the heterogeneity of the pre-rift mantle. These fertile source rocks were rapidly consumed, limiting the volume and duration of early syn-rift magmatism. Continued extension in the southern RGR has led to incipient melting of the underlying asthenosphere and production of less enriched magmas. We present geodynamic models that show that fertile mafic and hydrous rocks in the upper mantle begin melting at the onset of extension under a range of likely RGR geotherms. Melt production peaks between 60-80% extension, depending on rock composition, lithosphere thickness, and mantle geotherm. Dry lherzolite asthenosphere requires >75% extension prior to melting in all models. The geodynamic models support suggestions that early syn-rift magmatism in the RGR resulted from melting fertile compositions in the lithosphere, with asthenosphere melts appearing later in the more highly extended southern parts of the rift.