SYN-RIFT MAGMATISM AND INCIPIENT RIFTING IN NW COLORADO: IMPLICATIONS FOR THERMAL EVOLUTION AND PRE-RIFT LITHOSPHERE COMPOSITION IN THE NORTHERN RIO GRANDE RIFT FROM GEODYNAMIC MODELS

We present geodynamic models simulating magma production in the northernmost part of the Rio Grande Rift (RGR) tectonic province in northwestern Colorado. This is a region of diffuse incipient extension (<2%) cut by small normal faults and volcanic centers. The region differs from the RGR further south, which consists of a series of tectonically linked rift basins and flanking uplifts spanning from central Colorado into northern Mexico (where extension approaches 100%). Tectonism throughout the RGR system was most rapid during the Miocene and has continued at a slower pace since. Syn-rift magmatism is minor and mostly concentrated along NE trending structural fabrics that offset rift segments in New Mexico and Colorado and in smaller volcanic centers scattered along the length of the rift. Trace element and isotopic characteristics of syn-rift mafic rocks indicate that most were sourced from relatively fusible compositions in the uppermost mantle, which have been attributed to intrusion and/or hydration during the long prior period of subduction beneath the western U.S. during the Mesozoic. We infer that the upper mantle geotherm beneath the RGR system lay close to the solidi of relatively fusible rocks prior to rifting, and that these rocks began to undergo decompression melting at the onset of extension. Our geodynamic models show that anhydrous mafic (eclogite) and fertile ultramafic (pyroxenite) rocks and hydrous peridotite in the upper mantle would have begun melting at the onset of extension, which accounts for the volcanic rocks erupted in the incipient portions of the rift in northwest Colorado. However, the models also predict that melt production should continue as rifting progresses, peaking after 60-80% extension. In northwest Colorado, volcanism ceased after emplacement of a few relatively small volcanic centers. We conclude that either i) the degree of hydration and/or the amount of fusible materials present in the mantle prior to rifting were minor, such that they were consumed by melting after very little extension; or ii) the lower lithosphere became cooler during extension, inhibiting further melting. In the latter case, syn-rift cooling of the mantle may have resulted from either lateral heat conduction during rifting, which we deem to be minimal in this broad portion of the RGR system, or from consumption of latent heat during melting.