LITHOSPHERIC MANTLE MELTING AND THE ORIGIN OF THE MID-TERTIARY IGNIMBRITE FLARE-UP, SOUTHERN ROCKY MOUNTAINS

Voluminous intermediate to silicic composition volcanic rocks were generated throughout the southern Rocky Mountains, western U.S., during the mid-Tertiary “ignimbrite flare-up”, principally at the San Juan and Mogollon-Datil volcanic fields. At both volcanic centers, radiogenic isotope data have been interpreted as evidence that 50% or more of the volcanic rocks (by mass) were derived from mantle-derived, mafic parental magmas, but no consensus exists as to whether melting was largely of lithospheric or sublithospheric mantle. Recent xenolith studies, however, have revealed that thick (> 100km), fertile, and hydrated continental lithosphere was present beneath at least portions of the southern Rocky Mountains during the mid-Tertiary (Lee et al., 2001; Smith and Griffin, 2005). The presence of such thick mantle lithosphere, combined with an apparent lack of syn-magmatic extension, leaves conductive heating of lithospheric mantle as a potential method of generating the mafic magmas that fueled the ignimbrite flareup in this inland region. To address this possibility, we estimated the minimum volume of mantle needed to generate the mafic magmas parental to the preserved mid-Tertiary igneous rocks. Conservative estimates of the mantle source volumes that supplied the Mogollon-Datil and San Juan volcanic fields are ~2 Mkm³ and ~7 Mkm³, respectively. Such mantle sources volumes can be accommodated solely within the lithospheric mantle but only if at least the lower ~20 km of the mantle lithosphere beneath the entire southern Rocky Mountains region underwent partial melting during the mid-Tertiary and if the resulting mafic magmas were drawn laterally for distances of up to ~300 km into each volcanic center. Such widespread melting of lithospheric mantle also requires that the mantle have been uniformly fertile and primed for melting in the mid-Tertiary, a possibility if it had experienced widespread hydration and refrigeration during early Tertiary low angle subduction. In this case, exposure of the mantle lithosphere to upwelling sublithospheric mantle during mid-Tertiary slab roll back represents a plausible trigger for partial mantle melting and generation of the ignimbrite flare-up in this region.