MAGMATIC VOLUMES AND THE ORIGIN OF THE MID-TERTIARY IGNIMBRITE FLAREUP IN THE SOUTHERN ROCKY MOUNTAINS, WESTERN U.S

Silicic composition ignimbrites were generated throughout the southern Rocky Mountains (western U.S.) during the mid-Tertiary, although 90% of the total silicic volcanism was at only two centers, the San Juan and Mogollon-Datil volcanic fields. At both volcanic centers, existing radiogenic isotope data suggest that 80% or more of the silicic magmatism was derived from mantle-derived parental magmas. The low eNd values (<-2) inferred for these parental magmas can be interpreted as evidence of involvement of “lithospheric” mantle in magma genesis. Such an origin is only consistent with the combined volume of erupted intermediate to silicic volcanic material at the San Juan and Mogollon-Datil volcanic fields (>55,000 km3) if at least the lower 5 km of preexisting, low eNd, mantle lithosphere partially melted beneath the entire southern Rocky Mountains during the mid-Tertiary. A wide areal extent of lithospheric mantle melting is consistent with the hypothesis that the ignimbrite flareup, even deep within the continental interior of North America, involved widespread hydration and refrigeration of mantle lithosphere during early Tertiary low angle subduction. Upwelling of subcontinental mantle and conductive heating of the hydrated “lithospheric mantle” induced by slab rollback then triggered the magmatism, itself. However, given the wide spacing between the San Juan and Mogollon volcanic fields, this model requires that mafic partial melts were drawn laterally for distances of up to ~ 300 km into each center and it is unclear that such long transport distances are plausible. An alternative model is that slab rollback induced convective instabilities and decompression melting in the sublithospheric mantle. In this case, mantle melting could have been localized beneath the San Juan and Mogollon-Datil volcanic fields by “fingers” of upwelling (low eNd?) mantle. This model is particularly attractive for the Sierra Madre Occidental (SMO) in northern Mexico, where six times as much rhyolite erupted as that in the southern Rocky Mountains. Mantle source volumes estimated for the SMO rhyolites exceed by at least a factor of two even maximum estimates of the volume of lithospheric mantle present beneath this region in the mid-Tertiary, requiring magma production in upwelling subcontinental mantle.