WESTWARD MIGRATING IGNIMBRITE CALDERAS AND A LARGE RADIATING MAFIC DIKE SWARM OF OLIGOCENE AGE, CENTRAL RIO GRANDE RIFT, NEW MEXICO: SURFACE EXPRESSION OF AN UPPER MANTLE DIAPIR?

A cluster of five large overlapping ignimbrite calderas is moderately well exposed in strongly extended tilted fault-block mountain ranges of the central Rio Grande rift, southwest of Socorro NM. The Socorro-Magdalena caldera cluster (SMCC) is 85 km long and 20-25 km wide. It parallels the southeastern flank of the Colorado Plateau and the WSW-trending San Agustin arm of the rift. Precise ⁴⁰Ar/ ³⁹Ar ages of sanidines from the rhyolite ignimbrites demonstrate that the distended calderas become progressively younger to the west-southwest. Large volume ignimbrite eruptions occurred at 31.9, 28.7, 27.9, 27.4 and 24.3 Ma. A large satellitic caldera, formed at 28.4 Ma, is located 20 km south of the main overlapping trend.

Moderately alkaline to sub-alkaline basaltic andesite and basalt dikes of Oligocene age (31-24 Ma, K-Ar, ⁴⁰Ar/ ³⁹Ar in progress) form a large semi-continuous radial array that is broadly focused on the SMCC. The Magdalena radial dike swarm (MRDS) fans through 220° of arc from Pie Town clockwise to Acoma, Chupadera, Bingham and Elephant Butte. The maximum radius of the MRDS is 125 km; subswarms of near parallel dikes are commonly 20-75 km long. Longer dikes are typically basaltic andesites (53 -56 wt. % SiO₂). Within the NW-striking Pie Town subswarm, exposed dike length (75, 24 and 2 km) decreases with increasing MgO content (3.1, 5.9 and 8.0 wt. %, respectively). This is consistent with dominantly horizontal flow and lateral intrusion of magma at different levels of neutral buoyancy, as controlled by magma density. Cross cutting relationships of undated dikes near Riley and La Joya imply a westerly migrating focus of mafic magmatism. Within 40 km of the northeast margin of the caldera cluster, rhyolite ignimbrites are intercalated with a 400-700 m thick plateau-like belt of basaltic andesite lavas. We suggest this 7000 km³ bimodal volcanic pile is the surface expression of a diapiric upwelling from the upper mantle that formed during westward retreat and disintegration of the Farallon slab in Oligocene time. Westward flow in the surrounding mantle may have tilted the rising diapir and sheared it into multiple segments, thus producing the apparent westward migration of magmatism in the overlying crust. The Socorro-Magdalena magma system exhibits characteristics similar to recently described "miniplumes".