Richard M. Chamberlin, William C. McIntosh and Ted L. Eggleston

Abstract40Ar/39Ar age determinations help provide a precise chronologic framework for volcanism in and near the 24-km-diameter Socorro caldera, previously established as the source of the 1200 km3, phenocryst-rich Hells Mesa Tuff erupted at 31.9 Ma. The Socorro caldera is the easternmost member of a westward-younging 31.9-24.3 Ma cluster of six large silicic calderas in the northern Mogollon-Datil volcanic field. Strongly east-tilted fault blocks of the Rio Grande rift, within the Chupadera Mountains, provide a cross sectional view of the eastern sector of the Socorro caldera. 

New 40Ar/39Ar ages from this area, including 15 precise single-crystal laser-fusion ages from sanidine-bearing rhyolites, suggest a somewhat unusual eruptive history for the Socorro caldera. Resurgent uplift and eruptive activity soon after caldera formation was minimal. Only one phenocryst-rich, 31.9-Ma, ring-fracture lava dome has been identified. Significant uplift of the caldera core began shortly before 30.0 Ma, probably about 1.5 Ma after caldera collapse.  The moat-filling sequence, Luis Lopez Formation, consists of interlayered volcaniclastic sediments, phenocryst-poor rhyolitic tuffs, and basaltic to rhyolitic lavas. The basal sedimentary member is bracketed between 31.9 and 30.0 Ma. Two flow-banded rhyolite cobbles in this interval were dated at 33.7 Ma, suggesting derivation from an otherwise unknown precaldera lava flow near the southeast rim of the caldera. Following prolonged sedimentation, a primitive trachybasalt (170 ppm Ni, 9.3 % MgO) ponded in the southeastern moat of the caldera shortly before 30.0 Ma; more differentiated basaltic-andesite to andesite lavas were also erupted in the northeastern moat at about this time. Marked uplift and moderate east tilting of the central Socorro caldera occurred shortly before eruption of the basaltic lavas and prior to eruption of  >10 km3 of rhyolitic pumiceous tuffs in the medial Luis Lopez Formation at 30.0 Ma. Coarse lithic-rich vent facies and thickness variations suggest that the upper pumiceous tuff was erupted from a small collapse structure nested within the central Socorro caldera, northwest of Black Canyon (Black Canyon vent area). The pumiceous tuffs locally contain mafic rhyolite flows adjacent to the central horst block; these anomalously Cr-rich (70 ppm Cr) low-silica rhyolite flows apparently represent mixing of basaltic andesite and silicic rhyolite magmas.  Upward coarsening intermediate porphyry lavas, primarily erupted from fissure vents in the northeast moat of the Socorro caldera, conformably overlie the pumiceous tuffs.  At 28.8 Ma, rhyolitic activity in the eastern Socorro caldera intensified, erupting crystal-poor, high-silica rhyolite lava domes along preexisting ring fractures and intruding compositionally similar rhyolite dikes along the north flank of the central horst block, south of Black Canyon.  Emplacement of ring-fracture lava domes was soon followed at 28.7 Ma by eruption of the 1250 km3, phenocryst-poor, La Jencia Tuff and collapse of the Sawmill Canyon caldera, which obliterated most of the western sector of the Socorro caldera. A moderately porphyritic rhyolite dike exposed north of Black Canyon was emplaced at 28.3 Ma, probably during resurgence of the Sawmill Canyon caldera. Lemitar Tuff dated at 27.8 Ma locally overlies the eastern wall of the Sawmill Canyon caldera where it truncates the northeastern moat deposits of the Socorro caldera. One rhyolite dike in the study area, once thought to be related to Oligocene caldera volcanism, is actually 11.0 Ma and therefore related to volcanism along the Socorro accommodation zone, a transverse structural element of the Rio Grande rift.

Crystallization trends, field relationships, and eruption age data imply that the 31.9-Ma Hells Mesa magma body crystallized within a few hundred thousand years after caldera collapse. Eruption of a primitive trachybasalt at about 30.5 Ma, signaled initiation of a new crustal magmatic system that apparently evolved by fractionation, assimilation and possibly more basaltic replenishment into the large rhyolite body that ultimately fed eruption of the La Jencia Tuff at 28.7 Ma. A relatively high Ni/MgO ratio (18.3) indicates the trachybasalt is similar to some plume-fed basalts. We suggest that similar magmatic cycles in younger calderas to the west also represent periodic replenishment of the central magmatic system by basaltic underplating associated with diapiric upwellings of asthenospheric upper mantle.