THE EXHUMED GOLETA CALDERA IN SOUTHERN MEXICO: RECORD OF VOLUMINOUS EOCENE IGNIMBRITIC VOLCANISM FED BY WIDE PYROCLASTIC DIKE COMPLEXES

The Sultepec-Goleta volcanic area represents one of the major Eocene volcanic centers related to the Eocene-Oligocene ignimbrite flare-up of the north-central part of the Sierra Madre del Sur in southern Mexico. It is constituted by a dissected silicic volcanic succession that covers an area of ~400 km². The most conspicuous and extended unit is the crystal-rich basal Goleta ignimbrite, with a thickness that ranges from ~200 m in the north to at least 600 m in the south of the volcanic center. The overlying ignimbrite units, which are distributed in the northern sector of the volcanic field, are pumice-rich.

A remarkable feature of the Sultepec-Goleta Volcanic Center is the exposure of a complex of pyroclastic dikes up to 1 km wide that extend almost continuously along the western and southern flanks of the study area and represent the feeding conduits of the ignimbrites. The broad distribution of the feeding dikes suggests the existence of a huge and probably continuous magma chamber underneath the volcanic area. The semi-curvilinear trend of the pyroclastic conduits along the western and southern flanks of the Goleta range, coupled with the greater thickness of the Goleta ignimbrite in this area, are indicative of the development of a partial collapse caldera. Conversely, the reduced thickness of the Goleta ignimbrite and the higher elevation of the pre-volcanic basement in the northern sector preclude the occurrence of a collapse in the northern part of the study area.

Component analogies indicate that the central and southern dike complexes fed the crystal-rich Goleta ignimbrite, whereas the pumice-rich units of the northern sector were extruded through pyroclastic conduits distributed in the northern area.

The marginal breccias rich in wallrock fragments at the borders of the pyroclastic dikes indicate that the extraordinary widening of the conduits was produced mainly by erosion and stoping of the conduit walls due to explosive gas-rich emissions.