FORMATION PROCESS OF PHENOCRYST-RICH AND PHENOCRYST-POOR IGNIMBRITES OF THE EXHUMED EOCENE SULTEPEC-GOLETA VOLCANIC CENTER OF SOUTHERN MEXICO

The Sultepec-Goleta area in southern Mexico hosts one of the major Eocene silicic volcanic centers that make up the Cenozoic volcanic record of the north-central part of the Sierra Madre del Sur. This center is represented by a partially exhumed NNE-SSW trending ignimbritic field that covers an area of ~400 km² with a preserved volume of ~200 km³. A remarkable feature of volcanic center is the exposure of pyroclastic dike complexes interpreted as the remnants of the conduits that fed volcanic activity in the area, which are up to 1 km wide and extend almost continuously along the western and southern flanks of the volcanic field.

The ignimbritic record comprises the phenocryst-rich Goleta ignimbrite, which has a thickness that ranges from ~200 m in the north to at least 600 m in the south of the volcanic center, and the phenocryst-poor Cienega, Lobera and Potrero ignimbrites overlying the phenocryst-rich unit. Based on spatial relationships as well as analogies in the nature and abundances of the components, it has been recognized that the central Sultepequito-Picacho and southern Cofradia-Zacatones dike complexes fed the relatively phenocryst-rich Goleta ignimbrite, whereas the phenocryst-poor units of the northern sector were extruded through the Ahuacatitlán and Ocotzol-Ornamento pyroclastic conduits distributed around Sultepec. Although phenocryst abundances were analyzed in juvenile pumice clasts for establishing connections between feeding dikes and ignimbrites because they are less sensitive to changes during ash-flow emplacement, an exact correspondence in component abundances between extrusive ignimbrite units and genetically related feeding dikes is not expected due to processes such as “ash winnowing” and non uniform deposition from pyroclastic flows that can modify component abundances.

From the origin of phenocryst-rich and -poor ignimbrite units we suggest that the stirring produced by the partial collapse of the magma chamber roof beneath the southern sector of the study area triggered the tapping of phenocryst-rich portions of the zoned magma chamber, resulting in an increase of phenocryst content in the Goleta ignimbrite, in contrast with the northern area where no collapse occurred and the Cienega, Lobera and Potrero ignimbrites are phenocryst-poor.