Caldera formation and progressive batholith construction: Geochronological, petrographic and stratigraphic constraints from the Coxcatlán-Tilzapotla area, Sierra Madre del Sur, Mexico

Abstract

A feature of great interest in the late Eocene-early Oligocene volcanic province of the Sierra Madre del Sur is an elliptical NW-SE oriented dome structure (52 × 30 km) in the Coxcatlán-Tilzapotla region. The elliptical structure encompasses the Tilzapotla collapse caldera, rhyolitic domes, large volumes of ignimbrites, as well as the Buenavista intrusive body, and the Coxcatlán and Chautle plutons located west and east of the structural margin of the caldera, respectively. Previous geochronological studies carried out on the silicic and intermediate magmatic rocks intercalated with pre-caldera tilted terrestrial sediments, as well as the occurrence of an angular unconformity separating fine-grained and coarse-grained beds, constrain uplift in the dome area to the late Eocene (~38-34 Ma). This suggests that doming was related to emplacement of magmas into the crust prior to collapse of the Tilzapotla caldera at 34.3 Ma.

New LA-ICPMS zircon U-Pb and 40Ar-39Ar mineral ages for the key magmatic units combined with field observations, petrographic studies and geochemical analyses reveal the geochronology of magma emplacement and development of the volcano-plutonic complex, and its connection with the dome. Zircon 206Pb/238U ages of the Coxcatlán pluton ranging from 39.5 ± 0.6 to 35.2 ± 0.2 Ma suggest that the Coxcatlán pluton was constructed gradually over ~4 m.y. and overlapped in time with the beginning of the development of the Tilzapotla ignimbrite (36.6 ± 0.4 to 33.3 ± 0.5 Ma). Significant overlap of the population density distributions is found indicating temporal continuity of the zircon crystallization record. Zircons from the mafic Chautle pluton (34.37 ± 0.26 Ma) yield the same weighted mean age within uncertainty as zircon from the Tilzapotla ignimbrite. These data are best reconciled with a continuous but incremental assembly of a volcano-plutonic system that climaxed with the caldera collapse and the eruption of the Tilzapotla ignimbrite. The evidence of mafic inputs throughout the history indicates that recharge provided the thermal engine for the prolonged history of the system. Mutual intrusion relationships observed between mafic enclaves and granodiorite facies indicate magma mingling within the magma chamber when both host and enclaves were not completely crystallized.

Development of the Coxcatlán-Tilzapotla system resulted in updoming of the surface, extending the “roof” above the melt-rich part of the system in the southeast. The spatial and temporal relationship between the crystal-rich Tilzapotla ignimbrite and the less differentiated Chautle intrusion suggests that mafic magma injection might have disrupted crystal mushes in the chamber, and destabilized the “perched” magma reservoir and thereby triggering the eruption in the southeastern part of the dome.