TECTONOTHERMAL EVOLUTION OF THE NORTHERN ACATLÁN COMPLEX, SOUTHERN MEXICO

Basement units of the northern Acatlán Complex, southern Mexico, include the Piaxtla Suite and Cosoltepec Formation. These units record a complex tectonothermal history that spans much of the Paleozoic. Units of the Piaxtla Suite record retrograde stages of eclogite to amphibolite to greenschist, and blueschist to greenschist metamorphic conditions. The high-pressure assemblages have resulted in the interpretation of these units as pieces of a subduction zone. The Cosoltepec Formation, which is interpreted as a continental rise prism, records a prograde greenschist metamorphic event correlative with the greenschist retrogression of the Piaxtla Suite, and is accompanied by a penetrative deformation fabric. Both the Piaxtla Suite and Cosoltepec Formation are affected by two later subgreenschist facies deformational events. Eclogite metamorphism has been dated ca. 346 Ma, and was followed by exhumation under greenschist facies conditions in the early Carboniferous, a history consistent with Rheic Ocean closure. The two later phases of deformation are correlated with convergence along the paleo-Pacific margin of Pangea in the Permo-Triassic.