The enigmatic Ayú metamorphic complex of southern Mexico: A late Palaeozoic polyorogenic metasedimentary prism of the Acatlán Complex reworked in the Jurassic

Abstract

This contribution deals probably with the most enigmatic metamorphic unit (Ayú Complex) closely associated but in controversial contact relationships with the Palaeozoic Acatlán Complex of southern Mexico, and this is because it represents its deepest, and yet youngest roots now exposed as a wide erosional window in the eastern region of the Acatlán Complex close to the tectonic contact with the Grenvillian Oaxacan Complex. Based on detailed structural geologic field mapping and petrographic analyses of the involved lithologies, including two representative samples from which their detrital zircons were separated and dated to reveal their maximum ages of deposition, our main hypothesis that the Ayú Complex sedimentary protolith constitutes a Palaeozoic polyorogenic unit akin to the Acatlán Complex and metamorphosed twice, in the early Permian (285-270 Ma) and in the Middle Jurassic (174-163 Ma), is supported by our new data instead of an original Triassic-Jurassic sedimentary succession metamorphosed only once in the Middle Jurassic. Our new model for the origin and evolution of the Ayú Complex introduces a paleogeographic late Palaeozoc setting that involved the sequential accretion against the Mexican Grenvillian margin of several tectonic blocks constituting the western Acatlán Complex, which is characterized by a protracted Ordovician-Mississippian deep polyorogenic history. The model also attempted to explain the Middle Jurassic origin of the very high temperature and partial melting metamorphic event that migmatized and pervasively reworked the late Palaeozoic Ayú basin protoliths and its underlying probably Precambrian crust, reinforcing the prevailing view of an extensional, magma-assisted tectonic environment directly associated with the Middle Jurassic breakup of Pangea in the southern region of Mexico.