Metamorphic belts of southern Mexico and their tectonic significance

Abstract

Crystalline terranes of southern Mexico are sllbdivided into middle Proterozoic Oaxacan, Paleozoic Acatlan and Paleozoic(?)-Mesozoic Xolapa and "Tierra Caliente" complexes. The former consists of granulite facies banded gneisses, with a basal part of anorthosite-gabbro orthogneiss and an upper part of calcareous, pelitic and quartzo-feldspathic paragneisses, including several intervals of charnockite. The age, deformation, stratigraphic and metamorphic characteristics of the Oaxacan Complex are interpreted in terms of a Grenvillian tectonic cycle. The polymetamorphic Acatlán Complex consists of two parts: (1) a parautochtonous plate formed by migmatite at the base, pelitic and psammitic schist with layered metagabbro in the middle, and carbonate-free quartzose metasediments in the upper part. (2) An allochtonous plate formed by eclogitized ophiolitic rocks, together with abundant mylonitized granitoids, all covered by immature flyshoid metasediments. The Acatlán Complex is interpreted in terms of a Paleozoic Wilson cycle, related to the opening and closure of a pre-Atlantic ocean or lapetus of the Appalachian-Caledonian orogenic system. The Xolapa and "Tierra Caliente" metamorphic complexes, of most probable Mesozoic age, appear to represent respectively the polydeformed, infracrustal (roots) and supracrustal parts of magmatic arcs and related sedimentary basins, that developed in response to Pacific subduction mainly during the Mesozoic phase of the Cordilleran tectonic regime. The geologic, gradual building of southern Mexico is thus attributed to the Grenvillian, Appalachian and Cordilleran tectonic regimes, each one of which left its unmistakable signature in the rocks briefly described in this paper.