Geochemistry and characterization of intermediate temperature eclogites from the Acatlan Complex, southern Mexico

Abstract

The Acatlan Complex of southern Mexico is one of two high-pressure metamorphic belts reported in Mexico. This complex contains an extensive sequence of metabasic, pelitic, granitic, mafic, ultramafic and carbonate rocks affected by an eclogite-facies metamorphic event of apparently Devonian age and subsequent overprinting of amphibolite and greenschist metamorphism. The purpose of this study was to characterize the metabasic eclogites from the Paleozoic Acatlan Complex, southern Mexico in terms of their thermobarometric conditions of formation as well as to identify the nature of the magmatic precursors. The approach was to place constraints on the petrogenetic history of the eclogites. Microtextures indicate that equilibrium existed among mineral phases of garnet, sodic pyroxene and sodic-calcic amphibole in the eclogites. This indicates that a complete equilibration to a sensu-stricto named eclogite did not occur and eclogitic assemblages were only locally developed. According to electron microprobe analyses conducted in high-pressure mineral assemblages from two-type areas, the petrogenetic evolution of these rocks is bracketed between a maximum P-T conditions of 565 ± 37°C and 11-15 kbar for basic metaeclogites from the areas of Izucar de Matamoros and Piaxtla-Tecomatlan. Minimum P-T conditions of 343°C, 3.5 kbar were constrained for greenschist metamorphism. The obtained P-T path for the polymetamorphic rocks from the Acatlan Complex has similarities with those of metamorphic rocks from New Caledonia which has a less complicated retrogressional history than alpine eclogites. Analyses of major, trace and REE abundances were determined for representative samples. The whole-rock REE-patterns suggest that there was no significant REE mobility of the protolith system. Consistent geochemistry allows identification of the protoliths as Mid Ocean Ridge Basalts (MORB), Oceanic Island Basalts (OIB) and Island Arc Volcanism (IAV). Geochemical evidence also indicates that the Acatlan Complex eclogites are tectonically emplaced slices of oceanic material suggesting a more complex tectonic regime. The data from this work suggest that the basic metaeclogites could be the product of two different convergent tectonic scenarios: (a) a collision of a small block(s) such as an island arc or a microcontinent or (b) lateral or non-frontal collision of a continent.