Geochemistry of Carboniferous low metamorphic grade sedimentary and tholeiitic igneous rocks in the western Acatlán complex, southern Mexico: deposition along the active western margin of Pangea

Abstract

Low grade metasedimentary rocks in the western Acatlán complex (Olinalá area) of southern Mexico occur in two units termed here the Progreso and Zumpango units of Lower and middle Carboniferous ages, respectively. The Zumpango Unit is dominated by phyllite and quartzites, but the Progreso Unit is more highly deformed and is dominated by quartzite that is locally interbedded with pillow basalt, and intruded by a suite of mafic dikes. Geochemical and Sm-Nd isotopic data indicate that the basalts and the mafic dikes are tholeiitic, have N-MORB affinities and are moderately fractionated. Deposition of the Progreso Unit was coeval with Carboniferous extrusion of high metamorphic grade rocks of the Acatlán complex above of an active subduction zone, and deposition of Mississippian rocks in the adjacent Oaxaquia terrane. The tholeiitic affinity of mafic rocks, suggests extension on this active margin. The Mid-Continent (USA) affinities of the fauna in the latter rocks suggest that Pangea had already amalgamated and that these events took place on the western margin of Pangea.