| Summary: | More than 80% of the exposed area of the Acatlan Complex was mapped and new outcrops of eclogites (Xayacatlan Formation), granitoids (Esperanza, Hornos, Teticic), and volcanic rocks (Cosoltepec and Tecomate Formations) were identified. 44 selected samples from the new outcrops were geochemically analyzed to establish the tectono-magmatic affinity. Volcanic rocks of the Petlalcingo Group suggest an oceanic setting (MORB and OIB), while bimodal rocks of the Tecomate Formation represent an intraplate event (WPB), in addition to an inherited arc signature. Most granitoids of the Acatlan Complex are peraluminous and classify as S type intrusions derived from crustal melting. These plutons straddle the VAG+ Syn-ColG and WPG fields in a Nb-Y discriminant diagram and are considered as syn-tectonic, late-tectonic and post-tectonic granites. Based on the main geologic, stratigraphic, structural and geochemical data it is suggested that the Petlalcingo Group represents a sedimentary sequence deposited in distal passive margin correlated with those siliciclastic deposits of the Early Paleozoic bordering the western margin of Gondwana. The Piaxtla Group (Xayacatlan and Esperanza Granitoids) represents an allochthonous unit deformed, metamorphosed and emplaced over the Petlalcingo Group during the Silurian orogeny (Caledonian) caused by the collision of peri-Gondwanan terranes (Avalon, Oaxaquia) and the subsequent approaching of Gondwana. During the Early Devonian, the Tecomate Formation was deposited because of the relaxation of the Silurian orogen. Finally the subsequent southward translation of Gondwana produced an intense deformation and low grade metamorphism into the Tecomate Formation (Acadian orogeny). Upper Paleozoic sediments deposited over the Acatlan Complex delimit its evolution.
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