U-Pb geochronology of the Acatlán Complex and implications for the Paleozoic paleogeography and tectonic evolution of southern Mexico

Even though the Acatlán Complex in southern Mexico contains the largest exposure of Paleozoic rocks in Mexico, it is commonly ignored in reconstructions of Pangea because of poor geochronologic data. Presently, this complex is understood to be composed of metasedimentary units (Cosoltepec, Magdalena, Chazumba and Tecomate Formations), a major magmatic suite (Esperanza Granitoids), and a suite with eclogites and blueschists (Xayacatlán Fm). Sedimentary cover includes unmetamorphosed upper Paleozoic units. Here we provide single-crystal laser ablation U-Pb geochronology of the metasedimentary and magmatic suites of the Acatlán Complex and its upper Paleozoic sedimentary cover. The data reveal a complex geological evolution recording tectonic events from the assembly of Rodinia to the break-up of Pangea. Data for the Esperanza Granitoids record three major tectonothermal events: (1) a Grenvillian (1165 ± 30 to 1043 ± 50 Ma), (2) a Taconian (478 ± 5 to 471 ± 5 Ma), and (3) a Salinian (Acatecan; 461 ± 9 to 440 ± 14 Ma). Eclogitic rocks from the Xayacatlán Formation of Neoproterozoic-Early Ordovician age contain detrital zircons derived most probably from the southwestern North America Grenville province. Data for the blueschists are consistent with a Middle Ordovician depositional age and derivation from Laurentian sources. The Tecomate Formation is composed of two unrelated units of contrasting age and lithology: a Neoproterozoic-Early Ordovician, arc- and rift-related volcanosedimentary unit containing detrital zircons derived from the southwestern North America Grenville province; and an essentially sedimentary unit containing Early Permian fauna. The Cosoltepec Formation has a maximum Devonian depositional age and contains detrital zircons consistent with derivation from South American sources. The age of the Magdalena and Chazumba Formations is established to be Late Pennsylvanian-Early Permian. These units contain detrital zircons indicating ultimate derivation from both North and South America crustal sources. The Late Paleozoic sedimentary cover contains detrital zircons shed mainly from Grenvillian sources with a significant contribution of Pennsylvanian magmatic rocks. The new U-Pb geochronologic data indicate that the traditional stratigraphic scheme used for the Acatlán Complex needs complete revision. Data further indicate that the earliest stages of the tectonic evolution of the Acatlán Complex are tied to the evolution of Rodinia and that the actual configuration of the Acatlán Complex was ultimately achieved by amalgamation of the Magdalena-Chazumba suite during the final stages of Pangea assembly. The Early Jurassic tectonothermal event affecting only the Chazumba and Cosoltepec units to produce the Magdalena Migmatite is related to the break-up of Pangea and the opening of the Gulf of Mexico. The Acatlán Complex contains a section of the suture between Laurentia and Gondwana with some sediments arriving from Laurentia and others from Gondwana and mirrors the structure and evolution of the Appalachian-Caledonian chains of North America.