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Pressure-temperature-time evolution of Paleozoic high-pressure rocks of the Acatlán Complex (southern Mexico): Implications for the evolution of the Iapetus and Rheic Oceans

Ricardo Vega-Granillo^,1, Oscar Talavera-Mendoza², Diana Meza-Figueroa³, Joaquin Ruiz⁴, George E. Gehrels⁴, Margarita López-Martínez⁵ and Julio C. de la Cruz-Vargas⁶

¹ Departamento de Geología, Universidad de Sonora, Rosales y Encinas S/N, Hermosillo, Sonora, México 83000
² Unidad Académica de Ciencias de la Tierra, Universidad Autónoma de Guerrero, A.P. 197, Taxco, Guerrero, México 40200
³ Departamento de Geología, Universidad de Sonora, Rosales y Encinas S/N, Hermosillo, Sonora, México 83000
⁴ Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA
⁵ Departamento de Geología, CICESE, Km 107 Carr, Tijuana–Ensenada, Ensenada, Baja California, México
⁶ Departamento de Geología, Universidad de Sonora, Rosales y Encinas S/N, Hermosillo, Sonora, México 83000

New thermobarometric and U/Pb and ⁴⁰Ar/³⁹Ar geochronologic data coupled with ages obtained from the Acatlán Complex, the basement of the Mixteco terrane of southern Mexico, reveal the existence of three distinctive high-pressure metamorphic events of early to middle Paleozoic age, each recorded in a separate lithological suite. Xayacatlán suite eclogites with oceanic affinity underwent peak metamorphism at 609–491 °C and 13–12 kb during the Early Ordovician (ca. 490–477 Ma, U-Pb zircon), followed by a partial overprint at 600 °C and 9.6 kb and then at 500 °C and 6.7 kb. An overprinting event at 525–500 °C and 9.5 kb is ascribed to the Devonian. The pressure-temperature (P-T) path of the Xayacatlán suite indicates a subduction-exhumation process followed by tectonically related reburial. Ixcamilpa suite blueschists with oceanic affinity underwent epidote-blueschist metamorphism (T, 200–390 °C; P, 6–9 kb) and then epidote-amphibolite (T, 390–580 °C; P, 9–6 kb) events ascribed to the Late Ordovician–Early Silurian. Esperanza suite eclogites with continental affinity underwent peak metamorphism at 830–730 °C and 17–15 kb. Amphibole from eclogite yields a 430 ± 5 Ma ⁴⁰Ar/³⁹Ar age, dating the high-pressure (HP) event. P-T paths of high-temperature (HT) eclogites like those of the Esperanza suite have been related to the collision of continental blocks. Partial overprinting occurred at 690–640 °C and 14–10 kb prior to 374 ± 2 Ma (⁴⁰Ar/³⁹Ar, phengite). The three HP suites were tectonically juxtaposed at different times before the Mississippian Period, resulting in the closure of the Iapetus Ocean. Phengite ⁴⁰Ar/³⁹Ar geochronology reveals the existence of a widespread tectonothermal event between 345 and 323 Ma, which may be related to the juxtaposition of the HP-composed block and the Gondwanan-affinity Cosoltepec suite, causing the closure of the Rheic Ocean. The tectonothermal events in the Acatlán Complex coincide in time, physical conditions, and tectonic setting with events in the Appalachian-Caledonian orogen, suggesting their relation. On that basis the geology of the Acatlán Complex can lead to a more comprehensive understanding of the tectonic evolution of the Appalachian orogen and of the Gondwana-Laurentia interactions preceding the Pangean assembly.

Key Words: eclogite • blueschist • Paleozoic • Appalachian • Acatlán Complex • Mexico

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