Abstract

The Acatlan Complex of southern Mexico is linked to the evolution of the Appalachian–Caledonian chains and records events related to the Taconian, Acadian, and Alleghanian orogenies of northeastern North America. Mafic eclogites and garnet amphibolites from two selected localities are used to partially reconstruct the tectonometamorphic evolution of this complex. Eclogites contain garnet (almandine) + Ca–Na pyroxene + phengitic mica + zoisite–clinozoisite + quartz ± Ca–Na amphibole (barroisite, katophorite) ± albitic plagioclase ± rutile. Phase and textural relationships, thermobarometric determinations, and available radiometric ages indicate that eclogite-facies metamorphism took place during the Ordovician at temperatures around 560 ± 60°C and pressures between 11 and 15 kbar (1 kbar = 100 MPa). Eclogites underwent widespread retrogression to epidote-amphibolite then greenschist facies during exhumation, most probably during Devonian times. Epidote–amphibolite facies include the critical assemblage calcic pyroxene + calcic amphibole (magnesiohornblende and pargasite) + muscovite + garnet + plagioclase + epidote ± quartz, whereas greenschist facies is defined by the assemblage actinolite + albitic plagioclase + epidote + chlorite. Thermobarometric data suggest that retrogression occurred at temperatures between 510 ± 20°C and 300 ± 25°C and pressures ranging from 6 to 3.5 kbar. The obtained P–T (pressure–temperature) path suggest that the Acatlan Complex evolved in a more complex continental collisional setting, including intraoceanic arcs, than shown in previously proposed models.

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