Late Paleozoic subduction and exhumation of Cambro-Ordovician passive margin and arc rocks in the northern Acatlán Complex, southern Mexico: Geochronological constraints

Abstract

The origin and age of high pressure (HP) rocks are crucial for paleogeographic reconstructions because they either define an oceanic suture or an extrusion zone within the upper plate. HP rocks in the San Miguel Las Minas area in the northern part of the Acatlán Complex, southern Mexico have been inferred to be of early Paleozoic age and to mark oceanic sutures. However, blueschists in the northern part of the complex have yielded Mississippian 40Ar/ 39Ar plateau ages of 344 ± 5 Ma for glaucophane and 338 ± 3 Ma and 337 ± 2 Ma for muscovite. These ages are slightly younger than recently published ages: a U–Pb zircon age of 353 ± 1 Ma from associated eclogite, and a 347 ± 3 Ma muscovite age from the tectonically overlying, greenschist facies Las Minas Unit. Taken together, these data indicate rapid cooling between 700° and 340 °C in ca. 17 Myr. On the other hand, associated Ordovician Anacahuite Amphibolite cooled through ca. 500 °C at 299 ± 6 Ma ( 40Ar/ 39Ar on hornblende) suggesting a second, Permian phase of exhumation. Protoliths of the high grade rocks include Cambrian–Ordovician, rift-passive margin, psammites, pelites, and tholeiitic dykes, an Ordovician mafic intrusion (Anacahuite Amphibolite dated at 470 ± 10 Ma: U–Pb zircon) and megacrystic granite (dated at 492 ± 12 Ma: U–Pb zircon), and arc-related mafic rocks of unknown age. These rocks are interpreted to have been part of the upper plate rocks that was removed by subduction erosion and taken to depths between 35 and 55 km where they underwent blueschist–eclogite facies metamorphism. This was followed by rapid extrusion along a channel bounded by an easterly dipping, Mississippian, listric normal shear zone, and a thrust modified by a Permian dextral fault. Rocks above and below the extrusion zone are mainly Cambro-Ordovician rift-passive margin units, but a small vestige of the arc is preserved as dikes cutting rocks lying unconformably beneath the fossiliferous latest Devonian–Lower Permian Patlanoaya Group. Since faunal data indicate that Pangea had begun amalgamation by the Mississippian, at which time the Acatlán Complex lay 1500–2000 km south of the Ouachita collisional orogen between Gondwana and Laurentia, it is inferred that subduction and extrusion of the high pressure rocks occurred on the active western margin of Pangea.