High pressure rocks of the Acatlán Complex, southern Mexico: Large-scale subducted Ordovician rifted passive margin extruded into the upper plate during the Devonian–Carboniferous

High pressure (HP) rocks in the Paleozoic Acatlán Complex of southern Mexico occur in a median belt extruded into the complex, which is the root zone of a klippe to the west. These HP rocks are predominantly clastic metasedimentary rocks and include a bimodal suite of amphibolites and granitoids. U–Pb LA-ICPMS zircon dating indicates that one of the amphibolites was intruded at or before 461 ± 2 Ma, and a megacrystic granitoid yielded an intrusive age of 488 + 8/− 7 Ma, which is within the 490–450 Ma range of other dated HP megacrystic granitoids. The amphibolites have a tholeiitic (transitional to alkalic), within-plate chemistry derived from a heterogeneous mantle by various degrees of partial melting. The host metasedimentary rocks contain detrital zircons derived both from the Acatlán granitoids and the ca. 1.0–1.3 Ga Oaxacan Complex, and are inferred to represent rifted passive margin sediments. An age of 348 + 4/− 1 Ma from zircons in a migmatized amphibolite is interpreted as the time of migmatization and dates decompression melting following peak pressure at depths of 45–55 km. Retrogression from 530 °C to 370 °C occurred between 343 and 335 Ma. The predominance of reasonably coherent, rifted passive margin rocks over a surface area of 10 × 90 km suggests removal of a large slice of the upper plate during flat slab subduction. This slice was subducted to depths of ca. 45–55 km where it was underplated and transferred to the overriding plate. The absence of Oaxacan Complex rocks in the HP belt suggests the Oaxacan basement was subducted to greater depths. Underplating was followed by extrusion facilitated by extension in the upper plate that produced Mississippian rift basins east of the median HP belt. Westward overthrusting of the HP rocks produced a klippe that overrode Mississippian clastic wedge sediments.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► High pressure metamorphism of Ordovician rift tholeiites in Acatlân Complex, Mexico. ► U–Pb zircon ages indicate Ordovician protolith and Carboniferous migmatization. ► Subduction erosion and extrusion of big rifted passive margin slices into upper plate. ► Devonian–Carboniferous subduction tectonics on the western margin of Pangea.