Constraints on the subduction erosion/extrusion cycle in the Paleozoic Acatlán Complex of southern Mexico: Geochemistry and geochronology of the type Piaxtla Suite

摘要

The type high-pressure (HP) Piaxtla Suite in the Acatlán Complex of southern Mexico consists of retrogressed eclogite (amphibolite), megacrystic granitoids and high-grade meta-sedimentary rocks. Exhumation of these HP rocks has recently been interpreted as the result of extrusion into the upper plate, rather than by return flow up the subduction zone. Geochemical analyses of the retrograde eclogites indicate that they have a rift tholeiitic-transitional alkalic composition. These are closely associated with a megacrystic meta-granitoid that has yielded an intrusive age of 452±6Ma (concordant U-Pb zircon analyses) with inherited zircon populations at ca. 800-950Ma and 1000-1200Ma derived from the underlying basement, probably the Oaxacan Complex which borders the Acatlán Complex to the east. The bimodal nature of these igneous rocks and their spatial and temporal close association with continentally-derived sedimentary rocks is similar to most HP rocks in the Acatlán Complex derived from a rifted passive margin. The youngest detrital zircon population in a meta-psammite sample yielded a U-Pb age of 365±15Ma with older analyses distributed along a chord with an upper intercept of 1287±29Ma. The ca. 365Ma age provides a maximum age for the time of deposition of this sample. ~(40)Ar/ ~(39)Ar ages from the retrogressed eclogites provided hornblende plateau ages of 342±2Ma and 344±2Ma, whereas muscovite from the granitoid and meta-psammite yielded 334±2Ma plateau ages. These data constrain the subduction erosion-extrusion cycle to &le35Ma during which the rocks were taken to a depth of ca. 40km at a rate of 2.7km/Ma and back to the surface at 2.4km/Ma. Such exhumation rates are slower than those in continent-continent collision zones, but similar to those in the Iberia-Czech Variscan belt where tectonic interpretation also suggests extrusion into the upper plate.