HIGH PRESSURE KLIPPE ON THE ACTIVE WESTERN MARGEN OF PANGEA DURING INITIAL STAGES OF ITS AMALGAMATION
The granite and mica schist are inferred to be high-grade equivalents of lower Paleozoic, low-grade rocks elsewhere in the Acatlán Complex, from which they are inferred to have been removed by subduction erosion. The associated metabasites represent a juvenile cumulate magma (εNdt=450 Ma= 5.3-8.9) generated in an island arc environment of unknown age (pre-Mississippian), and the serpentinite body is an ophiolotic harzbugite generated in a suprasubduction environment. This set of HP rocks was subducted, undergoing HP metamorphism and polyphase deformation at depths of ~50 km (~16 kbar and 750 °C: eclogite facies), and subsequent retrogression passed through amphibolite-epidote to greenschist facies, which was synchronous with W-vergent Mississippian thrusting over the low-grade clastic rocks. Cooling through ca. 385ºC is indicated by ~329 Ma (metabasite), ~316 Ma (granitoid), and ~317 Ma (mica schist) 40Ar-39Ar muscovite plateau ages, which are 5-17 my younger that those of the adjacent Piaxtla eclogites suggesting younger thrusting. These data are consistent with a subduction-extrusion process on the active western margin of Pangæa during initial stages of its amalgamation, instead of collision as was previously suggested. Igneous detrital zircon populations in the low-grade psammite and the HP schist indicate derivation from the Ordovician Acatlán granitoids (490-440), Neoproterozoic Brasiliano orogens (750-500), 900-750 Ma Goiás arc (Amazonia), 1.3-1.0 Ma Oaxaquia, and more ancient sources suggest derivation from Oaxaquia/Amazonia.