GEOCHEMICAL, GEOCHRONOLOGICAL AND STRUCTURAL CONNECTIONS BETWEEN THE WESTERN AND CENTRAL HIGH PRESSURE BELTS IN ACATLÁN COMPLEX, SOUTHERN MÉXICO: ONE CYCLE OF METAMORPHIC PETROGENESIS AND EMPLACEMENT DURING LATE DEVONIAN-MISSISSIPPIAN SUBDUCTION-EXTRUSION ON THE WESTERN MARGIN OF PANGEA

The Acatlán Complex contains an assemblage of Paleozoic metamorphic rocks deformed under high pressure conditions (eclogite- blueschist facies). Such rocks are critical for paleogeographic reconstructions and knowledge of subduction processes. Two high pressure belts occur in the Acatlán Complex: Piaxtla-Mimilulco belt in the middle and the Ixcamilpa-Olinalá belt in the west that were previously considered to have a distinct origin in both time and space. However, new detailed mapping has allowed make reconsiderations about protolith nature, petrogenesis and emplacement.

At Ixcamilpa area, the high grade assemblage of Neoproterozoic-Ordovician, metapsammites-pelites intruded by metabasites (Piaxtla Suite) and Ordovician granitoids occurs in a W-vergent klippe that override metasediments of the low grade Zumpango Unit. The high grade rocks of Ixcamilpa-Olinalá are similar lithologically, geochronologically and geochemically to those rocks in the middle belt. In both belts the Ordovician mafic rocks have a sub-alkaline tholeiitic signature formed during extension along with the granitoids. Such magmatism is consistent with formation on the rift-passive margin along the southern margin of the Rheic Ocean.

Fabrics of glaucophane and retrogressed eclogite at Ixcamilpa are regarded as products of progressive deformation. Metamorphic peaks were defined in 12-16Kbar – 550°-600°C, and cooling ages (Ar-Ar) of ~344 Ma in amphibole and ~318 Ma in glaucophane. The age and metamorphic peaks are similar with those in middle HP belt. The eastern boundary of the middle HP belt is a ductile shear zone with normal kinematics. These features and geochronology are consistent with the architecture of ductile extrusion from depths of 45-20 km during the Mississippian. Prior subduction removed the protoliths from the northwestern margin of Gondwana by subduction erosion, during Late Devonian-Carboniferous get inside subduction zone, and subsequently extruded into upper crust.