DEVONO-CARBONIFEROUS AND PERMIAN TECTONOTHERMAL EVENTS IN THE NORTHERN ACATLÁN COMPLEX, SOUTHERN MEXICO: A RECORD OF CLOSURE OF THE RHEIC OCEAN FOLLOWED BY CONVERGENCE ON THE PACIFIC MARGIN OF PANGEA

Recent mapping in the northern part of the Acatlán Complex (southern Mexico) has distinguished two lithological units: a high-grade unit assigned to the Piaxtla Metamorphic Suite, and a low-grade unit assigned to the Cosoltepec Formation. Along-strike correlatives of the latter have recently yielded detrital zircons as young as ~410 Ma. Two major Paleozoic tectonothermal events have been identified in these rocks. The first event, which is recorded only in the Piaxtla Metamorphic Suite, produced a penetrative deformation fabric (S₁) parallel to a compositional banding during blueschist and amphibolite facies metamorphism. This high-grade metamorphism has recently been dated as ~346 Ma in a neighboring area. The second event, which is recorded in both the Piaxtla Metamorphic Suite and Cosoltepec Formation, produced three penetrative deformational fabrics under greenschist metamorphic conditions. In the Piaxtla Metamorphic Suite this sequence is defined as: a SE-dipping foliation of chlorite and muscovite (S₂) which is deformed by S-vergent sheath folds producing an axial planar fabric (S₃) which is deformed by north-south chevron folds producing kink bands (S₄). In the Cosoltepec Formation this sequence is defined as: a bedding-parallel foliation of muscovite and quartz (S₁) which is deformed by sheath folds producing an axial planar fabric (S₂) which is deformed by north-south, east-vergent, upright folds producing a NW-dipping crenulation/solution cleavage (S₃). The second tectonothermal event is inferred to have accompanied tectonic juxtaposition of the two units during exhumation of the high-grade unit in the Permian. The first, high-grade, tectonothermal event is believed to have accompanied closure of the Rheic Ocean in the Mississippian, whereas the second, low-grade, event probably records convergence along the paleo-Pacific margin of Pangea.