THE METAMORPHIC RETROGRESSION OF THE ECLOGITIC ROCKS FROM ACATLÁN COMPLEX, MÉXICO: UNRAVELING ITS EXHUMATION COMBINING FIELD OBSERVATIONS WITH MICROSTRUCTURAL ANALYSIS

The Acatlán Complex is the largest outcrop of metamorphic Paleozoic rocks in Mexico. This Complex is built by polydeformed metaigneous and metasedimentary units that registered, at least, three independent orogenic events occurred between the Ordovician-Silurian, Early Mississippian, and Early Permian. In particular, the Mississipian event reached metamorphic peak conditions in the eclogite facies at ca. 22 kbar and 760 °C.

This research focuses on the understanding of the processes involved in the exhumation of high-P rocks (i.e., eclogitic rocks) and related units outcropping in the central part of the Acatlán Complex, at the Piaxtla–Tecomatlán area. A preliminary study using a multidisciplinary approach, combining macroscopic field observations and multi-technique microstructural analysis –i.e., electron microprobe analysis and detailed compositional mapping processing–, is presented here to provide new elements to solve how high-P rocks generated under more than 75 km deep where exhumed and brought back to the surface.

The metamorphic retrogression of the eclogitic units from the Acatlán Complex is shown by two main types of fingerprints: 1) microstructures including exsolutions, coronae and pseudomorphs involving plagioclase, epidote, clinopyroxene, chlorite, garnet, titanite and ilmenite; and 2) mineralogical and chemical fingerprints such as compositional zoning of single crystals of garnet, tourmaline, epidote, amphibole and phengite, as well as several compositional generations of clinopyroxene, garnet, epidote, zoisite, amphibole, plagioclase, chlorite and calcite in different microstructural domains. It should be noted that the microstructures are better preserved in the northern part of the studied area (i.e., Piaxtla area), whereas the mineralogical and chemical fingerprints are predominant in the southern portion, at Tecomatlán area.

Hence, our novel data could help to reconstruct the retrograde trajectories of the high-P lithologies from the Acatlán Complex, which are key to unravel the post peak history of this extraordinary metamorphic complex.