Multi-thermobarometry and microstructures reveal ultra-high temperature metamorphism in the Grenvillian Oaxacan Complex, Southern Mexico

The Oaxacan Complex represents the largest outcrop of the Mesoproterozoic microcontinent Oaxaquia. It experienced a major tectonometamorphic event, locally known as Zapotecan Orogeny (ca. 1004-980 Ma), during the latest stages of the amalgamation of the supercontinent Rodinia. It is typically interpreted that the syn-tectonic metamorphism occurred at granulite facies conditions (<800 degrees C and approximate to 0.7 GPa). However, the mineralogical record and recent classic geothermobarometry suggest that the Oaxacan Complex reached peak ultra-high-temperature metamorphism (UHTM) but no conclusive results have been obtained. In this work, we present a combination of microstructural observations, mineral chemistry, phase equilibria thermobarometry, and trace element thermometry to document the processes associated with the granulite petrogenesis and calculate the peak metamorphic conditions. Petrographic observations show that the peak syn-tectonic metamorphism was contemporaneous to partial melting. This is revealed by the presence of euhedral and subhedral plagioclase and feldspar growing with quartz, melt relics included in garnets, films, or veinlets surrounding peak phases, among others. Exsolved clinopyroxene on orthopyroxene, exsolved rutile needles in garnet, ubiquitous mesoperthites, and F- and Ti-rich biotite grains indicate UHTM. The combination of phase equilibria modelling in two metapelites and one garnet-charnockite, along with Zr-in-Rutile and Ti-in-Quartz thermometry, confirms UHT conditions for the Zapotecan Orogeny (T = 895-916 degrees C and P = 0.83-0.99 GPa). Preconditioning of the crust during the earlier Olmecan tectonothermal event (ca. 1100 Ma) and the ascent of the sub-lithospheric mantle follwed by the Zapotecan crustal thickening might account for the UHTM recorded in the Oaxacan rocks. The recognition of syn-tectonic UHTM in the Oaxacan Complex supports palaeographic reconstructions that place Mexican-Colombian terranes entrapped among Amazonia and Baltica during their collision as part of the amalgamation of Rodinia. Our novel results provide new avenues for petrological research for a better understanding of Oaxaquia and its role in the final configuration of Rodinia.