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

ABSTRACT

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°C and ≈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°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.

Graphical-Abstract

KEYWORDS:

• phase equilibria thermobarometry
• trace element geothermometry
• ultra-high-temperature metamorphism
• partial melting
• Oaxaquia

Highlights

• The Oaxacan Complex experienced peak ultra-high temperature metamorphism (T = 895–916°C and P = 0.83–0.99 GPa) during the Grenvillian Zapotecan Orogeny (1004-980 Ma).

• Melt microstructures reveal syn-tectonic metamorphism contemporaneous to partial melting.

• Oaxaquia and the Colombian terranes were entrapped between Amazonia and Baltica during the amalgamation of Rodinia.

Acknowledgments

This research was funded by CONACyT 164454 and UNAM-PAPIIT IN107320 grants provided to FOG and by CONACyT-Ciencia Básica A1-S-14574 and UNAM-PAPIIT IA102121 to VC. We would like to thank Luigi Solari for providing some of the samples used in this work, Margarita Reyes Salas, Consuelo Macías Romo, and Sonia Ángeles García for helping us with the sample preparation, and Carlos Ortega-Obregón for his help in the acquisition of the LA-ICP-MS data. Special thanks to Artur Benisek for providing the spreadsheet to make the two-feldspar calculation and to Laura Luna González for providing (Figure 1). We thank Mihai Ducea and an anonymous reviewer for their comments that improved the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/00206814.2022.2084648

Additional information

Funding

This work was supported by Consejo Nacional de Ciencia y Tecnología [Grant 164454 and Ciencia Básica grant A1-S-14574]; Universidad Nacional Autónoma de México [UNAM-PAPIIT grant IN107320, UNAM-PAPIIT grant IA102121].