P-T conditions of hydrous retrograde metamorphism of UHT granulites from thermodynamic modeling of effective local compositions: Insights from the Grenvillian Oaxacan Complex, southern Mexico

Almazán-López, María del Mar ; Ramírez-Salazar, Anthony ; Colás, Vanessa (Universidad de Zaragoza) ; Ortega-Gutiérrez, Fernando

Resumen: The pressure, temperature, and composition retrogression of granulites is a common fate of many high temperature (HT)-ultra-high temperature (UHT) terranes accompanying their exhumation and coeval interaction with water-rich, CO2-rich and other fluids. This process generates complex coronitic and pseudomorphic microstructures indicating cooling and hydration, progressing from the UHT granulite to the low-greenschist (chlorite) facies assemblages. Nonetheless, the systematic study of the P-T evolution accompanying these highly significant retrograde systems have been commonly overlooked since they are formed by disequilibrium reactions and hence, not well understood. In this work, we used a combination of petrologic computing tools with thermodynamic modeling and conventional thermometry of bulk rock and recalculated local effective composition to determine the P-T conditions of retrogressive microstructures of an enderbite (i.e., orthopyroxene-bearing metatonalite) from the Grenvillian Oaxacan Complex, southern Mexico. The textural evidence described here, coupled with mineral chemistry data and our petrological modeling results show that, after reaching the metamorphic peak at 910 °C and 7.4 kbar, the enderbite registered three stages of hydrous metamorphic retrogression: MR1 defined by Mg-cummingtonite coronae on orthopyroxenes; MR2 defined by actinolite corona on MR1; and MR3 mainly defined by formation of biotite and chlorite. P-T conditions of MR1, MR2 and MR3 constrained a retrogressive clockwise path characterized by a well-defined isobaric cooling from 910 °C to 7.4 kbar (metamorphic peak conditions of the studied sample) to ∼825 and 770 °C at 7.4 kbar (MR1 and MR2) that was followed by a later cooling to ∼560 °C and decompression to ∼5.7 kbar (MR3). These P-T data represent the first systematic approach for tracing the retrogressive P-T path of the metamorphic evolution of the Oaxacan Complex, which is the core of the microcontinent Oaxaquia, and therefore, a key element in the terminal assembly and dispersion of the supercontinent Rodinia.

Idioma: Inglés

DOI: 10.1016/j.jsames.2025.105616

Año: 2025

Publicado en: JOURNAL OF SOUTH AMERICAN EARTH SCIENCES 164 (2025), 105616 [17 pp.]
ISSN: 0895-9811

Tipo y forma: Article (PostPrint)

Área (Departamento): Área Cristalografía Mineralog. (Dpto. Ciencias de la Tierra)