Thermodynamic modelling of coesite-bearing gneisses from the Três Pontas-Varginhas nappe, Minas Gerais - Brazil.

Abstract

This project focuses on obtaining pressure and temperature conditions (P-T) to which the granulitic metassedimentary rocks of the Três Pontas-Varginha nappe, at the southern end of the Brasília Orogen, were subjected. Recent identification of coesite micro-inclusions in garnet porphyroblasts indicate that the rocks from the nappe have been under ultra-high pressure metamorphic conditions. Such evidence, unprecedented in literature, calls for new detailed thermodynamic modelling to be carried out on these granulites in order to calibrate the ultra-high pressure P-T conditions.The objectives of this project will be achieved through careful petrographic analysis, punctual WDS chemical analysis and quantitative compositional mapping of oxides in wt% through electron microprobe. Compositional maps and the extraction of the chemical mineral composition and local average composition will be processed using the XMapTools software. Metamorphic conditions will be estimated through phase equilibrium diagrams and compositional isopleths calculated using the DOMINO software. Obtaining new data related to ultra-high pressure granulites from the Três Pontas-Varginha nappe will offer a better understanding of the initial stages of the collision, adding new evidence about the amalgamation processes of West Gondwana and the geotectonic evolution of Neoproterozoic collisional orogens.