Comment on “Petrology, geochemistry and Sm-Nd systematics of the Paleoproterozoic Itaguara retroeclogite from Sao Francisco/Congo Craton: One of the oldest records of the modern-style plate tectonics”

Abstract

The recent paper by Chaves and Porcher published in Gondwana Research (2020, v.87, 224–237) asserts the presence of 2.2 Ga eclogite facies rocks in the Itaguara Sequence, São Francisco craton (SFC), Brazil. They present images in which the diagnostic high-pressure minerals either occur on cracks (rutile), or as fragmentary matrix grains with unclear mineral relationships (omphacite and phengite). Here I discuss the sample petrology in order to identify the equilibrium assemblage, and utilise a P-T phase modelling approach, integrating the major mineral compositions, to show that these rocks are not retrogressed eclogites.The modelling presented here indicates that peak conditions were likely in the amphibolite facies, reaching maximum pressures of no more than 8 kbar and peak temperatures of ca. 700 °C (at 5 kbar). This result is consistent with that found by Massonne (2020, JSAMES, v.99, 102495) who suggested that mica schists from Itaguara reached maximum pressures of 13–14 kbar, and possibly lower than 10 kbar at 600 °C (i.e. within the medium to high-pressure amphibolite facies). This work shows that declarations of eclogite facies conditions, particularly from Paleoproterozoic terrains, should be accompanied by carefully evaluated petrography and thermodynamic investigations based on minerals (or mineral replacement textures) that indicate equilibrium relationships.