PALEOPROTEROZOIC GRAPHITIZATION IN THE CLÁUDIO SHEAR ZONE (SOUTHERN SÃO FRANCISCO CRATON - BRAZIL) INVESTIGATED BY RAMAN SPECTROSCOPY AND X-RAY DIFFRACTION

Grafitização paleoproterozoica na Zona de Cisalhamento Cláudio (Sul do Cráton do São Francisco - Brasil) investigada por Espectroscopia Raman e Difração de Raio-X

Abstract

The southern São Francisco craton hosts some of the largest graphite mines in Brazil, located in the city of Itapecerica. This paper addresses graphite crystals that occur within the regional structure known as the Cláudio shear zone in terms of thermometry. Raman spectroscopy and X-ray diffraction data showed that, on average, graphite crystals of the Cláudio shear zone have high crystallinity and are similar to those from mines in Itapecerica. The average temperature found (702 ºC) in this study is close to the samples from Itapecerica. However, this temperature is still lower than the expected (~900 ºC) from previous studies. The hypothesis for this would be that the graphite devolved high crystallinity, but was modified during the evolution of the metamorphism, or during the post-collisional stage fluid percolation. Thus, the modified crystallinity parameters lead to the relatively low-temperature values found. The proximity and similarity of geological framework suggest correlation between the precursor basins that formed the Itapecerica and the Cláudio shear zone graphite occurrences. From a global perspective, the closure of precursor basins and formation of the Cláudio shear zone are part of the collisions of Archean and Paleoproterozoic crustal blocks that formed the Columbia supercontinent.