Low-temperature metamorphism in the Capiru Formation, Morro Grande Synform, Southern Ribeira Belt

Abstract

The Capiru Formation consists of a low-grade meta-sedimentary succession of slates, phyllites, quartzites and marbles, disposed in blocks delimited by thrust and strike-slip faults in the Curitiba Terrane, Southern Ribeira Belt, Southern Brazil. The metamorphic and deformation records are heterogeneous, with deformed zones tectonically interbedded by domains with preserved sedimentary features. The present work aims to understand the development of mineral paragenesis based on geochemical and petrographic analysis. We selected samples of metasandstones, slates, phyllites and rhythmic phyllites, with different metamorphic and deformation records to be analysed. We selected samples with different metamorphic records to be analysed. The petrographic characterization of foliations was made through field studies supported by microtectonic analysis. The mineralogical and chemical compositions were determined by X-ray diffractometry and X-ray fluorescence, respectively. The results show a preserved sedimentary bedding (S0), defined by sedimentary structures with top-and-bottom indicators. The S1 foliation is related to a thrust tectonics, and it is characterized by a continuous slaty cleavage. The S1 slaty cleavage is crenulated and cut by a millimetric S2 cleavage. The mineral assemblage is composed by quartz, sericite, magnetite/ goethite and carbonaceous material. The metamorphism was developed under low-temperature conditions (between 250–350ºC) and low pressure gradients (2.5–4.5 kbar). The geotectonic environment is considered as a thrust-and-fold-belt system, with structures developed in superior crust levels.