Microstructural and metamorphic evolution of the Carrancas Klippe, interference zone of the Neoproterozoic southern Brasilia and Ribeira orogens, SE Brazil

Abstract

The Carrancas Klippe is the basal structure of a nappe system, at the southwestern edge of the São Francisco Craton, in the southern part of the Southern Brasília Orogen. The klippe comprises three metasedimentary units: from base to top, 1) a micaceous quartzite (São Tomé das Letras Unit), 2) chloritoid phyllite grading to staurolite schist with quartzite and gray phyllite intercalations (Campestre Unit), and 3) biotite schist (Santo Antônio Unit), all belonging to the Neoproterozoic Andrelândia Group. This paper describes the results of detailed mapping, structural and metamorphic analyses, including microtectonics, mineral chemistry and related geothermobarometry, and Sm–Nd dating of garnet. Deformational structures were grouped into three sets, D1, D2 and D3. D1 generated thrusting to the east-southeast with associated stretching lineation and tight recumbent folds. D2 produced tight to close asymmetrical folds with axial planes dipping to the SSW, S and SE, and subhorizontal axes. Steep north-south trending axial planes and shallow fold axes characterize open D3 folds and crenulations. In the northwestern part of the klippe garnet chloritoid phyllites formed during upper greenschist facies metamorphism of D1 age with syn-D1 snowball garnet. Towards the southeast the phyllites grade into amphibolite facies schists with syn-D2 garnet and staurolite. Geothermobarometric calculations based on mineral chemical compositions resulted in the following average values for D1 equilibria: 470 ± 45 °C and 12.3 ± 1 kbar. For D2 the following average values were calculated: 580 ± 35 °C and 8.4 ± 1.4 kbar. An age of 562 ± 24 Ma was obtained by Sm–Nd dating of D2 garnet growth. In conclusion, D1 is characterized by east to southeastward thrusting, including the emplacement of the klippe, related to the Southern Brasília Orogen tectonics, at upper greenschist facies conditions with relatively high pressures. During D2, the direction of tectonic transport changed to north, under greenschist facies conditions in the northwest and medium pressure amphibolite facies conditions in the southeast, producing staurolite and kyanite isograds that cut the klippe structure. This phase is attributed to the NNW-SSE shortening in the Ribeira Orogen. D3 reflects late E-W shortening also recorded in major NE-SW dextral shear zones under upper greenschist facies conditions with lower pressures.