C, Sr and O isotopic compositions and REE geochemistry of São Gabriel Block carbonate rocks, Rio Grande do Sul

Abstract

The São Gabriel Block, located in the central zone of the crystalline basement in the state of Rio Grande do Sul has extensive Neoproterozoic records of the Brasiliano Cycle, recorded in igneous and metamorphic complexes. These complexes also keep sedimentary records of metamorphosed carbonate successions whose tectonic evolution is poorly understood. Therefore, this study aimed to investigate the origin and evolution of carbonate rocks that occur in São Gabriel Block by geochemical characterization of major, trace, REE and Sr, C and O isotopes. Hence, Passo Feio Formation, Cambaí Complex and Coxilha do Batovi Complex were chosen as research areas. The Passo Feio Formation carbonate rocks consist of impure dolomite marbles. The Cambaí Complex samples are classified as impure calcite marbles. Coxilha do Batovi Complex samples include calcite and calc-silicate marbles, some of them with graphite, with quartz rates up to 10% showing the siliciclastic contribution to the protolith. Ages from detrital zircon of chlorite schists rocks from previous works indicate an interval between 770-700 Ma for carbonate deposition in São Gabriel Block. The carbonates from Passo Feio Formation were affected by dolomitization process during or early after its deposition, while the carbonates from Cambaí Complex were not affected by dolomitization. Trace elements and REE geochemistry of marbles indicate preservation of original composition of carbonate protoliths along different 76 metamorphic grades; Coxilha do Batovi Complex (low grade), Passo Feio Formation and Cambaí Complex (medium grade). The integrated analysis of Sr, C and O isotopes from Passo Feio Formation and Cambaí Complex revealed the preservation of primary isotopic values or very close to the original isotopic composition. For the Passo Feio Formation the values are: 0,7074 (Sr⁸⁷/Sr⁸⁶), -0,26‰ and 2,44‰ (δ¹³C_PDB) and -5,68 (δ¹⁸O_PDB). For the Cambaí Complex, these values are: 0,7069 (Sr⁸⁷/Sr⁸⁶), 5,75 (δ¹³C_PDB) and -11,64 (δ¹⁸O_PDB). When compared with seawater Sr, C and O isotopes variations along the Neoproterozoic it appears that Passo Feio Formation and Cambaí Complex are situated in the range between 770 and 730 Ma. Based on these data it is suggested that the São Gabriel Block depositional event occurred between 770-700 Ma. The contextualization of isotopic data in the chronology of the tectonic events recognized in São Gabriel Block allowed estimation of periods of deposition of carbonate rocks of Passo Feio Formation between 770 and 730 Ma, and between 740 and 730 Ma for Cambaí Complex.