Eventos diagenéticos e hidrotermais do sul do Cráton Amazônico: desvendando a origem das brechas carbonáticas e os processos de dolomitização da Formação Serra do Quilombo do Ediacarano, sudeste do Estado de Mato Grosso

Abstract

The Ediacaran period was characterized by the establishment of extensive platforms during CaCO3 supersaturation events, and by the exceptional creation of accommodation space associated with the aftermath of the Marinoan glaciation (~635 Ma). The carbonate deposits in the Araras Group recorded sedimentation from the beginning of the Ediacaran period, stage 1, in the Amazon Craton. The Serra do Quilombo Formation is a distinct unit characterized by the occurrence of strongly fractured dolomites and faults, along with the presence of dolomitecemented breccias (BC) overlying thick packages of limestone associated with diagenetic and hydrothermal modifications. This evidence introduces complexity to these scenarios. The origin of this deposit is still uncertain, as work has mainly focused on paleoenvironmental issues. This study aims to unravel the origin of cemented breccias and diagenetic/burial processes within the unit, focusing on dolomitization processes. The dolomite samples were analyzed using petrography, scanning electron microscopy, microprobe, Raman microscopy, cathodoluminescence, and isotopic analyses (δ13C, δ18O, 87Sr/86Sr) to unravel their burial history. BCs occur as subvertical to subhorizontal bodies with complex geometries, generally cutting through bedding at a high angle, indicative of hydrofracturing processes related to vertical flows of hydrothermal fluids (hydraulic breccia), in addition to exhibiting the typical cockade texture of expansion breccias in dilatational faults. The paragenetic assemblage of these rocks includes dolomite, quartz, calcite, alkali-feldspar, apatite, pyrite, chlorite, bitumen, and iron oxide, with the replacement features (RD) and dolomite cementation (DC) being the main targets of analysis. The almost micritic substitutive matrix (RD1/RD2) is the primary constituent of the Serra Quilombo Formation, with its low correlation between the values of δ13C and δ18O (R²=0.009), well-preserved fabric, and similarity to the isotopic values (C and Sr) documented for Ediacaran carbonates, suggesting syndepositional dolomitization of this constituent under conditions of shallow burial, possibly involving seawater. The first generation of dolomite cement (DC1) and the last phase of dolomite cementation (saddle dolomite - DC3) occur by filling pores, BCs, and fractures. The cockade texture of the breccias highlights a low precipitation rate or a pause in precipitation between DC1 and DC3. Concomitantly, DC1 has isotopic signals of δ18O = -4.34 ± 1.32‰ (n=18) and 87Sr/86Sr = 0.708831 (n=2), while DC3 has values of δ18O = -9.57 ± 2.51‰ (n=15) and 87Sr/86Sr = 0.711464 (n=3). The significant isotopic difference between the two cementation phases indicates distinct dolomitizing fluids. This relationship shows an increase in 87Sr in the fluid as the temperature increases. Furthermore, the enrichment of 87Sr in the fluid can be explained by its interaction with crystalline basement rocks. Thus, faults with deep roots spatially close to tectonically active zones are likely the main conduits for the ascent of this radiogenic fluid. The occurrence of BCs, essentially in the Serra do Quilombo Formation, is attributed to the limestone-dolomite contact and was interpreted as facilitating the development of fracture corridors, which assist in the percolation of hydrothermal fluids. Finally, it is understood that the conduits with carbonate breccia are subsequent to the silicification event of the evaporites of the Nobres Formation, which acted as a sealing rock for hydrothermal fluids. The presence of tectonic stylolites cutting the cemented breccias and the subverticalized zebra-like stratified structures indicates that the BCs were already formed during the installation of post-Ordovician transtensional structures, preceding the installation of the Paleozoic Basins on the South American Platform.