Dolomitização multifásica em rochas metacarbonáticas do Grupo Açungui, Proterozóico do Estado do Paraná, Brasil

Multistage dolomitization in the Açungui Group metacarbonate rocks, Proterozoic of Paraná State, Brazil

Abstract

stract
The carbonate rocks of Rio Bonito Quarry, at the State of Paraná, are represented by metamarls, mylonized rocks, calcite and dolomite marbles and dolomite breccias, which were formed in several burial stages. The first dolomitization process which affected the marbles had its origin in a burial environment. The process occurred during Mesoproterozoic, where a precedent calcitic rock had its matrix widely replaced by fine to medium sized dolomite crystals with cloudy aspect, and where styolites and fractures formed the principal conduits for the circulation of Mg-rich solutions. The dolomite breccia were formed during the Brazilian Cycle (Neoproterozoic), in conditions of hydraulic fracturing, originated by devolatilization which occurred during the metamorphism of green schist facies. The second dolomitization process, caused the formation of Fe-rich microcrystalline dolomite (Dp1), originated through a fluid at chemical disequilibrium with the fluid responsible for the former dolomitization. In the Paleozoic occurred a new phase of dolomitization, developing the Dp2-type dolomite, as the consequence of reactivation of deep crustal faults or igneous intrusions (dykes of microgabbro). This dolomite is constituted by saddle dolomite, with non-planar and coarse crystals, strong undulate extinction, which fills fractures in breccias as well as in dolomitic marbles. Superficial processes at the Paleogene, related to the topographic evolution, promoted the percolation of low-temperature fluids, occasioning the process of dedolomitization via dissolution and posterior filling of cavities. The process of dedolomitization occurred in dolomitic breccia with precipitation of late calcite, which filled the cavities, associated to dolomite of generation Dp2. Quartz completes the final phase of the void-filling process.