Metamorfismo de contato nas rochas da Formação Irati (Permiano), norte da Bacia do Paraná

Abstract

The Permian Irati Formation is a black shale-carbonate sequence that is intruded by Jurassic-Cretaceous basic rocks. In Goiás State, near Perolândia town, the SUCAL quarry exposes the Irati Formation with a diabase sill, less than thirteen meters thick. To investigate the mineral modifications caused by the heat of the sill, pelites and carbonate rocks were studied through X rays diffraction, electronic microprobe and scanning electronic microscopy. The results show that the pelites and carbonate rocks without sill influence are constituted by saponite and dolomite respectively. Near the sill talc, serpentine, pyroxene, calcite and plagioclase were formed. The first three minerals are considered index minerals and named the metamorphic zones: Talc Zone, Serpentine Zone and Pyroxene Zone that are heterogeneously distributed below and above the sill. The confinement of aqueous fluid below the intrusion favour the formation of hydrated minerals (talc, serpentine), but the fluid escape above the sill allowed the formation of pyroxene and plagioclase. Talc, serpentine and calcite were formed by the reaction of dolomite, quartz and aqueous fluid. Pyroxene and plagioclase are the products of the reaction of saponite with quartz and calcite. The temperature in the Talc and Serpentine zone is not clear, because the index minerals can be stable from room temperature up to 400-600ºC. In the Pyroxene Zone the minimum temperature for the formation of pyroxene could be estimated between 300 and 500ºC.