Structural geology and stratigraphy at the junction of the curral anticline and the moeda syncline, Quadrilatero Ferrifero, Minas Gerais, Brazil

Abstract

In the map area the rocks are divided into two large subdivisions: the Archaean Nova Lima greenstone and the Bonfim granitoid gneisses, and the overlying Proterozoic Minas metasedimentary and metavolcanic rocks, including abundant iron formation. These groups are separated by a major unconformity. These new studies in the Minas Series culminated with the following results: reassignement of the conglomerate -quartzite sequence of the Mutuca area to the Moeda Formation rather than to the Nova Lima Group; identification of a suite of greenstones and peraluminous potassic schists in the Batatal and Gandarela Formations and Piracicaba Group ; recognition of the riebeckite in one of the itabirite sequences; and redefinition of the Gandarela Formation through the identification of stromatolites and greenstones. The structure of the junction of the Curral and Moeda structures consists of two elements: on the north is the recumbent northeast -trending Curral anticline with its lower limb faulted against Archaean Nova Lima rocks, and the upper limb deformed into the Moeda syncline to the south. In the south, the westward overturned Moeda syncline trends north -south, with both of the limbs strongly deformed. The western limb is folded into recumbent folds indicating a northerly transport direction. The eastern limb has become rotated westward during thrusting until now it is overturned by Nova Lima greenstones. This overturned limb of the Moeda syncline is separated from the Curral anticline by the Barreiro-Agua Quente zone of tear faults. The features demonstrate two major phases of deformation, the earlier D1 with a north-westward vergence and the later D2 -episode, with a westward vergence. Extensive development of quartz and muscovite in the sediments and of chlorite and talc in the mafic and ultramafic greenstones, and the quartz grain-size in itabirite, in general, smaller than 0.1 mm are indicative of low -grade metamorphism in the area. The Quadrilátero Ferrifero and the Michigan iron ore district show similarities in the occurrence of peraluminous potassic schists indicated by the log (SiO2/A1203)-(Na20 + Ca0/K20)- diagrams, the presence of greenstones, the strain-induced quartz grain-size variation in itabirites, and the basement controlling tectonics.