EVOLUÇÃO SEDIMENTAR DO SUPERGRUPO MINAS: 500 MA. DE REGISTRO GEOLÓGICO NO QUADRILÁTERO FERRÍFERO, MINAS GERAIS, BRASIL
Keywords:Quadrilátero Ferrífero, Supergrupo Minas, Transvaal Sequence, Geocronologia, Litoestratigrafia, Grupo Sabará, Paleoproterozóico
AbstractIt is now possible to bracket the age of the Minas Supergroup with reasonable precision between about2580 Ma. (base of the Moeda Formation) and 2050 Ma. (top of the Sabará Formation) on the basis of precisezircon U-Pb age determinations from both granitic rocks of the basement and detrital zircons from quartzitesand graywackes of the Proterozoic metasedimentary sequences of the Quadrilátero Ferrífero.Three major unconformities subdivide the stratigraphic column into three sequences. The first sequencecomprises the Moeda, Batatal, Cauê and Gandarela Formations from 2580 to 2400 Ma.. The second sequencecomprises the Cercadinho, Fecho do Funil, Taboões and Barreiro Formations (the Piracicaba Group) datedat 2400 to 2150 Ma.. The third sequence contains only the synorogenic sediments of the Sabará Formation,deposited during the Transamazonian Event between 2150 and 2050 Ma.. This event folded andmetamorphosed the Minas Supergroup for the first time.The Sabará Formation of the Piracicaba Group has very different sedimentological characteristics fromthe other formations of this group and is exceptionally thick (about 3500 m), we therefore propose to upgradeit to the rank of a Group.Zircon age histograms of the Moeda quartzite (sample QF-1) and Sabará graywacke are similar, indicatinga common Archean source, i.e. the Belo Horizonte Complex north of the Quadrilátero Ferrífero. This impliesthat sedimentation of the Minas Supergroup in the western Quadrilátero Ferrífero is autochthonous.Periods of carbonate and Iron Formation platform sedimentation (Cauê, Gandarela and partly Fecho doFunil Formations) coincide with minor frequency of age plots of detrital zircons, indicating also a stabletectonic environment.Comparison of the Minas Supergroup with the Transvaal Sequence of South Africa revealscorrespondence not only in age but also in thickness, position of the major unconformities, lithologicalcompositions and in the position of gold and pyrite bearing quartz-pebble conglomerates.
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