Age, provenance and tectonic setting of the Canastra and Ibia Groups (Brasilia Belt, Brazil): Implications for the age of a Neoproterozoic glacial event in central Brazil

摘要

The Brasilia Belt is one of the best preserved Neoproterozoic orogens in Brazil. It comprises a thick Meso-Neoproterozoic sedimentary/metasedimentary pile including the Canastra and Ibia Groups, which are the object of this study. The Canastra Group constitutes a regressive sedimentary sequence made mainly of greenschist-facies metapelitic and metapsammitic rocks, including phyllite, sandy metarhythmite and quartzite, with minor intercalations of limestone, as well as carbonaceous and carbonatic phyllite. The Ibia Group is formed of a basal diamictite followed upwards by phyllites and calc-schists. It rests on an erosional unconformity on top of the Canastra Group.rnA provenance study based on U-Pb zircon geochronology on a selection of seven samples helped to establish the various source areas and maximum depositional ages of the original sediments. In addition, seven new Sm-Nd analyses are presented and discussed together with previously published data.rnLAM-ICP-MS U-Pb dating of detrital zircon grains indicates a maximum depositional age of the Canastra and Ibia Groups of ca. 1030 and 640 Ma, respectively. The provenance signature of the Canastra Group comprises a wide range of detrital zircon ages with a significant Paleoproterozoic component (～1.8 and ～2.1 Ga) and an important Mesoproterozoic source (1.1-1.2 Ga), especially for the Paracatu Formation, indicating the Sao Francisco-Congo Craton as main source. These provenance data, in particular the absence of Neoproterozoic zircon grains, typical of the active margin of the Brasilia Belt, allied with the homogeneous Paleoproterozoic T_(DM) values are consistent with the previous interpretation that the Canastra Group represents a sedimentary sequence deposited on a passive margin setting.rnZircon grains from the diamictite of the Ibia Group yielded ages ranging from 936 to 2500 Ma. In contrast, the overlying calc-phyllite of the Rio Verde Formation reveals a dominant Neoproterozoic provenance pattern with important peaks at 665, 740 and 850 Ma. The Sao Francisco-Congo Craton and Goias Magmatic Arc are, most probably, the two main source regions for the Ibia Group which may represent, therefore, a former fore- or back-arc sedimentary sequence. Tectonically, therefore, the Ibia Group is equivalent to the Araxa Group exposed in central Goias and both represent syn-orogenic sedimentary sequences formed with important detrital contributions derived from the Neoproterozoic Goias Arc.rnThe provenance data presented here indicate that the Cubatao Formation is most possibly representative of a Marinoan or younger glacial event.