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

作者：Rodrigues J B^*; Pimentel M M; Dardenne M A; Armstrong R A

来源：Journal of South American Earth Sciences, 2010, 29(2): 512-521.

DOI：10.1016/j.jsames.2009.08.008

• Provenance
• Zircon
• Brasilia Belt
• Ibia Group
• Canastra Group
• TDM
• Neoproterozoic glacial event

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.
A 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.
LAM-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 (similar to 1.8 and similar to 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 TOM values are consistent with the previous interpretation that the Canastra Group represents a sedimentary sequence deposited on a passive margin setting.
Zircon 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.
The provenance data presented here indicate that the Cubatao Formation is most possibly representative of a Marinoan or younger glacial event.

• 出版日期2010-3