摘要
The Lajeado Group in the Ribeira Belt, southeastern Brazil, corresponds to an open-sea carbonate platform, comprised of seven overlapping siliciclastic and carbonatic formations, intruded in its upper portion by the Apiai Gabbro. These rocks have a Neoproterozoic tectonometamorphic overprint related to arc magmatism and the Brasiliano collisional orogeny. Geochronological constraints are given by new U- Pb SHRIMP and LA-ICP-MS data for Lajeado Group detrital zircons and for magmatic zircons from the Apial Gabbro. The youngest detrital zircons in the Lajeado Group are 1400-1200 Ma, and constrain its maximum age of deposition to be 〈1200 Ma, whereas the 877 ~ 8 Ma age for magmatic zircons in the Apiai Gabbro give the minimum age. Detritus source areas are mainly Paleoproterozoic （2200-1800 Ma） with some Archean and Mesoproterozoic contribution （1500-1200 Ma）, with distal or tectonic stable cratonic character. The Lajeado Group should be a Stenian-Tonian carbonate platform passive margin of a continent at this time, namely the Columbia/Nuna or the Rodinia. The Apial Gabbro displays similar age to other intrusive basic rocks in the Lajeado and Itaiacoca groups and represents tholeiitic MORB-like magmatism that we relate to the initial break-up of a Mesoproterozoic continent and the formation of the Brasiliano oceans.
The Lajeado Group in the Ribeira Belt, southeastern Brazil, corresponds to an open-sea carbonate platform, comprised of seven overlapping siliciclastic and carbonatic formations, intruded in its upper portion by the Apiai Gabbro. These rocks have a Neoproterozoic tectonometamorphic overprint related to arc magmatism and the Brasiliano collisional orogeny. Geochronological constraints are given by new U- Pb SHRIMP and LA-ICP-MS data for Lajeado Group detrital zircons and for magmatic zircons from the Apial Gabbro. The youngest detrital zircons in the Lajeado Group are 1400-1200 Ma, and constrain its maximum age of deposition to be 〈1200 Ma, whereas the 877 ~ 8 Ma age for magmatic zircons in the Apiai Gabbro give the minimum age. Detritus source areas are mainly Paleoproterozoic （2200-1800 Ma） with some Archean and Mesoproterozoic contribution （1500-1200 Ma）, with distal or tectonic stable cratonic character. The Lajeado Group should be a Stenian-Tonian carbonate platform passive margin of a continent at this time, namely the Columbia/Nuna or the Rodinia. The Apial Gabbro displays similar age to other intrusive basic rocks in the Lajeado and Itaiacoca groups and represents tholeiitic MORB-like magmatism that we relate to the initial break-up of a Mesoproterozoic continent and the formation of the Brasiliano oceans.
基金
the Fundacao de AmparoàPesquisa do Estado de Sao Paulo e FAPESP,grants 01/13457-1 and 06/01327-0,for financial support
the GeoQuEST Research Centre of the University of Wollongong for support
