The Mesoproterozoic to early Neoproterozoic passive margin Lajeado Group and Apiaí Gabbro, Southeastern Brazil

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 Apiaí 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 Apiaí 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 Apiaí 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 Apiaí 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. Graphical abstract Display Omitted Highlights ? The Apiaí Gabbro intruded the top units of the Lajeado Group at 877?±?8?Ma. ? Youngest detrital zircons of 1400–1200?Ma constrain the maximum Lajeado deposition age. ? Detrital zircons show main Palaeoproterozoic and minor Archean and Mesoproterozoic sources. ? The Lajeado Group represents a carbonate platform passive margin of a Mesoproterozoic continent. ? The mafic magmatism records the initial break-up of the Rodinia continent and the formation of the Brasiliano oceans.