Contribution of SHRIMP U-Pb zircon geochronology to unravelling the evolution of Brazilian Neoproterozoic fold belts

Abstract

The South-American continent is constituted of three major geologic-geotectonic entities: the homonym platform (consolidated at the end of the Cambrian), the Andean chain (essentially Meso-Cenozoic) and the Patagonian terrains, affected by tectonism and magmatism through almost all of the Phanerozoic. The platform is constituted by a series of cratonic nuclei (pre-Tonian, fragments of the Rodinia fission) surrounded by a complex fabric of Neoproterozoic structural provinces.Two major groups of orogenic processes (plate interaction cycles) constitute the evolution of these provinces: the older occurred in the Tonian (smaller in area) and the younger, Brasiliano that is present in all provinces. The Tonian cycles (pre-Rodinia fission?) are still being sorted out and many questions still need to be answered.The Brasiliano orogenic collage events (post-Rodinia fission?) developed in three main stages, in part coeval from a province to another, and are 650-600, 580-560 and 540-500 Ma, respectively (the late event reaching the Ordovician). The first group of orogenies is recorded in practically all provinces. The third group is restricted to part of the Mantiqueira Province (southeast of the platform, Búzios Orogeny) and present in the Pampean province (SW of the platform). For all these groups of orogenic events there are considerable records of rock assemblages related to processes of convergent plate interaction, opening, accretion, collision and further extrusion. There is a good correlation between the geologic and geotectonic data and geochemical and isotopic data. The late tectonic processes (post-orogenic magmatism, foreland basins, etc.) of the first two groups compete in time, in distinct spaces, with the peak of orogenic processes in the third group.The introduction of the SHRIMP U-Pb methodology was fundamental to separate the Tonian and post-Tonian orogenic groups and their respective divisions in time and space. Thus, there are still many open points/problems, which lead to expectations of addressing these issues in the near future with the more intense use of this methodology.