Estudo da Faixa Granulítica entre Rio Claro e Mangaratiba e seu significado no contexto da Zona de Interferência entre as Faixas Brasília e Ribeira

Abstract

The study area is related to the evolution of the central segment of the Ribeira Belt, which would be the result of a collision between distinct terranes, in two major events, the first between 595 and 570 Ma and the second in approximately 525 Ma. However, recent studies raise the hypothesis of this region being associated with the evolution of other neoproterozoic mobile belts, such as the Southern Brasilia and Dom Feliciano. In this new context presented the objective of this work is to study the rocks of the Ponte do Zinco granulite unit and other associated rocks, in order to verify if the area is inserted in the interference zone between the Brasília and Ribeira Belts. The region studied was mapped by company works and academic works between 1978 and 1995 and was inserted in regional works at the end of the year 2000. The research carried out here is based on geological mappings in 1:50,000 scale, petrographic and geochronological analyzes what resulted in new data that allows an interpretation of one different evolutionary history of the traditional works and corroborating with the geodynamic models presented recently. The identified metasedimentary rocks were separated into two different depositional sequences: the oldest with a maximum deposition age of 1.2 Ga, composed of biotite-gneiss with amphibolite intercalations and layers of quartzite, calcissilicatic and metaultramafic rocks closely associated in centimetric to metric intercalations; the youngest sequence presents a maximum deposition age of approximately 700 Ma, being composed of (Opx) -Sill-Grt-Bt-Gneiss with intercalation of biotite-gneiss and feldspathic quartzite, amphibolite, metaultramafic and calcissilicatic rocks, as well as iron formation possibly associated with metabasic rocks. Despite the difference in the deposition period, both sequences have similar source area, with contribution in similar proportions of archaean, paleoproterozoic and mesoproterozoic rocks and the youngest contribution of neoproterozoic rocks. The two metasedimentary sequences and the basement rocks presents a record of three metamorphic events: 640 Ma, associated with the evolution of a magmatic arc; 625 and 610 Ma, contemporary to the main phase of deformation and early to sin-collisional granitogenesis and; 580 Ma. The main metamorphic event occurred in amphibolite facies with intermediate pressure, recorded in all units mentioned above, being superimposed to a metamorphism of granulite facies, recorded in the younger depositional sequence and in the rocks of the basement. The main deformation phase generated metamorphic foliation, locally forming mylonites in shear zones, with intermediate to locally steep dipping for NW, with NE lineation and indication of top oblique movement towards NE. These metamorphic, structural and geochronological characteristics are similar to the evolutionary history observed in the Socorro-Guaxupé Nappe, a tectonic unit associated with the evolution of the Southern Brasília Belt. The younger metamorphism presents ages that are compatible with the first metamorphic event of the Ribeira Belt, thus corroborating with the recent model that inserts this region in the Interference Zone between the Brasilia and Ribeira Belts, specifically in the upper plate domain during the processes of convergence and continental collision, being the southernmost suture of the studied area. Later, there was the intrusion of gabbroic dykes, sub-horizontal granite bodies, faulting and diabase dikes.