Evolução metamórfica da formação ferrífera São João Marcos, Estado do Rio de Janeiro

Abstract

The São João Marcos Iron Formation (SJMIF) has been previously studied between Rio Claro and Mangaratiba-RJ. New occurrences of the iron formation has been found towards Southwest, until the region of Angra dos Reis and Parati, and towards Northeast, until the region of Cacaria/Monumento, following the trend NE-SW. At least two deformational events have affected the rocks studied in this work, Dn and Dn+1, to which four metamorfic stages are related. The oldest (E1) marks the entrance in granulite facies conditions, followed by a second retrometamorfic assemblage (E2) (main stage) of granulite to amphibolite facies. In the final moments of E2, the cooling occurred under relatively constant pressure, in a isobaric cooling path (IBC). The third stage (E3), possibly related to orogenic colapse, is marked by textures that indicate isothermal decompression (ITD path). Finally, a fourth stage (post-Dn or sin-Dn+1) (E4) is characterised by higher metamorphic grade, possibly linked to a new collision. Thermometric calculations for E1 using clinopyroxene composition have reached an average of 792.22°C. For the beggining of E2, it has been obtained an average of 801°C and 5.59 Kbar, being very close to E1. For the final moments of E2, it has been calculated 566.77°C and 4.16 Kbar. There were not thermobarometers for E3 nor E4. Regarding geochronology, the metamafic-metaultramafic rocks have presented paleoproterozoic-archean ages of provenance, and subordinate neoproterozoic. Crystalisation of their protolith has been calculated as 641±2.9 Ma, as well as 3 groups of metamorphic ages: 622-613Ma; 599-598Ma and 587-586Ma. On Other hand, metassedimentary rocks associated with the SJMIF present detritic grains since Archean to Neoproterozoic, with maximum sedimentation ages in 686 Ma, and 3 groups of metamorphic ages: 647-640Ma; 609-605Ma and 599Ma. The E1 stage would be linked to the 647-640 Ma metamorphism, which is contemporary to the crystalization of the metamafic-metaultramafic rocks, while E2 and E3 would be associated to metamorphic events between 622-598Ma. These metamorphic events would be related to pre-collisional to tardi-collisional stages of the development of Southern Brasília belt. Finally, E4 would be linked to the youngest 586Ma metamorphism, which enunciates a collisional stage of Ribeira Orogen.