Geologia do Granito Silva Jardim (RJ): Implicações na evolução tectônica dos terrenos Oriental e Cabo Frio

Abstract

Silva Jardim Granite is a monzogranitic pluton that crops out at the limit between the Cabo Frio and Oriental terranes, southeast Ribeira Belt, in the municipalities of Silva Jardim and Rio Bonito (Rio de Janeiro State). Its an intrusive igneous body coarse grained with a widespread porphyritic texture, and two main lithofacies: the porphyritic, which predominates in the área, and a fine grained with sparse phenocrysts, rich in matrix. Generally the lithotype shows no orientation, with randomly oriented phenocrysts, but can be also oriented, with a magmatic flow foliation determined by the phenocrysts, usually parallel to the tectonic foliation of the wall rocks. The main wall rock is a Neoproterozoic garnet biotite orthogneiss from the Oriental Terrane, but at the north-northeast portion of the region, the pluton cross cuts the contact between these gneisses and a biotite orthogneiss from the Cabo Frio Terrane. Based on U-Pb geochronology on zircons, we present here an age of 505 Ma (+ 2 Ma) interpreted as the crystallisation of the Silva Jardim Granite, coherent with the fact that it cross cuts the contact between both terranes that initiated collision at ca. 510 Ma. Therefore, although Silva Jardim Granite is texturally and compositionally similar to the syncollisional granitoids of the Oriental terrane, it is actually a late-collisional body, related to D4 deformational phase from the Cabo Frio Terrane (505 to 490 Ma). This partially explains the regular igneous oriented flow, following the wall rocks foliation, and responding to the field of stresses related to the final episodes of the Cabo Frio Terrane collision, responsable for its late docking into the Ribeira Belt during the Cambrian.