Caracterização estrutural e geocronológica da sequência metassedimentar no limite entre os terrenos Cabo Frio e Oriental, Região dos Lagos Fluminense

Abstract

The collisions documented in the central area of the Ribeira Fold Belt are marked by a singular deformational history, caused by the overlapping of several thrust sheets that comprises four tectonostratigraphic terranes. The collision between the Cabo Frio and Oriental terranes resulted in a complex structure of the rocks located at the boundary between these two terranes. This border is marked by a thrust contact between the Paleoproterozoic basement, constituted by the remnant portion of the Angola Craton on the South American continent called Região dos Lagos Complex, and a Neoproterozoic metasedimentary sequence. This thrust presents two senses of movements: NW and E-NE. The structural analysis in the southwest portion of this boundary demonstrates that the variation in the attitude of the thrust is occasioned by a sequence of folding, resulted from a continuous crustal shortening with the direction of transport to NW. Three deformation phases were described, the phase D1 was interpreted as associated with the basement placement on the metasedimentary sequence, while the D2 phase was responsible for the main structuring of the area. An antiformal structure was developed in the early/sin-D2 stages, and then refolded in the late-D2 stage, during this period the linear and the planar elements (which were previously generated) have rotated to a new position. During the D3 phase, a series of irregular folds were developed in the metasedimentary rocks, besides NE-SW shear zones. The responsible factor for the main geometry is inferred as related to the batholith granitic rocks of the Tinguí Complex, whose form and rheological contrast caused the refolding of the described antiform. Two metasedimentary sequences were individualized: cordierite-garnet-biotite gneiss, described as kinzigitic gneiss in the base, and sillimanite-biotite gneiss, called Serra do Palmital gneiss at the top. The geochronological analysis performed in the two units revealed a predominance of Neoproterozoic detrital zircon grains, which are attributed to the magmatic arcs of Serra do Prata and Rio Negro. The provenance of Mesoproterozoic and Paleoproterozoic detrital zircon grains are considered to relate to the Angola Craton. The maximum age of sedimentation identified for the two units was 623 years ± 6 Ma, from which the protoliths of the kinzigitic gneiss and sillimanite-biotite Serra do Palmital gneiss were deposited in a back-arc basin, related to the Rio Negro magmatic arc. Two sets of metamorphic ages were identified, the first with ages of ~ 590 Ma and the second one with ages around 530 Ma. The older ages are inferred as being related to the metamorphism generated during the syn-collisional stage of the Rio Negro magmatic arc, while the younger ones are related to the collision between the Oriental and Cabo Frio terranes, known as Búzios Orogeny. Based on the data obtained in this research, it is suggested that the metasedimentary sequence studied is related to the metasedimentary rocks of the São Fidélis Group, which are constituents of the Oriental terrane.