Gnaisse Shangri-lá - Complexo Chiquitania : magmatismo de arco de ilha estateriano - Terreno Paraguá - SW do Cráton Amazônico

Abstract

The Paraguá Terrane, in the Rondonian-San Ignacio Province it is composed of paleoproterozoic basement, consisting of the Chiquitania Complex, Lomas Manechis Complex, San Ignacio Group, formed during the Lomas Manechis Orogeny. These units have intrusive contact with the Pensamiento Suite, of mesoproterozoic age, generated in the San Ignacio Orogeny. In the west of the terrane, the Sunsás Belt (̴1.0 Ga) is responsible for intense orogenic rework and N70- 50W preferential direction and granitic magmatism. The studied region is located to the north of Front Santa Catalina, in a sector of the Paraguá Terrane not reworked by the Sunsás Orogeny, thus preserving the crustal conditions come before. The Shangri-lá gneiss intrudes the remnants of supracrustal sequence, the Ricardo Franco Complex, characterized by narrow strips of NW direction amphibolite rock and partially assimilated xenolith swarms in gneiss. Petrographically the Shangri-la Gneiss has tonalitic, granodioritic and granitic composition with granoblastic, nematoblastic and lepidoblastic textures and is formed by felsic levels of quartz, alkaline feldspar, plagioclase and mafic levels composed by biotite, and sometimes, amphibole. The accessory fhases are opaque minerals, allanite, apatite, zircon and titanite; while the phases of alteration and metamorphism correspond to chlorite, muscovite, sericite, garnet, fluorite, clay minerals and epidote / clinozoizite. The deformational evolution of Shangri-lá Gneiss is polycyclic, the F1 phase is responsible for the formation of gneissic banding; phase F2 is marked by axial plane of closed intrafolial folds and transposition to N25W; phase F3 generated sinister shear zones and disjunctive cleavage with direction to N80E. Taking into consideration the metamorphic paragenesis, the studied gneiss reached conditions compatible with the amphibolite facies (quartz + oligoclase + biotite ± garnet ± hornblende) and retrometamorphic in greenshist facies (biotite + epidote + muscovite + sericite + chlorite). The protolites were formed by a predominantly peraluminous and magnesian calcic-alkaline magmatism, with evident Nb, Ta, P and Ti anomalies. Possibly formed in island arc with remnants of oceanic crust represented by Ricardo Franco Complex amphibolites. These rocks must have been formed during the Lomas Manechis Orogeny (1.76-1.64 Ga), being, subsequently reworked, tectonically to the San Ignacio Orogeny (1.37-1.30 Ga). This rework generates intense magmatism, corresponding to Pensamiento Suite, and also marks the Paraguá Terrane agglutination to the Amazon Proto-Craton.