Petrologia, geoquímica e geocronologia dos granitos Presidente Kennedy e Barrolândia: contextualização na evolução do Cinturão Araguaia

Abstract

Several granitic bodies have been identified in the easternmost part of the Araguaia Belt, located in the highest metamorphic grade domain (Estrondo Group). These granites are interpreted as late kinematic in relation to the main tectonic and regional metamorphism in the evolution of this belt, at the end of the Neoproterozoic. The Presidente Kennedy Granodiorite (PKG) and the Barrolândia Granite (BG), cropping out in the central portion of the Araguaia Belt, are plutons of this granitogenesis. PKG and BG are small-elongated stocks concordant with foliation of mica schist and feldspathic schist of the Estrondo Group, without metamorphic thermal contact effects. They are hololeucocratic rocks of granodioritic and monzogranitic composition, containing two micas (biotite and muscovite), and eventually garnet. In general, the granites are grayish-gray, slightly pink, equigranular, with hipidiomorfic or alotriomorfic granular textures. However, tectono-metamorphic features are common. These are the polygonal granoblastic texture and foliation defined by the preferred orientation of micas and quartz and elongated feldspars. Their essential mineralogical constituents are oligoclase, microcline, quartz, biotite and muscovite, and the accessories are zircon, apatite, opaque minerals and, rarely garnet. Lithochemical data indicate the peraluminous character of these granites, and high values of SiO 2 , Al 2 O 3 and alkalis, and low values of MgO, Fe 2 O 3T and TiO 2 . The presence of normative corundum along with petrographic and geochemical data, allows characterize them as S-type granites. The single zircon Pb-evaporation age of 539 ± 5 Ma for the PKG is interpreted as minimum age of crystallization and crustal emplacement at the end of Neoproterozoic (Ediacaran). Sm-Nd T DM model age for the PKG of 2.13 – 2.17 Ga, with negative eNd (550Ma) values (-17.58; -24.31), and of 2.11 Ga, 2.24 Ga and 1.39 Ga, for the BG, also with negative values of eNd (550Ma) (-18.75; -18.38; -11.90), suggest important contribution of Paleoproterozoic crust. Thus, the Archaean basement rocks of the Araguaia belt cannot be the unique source of this granitic magmatism. The most likely hypothesis would be the mixing of sources involving both the partial melting fusion of metassedimentary rocks from the Estrondo Group with a minor contribution of the Archean basement rocks. The accumulation of these anatectic granitic liquids, the ascent and emplacement of these magmas in the supracrustal sequences were synchronous to the Araguaia Belt’s main tectonic event at the end of the Neoproterozoic. Both PKG and BG have geological, petrographic, geochemical and geochronological similarities to the Santa Luzia and Ramal do Lontra granites, located at the south and north segment of the Araguaia Belt, respectively. This indicates a regional scale granitic magmatism related to the tectonic and metamorphic events of the main phase of Araguaia Orogen’s evolution at the end of the Brasiliano Cycle.