Geochronology and structural and petrological features of the Bravo Pluton, Central Domain of the Borborema Province, Northeast Brazil: an early trans-alkaline granite in the post-collisional stage of the Brasiliano Orogeny
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ABSTRACT: The Bravo Pluton (State of Paraíba) is an ellipsoidal stock formed by porphyritic monzo/syenogranites, dioritic enclaves and their hybrid products. The granitic body is intruded in paleoproterozoic gneiss-migmatitic rocks of the Central Domain of the Borborema Province. The monzo/syenogranites are metaluminous to slightly peraluminous and exhibit high K2O/Na2O > 1.5 and FeOt/(FeOt + MgO) > 0.86. The diorites exhibit high contents of Zr (> 1,134 ppm), TiO2 ~ 1.6% and Nb > (49 ppm). (La/Yb)N is in between 14 and 19.4 and (Eu/Eu*)Nbetween 0.31 to 0.37. In the spiderdiagram, the data are moderately fractionated with peaks in La, Zr and strong negative anomalies in P, Ti and less in Sr. They plot in the field of post-tectonic/intraplate granites in the tectonic discriminant diagram, with some scattering. The estimated pressure (4.4 to 6.0 kbar) based on Alt content in amphibole suggest a range from the middle to the upper crust for the emplacement of the Bravo Pluton. The liquidus temperature according to Zr and SiO2 contents range from 847 to 893°C and for the crystallization, calculated by amphibole-plagioclase pairs range from 589-867°C. They show TDM model ages ranging from 2.35 to 2.18 and εNd(580 Ma) = -18.32 to -17.03. A crystallization age of 581 ± 2 Ma was obtained for the granite from LA-MC-ICP-MS U-Th-Pb method in zircon. The relationship between the crystallization age (~ 580 Ma), A-type geochemical characteristics of the granitoids, Nd model-ages greater than 2.1 Ga and association with the strike-slip regime contrasts with other similar granitoids, furthermore, slight younger in age (~ 570 Ma), which occur in Central, Rio Grande do Norte Domains and in eastern Nigeria. In this sense, the transition of the compressional to the directional regime (with an extensional component) has occurred just before in the case of this body showing an episodic and diachronic character of the Brazilian orogeny. We conclude that Bravo Pluton constitutes a good example of trans-alkaline post-collisional magmatism within Central Domain which together with other examples from Rio Grande do Norte Domain and in eastern Nigeria suggest a convective thinning of subcontinental lithosphere between 580-570 Ma during amalgamation of Gondwana Supercontinent.