Geologia, petrografia e geocronologia do Plúton Alcalino Brejinho - nordeste do Piauí / noroeste do Ceará

Abstract

The present work reports geological, petrographic and geochronological data of the alkaline pluton Nefelina Sienito Brejinho. The Nefelina Sienito Brejinho is inserted in the northwest portion of the Borborema Province, geographically located on the border of the states of Piauí and Ceará, approximately 60 km southeast of the city of Parnaíba. The pluton has approximate dimensions of 16 km long and 8 km wide and has a roughly triangular shape, elongated in the NNE direction. It is a typically igneous body, where its intrusive character was evidenced in gneisses and amphibolites of the Granja Complex, with cooling edges, xenoliths and small zones of hornfels. In general, the rocks along the plutonic body present magmatic feeding structures defining a preferential orientation in the NNE-SSW direction marked by tubular alkaline feldspar crystals, prismatic pyroxenes, lamellar biotites and, in some cases, this foliation is also evidenced by amphiboles and titanite. The petrographic and field studies allowed identifying three petrographic facies internally in the pluton: Pyroxene Microclimate Syenite, Pyrogen Nepheline Syenite and Leucosienites. The first two are dominant, and the other has restricted occurrences. Mineralogically the pluton presents a variation, qualitatively and quantitatively. Rocks in general consist essentially of microclimate, nepheline and aegirine-augite as essential minerals. In smaller percentages occur biotite, amphibole (arfvedsonite and riebeckite) and titanite. Accessory to apatite, opaque and fluorite. With regard to textural aspects, trachytoid textures are predominant in the pluton, defined by crystals of alkaline feldspar, pyroxene, biotite, titanite and amphibole. Subordinately, simplectitic intergrowth textures between alkaline and nepheline feldspar are present. This textural aspect suggests subsolvus crystallization for Nefelina Sienito Brejinho. The geochronological studies, carried out using the Rb-Sr methodology in eight rock samples, provided the Nefelina Sienito Brejinho with an age of 545 ± 11 Ma, with an initial ratio of 0.7173 ± 0.0002 and MSWD of 1.32. Based on this age, interpreted as the minimum crystallization age, it is suggested that the syenitic pluton was placed in the Lower Cambrian, during the distensive tectonics that occurred in this portion of NE Brazil. This tectonic was marked by the implementation of the Ubajara-Jaibaras System and by plutonism and volcanism, with an alkaline tendency, such as the Meruoca, Mucambo, Pajé, Anil, Morrinhos and São Paulo granites, among others; and the alkaline volcanism of the Parapuí Suite.