Petrografia, geoquímica e geocronologia do granito Chaval, noroeste da Província Borborema

Abstract

The northwestern region of the Borborema Province is represented by at least two crustal blocks (Central Ceará and Middle Coreaú) connected by the Transbrasiliano Lineament in a complex geological context, joining geological units of nature, origin, and ages that vary from the Archean to the Paleozoic. In this context, a large amount of granite bodies of diverse nature and ages usually a product of more intense granitogenesis in the Neoproterozoic emplaced in different stages of the Brasilian/Panafrican Orogeny stand out. The Chaval Granite in the northwestern edge of the Boroborema Province outcrops near on the Atlantic coast between Ceará and Piauí. It is exposed in a batholith body with intrusive relationship in Paleoproterozoic orthogneisses of the Granja Complex and Neoproterozoic supracrustal rocks of the Martinópole Group, covered by the sedimentary rocks of the Paleozoic Parnaíba Basin (Serra Grande Group) and recent coastal sedimentary deposits. Field data and petrographic studies highlight the porphyritic texture as a remarkable feature due to the presence of microcline megacrysts (centimeter scale) immersed in the coarse phaneritic matrix. Granodiorites with tonalitic and monzogranitic variations predominate in the body. Another peculiar characteristic are the deformational features related to the Santa Rosa Shear Zone placement whose transcurrent shear effects caused the magmatic fabric to change in much of the eastern portion of the body, generating mylonitic rocks. Thus, the typically igneous textures are gradually replaced by a tectonic fabric, initially evolving into protomylonites, in the body central and eastern portions, evolving into mylonitic granites, with strong deformation and comminution of the minerals constituting the matrix and stretching of the porphyroclasts, originally megacrysts. The geochemical studies reveal compositional similarities compatible with the petrographic classifications, consisting especially of granodiorite, followed by monzogranites and tonalities. The geochemical data also indicate the affinity of peraluminous type I granites with metaluminous tendency and calcium-alkaline series. The geochemical signatures indicate that the Chaval Granite is compatible with normal-type magma arc granites. The U-Pb zircon analysis indicates 632 Ma as the crystallization age, indicating placement in the Neoproterozoic, at the end of the Cryogenic period, making it one of the oldest granitoids in the region compatible with the granitoids of the Santa Quitéria Magmatic Arc, in Ceará Block. The Sm-Nd isotopic studies in total rock show model ages (TDM) between 1.27 and 2.04 Ga, and negative ɛNd values indicating Paleoproterozoic and Mesoproterozoic sources, with considerable time of crustal residence implying a more evolved nature.