Brazilian Journal of Geology ()
Magmatic and tectonic evolution of the Chaval Granite at the end of the Neoproterozoic, northwestern border of the Borborema Province
Abstract
Abstract The northwestern region of Borborema Province is represented by the Ceara Central and Northwest Ceara crustal blocks connected by an extensive transcurrent shear zone that represents the northern portion of the Transbrasiliano Lineament in a complex geological context, which brings together geological units of nature, origin, and ages of the Archean to the Paleozoic. In this scenario, there is a large amount of granitic bodies, with different natures, age and tectonic environment, but predominantly representing a more intense granitogenesis in the Paleozoic Neoproterozoic and early Paleozoic, with plutons emplaced in different stages of the Brazilian Orogeny. In this context, the Chaval Granite corresponds to an isolated body in the extreme northwest of Borborema Province, located near the Atlantic coast, in the northern states of Ceará and Piauí. It is an intrusive batholith housed in Siderian orthogneisses (Granja Complex) and in Neoproterozoic supracrustal rocks (Martinopole Group), but is partly covered by sedimentary rocks from the Parnaíba Paleozoic Basin (Serra Grande Group), and coastal Cenozoic deposits. Field data and petrographic studies highlight the porphyritic texture with microcline megacrystals involved in coarce matrix as a remarkable feature. The predominant petrographic types are granodiorites, with variations for monzogranites and tonalites. The predominant primary mineral constituents are pertitic microcline, oligoclase, and quartz; biotite and rarely hornblende as qualified minerals and additionally titanite, apatite, zircon, alanite, and opaque minerals. Another peculiar characteristic is the deformational features related to the installation of the Santa Rosa Transcurrent Shear Zone along its eastern flank. The effects of the transcurrent shear deformation led to the modification of the original magmatic fabrics in much of the eastern half of the batolith, generating various types of mylonites. Thus, the typically igneous textures preserved in the western half of the pluton were gradually replaced by tectonic fabrics, which initially evolved into proto-mylonites in the central portion of the body, grading to mylonites eastward, highlighting strongly stretching, comminution associated with dynamic recrystallization, highlighting the formation of microcline and plagioclase porphyroclasts in a mylonitic matrix, which are involved by mylonitic foliation. Geochemical studies reveal compositional similarities, compatible with petrographic classifications, in which they mostly present granodioritic composition, followed by monzogranites and tonalites, classified as I-type granites, peraluminous with compatible with the calcium-alkaline series. The geochemical signatures of the Chaval Granite indicate the character of the tectonic magmatic arc environment. U-Pb zircon analysis by Mass Spectrometry (LA-ICP-MS) indicates crystallization age of 633 Ma, placing it in the Neoproterozoic, late Cryogenic-Early Ediacaran period, being one of the oldest granitoids in northwestern Borborema Province, correlated to the granites of the Santa Quitéria Magmatic Arc in the Ceara Central Domain. Hf-TDM model ages (2.65 to 2.13 Ga) and εHf(t = 633 Ma) values from -9.6 to -18.1 suggest incorporation of neoarchean and paleoproterozoic crustal sources in their formation with a long crustal residence time. Similar Sm-Nd data in whole-rock indicate Nd-TDM model ages of 1.27, 1.72 and 2.04 Ga and negative εNd(t = 633 Ma) values of -2.64 to -9.13, indicating paleoproterozoic and mesoproterozoic sources, with considerable crustal residence time implying a more evolved nature.
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