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 Ceara and Piaui. 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 Parnaiba 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 Quiteria Magmatic Arc in the Ceara Central Domain. Hf-TDM model ages (2.65 to 2.13 Ga) and eHf(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 eNd(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.
Highlights
The Chaval Granite, the main object of the present study, outcrops near the Atlantic coast, north of Piauí and Ceará States, having as reference Chaval, Parnaíba, and Bom Princípio cities. It is a batholith of approximately 2,000 km2, inserted in the Northwestern Ceará Belt (NWCEB) (Abreu et al 1988), in the Northwestern Ceará Domain, on the northwest border of Borborema Province (Figs. 1 and 2)
Geochronological and geochemical data allow us to conclude that the Chaval Granite occupies a vast outcrop area, featuring a batholith of more than 60 km wide, representing the most important magmatic body of the Northwest Ceará Domain
Its magmatic evolution with housing in plutonic conditions at medium crustal level may be related to a magmatic arc environment at the end of Neoproterozoic, whose southwest continuity hides under sedimentary cover (Parnaíba Block) (Brito Neves et al 1984, Daly et al 2014)
Summary
The Chaval Granite, the main object of the present study, outcrops near the Atlantic coast, north of Piauí and Ceará States, having as reference Chaval, Parnaíba, and Bom Princípio cities. The northwestern portion of the Borborema Province (BP), presents a wide variety of rocks from Archean to Paleozoic, with an emphasis on granite magmatism, highlighting bodies of varying sizes, natures, and ages, which are temporal markers of regional geological processes
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