Navegando por Assunto "Granito Chaval"

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Aspectos texturais do magmatismo e tramas da tectônica impostas ao Granito Chaval na Zona de Cisalhamento Santa Rosa, extremo Noroeste da Província Borborema
(2014-12) GORAYEB, Paulo Sérgio de Sousa; LIMA, Aline Maria Meiguins de
The Northwestern region of Borborema Province features a wide variety of granitic bodies with diverse nature and tectonic evolution, since the Paleoproterozoic to Early Paleozoic, presenting higher incidence related to Neoproterozoic emplaced at different stages of Brasiliano (Panafrican) orogeny. One of these examples is the Chaval Granite representing a batholith body intruded in orthogneisses of Granja Complex and supracrustal rocks of Martinópole Group, outcropped near the Ceará and Piauí Atlantic coasts, Northeastern Brazil. The batholith is partly covered by Cenozoic coastal deposits and sedimentary rocks of the Paleozoic Parnaíba Basin. The Chaval Granite has a striking textural feature, which is represented by porphyritic highlighting microcline megacrysts in syenogranites and monzogranites, and many others textural/structural features of magmatic origin. They allowed interpreting the plutonic evolution at relatively shallow crustal level emplacement acting processes of fractional crystallization, magma mixing with intense magmatic flow, and gravitational force due to density difference of magma that lead to fluctuation of megacrysts of microcline and plagioclase in the residual magma, and accommodation of small bodies of leucogranite and pegmatites in the final stages of plutonic magmatic evolution of such plutonism. On the other hand, across the Eastern half of the pluton, there is a rich collection of shear structures related to the tectonic construction of the Santa Rosa Transcurrent Shear Zone, which led to tectono-metamorphic transformations superimposed on magmatic features that reached peak metamorphic conditions in the low amphibolite facies. Cartographically, three homogeneous structural domains were individualized, and each of them presents a variety of petrographic-structural features of the Chaval of igneous or tectonic nature. The plutonic rocks were deformed and progressively modified Eastward, in which the rocks change to darker gray, and comminution and dynamic recrystallization processes are reduced, as well as the coarse-grained granites are progressively, as well as the size of the phenocrysts to thinner dimensions, keeping their porphyroid characteristics, being accentuated the mylonitic features to achieve the main portion of Santa Rosa Transcurrent Shear Zone. As main structural features, there is the ondulatory extinction, cleavage and twining bending, twinning deformation, anastomosing foliation, badinage, S-C foliation, mineral stretching lineation, rotated and segmented crystals, lenticular, almondy or fish forms of porphyroclasts, pressure shadow, and ribbon quartz. The products of this shear deformation result in the formation of protomylonites, mylonites, and ultramylonites. These mylonite zones represent areas with the highest concentration of deformation that can gradually follow their textural and mineralogical changes, setting up a classic sequence of progressive heterogeneous deformation by simple shear in brittle-ductile and ductile conditions. The emplacement of Chaval Granite happened at the end of the Cryogenian (approximately 630 Ma) and can be interpreted as syn-tardi tectonic magmatism regarding the Brasiliano event. The process that generated the Santa Rosa Shear Zone resulted from lateral extravasation crustal masses in ductile flow took place in the final increments of deformation of a continental collision on an oblique thrust system, which built the Northwest Ceará Shear Belt, and happened at the end of Brasiliano (Panafrican) orogeny in Northwest Borborema Province.
Acesso aberto (Open Access)
Petrografia, geoquímica e geocronologia do granito Chaval, noroeste da Província Borborema
(Universidade Federal do Pará, 2018-04-17) ARAGÃO, Arthur Jeronimo Santana; GORAYEB, Paulo Sérgio de Sousa; http://lattes.cnpq.br/4309934026092502
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.