Navegando por Assunto "Sistema transcorrente"

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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)
Organização lito-estrutural do duplex Salobo-Mirim Serra dos Carajás
(Universidade Federal do Pará, 1990-06-06) SIQUEIRA, José Batista; COSTA, João Batista Sena; http://lattes.cnpq.br/0141806217745286
The Salobo Group embodies a sequence of volcanosedimentary rocks, which overlain a sialic basement, and is linked to the evolution of the Cinzento Transcurrent System, situated immediately to the north of the transcurrent system which provided the estabelishment of the Grão-Pará Group. The macrostructure is formed by two major shear zones which join at the west and east end parts of the studied area, defining a WNW-ESE eliptcal structure. This structure, which is interpreted as an extensional duplex, comprises, internally, minor shear zones that converge to the main zones. NW-SE, NNE-SSW and NE-SW strike-slipe zones modify the major geometric pattern. The duplex lithologies underwent metamorphism of the amphibolite facies during regional transpression, followed by expressive hidrotermalism and greenschist facies transformation trought the progress of the deformation. Significant mineralizations of bornite, chalcocite and in some cases chalcopirite, with molibdenite, gold and silver, are mainly hosted by lenses of iron formations, besides magnetite. These sulphides are distrubuted as inclusions in the grains of silicate gangue minerals and oxides or between them, as well as along the ductile and ductile-ruptile features, such as milonitic S-C foliations and R, R', p and y=D fractures, showing the manifold origin of such mineralizing fluids. Two kinematics models can be visualized to explain the main litho-structural units: sinistral stike-slip system involving transtension, followed by transpression and then transtension; and crotonic reworking related to transtension regime.