Microstructural evolution of the Seridó Belt, NE-Brazil: the effect of two tectonic events on development of c-axis preferred orientation in quartz |
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Authors: | M rio Neto Cavalcanti de Araú jo, Fernando C sar Alves da Silva, Emanuel Ferraz Jardim de S , Rodney J. Holcombe,Paulo Marcos de Vasconcelos |
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Affiliation: | a Departamento Geologia e Recursos Naturais, Instituto de Geociências, Universidade Estadual de Campinas, P.O. Box 6152, Campinas, SP 13083-970, Brazil;b Pós-Graduação em Geodinâmica e Geofísica, Universidade Federal do Rio Grande do Norte, Nata, RN-, Brazil;c Department of Earth Sciences, The University of Queensland/Brisbane, Queensland, 4072, Australia |
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Abstract: | The polyphase evolution of the Seridó Belt (NE-Brazil) includes D1 crust formation at 2.3–2.1 Ga, D2 thrust tectonics at 1.9 Ga and crustal reworking by D3 strike-slip shear zones at 600 Ma. Microstructural investigations within mylonites associated with D2 and D3 events were used to constrain the tectono-thermal evolution of the belt. D2 shear zones commenced at deeper crustal levels and high amphibolite facies conditions (600–650 °C) through grain boundary migration, subgrain rotation and operation of quartz c-prism slip. Continued shearing and exhumation of the terrain forced the re-equilibration of high-T fabrics and the switching of slip systems from c-prism to positive and negative a-rhombs. During D3, enhancement of ductility by dissipation of heat that came from syn-D3 granites developed wide belts of amphibolite facies mylonites. Continued shearing, uplift and cooling of the region induced D3 shear zones to act in ductile-brittle regimes, marked by fracturing and development of thinner belts of greenschist facies mylonites. During this event, switching from a-prism to a-basal slip indicates a thermal path from 600 to 350 °C. Therefore, microstructures and quartz c-axis fabrics in polydeformed rocks from the Seridó Belt preserve the record of two major events, which includes contrasting deformation mechanisms and thermal paths. |
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Keywords: | Microstructures Deformation mechanisms Mylonites Quartz c-axis fabrics |
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