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Deformation mechanisms and chronology of events in the Patos shear zone (Borborema Province, NE Brazil

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Luis Gustavo Ferreira Viegas
Total Authors: 1
Document type: Doctoral Thesis
Press: São Paulo.
Institution: Universidade de São Paulo (USP). Instituto de Geociências
Defense date:
Examining board members:
Carlos Jose Archanjo; Marcos Egydio-Silva; Leonardo Evangelista Lagoeiro; Ruy Paulo Philipp
Advisor: Carlos Jose Archanjo

The Patos shear zone is an E-W transcurrent structure that deformed Precambrian rocks from the Borborema Province during the Neoproterozoic. The relationships between partial melting and deformation, the nucleation of the shear zone and the thermochronological evolution of mylonites were investigated by means of detailed field work coupled with Anisotropy of Magnetic Susceptibility (AMS), U-Pb SHRIMP geochronology and crystallographic fabrics measured through electron backscatter diffraction (EBSD). In the Santa Luzia dome, located within the Seridó Belt, the magnetic lineations are slightly oblique to the dome NE elongation. Zircon recrystallized rims yield U-Pb ages of 575 ± 3.4 Ma. These data are consistent with ages and magnetic fabrics found in the Acari granite, suggesting that magma emplacement and shearing were coeval in the Seridó belt. In the Espinho Branco anatexite, emplaced within the mylonites of the Patos shear zone, complex field leucosome geometries are not reproduced in the magnetic fabric, which is consistent with the kinematics of the shear zone. However, low susceptibilities (< 0.5 mSI) display a scattering of magnetic orientations and do not correlate with biotite lattice fabrics and field foliations. Nevertheless, anisotropy of anhystheretic remanence fabrics remain parallel to the external strain field, suggesting that AMS dispersions may be related to late percolation of hydrothermal fluids. Mylonitic orthogneisses from the northern border of the Patos shear zone are marked by grain boundary migration, intracrystalline fracturing and myrmekitization at the periphery of grains. Quartz [0001] lattice fabrics form maxima mainly on Y, suggesting activation of prism-a slip systems. Towards the contact with migmatites, the microstructures progressively change from solid-state to magmatic, characterized by widespread interstitial quartz and random orientations of quartz c-axis fabrics. Zircons from the leucosomes yield an age of 566 ± 5 Ma, which is attributed to magma crystallization. The southern border of the Patos shear zone is marked by progressive grain size reduction and formation of fine-grained mylonites to ultramylonites without traces of partial melting. An age of 545 ± 14 Ma, obtained in zircons from an ultramylonitic augen granite, constrains the timing of the low-grade metamorphism. These Southern mylonites show quartz [0001] fabrics with maxima spreading between Z and Y, suggesting activation of prism and basal slip systems. In feldspars, the main activated slip systems are (010)[001] and (010)[100] in all rock types, except for the (100)[010] slip system, observed in the transition from migmatites to low-grade mylonites. TitaniQ temperature estimates record mean temperatures of ~ 490ºC for the southern mylonites. These data allows us to conclude that: i) AMS reflects the final strain increments recorded in migmatites during regional deformation, ii) partial melting and dextral shearing in the Patos shear zone occurred at ~ 565 Ma, while low-grade mylonitisation dated at 545 Ma defines a late reworking at ductile-brittle conditions, iii) this reworking event is observed in other areas of the Borborema Province, defining a regional event possibly associated with late collisions in the western margin of Gondwana, iv) the Patos shear zone was nucleated after the collision between the Congo-São Francisco and West-African Cratons, through accommodation of strike-slip displacements localized in crustal discontinuities previously formed in an early pre-collisional stage. (AU)

FAPESP's process: 09/17537-1 - Deformation mechanisms and event chronology in the Patos shear zone, Borborema Province (Northeastern Brazil)
Grantee:Luís Gustavo Ferreira Viegas
Support type: Scholarships in Brazil - Doctorate