Deformational, crystallographic and petrochronological analyses of the mylonites from Taxaquara shear zone in Ribeira Belt (São Paulo)

Author(s)

Bruno Vieira Ribeiro

• Document type: Master's Dissertation
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto de Geociências (IG/BT)
• Defense date: 2019-11-18

Examining board members

Ginaldo Ademar da Cruz Campanha; Sergio Wilians de Oliveira Rodrigues; Luis Gustavo Ferreira Viegas

• Advisor: Ginaldo Ademar da Cruz Campanha

Abstract

The Taxaquara Shear Zone (TSZ) is a major transcurrent structure in the Ribeira Belt (RB), SE Brazil. In the study area, around Pilar do Sul city in the countryside of São Paulo, the TSZ display a strong NE-SW mylonitic foliation preferentially dipping to W and stretching lineation with variable plunge. It separates the Votuverava and the São Roque Groups and cross-cut granitic rocks from the Pilar do Sul and Piedade Suits giving rise to quartz-feldspathic mylonitic rocks with newformed matrix assemblage composed of quartz + oligoclase + Kfeldspar + biotite + muscovite + rutile + magnetite. Pseudosection with Si (p.f.u) in muscovite and sodium molar number in plagioclase (\'X IND.Na\') isopleths, couple with muscovite-biotite geothermometer, indicates that the mineral assemblage constrains the deformation condition at 513 - 525°C and 3.9 - 4.4 kbar. Microstructures and crystallographic preferred orientation (CPO) indicate that quartz from monomineralic ribbons accommodates deformation through dislocation creep achieving the transition between subgrain rotation and grain boundary migration, at a mean strain rate of 10-12 to 10-12 s-1, with monoclinic CPO patterns and activation of \'basal-\' + \'rhomb-\' slip system during deformation. CPO from fine-grained quartz and feldspar from polyphase matrix, however, indicate a switch in deformation mechanism from dislocation to diffusion creep. Vorticity and three-dimensional finite strain data indicate that the TSZ was developed under moderate to high strain within a transpressional sub-simple shear environment. Petrochronological analyses using muscovite 40Ar/39Ar (step heating) and apatite, titanite and zircon U-Pb ages (LASS - ICP - MS) indicates that the TSZ was active from 544 ± 4.0 Ma to 534 ± 1.5 Ma, continuously or not. These ages, coupled with closure temperature estimates, suggest a cooling rate of ~5°C/Ma as indicated by regional studies. The geochronological data from the TSZ contrast with those from other shear zones in the RB, generally constrained by single mineral dating lacking textural and chemical data between the mineral and deformation. Thus, a detailed revision using multi-mineral method approach is required to improve the understanding the tectonic significance of the shear zones in the RB. (AU)

• FAPESP's process: 18/00320-9 - Crystallographic, deformational and thermocronological analysis of mylonites of Taxaquara shear zone (São Paulo)
• Grantee: Bruno Vieira Ribeiro
• Support Opportunities: Scholarships in Brazil - Master