Home

Search Abstracts

Author Index

Symposia Programmes

Sponsors

Help

MPC-02 Geochronology of metamorphic reactions and deformation in high-grade orogenic settings

Using structural analysis and thermochronology to depict the geodynamic evolution of Ribeira Belt

Telmo Bento dos Santos, Centro de Geologia da Universidade de Lisboa (Portugal)
Paulo Fonseca, Centro de Geologia da Universidade de Lisboa (Portugal)
José Munhá, Centro de Geologia da Universidade de Lisboa (Portugal)
Colombo Tassinari, Instituto de Geociências, Universidade de São Paulo (Brazil)
Coriolano Dias Neto, Instituto de Geociências, Universidade de São Paulo (Brazil)
 

This study integrates new thermochronological data (zircon U-Pb, garnet-WR Sm-Nd, plagioclase-WR Rb-Sr, biotite-WR Rb-Sr and biotite K-Ar at respective isotopic closure temperatures of 750°C, 650°C, 500°C, 350°C and 250°C) on the central segment of Ribeira Belt (SE Brazil) with structural analysis of the Paraiba do Sul River megashear in order to constrain the thermotectonic evolution of this Panafrican granulite belt. Combining T-t informations taken from the geochronological methods and the observed structural features, we suggest that the main regional high grade thrust deformation (D₂: 250°, 55-70° NW; stretching lineation 55-65°, 5-20°) was coeval with peak metamorphism at ∼565 Ma (mean zircon U-Pb dating), post-dating earlier collision related imbrication nappe thrusts (D₁) at 630-600 Ma (oldest U-Pb dating). D₁ and D₂ strain markers were mostly erased by D₃ thrusting and long-term dextral transpressional shearing (50-65°, 70-85° NW; stretching lineation 5-15°, 172-178°), simultaneous with slow-cooling (1 to 5°C/Ma) of the orogen until ∼ 500 Ma (Garnet-WR Sm-Nd ages). Brittle, extensional, tectonic event D₄ (290-320°, sub-vertical) and thermal relaxation is associated with late granite emplacement at 490Ma (zircon U-Pb), being followed by regional tectonic collapse that resulted on rapid exhumation/cooling (∼30°C/Ma) of the high grade rocks at ∼470 Ma (Biotite-WR Rb-Sr ages).
Results suggest that a ∼35 Ma period of nearly orthogonal shortening between the San Francisco and West Congo cratons occured until 565 Ma with development of a D₂ flower thrust system, coeval with high-grade granulitic metamorphism. Afterwards, orthogonal shortening became rheologically impossible (because rocks could not absorb further shortening) and a (D₃) dextral transpressive regime became dominant, turning the flower structure asymmetric.

Specific positioning within the flower structure and strong partition of deformation induced "local" antithetical sinistral kinematics within the main regional dextral regime and differential exhumation; thus, granulites in the central axis were rapidly exhumed, whereas along the lateral branches exhumation was much slower (because of the small dip angle: 5 to 10°) resulting in very slow cooling on the lateral branches, that lasted for almost 100Ma.

FAPESP, POCA-PETROLOG (CEGUL, UI: 263; POCTI/FEDER) and SFRH/BD/17014/2004 FCT PhD scholarship co-financed by FEDER provided support for field and analytical work.