Reno, Barry L. et al. (2009) Eclogite-high-pressure granulite metamorphism records early collision in West Gondwana: new data from the Southern Brasilia Belt, Brazil, Journal of the Geological Society, London, 166, 1013-1032, doi: 10.1144/0016-76492008-140. < 20100403102858849.admin > pub
  • abstract:

    Nappes in the southern sector of the Southern Brasilia Belt record suturing of the Paranapanema Block and Socorro–Guaxupe Arc with a subducted passive margin on the western side of the Sao Francisco Craton. We report secondary ion mass spectrometry U–Pb zircon ages that for the first time constrain the age of: (1) retrograded eclogite from a block along the tectonic contact beneath the uppermost nappe in a stack of passive margin-derived nappes; (2) high-pressure granulite-facies metamorphism in the uppermost passive margin-derived nappe; (3) high-pressure granulite-facies metamorphism in the overlying arc-derived nappe. Rare zircons from a retrograded eclogite yield a 206Pb/ 238U age of 678 ± 29 Ma, which we interpret as most likely to date close-to-peak-P metamorphism and to provide a minimum age for detachment of the overlying passive margin-derived nappe from the subducting plate. Zircon associated with ilmenite in samples from two structural levels in the passive margin-derived high-pressure granulite nappe yields 206Pb/ 238U ages of 648 ± 12 and 647 ± 11 Ma, and Ti-in-zircon crystallization temperatures from c. 860°C down to c. 785°C, but skewed toward the lower part of the range. These data indicate zircon formation during cooling from around peak T to the solidus, consistent with the high-T retrograde P–T path deduced from microstructures linked to phase assemblage fields in isochemical phase diagrams. Rb–Sr multi-mineral–whole-rock isochrons from two samples from close to the bottom of this nappe date formation of a retrograde sillimanite-bearing penetrative fabric to c. 590 Ma at temperatures of c. 750°C (based on Ti-in-quartz thermometry). Rare zircons from leucosome in high-pressure granulite from the overlying arc-derived nappe yield a 206Pb/ 238U ages of 622 ± 28 Ma and Ti-in-zircon crystallization temperatures from c. 970°C down to c. 820°C, which we interpret to record formation of zircon during cooling from peak high-pressure granulite-facies conditions. These ages indicate that the first stage of craton amalgamation in West Gondwana may have occurred earlier than previously inferred.

  • doi: 10.1144/0016-76492008-140
  • modified at 2020-05-11
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  • name
  • Reno, Barry L.
  • Brown, Michael
  • Kobayashi, Katsura
  • Nakamura, Eizo
  • Piccoli, Philip M.
  • Trouw, Rudolph A. J.