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Eclogite–high-pressure granulite metamorphism records early collision in West Gondwana: new data from the Southern Brasília Belt, Brazil

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posted on 21.06.2016 by BARRY L. RENO, MICHAEL BROWN, KATSURA KOBAYASHI, EIZO NAKAMURA, PHILIP M. PICCOLI, RUDOLPH A.J. TROUW

Nappes in the southern sector of the Southern Brasília Belt record suturing of the Paranapanema Block and Socorro–Guaxupé Arc with a subducted passive margin on the western side of the São 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 PT 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 age 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.

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