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