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

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°8C down to c. 820°8C, 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.