Eclogite-high-pressure granulite metamorphism records early collision in west Gondwana; new data from the southern Brasilia Belt, Brazil

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 (super 206) Pb/ (super 238) U age of 678+ or -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 (super 206) Pb/ (super 238) U ages of 648+ or -12 and 647+ or -11 Ma, and Ti-in-zircon crystallization temperatures from c. 860 degrees C down to c. 785 degrees 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 degrees C (based on Ti-in-quartz thermometry). Rare zircons from leucosome in high-pressure granulite from the overlying arc-derived nappe yield a (super 206) Pb/ (super 238) U age of 622+ or -28 Ma and Ti-in-zircon crystallization temperatures from c. 970 degrees C down to c. 820 degrees 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.