Age and duration of ultrahigh-temperature metamorphism in the Anapolis-Itaucu Complex, Southern Brasilia Belt, central Brazil - constraints from U-Pb geochronology, mineral rare earth element chemistry and trace-element thermometry

Integrated petrographic and chemical analysis of zircon, garnet and rutile from ultrahigh-temperature (UHT) granulites in the Anapolis-Itaucu Complex, Brazil, is used to constrain the significance of zircon ID-TIMS U-Pb geochronological data. Chondrite-normalized rare earth element (REE) profiles of zircon cores have positive-sloping heavy-REE patterns, commonly inferred to be magmatic, whereas unambiguous metamorphic grains and overgrowths have flat to slightly negatively sloping heavy-REE patterns. However, in one sample, a core of zircon interpreted as having formed prior to garnet crystallization and a metamorphic zircon formed within microstructures involving garnet breakdown both display elevated heavy-REE (and Y) with positive-sloping patterns. D-REE(zrc/grt) partition coefficients suggest an approximation to equilibrium between zircon and garnet cores, although progressive enrichment in heavy REE towards garnet rims occurs in two of the samples investigated. Titanium-in-zircon thermometry indicates zircon growth during both the prograde and post-peak evolution, but not at peak temperatures of the UHT metamorphism. By contrast, zirconium-in-rutile thermometry of inclusions armoured by garnet records crystallization temperatures, based on the upper end of the interquartile range of the data, of similar to 890 to 870 degrees C and maximum temperature around 980 degrees C, indicating prograde and retrograde growth close to and after peak conditions. Rutile located in retrograde microstruct records crystallization temperatures of 850 to 820 degrees C. Rutile intergrown with ilmenite and included within orthopyroxene, which is associated with exsolved zircon, records temperatures similar to 760 degrees C, consistent with Ti-in-zircon crystallization temperatures. ID-TIMS U-Pb geochronological data from two of the four samples investigated define upper intercept ages of 641.3 +/- 8.4 Ma (MSWD 0.91) and 638.8 +/- 2.5 Ma (MSWD 1.03) that correlate with periods of zircon growth along the prograde segment of the P-T path. Individual zircon U-Pb dates retrieved from all samples range from 649 to 634 Ma, indicating a maximum duration of c. 15 Myr for the UHT event. This period is interpreted as recording modest thickening of hot backarc lithosphere located behind the Arenopolis Arc at the edge of the Sao Francisco Craton consequent upon terminal collision of the Parana Block with the arc during the amalgamation of West Gondwana.