IN-SITU U-PB AND AR-AR AGE CONSTRAINTS ON UHT METAMORPHISM IN THE BRASILIA FOLD BELT: A CASE STUDY FOR THE DOMINANCE OF POST-PEAK ZIRCON IN HIGH GRADE ROCKS

Ideally, in-situ U-Pb dating and trace element analyses allow direct correlation of age data with prograde and retrograde stages of the metamorphic evolution. With an intact textural context, growth reactions and petrologically relevant textures can be observed (1), and trace element analyses be used to test equilibrium. Zircon and rutile in the UHT granulites from the Brasília fold belt are thus investigated in-situ and supplemented by in-situ Ar-Ar dating of biotite to constrain age and duration of the metamorphic event. The results challenge older concepts that interpret zircon ages to date the peak of metamorphism.

Peak conditions of UHT granulites in the Anápolis-Itauçu Complex of the Brasília fold belt attained are ca. 1050°C at >10 kbar (2, 3), followed by initial decompression and near-isobaric cooling to below 850°C, when opx- and zircon-bearing leucosomes crystallized. Euhedral zircon from these leucosomes have an age of 631±6 Ma and are characterized by flat zircon/opx HREE distribution patterns with a D of ca. 10, interpreted to indicate equilibrium. Post-peak zircon growth also occurred as rims around rutile the same time. High Zr peak-metamorphic rutile is interpreted to have recrystallized during the early retrograde history to about 850°C, shedding ca. 80% of its peak-T Zr content. Rare zircon cores (with negative Eu anomalies and near-flat HREE patterns indicating equilibrium with Grt and Fsp) and high-Zr rutile in garnet are 50 m.y. older and may be interpreted as prograde to peak, or alternatively as an earlier event.. In-situ laser-ablation Ar-Ar analyses of biotite dates cooling through ca. 300°C at 596.0±2.0 Ma, which yields a fast cooling rate of ca. 15°C/m.y., possibly indicating tectonic exhumation.

The potential processes to grow the dominant zircon population of these UHT rocks during the post-peak history are thus identified by texture, trace element analysis and in-situ dating as post-UHT rutile recrystallization and crystallization of decompression melts.

[1] Möller A., O'Brien, P. J., Kennedy, A. & Kröner, A. (2003) Geol. Soc. London, Sp. Publ. 220, 65-81.

[2] Moraes, R., Brown, M., Fuck, R.A., Camargo, M.A. & Lima, T.M. (2002) J. Petrol. 43, 1673-1705.

[3] Baldwin, J.A., Powell, R., Brown, M., Moraes, R. & Fuck, R.A. (2005) J. met. Geol. 23, 511-531.