U-Pb chronostratigraphy of the granitic magmatism in the Agudos Grandes Batholith (west of Sao Paulo, Brazil) - implications for the evolution of the Ribeira Belt

U-Pb zircon and monazite dating was used to investigate the emplacement ages of granitic bodies in the Agudos Grandes Batholith, Central Ribeira Belt, southeast Brazil. The most voluminous granite type in the batholith, a syn-orogenic, isotropic to strongly foliated porphyritic hornblende-biotite quartz monzonite to monzogranite (Ibiuna type), was dated at 610 +/- 2 Ma. The less abundant Turvo type, a foliated peraluminous (muscovite + biotite +/- garnet +/- tourmaline) leucogranite appearing as scarce bodies intruding the Ibiuna type, has an identical U-Pb monazite age (610 +/- 1 Ma), suggesting a short duration for the syn-orogenic period in the batholith. The late-orogenic Piedade massif, a zoned elliptical body cropping out in the NW border of the batholith, was also dated and yielded three similar ages in the range 600-605 Ma. A more precise age was obtained for a porphyritic muscovite-bearing facies (601 +/- 2 Ma in monazite); other two ages, less precise owing to inheritance and Pb loss, were obtained: a porphyritic titanite-bearing biotite granite yielded 604 +/- 8 Ma (zircon), and an inequigranular, more fractionated pink biotite granite yielded 605 +/- 7 Ma (zircon and monazite). The post-orogenic Serra da Bateia massif, made up of biotite syenogranites with a strong magmatic foliation and A-type geochemical affinities, was dated at 564 +/- 8 Ma (zircon). The U-Pb ages are consistent with field relations previously established and show that the evolution of the Agudos Grandes Batholith spanned at least 45 million years. The ages of the syn-orogenic granites (Ibiuna and Turvo types, ca. 610 Ma) are older than those of similar rocks in the northern part of the Mantiqueira Orogenic System (590-565 Ma in the north Ribeira and Aracuai Fold Belt in Rio de Janeiro and Espirito Santo States), demonstrating the diachronous character of the orogenies in the belt. (C) 2001 Elsevier Science Ltd. All rights reserved.