Geologia USP. Série Científica (Oct 2009)
In Situ Isotopic Analyses of U and Pb in Zircon by Remotely Operated SHRIMP II, and Hf by LA-ICP-MS: an Example of Dating and Genetic Evolution of Zircon by 176Hf/177Hf from the Ita Quarry in the Atuba Complex, SE Brazil
Abstract
Remotely-operated SHRIMP dating of zircon is an interesting alternative for dating of zircon crystals. Althoughit does not represent any technical progress of the geochronological method using the U-Pb system in zircon it is avery useful and cheap facility. The procedure was fi rst used for mass spectrometric analyses involving two internationallaboratories in São Paulo, Brazil and Beijing, China. It was applied to samples of three gneiss-migmatitic rocks from theIta quarry in the Atuba Complex (located between the Luis Alves and the Apiaí Domain) to test previous controversialhypotheses about its evolution. The presence of important archean and paleoproterozoic components in the complex isconfi rmed by analyses of zircon found in probably neoproterozoic leucosomes. Diorite intrusion also occurred during theneoproterozoic, associated with the 0.6Ga continental collisions involved in the assembly of Gondwana. The determinationof Hf isotope ratios by LA-ICP/MS represents a new option for checking the relative importance of mantle (εHf > 0) andcrustal contributions (εHf < 0) during the growth of the zircon crystals. While the archean component in the complex wasderived from the mantle (εHf +1.5 to +8.7) the paleoproterozoic component had a crustal contribution (εHf-9.1 to -10.1).