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. Although it does not represent any technical progress of the geochronological method using the U-Pb system in zircon it is a very useful and cheap facility. The procedure was fi rst used for mass spectrometric analyses involving two international laboratories in So Paulo, Brazil and Beijing, China. It was applied to samples of three gneiss-migmatitic rocks from the Ita quarry in the Atuba Complex (located between the Luis Alves and the Apia Domain) to test previous controversial hypotheses about its evolution. The presence of important archean and paleoproterozoic components in the complex is confi rmed by analyses of zircon found in probably neoproterozoic leucosomes. Diorite intrusion also occurred during the neoproterozoic, associated with the 0.6Ga continental collisions involved in the assembly of Gondwana. The determination of Hf isotope ratios by LA-ICP/MS represents a new option for checking the relative importance of mantle ( Hf > 0) and crustal contributions ( Hf < 0) during the growth of the zircon crystals. While the archean component in the complex was derived from the mantle ( Hf +1.5 to +8.7) the paleoproterozoic component had a crustal contribution ( Hf -9.1 to -10.1).