Datações U-Pb convencional versus SHRIMP do Maciço estanífero Santa Bárbara, Suíte Granitos últimos de Rondônia, Brasil

Abstract

The Santa Bárbara Granite Massif is part of the Younger Granites of Rondônia (998 - 974 Ma) and is included in the Rondônia Tin Province (SW Amazonian Craton). It comprises three highly fractionated metaluminous to peraluminous within-plate A-type granite units emplaced in older medium-grade metamorphic rocks. Sn-mineralization is closely associated with the late-stage unit. U-Pb monazite conventional dating of the early-stage Serra do Cícero facies and late-stage Serra Azul facies yielded ages of 993 ± 5 Ma and 989 ± 13 Ma, respectively. Conventional multigrain U-Pb isotope analyses of zircon demonstrate isotopic disturbance (discordance) and the preservation of inherited older zircons of several different ages and thus yield little about the ages of Sn-granite magmatism. SHRIMP U-Pb ages for the Santa Bárbara facies association yielded a 207Pb/206Pb weighted-mean age of 978 ± 13 Ma. The textural complexity of the zircon crystals of the Santa Bárbara facies association, the variable concentrations of U, Th and Pb, as well as the mixed inheritance of zircon populations are major obstacles to using conventional multigrain U-Pb isotopic analyses. Sm-Nd model ages and µNd (T) values reveal anomalous isotopic data, attesting to the complex isotopic behaviour within these highly fractionated granites. Thus, SHRIMP U-Pb zircon and conventional U-Pb monazite dating methods are the most appropriate to constrain the crystallization age of the Sn-bearing granite systems in the Rondônia Tin Province.