U-Pb conventional dating compared to SHRIMP of the Santa Barbara Granite Massif, Younger Granites Suite of Rondonia, Brazil

The Santa Barbara Granite Massif is part of the Younger Granites of Rondonia (998 - 974 Ma) and is included in the Rondonia 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 Cicero 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 yielded a 207 Pb/206 Pb weighted-mean age of 978 +- 13 Ma. The textural complexity of the zircon crystals of the Santa Barbara 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 epsilonNd(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 Rondonia Tin Province. (author)