Contribution to the understanding of the Rondonia Tin Province granites (SW Amazonian Craton) origin using U-Pb and Lu-Hf in zircon by LA-ICP-MS: implications to A-type granite genesis

Abstract

The study of granite rocks is important in order to understand chemical evolution of the continental crust. Their isotopic composition provides information on their magmatic sources, whether being mantle, crust or a mixture of both. In the SW border of the Amazonian Craton, suites of rapakivi granites related with tin-polymetallic mineralization emplaced within heterogeneous Proterozoic crust. Zircons from twelve rocks from three granite intrusions, Massangana, São Carlos and Caritianas, representative of the Younger Granites of Rondônia have been studied by using laser ablation inductively coupled plasma source mass spectrometry (LA-ICP-MS) in order to obtain U-Pb ages and Hf isotopic compositions. The samples from Massangana massif, show greater range in age (between 1026 – 993 Ma) and initial 176Hf/177Hf = 0.2817-0.2823. The ԐHf values are both negative and positive (-6.2 to +3.4) in some samples, and only negative in others (-14.1 to -1.6) which reflects the heterogeneity of crust that they intruded. The TDM age varying between 2.40 to 1.61 Ga also indicates that different sources were involved in the formation of these rocks. The samples from São Carlos massif show U-Pb ages between 996 to 974 ± 10 Ma and initial ԐHf between -15 and +11, corresponding to a TDM age range between 2.65 and 1.08 Ga. The samples from Caritianas massif with U-Pb ages of 1001 and 999 Ma, show more initial ԐHf positive values (13 zircon grains) than negative (6 zircon grains), different from the other massifs. The range of initial ԐHf of the Caritianas massif is -1.5 to +8.2, and residence crustal ages between 1.76 and 1.25 Ga. The great variation in the ԐHf values indicates heterogeneity of sources; the Massangana and São Carlos massifs represent mainly crustal melts with a subordinate mantle contribution. The Caritianas massif, which shows more positive values of the ԐHf parameter, seems to have had more mantle contribution than the other massifs studied here. The characterization of the sources of the rapakivi rocks may play an important role in the genesis of cassiterite ore and could represent an important tool for mineral exploration.

Keywords

• granites; Amazonian craton; U-Pb ages/ LA-ICP-MS