Isotopic clues tracking the open-system evolution of the Ponte Nova mafic-ultramafic alkaline massif, SE Brazil: The contribution of Pb isotopes

Abstract

The Pb isotope ratios of olivine- and nepheline-normative rocks from the Ponte Nova mafic-ultramafic alkaline massif (SE Brazil) corroborate evidence that the assimilation of crustal components played an important role in this small magma chamber. New whole-rock 206Pb/204Pbi (17.655–18.362) and 208Pb/204Pbi (38.029–38.591) ratios correlate negatively with 143Nd/144Ndi (0.512216–0.512555) and positively with 87Sr/88Sri (0.70432–0.70641). The whole-rock ratios of 207Pb/204Pbi (15.494–15.556) are not clearly correlated with other isotopic pairs. Most of Ponte Nova samples straddle the curve for μ = ∼8.7, but the majority plot to the right of the geochron, indicating U enrichment. The least contaminated samples plot within the Tristan-type isotopic composition and close to the Trindade isotopic field and might have formed through mixing between OIB and EM-I components. Mixing model isotope curves correlating Sr–Nd–Pb isotope ratios, considering the most pristine compositions (central plug) and the main host rocks of this area, indicate low crustal contributions for some intrusions and high contributions for others. More pronounced contributions from local metagranitic host rocks are associated with intrusions showing high amounts of crustal components. The wide interval of calculated Nd depleted-mantle model ages (TDM: 703–1217 Ma) for Ponte Nova rocks indicates that crustal contamination and clinopyroxene accumulation can strongly modify these values versus those from liquid compositions (nearby alkaline dikes). Therefore, rocks affected by crustal contamination and clinopyroxene accumulation should not be considered for periods of enrichment/metasomatism of the mantle source.