Barium isotope evidence for the generation of peralkaline granites from a fluid-metasomatized crustal source

Abstract

The origin of peralkaline granitic rocks with higher concentrations of alkali than Al2O3 (with molar (Na + K)/Al >1.0) is still debated. The debate focuses on whether they result from prolonged magmatic fractionation of alkali-rich metaluminous melts or partial melting of the fluid-fluxed, alkali-metasomatized lower crust. In order to decipher the petrogenesis of peralkaline granitic rocks, this study presents Ba isotope data for spatiotemporally associated metaluminous and peralkaline granites from the Zhoushan archipelago (East China). The δ138/134Ba values of the metaluminous granites (−0.09‰ to 0.05‰ with a mean of −0.02‰) are comparable to those of the average continental crust, while those of the peralkaline granites are significantly lower (−0.60‰ to 0.11‰ with a mean of −0.20‰) and more variable. The δ138/134Ba of the metaluminous granites record magmatic fractionation and minimal volatile exsolution at the intrusion level. In contrast, the δ138/134Ba of the peralkaline granites record fluid-fluxing at the source level and re-melting of a granulitic crustal source. We propose that the distinctive Ba isotopic compositions of the peralkaline granites in the Zhoushan area were produced by fluxing a Ba-poor source with Na-rich fluids and garnet formation, not by extreme magmatic fractionation. This study indicates that Ba isotopic compositions of granites can generally be used to interpret fluid evolution from source to final crystallization of granitic magmas, thus providing new constraints for Earth's volatile cycle.