Decoupling of U-Pb ages and compositional zoning of garnet in a high-pressure marble from the eastern Iratsu body, Sanbagawa metamorphic terrane, Japan

Here we first report the in situ U-Pb dating of metamorphic grossular garnet (Grs) with distinction between internal zonation textures. The studied Grs occurs in an eclogite-facies marble collected from the eastern Iratsu body of the Sanbagawa metamorphic terrane, Japan. The Grs has a patchy texture, predominantly with pure Grs cores and andradite (Adr)-rich rims formed during eclogite-facies and exhumation stages, respectively. The UPb ages for the Grs core and Adr-rich rim were 97 +/- 10 and 106 +/- 16 Ma (95% confidence level), respectively. Despite the compositional zoning formed under different P-T conditions, the U-Pb ages of the core and rim were in similar values within analytical uncertainties. This decoupling of chemical zonation and U-Pb ages implies that the U-Pb chronological signatures of rims were inherited from cores owing to the redistribution of radiogenic Pb in cores during the rim formation through fluid-mediated dissolution and reprecipitation. The Grs U-Pb age (97 +/- 10 Ma) thus directly corresponds to previously reported P-T conditions of the core formation during the eclogite-facies metamorphism. This advantage of Grt petrochronology as the combination of radiometric ages obtained by in situ analysis and P-T conditions deduced from paragenesis can contribute to reconstruct reliable metamorphic histories.