Zircon U-Pb ages and oxygen isotopes of high-grade metamorphic rocks in the eastern part of the Shandong Peninsula

LA-ICP-MS and SHRIMP zircon U-Pb ages and the oxygen isotope compositions of mineral separates were determined for UHP and HP metamorphic rocks, including granitic gneisses, eclogites and amphibolites in the eastern part of the Sulu terrane (the Shandong Peninsula), which constitutes the most eastern part of the Dabie-Sulu orogenic belt in China. The results show: (1) Igneous zircons from three samples of granitic gneiss have U-Pb ages of 723 +/- 36Ma, 738 +/- 17Ma and 744 +/- 63Ma, respectively, suggesting Neoproterozoic protolith. Most of the igneous zircons have unusually low delta(18)O values (0.42 to 4.14parts per thousand), indicating that gneiss protolith was intruded as low delta(18)O magma during the Middle Neoproterozoie. Oxygen isotope geothermometry and minor occurrences of the Triassic metamorphic zircons in the granitic gneiss both indicate that the granitic gneiss and eclogite were subducted as a coherent slab during the Triassic collision between the Sino-Korean and Yangtze cratons. (2) Metamorphic over-growth zircons from one eclogite sample hosted in the granitic gneiss (G-type) give an U-Pb age of 229 +/- 3Ma and a low delta(18)O value of 0.22parts per thousand, pointing to the Triassic UHP metamorphism age and low delta(18)O protolith of the G-type eclogite. Residual igneous zircons give an U-Pb age of 806 +/- 79Ma, suggesting the Neoproterozoic protolith of the G-type eclogite. Zircons from the other G-type eclogite sample give discordia intercept ages of 1838 +/- 41 Ma and 242 +/- 21 Ma, respectively, indicating the Palaeoproterozoic protolith and the Triassic UHP metamorphism. The zircons also have a low delta(18)O value of 3. 40parts per thousand, suggesting that the eclogite protolith was intruded as low delta(18)O mafic magma in the Late Palaeoproterozoic. (3) Both whole-rock and mineral separates of eclogites hosted in marbles (M-type) have very high delta(18)O values. Metamorphic zircons from an eclogite sample give an U-Pb age of 229 +/- 4Ma, corresponding to the Triassic UHP metamorphism. The metamorphic zircons have unusually high delta(18)O value of 15.9parts per thousand, indicating that the protolith of M-type eclogite had high delta(18)O value before the Triassic eclogite-facies metamorphism. (4) Igneous zircons from a HP amphibolite in the Haiyangsuo region give a protolith age of 1719 +/- 18Ma. Together with eclogite protolith of similar ages, they acted as rift shoulders during Neoproterozoic magma emplacement along the northern edge of the Yangtze plate, but suffered the Triassic metamorphic event when the Yangtze plate was subducted beneath the North China plate. The present study provides tight constraints on the protolith nature of eclogites and associated granitic gneisses as well as amphibolite in the eastern part of the Sulu orogen.