The combined Lu-Hf and Sm-Nd mineral-whole rock, and zircon U-Pb, REE and Hf isotope study of ultra high pressure rocks from the CCSD program

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Ultrahigh pressure (UHP) rocks from the Chinese Continental Scientific Drilling (CCSD) program were chosen to study the Sm-Nd and Lu-Hf isotope systems in combination with U-Pb LA-ICP-MS and SHRIMP dating of distinct zircon domains. First results show that the combined use of trace element and U-Th-Pb isotope compositions of zircon and PT information from their mineral inclusions, allow the linkage between U-Pb ages and metamorphic reactions. Inherited and metamorphic zircons were distinguished based on CL imaging, mineral inclusions, trace element contents and Hf isotope composition. Up to three phases of zircon growth or re-crystallisation can be identified in a single sample. The ages are interpreted to date the time of (1) prograde and (2) UHP metamorphism during Middle Triassic subduction, and (3) retrograde metamorphism during Late Triassic exhumation. Ongoing studies of the Hf isotope analyses of UHP zircon domains from different samples promise new insights in the understanding of the behaviour of the Lu-Hf system during UHP metamorphism. Preliminary Lu-Hf and Sm-Nd grt-cpx isochrones from CCSD and Dabie eclogites yield surprisingly uniform ages that are in average about 6-8 Myr younger than UHP zircons. The interpretation of these ages is yet not clear for us. A major focus of the applied period of the proposal will be the application of the different chronometers (U-Pb on zircons and Lu-Hf / Sm-Nd on garnet-cpx) on minerals from the same samples. These studies should clarify if the different chronometers applied to different rocks on various sections of the drill hole yield consistent results or date different events. They will thus (1) test the limitations and robustness of the analytical methods, (2) allow to distinguish between analytical and geological scatter, and (3) will contribute to a better understanding of the metamorphic evolution of the Sulu terrane.