Thursday, August 25, 2016

Evidence of Glacial Meltwater/Volcanic Interaction From Rifting of Supercontinent Rodinia During Tonian/Crogenian NeoProterozoic


He et al


To seek the occurrence of negative δ18O magmas in the Neoproterozoic, we conducted in-situ zircon O isotope analysis and U-Pb dating for granitic gneisses from the northeastern Sulu orogen, east-central China. Zircon U-Pb dating yields protolith ages of 753±15 Ma to 780±13 Ma and metamorphic ages of 209±3 to 244±7 Ma. The Neoproterozoic cores with concordant U-Pb ages exhibit a wide δ18O range from -11.0 to 5.8‰, which is nearly the same as those for cores with discordant U-Pb ages. The Triassic rims of some samples have homogeneous δ18O values of around -10‰ whereas the rims of the other samples show a wider range from -9.8 to 5.0‰. The δ18O values as negative as -11.0‰ for zircons with concordant Neoproterozoic U-Pb ages are reported for the first time, representing the primary record of negative δ18O magma in the Neoproterozoic. The continental subduction-zone metamorphism in the Triassic did not erase the abnormal δ18O record in the protolith cores despite metamorphic dehydration and partial melting under high-pressure to ultrahigh-pressure conditions. A conservative estimate suggests that the hydrothermal fluid reacted with the rocks should have δ18O values lower than -9.2‰, corresponding to the meteoric water in cold paleoclimate or the meltwater of local continental glaciation. The spatial variation in the O isotope compositions of Neoproterozoic zircons is a manifestation of the O isotope heterogeneity in the extinct hydrothermal-magmatic system. The hydrothermal alteration during the Neoproterozoic was incongruent, which was lately recorded by the wide range of δ18O values in the metamorphic zircons of Triassic age. The extensive O isotope exchange between the surface water and the deep rock requires high temperature and high water-rock ratios in continental rifting zones. This is ascribed to Neoproterozoic splitting of the South China Block from the Rodinia supercontinent.

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