Late Mesoproterozoic granulite-facies metamorphism in the Oulongbuluke block of the North Qaidam Mountains:Constraints from petrology,phase equilibrium modeling and U-Pb dating of garnet-sillimanite-cordierite gneisses
Late Mesoproterozoic granulite-facies metamorphism in the Oulongbuluke block of the North Qaidam Mountains:Constraints from petrology,phase equilibrium modeling and U-Pb dating of garnet-sillimanite-cordierite gneisses
The garnet-sillimanite-cordierite gneiss,which has been identified from the Chahanhe Group in the northern region of the Oulongbuluke block,consists of garnet,sillimanite,cordierite,biotite,plagioclase,ilmenite,and a small amount of K-feldspar.Petrographic observation shows that three stages(M1,M2 and M3)of mineral assem-blage can be recognized.They are M1:Pl±Kfs+Grt+Sill+Qz,M2:Pl±Kfs+Grt+Sill+Qz+Ilm+Bt,and M3:Crd+Bt+Ilm+Qz+Grt+Pl±Kfs.Phase equilibrium modelling shows that the peak p-T conditions of the garnet-sillimanite-cordierite rock are p=0.92～1.08 GPa and t＞790℃,and experienced a heating decompression during exhuma-tion.The metamorphic ages acquired from zircon and monazite U-Pb dating by LA-ICP-MS are 1 133±14 Ma and 1 125±37 Ma,respectively,which represent the timing of granulite-facies metamorphism.Combining regional ge-ology with our data,we suggested that the garnet-sillimanite-cordierite rock in the Chahanhe Group formed in an arc or back-arc system related to oceanic subduction.The magmatic-metamorphic complex belt in the northern region of the Oulongbuluke block underwent an evolutionary process from subduction-accretion to collision orogeny during late Mesoproterozoic to early Neoproterzoic,which is a response to the convergence process of Rodinia supercontinent.
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