An unusual zircon SHRIMP dating result of a granitic gneiss from the Qinglongshan eclogite-gneiss roadcut section is presented in this paper. The very peculiar and complicated internal structures, as well as the very ...An unusual zircon SHRIMP dating result of a granitic gneiss from the Qinglongshan eclogite-gneiss roadcut section is presented in this paper. The very peculiar and complicated internal structures, as well as the very low Th/U ratios (0.01-0.08) of the zircons indicate that they were formed by metamorphic recrystallization. Strongly in contrast with previously published zircon U-Pb ages of the Dabie-Sulu UHP metamorphic rocks where protolith ages of 600-800 Ma are commonly recorded, only metamorphic age of 218±5 Ma, defined by 18 analytical spots either in rim or in core of zircons, are recorded in this granitic gneiss. This age represents the time of the complete metamorphic recrystallization overprint on primary magmatic zircons. The recrystallization was derived by the UHP metamorphism, and was strengthened by the early stage of retrograde metamorphic fluid activity.展开更多
A continuous flow method,by a combination of thermal conversion elemental analyzer(TC/EA)with isotope ratio mass spectrometry(MS),was developed to determine both H isotope composition and H2O concentration of ultrahig...A continuous flow method,by a combination of thermal conversion elemental analyzer(TC/EA)with isotope ratio mass spectrometry(MS),was developed to determine both H isotope composition and H2O concentration of ultrahigh-pressure(UHP)metamorphic rocks in the Dabie-Sulu orogenic belt.By using the developed step-heating technique,we have studied H2O concentration and H isotope composition of the different forms of water(structural OH and molecular H2O)in garnet.The quantitative measurements of H2O concentration and H isotope composition of minerals in UHP metamorphic rocks from several typical outcrops indicate that the gneisses can release more amounts of water than the eclogites during exhumation of the deeply subducted continental crust.Therefore,by decompression dehydration at the contact between eclogite and gneiss,the released water could flow from the gneiss to the eclogite and result in significant hydration of the eclogite adjacent to the gneiss.The measured maximum water contents of minerals in eclogites indicate that garnet and omphacite have the maximum water solubilities of 2500and 3500 ppm,respectively,under the peak UHP metamorphic conditions.展开更多
基金the National Natural Science Foundation of China(No.40399142 , No.40372037) Centre National de la Recherche Scientifique(CNRS)of France.
文摘An unusual zircon SHRIMP dating result of a granitic gneiss from the Qinglongshan eclogite-gneiss roadcut section is presented in this paper. The very peculiar and complicated internal structures, as well as the very low Th/U ratios (0.01-0.08) of the zircons indicate that they were formed by metamorphic recrystallization. Strongly in contrast with previously published zircon U-Pb ages of the Dabie-Sulu UHP metamorphic rocks where protolith ages of 600-800 Ma are commonly recorded, only metamorphic age of 218±5 Ma, defined by 18 analytical spots either in rim or in core of zircons, are recorded in this granitic gneiss. This age represents the time of the complete metamorphic recrystallization overprint on primary magmatic zircons. The recrystallization was derived by the UHP metamorphism, and was strengthened by the early stage of retrograde metamorphic fluid activity.
基金supported by the Ministry of Science and Technology of China(2009CB825004)the National Natural Science Foundation of China(41273006 and 41221062)the Chinese Academy of Sciences(KZCX2-EW-QN502)
文摘A continuous flow method,by a combination of thermal conversion elemental analyzer(TC/EA)with isotope ratio mass spectrometry(MS),was developed to determine both H isotope composition and H2O concentration of ultrahigh-pressure(UHP)metamorphic rocks in the Dabie-Sulu orogenic belt.By using the developed step-heating technique,we have studied H2O concentration and H isotope composition of the different forms of water(structural OH and molecular H2O)in garnet.The quantitative measurements of H2O concentration and H isotope composition of minerals in UHP metamorphic rocks from several typical outcrops indicate that the gneisses can release more amounts of water than the eclogites during exhumation of the deeply subducted continental crust.Therefore,by decompression dehydration at the contact between eclogite and gneiss,the released water could flow from the gneiss to the eclogite and result in significant hydration of the eclogite adjacent to the gneiss.The measured maximum water contents of minerals in eclogites indicate that garnet and omphacite have the maximum water solubilities of 2500and 3500 ppm,respectively,under the peak UHP metamorphic conditions.
