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U-Pb zircon age of the foliated garnet-bearing granites in western Dabie Mountains,Central China

Chinese Science Bulletin 2001年第19期46卷 1657-1660页

作者： ZHANG Hongfei ZHONG Zengqiu GAO Shan ZHANG Benren LI Huiming Faculty of Earth Sciences China University of GeosciencesWuhan 430074China Department of Geology Northwest UniversityXi’an 710069China Tianjin Institute of Geology and Mineral Resource Tianjin 300170China

U-Pb zircon dating on two foliated garnet-bearing granite samples in the western Dabie ultra-high-pressure (UHP) metamorphic unit yields concordant ages of (234±4) Ma and (227±5) Ma, respectively. These ages, following the UHP peak metamorphism, represent the magma emplacement ages for the foliated garnet-bearing granites. This, for the first time, shows that there are the Triassic granites in the Dabie Mountains. The foliated garnet-bearing granites resemble A-type granite in geochemical characteristics, indicating that they were formed in exten-sional geodynamic setting. The magma formation reflects a reheating event in the Dabie orogenic belt and it enhances the transfer of tectonic regime from collision into extension and promotes the rapid exhumation into lower crust for the UHP metamorphic rocks.

关键词： Dabie orogenic belt foliated garnet-bearing granite U-Pb zircon age post-collisional granite.

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Electron Probe Microanalysis of Monazite and Its Applications to U-Th-Pb Dating of Geological Samples

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Journal of Earth Science 2019年第5期30卷 952-963页

作者： Wenbin Ning Junpeng Wang Deng Xiao Fenfang Li Bo Huang Dong Fu State Key Laboratory of Geological Processes and Mineral Resources Hubei Key Laboratory of Critical Zone Evolution Center for Global Tectonics School of Earth Sciences China University of Geosciences

Electron probe microanalysis(EPMA) dating of monazite has been developed over decades. However, limited by the detectability and analytical sensitivity of dating-related elements(Th, Pb, U and Y), the EPMA dating has been restricted to geological research. In this study, various probe currents, beam diameters and counting times have been utilized on a JEOL JXA-8230 electron microprobe to determine the optimal experimental conditions for measuring Th, Pb, U and Y in monazite. The optimal conditions are:(1) accelerating voltage is 15 k V;(2) probe current is 100 n A;(3) beam diameter is 1 μm;(4) the peak and background counting time of U and Pb are 200 and 100 s;and(5) the peak and background counting time of Th and Y are 100 and 50 s. We apply this method to monazite from garnet-bearing biotite gneiss in the Zanhuang area of the Central Orogenic Belt of the North China Craton. The Pb O-Th O2* isochron age calculated by EPMA data is 1 812±17 Ma(MSWD=2.06), which is similar to the weighted mean 207 Pb/206 Pb age(1 805±12 Ma, MSWD=1.07) obtained by LA-ICP-MS. This study suggests that EPMA dating of monazite as a powerful dating technique can be widely used in geochronological study.

关键词： monazite EPMA dating garnet-bearing biotite gneiss Zanhuang area North China Craton geochemistry

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