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    Timing of metamorphism in the Paleoproterozoic Jiao-Liao-Ji Belt New SHRIMP U-Pb zircon dating of granulites, gneisses and marbles of the Jiaobei massif in the North China Craton [查看] PuiYukTam、GuochunZhao、FulaiLiu、XiwenZhou、MinSun、SanzhongLi
    The Paleoproterozoic Jiao-Liao-Ji Belt lies in the Eastern Block of the North China Craton, with its southern segment extending across the Bohai Sea into the Jiaobei massif. High-pressure pelitic and mafic granulites have been recently recognized in the Paleoproterozoic Jingshan Group (Jiaobei massif). New SHRIMP U–Th–Pb geochronology combined with cathodoluminescence (CL) imaging of zircon has been applied to the determination of the timing of the metamorphism of the high-temperature and high-pressure granulites and associated gneisses and marbles. Metamorphic zircons in these high-pressure granulites, gneisses and marbles occur as either single grains or overgrowth (or recrystallization) rims surrounding and truncating oscillatory-zoned magmatic zircon cores. Metamorphic zircons are all characterized by nebulous zoning or being structureless, with high luminescence and relatively low Th/U values. Metamorphic zircons from two high-pressure mafic granulites yielded 207Pb/206Pb ages of 1956±41 Ma and 1884±24 Ma. One metamorphic zircon from a garnet–sillimanite gneiss also gave an apparent 207Pb/206Pb age of 1939±15 Ma. These results are consistent with interval of ages of c. 1.93–1.90 Ga already obtained by previous studies for the North and South Liaohe Groups and the Laoling Group in the northern segment of the Jiao-Liao-Ji Belt. Metamorphic zircons from a high-pressure pelitic granulite and two pelitic gneisses yielded weighted mean 207Pb/206Pb ages of 1837±8 Ma, 1821±8 Ma and 1836±8 Ma respectively. Two diopside–olivine–phlogopite marbles yielded weighted mean 207Pb/206Pb ages of 1817±9 Ma and 1790±6 Ma. These Paleoproterozoic metamorphic ages are largely in accordance with metamorphic ages of c. 1.85 Ga produced from the Ji'an Group in the northern segment of the Jiao-Liao-Ji Belt and c. 1.86–1.80 Ga obtained for the high-pressure pelitic granulites from the Jingshan Group in the southern segment. As this metamorphic event was coeval with the emplacement of A-type granites in the Jiao-Liao-Ji Belt and its adjacent areas, it is interpreted as having resulted from a post-orogenic or anorogenic extensional event.
    EPMA U-TH-Pb MONAZITE AND SHRIMP U-Pb ZIRCON GEOCHRONOLOGY OF HIGH-PRESSURE PELITIC GRANULITES IN THE JIAOBEI MASSIF OF THE NORTH CHINA CRATON [查看] XIWENZHOU、GUOCHUNZHAO、CHUNJINGWEI、YUANSHENGGENG、MINSUN
    ABSTRACT. The Jiaobei massif constitutes the southern segment of the Paleoproterozoic Jiao-Liao-Ji belt in the Eastern Block of the North China Craton, and contains high-pressure (HP) mafic and pelitic granulites. The HP pelitic granulites contain four distinct mineral assemblages: garnet kyanite biotite muscovite rutile (M1),garnet kyanite ternary-feldspar biotite muscovite rutile (M2), garnet sillimanite biotite perthite anti-perthite (M3), and biotite muscovite sillimanite plagioclase (M4), of which M2 is a typical HP pelitic assemblage. The origin of these HP granulites is important for understanding the tectonic evolution of the Jiao-Liao-Ji Belt, but the timing of the HP event still remains unknown. This study applied the EPMA U-Th-Pb monazite and SHRIMP U-Pb zircon dating techniques to determine the metamorphic ages of the HP pelitic granulites in the Jiaobei massif. CL images and Th/U ratios show that all monazite and zircon grains in the analyzed samples are of metamorphic origin. Eighty-two EPMA analyses on 16 monazite grains from two samples yielded three age groups at 1895 to 1882 Ma, 1814 to 1813 Ma and 1706 to 1692 Ma, and the ages of 16 SHRIMP analyses on 14 zircon grains from one sample range from 1864 to 1803 Ma. The oldest ages of 1895 to 1882 Ma were yielded from high-Y monazite grains enclosed in garnet and sillimanite pseudomorph after kyanite, and thus interpreted as the time of the HP (M2) granulite-facies metamorphism.The EPMA monazite ages of 1814 to 1813 Ma and SHRIMP zircon ages of 1864 to 1803 Ma are interpreted as the approximate ages of the post-HP granulite-facies metamorphism (M3), most probably resulting from the exhumation of the HP granulites to the medium-pressure granulite-facies levels. The youngest age group of 1706 to 1692 Ma obtained from low-Y monazite grains in the matrix can be interpreted as the age of late cooling and retrograde metamorphism (M4) that occurred when the HP pelitic granulites were exhumed to the upper crust. The presence of Paleoproterozoic HP mafic and pelitic granulites in the southern segment of the Jiao-Liao-Ji Belt suggests that the evolution of the Jiao-Liao-Ji Belt in the Eastern Block of the North China Craton must have been involved in subduction- or collision-related tectonic processes.
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