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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.
    SHRIMP U–Pb dating, trace elements and the Lu–Hf isotope system of coesite-bearing zircon from amphibolite in the SW Sulu UHP terrane, eastern China [查看] FulaiLiu、AxelGerdes、LingsenZeng、HuaiminXue
    In this study, we link mineral inclusion data, trace element analyses, U–Pb age and Hf isotope composition obtained from distinct zircon domains of complex zircon to unravel the origin and multi-stage metamorphic evolution of amphibolites from the Sulu ultrahigh-pressure (UHP) terrane, eastern China. Zircon grains separated from amphibolites from the CCSD-MH drill hole (G12) and Niushan outcrop (G13) were subdivided into two main types based on cathodoluminescence (CL) and Laser Raman spectroscopy: big dusty zircons with inherited cores and UHP metamorphic rims and small clear zircons.Weakly zoned, grey-white luminescent inherited cores preserve mineral inclusions of Cpx + Pl + Ap ± Qtz indicative of a mafic igneous protolith. Dark grey luminescent overgrowth rims contain the coesite eclogite-facies mineral inclusion assemblage Coe + Grt + Omp + Phe + Ap, and formed at T = 732–839 C and P = 3.0–4.0 GPa. In contrast, white luminescent small clear zircons preserve mineral inclusions formed during retrograde HP quartz eclogite to LP amphibolite-facies metamorphism (T = 612–698 C and P = 0.70–1.05 GPa). Inherited zircons from both samples yield SHRIMP 206Pb/238U ages of 695–520 Ma with an upper intercept age of 800 ± 31 Ma. The UHP rims yield consistent Triassic ages around 236–225 and 239–225 Ma for G12 and G13 with weighted means of 229 ± 3 and 231 ± 3 Ma, respectively. Small clear zircons from both samples give 206Pb/238U ages around 219–210 Ma with a weighted mean of 214 ± 3 Ma, interpreted as the age of retrograde quartz eclogite-facies metamorphism. Matrix amphibole from both samples indicate Ar–Ar ages of 209 ± 0.7 and 207 ± 0.7 Ma, respectively, probably dating late amphibolite-facies retrogression. The data suggest subduction of Neoproterozoic mafic igneous rocks to UHP conditions in Middle Triassic (~230 Ma) times and subsequent exhumation to an early HP(~214 Ma) and a late LP stage (~208 Ma) over a period of ~16 and 6 Myr, respectively. Thus, early exhumation from a mantle depth of 120–100 km to about 60 km occurred at an average rate of 0.3 cm/y, while subsequent exhumation to a middle crustal level took place at approximately 0.54 cm/y. These exhumation rates are considerably slower than those obtained for UHP rocks in the Dora Maira and Kokchetav massifs (2–3 cm/y). Based on similar P–T estimates and trace element and Hf isotope compositions, Sulu amphibolites can be identified as retrograde UHP eclogites. The eHf(800) of +8 implies a significant input from the depleted mantle to the Sulu–Dabie terrane during the middle Neoproterozoic. Overgrown rims are characterized by a distinct trace element composition with low Lu/Hf and Th/U and significantly higher 176Hf/177Hf ratios than inherited cores, consistent with formation during/after garnet (re-)crystallization and fractionation of the Lu–Hf system during UHP metamorphism. The combined dataset suggests homogenization of the 176Hf/177Hf ratio within the metamorphic mineral assemblage and during protolith formation. Observed variations are explained by mixing of material from both domains during laser ablation, e.g., due to partial recrystallization of inherited cores.
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