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Temperature of Prograde Metamorphism, Decompressional Partial Melting and Subsequent Melt Fractional Crystallization in the Weihai Migmatitic Gneisses,Sulu UHP Terrane:Constraints from ti-in-zircon thermometer

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Journal of Earth Science 2012年第6期23卷 813-827页

作者：续海金叶凯章军锋 State Key Laboratory of Geological Processes and Mineral Resources China University of Geosciences Faculty of Earth Sciences China University of Geosciences State Key Laboratory of Lithospheric Evolution Institute of Geology and GeophysicsChinese Academy of Sciences

In order to constrain temperature during subduction and subsequent exhumation of fel- sic continental crust, we carried out a ti-in-zircon thermometer coupled with zircon internal structure and U-Pb age on migmatitic gneisses from the Weihai region in the Sulu ultra-high pres- sure （UHP） metamorphic terrane, eastern China. The Weihai migmatitic gneisses are composed of in- tercalated compositional layers of melanosome and plagioclase （Pl）-rich lencosome and K-feldspar （Kfs）-rich pegmatite veins. Four stages of zircon growth were recognized in the Weihai migmatitic gneisses. They successively recorded informations of protolith, prograde metamorphism, decompres- sional partial melting during early stage exhumation and subsequent fractional crystallization of pri- mary melt during later stage cooling exhumation. The inherited cores in zircon from the melanosome and the Pl-rich leucosome suggest that the pro- tolith of the migmatitic gneiss is Mid- Neoproterozoic （-780 Ma） magmatic rock. Metamorphic zircons with concordant ages ranging from 243 to 256 Ma occur as over- growth mantles on the protolith magmatic zir- con cores. The estimated growth temperatures （625-717 ＂C） of the metamorphic zircons have a negative correlation with their ages, indicating a progressive metamorphism in HP eciogite-facies condition during subduction. zircon recrystal- lized rims （228-2 Ma） in the PI-rich ieucosome layers provide the lower limit of the decompress-sional partial melting time during exhumation. The ages from 228^-2 to 219~2 Ma recorded in the Pl-rich leucosome and the Kfs-rich pegmatite vein, respectively, suggest the duration of the fractional crystallization of primary melt during exhumation. The calculated growth temperatures of the zircon rims from the Pl-rich leucosome range from 858 to 739 , and the temperatures of new growth zircon grains （219±2 Ma） in Kfs-rich vein are between 769 and 529 . The estimated temperatures have a positive correlation with ages from the Pl-r

关键词： ti-in-zircon thermometer prograde metamorphism decompressional partial melting,fractional crystallization migmatitic gneiss Sulu UHP metamorphic terrane.

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zircon Trace Element Geochemistry and ti-in-zircon Thermometry of the Ngazi-tina Pan-African Post-Collisional Granitoids, Adamawa Cameroon

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International Journal of Geosciences 2021年第4期12卷 307-328页

作者： Alexis Hamdja Ngoniri Habib Dadjo Djomo timoleon Ngnotue Patrick Ayonta Kenne Ghislain Ngassam Mbianya Sylvestre Ganno Jean Paul Nzenti Department of Earth Sciences University of Dschang Dschang Cameroon Institute for Geological and Mining Researches (IGMR) Yaounde Cameroon Department of Earth Sciences University of Yaoundé I Yaounde Cameroon

In Cameroon, the Ngazi-tina region belongs to the Adamawa-Yade domain of the Pan-African Central African Fold Belt (CAFB). It is composed of two petrographic types: quartz-monzonites (majority) and nepheline syenites. Two morphological types, prismatic and pyramidal, were recognized in the zircon grains samples. These zircon types display internal structures typical of magmatic zircons. Zircons separated from the Ngazi-Tina samples contain higher abundances of Hf (close to 8000 ppm) and moderate trace elements (Y, Th, U, Nb, Ta) and REE contents, suggesting a variable degree of magmatic evolution. The chondrite-normalized REE patterns of zircons are characterized by LREE depletion relative to HREE with positive Ce and negative Eu anomalies, typical of magmatic zircons. The high Hf content together with high Ce/Ce*, Th/U, Zr/Hf ratios suggest magma crystallization under variable oxidation and oxygen fugacity. The application of Ti-in-zircon thermometer reveals crystallization temperatures ranging from 678°C to 811°C and 658°C to 768°C for quartz monzonites and nepheline syenites respectively. These features indicate probably a partial melting of continental crust as the source of these zircons grains and emplacement in the magmatic-arc setting.

关键词： Granitoids zircon Geochemistry ti-in-zircon thermometer Continental Crust Ngazi-tina Adamawa-Yade Cameroun

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