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Petrogenesis of an Alkali Syenite-Granite-Rhyolite Suite in the Yanshan Fold and Thrust Belt, Eastern North China Craton: Geochronological, Geochemical and Nd-Sr-Hf Isotopic Evidence for Lithospheric Thinning |
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| Authors: | Yang Jin-Hui; Wu Fu-Yuan; Wilde Simon A; Chen Fukun; Liu Xiao-Ming; Xie Lie-Wen |
| Institution: | 1State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Po Box 9825, Beijing 100029, China 2Department of Applied Geology, Curtin University of Technology, Po Box U1987, Perth, WA 6845, Australia 3State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, XI'AN 750069, China |
| Abstract: | The Yanshan Fold and Thrust Belt in eastern China has been intrudedby a series of alkalic igneous rocks, ranging in compositionfrom granite and rhyolite to syenite and trachyte. Laser ablationinductively coupled plasma mass spectrometry U–Pb analysesof zircon from three alkaline suites yield Early Cretaceousages of 130–117 Ma. Three groups of rocks have been identifiedbased on their mineralogical, geochemical and Sr–Nd–Hfisotope characteristics. The alkali granites and rhyolites areferroan and have low Al2O3, MgO, CaO, Sr, Ba and Eu concentrationsand high SiO2, total Fe2O3, K2O, Nb, Ga, Ta, Th and heavy rareearth element abundances and Ga/Al ratios. Geochemical dataand Sr-, Nd- and zircon Hf-isotopic compositions (87Sr/86Sr)i= 0·7050–0·7164,  Nd(t) = –8·4to –13·6 and Hf(t) = –5·7 to –16·8]indicate that they were probably generated by shallow dehydrationmelting of biotite- or hornblende-bearing granitoid crustalsource rocks and then mixed with contemporaneous magma froma mantle and/or lower crustal source. Ferroan syenites havedistinct geochemical features from those of the alkaline granitesand rhyolites, suggesting that they were produced by clinopyroxeneand plagioclase fractionation of melt derived from an enrichedmantle source, mixed with lower and upper crustal-derived magmas.The magnesian syenites and trachytes have Sr-, Nd- and zirconHf-isotopic compositions that are distinct from those of theferroan syenites. They were mainly derived from partial meltingof lower crustal materials, mixed with enriched mantle-derivedalkali basaltic magma. The emplacement of an alkali syenite–granite–rhyolitesuite, coeval with the formation of metamorphic core complexesand pull-apart basins in eastern China, indicates they formedin an extensional setting, possibly as a result of lithosphericthinning. KEY WORDS: alkaline rocks; zircon U–Pb dating; petrogenesis; crustal extension; Yanshan Fold and Thrust Belt; North China Craton |
| Keywords: | : alkaline rocks zircon U-Pb dating petrogenesis crustal extension Yanshan Fold and Thrust Belt North China Craton |
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