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

摘要

The Yanshan Fold and Thrust Belt in eastern China has been intruded by a series of alkalic igneous rocks, ranging in composition from granite and rhyolite to syenite and trachyte. Laser ablation inductively coupled plasma mass spectrometry U–Pb analyses of zircon from three alkaline suites yield Early Cretaceous ages of 130–117 Ma. Three groups of rocks have been identified based on their mineralogical, geochemical and Sr–Nd–Hf isotope characteristics. The alkali granites and rhyolites are ferroan and have low Al₂O₃, MgO, CaO, Sr, Ba and Eu concentrations and high SiO₂, total Fe₂O₃, K₂O, Nb, Ga, Ta, Th and heavy rare earth element abundances and Ga/Al ratios. Geochemical data and Sr-, Nd- and zircon Hf-isotopic compositions [(87Sr/86Sr)_i = 0·7050–0·7164, ε_Nd(t) = −8·4 to −13·6 and ε_Hf(t) = −5·7 to −16·8] indicate that they were probably generated by shallow dehydration melting of biotite- or hornblende-bearing granitoid crustal source rocks and then mixed with contemporaneous magma from a mantle and/or lower crustal source. Ferroan syenites have distinct geochemical features from those of the alkaline granites and rhyolites, suggesting that they were produced by clinopyroxene and plagioclase fractionation of melt derived from an enriched mantle source, mixed with lower and upper crustal-derived magmas. The magnesian syenites and trachytes have Sr-, Nd- and zircon Hf-isotopic compositions that are distinct from those of the ferroan syenites. They were mainly derived from partial melting of lower crustal materials, mixed with enriched mantle-derived alkali basaltic magma. The emplacement of an alkali syenite–granite–rhyolite suite, coeval with the formation of metamorphic core complexes and pull-apart basins in eastern China, indicates they formed in an extensional setting, possibly as a result of lithospheric thinning.