Origin of the Late Cretaceous syenite from Yandangshan, SE China, constrained by zircon U-Pb and Hf isotopes and geochemical data

The Yandangshan syenite is a representative Late Cretaceous igneous pluton cropping out in SE China. U-Pb zircon dating using LA-ICP-MS yielded a crystallization age of 98±1 Ma for the syenite. Petrographically and geochemically of shoshonitic affinity, it is enriched in LREE and LILE, and has a pronounced Nb-Ta trough in the primitive mantle-normalized trace element spider diagram. Zircon σ_Hf(t) values vary from -3.04 to -7.71, displaying a unimodal distribution. The syenite also has high Sr [( ⁸⁷Sr/⁸⁶Sr)_i=0.7086-0.7089], low Nd [σ_Nd(t)=-6.57 to -7.64] isotopic ratios, plotting in the enriched mantle field on an σ_Nd(t) versus (⁸⁷Sr/ ⁸⁶Sr)_i diagram. We propose that the Yandangshan syenite was generated by pyroxene-dominated high-pressure fractional crystallization from basaltic magma that was derived from an enriched mantle source. Although coexisting Yandangshan rhyolites have Sr-Nd isotopic compositions similar to the Yandangshan syenite, they were not derived from the same source. Instead, the rhyolitic magma was produced by partial melting of crustal materials as a result of the underplating of basaltic magma. The crust-like Sr-Nd-Hf isotopic signature of the Yandangshan syenite is ascribed to mantle sources that were enriched by subducted sediments. Formation of Yandangshan syenite may represent roll-back of the subducting palaeo-Pacific plate and migration of the arc front to the Yandangshan area at ∼98 Ma.