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

摘要

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 epsilon_(Hf){t) values vary from —3.04 to -7.71, displaying a unimodal distribution. The syenite also has high Sr [(~(87)Sr/~(86)Sr),=0.7086-0.7089], low Nd [epsilon_(Nd)(t)=-6.57 to -7.64] isotopic ratios, plotting in the enriched mantle field on an epsilon_(Nd)(t) versus (~(87)Sr/~(86)Sr), 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.