Multiple crust-mantle interactions for the destruction of the North China Craton: Geochemical and Sr-Nd-Pb-Hf isotopic evidence from the Longbaoshan alkaline complex

In situ zircon U-Pb ages and Hf isotopic data, major and trace elements, and Sr-Nd-Pb isotopic compositions are reported for the Longbaoshan alkaline intrusive complex in the western Shandong Province (Luxi Block), southeastern North China Craton. The Longbaoshan complex, which consists of quartz syenite, aegirine-augite syenite, hornblende syenite, monzonite, and syenodiorite. was emplaced at 129.4-131.7 Ma. The complex is characterized by high concentrations of SiO2, K2O + Na2O, Al2O3, LILEs (e.g., Sr and Ba), and LREEs, and low concentrations of CaO, Fe2O3, MgO, and HFSEs (e.g., Nb, Ta, P and Ti). The Sr, Nd, Pb, and Hf isotopic values of this complex are similar to the nearby mafic rocks, which were derived from EM2-type lithospheric mantle, indicating that the Longbaoshan complex was mainly derived from partial melting of the EM2 source. Combined with geochemical and isotopic features, the high Nb/Ta ratios (average 19.2) of the Longbaoshan complex suggest that the EM2 source was induced by modification of crust-derived melts, which were in equilibrium with rutile-bearing eclogite. This was attributed to the subduction of the Yangtze continental crust. The presence of inherited zircons (2.51-2.64 Ga) with positive epsilon(Hf)(t) values (0.2-6.2) suggest that the ancient crust of the North China Craton was also involved in the formation of the Longbaoshan complex. Magma mixing modelling shows that those 10-35% crustal materials were assimilated in the complex. This complex may have experienced hornblende, apatite/monazite, and titanite crystal fractionation before emplacement in the shallow crust level. Two stages of crust-mantle interaction can be identified through the formation of the Longbaoshan complex, including (1) the lithospheric mantle was metasomatized by the melts/fluids derived from the subducted crust and then transformed into an EM2 counterpart, and (2) magma derived from the EM2 source underplated the ancient crust and assimilated the crust-derived materials. We thus propose that multiple crust-mantle interactions are the essential mechanism for the destruction of the NCC. The first intensive crust-mantle interaction was aroused by crustal subduction and was responsible for the lithospheric mantle destruction, and the second major crust-mantle interaction was induced by mantle-derived magma underplating and was responsible for the crustal activation. (c) 2010 Elsevier B.V. All rights reserved.