Late Mesozoic adakitic intrusive rocks in Ningzhen district,Lower Yangtze River Reaches: ages, geochemical and genesis
Institute of Geology, Chinese Academy of GeologicalSciences, Beijing, 100037
Abstract:The late Mesozoicintermediate- acid intrusive bodies developed in Ningzhen district, the easternmost of the Middle- Lower Yangtze River tectonic- magmatic- metallogenic belt, are characterized by high- Sr and Ba, and enriched in Na, strong incompatible elements such as Rb, Th, U as well as light rare earth elements (LREE), depleted in high- field- strong- elements (HFSE) such as Ti, Nb and Ta, similar to the high Sr/Y adakitic rocks in geochemistry. The high Sr/Y bodies in Ningzhen district exhibit low radiogenic Pb isotopic compositions, lower HREE, higher Mg, Mg# and Cr- Ni contents for a given SiO2 contents compared to those high Sr/Y bodies developed in other area of the Middle- Lower Yangtze River reaches, inferring their parent magma was formed by magma mixing between the melt from a re- enriched mantle wedge and the melt from an ancient lower continental crust. The mixed magma had undergone strong fractional crystallization, and may be some assimilation of upper crust. The re- enriching event may be related with the mantle wedge metasomatized by melts and/or fluids from the paleo- Pacific plate, which result in more enriched in incompatible elements and fluids, but the radiogenic Sr and Nd isotope ratios cumulated in ancient enriched subcontinental lithospheric mantle were “neutralized”. The event only affected Ningzhen district, the easternmost of the Middle- Lower Yangtze River reaches, may not influenced other area more inland. The emplacement ages of the adakitic bodies in Ningzhen district are about 100 to 108 Ma, later about 30 Ma than those high Sr/Y intrusive bodies developed in other districts in the Middle- Lower Yangtze River reaches, may imply different dynamic setting between them. The latter may be related with the partial melting of thickened lower crust under the stress relaxation after intracontinental compressional orogeny in about 200~160 Ma. This event of lithospheric thinning was accompanied by compression stress relaxation- extension developed in the Lower Yangtze River reaches may not affected the easternmost (Ningzhen district). But the former may be related with change of the Pacific plate affect the Continental plate in late period of early cretaceous- late cretaceous, in this period, the tectonic setting of eastern margin of China continent entry into extension- dissociation, but only effect the easternmost part (Ningzhen district), no obvious magmatic response for this tectonic event in other area of the Lower Yangtze River reaches.