Record of Late Yanshanian Mafic Magmatic Activity in the Middle-LowerYangtze River Metallogenic Belt

Sun Yang; Ma Changqian; Liu Bin

doi:10.3799/dqkx.2017.077

Abstract：The Ningzhen region in Jiangsu Province, representing the easternmost magmatic region within the Middle-Lower Yangtze River Metallogenic belt, and is an important component of the Middle-Lower Yangtze River magmatic rock belt.The Jiangmiao pluton is the only mafic intrusive body in the Ningzhen region, and its zircon U-Pb dating,petrological,mineralogical and geochemical data were obtained in this paper to constrain its origin and mantle source characteristics.Olivine gabbros,hornblende gabbros and pyroxene diorites are the main rock types of the pluton.The zircons from the pyroxene diorite yield a U-Pb age of 121±1Ma.The zircon εHf(t) values are from-2.9 to-6.4.The Jiangmiao mafic rocks in the Ningzhen region are sodic,and are characterized by an enriched light rare earth elements(LREEs) pattern.They are enriched in large ion lithophile elements (LIEEs) such as K,Ba,Sr,but depleted in high field strength elements such as Nb,Ta,Ti,Zr and Hf.Jiangmiao mafic intrusive rocks have the highest εNd(t) and the lowest value of (87Sr/86Sr)i values among Mesozoic magmatic rocks in the Middle-Lower reaches of Yangtze River belt.Sr-Nd isotopic data suggest that asthenosphere-lithosphere interaction plays an important role in the generation of the Jiangmiao pluton.The trace element and isotopic data are consistent with crystallization fractionation and there is no significant crustal contamination during magma ascending and crystallization.Enriched LREEs and LILEs,together with depleted Nb,Ta,Ti,Zr and Hf,suggest that the mantle sources were metasomatized by slab-derived fluid/melt.Combining with regional geological,geochronological,mineralogical and geochemical data,the Jiangmiao maflc rocks in the Ningzhen region were probably formed in an extensional regime associated with westward subduction of the paleo-Paciflc plate.