长江中下游地区燕山晚期基性岩浆活动的记录

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

江苏省宁镇地区位于长江中下游沿江成矿带的最东端,是长江中下游成矿带的重要组成部分,蒋庙岩体是宁镇地区唯一的基性岩体.对此岩体进行了锆石U-Pb年代学、矿物化学、岩石地球化学和Sr-Nd-Hf同位素研究,讨论了其成因及地幔源区性质.岩体主要由橄榄辉长岩、角闪辉长岩和辉石闪长岩组成,其中辉石闪长岩的锆石LA-ICP-MS(laser ablation inductively coupled plasma mass spectrometry)U-Pb年龄为121±1Ma.锆石εHf(t)值介于-2.9^-6.4.全岩地球化学研究显示岩石样品富钠低钾,富集轻稀土元素,重稀土元素亏损不明显,富集大离子亲石元素(如K、Ba、Sr等),亏损高场强元素(如Nb、Ta、Ti、Zr、Hf等).蒋庙基性侵入岩在长江中下游地区中生代岩浆岩中具有最高的εNd(t)值以及最低的(87Sr/86Sr)i值,其Sr-Nd同位素组成介于软流圈地幔(depleted MORB mantle,DMM)-富集岩石圈地幔(enriched mantleⅡ,EMⅡ)之间.研究认为,蒋庙岩体应来源于软流圈地幔和富集岩石圈地幔的混合组分,可能有少量的俯冲板片物质加入源区,并在成岩过程中经历了一定程度的结晶分异.结合区域地质、年代学、矿物学和地球化学资料,表明蒋庙岩体的形成可能与古太平洋板块俯冲事件密切相关.

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 zirconεHf（t）values are from-2.9to-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 highestεNd（t）and the lowest value of（87Sr/86Sr）ivalues 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 andcrystallization.Enriched LREEs and LILEs,together with depleted Nb,Ta,Ti,Zr and Hf,suggest that the mantle sources were metasomatized by slab-deriveduid/melt.Combining with regional geological,geochronological,mineralogical and geochemical data,the Jiangmiao mac rocks in the Ningzhen region were probably formed in an extensional regime associated with westward subduction of the paleo-Pacic plate.