江西中部新丰街花岗质岩体LA-ICP-MS锆石U-Pb年代学、元素和Sr-Nd-Hf同位素地球化学特征及岩石成因

LA-ICP-MS Zircon U-Pb Geochronology,Elemental and Sr-Nd-Hf Isotopic Geochemistry of the Xinfengjie Granitic Intrusion in Central Jiangxi Province,SE China,and Its Petrogenesis

对江西中部新丰街花岗质岩体开展了岩相学、LA-ICP-MS锆石U-Pb年代学以及元素和Sr-Nd-Hf同位素地球化学研究,并探讨了岩石成因及其构造意义。结果表明,新丰街岩体由二云母花岗岩和黑云母花岗组成;两者均形成于晚侏罗世(~148 Ma);二云母花岗岩SiO_2含量为75.71%~78.36%,为弱过铝质—强过铝质岩石,属高钾钙碱性系列,Mg#变化于0.26~0.34,具有较低的Ga/Al比值(绝大部分<2.6×10^(-4))和较低的Zr+Nb+Ce+Y含量(<350×10^(-6)),全岩εNd(t)为-10^-8.2,锆石原位ε_(Hf)(t)为-15.7^-9.4;黑云母花岗岩SiO_2含量为71.25%~74.41%,主要为准铝质—弱过铝质岩石,也属于高钾钙碱性系列,Mg#变化于0.32~0.37,同样具有较低的Ga/Al比值(绝大部分<2.6×10^(-4))和较低的Zr+Nb+Ce+Y含量(<350×10^(-6)),全岩初始87Sr/86Sr比值为0.7136~0.7153,εNd(t)为-10.0^-8.9,锆石原位ε_(Hf)(t)值为-16.5^-10.9。通过综合研究认为二云母花岗岩具有S型花岗岩特征,是由下地壳中变质泥岩在相对较低温度下发生部分熔融而形成的;黑云母花岗岩具有I型花岗岩特征,是由下地壳中长英质火成岩在相对较高温度下发生部分熔融而形成的。岩体侵位于由古太平洋板块俯冲引起的陆缘弧构造环境。

In this study, we present detailed LA-ICP-MS zircon U-Pb chronology, major and trace elements, and Sr-Nd-Hf isotopic data for the Xinfengjie pluton in central Jiangxi province, and discuss its petrogenesis and tectonic implications. Our results show that the Xinfengjie granites are composed of two-mica granites and biotite granites, both of which were formed in late Jurassic （ca. 148 Ma）. The two-mica granites have SiO2 contents of 75.71~78.36%. They are weakly peraluminous to strongly peraluminous rocks, and belong to high-K calc-alkaline series. Their Mg# vary from 0.26 to 0.34, and they have low Ga/Al ratios （mostly 〈 2.6×10^-4） and low Zr＋Nb＋Ce＋Y contents （〈350×10^-6）. They have whole-rack εNd（t） of -10^-8.2, and zircon in-situ εHf（t） of -15.7^-9.4. The biotite granites have SiO2 contents of 71.25~74.41% . They are metaluminous to weakly peraluminous rocks, and also belong to high-K calc-alkaline series. Their Mg# vary from 0.32 to 0.37, and they also have low Ga/Al ratios （mostly 〈 2.6×10^-4） and low Zr＋Nb＋Ce＋Y contents （〈350×10^-6）. They have initial 87Sr/86Sr ratios of 0.7136~0.7153, εNd（t） of -10.0^-8.9, and zircon in-situ εHf（t ） of -16.5^-10.9. Through our comprehensive research, we consider that the two-mica granites show characteristics of S-type granites, and were formed at lower temperature by partial melting of meta-pelite in the lower crust, while the biotite granites exhibit I-type affinity, and were formed at higher temperature by partial melting of felsic rocks in the lower crust. This pluton emplaced in a continental-arc tectonic regime which was induced by the subduction of the paleo-pacific plate.