旌德复式岩体位于安徽南部,主体相花岗闪长岩中发育暗色包体.本文对花岗闪长岩与暗色包体进行了岩相学观察、全岩主微量元素分析、锆石U-Pb定年与Hf同位素测试.岩相学观察发现暗色包体为典型岩浆岩结构,且发育针状磷灰石.主量元素分析数据表明花岗闪长岩的SiO2含量为66.04％～67.80％；暗色包体的SiO,含量为54.63％～ 54.77％,为二长闪长岩.花岗闪长岩的Mg#=38 ～ 40；暗色包体的Mg#=44 ～ 45.微量元素分析数据表明花岗闪长岩与暗色包体的REE球粒陨石标准化图呈右倾型,Eu负异常；大离子亲石元素富集,高场强元素亏损.锆石U-Pb年代学与Hf同位素研究表明,花岗闪长岩与暗色包体的年龄分别为139.7±1.3Ma和142.3±1.7Ma,在误差范围内一致.花岗闪长岩锆石的εHf(t)为-2.5 ～0.4,地壳模式年龄(tDMC)为1170 ～1350Ma;暗色包体锆石的εHf(t)为-5.2～1.8,地壳模式年龄(tDMC)为1090～1530Ma.两者的tDMC峰值都在1.2 ～ 1.3Ga.这些数据表明花岗闪长岩中的暗色包体为同源岩浆混合成因,源区为年轻地壳,有可能为中新元古代古华南洋壳俯冲扬子板块形成的火山岛弧.旌德花岗闪长岩在Pearce et al.(1984)的构造判别图上落在岛弧花岗岩区.在Sr/Y-Y图解上落在经典岛弧岩浆岩区.花岗闪长岩的岩浆Zr饱和温度低(630 ～ 680℃),与锆石钛温度计(630～720℃)给出的结果基本一致.锆石的Ce(Ⅳ)/Ce(Ⅲ)高(240～530),指示岩浆具有高的氧逸度.旌德岩体的低温与高氧逸度特征说明岩体的源区物质受到过洋壳俯冲的影响.旌德岩体的成因可能与太平洋板块后撤诱发的地壳部分熔融有关.%Petrographic features, whole-rock major and trace elements, zircon U-Pb dating and Hf isotopic compositions are reported for the granodiorite and its mafic microgranular enclaves (MMEs) from the Jingde pluton, South Anhui Province. The MMEs have classical igneous petrographic textures with abundant acicular apatites which indicate a rapid cooling process. The low Mg# ( < 45) indicates that MMEs may not come from the mantle source. Zircon U-Pb dating yields an age of 139. 7 ± 1. 3Ma for the host granodiorite and 142. 3 ± 1. 7Ma for the MMEs, which are identical within analytical errors. The εHf({t) for host granndiorite ranges from -2. 5 to 0.4, corresponding to crust Hf model ages (tDMc) of 1170 ~ 1350Ma. Similarly, the MMEs have εHf (t) of -5.2～1.8 and corresponding crust Hf model ages (tDMc) of 1090 ~ 1530Ma. Both spectra of Hf model age peak between 1.2 ~ 1.3Ga. The consistency of U-Pb age and the similar ranges of Hf isotope data indicate that the MMEs in granodiorite are formed by mingling of magmas from the same source, partial melting of crustal materials which likely formed during the Neoproterozoic convergence along the Jiang-Shao suture. On tectonic discrimination diagrams, Jingde granodiorite plots in the VAG region. In a Sr/Y versus Y diagram, Jingde granodiorite also plots in arc field. Its magma zirconium saturation temperature ranges between 630℃ and 680℃ and the Ce (Ⅳ)/Ce( Ⅲ) of zircon ranges from 240 to 530. Low temperature and high oxygen fugacity are consistent with its arc characteristics. Considering the tectonic evolution of eastern China in the Early Cretacenus-Late Jurassic era, we proposed that the Jingde granodiorite was formed by lower crust partial melting triggered by slab rollback.
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