榴辉岩类捕虏体的地球化学特征及其构造意义——来自徐淮地区中生代侵入杂岩中的信息

Geochemical Characteristics of Eclogite Xenoliths in Mesozoic Intrusive Complex from Xu-Huai Area and Its Tectonic Significance

利用X荧光光谱和ICPMS测试了徐淮地区中生代侵入杂岩中榴辉岩类捕虏体的组成.主量元素为:SiO2的含量为40.88%～47.82%,TiO2为0.78%～2.00%,Al2O3为11.72%～16.08%,FeO(全铁)为12.77%～20.29%,MgO为5.23%～11.47%,CaO为7.80%～13.11%,Na2O为0.30%～2.09%,K2O为0.13%～0.69%.榴辉岩类捕虏体具有较低的稀土元素丰度(REE=(34.26～115.51)×10-6),且具有明显的Nb、P、Zr和Ti的负异常.捕虏体的I(Sr)和ε(Nd,t)值分别为0.7042～0.7085和-5.77～2.99(L-4为-13.05).上述特征暗示本区榴辉岩类捕虏体的原岩具有岛弧拉斑玄武岩的特征.榴辉岩类捕虏体Nd的模式年龄和其中锆石SHRIMPUPb年龄显示它们应属于华北克拉通的基底物质.华北克拉通基底物质发生的榴辉岩相变质作用暗示中生代早期华北克拉通东部曾存在一次重要的陆壳加厚过程,这应与扬子克拉通沿北西或北西西方向深俯冲于华北克拉通之下有关.

Using X-fluorescence spectrum and TCP-MS tests eclogite xenoliths components in Mesozoic intrusive complex from Xu-Huai area. The major element components are as following, SiO2: 40.88%-47.82%, TiO2: 0.78%-2.00%, Al2O3: 11.72%-16.08%, FeO (total iron): 12.77%-20.29%, MgO: 5.23%-11.47%, CaO: 7.80%-13.11%, Na2O: 0.30%-2.09%, K2O: 0.13%-0.69%. Eclogite xenoliths have a relatively low REE abundance [∑REE=(34.26-115.51) × 10-6] and obvious Nb, P, Zr and Ti negative anomaly. The I(Sr) and Ε(Nd, t) values of eclogite xenoliths are 0.7042-0.7085 and -5.77-2.99 (L-4:-13.05) respectively. These characteristics indicate that their protoliths could be the tholeiitic rocks formed in an island arc environment. The Nd model ages and zircon SHRIMP U-Pb dating of the eclogite xenoliths suggest that they should belong to the basement of the North China craton (NCC). The eclogite phase metamorphism of the basement of the NCC implies that an important continental crustal thickening could have taken place in the eastern part of the NCC in early Mesozoic, which could be attributed to the deep subduction of the Yangtze craton beneath the NCC in an NW or NWW direction.