皖东池河地区魏岗山岩体地球化学特征、锆石U-Pb年龄及构造意义

Geochemical Characteristics,Zircon U-Pb Age and Tectonic Significance of the Weigangshan Intrusion in the Chihe Area,Eastern Anhui Province

魏岗山岩体出露于池河-太湖断裂以西,岩体变形较弱,变质作用明显。岩石具较高的SiO2和碱质含量,钾钠含量基本相似,岩浆分异演化不彻底。较低的MgO,较高的Yb、Y含量以及Sr元素亏损和强烈的Eu负异常暗示魏岗山岩体的源区相对较浅。非常低的Mg^#[Mg/(Mg+Fe^*)]值表明岩浆经历过铁镁质矿物的分异作用。A1-A2图解显示岩体形成于拉张环境。岩体的形成应与新元古代超大陆裂解过程相关。通过LA-ICP-MS锆石U-Pb定年,获得206Pb/238U变质年龄为210.3±1.8 Ma,表明该岩体经历了印支期变质,由此推断魏岗岩体可能为大别造山带的组成部分。据此推断,郯庐断裂带池河段的西界断裂可能并非现在的池河-太湖断裂,而是在魏岗山岩体以西、炼铺以东,隐伏在第四系之下。现在的池河-太湖断裂属于叠加于印支期韧性剪切带东界之上的脆性断裂带。

The Weigangshan magmatic intrusion is exposed to the west of the Chihe-Taihu fault,with weak deformation and obvious metamorphism.The rocks have high SiO2 and alkali content,similar K and Na content,and incomplete magmatic differentiation and evolution.The lower MgO,the higher Yb and Y contents,the depletion of Sr element and the strong Eu negative anomaly suggest that the source area of Weigang mountain rock body is relatively shallow.The very low Mg^#[Mg/(Mg+Fe^*)]value indicates that the magma has experienced the differentiation of Fe-Mg minerals.A1-A2 diagram shows that the intrusion was formed in the extension environment.The formation of the intrusion should be related to the process of Neoproterozoic supercontinent breakup.The U-Pb dating of metamorphic zircons by LA-ICP-MS shows that ^206Pb/^238U age is 210.3±1.8 Ma,indicating that the intrusion underwent Indosinian metamorphism,from which it can be inferred that Weigangshan intrusion may be a part of Dabie orogenic belt.It can be concluded that the west boundary fault of the Chihe section of the Tan-Lu fault zone is not the present Chihe-Taihu fault,but lies in the west of the Weigangshan intrusion,the east of Lianpu and under the Quaternary system.The present Chihe-Taihu fault belongs to a brittle fault zone superimposed on the eastern boundary of the Indosinian ductile shear zone.