华北克拉通南缘中条山-嵩山地区1.78Ga基性岩墙群的地球化学特征及构造环境

幔源岩浆侵位产生的基性岩墙群是地壳伸展裂解的重要标志。华北克拉通南缘中条山-嵩山地区出露有大量的中元古代基性岩墙群,对其研究将有助于深入了解华北克拉通中元古代构造演化特征。其岩石类型以辉绿岩为主,少量辉长辉绿岩和辉绿玢岩;主要造岩矿物为斜长石和单斜辉石,其它矿物包括角闪石、Fe-Ti氧化物、磷灰石、黑云母、碱性长石和石英。锆石SHRIMPU-Pb年龄为1785±18Ma,代表岩墙的结晶年龄。岩石K2O+Na2O含量为3.63%～6.18%,K2O/Na2O比值为0.73～1.38,FeOT含量较高(10.03%～13.59%),属于拉斑玄武岩系列。岩石的稀土元素含量高(142×10-6～381×10-6),亏损Nb、Ta、Zr、Hf等高场强元素,富集Rb、Ba、La等大离子亲石元素,全岩εNd(t)值为-6.2～-8.1。岩墙的固结指数(SI)和MgO含量呈明显的正相关关系,说明幔源岩浆发生过明显的结晶分异作用。地球化学分析表明,研究区基性岩墙属于板内拉斑玄武岩系列,与熊耳群火山岩有相似的地球化学特征;基于两者紧密的产出关系,我们认为该区岩墙可能是熊耳群火山岩的通道岩墙,共同代表了华北克拉通东、西陆块碰撞后伸展事件。

Geochemistry and tectonic setting of the 1.78 Ga mafic dyke swarms in the Mt. Zhongtiao and Mt. Song areas, the southern margin of the North China Craton

Emplacement of the mafic dykes from mantle magmas represents the extension and rift event of crust. Many Mesoproterozoic mafic dyke swarms occur in Mt. Zhongtiao and Mt. Song areas, southern margin of North China Craton (NCC). They are mainly diabase with minor gabbroic diabase and algovite, which consist of plagioclase and chinopyroxene with minor hornblende, Fe-Ti oxides, apatite, biotite, alkali feldspar and quartz. Zircon SHRIMP U-Pb dating indicates that the crystallization age of the dykes is 1785±18Ma. These dykes, similar to the Xiong'er Group volcanic rocks, belong to tholeiitic series. They are characterized by enrichment in large ion lithophile elements (LILEs, e.g. Rb, Ba, REEs), but depletion in high field strength elements (HFSEs, e.g. Nb, Ta, Zr, Hf). Solidification index (SI) shows a positive correlation with MgO contents of rocks, indicating an obviously fractional crystallization process. These dykes are plotted into the field of Within Plate Basalt (WPB) in the discrimination diagrams, together with the Xiong'er Group volcanic rocks, indicating that they have formed in an extension setting after the amalgamation of the eastern and western blocks of the North China Craton. The mafic dykes are possibly the counterpart of the Xiong'er Group volcanic rocks.