甘肃南祁连党河南山中奥陶世火山岩的地球化学特征

THE GEOCHEMICAL CHARACTERISTICS OF ORDOVICIAN VOLCANIC ROCKS IN SOUTHERN DANGHE MOUNTAIN IN SOUTHERN QILIAN MOUNTAINS,GANSU PROVINCE

甘肃南祁连褶皱带党河南山地区中奥陶世火山岩的岩石学和微量元素、稀土元素的地球化学等研究证明,本区火山岩由基性火山岩和中性火山岩组成。其中,基性火山岩分属碱性玄武岩系列和拉斑玄武岩系列;而中性火山岩为一套钙碱性安山岩,二者均具有低钾、高钠的特点。这些火山岩的稀土元素配分曲线均属于轻稀土富集型,(w(La)/w(Yb))N=2 73～7 777,(w(La)/w(Sm))N=1 43～3 21,(w(Gd)/w(Yb))N=1 01～1 92,明显地表现为轻稀土元素配分曲线陡倾,而重稀土元素配分曲线相对平坦的特征。微量元素配分曲线图表现为大离子亲石元素的富集和Nb、Ta、Zr、Hf及Ti不同程度的亏损。这些岩石学和地球化学的特征证明本区火山岩形成于岛弧环境。

It is suggested basic lava and intermediate lava constitute the volcanic rocks by the testing of rare earth elements and trace elements in southern Danghe Mt. in the southern Qilian fold belts. The basic lava can be divided into two types: they are the alkalic basalt series and the tholeiite series while the intermediate lava belong to calcic-alkalic series. Both of them have lower potassium and high sodium. The REE distribution patterns of the volcano indicate that they have a rich light REE, (w(La)/w(Yb))_N=2.73～7.77, (w(La)/w(Sm))_N=1.43～3.21, (w(Gd)/w(Yb))_N=1.01～1.92. The curve is steep in light REE while relatively flat in heavy REE. The trace elements distribution patterns show that the element of Sr, K, Rb, Ba are relatively rich while the element of Nb, Ta, Zr, Hf, Ti are relatively poor. All these characteristics reflect that the volcano was formed in the island-arc environment.