柴北缘牛鼻子梁地区花岗闪长岩锆石U-Pb年代学及地球化学研究

Zircon U-Pb geochronology and geochemistry of Niubiziliang granodiorite in northern margin of Qaidam Basin

柴北缘牛鼻子梁地区的花岗闪长岩锆石U-Pb年代学及岩石地球化学研究结果表明,该花岗闪长岩的锆石LA-ICP-MS U-Pb谐和年龄为393.2~395.6Ma,对应的加权平均年龄为(394.4±2.4)Ma,表明该岩石形成于中泥盆世。岩石主量元素具有较高的SiO_2(65.70%~69.22%)和Al_2O_3(14.96%~16.03%)、较低的MgO(1.37%~1.89%)含量,A/CNK值为0.95~0.98,属于钙碱性、准铝质系列,具有I型花岗岩的特征,表明其岩浆起源于下地壳部分熔融。岩石稀土总量较高,无明显的Eu异常(δEu=0.95~1.30),富集轻稀土元素和K、Rb、Ba、U等大离子亲石元素,相对亏损重稀土元素和Nb、Ta、Ti、Th、Zr等高场强元素。综上可知,岩石具有碰撞后花岗岩的地球化学特征属性,结合区内晚泥盆世牦牛山组陆相火山岩和区域构造演化特征,认为牛鼻子梁地区的花岗闪长岩形成于柴达木地块与南祁连地块碰撞后的伸展环境。

In this study,the authors present new zircon U-Pb age and whole-rock major and trace element data for the Niubiziliang granodiorite in the northern margin of Qaidam Basin. LA-ICP-MS zircon U-Pb dating demonstrates that the concordant age and the corresponding weighted mean age of the granodiorite are 393. 2 -395. 6 Ma and（ 394. 4 ± 2. 4） Ma,respectively,indicating that the intrusion occurred in the Middle Devonian.The granodiorites are characterized by high SiO_2（ 65. 70% - 69. 22%） and Al_2O_3（ 14. 96% - 16. 03%）,but low MgO（1. 37% - 1. 89%）,the rocks fall into the calc-alkaline and metaluminous series with the A/CNK values of 0. 95 - 0. 98,being of the characteristics of I-type granite,suggesting that the primary magmas of granodiorites were derived from partial melting of the lower crust. They have high rare elements contents,without obvious Eu anomalies（ δEu = 0. 95 - 1. 30）. They are enriched in LREEs and large ion lithophile elements（ e. g. K,Rb,Ba and U）,depleted in HREEs and high field strength elements（ e. g. Nb,Ta,Ti,Th and Zr）. Meanwhile,thegranodiorites are similar to the post-collision granite geochemically. Integrating the continental volcanic rocks of Maoniushan Formation in the Late Devonian and regional tectonic evolution,we conclude that the granodiorite intrusion was formed in the extensional setting after the collision between the Qaidam block and Southern Qilian block.