西藏夏京地区花岗闪长岩锆石U-Pb年代学、岩石地球化学及其构造地质意义

夏京地区花岗闪长岩体是新特提斯洋演化过程中的重要岩浆活动产物。岩石的形成年龄为52Ma左右,属于早始新世。夏京地区早始新世花岗闪长岩体具富钠、富镁铁、低钛、低磷的特征,属于准铝质的钙碱性—高钾钙碱性系列Ⅰ型花岗岩。这些花岗岩具有高的La/Ta值(27. 61～97. 35),指示其岩浆起源于地幔或与之有关;同时夏京岩体具有高的Th/Nb值(1. 83～3. 95)和低的Sm/Nd值(0. 17～0. 21),反映岩浆中有地壳物质的加入; A/MF-C/MF源区判别图解显示为基性岩的部分熔融;以上表明,夏京岩体岩浆源区为壳幔混合源区基性岩的部分熔融的产物。结合区域上岩浆活动特征,认为夏京花岗闪长岩体是印度—欧亚板块碰撞过程中一复杂地质过程的岩浆作用产物,可能与新特提斯洋板片的回转断离有关。

Zircons U-Pb chronology, geochemistry and tectonic geological significance of granodiorite in Xiajing area,Tibet

The granodiorite pluton in Xiajing area is an important product of magmatic activity during the evolution of the Neo Tethys Ocean.The formation age of the rocks is about 52million, which belongs to the early Eocene. The early Eocene granodiorite in Xiajing area is characterized by rich sodium, magnesium and iron, low titanium and phosphorus. It belongs to the calc-alkaline - high potassium calc-alkaline series I the granites with quasi-aluminum property. These granites have high La/Ta value (27.61-97.35), indicating that the magma originatea from the mantle or was related to it. Meanwhile, Xiajing rock mass has high Th/Nb value (1.83-3.95) and low Sm/Nd value (0.17-0.21), reflecting the addition of crustal materials in magma. A/MF-C/MF source area discrimination diagram shows partial melting of basic rocks. The above results indicate that the magma source area of Xiajing rock mass is the product of partial melting of basic rocks in the crust-mantle mixed source area. Combined with the characteristics of regional magmatic activity, it is considered that the Xiajing granodiorite is the magmatic product of the complex geological process during the collision of Indo-Eurasia plate, which may be related to the rotation and dissociation of the Neo-Tethys oceanic plate.