狮泉河- 纳木错特提斯洋盆的俯冲极性：岛弧岩浆岩年代学和岩石地球化学证据

阿翁错- 盐湖复式岩体位于狮泉河- 纳木错蛇绿混杂岩带北缘，为狮泉河- 纳木错特提斯洋俯冲消减、碰撞造山过程中岩浆响应的重要组成部分，其研究对认识狮泉河- 纳木错特提斯洋盆的演化具有重要意义。本文以阿翁错- 盐湖复式岩体为研究对象，开展了系统的地质学、地球化学和锆石U- Pb年代学研究，总结了复式岩体的时空分布规律，探讨了复式岩体的成因类型、源区特征和构造背景。复式岩体主要由石英闪长（玢）岩、二长闪长岩、花岗闪长岩、黑云母正长花岗岩、（黑云母）二长花岗岩等组成，为一套岛弧岩浆岩组合，成岩年龄集中在120~104 Ma，属于早白垩世末期，空间分布上具从南到北，由早到晚的趋势。岩石地球化学特征显示从石英闪长岩到二长花岗岩具有由钙碱性系列岩石向钾玄岩系列演化的趋势，主体属于准铝质—弱过铝质I型花岗岩，轻稀土元素分馏程度较高，而重稀土近于平坦，具有弱的负铕异常；富集Rb、K等大离子亲石元素，Ta、Zr、Hf等高场强元素和轻稀土元素，相对亏损Nb、P、Ti等高场强元素和重稀土元素，具有岛弧岩浆岩的特征。基于本次研究结果，结合区域构造背景认为，阿翁错- 盐湖复式岩体形成于狮泉河- 纳木错特提斯洋盆北向俯冲的背景，来自俯冲板片的流体上升，引起上覆地幔楔物质发生部分熔融，形成幔源岩浆，在密度差的作用下幔源岩浆向上运移，底侵至新生下地壳，致其发生部分熔融，并与之发生混合作用，形成了阿翁错- 盐湖复式岩体的母源岩浆，早白垩世末期（104.8±1.4 Ma）狮泉河- 纳木错特提斯洋壳仍持续向北俯冲，下白垩统竟柱山组磨拉石建造的沉积才意味着洋盆彻底关闭。

Subduction polarity of Shiquanhe- Namco Tethys ocean basin: evidence from geochronology and geochemistry of island arc magmatic rocks

The Awengco- Yanhu complex pluton is located in the northern margin of the Shiquanhe- Namco ophiolite mélange zone. It is an important archive of magma response during the subduction, collision and orogeny of the Shiquanhe- Namco Tethys ocean. Research on the complex pluton is of great significance for understanding the evolution of the Tethys ocean basin on the Qinghai- Tibet Plateau. In this paper, a systematic study of geology, geochemistry and zircon U- Pb geochronology has been carried out in the Awengco- Yanhu complex pluton. The spatial- temporal distribution of the complex pluton is summarized, and the genetic type, source characteristics and tectonic background of the complex pluton are discussed. The complex pluton is mainly composed of quartz diorite, monzodiorite, granodiorite, syenite granite and monzogranite. It is a set of island arc magmatic rock assemblage. Its diagenetic age is 120~104 Ma, belonging to the end of Early Cretaceous, and its spatial distribution has a trend from south to North, and from early to late. The geochemical characteristics of the rocks show that from quartz diorite to monzogranite, there is a trend of evolution from calc alkaline series to shoshonite series. The main body belongs to quasi- aluminous to weakly peraluminous I- type granite, with high fractionation degree of light rare earth elements. However, there is no obvious differentiation of heavy rare earth elements, a weak negative Eu anomaly, enrichment of Rb, K and other large ion lithophile elements, Ta, Zr, HF and other high field strength elements. Light rare earth elements are relatively depleted in high field strength elements such as Nb, P, Ti and heavy rare earth elements, which are characterized by island arc magmatic rocks. Based on the results of this study, combined with the existing regional data, it is considered that the Awengco- Yanhu complex pluton was formed in the background of northward subduction of the Shiquanhe- Namco Tethys ocean basin. The fluid rising from the subducted plate resulted in the partial melting of the overlying mantle wedge material and the formation of mantle derived magma. Under the effect of density difference, the mantle derived magma migrated upward and underintruded into the new generation. During the end of Early Cretaceous (104. 8±1. 4 Ma), the Shiquanhe- Namco Tethys Oceanic crust continued to subduct northward, and the molasse formation in the Jingzhushan Formation in the early Late Cretaceous meant that the ocean basin was completely closed.