冈底斯曲水岩基岩浆混合:来自暗色岩浆包体角闪石显微结构的证据

冈底斯岩浆带位于拉萨地体南缘，其形成过程主要受中生代新特提斯洋板片俯冲和新生代印度-亚洲陆-陆碰撞控制，是揭示青藏高原形成过程和深化大陆动力学研究的天然实验室。曲水岩基位于冈底斯岩浆带中段，介于拉萨和曲水之间，主要由花岗闪长岩、花岗岩、闪长岩和辉长岩组成。岩基花岗质岩体中包含大量暗色岩浆包体，包体产出状态有同侵位岩墙、包体墙、包体群等，表明岩浆混杂与混合现象。前人关于曲水岩基做了大量研究工作，取得很多进展，比如，发现这些暗色岩浆包体与寄主岩具有相同的结晶时代，主要集中在55~45Ma。但是，关于曲水岩基形成在俯冲背景还是碰撞背景还存在着争论。这些广泛分布的暗色岩浆包体和寄主岩的关系，及其所代表的岩浆混合过程还需要精细的矿物学工作。因此，本文在综合分析野外岩性分布、暗色岩浆包体出露形态的基础上，重点选择花岗闪长质寄主岩和其中的暗色岩浆包体中的角闪石进行矿物显微结构和构造的分析，并结合电子探针数据，以探求曲水岩基的岩浆混合过程。我们初步认为曲水岩基的形成经历两期混合过程：早期的基性岩浆和酸性岩浆端元在深部的混合；晚期基性、酸性岩浆混合后的中性岩浆爆破、上升，并继续与酸性岩浆混合。此外，曲水岩基形成于俯冲到碰撞的转换过程，受控于俯冲板片作用所产生的弧型岩浆和板片回旋及稍后的断离所产生的地幔岩浆双重作用。

Magma mixing of the Quxu batholith in the Gangdese belt, southern Tibet: Evidence from microstructure of hornblende in microgranular enclaves

The Gangdese batholith in the southern margin of the Lhasa terrane provides important windows to the formation process of the Tibetan Plateau and continental geodynamics of Earth as its formation is closely related with the subduction of the Neotethyan oceanic lithosphere and the Indo-Asian collision. The Quxu batholith is located in the central part of the Gangdese belt, spanning from Lhasa City to Quxu City. This batholith is mainly composed of granodiorite, granite, diorite and gabbro. Voluminous microgranular enclaves are enclosed within the dioritic-granitic plutons, for example syn-plutonic dyke, enclave dyke, enclave swarm, etc, implying significant magma mingling and mixing. Lots of progresses about the formation of the Quxu batholith have been achieved by previous workers, for instance, the enclaves and their host plutons have synchronous crystallization timing, etc. However, questions remain regarding the geological setting of the Quxu batholith and the magma mixing process between the enclaves and their host magma. Here, we will provide detailed observations of the hornblende microstructures from the enclaves, as well as their mineral compositions, in combination with the field observations, to address the magma mixing process. The results reveal that the Quxu batholith experienced two stages of magma mixing: early mixing between mafic and granitic magmas and later mixing between dioritic and granitic magmas. The Quxu batholith was formed during the transitional period from ocean-continent subduction to continent-continent collision, whose magma source was dominated by the arc-type magma induced by the subduction of the Neotethyan oceanic slab and the upwelling of the asthenosphere triggered by the slab rollback and subsequent breakoff.