西藏冈底斯尼木后碰撞花岗岩的岩浆混合作用:显微结构证据

岩浆混合过程中不同熔体之间的相互作用会影响晶体的成核与生长,形成矿物内部复杂的成分变化,以及矿物之间的不平衡结构。尼木二长花岗岩位于冈底斯岩浆岩带中部,是代表性的形成于后碰撞构造演化时期的花岗岩体。本文对其中的斜长石与角闪石颗粒进行了详细的结构和成分分析,揭示了斜长石中的港湾状、浑圆状、筛孔状熔蚀结构以及斜长石成分的突然变化和角闪石包裹黑云母的不平衡结构,并探讨了它们的成因以及相关的岩浆混合作用。分析结果显示,斜长石中突变环带的An含量为37.6~40.6,熔蚀环带的An含量为48.2~59.5,均高于两侧斜长石的An含量(18.4~26.4),表明在形成这些结构时有外来基性岩浆的混合使得岩浆成分发生了突变。样品中的部分黑云母被自形的角闪石包裹,黑云母呈浑圆状并且具有港湾状的熔蚀边,这可能是基性岩浆的混合作用使得岩浆的温度升高导致黑云母发生部分熔融,混合后的岩浆在黑云母周围继续结晶形成角闪石。这些显微结构为揭示冈底斯岩浆岩带的岩浆混合作用提供了新证据。

Magma mixing of the Nyemo post-collisional granite from the Gangdese magmatic belt, Tibet: Evidence of microstructures

The interaction between different melts during magma mixing process would have an influence on the nucleation and growth of the crystals, and could generate complex compositional variations within mineral or form disequilibrium textures between minerals. The Nyemo monzogranite is located in the middle of the Gangdese magmatic belt, and was typically formed in a post-collisional tectonic setting. In this study, the internal texture and chemical composition of plagioclase and amphibole from the Nyemo monzogranite have been analyzed in detail, and many disequilibrium textures have been revealed, including the resorption zone in plagioclase with harbor, round or sieve shapes, the spike composition zone in plagioclase and the occurrence of biotite mantled by amphibole. The An contents of the spike zone (37.6~40.6) and the resorption zone (48.2~59.5) are evidently higher than the An contents of other domains in the same plagioclase grain (18.4~26.4), indicating a change of magma composition during the crystallization of the plagioclase which may be resulted from a magma mixing event. Some biotite grains are mantled by euhedral amphibole and show harbor shaped rims. The resorption of biotite may be related to the rise of magmatic temperature during the magma mixing process, and the amphibole mantle was probably crystallized from the mixed magma. Therefore, the microstructures revealed from the Nyemo monzogranite provide new evidence for the magma mixing and evolution processes of the Gangdese magmatic belt.