青藏高原大陆碰撞环境埃达克质岩浆富K2O形成机制研究:以马厂箐岩体为例

Previous studies have suggested that the input of ultrapotassic magma increased K2O in adakites during continiental collision zones. However, there is no direct evidence for the interaction between ultrapotassic and adakitic magmas. Machangqing is a typical porphyry Cu-Mo deposit, located in the eastern part of the Tibet. Magmatic activities are strong in the mining area, including adakitic rocks and ultrapotassic lamprophyre, which are closely related in time and space. The mineralization is related to the adakitic rocks, which occured during the continental collision zones. We newly discovered a mafic microgranular enclave(MME2) from the adakitie at Machangqing. The MME2, which most likely was derived from the mixing of ultrapotassic and adakitic magmas, shows both adakite and ultrapotassic affinities, provides an exceptionally excellent opportunity to better understand the mechanism of K2O-rich in adakites. In this proposal, we will conduct detailed mineralogy, petrology, whole rock geochemistry, isotope geochemistry and zircon Hf isotope analysis to the MEE2. When combined with the nature of adakite and lamprophyre, we will first discuss the change of elements in the MEE2 during the mixing process, then trace the source, explore the magmatic evolution of the MEE2, adakite, and lamprophyre, as well as the genetic relationships among them. And understand the roles of ultrapotassic magmas in the formation of adakitic magmas. Finally assess the mechanism of K2O-rich in adakites during the continental collision zones, which has important significance for the ore model of collision-related porphyry deposits, and the systematically comparison between subduction- and collision-related porphyry deposits.

前人认为同时代的超钾质岩浆注入是大陆碰撞环境下埃达克岩富钾的主要原因之一，但一直未找到两类岩浆相互作用的直接证据。马厂箐斑岩Cu-Mo矿床位于青藏高原东部，矿区内成矿埃达克岩与煌斑岩在时空上密切相关，前者中新发现一套类似超钾质岩的包体（MME2），初步研究表明该类包体可能是超钾质岩浆与埃达克质岩浆混合的产物，可以为探讨埃达克岩富钾机制提供直接证据。本项目拟对MME2开展精细的矿物学、岩石学、主微量元素、Sr-Nd-Pb同位素、锆石年代学和Hf同位素工作，结合埃达克岩、煌斑岩的特征，分析超钾质岩浆与埃达克质岩浆混合过程中MME2被改造的过程，探讨MME2、埃达克岩和煌斑岩的源区性质、岩浆演化过程以及三者之间的成因联系和差异，查明超钾质岩浆的注入是否导致埃达克岩富钾，进而揭示大陆碰撞环境埃达克岩富K2O的形成机制，为完善大陆碰撞环境下斑岩型矿床成矿模型、岩浆弧与碰撞环境下斑岩型矿床类比提供依据

与俯冲环境下形成的埃达克质岩浆相比，大陆碰撞环境成矿相关的埃达克质岩浆具有更高的K2O含量和K2O/Na2O比值，目前，大陆碰撞环境下埃达克岩富钾机制存在一定的争议。马厂箐斑岩Cu-Mo矿床位于青藏高原东南部，矿区内发育的侵入杂岩体包括成矿埃达克岩、镁铁质包体（含金云母的Type-I MMEs和不含金云母的Type-II MMEs）和煌斑岩。成矿埃达克岩斑状花岗岩和花岗斑岩的结晶年龄分别为35.4±0.4Ma和35.2±0.3Ma，具有高的SiO2、Al2O3含量和Sr/Y比值以及相对低的Y、Ni含量和Mg#值，其(87Sr/86Sr)i=0.7066~0.7071,εNd(t)=-5.5~-3.3,锆石εHf(t)=0.0~+2.2，表明埃达克岩斑状花岗岩和花岗斑岩均起源于俯冲改造的增厚基性下地壳的部分熔融。煌斑岩的成岩时代为34.1±0.2 Ma，并具有高的K2O、MgO量和K2O/Na2O）比值，以及高Rb/Sr值和低Ba/Rb、Nb/U比值，其(87Sr/86Sr)i=0.7073~0.7084, εNd(t) =-3.2~-2.8，表明煌斑岩为受俯冲板片流体交代的富集岩石圈地幔低程度熔融的产物。MMEs的结晶年龄约34.0Ma。在Type-I MMEs中识别出钾质-超钾质岩（如煌斑岩等）中存在的金云母。MMEs具有较低的SiO2含量，高的MgO、K2O含量及K2O/Na2O比值，显示出钾质-超钾质岩的特征。其(87Sr/86Sr)i=0.7064~0.7076,εNd(t)=-6.6~-4.1,206Pb/204Pb=18.722~19.071，锆石εHf(t)=-0.6~+3.2。岩相学和地球化学特征表明，马厂箐镁铁质暗色包体是幔源超钾质熔体与下地壳熔融的埃达克质熔体混合的产物。结合前人研究成果，我们提出岩浆混合模式解释碰撞环境成矿埃达克岩富K2O机制：①俯冲板片流体交代的岩石圈地幔部分熔融形成钾质-超钾质基性熔体；②钾质-超钾质镁铁质熔体导致加厚的新生下地壳（石榴石-角闪岩相）部分熔融，形成埃达克质岩浆熔体；③约80%的埃达克质岩浆熔体与约20%钾质-超钾质岩浆熔体在上地壳岩浆房和/或下地壳MASH带发生混合，导致埃达克质岩浆K2O含量升高，同时岩浆上升侵位过程中的分异作用使埃达克质侵入体的K2O含量进一步富集。