【摘要】 阿翁错复式岩体位于班公湖-怒江缝合带西段,是班公湖-怒江特提斯洋俯冲消减,造山过程中岩浆响应的重要组成部分,以广泛发育暗色微粒包体和岩浆混合、不协调现象为特征。本文以阿翁错复式岩体为研究对象,对寄主岩和暗色微粒包体开展了系统的地质学、地球化学和锆石U-Pb年代学研究,探讨了阿翁错复式岩体的岩浆混合成因。暗色微粒包体塑性变形特征明显,与寄主岩呈截然或渐变接触,偶见反向脉发育,包体具细-中粗粒结构,含斜长石、钾长石、角闪石、暗色镶边石英等斑晶,偶见角闪石斑晶横跨包体和寄主岩,在包体及包体周围寄主岩中见长柱状斜长石、角闪石和针状磷灰石等结构特征,表明暗色微粒包体为岩浆混合作用的产物。寄主岩与包体均为准铝质、钙碱性-高钾钙碱性系列岩石,主要氧化物含量在Harker图解上具有良好的线性关系,稀土元素配分曲线图和微量元素蛛网图具有高度一致性,表明二者具有强烈的地球化学亲源关系,且经历了相似的岩浆演化过程。寄主岩和暗色微粒包体的成岩年龄分别为109. 1±1. 0Ma和107. 4±0. 7Ma,岩浆混合作用发生在早白垩世晚期,处于班公湖-怒江特提斯洋由弧-陆碰撞向陆陆碰撞的转换阶段即软碰撞阶段。研究表明,在班公湖-怒江特提斯洋向北向羌塘地块之下俯冲的背景下,洋壳脱水,引起上覆地幔楔发生部分熔融,形成镁铁质岩浆,镁铁质岩浆向上运移,并底侵于壳-幔边界,引发下地壳物质发生部分熔融,形成长英质岩浆,当镁铁质岩浆从底部注入长英质岩浆房时,镁铁质岩浆快速冷凝,形成部分色率高、粒度细,具冷凝边的包体,与寄主岩呈截然型接触,随着端元岩浆之间的温差逐渐降低,包体色率降低,粒度变大,与寄主岩呈渐变过渡。更多还原

【Abstract】 Located at the western part of Bangong Co-Nujiang Suture Zone,the Awengcuo composite plutons are the important part result in the magmatic response during the subduction of the Bangongco-Nujiang Tethys Ocean,and is featured by widely developed mafic microgranular enclaves,the magma mingling and the magma incoordination phenomenon. Taking Awengcuo composite plutons as the research object,this paper conducts an in-depth study on the geological characteristics and geochronology of mafic microgranular enclave and its formation mechanism. Mafic microgranular enclave has obvious plastic deformation features,and has sharp or gradual contact with the host rock,presenting still a little reverse pulse development. With the structure of fine to medium-coarse,the enclave contains many phenocrysts such as plagioclase,potash feldspar,hornblende,and dark rimmed quartz. Besides,plagioclase phenocryst is occasionally found across the enclave and the host rock,and long columnar plagioclase,hornblende,acicular apatite are also existed in the enclave and the host rock. All of them show that mafic microgranular enclave is produced by magma mixing. Host rock and MMEs are transitional rock such as quasi-aluminum,high-calcium alkaline-potassium kaolin series,and the main oxide content in the Harker diagram has a good linear relationship. Rare earth element distribution curve and normalized spidergram for trace element primitive mantle are with high degree of consistency. It shows that there is a strong geophysical relationship between the two groups,and experience the similar magmatic evolution process. A good linear relationship is showed between the host rock and inclusions in the denominator covariant map of the principle and trace elements. And their relationship is closely related to the magmatic mixing. Both of them contain a lot of large ion lithophile elements such as Rb,K,U,Th,and lack high field strenghth elements Nb,Ta,P,Ti. Dark Microgranular is low-silicon,but rich with Mg,Fe,Ca,The ratio of Nb/Ta is 17. 6,indicating that the original magma of the inclusions may be derived from the mantle wedge by fluid of subduction zone. While the host rock is relatively high in silicon and with lower Mg,Fe and Ca,Nb/Ta ratio is 9. 85,indicating that the host rock is the product of mantle-derived magma mixed with the molten lower crust magma,mainly with molten lower crust magma. The LA-ICP-MS single-zircons U-Pb dating demonstrates that the magmatic age is 109. 1 ± 1. 0 Ma for the host rock and 107. 4 ± 0. 7 Ma for the enclave. The two ages are basically the same,suggesting that the mixing event took place during the period of Early Cretaceous,during the soft collision phrase which the Bangong Co-Nujiang Tethys Ocean is transitional from the ocean-continent subduction to continent-continent subduction. Research shows that during BangongcoNujiang Tethyan Oceanic plate north-forward subduction to the Qiangtang block,the dehydration of oceanic crust leads to the partial melting of the overlain mantle wedge,and produces mafic magma. The mafic magma transfers upward and eventually intrudes into crust-mantle boundary,which causes the partial melting of the lower crustal materials and forms felsic magma. When the mafic magma flows at the bottom into the felsic magma chamber,it will condensates rapidly and become some enclave with high color ratio,fine granularity and condensation rim. Such enclave is in sharp contact with the host rock,while with the gradually declining temperature between end-member magmas,the color ratio of the enclave is becoming lower and its granularity bigger,then the enclave has a gradational contact with the host rock.更多还原