【摘要】 本文在研究西藏北拉萨块体中段桑心日岩体中的暗色包体时发现了一种具有特殊岩石成因的暗色包体。暗色包体呈椭球状,在暗色包体和寄主岩的接触面上通常形成一个明显的可能由风化作用造成的间隙面。暗色包体为二长玢岩-花岗闪长玢岩,寄主岩为花岗岩,暗色包体明显较寄主岩更基性,更富Na₂O、CaO、MgO和Fe₂O₃^T。暗色包体和寄主岩具有明显不同的稀土元素特征,暗色包体的稀土元素含量变化较大,最基性样品具有最高的稀土含量,随着基性程度的降低稀土元素含量明显下降。随着岩浆的进一步演化,岩浆向花岗岩方向演化,稀土含量又逐步升高。寄主岩和暗色包体具有基本一致的微量元素组成,具有典型的弧岩浆岩的特征,富集Rb、Cs、K等大离子亲石元素和Th、U,亏损Nb、Ta、Ti等高场强元素。此外,暗色包体和寄主岩具有明显的Ba、Sr的负异常。暗色包体成岩年龄为75. 6±1. 2Ma,寄主岩的成岩年龄为71. 8±0. 6Ma,暗色包体成岩年龄较寄主岩早约4Myr。两者具有一致的锆石原位Lu-Hf同位素特征。综合以上岩相学、年代学、元素地球化学和同位素地球化学证据,我们认为桑心日暗色包体和寄主岩来源于同源母岩浆,初始岩浆在母岩浆房中经历了不同程度的含钾角闪石结晶分离作用,并沿早期较弱的构造裂隙侵入到地壳的某一层位,随着构造活动进一步加剧,经过进一步分异母岩浆大规模上侵,并将早先侵位处于半塑性状态的暗色包体侵吞、裹挟至近地表。桑心日暗色包体最可能的成因模式可以解释为同源岩浆不同期次间的物理混合。更多还原

【Abstract】 This paper proposes a special petrogenetic model for mafic microgranular enclaves( MMEs) based on a systematic study on the Sangxinri pluton in the middle part of the northern Lhasa Block,Tibet,China. The MMEs found in the Sangxinri host monzogranite are composed of monzonite porphyry to granodiorite porphyry,which usually present axiolite with an compactly cement with host rocks probably caused by weathering. The MMEs are more basic than the host rocks with higher Na2 O,Ca O,MgO and Fe2 O3 T contents. The MMEs have distinct REE characteristics from the host rocks and more variable REE contents than the host rocks.The most basic MME samples have the highest REE contents,while the most acid MME samples have lowest REE contents,indicative of reducing REE contents with increase of Si O2. With evolution of the magma from the MMEs to the host monzogranite,the REE contents rise to a high level. Both the MMEs and host rocks show characteristics of arc rocks with enrichments in large ion lithophile elements( LILEs) Rb,Cs,K,etc.,but depletions in high field strength elements( HFSEs) Nb,Ta and Ti etc. and Sr and Ba elements. Zircon LA-ICP-MS U-Pb dating obtains a 75. 6 ± 1. 2 Ma age and a 71. 8 ± 0. 6 Ma age for the MMEs and host rocks,respectively. Although the MMEs are 4 Myr earlier than the host rocks,they have consistent zircon Lu-Hf isotopes. Synthesizing above petrography,geochemistry and isotopic evidences,we suggest the MMEs are derived from the same source rocks with host rocks. The initial magma experienced various degree of K-bearing hornblende fractional crystallization in the parent magma chamber,and then emplaced along with the early tectonic fault and formed the MMEs. With aggravation of the tectonic motion,massive magma experienced further differentiation upwelled,wrapped and carried the earlier formed MMEs( in semiplastic) to subsurface. The most probable petrogenetic model for the Sangxinri MMEs is magma mingling between the magmas from different periods of the same sourced rock.更多还原