岩浆混合作用的识别:以义敦岛弧稻城岩体为例  

Identification of Magma Mixing: A Case Study of the Daocheng Batholith in the Yidun Arc

作　　者：张瑞刚 高雪[1] 杨立强[1] Zhang Ruigang;Gao Xue;Yang Liqiang(State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Beijing 100083,China)

机构地区：[1]中国地质大学地质过程与矿产资源国家重点实验室,北京100083

出　　处：《地球科学进展》2018年第10期1058-1074,共17页Advances in Earth Science

基　　金：国家重点基础研究发展计划项目"中国西南特提斯典型复合成矿系统及深部驱动机制"(编号:2015CB452605);高等学校学科创新引智计划"成矿作用动力学"(编号:B07011)资助~~

摘　　要：义敦岛弧稻城岩体由花岗岩、花岗闪长岩和钾长花岗岩组成。大量暗色镁铁质微粒包体发育于花岗闪长岩和钾长花岗岩中,且其与寄主岩石的接触界线截然。暗色镁铁质微粒包体具有细粒结构,发育石英眼构造、针状磷灰石和具环带结构的斜长石斑晶。通过暗色镁铁质微粒包体与寄主岩石的岩相学、斜长石环带和主微量稀土元素特征的研究,识别出稻城岩体的岩浆混合作用,并结合前人对该岩体Sr-Nd-Hf同位素的研究成果,对稻城岩体的岩石成因进行了讨论。暗色镁铁质微粒包体中斜长石牌号为29～44,寄主岩石中斜长石牌号为21～50,斜长石斑晶的成分突变和熔蚀结构与岩浆混合作用有关。暗色镁铁质微粒包体的Si O2质量分数较低(56.34%～60.91%),Al2O3质量分数较高(16.06%～17.98%)、富镁铁,具有准铝质特征(A/CNK=0.82～0.98);而寄主岩石属于高钾钙碱性系列,富碱(Na2O+K2O的质量分数为6.25%～7.79%)、贫钙(CaO质量分数为1.40%～3.22%)。暗色镁铁质微粒包体与寄主岩石均富集大离子亲石元素(Rb,K和Pb)和轻稀土元素,亏损高场强元素(Nb,Ta,Zr,Hf,P和Ti),与典型岛弧岩浆岩的微量元素特征一致。但与寄主岩石相比,暗色镁铁质微粒包体具有近平坦的稀土配分曲线((La/Yb)N=1.99～2.46)和较明显的Eu负异常(Eu/Eu*=0.30～0.50)。寄主岩石Rb/Sr值范围为1.0～1.9,与壳源(Rb/Sr>0.5)的物质范围一致; Zr/Hf值范围为27.5～36.9,与地壳和幔源岩石之间Zr/Hf值(33.0～36.3)接近,表明稻城岩体的成因与幔源物质和壳源物质的混合有关。暗色镁铁质微粒包体具有较低Si含量和较高Mg#,Nb/Ta值范围为11.0～14.5,且包体和寄主岩石的主要氧化物与其Si O2呈线性分布,表明暗色镁铁质微粒包体的形成与岩浆混合作用有关。稻城花岗质岩体的母岩浆起源于晚三叠世岛弧下地壳的部分熔融,且源区有少量幔源物质的加入;暗色镁铁质微粒�The Daocheng batholiths, located in the east of the Yidun arc, consist of granite, granodiorite and K-feldspar granite. Abundant massive mafic microgranular enclaves （MMEs） mainly developed within the grano- diorite and K-feldspar granite, and they have clear contacts with the hosted granites. The MMEs are characterized by the quartz eye structure, quenched apatite, and plagioelases phenocrysts with obvious oscillatory zones. Petro- graphical studies on MMEs and host granites, zoned plagioclase and whole-rock geochemical analysis were carried out to identify the presence of magma mixing. Combined with the previous studies on the whole-rock Sr-Nd-Hf iso- topic signatures, the petrogenesis of Daoeheng batholith was discussed. The zoned plagioclases from MMEs have An contents varying between 29 and 44, while those from the host granites have An contents of 21 - 50. The composi- tional variations and corrosion structure of plagioclase are probably related to magma mixing. Geochemically, the MMEs have relatively low SiO2 contents （56.34-60.91wt%）, high A1203 contents of 16.06-17.98wt%, and are enriched in magnesium and iron, belonging to metalumnious series （A/CNK= 0.82-0.98）. The Daocheng batholith belongs to high-K eale-alkaline series, which have high alkaline contents （Na20＋K20= 6.25-7.79wt%） and low CaO contents （ 1.40- 3.22wt%）. Furthermore, both the MMEs and hosted granites are enriched in LILEs （ K, Rb and Pb） and LREEs and depleted in HFSEs （Nb, Ta, Zr, Hf, P and Ti）, showing affinities of typical arc magmas. Compared with the host granites, the MMEs are characterized by lower （La/Yb） N ratios of 1.99 to 2.46, and much more obvious Eu depletions （Eu/Eu ＊ = 0.30-0.50）. The host granites have Rb/Sr ratios ranging from 1.0 to 1.9, and they are consistent with the crust-derived materials （Rb/Sr〉0.5）. Their Zr/Hf ratios range from 27.5 to 36.9, which are close to the transitional Zr/Hf ratios between mantle-and crust-derived materials. This indicates that the formation

关 键 词：岩浆混合 岩石成因 岩石地球化学 稻城岩体 义敦岛弧 

分 类 号：P588.1[天文地球—岩石学] P595[天文地球—地质学]