摘要
义敦岛弧形成于晚三叠世大规模俯冲造山作用过程中,位于松潘-甘孜地体和羌塘地体之间.稻城边部岩体是义敦岛弧带内规模巨大的复式花岗质岩体,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明显.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.文中以稻城岩体寄主岩石和暗色微粒包体中斜长石、黑云母和角闪石为研究对象,开展岩相学和电子探针原位化学成分分析,厘定了矿物形成的物理化学条件,探讨了岩浆混合作用过程及其形成的构造环境.研究表明:花岗闪长岩和暗色微粒包体中的斜长石主要为中长石,其核部呈浑圆状;前者核部的An值(21~50)显著高于幔部(21~34);后者则发育典型的突变环带,An值(29~44)呈波状变化且相对集中.暗色微粒包体与寄主花岗闪长岩中斜长石的An值部分重叠表明二者形成过程中存在含量的岩浆混合作用.斜长石环带中的An值随Al2O3、FeO、MgO和CaO含量的升高而升高,但随SiO2、Na2O和K2O含量的升高而降低.寄主岩石和暗色微粒包体中角闪石富镁铁,阳离子特征为:CaB=1.56~1.75,Ti=0.08~0.13,属于钙质角闪石,具壳源特征,其结晶温度分别为697~725℃和680~705℃.花岗闪长岩中黑云母的Mg/(Mg+Fe2+)为0.37~0.45,显示出富Fe贫Ca、Mg,属于典型的岩浆成因黑云母.黑云母TiO2含量变化范围为3.54%~4.62%,Al2O3含量变化范围为13.89%~15.15%;黑云母的氧化系数为0.08~0.11,Mg#为0.39~0.46,MF值为0.36~0.44,单位分子中阳离子数AlⅥ为0.03~0.11,以单位分子中Ti和Al阳离子数计算的黑云母结晶温度为584~624℃,表明其结晶温度较高,具壳幔混源特征.稻城岩体是以壳源为主的壳幔混源成因的Ⅰ型花岗岩,暗色微粒包体是由镁铁质岩浆与长英质岩浆不同程度的混合作用形成的.
The Yidun arc,located between the Songpan-Garze Terrane and Qiangtang Block of southwestern China,was formed in response to the Late Triassic large-scale subduction orogenic process.The Daocheng batholith consists of granite,granodiorite and K-feldspar granite.Abundant massive mafic microgranular enclaves(MMEs)developed mainly within the granodiorite and K-feldspar granite,forming clear contacts with the host granites.The MMEs are characterized by the quartz eye structure,quenched apatite,and plagioclases phenocrysts with obvious oscillatory zones.Based on the systematic petrographical,mineralogical and geochemical characteristics of plagioclase,hornblende and biotite from the host rock and MMEs of the Daocheng batholith of western Sichuan,we provide here not only the constraints on the physicochemical conditions for mineral and host rock formation,but also important insights into understanding the magma mixing process and structural setting.Our study shows that the plagioclases in the granodiorite and MMEs are all dominated by andesine with rounded or embayed cores.In the former,the An contents in the cores vary between 21 and 50,significantly higher than the An contents in the mantles(21-34),and in the latter,the An contents(29-44)are waved and concentrated in the mutation annulus.The overlap of An value between MMEs and host granite diorite indicates that magma mixing occurred during their formation.The An contents increase with the increase of Al2O3,FeO,MgO and CaO,but decrease with the increase of SiO2,Na2O and K2O.The hornblendes from the host rock and MMEs are rich in Mg and Fe,with CaBand Ti values of 1.56-1.75 and 0.08-0.13,respectively.They both belong to the calcamphiboles,crystallized at 697-725 ℃ and 680-705 ℃,respectively.The biotites of granodiorite are enriched in iron,depleted in calcium and magnesium,which belongs to typical magmatic biotite.These biotites have Mg/(Mg+Fe2+)ratios of 0.37-0.45,TiO2 contents of 3.54%-4.62%,oxidation coefficient of 0.08-0.11,Mg# values of 0.39-0.46,MF values of 0.36-0.44,AlⅥ value(molecular number of cations in one unit)of 0.03-0.11.The biotites from granodiorite display low CaO,MgO and AlⅥ values and high FeO and TiO2 contents,and were most likely crystallized between 584 ℃ and 624 ℃ as calculated by the cation numbers of Ti and Al.According to the tectonic setting discrimination diagrams and the geochemical features of biotites,we conclude that the Daocheng granodiorite belongs to the I-type granite-derived from the partial melting of the Late Triassic arc lower crust with minor mantle-derived materials,and the MMEs were generated by the mixing of mafic and felsic magma.
作者
张瑞刚
和文言
高雪
李萌萌
ZHANG Ruigang;HE Wenyan;GAO Xue;LI Mengmeng(State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Beijing 100083)
出处
《地学前缘》
EI
CAS
CSCD
北大核心
2018年第6期226-239,共14页
Earth Science Frontiers
基金
国家重点基础研究发展计划“973”项目(2015CB452605,2015CB452606)
高等学校学科创新引智计划(B07011)
中国地质调查局地质调查项目(12120114013501)
北京市优秀博士学位论文指导老师科研项目(20111141501)
关键词
斜长石环带结构
矿物化学
岩浆混合
稻城岩体
川西
zoned plagioclase
mineral chemistry
magma mixing
Daocheng batholith
western Sichuan
