云南六合富碱斑岩和包体岩石的显微岩相学与地幔流体作用示踪研究

Micro-petrography and traces of mantle fluid activities for alkalic-rich porphyry and deep xenoliths in Liuhe, Yunnan

云南六合霓辉正长斑岩含有较多镁铁-超镁铁质包体以及特殊的富铁熔浆包体。岩相学研究表明,伴随交代蚀变,寄主岩石和各类包体中普遍发育沿粒间和矿物晶体裂隙或解理纹贯入或穿插的呈网状和细脉浸染状分布的黑色不透明物质——富铁微晶玻璃,其性质与富铁熔浆包体类似;对该类物质采用电子探针及扫描电镜和能谱分析,发现它们是一种纳米-微米级的超显微隐晶固体,其组成以硅酸盐和石英为主,尤其含有地幔标型矿物碳硅石、自然铁和钛铁矿等;这些矿物间构成超显微熔离结构交生。研究认为,这种物质组成和特征是具超临界和熔浆性质的地幔流体作用的现实微观踪迹,其活动引发的交代蚀变表现为碱性蚀变和硅化并伴随不同程度的金属矿化,以及寄主岩石和包体中暗色矿物由辉石→角闪石→黑云母→绿泥石的退变序列。稀土和微量元素地球化学研究表明,霓辉正长斑岩及其包体和富铁熔浆包体具有相似的富集轻稀土元素和Ce负异常以及富集大离子亲石元素和高场强元素的特征。本文综合研究认为,富铁微晶玻璃和富铁熔浆包体是起源于富集地幔,但独立于富碱岩浆并被富碱岩浆裹挟的成分不混溶产物。它们是地幔流体作用现实微观踪迹的两种表现形式,是引发壳幔物质混染并导致寄主岩石和包体中交代蚀变和金属矿化的重要物质源和动力源。

There are abundant mantle xenoliths and one special Fe-rich melt xenolith in aegirine syenite porphyry,Liuhe,Yunnan province.Petrographic research shows that black opaque material is Fe-rich glass,which are reticulately distributed along intergranular cracks of minerals,crystal fissures and cleavage in association with metasomatic alteration in mafic-ultramafic xenoliths and host rocks.The Fe-rich melt xenolith has the similar characteristics.Intergrated analyses using electron microprobe analysis,SEM and energy spectrum Reveal that these materials are submicroscopic cryptocrystal solid of nanometer-micron,which are mainly composed of silicates,quartz and,especially,carborundum,native iron and ilmenite,typical of mantle origins.These minerals show liquation in the back-scattered electron image.It is concluded that the compositions and characteristics of the materials are actual traces of supercritical and molten-lava mantle fluid.The metasomatic alteration caused by mantle fluid expressed in alkaline alteration,silication together with metalliferous mineralization in varous degrees and retrograde reaction in the order of pyroxene→amphibole→biotite→chlorite in dark minerals of host body and xenoliths.The aegirine syenite porphyry,xenoliths and Fe-rich melt xenolith have similar trace element patterns,with enrichments of LREE and HFSE,negtive anomaly of Ce.Fe-rich glass and Fe-rich melt xenolith,which originate from enriched mantle,are the products of immixing components in the alkalic-rich magma,and two kinds of actual microcosmic traces manifestations of mantle fluids,which triggered the crust-mantle interaction via the alteration,metasomatism and mineralization of the host rocks and xenoliths.