摘要
在云南马关碧(钾)玄质火山角砾岩中发现了一类特殊包体,呈红色与黑色两种。经X射线粉晶衍射鉴定,红色者为以锰铝榴石为主要结晶相的隐晶—非晶质混晶,黑色者为以绿辉石为主要结晶相的隐晶—非晶质混晶。两种矿物混晶包体在一定程度上类似于熔浆玻璃或熔体囊,它们具有同源演化的相似地球化学特征。文中通过对两种矿物混晶包体的显微特征与地球化学分析研究表明,它们是亏损地幔部分熔融的产物,并作为一种不混溶熔体成分被碱性玄武岩浆携带、运移上升。它们代表了石榴石相(榴辉岩相)地幔源区组分,暗示新生代时期软流圈上涌除释放小体积交代熔体交代上地幔使其富集之外,还造成岩石圈地幔拆沉,尖晶石相地幔组分向石榴石相地幔组分转变。另外,据两种混晶包体与其他类型包体和寄主岩岩浆的不同来源,推测马关地区深部岩石圈地幔经交代作用发生过两次转换。首先是由原始地幔向亏损地幔转化,并发生部分熔融,其后是由亏损地幔转化为富集地幔,形成富碱岩浆和与其互不混溶的进一步富集成矿元素的地幔流体。由此暗示滇西地区与富碱斑岩有关的多金属成矿作用即受制于这一深部地质过程与壳幔混染机制。
A special kind of xenolith has been discovered from the basaltic(shoshonitic) volcanic breccia at Maguan area,Yunnan Province.There are two kinds of xenoliths: one is red and the other is black in color.It can be identified by X-ray crystal powder diffraction that the red one is a mixed crystal of aphanites-amorphous material,the major potential component of which is spessartine,while the black one is a mixed crystal of aphanites-amorphous material,major potential component of which is omphacite.To some extent the two kind of mixed crystals are similar to melting glass or melt pockets.It shows that they have the similar geochemical characteristics of cogenetic rock evolution.By analyzing their microscopic features and geochemical characteristics,the present paper suggests that they are partial melting products from the depleted mantle and are carried,transported and erupted by alkaline basaltic magma,as an immiscible melt.They represent the component of garnet(eclogite phase) mantle,and it is suggested that the Cenozoic asthenospheric upwelling,in addition to the release of small-volume of melt to replace upper mantle causing its enrichment,had also resulted in the delamination of lithospheric mantle,and the spinel mantle components were transformed to the garnet mantle components.Besides,according to the different origins of the two kinds of mixed crystals,and of other types of xenoliths and the host magma,it is suggested that the deep lithospheric mantle after metasomatism had undergone two stages of transformations at Maguan area.The first is the transformation of the primitive mantle to depleted mantle,and accompanied by partial melting;the second is the transformation of depleted mantle to enriched mantle,and then generated the alkali-rich magma and the immiscible mantle fluid which is further enriched in ore-forming elements.This implies that the alkali-rich porphyry-related polymetallic mineralization is constrained by this deep geological process and crust-mantle contamination mechanism in western Yunnan.