缅甸硬玉岩地区的热液型钠长石岩

Hydrothermal albitite from the Myanmar jadeite deposit

产于俯冲带内的低温高压带的由单矿物构成的硬玉岩通常伴有钠长石岩,目前对于硬玉岩研究的关注度较高,而对于钠长石岩则相对较低,很少有相关论文报导。产于缅甸翡翠矿区的钠长石岩,经常与硬玉岩相伴而生,是良好的研究样品。钠长石岩的主要矿物成分是低温钠长石,其次含有硬玉、绿辉石、透辉石等辉石类矿物和钠透闪石、蓝透闪石、镁钠闪石等闪石类矿物,此外还有钠沸石等。钠长石沿着解理和裂隙交代硬玉,说明钠长石形成晚于硬玉岩。钠长岩中的主要组成矿物钠长石的形成温度小于300℃,且其形成压力小于0.5kb,推测是在硬玉岩抬升程中通过交代与沉淀作用形成。其内的透辉石有两种类型,一类可能是被交代的硬玉中的透辉石组分会渐进增加,最终形成透辉石。另一类是被绿辉石包裹的透辉石残留,其很有可能是早期来自地幔楔或者俯冲带岩石中的矿物残留,即异剥钙榴岩或辉石岩类,可以视作硬玉化绿辉石岩和硬玉化异剥钙榴岩的矿物学证据。热液型钠长石岩的存在进一步说明缅甸翡翠矿区钠化热液存在现象的普遍性与穿越性。

Albitites occur commonly in the jadeitite area. Jadeitite, a rock made up almost entirely of jadeitic pyroxene, forms under a relatively high-P/T condition in the subduction zones. Jadeitites have attracted remarkable attention of geologists and gemologists from all over the world and many localities of jadeitites have been well-documented. However, there has been little understanding about the albitites. Albitites in the Myanmar jadeite area, which are always associated with jadeitites, are perfect samples for understanding formation of the jadeitite and related events. Albitite is mainly composed of low-temperature albite, pyroxene minerals such as jadeite, omphacite and diopside and amphibole minerals such as richterite, winchite and magnesio-riebeckite, natrolite. Replacement of the jadeite by albite in the albitite along its cleavage and crack indicates that albite forms later than the jadeite. Albite record conditions of lower pressure than the jadeitite, and it was formed at temperature lower than 300℃, thus it is concluded that albite forms during uplift of the jadeitite through hydrothermal filling and precipiation. Diopside in the albitite consists of two types： one formed by replacement of jadeite which contains some components of diopside, the other is inferred to be the residual which was part of the replaced rocks like rodingite or pyroxenite belonging to the host serpentine melánge. The latter can be used as mineralogical evidence for confirmation of jadeitized pyroxenite and jadeitized rodingite. Existence of the abitite shows that Na-rich fluids are thorough and traversing.