缅甸硬玉岩区的硬玉化绿辉石岩

Jadeitized omphacitite from Myanmar jadeite area.

绿辉石岩是缅甸硬玉岩区一种新的岩石类型,普遍遭受多期次硬玉化。未发生硬玉化的绿辉石含较高TFeO和CaO,但较低的MgO,按化学成分分类部分已属霓石-普通辉石系列的中间相。这种绿辉石很可能是硬玉质的流体/熔体交代地幔辉石岩类的产物,与主期大规模硬玉岩的结晶、钠质-钠钙质角闪石边和钠铬辉石的形成同期或稍后,是该区第Ⅰ期的硬玉化。沿绿辉石的解理或裂隙交代并伴有充填的硬玉是在相对拉张的构造背景下形成的,是该区第Ⅱ期的硬玉化。切割早期绿辉石和硬玉的充填硬玉细脉可能是最晚一期的硬玉化。硬玉化绿辉石岩的结构与显微构造特征指示了在缅甸硬玉岩区,至少存在不少于3期的硬玉化的交代充填作用。文中讨论了硬玉化物质的可能来源,认为其可能与俯冲板片上的沉积物有密切关系。

Omphacitite, a new type of rock found in Myanmar jadeitite area, had experienced multi-stage jadeitizations. The rock contains relict omphacites of high TFeO and CaO, but low MgO, and relict pyroxenes even belonging to aegirine- augite series. The omphacite is possibly a production of reaction between jadeitic material and mantle pyroxenite under high P-T conditions, the same stage as or a little later than the main formation of the Myanmar jadeitite and associated sodic- to sodic-calcic- amphibole and kosmochlor, which is considered to be the stage-Ⅰ jadeitization. The precipitation of jadeite filled along omphacite＇s crack or cleavage due to regional deformation and the formation of less pure jadeite as a reactant of replacement of omphacite by jadeite is regarded as the stage-Ⅱ jadeitization. The formation of fine jadeite vein crosscutting all minerals and textures is considered as the stage-Ⅲ jadeitization, and possibly the last one. The texture and microstructure of the omphacitite is indicative of three or more stages of mineralization in this area. Derivation of the jadeitic material is discussed and thought to be more likely related to sediments of subducted slabs.