辽宁岫岩透闪石质河磨老玉中石墨的成因及其指示意义

石墨常出现在岩浆作用、变质作用及热液蚀变交代作用过程中,是成岩成矿物理化学条件的指示性矿物。辽宁岫岩透闪石质玉、特别是其中河磨老玉(子料)中常常含有大量的石墨,目前,对这些石墨形成条件及其与闪石质玉石成因联系知之甚少。通过显微岩相学、拉曼光谱和X射线单晶衍射等分析,探讨了辽宁岫岩河磨老玉中石墨与透闪石质玉矿的成因联系。河磨老玉中的石墨呈条带状、星点状和云雾状3种分布形态,计算获得的石墨化度值在0.562~0.644之间,菱面体多形含量变化在19.52%~23.00%,温度范围为454~623℃,与角闪岩相变质峰期温度基本一致,明显高于前人获得的透闪石质玉的形成温度范围。结果显示,河磨老玉中的石墨与闪石质玉石是不同地质作用产物。结合野外地质产状,推测河磨老玉中的石墨形成早于闪石质玉石,是区域变质作用形成的变质残余矿物。透闪石质河磨老玉石墨的特征暗示辽河群含石墨方解石大理岩、含石墨的透闪变粒岩、透闪岩大理岩可能是河磨老玉主要的原岩,而透闪石玉是多期次/阶段成矿作用产物。

The Genesis of Graphites in Xiuyan Gravel Nephrite Jades and Its Constraint on Their Host Nephrite Jade Rocks

Graphites are widely generated in geological processes of magmatism, metamorphism and hydrothermal alteration & metasomatism, and considered to be an indicative mineral for these geological processes. Xiuyan nephrite jades (locally named Laoyu), especially gravel nephrite jades, in Liaoning province, commonly contain graphite as internal enclaves. The study of these graphites will contribute to understand the formation of their host nephrite jades and the relationship between graphite enclaves and their host nephrite jades. In this paper,microscopic observation, Raman spectra and X-ray single-crystal diffraction are used to study graphites in host nephrite jades from Xiuyan, Liaoning, and the genetic relationship between graphites and their host nephrite jades. The results show that crystalline graphites commonly occurred in three clustered patterns of stripped, star-like and nebulous, in their host nephrite jades, and contained 19.52%～23.00% graphites in rhombohedral polymorph. The crystallinity indexes of graphites range between 0.562～0.644, and calculated forming temperatures range between 454～623 ℃. The temperatures are identical to those for amphibolite facies, obviously higher than previously reported data of nephrite jades. These results infer that graphites and their host nephrite jades probably formed in different geological processes or periods. Combining field investigation, we think that graphites in Xiuyan gravel nephrite jades were probably the remnant minerals of primary regional metamorphism prior to their host nephrite jades-formation, and Xiuyan nephrite jades formed through multi-stage or multi-period metamorphic and hydrothermal metasomatism processes from Liaohe group primary strata such as graphite & calcite-containing marble, graphite-containing tremolite granulite, or tremolite marble.