摘要
罗甸玉的主要组成矿物为透闪石,具有典型的毛毡状纤维交织结构,次要矿物为透辉石和滑石,次要矿物的含量及结构是影响玉石质量的主要因素。罗甸玉微量元素中Cr-Ni-Co的含量远低于交代蛇纹岩型软玉,与交代白云岩型软玉相当。球粒陨石均一化的稀土元素配分模式为右倾型,具有强烈的Ce负异常(δCe=0.21~0.34)和弱Eu负异常,与我国典型的新疆和田玉、岫岩玉差别非常明显。罗甸玉、矿区辉绿岩、地层硅质岩的地球化学特征对比发现,辉绿岩的侵入主要提供了热源,罗甸玉形成过程中大量的Mg可能来源于海水循环,稀土元素主要继承了地层中二叠系硅质岩的特征,并在蚀变过程中进一步叠加了热循环海水的信息。综合区域地质、矿物学及地球化学的研究成果,我们认为在约255 Ma时黔南辉绿岩岩床侵入,上覆海水通过断裂体系形成大规模的热传导循环,形成厚大的外接触蚀变带,同时不断将海水中的Mg循环输送到下部,通过白云岩化、透辉石化、透闪石化及滑石化等蚀变反应,最终形成罗甸透闪石玉矿床。
Luodian nephrite consist of main tremolite with minor diopside and talc, with prevail interweave structure. The typical structure and mineral composion dominate the quality of nephrite. Value of trace elements content of Cr-Ni-Co in Luodian nephrite is much lower than that of serpentinite metasomatism type nephrite and has the same range of metasomatized dolomite type nephrite. The strong Ce negative anomaly(δCe=0.21-0.34) and not obvious Eu negative anomaly of the nephrite's right-slop REE patterns make it obviously different from famous nephrite in Hetian and Xiuyan. Through geochemical characteristics comparison of Luodian nephrite, diabase dyke, siliceous rock, it's reasonable to consider that, during the mineralization of Luodian nephrite, the intrusion of diabase plays the role of heat-source and massive Mg possibly come from thermal circle of seawater. Features of REE inherited from siliceous rock with alteration replenish the information of cyclic thermal seawater. By summarizing the research result of areal geology, mineralogy, geochemistry, we conclude that, at about 255 Ma, the up-intrusion of the diabase dyke caused the overlying seawater forming large-scale heat exchange circulation through the fracture system and then the upper thick massively contacted metamorphic zone produced; simultaneously, the Mg was continually transmitted to bottom to generate dolomitization, diopsidization, tremolitization, steatitization and in the final, the nephrite ore deposite occurrence.
出处
《矿物学报》
CAS
CSCD
北大核心
2014年第2期223-233,共11页
Acta Mineralogica Sinica
基金
国家自然科学基金项目(批准号:41072054)
