山东蒙阴金伯利岩中尖晶石族矿物特征及其地质意义

Characteristics and Their Geological Significances of Spinel Minerals of Kimberlite in Mengyin,Shandong Province

尖晶石作为副矿物存在于金伯利岩中,贯穿了金伯利岩的整个演化过程,其形态和成分特征可有效用于反映金伯利岩岩浆的演化过程。为了解蒙阴金伯利岩中尖晶石的矿物学特征及其地质意义,对其进行了偏反光显微镜下观察、电子探针（EMPA）、扫描电镜（SEM）和激光剥蚀电感耦合等离子体质谱（LA-ICP-MS）分析。结果表明,山东蒙阴金伯利岩基质中尖晶石的自形程度较高,粒径通常为20-60μm,多具有环带,为原生尖晶石。自尖晶石的核部至边部,Cr含量逐渐降低,Ti含量逐渐升高,Fe^2＋/（Fe^2＋＋Mg）呈逐渐上升状态。据其形态可分为自形-半自形颗粒和环礁结构颗粒。不同的环带特征可能由多个因素造成,如形成环境的热扰动、压力变化及融体成分变化等。这些差异特征反映华北克拉通金伯利岩经历了长时间复杂的演化过程。

Spinel minerals,as a group of accessory minerals in kimberlite,were formed throughout the whole evolution process of kimberlite. The morphological and compositional characteristics of spinel minerals can be effectively used to decipher the evolution process of kimberlite magma. In order to figure out characteristics of spinel minerals of kimberlite in Mengyin,Shandong Province,those samples were observed and analyzed by using optic microscope,electron microprobe analysis（ EMPA）,scanning electron microscope（ SEM） and LA-ICP-MS for their appearance characteristics and mineral compositions. The results show that spinel minerals in matrix of the Mengyin kimberlite generally are 20 - 60μm sized euhedral crystals with zoning texture. They are thought to be primary spinel. From the core to the edge of the zoning spinel minerals,the Cr contents are gradually decreased,whereas the Ti contents and Fe^2＋/（ Fe^2＋＋Mg） values are gradually increased. Based on their morphologies,there are enhedral-subhedral crystals and atoll grains of spinel minerals. The various zonings of the spinel could be caused by various factors including the thermal interference of the environment,variations of the pressure and melt composition. These difference characteristics of spinel minerals in the Mengyin kimberlite indicate that the kimberlite had experienced a long term complex evolution process.