Complex zoning texture of phenocryst plagioclase in andesitic rocks of Tancheng area, Shandong Province: Record of magma mixing
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摘要: 山东郯城地区的新生代地层中发现了大量大颗粒金刚石及多个金刚石砂矿体,但尚未发现金刚石原生矿。为探索金刚石的来源,对该地区出露的一套安山质火山岩开展岩相学研究,发现以下特征: ①斑晶斜长石多具有复杂环带结构,内核主要为微晶斜长石及细小橄榄石,外环的斜长石具有正常环带结构; ②橄榄石较均匀地分布于岩石中,含量大于20%,且具有3个世代特征,与金伯利岩中橄榄石的3个世代特征一致; ③岩石具有“橄榄石+金云母+石榴石”与“斜长石+辉石+角闪石”矿物组合。岩相学证据表明该地区的安山质火山岩是安山岩岩浆与金伯利岩岩浆发生混合作用的产物,而斑晶斜长石的复杂环带结构记录了混合作用的演化过程。研究对郯城地区金刚石原生矿的勘探具有指导意义。Abstract: A large amount of coarse grain diamonds and diamond placer deposits have been found in the Cenozoic strata in Tancheng area of Shandong Province, without primary diamond deposits so far. The petrography characteristics of outcrop of andesitic volcanic rocks in this area was investigated to explore the source of diamonds. The results show: ① Most of the phenocryst plagioclases have different complex zoning textures, and the core of phenocryst plagioclase is composed of microcrystalline plagioclases and fine olivines, with normal zoning texture crystallized by plagioclase in the outer rings. ② The olivines are evenly distributed in the rock, with content of more than 20%. It also has three generations characteristics, which is basically consistent with the three generations characteristics of olivine in kimberlite. ③ The mineral combinations of “olivine+phlogopite+garnet” and “plagioclase+pyroxene+hornblende” occurred in the rocks. These evidences indicate that the andesitic volcanic rocks in the area are the product of the mixing of andesite magma and kimberlite magma, which was recorded by the complex zoning texture of phenocryst plagioclase. The research in this paper is of enlightenment for the primary diamond ore research in Tancheng area.
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Key words:
- andesitic volcanic rock /
- plagioclase /
- complex zoning texture /
- olivine /
- kimberlite /
- magma mixing /
- Tancheng area
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