The character and genesis of anorthite as inclusions in spinel of mantle peridotites from the Purang ophiolite (southwestern Tibetan Plateau).
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摘要: 西藏普兰超镁铁岩体之东南缘与玄武岩接触界线附近的地幔橄榄岩中除有粒状半自形的钙长石产出外,还在尖晶石中发现有呈蠕虫状、浑圆状的钙长石包裹体存在。研究发现两种产状的钙长石An值都大于95且均无环带构造,说明钙长石从高Ca/Al比值的熔体中结晶时具有结晶时间短、结晶速度快的特点,可能形成于地壳较浅部位。从化学成分来看,包裹体形态的钙长石具有较高的Cr2O3含量,其寄主矿物尖晶石的Cr#值低且TiO2含量比深海橄榄岩中的尖晶石低得多,推断钙长石包裹体与寄主矿物尖晶石是在液相条件下几乎同时结晶的产物。综合研究表明钙长石包裹体的成因可能是玄武岩熔体在地壳较浅部位侵入方辉橄榄岩时,高温的玄武质熔体提供热源,使得方辉橄榄岩中尖晶石内的Cpx+Opx细粒矿物包裹体在高温环境下发生熔融,发生Opx+Cpx+Sp→Ol+Pl的反应,由于这种情况下尖晶石有剩余,故新生成的橄榄石和钙长石矿物仍然包裹于尖晶石内,从而形成尖晶石内部呈蠕虫状的钙长石包裹体。Abstract: The vermicular-shaped anorthite as inclusions in spinel and the subhedral anorthite were both discovered in the mantle peridotites from the Purang ophiolite in Tibet. Research shows that there exists no zoning forms and the An values of anorthite are greater than 95 in these two distinct anorthites, which indicating the rock's cooling time is short and its forming depth is low. The anorthite inclusions are relatively high in value of Cr2O3, and its host mineral spinel is with low Cr# and content of TiO2 is much lower compared with those in abyssal peridotites. Thus, it might be inferred that both anorthite and spinel have crystallized at the same time. The research presented that the genesis of the anorthite as inclusions in spinel is forming from a high temperature under closed system when basalt melt intruded to harzburgite at the shallow crust. The spinel and its inclusions (Cpx+Opx) melt under this high temperature environment and generate olivine and plagioclase according to the reaction: Cpx+Opx+Sp=Pl+Ol. The new formed minerals, olivine and plagioclase, are still wrapped in the spinel as inclusions because spinel is residual at this time that is the genesis of anorthite as inclusions in spinel .
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Key words:
- Purang ophiolite /
- Mantle peridotite /
- Anorthite /
- Inclusions
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