蛇绿岩中铬铁岩母岩浆的富Ca特征：矿物包裹体证据
Ca-Enrichment Characteristics of Parental Magmas of Chromitite in Ophiolite: Inference from Mineral Inclusions

铬铁矿作为蛇绿岩中的重要矿产，其成矿母岩浆性质及演化一直存在较大争议.铬铁矿的矿物包裹体同时或先于铬铁矿结晶，其成分和类别能很好地记录成矿母岩浆性质和演化过程.土耳其Pozant-Karsant 蛇绿岩不同类型铬铁岩的铬铁矿中发现了多种类型包裹体：不含水硅酸盐矿物(如橄榄石和单斜辉石)、含水硅酸盐矿物(如角闪石和金云母)、复合型矿物包裹体(如蛇纹石、硅灰石和单斜辉石的复合型包裹体)和不常见矿物(如磷灰石、铂族元素硫化物).含水矿物包裹体的出现以及矿物的高Mg#特征(如橄榄石Fo=95.4～97.1；单斜辉石Mg#=92.0～99.9；角闪石Mg#=88.9～99.8)表明结晶铬铁矿的母岩浆具有富水、富Mg的特征.同时，除钙铬榴石和磷灰石的包裹体外，在铬铁矿中首次发现富Ca矿物方解石和硅灰石，其中方解石和菱镁矿以复合型包裹体形式产出，硅灰石则分布于蛇纹石矿物包裹体中.这些富Ca矿物的出现以及硅酸盐矿物的高CaO含量均揭示了铬铁岩母岩浆的富Ca特征.母岩浆中的Ca组分可能来源于俯冲板块中富Ca岩石/矿物的部分熔融，Ca离子的大量出现使得Cr3+在熔体中更加稳定，同时富Ca矿物的结晶促进了岩浆中Cr的进一步富集而利于铬铁矿的大量结晶沉淀.
The origin and mechanisms involved in the formation of chromitite deposit in ophiolites remain a controversial subject of continuous debate. One of the important ways to address this issue is to investigate the nature and composition of parental magmas of chromitite, which may be revealed by mineral inclusions in chromite interpreted to have crystallized contemporaneously with or earlier than the host chromite. Various types of inclusions have been found in chromite ores with different textures in the Pozant-Karsant ophiolite in Turkey, which include (1) anhydrous silicate type such as olivine and clinopyroxene, (2) hydrous silicate type such as amphibole and phlogopite, (3) composite type such as the association of serpentine, wollastonite and clinopyroxene, (4) and uncommon mineral type such as apatite and platinum group element sulfide. The occurrence of hydrous mineral and the high Mg# of some minerals (e.g., olivine Mg# = 95.4-97.1, clinopyroxene Mg# = 92.0-99.0, amphibole Mg# = 88.9-99.8) suggest that parental magmas of the chromite are rich in Mg and water contents. Besides the inclusions of apatite and uvarovite, it is reported, for the first time, that calcite and wollastonite inclusions in chromite, which, together with high-CaO features in silicate minerals, indicate Ca-enrichment. The elevated Ca contents in melts are favorable in stabilizing Cr3+ in silicate melt, while crystallization of Ca-bearing minerals could result in Cr enrichment in the melts. The Ca-rich component was probably derived from Ca-enriched rocks in subducting slab