Controls on platinum-group elemental distributions of podiform chromitites : A case study of high-Cr and high-Al chromitites from Chinese orogenic belts
Controls on platinum-group elemental distributions of podiform chromitites : A case study of high-Cr and high-Al chromitites from Chinese orogenic belts
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DOI：
10.1016/s0016-7037(97)00382-7
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发表时间：
1998-02
影响因子：
5
通讯作者：
Mei‐Fu Zhou;M. Sun;R. Keays;R. Kerrich
中科院分区：
文献类型：
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作者：
Mei‐Fu Zhou;M. Sun;R. Keays;R. Kerrich
对亚洲造山带和祁连-强岭-昆仑-喜马拉雅构造域脚状铬铁矿床的研究，为脚状铬铁矿床中pge的地球化学特征和矿床成因提供了新的认识。大部分矿床赋存于蛇绿岩的地幔橄榄岩中，具有典型的蛇绿岩型PGE模式，相对于平均上地幔，其Pt和Pd含量呈负倾斜分布。I型（高铬）铬铁矿的Os、Ir、Ru和Rh含量高于II型（高铝）铬铁矿，尽管两者的Pd和Pt含量相似。大多数I型和II型铬铁矿的Pd和Pt含量低于它们所在的上地幔橄榄岩。脚状铬铁矿主要是上地幔熔体/岩石相互作用的产物；Cr和pge的贡献不仅来自于侵入岩浆，还来自于上地幔宿主；铬铁矿矿床部分为交代替代体。ⅰ型（高铬）铬铁矿PGE模式是由硫不饱和的硼质岩浆与贫哈尔茨伯尔岩相互作用形成的，而ⅱ型（高铝）铬铁矿PGE模式是由初始硫饱和的拉斑岩岩浆与贫哈尔茨伯尔岩相互作用形成的。I型和II型铬铁矿的低至极低的Pd和Pt含量要求形成铬铁矿的地幔组合在铬铁矿形成之前已经失去了硫化物，从而失去了Pd和Pt；此外，侵入岩浆没有或很少沉积Pd和Pt，这些岩浆要么保持s欠饱和状态（博长岩），要么由于与缺s的哈兹伯基地幔相互作用而变成（MORB） s欠饱和状态。本文认为，一般来说，低Ir、低Os、低Ru含量可能是由于在脚状铬铁矿形成过程中Ir的损失所致。具有ⅰ型PGE模式的足状铬铁矿形成于岛弧环境，而具有ⅱ型PGE模式的足状铬铁矿形成于弧后环境。
A study of podiform chromite deposits from the Asiatic Orogenic Belt and the Qilian-Qiangling-Kunlun-Himalaya Tectonic Domain provides new insights into the geochemistry of the PGEs in podiform chromite deposits and the genesis of the deposits themselves. The bulk of deposits, which occur in mantle peridotites of ophiolites, have typical ophiolitic PGE patterns that are depleted in Pt and Pd relative to the average upper mantle and have negatively sloping distributions on mantle-normalized diagrams. Type I (high-Cr) chromitites have higher Os, Ir, Ru, and Rh contents than Type II (high-Al) chromitites, although both have similar Pd and Pt. Most of the Type I and II chromite deposits have lower Pd and Pt contents than the upper mantle peridotites in which they occur. Podiform chromitites are essentially products of melt/rock interaction in the upper mantle; their Cr and PGEs were contributed by not only the invading magmas but also by the upper mantle host; the chromite deposits are, in part, metasomatic replacement bodies. The Type I (high-Cr) chromitite PGE patterns were produced by interaction between S-undersaturated boninitic magmas and depleted harzburgites, whereas the Type II (high-Al) chromitite PGE patterns were formed by interaction between initially S-saturated tholeiitic magmas and depleted harzburgites. The low to very low Pd and Pt contents of both Type I and Type II chromitites require that the mantle assemblage in which the chromite deposits were formed had lost their sulfides, and hence Pd and Pt, prior to formation of the chromite deposits; in addition, no or little Pd and Pt were deposited by the invading magma which either remained S-undersaturated (boninite) or became (MORB) S-undersaturated due to interaction with the S-depleted harzburgitic mantle. It is suggested that the very low Ir, Os, and Ru contents of boninites in general might be due to loss of Ir during the formation of podiform chromitites. It is suggested that podiform chromitites with Type I PGE patterns were formed in an island arc environment, whereas those with Type II PGE patterns were formed in a back-arc setting.