中国大陆科学钻探主孔榴辉岩中石榴石和绿辉石原位激光探针分析及其成岩成矿指示意义

In situ LA-ICP-MS analysis for garnet and omphacite of eclogites from the main hole of CCSD:Implications for petrogenesis and mineralization.

本文以中国大陆科学钻探主孔0～2000m岩芯中的榴辉岩为对象,运用EMPA和LA-ICP-MS技术,系统测定了榴辉岩中石榴石和绿辉石的主量与微量元素组成,并据此讨论了它们的成岩成矿意义。研究结果表明,CCSD主孔榴辉岩中石榴石富重稀土和Sc、Y、Co,而绿辉石则富中稀土和Pb、Sr、V,石榴石和绿辉石的高场强元素(特别是Nb、Ta)含量均很低。石榴石存在不同程度的Ce负异常,指示榴辉砉的形成过程中卷入有地表氧化条件下形成的风化沉积物。石榴石具有低的Zr/Y比值,绿辉石普遍具有高的Sr含量,这些特征说明榴辉岩(特别是高钛榴辉岩)的原岩可能为遭受过壳源物质混染与交代的富集地幔部分熔融的产物。高钛与低钛榴辉岩中石榴石和绿辉石在主量及微量元素组成上存在一定差别,总体而言,高钛榴辉岩中石榴石具高的MgO含量和较高的MgO/TFeO比值,以及较高的稀土和Sc含量,而绿辉石则相对富TFeO、MnO,并具有较高的Sr、Zr、Hf含量。高钛榴辉岩中石榴石和绿辉石常出现不同程度的Eu正异常,Cr含量均显著低于低钛榴辉岩。综合分析表明,高钛榴辉岩的原岩最可能为富斜长石的辉长质侵入岩,原岩组成的差异应是导致二类榴辉岩中石榴石和绿辉石矿物化学组成存在差异的主要原因。

In situ major and trace element analyses for garnet and omphacite in eclogites from 0 similar to 2000m of the CCSD main hole have been undertaken by using EMPA and LA-ICP-MS techniques, and the petrogenetic and metallogenic implications are accordingly discussed. Investigation results have shown that garnets in eclogites are enriched in HREE and Sc, Y, Co, and omphacites, in MREE and Pb, Sr, V. Both garnets and omphacites have low concentrations of HFSE (especially Nb and Ta). The garnets display various degree negative Ce anomalies, suggesting that weathering sediment components developed at the Earth's surface under oxidizing conditions had been involved in the formation of eclogites. The features that the garnets have low Zr/Y ratios and the omphacites have high Sr abundances imply that the protolith of eclogite (especially the high-Ti eclogite) might be partial melting products of enriched mantle which was generated by contamination and metasomatism of subducted crustal material in the source region. Garnets and omphacites from high-Ti and low-Ti eclogites display different major and trace element compositions. In general, garnets from high-Ti eclogites are enriched in MgO, and have higher MgO/TFeO ratios and REEs and Sc concentrations, whereas the omphacites show higher contents of TFeO, MnO and Sr, Zr, Hf. Both garnets and omphacites from the high-Ti eclogites display different degree positive Eu anomalies and have quite lower Cr abundances than those in low-Ti eclogites. Integrated study demonstrates that the protolith of high-Ti eclogite is most likely to he plagioclase-rich gabbroic intrusive rocks. The present authors suggest that the chemical composition differences of garnet and omphacite between high-Ti and low-Ti eclogites were likely to have been induced by different protolith.