黔东南与金刚石有关的石榴石精细地球化学研究

In China, primary diamond was first discovered in Southeastern Guizhou. Given the rarity of the diamond along with the limited analytical technique used by the previous studies in China, their origin, source nature and mineralizing potential remain unresolved in this area. In this case, seeking a substitute such as a type of mineral derived from the same source with the diamond and then carrying out detailed study are crucial to decipher their genesis and origin. Fortunately, we have found such a mineral of garnet that developed as an inclusion in diamond and also as xenocrysts in its host rock. These garnets could have a common origin with the diamond. This finding thus provides a good opportunity to unravel the genesis and origin of the diamond and reassess its mineralizing potential in Southeastern Guizhou. In this project, we will carry out systematic investigation and new in-situ analytical methods, including laser raman spectrometer, EPMA and ICPMS/SIMS/SHRIMP analyses, to achieve their chemical compositions such as the major-trace elements and O isotopic compositions of the two types of garnets, and then further discuss the differences and similarities of their chemical compositions. By comparison with their geochemical characteristics, separating some garnet xenocrysts that are closely related to the formation of the diamond in genesis. We will further investigate their Sm-Nd and Lu-Hf isotopic compositions by MC-ICPMS analysis. Finally, we will integrate all our results with previous published data to 1) elucidate the genesis and origin of the diamond, 2) reveal the source nature of the deep mantle in the Southeastern Guizhou, and 3) reevaluate the mineralizing potential of primary diamond in this area.

黔东南地区是我国最早发现原生金刚石的产地，囿于金刚石样品稀缺和测试条件有限，关于其源岩特征和原生金刚石成矿潜力存在明显争议。找到能替代的样品并开展精细地球化学研究是解决问题的关键。申请者在金刚石及其寄主岩石中分别发现了石榴石包裹体和石榴石捕虏晶，为探究黔东南地区金刚石的形成机制和评估其成矿潜力提供了新契机。本项目将利用拉曼光谱、EPMA、ICPMS/SIMS/SHRIMP等现代原位分析技术，对两类石榴石进行主、微量元素和O同位素对比，探讨它们的化学组成异同，进而揭示金刚石形成的地质地球化学环境，初步限定其源岩性质；在此基础上，通过地球化学特征的比对，筛选出与金刚石有成因联系的石榴石捕虏晶，利用MC-ICPMS对其进一步开展Nd-Hf同位素地球化学研究，以此约束金刚石的形成机制，阐述其源区特征，探讨黔东南深部地幔演化特点，为评估该地区原生金刚石的成矿潜力提供新的制约。

囿于金刚石样品稀缺和早期测试条件的局限，关于黔东南地区原生金刚石的源岩特征和成矿潜力尚存争议。鉴于此，我们一方面选取黔东南地区与金刚石伴生的石榴石和铬尖晶石捕虏晶进行详细的矿物学和地球化学研究，阐述其形成的地质地球化学环境，初步限定其源岩性质；另一方面对原生金刚石进行细致的矿物学、谱学和同位素地球化学研究，约束金刚石的形成机制。取得的初步认识如下：（1）黔东南与金刚石伴生的石榴石均为镁铝榴石，来自橄榄岩地幔源区，且与二辉橄榄岩关系密切；（2）铬尖晶石与受俯冲作用改造的地幔橄榄岩有关，对应于99~175km的深度，部分达到了金刚石稳定区的上限，暗示苍蒲塘地区具有较好的金刚石找矿前景；（3）大多数石榴石具有较高的δ18O值（> 6‰），结合石榴石较低的Zr和Y含量，以及较低的Zr/Hf以及Ti/Eu比值，显示源区经历了硅酸盐熔体交代作用；（4）黔东南原生金刚石具有“双峰式”碳同位素分布特点，与同属于扬子克拉通的湖南砂矿金刚石具有相似的碳同位素分布特点，暗示这二者的碳来源具有密切的联系。

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