| 西藏班公湖地区含镍碳酸岩成因探讨 |
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| 引用本文：.2010.A tentative discussion on the genesis of Ni-bearing carbonatites in Bangong Lake area, Tibet[J].Mineral Deposits,29(6):1029~1042 |
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| 基金项目:本文是“十一五”国家科技支撑计划项目（编号：2006BAB01A05）和中国地质调查局大调查项目（编号：1212010818097）联合资助的阶段性成果 |
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| 中文摘要:野外调查发现西藏班公湖地区分布着众多的碳酸岩体，它们呈长几十米至几百米的岩株或岩墙产出，与蛇绿岩带中的超基性岩密切共生。这些碳酸岩体在成分上可分为2种：一种以方解石为主，w(CaO) 在42.74%～45.78%之间，为钙质碳酸岩；另一种以菱镁矿和菱铁矿为主，w(MgO)和w(Fe2O3) (全铁)分别为23.83%～29.85%和7.75%～16.85％，为镁质碳酸岩。微量元素地球化学显示钙质碳酸岩的原始地幔标准化曲线（蛛网图）与华北地块北缘内蒙古大青山地区壳源岩浆碳酸岩的原始地幔标准化曲线高度吻合；Nd、Sr、Pb同位素表明这些钙质碳酸岩最终仍然来源于地幔，接近EMⅡ地幔端员，TDM集中在697.4～881.9 Ma之间，说明它们最初是在晚元古代从地幔中分离出来，经喷发、沉积、变质，于中生代经镁铁质岩浆侵入、受热重熔形成。镁质碳酸岩在地球化学上既不同于幔源岩浆碳酸岩，也不同于壳源岩浆碳酸岩，而是与共生的含镍超基性岩具有完全一致的稀土元素特征，Pb同位素也显示出它们有一致的物质来源，εNd-ISr图则显示下地壳组分在岩体中有重要作用，这些特征说明它们是富CO2的深部流体在中下地壳对超基性岩交代的结果。 |
| 中文关键词:地质学 碳酸岩 元素地球化学 Nd-Sr-Pb同位素 岩石成因 班公湖 |
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| A tentative discussion on the genesis of Ni-bearing carbonatites in Bangong Lake area, Tibet |
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| Abstract:Numerous carbonatite bodies, associated intimately with ultrabasic rocks in the ophiolite zone, have been discovered through field survey. They occur as stocks or dikes with the width about tens to hundreds of meters, and can be subdivided into two types according to chemical composition, i.e., calciocarbonatite and magnesiocarbonatite. The former consists mainly of calcite with CaO content of 42.74～45.78％, while the latter is mainly comprised of magnesite and siderite with MgO and Fe2O3(T) content of 23.83～29.85％ and 7.75～16.85％ respectively. The abundance pattern of trace elements normalized by primitive mantle for calciocarbonatite coincides with that of the crust-sourced magmatic carbonatite in Daqingshan of Inner Mongolia along the north boundary of the North China Block. However, evidence from Sr-Nd-Pb isotopes indicates that calciocarbonatite was derived from the mantle, close to EMII. Besides, its TDM ranges from 697.4 Ma to 881.9 Ma. These characteristics suggest that calciocarbonatite firstly differentiated from mantle in late proterozoic, then went through eruption, sedimentation and metamorphism successively, and lastly remelted in Mesozoic as a result of being heated by the intrusion of mafic magma. Magnesiocarbonatite differs from either typical mantlesourced or crust-sourced carbonatites in geochemical characteristics, but its REE pattern is the same as the REE pattern of associated Ni-bearing ultrabasic rocks, with their Pb isotopes also indicating the same material source. In addition, the lower crust must have played an important role in the source of magnesiocarbonatite, as shown by the εNd-ISr plot. All these phenomena prove that magnesiocarbonatite is the product of metasomatism of ultrabasic rocks metasomatized by deep-seated CO2-rich fluid in the middle-lower crust. |
| keywords:geology, carbonatite, element geochemistry, Nd-Sr-Pb isotopes, petrogenesis, Bangong Lake |
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