罗布莎蛇绿岩的电磁学特征及其成因机理

Indus-Yarlung Zangbo (IYZ) ophiolite in southern Tibet, which represents the vanished Neo-Tethyan Ocean between India and Asia, had been used as an important petrological indicator to constrain the suture of the world famous continental collision. Luobusa ophiolite acts as the representative ophiolite of IYZ suture, and hosts the largest chromite resources in China, accounts for almost 90% of the production of the country. As a result, Luobusa has attracted great interest of the geologists from China and abroad. Significant progress has been made in the geological features, the geological ages of the rock units, its formative environment and the regional tectonics. However, poor geophysical understanding about the Luobusa ophiolite leads some key characteristics of the ophiolite, such as the tectonic occurrence, the emplacement mechanism, the interior of the rock body and the favorite areas for mining still remain under debate. In the past several years, geo-electromagnetic methods played an important role in chromite deposits exploration at Luobusha. However, the correlation between the geology and the electromagnetic properties of the ophiolite and the related podiform chromites, is still remained poorly understood. We proposed a project on the studying of the geo-electromagnetic property and its origin of Luobusa ophiolite based on the electromagnetic exploration data and geology wisdoms. An ultramafic rock physical model will be set up by measuring and analysis the electromagnetic properties, petrology, and geochemistry of the samples from Luobusa. Then we will study the geo-electromagnetic feature of the ophiolite by collecting and processing the AMT data and the MT observation in Luobusa. A geology-geoelectrical model will be carried out. Based on this model, we will study some basic scientific problems, such as the origin of the geo-electrical properties of the chromite deposit inside the rock complex, the geo-electrical properties of the ophiolite with different rock facies. Finally, an exploratory study on the correlation among the continental collision process, the interior feature of the Luobusa complex and the geo-electrical properties will be conducted in our proposed project.

罗布莎蛇绿岩位于雅鲁藏布缝合带东段，记录了印度－亚洲碰撞及构造演化过程，是我国研究程度最高的蛇绿岩，更是重要的战略资源铬铁矿产地。罗布莎地质研究和资源勘探已形成了丰富的成果，并不断有基础地质研究新发现。然而，地球物理研究的滞后制约了罗布莎蛇绿岩的整体认识和勘探突破，矿山一度陷入严重的资源危机，威胁着我国铬铁矿战略安全。为缓解资源危机，在罗布莎开展了高密度电磁法勘探，取得了较好的效果。但深入发掘罗布莎电磁学资料的地质含义并推广铬铁矿电磁法勘探模式，都亟待蛇绿岩和铬铁矿的电磁学属性与成因机理研究。本项目以超基性岩电磁学研究为基础，依托罗布莎电磁法数据，结合已有的地质资料，补充少量的大地电磁测量，通过电磁学与岩石地球化学、矿床学、构造地质学测量和分析，研究蛇绿岩与豆荚状铬铁矿含矿构造的电磁学特征及其成因机理，指导铬铁矿电磁法勘探，并探索陆陆碰撞对蛇绿岩就位机制、内部构造与岩相分布的影响。

超基性岩记录了岩石圈演化、大洋消亡的关键信息，在地球科学研究中有着特殊的地位。超基性岩也是铬铁矿、钴、镍等关键金属的赋矿围岩，在服务人类需求方面的地位也非常突出。罗布莎是我国铬铁矿的主产区，也是新特提斯洋残留的雅江蛇绿岩带的重要组成部分，是我国蛇绿岩研究的热点地区。电磁法在罗布莎铬铁矿勘探取得突破，但深入发掘罗布莎电磁学资料的地质含义并推广铬铁矿电磁法勘探模式，亟待蛇绿岩和铬铁矿的电磁学属性与成因机理研究。本项目针对超基性岩电磁学特征与机理、超基性岩岩石电磁学与铬铁矿赋存的关系等科学问题，通过岩石学、地球化学和电磁学等学科交叉，研究了岩石复阻抗精确测量方法、超基性岩的电磁学属性和机理以及罗布莎岩体的电磁学特征和成因。主要研究内容包括：（1）罗布莎超基性岩样品电磁学特征及成因机理；（2）蛇绿岩及铬铁矿控矿构造的电磁异常特征及成因机理；（3）电磁法探测铬铁矿控矿构造的应用基础研究。在先期工作的基础上，通过四年的研究，全部内容按计划完成。项目主要成果包括：(1) 完善了岩石电磁学测量方法。通过样品复阻抗实测分析，分析了测量方法、电极效应、装置参数等对数据测量的影响；(2) 通过罗布莎岩体超基性岩的岩石电磁学属性和机理研究，确认了蛇纹石含量、烧失量和岩石电阻率的负相关关系，蛇纹石化和蚀变是导致岩石电阻率降低的最主要因素。从岩相和构造两方面解释了罗布莎岩体高低阻异常的成因机理；(3) 在铬铁矿成矿预测和岩体结构研究方面取得新认识，通过超基性岩岩石学和电磁学交叉研究揭示了我国铬铁矿主产区－西藏罗布莎岩体的电性结构和铬铁矿发育分布的电磁学规律。(4) 综合铬铁矿地质特征并结合其它地球物理资料提出了隐伏豆荚状铬铁矿地质－地球物理探测模型。项目成果和形成的方法体系已在第二次青藏科考，白云鄂博稀土矿地球物理探测等项目中推广应用。