赣东北蛇绿混杂岩中铬尖晶石矿物学特征及其地质意义

The Northeastern Jiangxi ophiolitic mélange belt is the oldest ophiolite formed in the late Proterozoic. And a lot of researches concerning about the ophiolite's tectonic property have been carried out,while only a few concerning about mineralogical study. Taking genetic mineralogy, inclusion mineralogy, mantle mineralogy and mineral crystal chemistry as the theory guidances. This project selects mineralogy of Northeastern Jiangxi ophiolite as subject investigated. .With the application fo the most advanced in stiu analysis techniques, the major\minor elements and structure of the spinel and paragentic minerals in the Neoproterozoic ophiolite northeastern Jiangxi province are studied, and the distribution of the chrome spinel within the ophiolitic melange belt are identified. By Summarizing the association relation of spinel assemblage and the types and characteristics of it's inclusions, the temperature and pressure condition and their informations that spinel experienced are determined, so as to reveal the relationship between the spinels' micro-zone and their experienced deep geological processes, and to explore the geological significance which they represent. The completion of this study can not only enrich the research content of the northeastern Jiangxi ophiolitic mélange, but also provide a strong mineralogical evidence for the regional tectonic environment, which has a geodynamic significance in ophiolite genesis.

赣东北蛇绿混杂岩是中国少有的产于晚元古宙的蛇绿岩，也是我国最为古老的蛇绿岩带之一，已有的研究工作多围绕蛇绿岩的构造属性展开，而矿物学等基础性研究工作开展较少。本项目选择矿物学研究程度较低的江西赣东北蛇绿岩为研究对象，以成因矿物学、包体矿物学、地幔矿物学和矿物晶体化学为理论指导，以矿物主量组份、微量元素组成和矿物微观结构等为切入点，采用各种现代地球科学测试技术，对江西赣东北蛇绿混杂岩中的铬尖晶石及其共生矿物进行深入研究，查明蛇绿混杂岩带内铬尖晶石的空间分布特征；总结铬尖晶石矿物共生组合关系以及矿物内包裹体的种类和特征，确定矿物所经受的温压条件等信息；揭示尖晶石的微区环带特征与其所经历的深部地质过程之间的关系，进而探讨其代表的地质意义。本研究的完成不但能丰富赣东北蛇绿混杂岩的基础研究资料，也为区域构造环境判别提供了有力的矿物学证据，对赣东北蛇绿混杂岩带成因研究具有重要的地球动力学意义。

本项目的研究对象为江西赣东北蛇绿混杂岩带中的超基性岩，主要研究其中的铬尖晶石矿物学特征，从而探讨铬尖晶石代表的地质意义。项目组对研究区进行了详细的野外地质调查，在此基础上对不同类型橄榄岩进行了细致的岩相学鉴定和矿物成份的分析，确定了铬尖晶石形成时的温度、氧逸度等信息。同时项目组还开展了雅鲁藏布江蛇绿岩带与江西赣东北蛇绿岩带的矿物学对比工作，取得了一定的认识，也为本项目的顺利完成奠定了基础。四年的研究工作基本上是按照项目申请书中所提出的研究内容和研究计划进行的，研究工作基本完成，申请书中提及的研究目标基本达到，相关研究成果已陆续发表。主要取得如下几点认识：（1）通过铬尖晶石及其共生矿物的研究，计算了赣东北蛇绿混杂岩带中各类型橄榄岩的形成温度（纯橄岩为1120℃，方辉橄榄岩为972℃，含单斜辉石方辉橄榄岩为923℃）和氧逸度（对数氧逸度值在-3.5至-0.1之间），低氧逸度值可能与华南、扬子板块在新元古期间快速拼合过程中俯冲带内发生的熔体-岩石反应有关；（2）铬尖晶石寄主岩相橄榄岩及其伴生的玄武岩两者兼具有MORB性质和陆源物质的混染特征，指示赣东北蛇绿岩混杂岩在构造演化过程中经历了洋中脊（MOR）到俯冲带（SSZ）两种构造环境；（3）赣东北蛇绿混杂岩中的环带状铬尖晶石记录了岩体经历上述两期次的构造演化过程,即赣东北环带状铬尖晶石可能是俯冲环境的流体交代导致了先期形成的尖晶石，从核心到边缘发生元素的重新分配的结果。

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