西沙群岛中新世铁白云岩的空间变异和晶格结构特征与成因机制

The dolomite problem has been the crown of carbonate sedimentary petrology for more than 200 years, and it is still a frontier and hot topic of research. This study is aimed at the dolomite problem, supplementing and perfecting the mechanism of environmental iron dolomite formation in the isolated islands of the ocean and the information recorded in the paleo-oceanic environment, which has important theoretical and practical significance. It is proposed to base on that the Neogene iron dolomite in the Xisha Islands continues six layers of more than one hundred meters in the Xiyong-2 well of Yongxing Island and 16 deep-seated in the Xike-1 well of Shi Island and preliminary research results, which was newly discovered in recent years. Aimed at the two well iron dolomites, we conducted research and comparison of spatial variability between individual and interwells. High resolution measurement is conducted to analyze the mineralogical and geochemical characteristics of iron dolomite cores, revealing that specific depths of iron dolomitization included water-rock interactions and the information on the paleo-ocean evolution of the iron dolomite in the deep-water environment of the isolated carbonate platform of Xisha Islands in Neolithic, and constructing the gold-nail well research section of the iron dolomite. To further distinguish the types of iron dolomite and different ankerite, we carry out the five parameters of the crystal structure of iron dolomite: systematic test analysis of orderliness, cell parameters, lattice fringes, interplanar spacing, and lattice defects; The correlation analysis eliminates multiple solutions, clarifies the genetic mechanism of iron dolomite changes in the paleo-oceanic environment in the South China Sea during the Miocene, and proposes the establishment of genetic models.

白云岩问题至今200余年，是碳酸盐岩沉积岩石学的皇冠，至今还是研究前沿与热点。该研究针对白云岩问题，补充完善海洋孤立岛屿环境铁白云岩的成因机制及所记录古海洋环境变化的信息，具有重要的理论价值和实际意义。拟基于申请者近年来新发现的西沙群岛永兴岛西永2井延续百余米六层和石岛西科1井16个深度新近纪铁白云岩和初步研究成果，针对该两口井铁白云岩开展各自和井间空间变异特征研究与对比，高分辨率测试分析铁白云岩岩心岩石矿物学、地球化学特征，揭示特定深度铁白云石化作用包括水岩相互作用，新近纪西沙群岛孤立碳酸盐岩台地深水环境铁白云岩的古海洋演变信息，构建铁白云岩金钉子井研究剖面；进一步区分铁白云岩的类型及不同的铁白云石，开展这些铁白云石晶体结构五大参数：有序度、晶胞参数、晶格条纹、晶面间距、晶格缺陷的系统测试分析；通过相关性分析剔除多解性，阐明中新世南海古海洋环境变化的铁白云岩成因机制，提出构建成因模式。

白云岩问题至今232年，是碳酸盐岩沉积岩石学的皇冠，至今还是研究前沿与热点。项目针对白云岩问题补充完善岛屿环境铁白云岩成因特征和机制及其古海洋环境信息开展研究，具有重要理论价值实际意义。确认西科1井赋存12层435.1m白云岩含25层铁白云岩；井底侏罗纪火山岩上覆第一块岩心是藻礁白云岩，三米以后出现铁白云岩，百米以后是105m致密白云岩，三者授权国家发明专利。发现铁白云石铁离子加入改变了原白云石晶格结构，铁白云石晶胞参数a、c值较白云石偏大，具有晶格线缺陷；铁白云岩铁同位素特征与自然界各种来源宿主铁元素铁同位素标准数据对比，地球化学属性与宿主性铁白云岩铁同位素来源尚有待确认。铁白云岩电子探针和扫描电镜背散射相互印证；完成去白云化作用、西科1井和西永2井白云岩-铁白云岩空间变异对比：赋存层系深度、重金属含量、变异过程与结果不同。完成高尖石岛调查，登陆绘制六套火山岩地层层序，采集对应六块样品，完成岩相学矿物岩石学鉴定，未检获锆石故未测年；也未测试铁元素铁同位素及与铁白云岩铁同位素实现对比。.对比全球20个岛屿白云岩钻井含南沙美济岛南科1井，西沙琛航岛琛科2井等，发现唯一西科1井发育161.59m深层白云岩，提出浅层≦500m、中深层500-1000m和深层≥1000m白云岩划分标准。发现深层白云岩具有Mn、Fe含量极高，达771.30ppm和2476.24ppm；Sr含量和87Sr/86Sr最低，仅173.70和0.7082等异常；阴极发光分析证实形成于7-8个世代；流体包裹体均一化温度甚至超过沉积盆地5千米深层白云岩；稀土元素配分模式指示致密白云岩围岩与填屑物分别形成于热液流体和正常海水环境；说明深层白云岩是生物白云化，热液白云化，多期白云化和原生白云化共同作用的产物，但成因特征和机制研究尚为薄弱环节。发表论文14篇，国内外会议特邀报告3个，分会场报告11个，培养研究生9人。