南阿尔金地区含铀-富铀锆钛石的发现及其新成因机制  

Discovery of uranium-bearing srilankite and a new genetic mechanism in South Altyn Tagh, China

作　　者：周宁超[1,2,3] 刘亚非 魏小燕[1,2] 王立社 Ningchao Zhou;Yafei Liu;Xiaoyan Wei;Lishe Wang(Xi'an Center of China Geological Survey,Xi'an 710054,China;Northwest China Center fo r Geoscience Innovation,Xi’an 710054,China;Department of Geology,Northwest University,Xi'an 710069,China)

机构地区：[1]中国地质调查局西安地质调查中心,西安710054 [2]西北地质科技创新中心,西安710054 [3]西北大学地质学系,西安710069

出　　处：《科学通报》2019年第26期2717-2727,共11页Chinese Science Bulletin

基　　金：国家自然科学基金(41502034)资助

摘　　要：锆钛石(ZrTi2O6)为高级变质岩、交代地幔岩和岩浆岩中的一种罕见副矿物,是研究高场强元素迁移与富集机理的理想对象.中国南阿尔金高压-超高压变质带的高压基性麻粒岩中发现了含铀-富铀锆钛石,其主要化学组成为TiO2,ZrO2和UO2,但不同颗粒或同一颗粒不同区域成分变化较大,UO2含量最高可达20.83 wt%,且U与Zr具类质同象替代关系,晶体化学式可简写为(Zr,U)Ti2O6.低-中等UO2含量的含铀锆钛石与不含铀锆钛石相比晶体结构未发生显著变化,高UO2含量的富铀锆钛石由于放射性损伤已发生蜕晶化.矿物显微结构及成分分析表明南阿尔金含铀-富铀锆钛石及其伴生矿物组合为:金红石-钛铁矿-含铀-富铀锆钛石-白钨矿或钛铁矿-含铀-富铀锆钛石-白钨矿,其中钛铁矿为金红石退变形成,而含铀-富铀锆钛石和白钨矿是金红石退变分解过程中释放的Zr,Ti,W等元素与变质流体中的U,Ca结合直接结晶形成.该研究对矿物尺度高场强元素迁移富集机理具有重要的指示意义.The mineral srilankite(Zr Ti2O6)is a rare accessory phase in high-grade metamorphic,metasomatic mantle and igneous rocks.Due to being highly enriched in high field strength elements(HFSE),it provides an ideal basis to study the migration and enrichment mechanism of HFSE during geologic processes.Here,we present analyses of a cluster of uranium-bearing srilankite with UO2 up to 20.83 wt%discovered in the mafic granulite from the high-pressure(HP)to ultrahigh-pressure(UHP)belt in South Altyn Tagh,China.A new genetic mechanism of metamorphic srilankite is proposed based on the study of the mineralogical characteristics of the uranium bearing srilankite and its associated minerals through various methods such as scanning electron microscope(SEM),electron probe microanalysis(EPMA),focused ion beam(FIB)and transmission electron microscope(TEM).The uranium-bearing srilankite occurs as irregular to subhedral grains,up to 30μm in size,characterized by relatively light backscattered electron image and no cathodoluminescence under the electron beam in SEM.The results of EPMA show that the main chemical components of srilankite was Ti O2,Zr O2 and UO2,but with dramatic variation among different grains or in different compositional domains of one individual crystal.For example,the content of the Zr O2 ranges from 27.03 wt%to 42.31 wt%,TiO2 from 49.39 wt%to 56.55 wt%and UO2 from 0.65 wt%to 20.83 wt%.Characteristic variation of the chemical composition indicates the isomorphous substitution of U and Zr.Therefore,the crystallochemical formula of the uranium-bearing srilankite could be simply written as(Zr,U)Ti2O6.TEM analysis of the mineral slice prepared by FIB shows that the crystal structure of srilankite with low to medium UO2 content did not change significantly compared to those containing no uranium,whereas srilankite with high UO2 had been metamictized due to radioactive damage.Microstructure and chemical composition analysis show that the uranium-bearing srilankite and associated mineral assemblages are rutile-ilmenite

关 键 词：含铀-富铀锆钛石 南阿尔金 高压基性麻粒岩 金红石 钛铁矿 

分 类 号：P61[天文地球—矿床学]