内蒙古固阳绿岩带三合明BIF型铁矿的形成时代、地球化学特征及地质意义  

Formation age,geochemical signatures and geological significance of the Sanheming BIF-type iron deposit in the Guyang greenstone belt,Inner Mongolia

作　　者：刘利[1,2] 张连昌[1] 代堰锫[1,2] 王长乐[1,2] 李智泉[1,3] 

机构地区：[1]中国科学院地质与地球物理研究所矿产资源研究重点实验室.北京100029 [2]中国科学院大学,北京100049 [3]中国地质大学地质过程与矿产资源国家重点实验室,北京100083

出　　处：《岩石学报》2012年第11期3623-3637,共15页Acta Petrologica Sinica

基　　金：国家重点基础研究发展计划973项目(2012CB416601);中国科学院知识创新工程重要方向项目群(KZCX-2YW-Q04-07)联合资助

摘　　要：三合明BIF型铁矿位于华北克拉通西部陆块北缘,产出于固阳绿岩带。矿体赋存于新太古界色尔腾山群斜长角闪岩中。矿石主要呈粒状变晶结构、粒状-针柱状变晶结构,条带-条纹状构造;组成矿石的金属矿物主要为磁铁矿,非金属矿物主要为石英,次为角闪石等。对选自斜长角闪岩中的锆石进行SIMSU-Pb定年,具有核边结构、Th/U比大于0.4的锆石其核部给出了2562±14Ma的上交点年龄,可大致作为三合明BIF的形成时代。原岩恢复显示斜长角闪岩为正变质岩,Zr/Ti-Nb/Y图解显示为亚碱性玄武岩系列,Fe2O3T+TiO2-Al2O3-MgO图解落入高铁拉斑玄武岩区;斜长角闪岩主量元素特征与MORB相近,REE配分曲线和蛛网图都近于平坦,且介于N-MORB和E-MORB之间,LREE略微富集,Th、U相对亏损,Nb、Ta、Zr和Hf无明显异常;(La/Sm)N和Nb/U分别为0.76和50,由此推断原岩可能为T-MORB。Ti-V、Th-Hf-Ta构造环境判别图解中,分别落入MORB和弧后盆地的重叠区、N-MORB区。结合T-MORB形成的构造环境以及前人提出的岛弧叠加地幔柱模式,本文认为三合明BIF形成于弧后盆地并有地幔柱叠加的构造环境。铁矿石化学组分主要为SiO2、Fe2O3和FeO,较低的Al2O3(0.68%)、极低的TiO2(0.04%)和HFSE表明只有极少量陆源碎屑物质的加入。铁矿石的球粒陨石标准化REE配分模式与固阳绿岩带底部的科马提岩极为相似,PAAS标准化的铁矿石REY配分模式与高温热液海水混合物相似,即LREE亏损,HREE富集((La/Yb)SN=0.34),具有明显的正Eu异常(δEu=2.33)和微弱的正Y异常(δY=1.13),Y/Ho重量比29,摩尔比53。根据铁矿石兼具有与科马提岩和高温热液海水混合物相似的地球化学特征,本文推断海底高温热液淋滤科马提岩为三合明BIF型铁矿提供了大量的Fe和Si。The Sanheming BIF-type iron deposit occurs in the Guyang greenstone belt, which is located in the northern margin of Western Block of the North China Craton. The iron deposit is hosted in the Mount Se’erteng Group of Neoarchean Erathem with its ore bodies interlayered with amphibolites. The majority of the ores are characterized by granular crystalloblastic texture or granular-columnar crystalloblastic texture and banded or laminated structure. The ore consists predominantly of magnetite and quartz, secondary nonmetallic minerals include amphibole. SIMS U-Pb dating of zircons from the amphibolite interlayer shows that cores of the zircons characterized by core-rim texture and with a Th/U ratio of higher than 0.4 have an upper intercept age of 2562±14Ma, which is approximately the depositional age of the Sanheming BIF. Diagrams for protolith reconstruction reveal that the amphibolites are ortho-amphibolite and they are plotted within sub-alkaline basalt series and high-iron tholeiite in the Zr/Ti-Nb/Y diagram and Fe2O3T＋TiO2-Al2O3-MgO diagram, respectively. Major elements of the amphibolites are similar to those of MORB. Additionally, both REE profiles and spidergrams are flat in shape, roughly between those of N-MORB and E-MORB. LREE are slightly enriched with a （La/Yb）N ratio of 1.68, Th and U are relatively depleted, there are no apparent anomalies in terms of Nb, Ta, Zr and Hf, （La/Sm）N ratio and Nb/U ratio are 0.76 and 50, respectively. On the basis of such characteristics, it is deduced that protoliths of the amphibolites are T-MORB. Constraints from Ti-V and Th-Hf-Ta discrimination diagrams suggest the tectonic setting for the amphibolites are MORB or back-arc basin and N-MORB, respectively. Combining the general tectonic setting of T-MORB, i.e., mid-ocean ridge or back-arc basin, with the model of island arc-mantle plume interaction for the Guyang greenstone belt, it is speculated that the amphibolite interlayers were formed in a tectonic setting of back-arc basin, accomponied by mantle plume.

关 键 词：三合明BIF型铁矿 锆石U-PB年龄 地球化学 固阳绿岩带 

分 类 号：P618.51[天文地球—矿床学]