冀东杏山BIF铁矿形成时代及成因探讨  

Formation age and origin of the Xingshan BIFtype iron deposit in eastern Heibei Province

作　　者：郑梦天 张连昌[1] 王长乐[1,2] 朱明田[1] 李智泉[1,3] 王亚婷[1,3] 

机构地区：[1]中国科学院矿产资源研究重点实验室、地质与地球物理研究所,北京100029 [2]中国科学院大学,北京100049 [3]中国地质大学,北京100083

出　　处：《岩石学报》2015年第6期1636-1652,共17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：杏山BIF型铁矿位于冀东曹庄一带,矿体赋存于太古界三屯营组变质岩系中。矿石多为条纹-条带状构造,其围岩和夹层以斜长角闪岩为主,兼具少量黑云石英片岩和黑云钾长片麻岩。对BIF铁矿进行的地球化学分析表明,条带状铁矿石富集重稀土（（La/Yb）＊PAAS平均0.46）,Eu正异常（Eu/Eu＊PAAS平均2.19）、La正异常（La/LaPAAS平均2.14）和Y的正异常（Y/Y＊PAAS平均1.85）明显,加之高的Y/Ho值（平均46.81）,说明成矿物质来源于海底高温热液与周围海水的混合溶液;无Ce的负异常说明BIF形成于低氧逸度环境;BIF显示较低的Al2O3和TiO 2含量,且二者缺乏相关性,说明在BIF沉淀过程中几乎没有陆源碎屑物的加入。对矿体围岩斜长角闪岩、黑云石英片岩及黑云钾长片麻岩的原岩恢复,表明斜长角闪岩的原岩为玄武岩-安山质玄武岩,黑云石英片岩及黑云钾长片麻岩的原岩可能为泥质碎屑岩。元素地球化学分析表明,黑云钾长片麻岩具有较低的总稀土含量（59.46×10-6-66.99×10-6）、不明显的Eu负异常（Eu/Eu＊=0.77-0.91）、略微的Th和U富集、无Nb和Ta的亏损;而黑云石英片岩具有较高的稀土总量（168.7×10-6-171.0×10-6）、强烈的轻稀土富集（（La/Yb）N=6.81-7.07）和较弱的Eu负异常（Eu/Eu＊=0.64-0.68）,两者不一致的地球化学特征说明他们具有不同的物质来源。斜长角闪岩与N-MORB具有相似的特征,如富集大离子亲石元素（LILEs）,亏损Th和U、Nb、Ta、Ti、Zr和Hf等无明显亏损。综合分析表明斜长角闪岩的母岩浆来源于尖晶石二辉橄榄岩30%的部分熔融,且其母岩浆在分异结晶过程中可能受到陆壳物质的混染。对矿体夹层斜长角闪岩LA-ICP-MS锆石U-Pb定年,结果表明其原岩形成于2859±22Ma至2491±13Ma之间,最有可能形成于新太古代晚期,这间接约束了杏山BIF型铁矿的形成时代,暗示着迁西岩群主体可能仍形成于新太古�The Xingshan BIF-type iron deposit,which is located in the Caozhuang area of eastern Hebei,is hosted in Archean metamorphic rock series of the Santunying Formation. The majority of the ores are characterized by banded or laminated structure,while the wallrocks consist predominantly of amphibolites,which is interlayered with iron ore,and a small amount of biotite quartz schist and biotite k-feldspar gneiss. Geochemical analyses show the iron ores are enriched in HREE（（ La / Yb）PAAS= 0. 46）,and characterized by positive Eu anomaly（ Eu / Eu＊PAAS= 2. 19）,positive La anomaly（ La / La＊PAAS= 2. 14） and positive Y anomaly（ Y /Y＊PAAS= 1. 85） With a Y / Ho weight ratio of 46. 81,indicating the ore-forming materials were supplied through mixtures of hightemperature hydrothermal fluids and seawater. The lack of negative Ce anomaly also suggests the seawater was under relatively low oxygen fugacity during the deposition of the Xingshan BIF. Moreover,the low concentrations of Al2O3 and Ti O2,low correlation coefficient between Al2O3 and Ti O2 indicate that there was no significant terrigenous input. In addition,a protolith reconstruction for wallrocks shows that the protolith of amphibolites is basalt,and the protolith of biotite quartz schist and biotite k-feldspar gneiss may be the argillaceous clastic rock. Geochemical analyses show that the biotite k-feldspar gneiss is characterized by relatively low ΣREE（ 59. 46 × 10- 6~ 66. 99 × 10- 6）,no obvious negative Eu anomaly（ Eu/Eu＊= 0. 77 ~ 0. 91）,slightly depletion of Th,U and no depletion of Nb,Ta,Ti; while the biotite quartz schist is characterized by relatively high ΣREE（ 168. 7 × 10- 6~ 171. 0 × 10- 6）,enrichment in LREE（（ La / Yb）N= 6. 8 ~ 7. 1） and slightly negative Eu anomaly（ Eu / Eu＊= 0. 64 ~ 0. 68）,the different geochemical characteristics between them may imply the different material source. Meanwhile,the amphibolites have similar characteristics with NMORB,such as enrichment in LILEs,depletion of Th,U and n

关 键 词：杏山BIF铁矿 太古代 斜长角闪岩 锆石U-PB定年 地球化学 华北克拉通 

分 类 号：P597.3[天文地球—地球化学] P618.31[天文地球—地质学]