内蒙古固阳绿岩带条带状铁建造地球化学特征与沉积环境讨论  

Geochemistry and depositional setting of Banded Iron Formations in Guyang greenstone belt, Inner Mongolia

作　　者：南景博 黄华[2] 王长乐[2] 彭自栋 佟小雪[2,3] 张连昌[2] NAN Jingbo;HUANG Hua;WANG Changle;PENG Zidong;TONG Xiaoxue;ZHANG Lianchang(State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China;Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029,China;University of Chinese Academy of Sciences, Beijing 100049, China)

机构地区：[1]中国地质大学(北京)地球科学与资源学院,北京100083 [2]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 [3]中国科学院大学,北京100049

出　　处：《中国地质》2017年第2期331-345,共15页Geology in China

基　　金：国家自然科学基金(41602097);国家"973"计划项目(2012CB416601);中国地质调查局重点项目"华北克拉通乌拉山地区早前寒武纪变质作用研究"(121201121062)联合资助~~

摘　　要：Algoma型条带状铁建造(BIF)是一种海相化学沉积岩,它发育在太古宙绿岩带的火山-沉积岩序列中,大部分经历了后期变质变形作用。稀土元素具有稳定的地球化学性质,因而被广泛用于指示BIF的原始沉积环境。华北克拉通北缘固阳绿岩带中的BIF(如三合明、公益明、东五分子、公巨成等)赋存于新太古代色尔腾山群的变火山-沉积序列中,普遍经历了绿片岩相—低角闪岩相变质,呈条带状构造,其主要矿物组成为自形—半自形粒状磁铁矿和他形粒状石英,含少量角闪石,局部地区可见石榴石、绿泥石、斜长石等富Al矿物。根据BIF中惰性元素的含量及其相关性,可划分为纯净的BIF和遭受碎屑物质混染的BIF,并通过特征的Co/Th比值判断出这些碎屑物质主要来自于玄武质岩石。纯净及遭受轻微碎屑物质混染的BIF稀土元素特征通过后太古宙页岩(PAAS)标准化后,与现代海水及其他地区典型太古宙BIF相似,均表现为轻稀土相对重稀土亏损,正La、Y异常以及较高的Y/Ho比值(>29)。大多数样品发育不明显的负Ce_(PAAS)异常,说明BIF沉淀自缺氧的环境;正Eu_(PAAS)异常明显,说明海水中有高温(>250℃)热液的加入,其比例为0~10%。然而,遭受强烈碎屑物质混染的BIF稀土元素特征则具有明显差异。此种类型BIF的出现及分布不均,指示了在BIF沉积之前的局部地区或时间段内,深部海水处于相对动荡的环境。Algoma-type Banded Iron Formations(BIFs)are marine chemical sedimentary rocks that are generally interlayered with volcanic rocks and/or sedimentary sequences in Archean greenstone belts,mostly followed by metamorphism and deformation.Because rare earth elements(REEs)are usually not fractionated during diagenesis and metamorphism,REE patterns are thus regarded as a powerful tool to understand conditions under which BIF were deposited.Located in the northern North China Craton(NCC),BIFs such as Sanheming,Gongyiming,Dongwufenzi and Gongjucheng are hosted by the Guyang greenstone belt,of Seertengshan Group which consists of multiple sequences of metavolcanic rocks and metasedimentary rocks.The regional metamorphic grade ranges from upper greenschist?to lower amphibolite?facies.The BIFs are composed of laminated magnetite and white to gray quartz associated with amphibole layers.Moreover,minor chlorite,garnet and plagioclase,rich in Al,are observed interbedded with quartz and magnetite.Based on the content of immobile elements,the samples can be subdivided into two types:pure and contaminated.Cr/Th ratios in the contaminated BIF indicate that detritus is mainly from basaltic rocks.Normalized by post?Archean Australian Shale(PAAS),REE patterns of pure and weakly contaminated BIF are characterized by depletion of LREE,positive La and Y anomalies,and relatively high Y/Ho ratios(>29),which are typical of modern seawater and some other Archean BIF.Slightly negative CePAAS anomalies are present in most samples,suggesting its precipitation from anoxic waters.Large positive EuPAAS anomalies may indicate that,during deposition of BIF,the oceans were greatly influenced by high?temperature(>250℃)hydrothermal fluxes which accounted for0-10%.However,REE patterns of highly contaminated BIF are obviously distinct.Disordered destitution of this kind of BIF indicates that deep water in limited regions or time was not quiet before the deposition of BIF.

关 键 词：条带状铁建造 稀土元素 沉积环境 碎屑混染 绿岩带 

分 类 号：P588.2[天文地球—岩石学] P618.31[天文地球—地质学]