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1.
华北克拉通是我国早前寒武纪条带状铁建造(BIF)最重要的分布区。近年来大量锆石定年研究表明,华北克拉通BIF形成于始太古代到古元古代早期,但主要为新太古代晚期(2.50～2.55Ga)。最重要的BIF分布于华北克拉通东部的鞍本、冀东和鲁西地区,沉积环境相对稳定是大规模BIF形成的重要条件。华北克拉通新太古代晚期BIF形成的构造环境还不十分清楚,但可能为岛弧构造环境。BIF明显的时代专属性是地质和大气演化的结果。  相似文献   

2.
华北克拉通前寒武纪BIF铁矿研究:进展与问题   总被引：29，自引：18，他引：11  
研究表明,BIF铁矿在华北克拉通的分布具有一定规律性.大规模BIF铁矿主要发育在绿岩带分布区的鞍山-本溪、冀东、霍邱-舞阳、五台、鲁西和固阳等地;华北克拉通时代最古老的BIF形成于古太古代,最年轻BIF形成于古元古代早期,但BIF铁矿的峰期为新太古代晚期(2.52 ～2.56Ga);BIF铁矿类型可划分为阿尔戈马型和苏比利尔湖型两类,但华北以晚太古代绿岩带中的阿尔戈马型为主,仅吕梁的古元古代袁家村铁矿具典型苏比利尔湖型铁矿特征.根据BIF在绿岩带序列中的产出部位和岩石组合关系,可将华北BIF划分为:1)斜长角闪岩(夹角闪斜长片麻岩)-磁铁石英岩组合;2)斜长角闪岩-黑云变粒岩-云母石英片岩-磁铁石英岩组合;3)黑云变粒岩(夹黑云石英片岩)-磁铁石英岩组合;4)黑云变粒岩-绢云绿泥片岩-黑云石英片岩-磁铁石英岩组合;5)斜长角闪岩(片麻岩)-大理岩-磁铁石英岩组合等5种类型.华北克拉通BIF形成时代与早前寒武纪岩浆活动的时间基本一致(2.5～2.6Ga),但与华北克拉通陆壳增生的峰期(2.7～2.9Ga)有一定偏差,其原因可能与新太古代晚期华北克拉通构造-热事件十分强烈有关.华北克拉通新太古代BIF大多形成于岛弧环境,但局部地区(如固阳)BIF铁矿可能形成于深部有地幔柱叠加的岛弧环境.华北克拉通BIF富矿主要有三种类型:原始沉积、受后期构造-热液叠加改造和古风化壳等,但总体不发育富铁矿,国外发育的风化壳型富铁在我国甚为少见.本文认为在探讨BIF铁矿类型时,需要从绿岩带发育序列进行综合判别.阿尔戈马型铁矿一般产于克拉通基底(绿岩带)环境,苏比利尔湖型铁矿一般形成于稳定克拉通上的海相沉积盆地或被动大陆边缘.华北克拉通BIF铁矿地球化学研究结果表明,BIF铁矿无Ce负异常且Fe同位素为正值,从而暗示铁矿沉淀的环境为低氧或缺氧环境,而铕正异常可能指示BIFs为热水沉积成因,其机制可能为海水对流循环从新生镁铁质-超镁铁质洋壳中淋滤出F(e)和Si等元素,在海底排泄沉淀成矿,而条带状构造的形成可能归咎于成矿流体的脉动式喷溢.但对于BIF铁矿的物质来源、成矿条件和机制、富铁矿成因、华北克拉通不发育苏比利尔湖型铁矿的原因等方面,仍需深入研究.  相似文献   

3.
早前寒武纪BIF原生矿物组成及演化、沉积相模式研究进展   总被引：1，自引：0，他引：1  
条带状铁建造(BIF)原生矿物组成有助于约束其沉积相和沉积环境,当前主要认为三价铁氢氧化物或铁硅酸盐微粒(主要成分为铁蛇纹石或黑硬绿泥石)可能是BIF原生矿物的主要成分,在后期成岩或变质作用过程中转变为赤铁矿、磁铁矿、菱铁矿等矿物。根据BIF的矿物组合可将其沉积相划分为氧化物相、硅酸盐相和碳酸盐相。通过沉积地层学和地球化学等方法研究,以古元古代大氧化事件为标志将沉积相总结为"缺氧还原"和"分层海洋"2种相模式:大氧化事件前,古海洋整体处于缺氧还原环境,BIF沉积相从远岸到近岸呈赤铁矿相—磁铁矿相—碳酸盐相分布,如南非West Rand群BIF(2.96~2.78 Ga)和Kuruman BIF(约2.46 Ga);大氧化事件期间及之后,古海洋上部氧化、下部还原,BIF沉积相与之前截然相反,从远岸到近岸呈碳酸盐相—磁铁矿相—赤铁矿相分布,如中国袁家村BIF(2.2~2.3 Ga)和加拿大Sokoman铁建造(约1.88 Ga)。总体看来,只有特定的沉积环境才能形成这种特殊的地质历史上不再重复出现的沉积建造,而原生矿物组成的甄别和推导、沉积相的形成机制、BIF沉淀条件的准确限定和微生物活动与BIF的关联等问题是推测古海洋环境的关键所在,也是目前亟待解决的问题。  相似文献   

4.
华北陆块基底构造格局及早期大陆克拉通化过程   总被引：52，自引：22，他引：30  
依据区域构造分析及同位素年代娄数据库,华北克拉通普质基底主要可以区划为以处构造单元：１）鄂尔多斯陆块新太古代被动边缘沉积;２）恒山－－承德太古代末期构造带;３）太古代末期五台－－登封岛弧带杂岩及构造缝合带;４）鲁西－－冀东－辽吉新太古代活动大陆边缘岩浆杂岩带;５）胶辽陆块;６）冀北－－固阳古元代初造山带及内蒙－＝东再造麻粒岩要带;７）吕梁－－中条古元古代裂谷带;８）辽南古元古代裂谷带。华北克拉通早  相似文献   

5.
怀安地区构造变形强烈,是研究华北克拉通形成和演化的重要窗口。通过对该区基底变质岩系构造形迹进行研究,结合区域地质资料及同位素年龄数据,将该区早前寒武纪构造变形序列划分为4期: 新太古代阜平晚期(D1),桑干岩群韧性变形,形成片麻理、无根褶皱和韧性剪切带; 新太古代五台晚期(D2),在新太古代TTG/花岗岩中形成区域性片麻理和条带状构造,并在桑干岩群中形成近EW向的复式背形和向形构造; 古元古代吕梁中期(D3),集宁岩群沙渠村岩组形成区域性片麻理,并形成与片麻理一致的NE向韧性剪切带,在新太古代地质体中叠加近SN向的开阔复式背形和向形构造; 古元古代吕梁晚期(D4),红旗营子岩群太平庄岩组形成片理和片麻理,尚义—平泉断裂形成。建立了该区早前寒武纪构造演化模式,并将其划分为新太古代陆壳增生阶段和古元古代碰撞造山阶段,对理解华北克拉通的形成及演化具有重要意义。  相似文献   

6.
鞍本是华北克拉通最为重要的BIF铁矿集中区之一,BIF赋存于鞍山群表壳岩中.通过对广泛分布的鞍山群表壳岩的12个样品进行锆石SHRIMP U-Pb定年,大都获得2.50~2.55 Ga岩浆锆石年龄,一些岩石中存在2.7~3.5 Ga碎屑锆石或外来锆石.一些东北部（东部）岩石记录了新太古代晚期-古元古代早期变质锆石年龄.结合前人研究成果,可得出如下结论:（1）鞍本地区广泛分布的鞍山群含BIF表壳岩形成时代为新太古代晚期;（2）鞍山群表壳岩形成于陆壳基底之上.该研究进一步支持了已有认识:鞍本为东部古陆块西缘新太古代晚期巨型BIF成矿带的重要组成部分,稳定的构造环境有利于大规模BIF形成.   相似文献   

7.
华北克拉通的组成及其变质演化   总被引：7，自引：0，他引：7       下载免费PDF全文
华北克拉通早前寒武纪变质基底主要由五套不同类型的变质岩系组成。克拉通在形成过程中经历了多期构造活动、多期岩浆侵位、多期变质作用以及不同程度的混合岩化和深熔作用,岩石已遭受多次不同地质作用的叠加改造,因此华北克拉通具有复杂的演化历史。从太古宙到古元古代末的克拉通形成,华北克拉通主要经历了五期区域变质作用。鞍山地区的古—中太古代经历了角闪岩相变质作用改造,尚未获得变质年龄数据。但在TTG岩系中已获得3 560 Ma和3 000~3 300 Ma早期的变质年龄。河南鲁山太华杂岩的中太古代斜长角闪岩中获得2 776~2 792 Ma和2 671~2 651 Ma两期变质作用年龄信息,代表了新太古代早期的变质作用。新太古代麻粒岩-TTG岩系和新太古代花岗-绿岩系都经历了新太古代晚期—古元古代初的变质作用改造。在古元古代阶段,在华北克拉通北缘在1 965~1 900 Ma期间发生了中低压/高压麻粒岩相变质,局部发生超高温变质,这期变质作用与陆块间的俯冲碰撞及其后的地幔上涌有关。在古元古代晚期(1 890~1 800 Ma)在华北克拉通的中部及东部的胶—辽—吉带发生了高压麻粒岩相-角闪岩相的区域变质,代表了陆块间的碰撞拼合过程。不同变质岩系类型经历的变质作用反映了不同的构造背景。太古宙晚期大量的TTG岩系及呈面状分布的中/低压麻粒岩主要出露在华北克拉通的中北部,普遍具有逆时针的p-T轨迹,反映了地幔柱底板垫托的构造环境。新太古代的花岗-绿岩系在新太古代晚期—古元古代早期经历的变质作用多为顺时针的p-T演化轨迹,反映其发生可能与弧后+地幔柱联合作用的构造背景。古元古代晚期的两期变质作用多表现为高压麻粒岩相的顺时针p-T演化轨迹,反映了不同陆块(地块)之间碰撞拼合的过程,意味着类似显生宙的板块构造体制已经出现。  相似文献   

8.
详细报道了冀东、五台和吕梁地区条带状铁矿全岩样品的稀土元素分析结果。结果表明,研究区BIF具有非常相似的特征：稀土总量均较低;经页岩标准化的稀土元素配分模式均呈现轻稀土亏损、重稀土富集的特征;Y/Ho比值较高;具有明显的Eu、Y、La的正异常,且这些特征表明研究区BIF的稀土元素来源于火山热液和海水的混合溶液。虽然BIF均显示Eu正异常,但不同类型、不同沉积年龄BIF的铕异常程度不同：与吕梁地区Superior型铁矿相比,冀东和五台地区的Algoma型铁矿显示了更大的Eu正异常;并且自中太古代-新太古代-古元古代,BIF的铕正异常逐渐减小,这可能反映了随着BIF沉积年龄的减小,进入到该地区海水中的高温热液流体逐渐减少;同时,研究区BIF缺乏明显的Ce负异常,可能暗示在BIF沉积时海水的氧化还原状态为缺氧环境。  相似文献   

9.
中国BIF型铁矿床地质特征和资源远景   总被引：10，自引：0，他引：10  
沈保丰 《地质学报》2012,86(9):1376-1395
BIF型铁矿床是中国最重要的铁矿床类型,占全国总查明资源储量55.2％.BIF型铁矿床主要分布在华北陆块,其次在扬子陆块.在华北陆块鞍山—本溪,密怀—冀东,五台—吕梁矿集区中铁矿床尤为集中,约占全国铁总探明储量41.5％.BIF型铁矿床在古太古代、中太古代、新太古代、古元古代和新元古代均有产出,但主要在新太古代—古元古...  相似文献   

10.
国内外前寒武纪条带状铁建造研究现状   总被引：4，自引：0，他引：4  
条带状铁建造(BIFs)主要发育于早前寒武纪时期(3.8~1.8Ga),记录了早期地球演化的重要信息且蕴含丰富的铁矿石资源。本文梳理总结了国内外BIF相关领域的研究认识及存在问题:1统计对比显示,BIF沉积事件与地幔柱、地壳增生等重大地质事件相关;2稀土元素及Nd同位素示踪表明,Fe来源于海水与海底高温热液的混合溶液,其中高温热液与海水比例为1:1000;3 BIFs缺乏负Ce异常且富集重Fe同位素,暗示沉积时古海洋整体处于缺氧环境以避免Fe~(2+)发生氧化;4一些重要科学问题尚未解决,例如Si的主要来源、沉淀机制及条带成因等;5华北克拉通BIFs多形成于约2.54Ga,BIF类型、形成时代与富矿成因等问题有待深入研究。本文认为,加强国内外典型BIFs的对比研究并适当应用现代先进测试技术,有利于探索BIF沉积的精细过程及古老克拉通的早期演化。  相似文献   

11.
Banded iron formation(BIF)belongs to sedimentary rocks formed in Precambrian marine,which can directly reflect the redox state of the ancient oceans. Mineral composition and geochemistry of BIF can reveal the relative changes of oxygen contents of ancient atmosphere-ocean. The Neoarchean and Paleoproterozoic BIFs widely distributed in the North China Craton(NCC),are the ideal research objects for understanding the changes of the ancient ocean redox environment before and after the Paleoproterozoic Great Oxidation Event(GOE). Our previous studies indicated that the sedimentary facies of the Neoarchean BIF in the NCC are mainly magnetite-type oxide and silicate,with minor carbonate. The sedimentary facies of the Paleoproterozoic BIF are hematite- and magnetite-type oxide,silicate and carbonate,of which the hematite-oxide facies is unique to the Paleoproterozoic BIF. The above mineralogical features suggest that the redox conditions of the Neoarchean and Paleoproterozoic seawater are different. The rare earth element composition of the Neoarchean BIF in the NCC lacks a strong negative Ce anomaly,reflecting that the oxygen content of contemporary seawater is very low and the marine is anoxic. However,a small amount of BIFs in the NCC also present the negative Ce anomalies and a wide range of Th/U ratios,indicating that the local water of the Neoarchean ocean had relatively high oxygen content and was at a weak oxidation state. Compared with the Neoarchean BIFs,the Paleoproterozoic BIFs present a wide range of Ce anomalies(i.e.,no Ce anomalies,positive Ce anomalies and negative Ce anomalies). The hematite-bearing BIF has an obvious negative Ce anomalies,implying that the oxygen content and redox state of Paleoproterozoic seawater changed significantly. Combined with the ratios of Ni/Co,V/(V+Ni)and Th/U of the BIFs in the NCC,the Paleoproterozoic oceans exhibited a suboxidation to oxidation environment. Besides,Neoarchean BIF is strongly enriched in heavy iron isotopes and the non-mass fractionation of S isotope is obvious,whereas the Paleoproterozoic BIF is relatively enriched in light iron isotopes and the non-mass fractionation of S isotope is not obvious. It is summarized that the Neoarchean marine is anoxic,but the oxygen‘oasis' may exist locally,implying that photosynthetic oxygen production already existed in the Late Neoarchean. The ancient ocean presented a layered characteristics during and after the GOE,indicating that the shallow water was generally oxidized and the deep water was reduced.  相似文献   

12.
Banded iron formations (BIFs) within the Lvliang region of Shanxi Province, China, are hosted by sediments of the Yuanjiacun Formation, part of the Paleoproterozoic Lvliang Group. These BIFs are located in a zone where sedimentation changed from clastic to chemical deposition, indicating that these are Superior-type BIFs. Here, we present new major, trace, and rare earth element (REE) data, along with Fe, Si, and O isotope data for the BIFs in the Yuanjiacun within the Fe deposits at Yuanjiacun, Jianshan, and Hugushan. When compared with Post Archean Australian Shale (PAAS), these BIFs are dominated by iron oxides and quartz, contain low concentrations of Al2O3, TiO2, trace elements, and the REE, and are light rare earth element (LREE) depleted and heavy rare earth element (HREE) enriched. The BIFs also display positive La, Y, and Eu anomalies, high Y/Ho ratios, and contain 30Si depleted quartz, with high δ18O values that are similar to quartz within siliceous units formed during hydrothermal activity. These data indicate that the BIFs within the Yuanjiacun Formation were precipitated from submarine hydrothermal fluids, with only negligible detrital contribution. None of the BIF samples analyzed during this study have negative Ce anomalies, although a few have a positive Ce anomaly that may indicate that the BIFs within the Yuanjiacun Formation formed during the Great Oxidation Event (GOE) within a redox stratified ocean. The positive Ce anomalies associated with some of these BIFs are a consequence of oxidization and the formation of surficial manganese oxide that have preferentially adsorbed Ho, LREE, and Ce4 +; these deposits formed during reductive dissolution at the oxidation–reduction transition zone or in deeper-level reducing seawater. The loss of Ce, LREE, and Ho to seawater and the deposition of these elements with iron hydroxides caused the positive Ce anomalies observed in some of the BIF samples, although the limited oxidizing ability of surface seawater at this time meant that Y/Ho and LREE/HREE ratios were not substantially modified, unlike similar situations within stratified ocean water during the Late Paleoproterozoic. Magnetite and hematite within the BIFs in the study area contain heavy Fe isotopes (56Fe values of 0.24–1.27‰) resulting from the partial oxidation and precipitation of Fe2 + to Fe3 + in seawater. In addition, mass-independent fractionation of sulfur isotopes within pyrite indicates that these BIFs were deposited within an oxygen-deficient ocean associated with a similarly oxygen-deficient atmosphere, even though the BIFs within the Yuanjiacun Formation formed after initiation of the GOE.  相似文献   

13.
The primary mineral compositions of BIF are regarded as ferric oxyhydroxide or iron silicate nanoparticles (mainly greenalite and stilpnomelane ) whichcan transform into minerals like hematite, magnetite and siderite. On the basis of predominant iron minerals, three distinctive sedimentary facies are recognized in BIF: oxide facies, silicate facies and carbonate facies. Marked by the Great Oxidation Event (GOE, 2.4~2.2 Ga), sedimentary facies can be divided into two models: “anoxic and reducing” model and “stratified ocean” model. The ancient ocean was anoxic and reducing before GOE, and under this circumstance, BIF was distributed from the distal to proximal zones transforming from hematite facies through magnetite facies to carbonate facies, such as West Rand Group BIF (2.96~2.78 Ga) and Kuruman BIF (~2.46 Ga) in south Africa. However, the ancient ocean was a stratified ocean during and after GOE, which means that shallow seawater was oxidizing while deeper seawater was reducing, leading to an opposite sedimentary facies distribution compared to the former one: BIF was distributed from the distal to proximal zones transforming from carbonate facies through magnetite facies to hematite facies, such as Yuanjiacun BIF in China (~2.3 Ga) and Sokoman iron formation in Canada (~1.88 Ga). Overall, BIF is an unrepeatable formation in geological history, which can only form in specific sedimentary environment. The key point to speculate the paleo-ocean environment, namely the problems to be solved at the moment, is to identify and derive the primary mineral compositions, to make sure the genetic mechanism of sedimentary facies especially silicate facies, to restrict the sedimentary conditions and to study microbial activities contacting with BIF.  相似文献   

14.
The Shilu Fe–Co–Cu ore district is situated in the western Hainan Province of south China. This district consists of the upper Fe-rich layers and the lower Co–Cu ores, which are mainly hosted within the Neoproterozoic Shilu Group, a dominantly submarine siliciclastic and carbonate sedimentary succession that generally has been metamorphosed to greenschist facies. Three facies of metamorphosed BIFs, the oxide, the silicate–oxide and the sulfide–carbonate–silicate, have been identified within the Shilu Group. The oxide banded iron formation (BIF) facies (quartz itabirites or Fe-rich ores) consists of alternating hematite-rich and quartz-rich microbands. The silicate–oxide BIF facies (amphibolitic itabirites or Fe-poor ores) comprises alternating millimeter to tens of meter scale, magnetite–hematite-rich bands with calc-silicate-rich macro- to microbands. The sulfide–carbonate–silicate BIF facies (Co–Cu ores) contain alternating cobaltiferous pyrite, cobaltiferous pyrrhotite and chalcopyrite macrobands to microbands mainly with dolomite–calcite, but also with minor sericite–quartz bands. Blasto-oolitic, pelletoidal, colloidal, psammitic, and cryptocrystalline to microcrystalline textures, and blasto-bedding structures, which likely represent primary sedimentation, are often observed in the Shilu BIF facies.The Shilu BIFs and interbedded host rocks are generally characterized by relatively low but variable ∑ REE concentrations, LREE depletion and/or MREE enrichment relative to HREE, and no Ce, Gd and Eu anomalies to strongly positive Ce, Gd and Eu anomalies in the upward-convex PAAS-normalized REY patterns, except for both the banded or impure dolostones with nil Ce anomaly to negative Ce anomalies and negative La anomalies, and the minor sulfide–carbonate–silicate BIF facies with moderately negative Eu anomalies. They also contain relatively low but variable HFSE abundances as Zr, Nb, Hf, Th and Ti, and relatively high but variable abundances of Cu, Co, Ni, Pb, As, Mn and Ba. The consistently negative εNd(t) values range from − 4.8 to − 8.5, with a TDM age of ca. 2.0 Ga. In line with the covariations between Al2O3 and TiO2, Fe2O3 + FeO and SiO2, Mn and Fe, Zr and Y/Ho and REE, and Sc and LREE, the geochemical and Sm–Nd isotopic features suggest that the precursors to the Shilu BIFs formed from a source dominated by seafloor-derived, high- to low temperature, acidic and reducing hydrothermal fluids but with variable input of detrital components in a seawater environment. Moreover, the involved detrital materials were sourced dominantly from an unknown, Paleoproterozoic or older crust, with lesser involvement from the Paleo- to Mesoproterozoic Baoban Group underlying the Shilu Group.The Shilu BIFs of various facies are interpreted to have formed in a shallow marine, restricted or sheltered basin near the rifted continental margin most likely associated with the break-up of Rodinia as the result of mantle superplume activity in South China. The seafloor-derived, periodically upwelling metalliferous hydrothermal plume/vent fluids under anoxic but sulfidic to anoxic but Fe2 +-rich conditions were removed from the plume/vent and accumulated in the basin, and then variably mixed with terrigenous detrital components, which finally led to rhythmic deposition of the Shilu BIFs.  相似文献   

15.
The Neoproterozoic (593–532 Ma) Dahongliutan banded iron formation (BIF), located in the Tianshuihai terrane (Western Kunlun orogenic belt), is hosted in the Tianshuihai Group, a dominantly submarine siliciclastic and carbonate sedimentary succession that generally has been metamorphosed to greenschist facies. Iron oxide (hematite), carbonate (siderite, ankerite, dolomite and calcite) and silicate (muscovite) facies are all present within the iron-rich layers. There are three distinctive sedimentary facies BIFs, the oxide, silicate–carbonate–oxide and carbonate (being subdivided into ankerite and siderite facies BIFs) in the Dahongliutan BIF. They demonstrate lateral and vertical zonation from south to north and from bottom to top: the carbonate facies BIF through a majority of the oxide facies BIF into the silicate–carbonate–oxide facies BIF and a small proportion of the oxide facies BIF.The positive correlations between Al2O3 and TiO2, Sc, V, Cr, Rb, Cs, Th and ∑REE (total rare earth element) for various facies of BIFs indicate these chemical sediments incorporate terrigenous detrital components. Low contents of Al2O3 (<3 wt%), TiO2 (<0.15 wt%), ∑REE (5.06–39.6 ppm) and incompatible HFSEs (high field strength elements, e.g., Zr, Hf, Th and Sc) (<10 ppm), and high Fe/Ti ratios (254–4115) for a majority of the oxide and carbonate facies BIFs suggest a small clastic input (<20% clastic materials) admixtured with their original chemical precipitates. The higher abundances of Al2O3 (>3 wt%), TiO2, Zr, Th, Cs, Sc, Cr and ∑REE (31.2–62.9 ppm), and low Fe/Ti ratios (95.2–236) of the silicate–carbonate–oxide facies BIF are consistent with incorporation of higher amounts of clastic components (20%–40% clastic materials). The HREE (heavy rare earth element) enrichment pattern in PAAS-normalized REE diagrams exhibited by a majority of the oxide and carbonate facies BIFs shows a modern seawater REE signature overprinted by high-T (temperature) hydrothermal fluids marked by strong positive Eu anomalies (Eu/Eu1PAAS = 2.37–5.23). The low Eu/Sm ratios, small positive Eu anomaly (Eu/Eu1PAAS = 1.10–1.58) and slightly MREE (middle rare earth element) enrichment relative to HREE in the silicate–carbonate–oxide facies BIF and some oxide and carbonate facies BIFs indicate higher contributions from low-T hydrothermal sources. The absence of negative Ce anomalies and the high Fe3+/(Fe3+/Fe2+) ratios (0.98–1.00) for the oxide and silicate–carbonate–oxide BIFs do not support ocean anoxia. The δ13CV-PDB (−4.0‰ to −6.6‰) and δ18OV-PDB (−14.0‰ to −11.5‰) values for siderite and ankerite in the carbonate facies BIF are, on average, ∼6‰ and ∼5‰ lower than those (δ13CV-PDB = −0.8‰ to + 3.1‰ and δ18OV-PDB = −8.2‰ to −6.3‰) of Ca–Mg carbonates from the silicate–carbonate–oxide facies BIF. This feature, coupled with the negative correlations between FeO, Eu/Eu1PAAS and δ13CV-PDB, imply that a water column stratified with regard to the isotopic omposition of total dissolved CO2, with the deeper water, from which the carbonate facies BIF formed, depleted in δ13C that may have been derive from hydrothermal activity.Integration of petrographic, geochemical, and isotopic data indicates that the silicate–carbonate–oxide facies BIF and part of the oxide facies BIF precipitated in a near-shore, oxic and shallow water environment, whereas a majority of the oxide and carbonate facies BIFs deposited in anoxic but Fe2+-rich deeper waters, closer to submarine hydrothermal vents. High-T hydrothermal solutions, with infusions of some low-T hydrothermal fluids, brought Fe and Si onto a shallow marine, variably mixed with detrital components from seawaters and fresh waters carrying continental landmass and finally led to the alternating deposition of the Dahongliutan BIF during regression–transgression cycles.The Dahongliutan BIF is more akin to Superior-type rather than Algoma-type and Rapitan-type BIF, and constitutes an additional line of evidence for the widespread return of BIFs in the Cryogenian and Ediacaran reflecting the recurrence of anoxic ferruginous deep sea and anoxia/reoxygenation cycles in the Neoproterozoic. In combination with previous studies on other Fe deposits in the Tianshuihai terrane, we propose that a Fe2+-rich anoxic basin or deep sea probably existed from the Neoproterozoic to the Early Cambrian in this area.  相似文献   

16.
山西吕梁袁家村条带状铁建造沉积相与沉积环境分析   总被引：4，自引：1，他引：3  
山西吕梁作为华北克拉通上条带状铁建造(BIF)的重要产区之一,位于华北中央构造带中。袁家村BIF分布于吕梁岚县袁家村一带,极有可能是华北克拉通内最为典型的Superior型BIF。与华北克拉通其他大多数BIF相比,袁家村BIF具有明显的差异性,其中包括它的形成时代(2.3~2.1Ga)、铁建造类型和低级变质程度(低绿片岩相)等。因此,研究袁家村BIF具有特殊的研究意义,可为探讨大氧化事件之后古海洋氧化还原状态以及国内Superior型BIF的成因提供研究基础。袁家村BIF产于吕梁群袁家村组变沉积岩系的下部,前人根据上覆和下伏含火山岩地层的时代,推测袁家村组的形成时代为2.3~2.1Ga。BIF整体产状陡倾,沿北北东-北东东向呈L形带状分布。依据原生矿物的共生组合及产出特征,可将BIF沉积相划分为氧化物相(60%)、硅酸盐相(30%)和碳酸盐相(10%)。氧化物相是本区BIF最主要的沉积相,主要矿物为赤铁矿、磁铁矿和石英,从而可进一步划分为赤铁矿(24%)和磁铁矿(36%)亚相;硅酸盐相BIF以大量硅酸盐矿物出现为特征,散布于研究区,主要矿物组成除了石英和磁铁矿之外,还有铁黑硬绿泥石、绿泥石、铁滑石、镁铁闪石和阳起石等。在与碳酸盐相BIF构成过渡相的BIF中,还可发现大量的铁白云石。而碳酸盐相主要矿物为菱铁矿、铁白云石和石英等,主要发育于研究区的南部。依据含铁岩系构造格局特点复原获得了原始沉积相分布略图,沉积相主要呈南北向延展,自东向西显示出相变规律,西边为碳酸盐相,东边为氧化物相,其间是过渡的硅酸盐相。通过袁家村BIF的岩相学和含铁矿物化学成分的研究,可大致推测原始沉积的矿物组成为无定形硅胶、水铁矿、与铁蛇纹石和黑硬绿泥石组成类似的铁硅酸盐凝胶、富Al的粘土碎屑和含铁、镁、钙的碳酸盐软泥。这些沉积物在随后的成岩期和绿片岩相的区域变质作用下发生矿物之间的相互转变。BIF中主要含铁矿物的PO-P-Eh 2CO2和pH相关图解说明除了赤铁矿之外,其他矿物均是在较低氧逸度环境中形成的,且所有矿物共存的水体系为中性到弱碱性。袁家村BIF氧化物相中发育豆粒、内碎屑结构和板状交错层理等原始沉积构造,指示氧化相部分是在相对高能的浅水环境下沉积的。但BIF大部分应该形成于浪基面以下(200m)较为深水的环境中,沉淀可能同时发生于上部氧化和下部还原的水体之中,由于还原弱酸性的深部富铁海水在海侵的过程中上升到浅部相对氧化和弱碱性的浅水环境中,因为Eh、pH及氧逸度等物化条件的骤然变化,最终导致铁质的沉淀和沉积相自上而下的变化。  相似文献   

17.
Banded iron-formations (BIFs) form an important part of the Archaean to Proterozoic greenstone belts in the Southern Cameroon. In this study, major, trace and REE chemistry of the banded iron-formation are utilized to explore the source of metals and to constraint the origin and depositional environment of these BIFs. The studied BIF belongs to the oxide facies iron formations composed mainly of iron oxide (mainly magnetite) mesobands alternating with quartz mesobands. The mineralogy of the BIF sample consists of magnetite and quartz with lesser amount of secondary martite, goethite and trace of gibbsite and smectite. The major element chemistry of these iron-formations is remarkably simple with the main constituents being SiO2 and Fe2O3 which constitute 95.6–99.5% of the bulk rock. Low Al2O3, TiO2, and HFSE concentrations show that they are relatively detritus-free chemical sediments. The Pearson’s correlation matrix of major element reveals that there is a strong positive correlation (r = 0.99) of Al with Ti and no to weak negative correlation of Ti with Mn, Ca and weak positive correlation of Si with Ca, suggesting the null to very minor contribution of detrital material to chemical sediment. The trace elements with minor enrichments are transition metals such as Zn, Cr, Sr, V and Pb. This is an indicator of direct volcanogenic hydrothermal input in chemical precipitates. The studied BIF have a low ΣREE content, ranging between 0.41 and 3.22 ppm with an average of 0.87 ppm, similar to that of pure chemical sediments. The shale-normalized patterns show depletion in light REE, slightly enrichment in heavy REE and exhibit weak positive europium anomalies. These geochemical characteristics indicate that the source of Fe and Si was the result of deep ocean hydrothermal activity admixed with sea water. The absence of a large positive Eu anomaly in the studied BIF indicates an important role of low-temperature hydrothermal solutions. The chondrite-normalized REE patterns are characterized by LREE-enriched (Mean LaCN/YbCN = 8.01) and HREE depletion (Mean TbCN/YbCN = 1.61) patterns and show positive Ce anomalies. With the exception of one sample (LBR133), all of the BIF samples analyzed during this study have positive Ce anomalies on both chondrite- and PASS-normalized plots. This may indicate that the BIFs within the Elom area were formed within a redox stratified ocean. The positive Ce anomalies in the studied samples likely suggest that the basin in which Fe formations were deposited was reducing with respect to Ce, probably in the suboxic or anoxic seawaters.  相似文献   

18.
The ocean and atmosphere were largely anoxic in the early Precambrian, resulting in an Fe cycle that was dramatically different than today’s. Extremely Fe-rich sedimentary deposits—i.e., Fe formations—are the most conspicuous manifestation of this distinct Fe cycle. Rare Earth Element (REE) systematics have long been used as a tool to understand the origin of Fe formations and the corresponding chemistry of the ancient ocean. However, many earlier REE studies of Fe formations have drawn ambiguous conclusions, partially due to analytical limitations and sampling from severely altered units. Here, we present new chemical analyses of Fe formation samples from 18 units, ranging in age from ca. 3.0 to 1.8 billion years old (Ga), which allow a reevaluation of the depositional mechanisms and significance of Precambrian Fe formations. There are several temporal trends in our REE and Y dataset that reflect shifts in marine redox conditions. In general, Archean Fe formations do not display significant shale-normalized negative Ce anomalies, and only Fe formations younger than 1.9 Ga display prominent positive Ce anomalies. Low Y/Ho ratios and high shale-normalized light to heavy REE (LREE/HREE) ratios are also present in ca. 1.9 Ga and younger Fe formations but are essentially absent in their Archean counterparts. These marked differences in Paleoproterozoic versus Archean REE + Y patterns can be explained in terms of varying REE cycling in the water column.Similar to modern redox-stratified basins, the REE + Y patterns in late Paleoproterozoic Fe formations record evidence of a shuttle of metal and Ce oxides across the redoxcline from oxic shallow seawater to deeper anoxic waters. Oxide dissolution—mainly of Mn oxides—in an anoxic water column lowers the dissolved Y/Ho ratio, raises the light to heavy REE ratio, and increases the concentration of Ce relative to the neighboring REE (La and Pr). Fe oxides precipitating at or near the chemocline will capture these REE anomalies and thus evidence for this oxide shuttle. In contrast, Archean Fe formations do not display REE + Y patterns indicative of an oxide shuttle, which implies an absence of a distinct Mn redoxcline prior to the rise of atmospheric oxygen in the early Paleoproterozoic. As further evidence for reducing conditions in shallow-water environments of the Archean ocean, REE data for carbonates deposited on shallow-water Archean carbonate platforms that stratigraphically underlie Fe formations also lack negative Ce anomalies. These results question classical models for deposition of Archean Fe formations that invoke oxidation by free oxygen at or above a redoxcline. In contrast, we add to growing evidence that metabolic Fe oxidation is a more likely oxidative mechanism for these Fe formations, implying that the Fe distribution in Archean oceans could have been controlled by microbial Fe uptake rather than the oxidative potential of shallow-marine environments.  相似文献   

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