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1.
华北克拉通宣龙式铁矿是中国北方最重要的沉积型铁矿类型,形成于元古宙中期(1800～800 Ma)。河北大岭堡地区鲕状赤铁矿石发育大量碎屑锆石,对这些碎屑锆石及庞家堡地区侵入串岭沟组的花岗岩脉进行了LA-ICP-MS锆石U-Pb年龄分析。鲕状赤铁矿石碎屑锆石获得了2组主要的峰值年龄,分别为1873 Ma和2530 Ma,记录了华北克拉通约1850 Ma和约2500 Ma两次构造热事件,结合前人研究,表明其与围岩具有基本一致的碎屑锆石源区,与北京十三陵地区串岭沟组源区略有差异,推测宣龙式铁矿可能为华北克拉通响应Columbia超大陆裂解的产物。花岗岩脉锆石U-Pb年龄为202.3±1.4 Ma(n=27,MSWD=0.96),表明区内发育印支期岩浆侵入活动,暗示磁铁矿石可能并非前人认为的燕山期的产物,其成因还需要进一步深入研究。 相似文献
2.
华北古陆北缘中元古代串岭沟组是整个华北古陆沉积环境从以滨浅海陆源碎屑岩为主体的沉积体系向以碳酸盐岩为主体的陆表海沉积体系的转折点。串岭沟组发育了大量的软沉积物变形构造:水塑性变形构造,液化构造,流化构造,均是与地震驱动相适配的变形构造,它们赋存于特定的层内,并在空间上呈规律变化,为典型震积岩。地震中心主要为蓟县—兴隆区域,东西向展布的断裂控制相带分布,南北向控制沉积类型。软沉积变形构造可以作为重要沉积标志来限定大陆裂解引发的伸展构造运动时间。因此综合研究串岭沟组发育的软沉积物变形构造类型、特征、层位和空间位置对于认识原型盆地的断裂构造活动特征具有重要意义。 相似文献
3.
术语本文所采用的"铁岩(ironstone)"这一术语是指含铁15%以上的化学沉积岩."含铁建造(iron formation)"是指主要由铁质岩组成的、最上层和最底层均为铁质岩的可在地质图上标示出的岩石单元.化学沉积岩限于其体积占50%以上的无机和(或)有机化学沉淀物,这些沉淀物来自地表水体和(或)那些沉淀物的成岩交代产物.铁质岩可由主要的富铁和贫铁矿物来修饰,如:条带状菱铁矿-燧石铁岩和鲕状赤铁矿-石英铁质岩,其中"燧石"包括重结晶燧石, 相似文献
4.
古/中元古代界线:1.8Ga 总被引：1，自引：0，他引：1
年代地层表是我们描述地球历史演化的时间框架,也隐含着我们对地球演化过程的基本认识,承载着一系列核心科学问题。现有的国际前寒武纪地质年表存在不少问题,还没有被普遍接受的新方案。在现行国际地层年表(IUGS 1989—2004)中,20～18亿年被称为造山纪,18～16亿年被称为固结纪,16～14亿年被称为盖层纪,也即:造山运动结束到盖层发育之间的过渡时期为固结纪,而将盖层的广泛发育作为中元古代的开始。在我国,由于"吕梁运动"是华北克拉通结晶基底最终形成的标志性构造-热事件,此后发育以长城系-蓟县系-青白口系为代表的地台型沉积盖层,因此我国地质界一直以长城系的底界代表中元古代的开始,并根据"吕梁运动"结束时间,将古/中元古代的时间界线置于18亿年。2012年国际地层委员会提出了一份全新的全球前寒武纪地质年代表划分建议方案,其中2060～1780Ma阶段被称为古元古代哥伦比亚纪,主要以Columbia超大陆的聚合为特征;而之后17. 8～8. 5亿年的近10亿年间,则被定义为罗迪尼亚纪,代表了从Columbia超大陆裂解到Rodinia超大陆聚合的漫长阶段。即这一新建议方案的古/中元古代界线为17. 8亿年,与我国学者长期以来所坚持的古/中元古代分界是基本一致的。在华北克拉通,"吕梁运动"的结束时间在～18亿年,此后整体处于多期裂解的陆内伸展环境,长城系-蓟县系基本上属于连续的陆表海沉积。近年来的研究表明,燕山地区长城系的底界年龄约为17亿年,长城系与蓟县系的分界则为16亿年。如果一味按照现行国际地层划分方案,其古/中元古代的界线(16亿年)将对应于长城系-蓟县系的分界,华北克拉通这套盖层型沉积将被人为分割为两部分,这显然是很不合理的。值得注意的是,在华北克拉通中部吕梁地区发育的小两岭组火山岩和在其南部地区发育的、也是世界范围内同时期最大规模的火山活动——熊耳群火山-沉积岩系,是华北克拉通结晶基底之上最早发育的盖层沉积,其起始形成时间约为18亿年。这与新建议的古/中元古代分界非常接近。从全球地质演化来看,从18到16亿年,造山作用结束,Columbia超大陆开始裂解,岩浆作用方式和岩浆岩组合类型及其地球化学特征发生明显改变,如斜长岩、环斑花岗岩在世界主要克拉通均有发育。与此同时,稳定沉积盖层开始广泛发育,条带状铁建造(BIF)消失,代之以鲕状或粒状矿物集合体组成的浅海富铁沉积,另外海相硫化物沉积、有核原生生物等也首次出现。所有这些都标志着,在此前后,地球岩石圈、水圈和大气圈均发生了重大转折,生物圈也进入新的演化阶段,标志着地球进入"中年期"演化阶段。虽然地球演化发生重要转折的根本原因和细节还有待进一步深入探讨,但这一转变的起点或最重要的时间点,无疑就在18亿年前后,因此,本文认为,应将其作为全球古/中元古代的时间界线。 相似文献
5.
四川龙门山石炭纪鲕状赤铁矿及其古环境意义 总被引：2，自引：0，他引：2
四川龙门山地区石炭系总长沟组底部发育一层鲕状赤铁矿,其内的铁质鲕粒由各种不同类型的核心和同心纹层构成.鲕粒核心主要有3种:由纯赤铁矿颗粒构成的鲕粒核心、由破碎鲕构成的鲕粒核心、由夹有石英颗粒的赤铁矿构成的鲕粒核心.鲕粒同心纹层由赤铁矿及高岭石组成,存在着明显的颜色、成分差别.纹层颜色有暗黑色、铁红色及亮白色,与之相对应,Fe2O3含量分别为39.472%～58.200%、12.988%～24.275% 及3.617%～9.230%.三者交替出现,构成鲕粒圈层.通过对鲕状赤铁矿的成分、结构及其他相关特征分析,推测其形成于后砂坝环境.汤靶沟阶晚期存在一次规模较大的古暴露及古岩溶事件,其间风化剥蚀形成的铁铝质风化壳为鲕状赤铁矿提供了物质基础. 相似文献
6.
扬子陆块古-中元古代地质演化与Columbia超大陆重建 总被引：3，自引：0，他引：3
扬子陆块在古-中元古代时期经历了较为强烈的岩浆-变质-沉积-成矿等地质事件,这些事件是理解该陆块陆壳演化和成矿效应内在联系及动力学的关键,也是探讨该陆块在Columbia超大陆中古地理重建的前提。本文以古-中元古代地质单元出露较为完整的扬子西南缘为重点研究对象,在总结已有资料的基础上,对扬子陆块古-中元古代时期地质事件进行剖析和讨论,明确了扬子陆块西南缘在古-中元古代时期经历了由Columbia超大陆初始裂解引起的陆内裂谷相关的沉积作用,岩浆侵位及矿产富集等地质过程。通过与全球陆块进行对比,发现相似的裂谷的相关沉积-岩浆-成矿事件在劳伦大陆西北部、澳大利亚北部及Siberian克拉通都有体现。本文认为扬子陆块在2. 4～2. 3 Ga通过增生拼贴到劳伦大陆Rae克拉通。在共同经历过Columbia超大陆聚合的峰期变质作用(2. 03～1. 81 Ga)之后,超大陆开始逐步裂解并形成大陆内部裂谷,最终在古元古代后期(～1. 66～1. 60 Ga)扬子陆块和Columbia超大陆主体分离。 相似文献
7.
华北地块北缘中新元古界沉积构造演化 总被引：2，自引：0，他引：2
根据文献资料及对研究区8 条实测剖面资料的综合分析结果表明,Columbia 超级大陆的裂解导致华北陆块北缘大陆裂
谷盆地的形成。随着大陆进一步伸展和洋壳的形成,华北地块北缘逐渐发展为被动大陆边缘。在1400 Ma 左右,即铁岭组
沉积后,华北地块北缘转变为活动大陆边缘。早期洋壳向华北地块低角度的俯冲造成弧后地区发生挤压（芹峪上升）,导致
铁岭组抬升和剥蚀,而后期洋壳高角度的俯冲又造成弧后区域发生强烈的伸展和断陷,沉积了下马岭组,并伴随辉绿岩的
侵入。华北地块与相邻地体之间的碰撞导致下马岭组的抬升（或蔚县抬升）以及碰撞花岗岩的形成,挤压构造发生的时间
对应于Rodinia 超级大陆的形成期。新元古代沉积是Rodinia 超级大陆裂解的结果。龙山组石英砂岩和海绿石砂岩是Rodinia
超级大陆裂解后的最早期沉积,记录了海侵初期的超覆过程。 相似文献
8.
河北宣化姜家寨铁矿床串岭沟组底部碎屑锆石LA-MC-ICP-MS U-Pb年龄及其地质意义 总被引：1，自引：0，他引：1
宣龙式铁矿是我国北方最重要的沉积型铁矿床。华北克拉通长城系串岭沟组底部砂页岩是宣龙式铁矿床的赋存层位,对该地层的年代学研究有助于深入完善长城系地层年代框架、认识区域成岩成矿过程并反演克拉通的演化历史。本文对河北宣化姜家寨铁矿床串岭沟组底部铁矿体顶板砂页岩中碎屑锆石进行了LA-MC-ICP-MS锆石U-Pb年代学研究,获得了三组主要的峰值年龄,加权平均年龄分别为1774.1±7.9Ma、1849.0±7.8Ma和2453.0±7.8Ma。揭示出华北克拉通中部带经历了三期较为重要的地质构造、岩浆作用和变质作用事件。在串岭沟组下部砂岩中我们获得了4颗较年轻的岩浆碎屑锆石,年龄为1657.4～1694.4Ma,代表了串岭沟组底部形成的时间下限,制约了姜家寨宣龙式铁矿床的形成时代不早于1657Ma。对比研究得出,1774.1Ma的峰值年龄数据与华北克拉通内18亿年后广泛发育的基性岩墙群的形成时代一致,代表了华北克拉通的在拼合后的抬升事件时间。在1.8～1.6Ga华北克拉通拉张期间形成了大庙式等钒钛磁铁矿-磷灰石铁矿(1720Ma左右)。基于时间和区域的一致性,我们推断,遭受抬升剥蚀的富铁基性岩墙群不仅是串岭沟组的物源之一,极有可能也是宣龙式铁矿床中铁质的主要物源之一。 相似文献
9.
华北地块北缘中元古代沉积层序中出现多个沉积间断和不整合面。此次研究对其中5个不整合面进行了野外考察,分析了它们的地质特征和空间分布,认为它们与不同的地质过程有关。大红峪组底部不整合面是早期裂谷盆地演化为被动大陆边缘的记录,属于裂解不整合面性质;高于庄组底部和杨庄组底部的不整合面是海平面变化造成被动大陆边缘内部高地沉积间断和海岸超覆沉积的结果;铁岭组顶部不整合面可能与弧后环境发生挤压抬升有关;下马岭组沉积后的抬升可能是相邻大陆地块碰撞作用的产物。华北地块北缘中元古代层序中不整合面的形成与Columbia和Rodinia超大陆的形成和裂解过程密切相关。 相似文献
10.
阿尔金南缘冰沟南组火山岩锆石U-Pb年龄及其前寒武纪构造演化意义 总被引：2，自引：0，他引：2
在阿尔金南缘玉苏普阿勒克塔格北侧出露的冰沟组火山岩为一套与滨浅海相细粒陆源碎屑岩和碳酸盐岩共生的中基性火山岩,主要岩石类型有蚀变熔结凝灰岩、安山质凝灰熔岩和强蚀变玄武安山岩。该火山岩作为Columbia超大陆裂解阶段的物质记录,对其年代学和产出的地质构造背景的探讨,有助于探讨阿尔金南缘前寒武纪构造演化过程。LA-ICP-MS锆石U-Pb同位素年代学研究结果显示,强蚀变玄武安山岩样品具有两组锆石年龄,第一组年龄分别为944. 7±6. 4 Ma(n=8,MSWD=0. 057)和951. 2±6. 3 Ma(n=12,MSWD=0. 042),测点位于锆石震荡环带上,显示了典型的岩浆锆石的特征,代表火山岩的形成时代,属青白口纪早期;第二组年龄介于446±5～458±4Ma,与区域高压—超高压岩石的原岩的退变质时代(～450 Ma)一致,可能代表受该期陆壳深俯冲/折返事件改造。另外,228±6 Ma、229±4 Ma和130±1 Ma的年龄信息,可能代表火山岩还遭受了晚三叠世和早白垩世构造热事件改造。构造背景分析表明,该火山岩形成于板内裂谷环境,对应于Columbia超大陆裂解阶段的晚期,之后索尔库里群沉积盆地由拉张转换到汇聚的构造背景,转为Rodinia超大陆汇聚阶段。结合区域地质特征和该火山岩年龄信息,进一步制约了阿尔金南缘Columbia超大陆裂解和Rodinia超大陆汇聚的时限分别为945～1602 Ma和871～945 Ma。 相似文献
11.
冀西北铁岩的结构和构造类似于碳酸盐岩,因此,借鉴于现代碳酸盐相模式与沉积环境,有可能恢复古代铁岩的沉积环境。本文对铁岩分类及其沉积环境进行了探讨。 相似文献
12.
The microbial origin of Precambrian iron formations is debated due to the lack of direct fossil evidence. In order to reveal the genesis of ironstones under low-oxygen levels, integrative studies of sedimentology, petrography, mineralogy, and geochemistry were conducted on the intertidal to shallow subtidal ooidal and stromatolitic ironstones from the terminal Paleoproterozoic Chuanlinggou Formation (ca. 1.65–1.64 Ga) of North China, using microscopy, SEM, EDS, ICP-OES, ICP-MS and MC-ICP-MS techniques. Mineralogical study shows that the Fe-rich mineral is predominantly hematite that resulted from dehydration of amorphous Fe-oxyhydroxide during diagenesis. Petrographic observation indicates that the iron was oxidized and precipitated from seawater rather than sourced from terrestrial detritus. Basinward increases of the ironstone abundance, Eu anomalies (from 1.39 to 1.56) and δ56Fe values of the ironstones (from +0.5‰ to +1.0‰) suggest that the iron was mainly sourced from seafloor hydrothermal fluids, and partially oxidized and precipitated in shallow subtidal to intertidal environments. The common existence of Fe-oxide coated sheaths, spiral stalks, residual extracellular polymer substances (EPS) and other biogenic fabrics indicates that microaerophilic iron-oxidizing bacteria (FeOB) may have played an important role in precipitating the Chuanlinggou ironstones. The extremely low oxygen concentrations implied by the proliferation of microaerophilic FeOB in the shallow waters, the weak positive Ce anomalies (0.94–1.12) and low Mn concentrations in the ironstones are broadly consistent with the previous result of a Cr isotope study. Thus the establishment of a microaerophilic FeOB genetic model for the widespread Chuanlinggou ironstones in North China provides new insight into the origin of Precambrian iron formations and the redox evolution of ocean-atmosphere systems during the “Boring Billion”. 相似文献
13.
The Campanian-Maastrichtian Agbaja Ironstone Formation of the Nupe basin, Nigeria, forms a major part of the about 2 billion
tons of iron ore reserves of the Middle Niger Embayment. The ironstone deposits were previously reported to be similar to
the Minette-type ironstones because of their depositional patterns, composition and inferred origin. Four rock-types are recognized
within the Agbaja Ironstone Formation: ooidal pack-ironstone, pisoidal pack-ironstone, mud-ironstone and bog iron ore. In
the ironstones, kaolinite of both the groundmass and the ooids/pisoids is of lateritic origin, whereas the associated quartz,
mica and heavy minerals are of detrital origin. Ooids and pisoids were formed by mechanical accretion of platy kaolinite crystals
by rolling on the sea floor in a near-shore environment, and were subsequently transported and deposited together with a fine-grained
kaolinitic groundmass. Pyrite (mainly framboidal) and siderite (both exclusively occurring as pseudomorphs of goethite and/or
hematite) are diagenetic whereas goethite is post-diagenetic in origin, resulting from the ferruginization of the kaolinitic
precursor. Crandallite-gorxeicite-goyazite, bolivarite and boehmite are also post-diagenetic in origin. Hematite was formed
from the dehydration of goethite, whereas gibbsite (restricted to the upper part of the deposit) is of recent and in situ lateritic origin. The presence of newly formed authigenic pyrite and siderite (now replaced by hematite and goethite) are
indicators of a reducing environment during diagenesis. The absence of diagenetic chamositic clay minerals, evidently caused
by a low Mg concentration, suggests that fully marine conditions were not established during sedimentation. This is supported
by the lack of fossils, brecciated shell materials and bioturbation features in the deposit. Reworking and redeposition of
the primary constituents are inferred from broken pisoids, nuclei of pisoidal/ooidal fragments in pisoids and high iron concentrations
present in the pisoids and ooids compared to that of the groundmass. These observations indicate that the Agbaja ironstone
deposits of the Lokoja study area exhibit some environmental and mineralogical characteristics that are markedly different
from other known deposits of Minette-type, where primary chamositic clay minerals generally form the protore for the ironstones.
The recognition of kaolinite as the precursor constituent and the occurrence of similar deposits of the same age (Late Cretaceous)
in Nigeria, Sudan and Egypt have implications for the paleoenvironmental interpretations of Phanerozoic ironstone deposits.
Received: 16 February 1998 / Accepted: 8 July 1998 相似文献
14.
通过野外考察和前人资料分析发现,柴达木盆地北缘全吉群红藻山组之上存在一个区域性的不整合,其中发育铁质风化壳、沉积间断、地层的缺失、侵蚀作用特征。不整合面上、下地层存在古生物组合、沉积环境的突变。界面以下为碳酸盐台地沉积,界面之上代表了冰水沉积到海相潮上—潮间带沉积,上覆黑土坡组—红铁沟组—皱节山组,依次沿着不整合面向上超覆,反映了不整合发育之后的又一次海进过程。该不整合界面可能代表了新元古代晚期该地区裂谷盆地的伸展阶段的构造响应。该区域性不整合的发现,改变了以往传统的对全吉群内部整合关系的认识,说明全吉群需要解体。该不整合可能发生于成冰纪—埃迪卡拉纪。该时期的不整合面上、下生物面貌和沉积环境的变化或突变,可能与新元古代晚期罗迪尼亚超大陆的裂解和全球雪球事件相关。 相似文献
15.
The present study aims to shed light on the mechanism of formation of the Oligo-Miocene oolitic ironstones of Haddat Ash Sham area, Saudi Arabia. These ironstones are enclosed within the middle part of the Oligo-Miocene siliciclastic succession of the western part of the Arabian Shield, western Saudi Arabia. The ironstone beds were formed during marine incursion and creation of short-lived starved time periods of high organic matter activities, ferrous iron, and low clastic input. The depositional and diagenetic processes involved in the formation of Haddat Ash Sham ironstones are summarized here as follows: (1) the deposition of detrital components (i.e., amorphous iron-bearing clays admixed with silt and sand-sized quartz grains) and their distribution by the waves and current actions in areas of different water depths (bars and inter-bar areas); (2) the deposition of the iron-bearing clays in slightly reducing transgressive conditions (dysaerobic zone) led to the authigenesis of green marine chamositic clays of variable mineralogical and chemical compositions according to the predominated depositional environments; and (3) in the upper parts of the depositional cycles, the iron-bearing clays become admixed with detrital quartz grains which resulted in the formation of silty and sandy ironstones of low iron content. The diagenetic processes led to the oxidation of the green chamositic clays and formation of amorphous Fe-oxyhydroxides, ferrihydrites, goethite, and hematite. These iron mineral phases are related to each other and show progressive steps of transformation during the diagenetic processes. The iron ooids represent in situ formed irregular domains formed during the diagenetic crystallization and dehydration of the amorphous iron oxyhdroxides resulted from the diagenetic oxidation of green chamositic clays. This is supported by the absence of detrital cores of the iron ooids, the gradational contact between the iron ooids and the enclosing matrix and also by the presence of many ooids of unclear and ill-defined internal structure. 相似文献
16.
Lower and middle Eocene ironstone sequences of the Naqb and Qazzun formations from the north‐east Bahariya Depression, Western Desert, Egypt, represent a proxy for early Palaeogene climate and sea‐level changes. These sequences represent the only Palaeogene economic ooidal ironstone record of the Southern Tethys. These ironstone sequences rest unconformably on three structurally controlled Cenomanian palaeohighs (for example, the Gedida, Harra and Ghorabi mines) and formed on the inner ramp of a carbonate platform. These palaeohighs were exposed and subjected to subaerial lateritic weathering from the Cenomanian to early Eocene. The lower and middle Eocene ironstone sequences consist of quiet water ironstone facies overlain by higher energy ironstone facies. The distribution of low‐energy ironstone facies is controlled by depositional relief. These deposits consist of lagoonal, burrow‐mottled mud‐ironstone and laterally equivalent tidal flat, stromatolitic ironstones. The agitated water ironstone facies consist of shallow subtidal–intertidal nummulitic–ooidal–oncoidal and back‐barrier storm‐generated fossiliferous ironstones. The formation of these marginal marine sequences was associated with major marine transgressive–regressive megacycles that separated by subaerial exposure and lateritic weathering. The formation of lateritic palaeosols with their characteristic dissolution and reprecipitation features, such as colloform texture and alveolar voids, implies periods of humid and warm climate followed major marine regressions. The formation of the lower to middle Eocene ironstone succession and the associated lateritic palaeosols can be linked to the early Palaeogene global warming and eustatic sea‐level changes. The reworking of the middle Eocene palaeosol and the deposition of the upper Eocene phosphate‐rich glauconitic sandstones of the overlying Hamra Formation may record the initial stages of the palaeoclimatic transition from greenhouse to icehouse conditions. 相似文献
17.
K. A. Savko A. V. Samsonov V. M. Kholin N. S. Bazikov 《Stratigraphy and Geological Correlation》2017,25(2):123-145
The results of correlation between geological events in the period of 2.8?2.0 Ga provide grounds to assume that the Sarmatia lithospheric megablock definable in the southern part of the East European Craton belonged to the ancient Vaalbara supercontinent consisting of the Pilbara and Kaapvaal cratons. In the period of 2.8?2.6 Ga, all of them represented fragments of the continental crust consolidated at approximately 2.8 Ga and subjected to continental rifting, which was accompanied by intense basite volcanism. In the period of 2.50?2.45 Ga, these three cratons were characterized by similar tectonic settings and accumulation of banded iron formations. Precisely these banded iron formations of the largest Transvaal, Hamersley, Kursk, and Kremenchug?Krivoi Rog iron ore basins accumulated in the period of 2.50?2.45 Ga in a single oceanic basin serve as a basis for adequate paleotectonic reconstructions of the Vaalbara supercontinent. In the period of 2.45?2.20 Ga, all three cratons were subjected to a long-lasting break in sedimentation followed by activation of continental rifting with terrigenous sediment deposition, which terminated with basite volcanism ca. 2.2 Ga. These events gave start to the Vaalbara breakup, which represented a multistage process with alternating divergence and convergence phases of supercontinent fragments until the Kaapvaal and Zimbabwe, Pilbara and Yilgarn, and Sarmatia and Volgo-Uralia cratons, respectively, became eventually united. 相似文献
18.
吴功成丁嘉鑫王忠梅王庆飞韩春明 《地质科学》2017,(2):545-570
塔里木克拉通的研究可以有助于理解中亚造山带元古代构造框架和Rodinia 超大陆的形成过程。位于塔里木克拉通北缘的库鲁克塔格地区广泛分布有基性岩-超基性岩-碳酸岩,并且伴随有铜镍、铁磷和蛭石矿床,其典型矿床为兴地铜镍矿床、且干布拉克蛭石矿床、大西沟和卡乌留克铁磷矿床。SIMS 锆石U-Pb 年代学研究表明,大西沟铁磷矿床形成于古元古代(2 452±10 Ma),其它铜镍、铁磷和蛭石矿床(兴地、卡乌留克和且干布拉克)形成于新元古代(812～707 Ma)。古元古代成矿作用与哥伦比亚超大陆的汇聚碰撞有关,而新元古代成矿作用形成于Rodinia超大陆裂解导致塔里木地幔柱活动有关的成矿动力学背景。 相似文献
19.
以侵入于宜昌崆岭杂岩中的圈椅埫花岗岩体为研究对象,系统研究了其年代学和地球化学特征,并据此对岩石成因和扬子陆核古元古代构造演化过程进行探讨。锆石LA-ICP-MS U-Pb定年结果表明,圈椅埫花岗岩形成年龄为(1 822±44) Ma,说明其为扬子陆核古元古代岩浆活动产物。地球化学研究表明,该花岗岩体富Si,贫Al、Mg,微量元素组成上富集Rb、Th,具有Eu、Ba、Sr和高场强元素的负异常。岩石具有高Ga/Al比值和(Zr+Nb+Ce+Y)含量,其锆石饱和温度计算值较高(＞862 ℃),综合地质地球化学特征表明该岩体应属铝质A型花岗岩。岩体的εNd(t)值在-12.4~-10.3之间变化,对应两阶段Nd同位素模式年龄值为3.3~3.2 Ga,暗示岩体可能形成于扬子陆核深部古老的长英质地壳物质在后碰撞伸展构造背景低压、高温条件下部分熔融。结合前人已有的研究成果,认为其可能与区域上2.0~1.9 Ga板块碰撞造山后发生的由碰撞挤压向伸展作用的构造转换作用有关。扬子陆核古元古代构造岩浆事件与全球范围内2.1~1.8 Ga的与Columbia超大陆演化有关的碰撞造山-裂解事件时间吻合,表明扬子陆核可能是Columbia超大陆的重要组成部分之一。 相似文献
20.
There is considerable debate about the mode and age of formation of large (up to ∼200 m long) hematite and goethite ironstone bodies within the 3.2 to 3.5 Ga Barberton greenstone belt. We examined oxygen and hydrogen isotopes and Rare Earth Element (REE) concentrations of goethite and hematite components of the ironstones to determine whether these deposits reflect formation from sea-floor vents in the Archean ocean or from recent surface and shallow subsurface spring systems. Goethite δ18O values range from −0.7 to +1.0‰ and δD from −125 to −146‰, which is consistent with formation from modern meteoric waters at 20 to 25 °C. Hematite δ18O values range from −0.7 to −2.0‰, which is consistent with formation at low to moderate temperatures (40-55 °C) from modern meteoric water. REE in the goethite and hematite are derived from the weathering of local sideritic ironstones, silicified ultramafic rocks, sideritic black cherts, and local felsic volcanic rocks, falling along a mixing line between the Eu/Eu* and shale-normalized HREEAvg/LREEAvg values for the associated silicified ultramafic rocks and felsic volcanic rocks. Contrasting positive Ce/Ce* of 1.3 to 3.5 in hematite and negative Ce/Ce* of 0.2 to 0.9 in goethite provides evidence of oxidative scavenging of Ce on hematite surfaces during mineral precipitation. These isotopic and REE data, taken together, suggest that hematite and goethite ironstone pods formed from relatively recent meteoric waters in shallow springs and/or subsurface warm springs. 相似文献