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1.
北祁连山西段北大河岩群碎屑锆石SHRIMPU-Pb年代学研究   总被引:10,自引:0,他引:10       下载免费PDF全文
白云母石英片岩中碎屑锆石的成因矿物学研究及SHRIMP U-Pb同位素年龄测定表明,北大河岩群变质沉积岩的蚀源区存在1400~3035Ma构造热事件,除了年龄为约2457Ma的变质事件外,主要表现为岩浆事件,其中以1800Ma左右的岩浆事件最为强烈,1400~1700Ma事件次之,中-新太古代的岩浆事件也比较明显;北大河岩群可能遭受过863Ma左右的变质作用影响,其沉积岩的沉积作用发生于1400Ma(最小的碎屑锆石年龄)至863Ma(变质年龄)之间,而不是前人所认为的古元古代;根据46%的碎屑锆石为1800Ma左右这一事实,最起码可以肯定北大河岩群的沉积年龄新于1800Ma;北大河岩群的蚀源区存在中-新太古代的古老陆壳。同时,北大河岩群碎屑锆石年龄资料也为朱龙关群蛇绿岩的形成时代提供了有力的制约:朱龙关群蛇绿岩很可能形成于早古生代,而非中元古代早期。  相似文献   

2.
熬油沟蛇绿岩位于甘肃省肃南县祁青乡,构造上位于北祁连山西段加里东造山带上。该蛇绿岩由原岩为方辉橄榄岩的蛇纹岩、堆积的辉长岩和闪长岩、枕状熔岩、块状熔岩、辉绿岩岩墙和硅质岩组成。在辉绿岩中选取了其中的3颗单颗粒锆石进行蒸发法年龄测定,结果分别为1840±2 Ma,1783±2 Ma and 1784±2 Ma,表明该蛇绿岩属中元古代早期。辉绿岩和玄武质岩石具有轻稀土富集和相对高的  相似文献   

3.
大岔大坂MOR型和SSZ型蛇绿岩对北祁连洋演化的制约   总被引:9,自引:0,他引:9  
北祁连大岔大坂剖面保存了MOR型和SSZ型两类蛇绿岩,记录了两种构造环境的岩浆活动。剖面南段的蛇绿岩由蛇纹石化的超基性岩和辉长岩组成,其中辉长岩具有N-MORB地球化学特征,代表了扩张形成的洋壳。从辉长岩中分选出大量锆石,采用SHRIMP方法测得这些锆石的年龄为505±8Ma,代表了辉长岩的结晶年龄,它与熬油沟辉长岩(504Ma)和东草河辉长岩(497Ma)基本同时形成,表明寒武纪末期北祁连洋盆存在一次规模较大的海底扩张事件。剖面北段主要由枕状熔岩组成,其地球化学性质类似于玻安岩,代表了由洋壳向北俯冲形成的岛弧。枕状熔岩形成较晚(483Ma),这与其覆盖在辉长岩上的野外关系是一致的,说明从残留洋壳的形成到岛弧的出现间隔了大约20Ma。这项研究为认识北祁连洋盆的构造演化提供了重要依据。  相似文献   

4.
蛇绿岩及蛇绿岩中浅色岩的SHRIMP U-Pb测年   总被引:101,自引:2,他引:101  
文中简要评述了蛇绿岩的层状辉长岩,斜长岩和斜长花岗岩,以橄榄岩为主岩的花岗岩和蛇绿岩中的埃达克岩的锆石SHRIMP U-Pb年龄的地质意义。层状辉长岩(或堆晶层状辉长岩)通常起源于洋脊下的岩浆房,因而它的形成年龄代表洋壳形成的时代。斜长岩与层状辉长岩的时代相近或略晚。斜长花岗岩年龄的解释极其依赖锆石组成和地球化学证据。橄榄岩为主岩的花岗岩,可能记录蛇绿岩的侵位时代。蛇绿岩中的埃达克岩是消减洋壳在深部的部分熔融的产物。文中发表了新疆扎河坝蛇绿岩SHRIMP定年的中间成果,并简略地介绍了滇川西部金沙江和内蒙古图林凯等地的研究实例。根据层状辉长岩的测定结果,扎河坝蛇绿岩形成于(489±4)Ma,金沙江蛇绿岩形成于(328±8)Ma。内蒙古图林凯蛇绿岩中埃达克岩形成于(467±13)Ma~(429±7)Ma。块状辉长岩、斜长花岗岩和橄榄岩为主岩的花岗质岩石记录了蛇绿岩的复杂演化。新疆扎河坝蛇绿岩中的块状辉长岩中存在多组锆石年龄值。较老的一组为468~511 Ma,与层状辉长岩和斜长岩相似,记录了蛇绿岩或洋壳的形成时代,但是,岩石中的大部分锆石年龄为396~419 Ma,加权平均年龄为(406±4)Ma,可能反映了一次部分熔融事件。滇川西部金沙江蛇绿岩中的斜长花岗岩的形成年龄为约300~285Ma,晚于层状辉长岩和?  相似文献   

5.
北秦岭西段吕梁期构造岩浆事件的年代学及其构造意义   总被引:3,自引:0,他引:3  
北秦岭造山带西段宝鸡地区首次识别出中元古代早期的花岗岩体——陕西太白岩基巩坚沟变形侵入体和宝鸡岩基胡店变形侵入体,原岩均为中细粒二长花岗岩。依据锆石CL影像特征,结合锆石成因分析,进行锆石微区U-Pb同位素LA-ICPMS(激光剥蚀等离子体质谱)测定,分别获得1741Ma±12Ma和1770 Ma±13Ma的岩浆结晶年龄。岩石地球化学资料均显示变形侵入体形成于板块边缘的俯冲-碰撞构造环境,岩浆源于部分熔融的地壳物质,应代表北秦岭造山带西段中元古代早期一次强烈的构造岩浆事件。太白巩坚沟和宝鸡胡店变形侵入体的测年结果和陕西凤县红花铺英云闪长岩中捕获锆石的年龄信息(1765 Ma±8Ma),为较精确地限定北秦岭西段吕梁期构造岩浆事件的时限提供了依据。在分析北秦岭造山带西段早期构造属性的基础上,提出近1.8Ga的构造岩浆事件可能是Columbia 超大陆汇聚在北秦岭造山带西段的响应。  相似文献   

6.
湖北随州大狼山群片岩中碎屑锆石的U-Pb年龄及其意义   总被引:2,自引:0,他引:2  
湖北随州大狼山群片岩样品中锆石LA-ICP-MSU-Pb年龄结果表明,锆石属于碎屑锆石,未受到后期绿片岩相变质作用的明显改造,锆石年龄代表了其原岩的年龄,是不同构造-热事件的响应。本区锆石的U-Pb年龄多集中在新元古代早中期(884~758Ma),可分为4组,反映南秦岭地区在该时期至少经历了4次构造-热事件,可能与Rodinia超大陆的聚合与裂解过程相对应。其中1颗锆石还给出了2500Ma的年龄,反映在南秦岭地区可能发生过新太古代末期至古元古代早期的构造-热事件。通过地层和锆石年代学的限定,认为大狼山群的成岩时代应为早震旦世。  相似文献   

7.
东昆仑地区小庙岩组的形成时代长期存在争议,且缺少精确的年代学证据。以东昆仑东段塔妥地区小庙岩组变质碎屑岩为研究对象,采用LA-ICP-MS锆石U-Pb同位素定年方法,探讨其形成时代。结果显示,锆石具有较宽的年龄谱(1551~2624Ma),可划分为2624~2603Ma、2501~2290Ma、2196~2036Ma和1921~1551Ma年龄组,最后一组可进一步划分为1921~1690Ma和1641~1551Ma两个亚组。其中1921~1690Ma代表了物源区强烈的变质岩浆热事件,具有最强峰值特征(峰值年龄为1783Ma),而2501~2290Ma代表了源区一次变质事件,并有少量2196~2036Ma年龄记录,个别2600Ma的碎屑锆石暗示东昆仑地区存在新太古代的陆核。1641~1551Ma(加权平均年龄为1597Ma)代表了小庙岩组发生变质的时间,而物源区最晚变质年龄为1690Ma,从而确定小庙岩组形成于1690~1597Ma,属中元古代。对比碎屑锆石年龄频谱和东昆仑地区较老变质基底记录的构造-热事件信息,小庙岩组主要沉积物源可能来自于古元古界白沙河岩组,而东昆仑与华北在古元古代末—中元古代早期构造事件的对比揭示,在这一时期二者具有相似的构造演化特征。  相似文献   

8.
东南极Windmill群岛变质杂岩经历的变质和岩浆事件与西澳大利亚Albany-Fraser造山带在时间上相对应,并可能与罗迪尼亚超大陆的拼合有关。Windmill群岛Bailey半岛的镁铁质片麻岩(角闪石-单斜辉石-斜方辉石-黑云母-斜长石-石英-磁铁矿-锆石)被认为具有较早的形成年龄,其中还出露属于正片麻岩的淡色片麻岩(斜长石-钾长石-石英-黑云母-锆石)。对这两种片麻岩中的锆石分别进行了SHRIMP U-Pb年龄测定,首次获得该区镁铁质片麻岩锆石核部207Pb/206Pb加权平均年龄1403±28 Ma,该年龄记录了本区中元古代早期岩浆事件,这是Windmill群岛地区记录的最早一期岩浆事件,可能受到了东部莫森大陆(Mawson Continent)构造岩浆活动的影响。铁镁质片麻岩锆石增生边的年龄为1318±34 Ma,则记录了早期构造热事件。淡色片麻岩中锆石核部年龄为1257±51 Ma,与Bailey半岛的片麻状含石榴子石花岗岩侵位年龄一致,共同记录了该区的一期岩浆活动。淡色片麻岩中锆石增生边的年龄为1197±26 Ma,记录了晚期的变质事件。这些新的年龄数据强烈支持1375~1151 Ma期间东南极Windmill群岛与西澳大利亚Albany-Fraser造山带相连接的构造模型,同时也为罗迪尼亚超大陆拼合过程提供了重要的年代学约束。   相似文献   

9.
东昆仑东段巴隆地区哈图沟出露一套变质—变形的沉积地层,其北部为变复成分砾岩段、南部为变细碎屑岩段。根据变复成分砾岩段砾石成分统计、砾石的分选系数(1.24~1.42)及砾石的砾度等粒性特征得出该套变复成分砾岩地层为近源快速堆积的产物。利用LA-MC-ICP-MS锆石U-Pb微区测年技术,对变复成分砾岩段绢云钠长石英片岩的碎屑锆石进行U-Pb同位素测年,测试结果表明,碎屑锆石年龄谱可明显划分为6组:1南华纪—震旦纪年龄组,751~602Ma,峰值为674Ma,该组锆石年龄与东昆仑造山带新元古代中—晚期岩浆事件的年龄相对应,并且与Rodinia超大陆裂解事件相关,该组锆石中最小的年龄为602Ma,表明沉积地层形成时代应晚于602Ma;2中元古代晚期—新元古代中期年龄组,1146~783Ma,出现三个峰值,分别为788Ma、947Ma和1115Ma,该组锆石年龄与东昆仑造山带中元古代晚期—新元古代中期岩浆事件大致相对应,代表东昆仑地区响应了全球尺度的Rodinia超大陆的汇聚事件,且证实了柴达木—祁连—东昆仑等地(陆)块前寒武纪的演化特点与塔里木及扬子克拉通非常相似;3中元古代晚期年龄组,1399~1180Ma,峰值为1318Ma,该组锆石年龄可能与东昆仑构造带中元古代晚期岩浆事件相关;4中元古代早期年龄组,1712~1553Ma,峰值为1556Ma,该年龄谱段代表了源区在中元古代早期存在着一期热事件;5新太古代晚期—古元古代早期年龄组,2530~2347Ma,峰值为2518Ma,该组锆石年龄与东昆仑造山带内新太古代晚期—古元古代早期岩浆事件年龄大致相对应,表明东昆仑造山带新太古代晚期—古元古代早期岩浆物质也为哈图沟变质地层提供了物源;6太古宙年龄组,3230~2763Ma,表明变复成分砾岩的物源区可能存在古太古代陆核。通过对变质岩系的碎屑锆石年龄和物源特征的分析研究,认为这套变质的沉积地层并非前人所划归的泥盆系牦牛山组,其形成时代应归属于晚震旦世。砾石成分统计和年代学测试结果表明,巴隆地区哈图沟变复成分砾岩层物质主要来源于东昆仑造山带的前寒武纪白沙河岩组、小庙岩组、万宝沟岩群等变质地层和中、新元古代深变质的花岗质片麻岩等。  相似文献   

10.
对海南岛北西部戈枕地区侵位于古中元古代抱板群花岗质岩进行了CL成像制约下的SHRIMP锆石U-Pb年龄测定,获得了1455±12Ma的U-Pb谐和年龄、1454±12Ma有利的207Pb/206Pb中值年龄。结合该花岗质岩锆石内部结构、岩浆源区和变形变质特征,以及前人多种定年成果,认为该花岗质岩结晶年龄应为~1450Ma,随后于~1400Ma经历了一次构造-热变质事件;Rodinia超大陆聚合前,华夏地块(包括海南岛、或至少是海南岛北西部)最可能位于劳伦大陆南缘的西南延伸部分。研究结果还暗示华夏地块(包括海南岛)所经历的格林威尔造山事件可能较扬子地块偏早,古、中元古代抱板群沉积上限应约束在~1450Ma。  相似文献   

11.
北祁连山西段中元古代蛇绿岩的发现及其地质意义   总被引:5,自引:0,他引:5  
北祁连山西段中元古代蛇绿岩的发现及其地质意义张招崇(中国地质科学院地质研究所,北京100037)毛景文杨建民左国朝吴茂炳(中国地质科学院矿床地质研究所,北京100037)(甘肃省地矿局地质科学研究所,兰州730000)关键词中元古代蛇绿岩北祁连山西段...  相似文献   

12.
The Aoyougou ophiolite lies in an early Palaeozoic orogenic belt of the western North Qilian Mountains, near the Aoyougou valley in Gansu Province, northwestern China. It consists of serpentinite, a cumulate sequence of gabbro and diorite, pillow and massive lavas, diabase and chert. Ages of 1840±2 Ma, 1783±2 Ma and 1784±2 Ma on three zircons from diabase, indicate an early Middle Proterozoic age. The diabases and basalts show light rare-earth element enrichment and have relatively high TiO2 contents, characteristic of ocean island basalts. All of the lavas have low MgO, Cr, Ni contents and Mg numbers indicating a more evolved character. They are believed to have been derived from a more mafic parental magma by fractionation of olivine, Cr-spinel and minor plagioclase. Based on the lava geochemistry and regional geology, the Aoyougou ophiolite was probably believed to have formed at a spreading centre in a small marginal basin. Subduction of the newly formed oceanic lithosphere in the Middle Proteroz  相似文献   

13.
祁连山蛇绿岩带和原特提斯洋演化   总被引:2,自引:1,他引:1  
位于阿拉善地块和柴达木地块之间的祁连造山带记录原特提斯洋扩张、俯冲、闭合、大陆边缘增生和碰撞造山的完整过程。从南向北,祁连造山带发育有三条平行排列、不同类型的蛇绿岩带:(1)南部南祁连洋底高原-洋中脊-弧后蛇绿岩混杂带;(2)中部托勒山洋中脊型蛇绿岩带;(3)北部走廊南山SSZ型蛇绿岩带。南部南祁连蛇绿混杂岩带以拉脊山-永靖蛇绿岩为代表,为典型的洋底高原型蛇绿岩,是大洋板内地幔柱活动的产物,形成年龄为525~500Ma;中部托勒山蛇绿岩带沿熬油沟-玉石沟-冰沟-永登一线分布,为大洋中脊型蛇绿岩,蛇绿岩形成年龄为550~495Ma;北部蛇绿岩带包括弧前和弧后两种类型,弧前蛇绿岩以大岔大阪蛇绿岩为代表,形成时代为517~487Ma,反映初始俯冲/弧前扩张到弧后盆地的过程;弧后蛇绿岩以九个泉-老虎山蛇绿岩为代表,为典型的SSZ型蛇绿岩,是弧后扩张的产物,形成时代为奥陶纪(490~445Ma)。三个蛇绿岩带分别代表了新元古代-早古生代祁连洋演化历史不同环境的产物,对了解秦祁昆构造带原特提斯洋的构造演化过程有重要意义。蛇绿岩及弧火山岩的时空分布特征限定了原特提斯洋的俯冲极性为向北消减俯冲。  相似文献   

14.
We report the presence of a Grenvillian ophiolite on the northern margin of the Yangtze craton, drastically changing current ideas about South China's role in plate reconstructions of the Rodinia supercontinent. Strongly deformed amphibolites that locally show relict pillow lavas, isotropic and layered metagabbro, diabase dikes, serpentinized dunite and harzburgite with podiform chromite are dated at circa 1100–985 Ma (U–Pb zircon). The ophiolite is structurally dismembered and thrust over the Proterozoic shelf sequence that covers the north margin of the Yangtze craton, and overrode a flysch to conglomerate-wildflysch unit shed from the ophiolite and a magmatic arc terrane and deposited on the older Yangtze carbonate platform. The youngest clasts in the conglomerate are circa 861–813 Ma (U–Pb zircon), giving a maximum age for ophiolite emplacement. Fine-grained layered amphibolites exhibit slightly depleted-flat type REE curves with no obvious Eu anomalies, and are N-MORB type tholeiites. Metagabbro has typical cumulate textures, flat REE distributions and obvious positive Eu anomalies. The REE characteristics of serpentinized dunites show a U-shape of slight loss of middle REE, representing cumulates metasomatized by LREE slightly enriched mantle. All these features indicate that the metamafic–ultramafic rocks from the Proterozoic Miaowan Formation form a structurally dismembered ophiolite resting above an ophiolitic wildflysch, sitting on top of the Proterozoic shelf sequence on the Yangtze craton. The ophiolite is contemporaneous with an arc sequence preserved to the north on the edge of the Yangtze craton, suggesting that the entire ophiolitic forearc–arc was accreted to the Yangtze craton between 1000 and 850 Ma. Xenocrystic zircons in granite clasts in the basal wildflysch unit have ages consistent with Australian affinity, and detrital zircons in the arc sequence also show derivation from Australia, suggesting that the arc formed on the Australian segment of Rodinia before collision with the Yangtze craton. The discovery of the Proterozoic Miaowan ophiolite supplies important evidence for the existence of a Neoproterozoic oceanic basin on the north margin of the Yangtze craton, and demonstrates that the Yangtze craton first collided with Rodinia on its northern margin, with subsequent accretion of the Cathaysian block on the southern margin of the craton.  相似文献   

15.
西藏群让蛇绿岩辉长岩SHRIMP锆石U-Pb年龄及地质意义   总被引:6,自引:1,他引:5  
对雅鲁藏布江缝合带中段群让蛇绿岩中的辉长岩进行SHRIMP锆石U-Pb定年,得出加权平均年龄为125.6±0.88Ma(2σ,MSWD=1.9),即辉长岩结晶年龄。结合已有的关于雅鲁藏布江蛇绿岩形成年龄的报道,该结果表明群让地区特提斯洋海底扩张的时代与中段大竹卡、吉定地区一致;雅鲁藏布江西段与中段地区洋盆形成时代一致,但晚于东段发育时代;整个东特提斯洋盆发育时代存在东早西晚的特点。  相似文献   

16.
U/Pb zircon ages are reported for four ophiolites and three crosscutting arc-related plutons from the Norwegian Caledonides. Plagiogranite differentiated from gabbro of the Karmøy ophiolite is dated at 493+7/-4 Ma whereas arc-related trondhjemite cutting this ophiolite crystallized at 485+/–2 Ma. A crosscutting clinopyroxene-phyric gabbro intrusion is dated at 470+9/–5 Ma by near concordant magmatic titanite (sphene) and discordant U-rich (2903–6677 ppm) zircon. Lower intercepts of 247+/–68 and 191+/–70 Ma defined by the plagiogranite and clinopyroxene-phyric gabbro best-fit lines may reflect a real low-T alteration/rift-related event.A plagiogranite differentiate of the Gullfjellet ophiolite complex is dated at 489+/–3 Ma and a crosscutting arc-related tonalite is 482+6/–4 Ma. Both of these ages overlap with those of the correlative rocks at Karmøy suggesting that they are parts of one ophiolitic terrane with a common history.Trondhjemite associated with the Leka ophiolite is dated at 497+/–2 Ma, indicating that supra-subduction zone magmatism there may be coeval with spreading which formed the Karmøy axis sequence.The U/Pb zircon ages of Norwegian ophiolites reported here, combined with ages of other Appalachian-Caledonian ophiolite complexes in Britain and Canada, indicate a narrow age range for the generation of at least two marginal basins in the Tremadoc-Arenig. Two spreading episodes documented at Karmøy are separated in time by intrusion of arc-related trondhjemite magmas at 485+/–2 Ma and may correlate with two separate spreading events documented in other ophiolites.The Solund/Stavfjorden ophiolite, at 443+/–3 Ma, is the only late Ordovician ophiolite yet documented in the entire Appalachian-Caledonian Orogen and it probably represents a small, short-lived marginal basin late in the history of the Iapetus Ocean. It is correlative with Caradocian ensialic marginal basin magmatism in Wales and the Trondheim region, and with tholeiitic gabbro-diorite plutons that intruded Newfoundland ophiolites in a tensional regime after emplacement of the ophiolites over the continental margin.  相似文献   

17.
元古宙蛇绿岩研究中有关问题的讨论   总被引:1,自引:0,他引:1  
经过20余年的不懈努力,元古宙蛇绿岩研究已取得了巨大进展。元古宙蛇绿岩的保存特点、多样性、高压超高压变质作用、地质年代学及研究方法(方法论)等几个关键问题尚需进一步探索。我国元古宙蛇绿岩研究应加强构造—岩性的综合研究,并通过同位素测年和地球化学示踪,获取可信的成因信息;同时,还需考虑元古宙洋壳的地球化学特殊性及后期变形变质作用的影响,不能简单套用显生宙蛇绿岩的判别图解和研究方法。  相似文献   

18.
白云山蛇绿混杂岩是北山造山带中红柳河-牛圈子-洗肠井蛇绿岩带的一部分,由蛇纹石化二辉橄榄岩、蛇纹岩、辉长岩(堆晶辉长岩、块状辉长岩)、基性枕状熔岩(变玄武岩)及深海-次深海远洋、半远洋相沉积物(硅质岩、板岩)、白云岩等组成。其中辉长岩LA-ICP-MS锆石U-Pb测年结果为496.4±2.2 Ma,指示蛇绿岩形成于晚寒武世;混杂带中玄武岩地球化学特征为洋底玄武岩,兼有洋岛碱性玄武岩(OIB)和洋脊玄武岩(MORB)的地球化学性质,枕状熔岩与硅质岩交互产出,指示其形成环境为深水洋盆。结合区域地质资料,推测该蛇绿混杂岩带为寒武纪洋中脊存在的遗迹。  相似文献   

19.
A subdivision of the Precambrian of China   总被引:1,自引:0,他引:1  
Precambrian rocks are widely distributed in China. The Precambrian is divided into two time units, i.e., the Archaean and Proterozoic Eon, each of these is separated into three chronological intervals, also with the status of eras, with the prefixes early, middle or late. The time boundary between the Archaean and Proterozoic Eon is placed at ~ 2500 Ma.According to the present isotopic data, the proposed subdivision for the Archaean of China is two-fold. The age of the Fuping Group is younger than 2800–2900 Ma, and that of the Qianxi Group and the corresponding stratigraphic units of eastern Liaoning are older than 2800 Ma, so that 2800+ Ma is selected as the boundary between the early—middle and late Archaean.Based on the representative stratigraphic units, the Wutai and Huto Groups, and an intervening major unconformity formed by the Wutaiian orogeny at 2200–2300 Ma, the early Proterozoic is further divided into two periods, with a time demarcation at 2200+ Ma. A major episode of orogeny known as the “Luliangian Movement” occurred at the end of the early Proterozoic at ~ 1900 Ma. This disturbance was very extensive and is, in a way, responsible for the difference in geological conditions between the lower and middle—upper Proterozoic in China. The boundary (1900 Ma) that relates to the Luliangian Movement is more important than the boundary corresponding to the age of 1600 Ma, which is recommended as the time boundary between Proterozoic I and II, so we propose to use 1900 Ma as the boundary between the early and middle Proterozoic in China.The time boundary between the middle Proterozoic, including the Changcheng System and the Jixian System, and the late Proterozoic, which is composed of the Qingbaikou and Sinian Systems, is ~ 1000 Ma. The age for the boundary between Cambrian and Precambrian, based upon the recent isochron data, is inferred to be 610 Ma.  相似文献   

20.
元古代蛇绿岩及铬铁矿   总被引:2,自引:1,他引:1  
鲍佩声 《岩石学报》2019,35(10):2971-2988
本文总结了国外典型元古代蛇绿岩的岩石组合、野外产状、地球化学资料以及成矿特征,并与显生宙蛇绿岩进行了对比,继而探讨元古代板块构造演化和铬铁矿成因。资料表明,早元古代和中-新元古代均有蛇绿岩的存在,但前者较少,仅见于Canadian地盾的Cape Smith Belt中的Prutuniq蛇绿岩(2. 05~2. 0Ga)和芬兰Fennoscandian地盾的Outokumpu和Jormua蛇绿岩(时代为1. 97~1. 96Ga),而中-新元古代的蛇绿岩则见于世界许多地区,如埃及东部沙漠区(~750Ma)和非洲东北部地区(ca.900~800Ma)等。与显生宙蛇绿岩相比,这些老蛇绿岩具如下特征:(1)它们均为被肢解的蛇绿岩,大多与"弧火山岩"和(或)混杂岩伴生,经历不同程度的变形和变质(具绿片岩相-角闪岩相组合);(2)岩石组合大多较齐全,壳层组合发育,以镁铁-超镁铁岩(堆晶岩)、辉长岩、镁铁质席状岩床(墙)杂岩、火山岩为代表;层状镁铁-超镁铁岩的韵律层以及矿物的隐晶变化等均提示了岩浆多期次活动及开放岩浆房的特征;(3)元古代蛇绿岩中既有高铝型铬铁矿,也有高铬型铬铁矿,且主要寄主于纯橄岩(或蛇纹岩)中;高铝型和高铬型直接受控于熔体的熔融程度及含水流体的参与,反映了铬铁矿形成于俯冲带演化的不同阶段;铬铁矿规模均较小,且均以低TiO 2为特征,均为岩浆分异作用的产物,明显区别于显生宙熔融残余成因的豆荚状铬铁矿;(4)元古代蛇绿岩常伴有硫化物Cu-Co-Zn-Au矿,且铬铁矿含Zn较高(Zn=0. 11%~0. 18%)(如芬兰Outokumpu蛇绿岩);橄榄岩及铬铁矿中常含较高的MnO (高达1. 79%,如埃及东部的Wizer蛇绿岩);(5)元古代蛇绿岩具多种成因:主要为俯冲带成因(如埃及蛇绿岩、北东非蛇绿岩、芬兰Outokumpu蛇绿岩),少量为洋中脊成因(加拿大Purtuniq蛇绿岩)及裂谷成因(芬兰Jormua蛇绿岩)等。  相似文献   

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