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1.
由于前人在桐柏地区秦岭岩群高级变质岩石中获得的变质年龄比较分散,所以有关麻粒岩相变质作用的时代问题存在不同的认识。本文使用LA-MC-ICPMS方法对侵入于秦岭岩群中的弱变形和未变形碱性岩脉进行了锆石U-Pb定年,获得其侵位时代分别为429.9±1.5Ma和430.3±1.3Ma。结合区域上已发表的同位素年代学资料,我们推测,秦岭岩群麻粒岩相变质作用发生于约440~430Ma,与华北陆块南缘的弧-陆碰撞作用有关。碰撞后的岛弧岩浆作用主要发生在约430Ma,从而造成了秦岭岩群的缓慢冷却,这可能是麻粒岩相变质锆石的同位素年龄比较分散的主因。 相似文献
2.
柴北缘欧龙布鲁克地块东段古元古代基性麻粒岩:岩石学、锆石U-Pb年代学和Lu-Hf同位素证据 总被引：2，自引：2，他引：0
在欧龙布鲁克地块东部地区的正片麻岩中识别出呈透镜状产出的基性麻粒岩,部分已转变为斜长角闪岩。其主要矿物组合为单斜辉石、斜方辉石、斜长石、角闪石等,为典型中低压麻粒岩相组合。锆石SHRIMP U-Pb定年得到基性麻粒岩1928±9Ma的变质年龄,片麻岩围岩得到了1927±20Ma的变质年龄,以及2368±5Ma、2377±7Ma的岩浆结晶年龄。片麻岩锆石Hf同位素数据显示变质锆石及岩浆锆石均具有相似的Hf同位素成分,其二阶段模式年龄为2590~2830Ma,显示其可能源于太古代地壳物质的再造。欧龙布鲁克地块古元古代岩浆及变质演化历史与塔里木克拉通及华北克拉通很高的相似性,预示着在古元古代三者可能具有一定的亲缘性。 相似文献
3.
胶北早前寒武纪变质基底多期岩浆-变质热事件:来自TTG片麻岩和花岗质片麻岩中锆石U-Pb定年的证据 总被引：28，自引：25，他引：3
本文采用LA-ICP-MS技术,对胶北地体TTG片麻岩和花岗质片麻岩中锆石进行系统原位U-Pb定年和稀土、微量元素的分析,发现研究区早前寒武变质结晶基底存在多期岩浆-变质热事件。4件TTG片麻岩和2件花岗质片麻岩锆石样品记录了2909±13Ma、2738±23Ma、2544±15~2564±12Ma和2095±12Ma 4组岩浆事件年龄,以及2504±16~2513±32Ma和1863±41Ma 2组变质事件年龄。结合以往TTG片麻岩和花岗质片麻岩的地球化学及Nd同位素研究发现,约2738Ma的TTG岩浆事件可能代表胶北地体地壳最主要的生长事件,而2544~2564Ma的岩浆事件则可能代表古老地壳重熔的最强烈岩浆事件,约2095Ma岩浆事件则反映了胶-辽-吉构造带内部在该时期与地壳拉张作用有关的岩浆活动。2504~2513Ma是研究区以及华北克拉通早前寒武基底最主要的一期变质热事件,可能与地幔柱(热点)岩浆的底侵作用有关,而TTG片麻岩记录的约1863Ma的变质年龄与研究区基性和泥质高压麻粒岩相岩石记录的麻粒岩相变质时代一致,暗示TTG片麻岩可能也经历了古元古代高压麻粒岩相变质作用,上述研究进一步表明胶北地体在古元古代的确存在一期陆-陆碰撞的重要造山事件。该项研究成果对于进一步深入探讨胶北乃至华北克拉通早前寒武纪变质基底的形成演化、岩浆-变质热事件序列及其构造背景具有重要的科学意义。 相似文献
4.
北秦岭造山带早古生代多期变质与深熔作用：锆石U-Pb年代学证据 总被引：5，自引：4，他引：1
北秦岭造山带中的秦岭群以多期变质作用和强烈混合岩化为特征,出露有早古生代的榴辉岩和高压麻粒岩,以及区域性分布的中低压麻粒岩-角闪岩相变质岩石和混合岩。本文对西峡双龙地区混合岩化片麻岩(中色体)、石榴子石黑云母片岩(暗色体)和淡色花岗岩脉体进行了详细的锆石U-Pb定年和微量元素研究。结果表明片麻岩的原岩年龄为941±11Ma(2σ,MSWD=0.59),石榴子石黑云母片岩的原岩年龄为756±9.9Ma(2σ,MSWD=1.07)。石榴子石黑云母片岩中的变质锆石获得了484±9.6Ma(2σ,MSWD=0.88)的年龄,与片麻岩中锆石变质增生边获得的单点年龄498±11Ma在误差范围内一致。这些变质锆石多数具有平坦的HREE分配模式,弱Eu负异常,与高压变质岩中的变质锆石特征相似。该年龄与秦岭榴辉岩和高压麻粒岩的变质年龄相一致。石榴子石黑云母片岩中锆石的最外层增生边记录了424±9.1Ma的年龄,同时淡色脉体中的锆石也给出了422±4.0Ma(2σ,MSWD=0.77)的加权平均年龄,代表了深熔脉体的结晶年龄。这些锆石均为新生锆石,阴极发光弱,具有低的Th/U比值,平坦的HREE分配模式,强烈的Eu负异常,指示部分熔融过程中存在大量石榴子石和斜长石。深熔脉体的年龄与秦岭群中的中低压高温-超高麻粒岩相变质的年龄相符。结合已有的结果,我们认为秦岭群普遍记录了500~480Ma的高压、超高压变质作用,并叠加了440~400Ma的中压-高温变质和深熔作用。秦岭群可能是Rodinia超大陆裂解过程中从华南陆块或相似构造属性陆块分离并漂移到华北克拉通南缘的微陆块。在500~480Ma时发生碰撞造山作用导致北秦岭微陆块深俯冲并发生了高压-超高压变质作用,在志留纪由于商丹洋壳北向俯冲导致秦岭微陆块发生了以中压-高温变质、深熔和同时的岩浆作用为特征的增生造山作用。 相似文献
5.
本文对丹凤地区秦岭岩群含柯石英超高压变质地体长英质片麻岩中的混合岩化长英质浅色体和含石榴子石暗色包体的花岗质脉体进行了详细的矿物学、地球化学和锆石U-Pb年代学以及Lu-Hf同位素研究。其中,长英质浅色体显示了近原位熔融的高硅、富钾的过铝质花岗岩地球化学特征;锆石的CL图像呈灰黑色,均匀无结构或云雾状内部结构,Th/U比值0. 008,并含有钾长石、斜长石、石英和磷灰石等包裹体,显示深熔锆石的特征;花岗质脉体暗色包体中的石榴子石显示核-边成分环带,其中核部成分与秦岭岩群长英质片麻岩中石榴子石成分一致,边部Sps含量升高,显示熔体改造或退变质扩散特征,寄主花岗质脉体显示重稀土强烈亏损的与石榴子石平衡的熔体特征,指示它们是秦岭岩群含石榴子石长英质片麻岩部分熔融的产物。锆石LA-ICP-MS定年得到长英质浅色体和花岗质脉体的结晶年龄分别为445±4Ma和420±1Ma,明显晚于本区的超高压变质时代,而与折返过程中麻粒岩相和角闪岩相退变质叠加的时代基本一致。结合区域地质和前人的研究成果,提出秦岭岩群在深俯冲板块的折返过程中,分别在445Ma和420Ma发生了两期部分熔融作用。 相似文献
6.
东昆仑南地体最古老的变质基底为苦海岩群,出露于苦海-温泉一带。采用LA-ICP-MS方法对苦海岩群副片麻岩中的锆石进行测试,得出苦海岩群副片麻岩的最大沉积时代为新元古代早期且在志留纪(423 Ma)遭受变质。通过与东昆仑北地体变质基底金水口岩群的锆石年龄谱对比,苦海岩群和金水口岩群的锆石年龄谱中都存在1 550~1 650 Ma、1 900~2 100 Ma、2 350~2 550 Ma的年龄段峰值,两岩群可能存在相似的物源区;年龄谱中最年轻的岩浆锆石峰值年龄分别为750~800 Ma和900~1 250 Ma,两岩群的最大沉积时代相近,分别为新元古代早期(Pt_3)、中元古代晚期-新元古代早期(Pt_(2-3));年龄谱中都存在400~450 Ma的变质年龄峰值,两岩群在志留纪均发生角闪岩相变质,说明它们在前志留纪有着相似的演化历史。即使昆南地体和昆北地体之间在早古生代期间存在洋盆,该洋盆也不会太大。 相似文献
7.
青藏高原拉萨地体南部林芝岩群的物质来源与形成年代：岩石学与锆石U-Pb年代学 总被引：26，自引：17，他引：9
本文对位于青藏高原拉萨地体东南部林芝岩群中的变质岩进行了岩石学和年代学研究。研究表明,林芝岩群由角闪岩相的变质沉积岩和正片麻岩组成。变质沉积岩主要为含石榴石白云斜长角闪片岩、含石榴石云母石英片岩、含石榴石黑云钾长片麻岩、大理岩和石英岩等,代表性矿物组合包括石榴石+斜长石+角闪石+石英+黑云母+白云母,或石榴石+斜长石+钾长石+石英+夕线石+黑云母+白云母。花岗质片麻岩（含二云母片麻岩）的矿物组合是石英+斜长石+钾长石+黑云母+白云母。锆石U-Pb年代学分析表明,变质沉积岩中的碎屑锆石主要为岩浆成因,获得了2708~63Ma的206Pb/238U年龄范围,在~1100Ma和~550Ma出现两个年代峰值。碎屑锆石的变质增生边给出了35Ma的变质年龄。正片麻岩获得了496Ma的锆石结晶年龄和1158Ma的继承年龄。基于上述研究结果、区域对比和相邻变质岩石中获得的多期变质年龄,我们认为林芝岩群的原岩很可能形成在早古生代,其沉积物质主要来源于印度陆块,与特提斯喜马拉雅早古生代的岩石一起同为印度大陆北缘的沉积盖层,在环冈瓦纳大陆周缘造山过程中被寒武纪花岗岩侵入。在新特提斯洋向北的俯冲过程中,林芝岩群经历了晚中生代的安第斯型造山作用,在印度与欧亚大陆的俯冲-碰撞过程中,林芝岩群部分地经历了新生代的变质和岩浆作用再造。本研究证明,林芝岩群并不是传统上认为的拉萨地体的前寒武纪变质基底,其角闪岩相至麻粒岩相变质作用发生在中、新生代。 相似文献
8.
西峡北部秦岭群变质沉积岩锆石SHRIMP定年:物源区复杂演化历史和沉积、变质时代确定 总被引：21，自引：15，他引：6
本文对秦岭群典型分布区西峡北部一个遭受高级变质作用的碎屑沉积岩样品(黑云斜长片麻岩)进行了锆石年代学研究。阴极发光图像显示锆石普遍具有核-边结构。在U-Pb年龄谐和图上数据点分散分布,年龄变化范围为>3.0Ga到~400Ma。结合以往研究,获得如下结论和认识:(1)秦岭群变质沉积岩的变质原岩形成于古元古代之后,其中一些可能形成于中元古代晚期甚至更晚,原秦岭群是由不同时代地质体组成,需进一步解体; (2)锆石特征表明,岩石遭受加里东期强烈变质,变质作用经历了高角闪岩相-麻粒岩相向角闪岩相的转变过程;(3)秦岭造山带及邻区存在长期复杂的早期演化历史。 相似文献
9.
冀北单塔子杂岩的地质学和锆石U-Pb年代学 总被引：14，自引：0，他引：14
冀北单塔子杂岩位于华北克拉通北缘中段隆化-赤城断裂以南,主要由高角闪岩相-麻粒岩相变质的英云闪长质、奥长花岗质、花岗闪长质片麻岩(TTG岩套)和铁镁质二辉麻粒岩、辉石斜长角闪岩、石榴石斜长变粒岩、石榴石夕线石黑云母片岩及少量的大理岩等表壳岩残留体组成。详细的SHRIMP和LA-ICPMS锆石U-Pb同位素地质年代学研究表明,冀北单塔子杂岩的形成主要经历了7个演化阶段:(1)2600～2530Ma之间形成了单塔子杂岩中英云闪长质和奥长花岗质片麻岩的围岩,其中一部分铁镁质表壳岩,即铁镁质二辉麻粒岩,可能为英云闪长质—奥长花岗质岩浆的源岩;(2)2517～2505Ma主要为英云闪长质岩浆的侵位结晶阶段;(3)2494～2473Ma,奥长花岗质片麻岩侵位结晶;(4)2427～2404Ma,发生了早期麻粒岩相变形变质作用和深溶混合岩化作用,使这一杂岩经历了强烈的改造;(5)～1859Ma,铁镁质岩浆侵位,形成一些铁镁质岩株和岩脉;(6)～1834Ma到1793Ma,单塔子杂岩经历了最强烈的变质作用改造,形成了晚期麻粒岩相变质作用;(7)～1730Ma经历了强烈的流体活动,使得一些锆石发生了流体交代和改造。但是总体上单塔子杂岩没有经历～300Ma的古生代的变质作用。 相似文献
10.
青藏高原拉萨地块西部念青唐古拉岩群的地球化学特征及构造意义 总被引：1，自引：0，他引：1
拉萨地块西部呈断块状沿狮泉河-申扎-嘉黎蛇绿混杂岩带附近分布的念青唐古拉岩群被认为是前寒武纪变质基底。本文对念青唐古拉岩群进行了系统的岩石学、地球化学、同位素年代学及构造地质学研究。研究结果表明片岩-片麻岩-变粒岩含十字石、石榴子石等特征变质矿物,遵循粒度分异规律,其原岩可能为来自冈瓦纳古陆核北缘中新元古代弧盆体系的活动大陆边缘浊积岩。斜长角闪岩具低硅、高铁镁、富钙的基性岩特征,其原岩为岛弧型基性火山岩。念青唐古拉岩群中的花岗伟晶岩锆石LA-ICPMS U-Pb年龄为1150±13Ma,具过铝质S型花岗岩地球化学特征,可能为中元古代(1150±13 Ma)以前就开始沉积的念青唐古拉岩群基底岩石通过部分熔融形成。与花岗伟晶岩渐变过渡接触的二云斜长片麻岩第一组变质重结晶锆石U-Pb年龄为701±15 Ma,结合十字石特征变质矿物,暗示了该地区中温高压变质作用的峰期变质,变质程度达角闪岩相;第二组热液流体锆石UPb年龄为301±8.4 Ma,可能与冈瓦纳大陆北缘古特提斯洋演化过程中的岩浆热液作用有关。 相似文献
11.
豫西秦岭杂岩变质带的分布及主期变质时代的限定 总被引：2，自引：1，他引：1
豫西秦岭杂岩中变质分级可呈与造山带大致平行的带状分布,从两侧向中心变质级别升高,尤其南侧分带明显:由南向北,依次为黑云母带-石榴子石带-蓝晶石带-夕线石带,直至斜方辉石带,而不是整体上经历了麻粒岩相变质作用。局部发生的麻粒岩相变质未见明显向角闪岩相变质转化的退变结构。通过几种岩石的锆石LA-MC-ICPMS测年研究,多数样品中的锆石经受了后期强烈的改造,同位素体系或多或少被重置。尽管如此,侵位花岗岩和伟晶岩年龄限定了主期变质作用的时代应老于484±3Ma,并可能与早期的榴辉岩相变质作用在演化上有联系。主期变质(不包括榴辉岩相变质)性质与经典的巴罗式变质带可以对比;此外,研究区未经历明显的地壳增厚,与高喜马拉雅结晶岩系类似,秦岭杂岩可能经历了中、下地壳物质沿隧道流上升过程。 相似文献
12.
中国与蒙古之地质 总被引：26，自引：0，他引：26
东昆仑中部缝合带清水泉一带发育石榴斜长紫苏麻粒岩、紫苏辉石黑云母石榴子石麻粒岩、石榴二辉斜长麻粒岩和石榴单斜辉石麻粒岩,它们与混合岩化黑云母石榴子石变粒岩、黑云母辉石变粒岩、石墨大理岩、含透辉石透闪石大理岩、透辉石大理岩、黑云斜长角闪岩和片麻岩等高级变质岩系以及纯橄岩、辉橄岩、橄长岩、辉长岩、辉绿岩和玄武岩等共同构成蛇绿混杂岩。麻粒岩相变质作用的温压条件为T=760~880℃,p=830~1200MPa,为高温中高压麻粒岩相变质作用,估算其形成深度为40~45km。麻粒岩相变质作用的SHRIMP锆石U-Pb年龄为(507·7±8·3)Ma。清水泉地区蛇绿岩形成于~520Ma,到~508Ma时俯冲至地下40~45km深处而发生中高压麻粒岩相变质作用,然后发生构造折返而剥露至地表。证实了清水泉高级变质岩和基性—超基性岩片是形成于早—中寒武世的蛇绿混杂岩,标志一个古生代早期的非常重要的板块汇聚边界,这对于进一步研究东昆仑造山带构造演化、乃至中国西部大地构造格局具有非常重要的意义。 相似文献
13.
北秦岭高压-超高压岩石的多期变质时代及其地质意义 总被引：14，自引：12，他引：2
在岩相学观察和锆石CL图像研究的基础上,利用LA-ICP-MS原位微区定年分析方法,本文确定北秦岭清油河退变榴辉岩的峰期变质时代为490±6Ma,退变质时代为453±9Ma,原岩形成时代为655±9Ma;松树沟超高压长英质片麻岩的峰期变质时代为497±8Ma,两期退变质时代分别为448±4Ma和421±2Ma,原岩形成时代上限832±25Ma;寨根石榴石辉石岩的峰期变质时代为498±2Ma,中压麻粒岩相退变质时代为450±3Ma,角闪岩相退变质时代为426±1Ma,原岩形成时代为573±40Ma;西峡北榴闪岩的角闪岩相变质时代为423±3Ma,原岩形成时代为843±7Ma。新确定的这些岩石的峰期变质时代与前人已报导的区内高压-超高压岩石的峰期变质时代在误差范围内基本一致,结合区内高压-超高压岩石不仅分布在秦岭岩群北缘的官坡-双槐树一带,而且断续出露在秦岭岩群中部或偏南侧的清油河北-松树沟-寨根北甚至西峡北东西一线,进一步表明它们应是同一期构造地质事件的产物。北秦岭已发现的全部正变质的高压-超高压岩石均呈透镜体状分布在围岩片麻岩中,松树沟超高压长英质片麻岩的原岩为典型的陆壳沉积物,因此,这些高压-超高压岩石的形成可能都是陆壳俯冲-深俯冲作用的产物。结合岩相学观察、锆石CL图像和锆石U-Pb定年表明,这些高压-超高压岩石在～500Ma经历了峰期变质作用后,又分别在～450Ma和～420Ma遭受了中压麻粒岩相和或角闪岩相退变质作用的叠加,充分说明这些高压-超高压岩石经历了一个完整的由陆壳俯冲-深俯冲、之后连续两次抬升的构造演化过程。另外,本次研究新获得的这些岩石的原岩形成时代介于843±7Ma～573±40Ma之间,结合官坡榴辉岩的原岩形成时代为791～814Ma以及松树沟榴闪岩原岩时代为787±16Ma的研究,共同表明北秦岭高压-超高压岩石的原岩形成时代均为新元古代,因此,限定俯冲-深俯冲的陆壳物质应来自形成时代为新元古代的大陆地壳或地质体。结合区域地质背景和前人研究成果综合分析,本文初步认为,北秦岭高压-超高压变质岩带的形成是商丹洋向北俯冲拖曳南秦岭新元古代陆壳物质在～500Ma发生陆壳俯冲-深俯冲作用的产物,之后在～450Ma与～420Ma经历了两期抬升。 相似文献
14.
Foliated garnet-bearing amphibolites occur within the West Bore Shear Zone, cutting through granulite facies gneisses of the Strangways Metamorphic Complex. In the amphibolites, large euhedral garnet (up to 3 cm) occurs within fine-grained recrystallized leucocratic diffusion haloes of plagioclase–quartz. The garnet and their haloes include a well-developed vertical foliation, also present in the matrix. This foliation is the same as that cutting through the unconformably overlying Neoproterozoic Heavitree Quartzite. The textures indicate syn- to late kinematic growth of the amphibolite facies mineral assemblages.
All mineral assemblages record an arrested prograde reaction history. Noteworthy is the growth of garnet at the expense of hornblende and plagioclase, and the breakdown of staurolite–hornblende to give plagioclase–gedrite. These dehydration reactions indicate increasing P – T conditions during metamorphism, and suggest heating towards the end of a period of intense deformation. Temperature estimates for the garnet–amphibolite and related staurolite–hornblende assemblages from the shear zone are about 600 °C. Pressure is estimated at about 5 kbar.
An Sm–Nd isochron gives an age of 381±7 Ma for the peak metamorphism and associated deformation. This age determination confirms that amphibolite facies conditions prevailed during shear zone development within the Strangways Metamorphic Complex during the Alice Springs Orogeny. These temperature conditions are significantly higher than those expected at this depth assuming a normal geothermal gradient. The Alice Springs Orogeny was associated with significant crustal thickening, allowing exhumation of the granulite facies, Palaeoproterozoic, lower crust. Along-strike variations of the tectonic style suggest a larger amount of crustal shortening in the eastern part of the Alice Springs Orogeny. 相似文献
All mineral assemblages record an arrested prograde reaction history. Noteworthy is the growth of garnet at the expense of hornblende and plagioclase, and the breakdown of staurolite–hornblende to give plagioclase–gedrite. These dehydration reactions indicate increasing P – T conditions during metamorphism, and suggest heating towards the end of a period of intense deformation. Temperature estimates for the garnet–amphibolite and related staurolite–hornblende assemblages from the shear zone are about 600 °C. Pressure is estimated at about 5 kbar.
An Sm–Nd isochron gives an age of 381±7 Ma for the peak metamorphism and associated deformation. This age determination confirms that amphibolite facies conditions prevailed during shear zone development within the Strangways Metamorphic Complex during the Alice Springs Orogeny. These temperature conditions are significantly higher than those expected at this depth assuming a normal geothermal gradient. The Alice Springs Orogeny was associated with significant crustal thickening, allowing exhumation of the granulite facies, Palaeoproterozoic, lower crust. Along-strike variations of the tectonic style suggest a larger amount of crustal shortening in the eastern part of the Alice Springs Orogeny. 相似文献
15.
北秦岭造山带的早古生代多期变质作用 总被引：15，自引：13，他引：2
北秦岭造山带的秦岭岩群以高级变质岩石为特征,主要包括少量榴辉岩、高压麻粒岩和区域上广泛分布的麻粒岩-角闪岩相变质岩石。年代学研究显示秦岭岩群中不同岩石记录了多期变质作用。已有的定年资料给出北秦岭官坡地区的榴辉岩的年龄为500Ma左右,代表榴辉岩相的变质时代。结合岩相学资料,对两个高压麻粒岩样品的SHRIMP和LA-ICPMS U-Pb测定分别获得504±7Ma 和506±3Ma的年龄,应代表高压麻粒岩相变质时代。这表明高压麻粒岩和相邻的榴辉岩有相近的变质时代,但形成在造山带中不同的构热造环境中。西峡地区的角闪二辉麻粒岩的U-Pb定年给出两组早古生代年龄,一组为440±2Ma,可能代表了中低压麻粒岩相的变质时代,另一组为426±1Ma,应代表区域角闪岩相的变质时代。桐柏山北部的石榴二辉麻粒岩的U-Pb定年数据给出436±1Ma的年龄,为中压麻粒岩相的变质时代。这些资料表明北秦岭造山带经历了早奥陶世的俯冲和地壳增厚作用,并在晚志留世遭受了广泛的巴罗式区域变质作用。 相似文献
16.
贺兰山——阿拉善地区下、中前寒武系的划分对比及其变质、成矿作用特征 总被引：8，自引：0，他引：8
据近期成果,贺兰山—阿拉善地区出露的巨厚变质杂岩可划分为中太古界贺兰山群和叠布斯格群(其全岩Rb—Sr等时年龄为3108.3和3218.8Ma),上太古界阿拉善群和下元古界的赵池沟群、阿拉坦敖包群;它们具不同的变质矿物共生组合,太古界变质岩属低压高温变质的麻粒岩相;下元古界为低—低中压区域动力(热流)变质的绿片岩相岩石。太古界有较强的混合岩化、花岗岩化作用,并蕴藏有铁、石墨、矽线石、刚玉等多种矿产。 相似文献
17.
The Xes-Xen dating of zircons from rocks of the Rayner Complex of the Enderby Land at the Molodezhnaya Station area (coast of the Alasheyev Bight) yielded age estimates of 550 ± 50 and 1040 ± 30 Ma. The metamorphic rocks of the Rayner Complex record two main events: first, the crystallization of the magmatic protoliths of charnockitic and enderbitic gneisses and, second, superimposed structural and metamorphic alterations under conditions transitional from the amphibolite to granulite facies (metamorphism manifested regionally in the rocks of the Rayner Complex). The most reliable Xes-Xen age estimates for magmatic zircons from the charnockitic and enderbitic gneisses correspond to the Grenville stage of the development of the Rayner Complex (～1.0 Ga). The Xe isotopic systematics of metamorphic zircons reflect a pan-African stage in the evolution of the Rayner Complex (600–550 Ma). Pan-African events are reflected in the U-Xe isotope system in two cases: if metamorphic zircons crystallized at the same time (which probably resulted in the formation of a plateau in the Xes-Xen age spectrum) and if the initial isotopic systems were disturbed (which resulted in a decrease in apparent age toward low-temperature gas fractions). It is important that secondary alterations and a decrease in apparent ages to 600–550 Ma affected only those components (i.e., caused xenon release only from those traps) that were unstable under the maximum metamorphic temperatures and yielded T cl values lower than 750°C (conditions transitional from the amphibolite to granulite facies). At a higher xenon retention, “primary” isotopic systems are preserved. Consequently, the age of metamorphism transitional between the amphibolite and granulite facies can be estimated at 600–550 Ma on the basis of Xes-Xen dating. In general, the results of our study indicate that the age of regional metamorphism of the Rayner complex at the Molodezhnaya area is approximately 600–550 Ma rather than ～1.0 Ga, as was previously supposed. 相似文献
18.
New U-Pb single-zircon geochronology undertaken on tonalitic gneisses, granite sheets, migmatites and metasediments from the Lewisian Gneiss Complex on the mainland and the northern part of the Outer Hebrides, NW Scotland, have been used to test the correlation of so-called Laxfordian events across the complex from the Outer Hebrides to the mainland, and the current model for the evolution of the complex as a whole. The study has revealed that the granite sheets originated in two quite different melting events. Those on the mainland at Loch Laxford are ca. 1,855 Ma old whereas those on Harris and Lewis, with which they are presently correlated, are ca. 1,675 Ma old. Grey gneisses associated with granites on the south side of Loch Laxford are confirmed to belong to the 'northern region'. A migmatitic grey gneiss on Harris has given a protolith age of ca. 3,125 Ma, the currently oldest recognised in the complex. Detrital zircons in the Leverburgh and Langavat belts range in age from 2,780 to 1,880 Ma and unequivocally demonstrate deposition in the Palaeoproterozoic. The granulite facies metamorphism in this block is dated from zircon overgrowths at ca. 1,880 Ma. The Laxford Shear Zone which separates the northern and central regions is interpreted to have evolved post-1,860 Ma, during amphibolite facies metamorphism accompanying deformation which took place at ca. 1,740 Ma in both regions. On Harris, the Langavat-Finsbay shear zone developed after 1,675 Ma when a ca. 1,880-Ma granulite facies Proterozoic arc was juxtaposed against amphibolite facies Archaean rocks to the north. Therefore, the shear zones which bound tectonic blocks in the Lewisian Complex evolved at different times and can be interpreted as terrane boundaries. The new data confirm that the Lewisian Complex was not constructed from one contiguous piece of Archaean crust reworked in the Proterozoic but was progressively assembled from several discrete terranes during the Proterozoic. Accordingly, the former regional divisions of the Lewisian Complex are here renamed as follows. On the mainland, the northern region is called the Rhiconich terrane, and the central region the Assynt terrane. On the Outer Hebrides, the Archaean gneisses of Lewis and the northern part of Harris comprise the Tarbert terrane, whereas the newly accreted Proterozoic blocks are called the Roineabhal terrane in Harris and the Niss terrane in the north on Lewis. Wider correlations show that the geology of the Outer Hebrides has more in common with East Greenland than mainland Scotland on the eastern side of the Minch Fault. 相似文献
19.
The Eger Complex in the northwestern Bohemian Massif consists mainly of amphibolite facies granitic gneisses containing a subordinate volume of felsic granulites. Microstructural changes and modelling of metamorphic conditions for both rock types suggest a short‐lived static heating from ~760 to ~850 °C at a constant pressure of ~16 kbar, which led to the partial granulitization of the granitoid rocks. Detailed study of the protolith zircon modifications and modelling of the Zr re‐distribution during the transition from amphibolite to granulite facies suggests that the development of c. 340 Ma old zircon rims in the granulite facies sample is the result of recrystallization of older (c. 475 Ma) protolith zircon. This study suggests that the partial granulitization is a result of a short exposure of the Eger Complex metagranitoids to a temperature of ~850 °C at the base of an arc/fore‐arc domain and their subsequent rapid exhumation during the Lower Carboniferous collision along the western margin of the Bohemian Massif. 相似文献
20.
Deep in the Heart of Dixie: Pre-Alleghanian Eclogite and HP Granulite Metamorphism in the Carolina Terrane, South Carolina, USA 总被引：2，自引：1，他引：2
The central part of the Carolina terrane in western South Carolina comprises a 30 to 40 km wide zone of high grade gneisses that are distinct from greenschist facies metavolcanic rocks of the Carolina slate belt (to the SE) and amphibolite facies metavolcanic and metaplutonic rocks of the Charlotte belt (to the NW). This region, termed the Silverstreet domain, is characterized by penetratively deformed felsic gneisses, granitic gneisses, and amphibolites. Mineral assemblages and textures suggest that these rocks formed under high‐pressure metamorphic conditions, ranging from eclogite facies through high‐P granulite to upper amphibolite facies. Mafic rocks occur as amphibolite dykes, as metre‐scale blocks of coarse‐grained garnet‐clinopyroxene amphibolite in felsic gneiss, and as residual boulders in deeply weathered felsic gneiss. Inferred omphacite has been replaced by a vermicular symplectite of sodic plagioclase in diopside, consistent with decompression at moderate to high temperatures and a change from eclogite to granulite facies conditions. All samples have been partially or wholly retrograded to amphibolite assemblages. We infer the following P‐T‐t history: (1) eclogite facies P‐T conditions at ≥ 1.4 GPa, 650–730 °C (2) high‐P granulite facies P‐T conditions at 1.2–1.5 GPa, 700–800 °C (3) retrograde amphibolite facies P‐T conditions at 0.9–1.2 GPa and 720–660 °C. This metamorphic evolution must predate intrusion of the 415 Ma Newberry granite and must postdate formation of the Charlotte belt and Slate belt arcs (620 to 550 Ma). Comparison with other medium temperature eclogites and high pressure granulites suggests that these assemblages are most likely to form during collisional orogenesis. Eclogite and high‐P granulite facies metamorphism in the Silverstreet domain may coincide with a ≈570–535 Ma event documented in the western Charlotte belt or to a late Ordovician‐early Silurian event. The occurrence of these high‐P assemblages within the Carolina terrane implies that, prior to this event, the western Carolina terrane (Charlotte belt) and the eastern Carolina terrane (Carolina Slate belt) formed separate terranes. The collisional event represented by these high‐pressure assemblages implies amalgamation of these formerly separate terranes into a single composite terrane prior to its accretion to Laurentia. 相似文献