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1.
西藏改则县冈玛错地区发现榴辉岩 总被引：4，自引：1，他引：3
青藏高原羌塘中部出露有一条东西长约500km的高压变质带,高压变质带中段片石山地区已有榴辉岩的报道,然而其他地区至今尚没有发现榴辉岩出露。首次在羌塘中部高压变质带西段冈玛错地区发现了典型的榴辉岩,岩石呈透镜状或块状,主要矿物成分为石榴子石、绿辉石、多硅白云母和角闪石。地质温压计估算结果表明,榴辉岩相变质作用发生的温度和压力条件为427~472℃和约2.3GPa。冈玛错榴辉岩是藏北羌塘地区第二例榴辉岩,它的发现进一步证明了该地区板块缝合带的存在,同时也暗示羌塘中部地区可能存在一条榴辉岩带,这对研究特提斯洋的闭合与青藏高原的早期演化具有重要意义。 相似文献
2.
羌塘中部高压变质带的形成过程 总被引：1，自引：1，他引：0
羌塘中部高压变质带由榴辉岩、石榴石白云母片岩和蓝片岩等组成,与蛇绿混杂岩、晚古生代浅变质地层岩片等共同构成了龙木错-双湖板块缝合带这一构造混杂岩带,是伴随古特提斯洋闭合的深俯冲作用及后期构造作用的产物。通过对其野外地质特征、不同岩石类型岩石学、矿物学以及同位素年代学等的研究,确认榴辉岩和石榴石白云母片岩在早期分别经历了各自的形成过程,在榴辉岩形成之后的折返过程中二者共同构成了高压变质带,并且在折返过程中榴辉岩发生蓝片岩相退变质作用,同时导致了带内蓝片岩的形成。同位素年代学研究结果表明,龙木错-双湖板块缝合带闭合过程中的榴辉岩相变质作用发生于240Ma左右,折返过程中的蓝片岩相退变质作用及蓝片岩的形成应在220～200Ma,高压变质带最终在214Ma之前抬升出露地表。 相似文献
3.
拉萨地块东部松多(超)高压榴辉岩记录了古特提斯洋俯冲及折返过程。松多榴辉岩带已发现松多、新达多、白朗和吉朗4个榴辉岩出露区,它们的峰期温压条件及变质p-T轨迹的研究对揭示拉萨地块古特提斯时期的俯冲及折返过程有重要意义。松多榴辉岩带东段吉朗榴辉岩的主要矿物为石榴子石、绿辉石、多硅白云母、角闪石、金红石、绿帘石、石英以及退变形成的后成合晶结构(透辉石+角闪石+斜长石)和少量的黑云母。石榴子石具有含丰富矿物包裹体的"脏"核和极少包裹体的"净"边,具有典型的进变质成分环带特征,从核部到边部镁铝榴石组分升高,锰铝榴石和钙铝榴石组分降低。石榴子石边部发育窄的角闪石+斜长石(An=28)组成的冠状体,表明石榴子石边部发生了后期角闪岩相退变质作用。通过变质相平衡模拟计算得到石榴子石以及多硅白云母记录的峰期温压条件为563℃、2. 4 GPa。结合岩相学特征,确定吉朗榴辉岩经历了4期变质演化阶段:(1)进变质阶段以石榴子石核部及其包裹体为代表性矿物组合;(2)峰期变质阶段矿物组合为石榴子石边部、绿辉石、多硅白云母、蓝闪石、硬柱石、金红石和石英;(3)早期退变质阶段以硬柱石分解产生绿帘石为特征;(4)晚期退变质阶段以绿辉石发育后成合晶和石榴子石生长冠状体为特征。认为吉朗榴辉岩为典型的低温高压榴辉岩,经历了顺时针p-T演化轨迹,折返过程为近等温降压过程。与松多带内其他(超)高压岩石相比,吉朗榴辉岩峰期温压条件较低,其围岩为变石英岩,区别于区内其他(超)高压榴辉岩的石榴子石白云母片岩及蛇纹岩围岩。推测吉朗榴辉岩来自于俯冲带浅部,由俯冲隧道中低密度沉积物裹挟折返。 相似文献
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5.
藏北羌塘中部戈木日榴辉岩的岩石学、矿物学及变质作用pTt轨迹 总被引：5，自引：0，他引：5
羌塘中部榴辉岩位于龙木错－双湖缝合带中段,改则县古姆乡片石山地区。榴辉岩的主要矿物成分为石榴子石、绿辉石、多硅白云母、金红石、角闪石等,围岩为石榴白云母片岩和蓝片岩,石榴白云母片岩主要由石榴子石、多硅白云母和石英构成,蓝片岩由石榴子石、角闪石（含蓝闪石）、多硅白云母等构成。岩石学和矿物学研究显示,榴辉岩主要经历了3期变质作用：①峰期榴辉岩相变质作用阶段,以石榴子石、绿辉石和多硅白云母为特征,变质温度和压力分别为500℃和2.3GPa;②绿帘角闪岩相变质作用阶段,以后期形成的冻蓝闪石、镁红闪石、绿帘石、钠长石等交代早期矿物为特征;③绿片岩相变质作用阶段,以毛发状阳起石等为特征。榴辉岩的变质演化过程代表了青藏高原北部古特提斯洋俯冲消减和冈瓦纳与劳亚大陆碰撞造山的过程。 相似文献
6.
位于藏北羌塘中部的冈玛日-桃形错地区首次发现含典型蓝闪石的蓝片岩。蓝片岩是由蓝闪石、铁蓝闪石、青铝闪石、多硅白云母、阳起石、黝帘石、绿帘石、方解石、榍石、金红石等矿物组成,变质温压条件为400℃±和7kbar±,属于高压变质作用。这是一条高压变质带,推测它可能向东与滇西的澜沧江高压变质带相连。冈玛日-桃形错地区含典型蓝闪石的蓝片岩的发现对羌塘陆块的重新认识以及冈瓦纳大陆与劳亚大陆的板块边界及晚古生代古特提斯洋的闭合,具有重要的大地构造意义。 相似文献
7.
青藏高原羌塘中部榴辉岩Ar-Ar定年 总被引：25，自引：4，他引：25
羌塘榴辉岩产于红脊山-双湖高压变质带中段的片石山地区,是目前青藏高原内部唯一的榴辉岩产地。榴辉岩多硅白云母~(40)Ar-~(39)Ar年龄 t_p=219.3±1.5Ma;榴辉岩围岩的多硅白云母~(40)Ar-~(39)Ar 年龄 t_p=217.2±1.8Ma,定年结果表明,榴辉岩与围岩经历了相同的高压变质作用,与红脊山-双湖地区蓝闪片岩的高压变质时间是一致的。根据已有高压变质年代学资料,从羌塘的红脊山到双湖、那曲地区的巴青以北、昌都的酉西,直到滇西地区,断续分布长达2000余公里的高压变质带为同一期构造事件的产物,是龙木错-双湖板块缝合带向南延伸到滇西的重要依据。榴辉岩的定年数据进一步揭示,冈瓦纳板块与欧亚(扬子)板块的主体碰撞时间在220Ma 左右,同时也是古特提斯洋消亡的时间记录。 相似文献
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澜沧谦迈地区位于西南三江昌宁-孟连结合带中段。新近在该地区发现榴辉岩,与双江县勐库地区退变榴辉岩一样,谦迈地区榴辉岩也产于湾河蛇绿混杂岩带内,岩石新鲜,后期退变质作用改造弱,岩石学信息保留完整。主要矿物成分为石榴子石、绿辉石、多硅白云母、金红石、角闪石、绿帘石、石英等,直接围岩为白云(钠长)片岩和斜长角闪岩,白云(钠长)片岩主要由多硅白云母、石英和长石构成。根据岩石学和矿物学特征将谦迈地区榴辉岩划分为4个变质阶段,其中峰期矿物组合为石榴子石+绿辉石+多硅白云母+硬柱石+蓝闪石+金红石+石英。初步研究表明,峰期榴辉岩相变质温度和压力分别为600℃和2.5GPa。新发现的榴辉岩为研究三江地区特提斯构造演化提供了关键性资料,该榴辉岩变质演化p-T轨迹的研究对探讨古特提斯洋的俯冲-造山过程具有重要意义。 相似文献
9.
青藏高原羌塘高压变质带的特征及其构造意义 总被引：2，自引：0，他引：2
羌塘高压变质带分布于龙木错-双湖板块缝合带的南侧．西起红脊山．经片石山、蓝岭、角木查尕日、纳若．到双湖以东的才多茶卡,长约500km。向东已经延伸到巴青以北和昌都的吉塘地区,再向南进入滇西与澜沧江蓝片岩带共同构成一条断续延伸近2000km的高压变质带。羌塘高压变质带主要由蓝片岩和榴辉岩构成,蓝闪石片岩中蓝闪石和多硅白云母同位素定年结果为223-215Ma,榴辉岩的变质年龄为243-217Ma。估算蓝片岩的变质温度为410-460℃,变质压力为0．67-0．75GPa：榴辉岩相的变质作用温度不超过5000C,压力为1．56-2．35GPa。羌塘高压变质岩定年研究结果确定龙木错-双湖板块缝合带最终碰撞闭合发生在晚三叠世早期。羌塘高压变质带是冈瓦纳与欧亚大陆汇聚事件的重要记录。 相似文献
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青藏高原拉萨地块松多榴辉岩的岩相学特征和变质演化过程 总被引：4，自引：2，他引：4
松多榴辉岩出露于拉萨地块的石英片岩中,主要由较为基性的金红石榴辉岩和较为酸性的石英榴辉岩组成。榴辉岩相矿物组合为石榴子石 绿辉石 绿帘石±多硅白云母±石英±金红石。岩石发生了较强烈的退变质作用,退变质矿物有角闪石、绿帘石、石英、钠长石及绿泥石。石榴子石变斑晶具有生长环带结构,变斑晶和基质石榴子石主要落入C类榴辉岩区,少数石榴子石变斑晶边部和基质石榴子石落入B类榴辉岩区;单斜辉石主要为绿辉石,少数Ⅰ世代和Ⅲ世代为普通辉石;角闪石均为钙质角闪石。根据石榴子石-绿辉石-多硅白云母矿物温压计计算,获得的温压范围为630～777℃和2.58～2.70GPa,峰期变质条件接近于石英-柯石英转变线。榴辉岩的原岩经历了从高绿片岩相、角闪岩相、榴辉岩相、角闪岩相到高绿片岩相的变质过程,这反映了与古特提斯洋闭合有关的俯冲进变质作用和随后的折返退变质作用。 相似文献
11.
东亚原特提斯洋(Ⅱ):早古生代微陆块亲缘性与聚合 总被引：2，自引：7，他引：2
原特提斯洋内存在塔里木、中祁连、柴达木、扬子、华夏、印支、兰坪-思茅等诸多陆块/微陆块,多数陆块之间在早古生代晚期发育有蛇绿岩带或高压-超高压带。原特提斯域形成于从Rodinia裂解到Pangea超大陆集结期间,存在复杂的洋-陆格局和聚散过程。原特提斯洋不同陆块/微陆块属性和关系及其拼合过程是恢复重建Pangea超大陆聚合前构造背景的关键,但对其认识迄今还存在争论。因此,本文采用综合对比方法,以期建立原特提斯洋陆块/微陆块的亲缘性和海-陆格局,厘定原特提斯微陆块拼合时序与方式。结果表明,早古生代早期除华北陆块不具有亲冈瓦纳大陆的特征外,扬子、华夏、塔里木、柴达木、阿拉善、北秦岭-中祁连-中阿尔金、欧龙布鲁克、北羌塘、南羌塘、拉萨、兰坪-思茅、印支等陆块/微陆块都具有亲冈瓦纳的特征。在450~400Ma左右这一系列陆块/微陆块都向南俯冲-增生,并逐步拼合于冈瓦纳大陆北缘东段,原特提斯洋关闭,并形成了原潘吉亚(Proto-Pangea)超大陆;原潘吉亚于380Ma以后裂离出塔里木-华北陆块和大华南陆块,分别出现勉略洋和古特提斯洋,直到240~220Ma逐步向北聚合,形成最终的劳亚古陆,此时才形成潘吉亚超大陆。 相似文献
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Formation Environment of the High-Pressure Metamorphic Rocks, Western Tianshan, Xinjiang, China 总被引：1，自引：0，他引：1
1.IntroductionFromexperimefltalphaseequilibrium,stableisotOPe,andthermo-barometricstudies,ProgradebineschistdineralparageneseshavebeenproducedexclusivelyatrelativelyhighPadratios(DeRoever,1956;Miyashiro,1961;DobretsovandSobolev,1984;Emst,1973,1988;Maruyamaetal.,1996).InthelastthreedeCades,withtheadvanceofplatetectonics,manygeologistssuggestedthatblueschists,representinghigh-Pressurelow-tCmperamre~rphism,areformedbysubductionofoceanicplate(Emst,1973).Blueschistshavealsobeenregardedasoneof… 相似文献
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青藏高原南部拉萨地体的变质作用与动力学 总被引：3，自引：0，他引：3
拉萨地体位于欧亚板块的最南缘,它在新生代与印度大陆的碰撞形成了青藏高原和喜马拉雅造山带。因此,拉萨地体是揭示青藏高原形成与演化历史的关键之一。拉萨地体中的中、高级变质岩以前被认为是拉萨地体的前寒武纪变质基底。但新近的研究表明,拉萨地体经历了多期和不同类型的变质作用,包括在洋壳俯冲构造体制下发生的新元古代和晚古生代高压变质作用,在陆-陆碰撞环境下发生的早古生代和早中生代中压型变质作用,在洋中脊俯冲过程中发生的晚白垩纪高温/中压变质作用,以及在大陆俯冲带上盘加厚大陆地壳深部发生的两期新生代中压型变质作用。这些变质作用和伴生的岩浆作用表明,拉萨地体经历了从新元古代至新生代的复杂演化过程。(1)北拉萨地体的结晶基底包括新元古代的洋壳岩石,它们很可能是在Rodinia超大陆裂解过程中形成的莫桑比克洋的残余。(2)随着莫桑比克洋的俯冲和东、西冈瓦纳大陆的汇聚,拉萨地体洋壳基底经历了晚新元古代的(～650Ma)的高压变质作用和早古代的(～485Ma)中压型变质作用。这很可能表明北拉萨地体起源于东非造山带的北端。(3)在古特提斯洋向冈瓦纳大陆北缘的俯冲过程中,拉萨地体和羌塘地体经历了中古生代的(～360Ma)岩浆作用。(4)古特提斯洋盆的闭合和南、北拉萨地体的碰撞,导致了晚二叠纪(～260Ma)高压变质带和三叠纪(～220Ma)中压变质带的形成。(5)在新特提斯洋中脊向北的俯冲过程中,拉萨地体经历了晚白垩纪(～90Ma)安第斯型造山作用,形成了高温/中压型变质带和高温的紫苏花岗岩。(6)在早新生代(55～45Ma),印度与欧亚板块的碰撞,导致拉萨地体地壳加厚,形成了中压角闪岩相变质作用和同碰撞岩浆作用。(7)在晚始新世(40～30Ma),随着大陆的继续汇聚,南拉萨地体经历了另一期角闪岩相至麻粒岩相变质作用和深熔作用。拉萨地体的构造演化过程是研究汇聚板块边缘变质作用与动力学的最佳实例。 相似文献
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Recently, a huge ultrahigh‐pressure (UHP) metamorphic belt of oceanic‐type has been recognized in southwestern (SW) Tianshan, China. Petrological studies show that the UHP metamorphic rocks of SW Tianshan orogenic belt include mafic eclogites and blueschists, felsic garnet phengite schists, marbles and serpentinites. The well‐preserved coesite inclusions were commonly found in eclogites, garnet phengite schists and marbles. Ti‐clinohumite and Ti‐chondrodite have been identified in UHP metamorphic serpentinites. Based on the PT pseudosection calculation and combined U‐Pb zircon dating, the P‐T‐t path has been outlined as four stages: cold subduction to UHP conditions before ～320 Ma whose peak ultrahigh pressure is about 30 kbar at 500oC, heating decompression from the Pmax to the Tmax stage before 305 Ma whose peak temperature is about 600oC at 22kbar, then the early cold exhumation from amphibolite eclogite facies to epidote‐amphibolite facies metamorphism characterized by ITD PT path before 220 Ma and the last tectonic exhumation from epidote amphibolite facies to greenschist facies metamorphism. Combining with the syn‐subduction arc‐like 333‐326 Ma granitic rocks and 280‐260 Ma S‐type granites in the coeval low‐pressure and high‐temperature (LP‐HT) metamorphic belt, the tectonic evolution of Tianshan UHP metamorphic belt during late Cambrian to early Triassic has been proposed. 相似文献
15.
The Duguer area represents one of the few occurrences of high-grade metamorphic rocks in the ‘Central Uplift’ zone of the Qiangtang terrane, central Tibet. The metamorphic rocks consist mainly of orthogneiss, paragneiss, and schist. To better understand the formation of these rocks, seven samples of gneiss and schist from the Duguer area were selected for in situ zircon U–Pb analysis and Ar–Ar dating of metamorphic minerals. The results suggest two distinct metamorphic stages, during the Late Triassic (229–227 Ma) and Late Jurassic (150–149 Ma). These stages correspond to the closure of the Palaeo-Tethys Ocean and northward subduction of the Bangong–Nujiang Neo-Tethys oceanic crust, respectively. We suggest that the Late Triassic metamorphic rocks of the Duguer area in the central South Qiangtang subterrane provide evidence of continental collision between the North and South Qiangtang subterranes, following the subduction of oceanic crust. It is likely that deep subduction of oceanic crust occurred along the Longmu Co–Shuanghu–Lancangjiang suture zone (LSLSZ), which would have hindered exhumation owing to the high density of oceanic crust. Subsequent break-off and delamination of the subducted oceanic slab at ~220 Ma may have resulted in exhumation of high-pressure and high-grade metamorphic rocks in the South Qiangtang subterrane. The Late Jurassic ages of metamorphism and deformation obtained in this study indicate the occurrence of an Andean-type orogenic event within the South Qiangtang subterrane. This hypothesis is further supported by an apparent age gap in magmatic activity (150–130 Ma) along the magmatic arc, and the absence of Late Jurassic sediments. 相似文献
16.
《Gondwana Research》2014,25(1):170-189
The Lhasa terrane in southern Tibet is composed of Precambrian crystalline basement, Paleozoic to Mesozoic sedimentary strata and Paleozoic to Cenozoic magmatic rocks. This terrane has long been accepted as the last crustal block to be accreted with Eurasia prior to its collision with the northward drifting Indian continent in the Cenozoic. Thus, the Lhasa terrane is the key for revealing the origin and evolutionary history of the Himalayan–Tibetan orogen. Although previous models on the tectonic development of the orogen have much evidence from the Lhasa terrane, the metamorphic history of this terrane was rarely considered. This paper provides an overview of the temporal and spatial characteristics of metamorphism in the Lhasa terrane based mostly on the recent results from our group, and evaluates the geodynamic settings and tectonic significance. The Lhasa terrane experienced multistage metamorphism, including the Neoproterozoic and Late Paleozoic HP metamorphism in the oceanic subduction realm, the Early Paleozoic and Early Mesozoic MP metamorphism in the continent–continent collisional zone, the Late Cretaceous HT/MP metamorphism in the mid-oceanic ridge subduction zone, and two stages of Cenozoic MP metamorphism in the thickened crust above the continental subduction zone. These metamorphic and associated magmatic events reveal that the Lhasa terrane experienced a complex tectonic evolution from the Neoproterozoic to Cenozoic. The main conclusions arising from our synthesis are as follows: (1) The Lhasa block consists of the North and South Lhasa terranes, separated by the Paleo-Tethys Ocean and the subsequent Late Paleozoic suture zone. (2) The crystalline basement of the North Lhasa terrane includes Neoproterozoic oceanic crustal rocks, representing probably the remnants of the Mozambique Ocean derived from the break-up of the Rodinia supercontinent. (3) The oceanic crustal basement of North Lhasa witnessed a Late Cryogenian (~ 650 Ma) HP metamorphism and an Early Paleozoic (~ 485 Ma) MP metamorphism in the subduction realm associated with the closure of the Mozambique Ocean and the final amalgamation of Eastern and Western Gondwana, suggesting that the North Lhasa terrane might have been partly derived from the northern segment of the East African Orogen. (4) The northern margin of Indian continent, including the North and South Lhasa, and Qiangtang terranes, experienced Early Paleozoic magmatism, indicating an Andean-type orogeny that resulted from the subduction of the Proto-Tethys Ocean after the final amalgamation of Gondwana. (5) The Lhasa and Qiangtang terranes witnessed Middle Paleozoic (~ 360 Ma) magmatism, suggesting an Andean-type orogeny derived from the subduction of the Paleo-Tethys Ocean. (6) The closure of Paleo-Tethys Ocean between the North and South Lhasa terranes and subsequent terrane collision resulted in the formation of Late Permian (~ 260 Ma) HP metamorphic belt and Triassic (220 Ma) MP metamorphic belt. (7) The South Lhasa terrane experienced Late Cretaceous (~ 90 Ma) Andean-type orogeny, characterized by the regional HT/MP metamorphism and coeval intrusion of the voluminous Gangdese batholith during the northward subduction of the Neo-Tethyan Ocean. (8) During the Early Cenozoic (55–45 Ma), the continent–continent collisional orogeny has led to the thickened crust of the South Lhasa terrane experiencing MP amphibolite-facies metamorphism and syn-collisional magmatism. (9) Following the continuous continent convergence, the South Lhasa terrane also experienced MP metamorphism during Late Eocene (40–30 Ma). (10) During Mesozoic and Cenozoic, two different stages of paired metamorphic belts were formed in the oceanic or continental subduction zones and the middle and lower crust of the hanging wall of the subduction zone. The tectonic imprints from the Lhasa terrane provide excellent examples for understanding metamorphic processes and geodynamics at convergent plate boundaries. 相似文献
17.
早古生代原特提斯洋在祁连造山带的分支本文称为古祁连洋。其洋内及邻区存在中祁连、阿拉善、柴达木、华北、扬子、塔里木等多个陆块、微陆块,处在一个复杂的多岛洋的环境中。祁连地区早古生代经历了较为复杂的俯冲拼合、碰撞造山过程。本文探讨了祁连造山带的几个构造单元构造属性,认为早古生代阿拉善微陆块南缘为被动大陆边缘,中祁连北缘为活动大陆边缘。阿拉善南部与之平行的龙首山构造单元为俯冲造山形成的增生楔体;北祁连构造带为一套俯冲增生杂岩,包含高压变质岩带、蛇绿岩带、岛弧岩浆和部分洋壳残片等,记录了古祁连洋壳从大陆裂解,洋壳形成,俯冲拼合,碰撞造山的造山过程。495Ma左右南祁连南部柴达木微陆块向北俯冲的影响,古祁连洋壳俯冲受阻,俯冲带向北后退,形成大岔大坂岛弧。弧前地区发生洋-洋俯冲事件,堆积增生大岔大坂、白泉门、九个泉等SSZ型北祁连蛇绿岩北带,并伴随第二期清水沟、牛心山、野牛滩等地岩浆事件。460Ma左右阿拉善微陆块和中祁连微陆块开始碰撞拼合,古祁连洋开始闭合。值得注意的是拼合过程不是均一的,存在自西向东斜向"剪刀式"的拼合方式,产生了由西向东年代变新的"S"型同碰撞岩浆岩。约440Ma古祁连洋闭合,进入陆内造山阶段。440Ma之后,拼合陆块处在一种拉伸的构造环境之下,金佛寺、牛心山、老虎山等地产生碰撞后岩浆岩。422~406Ma发生俯冲折返、高压榴辉岩和高压低温蓝片岩退变质作用,形成以紧闭不对褶皱为特征的第二幕变形。根据各陆块、微陆块碎屑锆石年龄谱分析对比,中祁连基底应与华北不同,而可能与扬子有关。Rodinia超大陆聚合之前,中祁连微陆块作为一个独立的微陆块与华北、扬子保持一定距离。1.0~0.8Ga Rodinia超大陆聚合过程中祁连微陆块与冈瓦纳北缘拼贴在一起,而距华北较远。随着Rodinia超大陆裂解,中祁连微陆块远离冈瓦纳,逐渐向华北靠近,500~400Ma原特提斯洋闭合,华北、阿拉善与中祁连拼合,并整体拼合到冈瓦纳大陆北缘。 相似文献
18.
Detailed geological mapping, structural, petrological and chronological investigation allow us to place new constraints on the tectono‐thermal evolution of the North Qilian high pressure/low temperature (HP/LT) metamorphic belt. The North Qilian HP/LT metamorphic belt manly consists of eclogite, blueschist, metasedimentary rocks and serpentinite. Most of eclogites and mafic blueschists occur as lenses within metasedimentary rocks, and minor eclogites within serpentinite. Petrological and geochemistical data indicate that the protoliths of eclogite and mafic blueschist includes E‐, N‐MORB, OIB and arc basalt. Geochronology and Lu‐Hf isotope of detrital zircons from metasedimentary rocks indicate the detritus materials are derived from Qilian block and likely deposit in continental margin or fore‐arc basin. Zircon U‐Pb datings show that the protolith ages of eclogites vary between 500 Ma and 530 Ma, and the metamorphic age of eclogite between 460 and 489 Ma. The detrital zircon ages of metasedimentary rocks distribute between 532 and 2700 Ma. The structural data show that the deformation related to the subduction during prograde is recorded in eclogite blocks. In contrast, the dominant deformation structures are characterized by tight fold, sheath fold and penetrative foliation and lineation, which are recorded in various rocks, reflecting a top‐to‐the‐south shear sense and representing the deformation related to the exhumation. The petrological data suggest that the different rocks in the North Qilian HP/LT metamorphic belt equilibrated at different peak metamorphic conditions and recorded different P‐T path. Synthesizing the structural, petrological, geochemical and geochronological data suggest a subduction channel model related to oceanic subduction during Paleozoic in the North Qilian Mountains. The different HP/LT metamorphic rocks formed in different settings with various protolith ages were carried by the subducted oceanic crust into different depth in subduction channel, and experienced independent tectono‐thermal evolution inside subduction channel. The North Qilian HP/LT mélange reflects a fossil oceanic subduction channel. 相似文献
19.
Sub-ophiolite metamorphic rocks from NW Anatolia, Turkey 总被引：4，自引：0，他引：4
The metamorphic rocks from near Kütahya in north-west Anatolia record different stages in the history of closure of the Neo-Tethyan İzmir–Ankara–Erzincan ocean. Sub-ophiolite metamorphic rocks within the Tavşanlı zone are a tectonically composite sequence of quartz–mica schists, amphibole schists, amphibolites and garnet amphibolites. They show increasing metamorphic grade towards the base of the ophiolite. A first metamorphic event, typical of sub-ophiolite metamorphic sole rocks, was characterized by high-grade assemblages, and followed by retrograde metamorphism. A second event was marked by a medium-to high-pressure overprint of the first-stage metamorphic assemblages with assemblages indicating a transition between the blueschist and greenschist facies. The chemistry of the sub-ophiolite metamorphic rocks indicates an ocean island basalt origin, and Ar–Ar dating indicates a high temperature metamorphic event at 93±2 Ma. Counter-clockwise P–T–t paths recorded by the sub-ophiolite metamorphic rocks are interpreted to result from intra-oceanic thrusting during the closure of the İzmir– Ankara–Erzincan ocean, initiating subduction, which formed the high-temperature assemblages. Further subduction then produced the widespread blueschists of the Tavşanlı zone during the Late Cretaceous. Later cold thrusting obducted the ophiolite (with the metamorphic sole welded to its base), ophiolitic melanges and blueschists onto the Anatolide passive margin in the latest Cretaceous. All these events pre-date the final Anatolide–Pontide continent–continent collision. 相似文献
20.
Geochemical, SrNd isotopic and geochronological methods were used to reveal the protoliths and geodynamic implications of the newly discovered Triassic Baqing eclogites, eastern Qiangtang terrane, central Tibet. We assessed the mobility of trace elements, and it turned out that high field strength elements (HFSEs) and rare earth elements (REEs) were immobile in the high-pressure (HP) metamorphic process and reliably employed to discriminate the protoliths of the Baqing eclogites. Whole-rock geochemical data (especially immobile elements) suggested both arc-related (negative NbTa anomalies, high light (L) REEs/heavy (H) REEs ratios) and mid-oceanic ridge basalt (MORB)-related (without negative NbTa anomalies, relatively low REEs contents) characteristics, indicating the contribution of different proportions of subduction material with a backarc basin origin. According to the SrNd isotopic ratios (initial 87Sr/86Sr ratios: 0.7078–0.7086; εNd(t) values: −0.58 to +1.96), the protoliths of the Baqing eclogites originated from mantle which experienced continental crustal contamination in the subduction zone. Compared with nearby subduction-related magmatic rocks from eastern Qiangtang terrane, backarc basin was the most likely tectonic environment for the Baqing eclogite protoliths. The Jinsha Paleo-Tethyan Ocean southward subduction underneath East Qiangtang block (EQB) and rollback led to the formation of this backarc basin between the Late Permian and Early Triassic, and subsequent northward subduction of the backarc basin formed the Baqing eclogites in early Late Triassic (227–221 Ma). 相似文献