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1.
The Muju area, located on the north–central margin of the Yeongnam Massif, mainly consists of Precambrian orthogneisses (granitic, leucogranitic, augen and dioritic gneisses) with minor migmatite. Zircon U–Pb dating indicates that the protoliths of the orthogneisses intruded at ca. 2.00–1.97 Ga and were metamorphosed at ca. 1.87–1.86 Ga. Magmatic zircon grains within the orthogneisses have positive to negative εHf(t) values (−7.63 to +3.3) and a Neoarchean two-stage model age (TDM2 = 2.78 Ga), indicating that the protoliths of most of the orthogneisses may have been derived from Archean crustal material. The results of geochemical analysis indicate that the protoliths of the orthogneisses formed by partial melting of metagraywacke and mafic igneous rocks in an arc-related tectonic setting. The intrusion ages and geochemical data of the Paleoproterozoic orthogneisses in the study area match well with those of Paleoproterozoic (ca. 2.00–1.97 Ga) orthogneisses in the northeastern Yeongnam Massif, indicating the presence of regional Paleoproterozoic subduction zones along the northern margin of the Yeongnam Massif at ca. 2.00–1.97 Ga. Meanwhile, ca. 2.00–1.97 Ga subduction-related magmatism has not been reported from the northern Gyeonggi and Nangrim Massifs in the Korean Peninsula or the Jiao–Liao–Ji belt in the eastern North China Craton, indicating that the Yeongnam Massif may not be correlatable with the northern Gyeonggi and Nangrim Massifs or the Jiao–Liao–Ji belt. The Yeongnam Massif may be correlated with the Cathaysia Block in the South China Craton and may have been located near Laurentia and the Siberian Craton within the Columbia supercontinent.  相似文献   

2.
赵燕  第五春荣  孙勇  朱涛  王洪亮 《岩石学报》2013,29(5):1698-1712
敦煌杂岩位于塔里木克拉通的东部,探寻和研究其中的早前寒武纪地质体对于探讨敦煌地块早前寒武纪地壳的形成和演化及其构造归属等问题具有重要的意义.甘肃敦煌水峡口地区的敦煌杂岩主要由英云闪长质片麻岩、花岗闪长质片麻岩以及表壳岩石组成.利用LA-ICP-MS锆石U-Pb定年方法测得水峡口英云闪长片麻岩和花岗片麻岩原岩的形成年龄分别为2561±16Ma和2510±22Ma,确证了在敦煌杂岩中存在太古宙岩石.此外,还获得斜长角闪岩的变质年龄为1806±14Ma,推测其原岩岩浆可能来自古老岩石圈地幔的部分熔融.根据已有的资料提出在古元古代晚期(1.80 ～ 1.85Ga)敦煌杂岩经历了一期较广泛的变质作用.锆石Hf同位素显示～2.5Ga的岩石年龄在敦煌地块代表新太古代晚期重要地壳生长时期,而～1.8Ga的构造-热事件则是以古老地壳物质循环再造为主.这些资料显示敦煌地块和华北克拉通在早前寒武纪经历了类似的地壳形成和演化过程,且共同记录了全球性的Columbia碰撞造山事件信息.  相似文献   

3.
Archean basement gneisses and supracrustal rocks, together with Neoproterozoic (Sinian) metasedimentary rocks (the Penglai Group) occur in the Jiaobei Terrane at the southeastern margin of the North China Craton. SHRIMP U–Pb zircon dating of an Archean TTG gneiss gave an age of 2541 ± 5 Ma, whereas metasedimentary rocks from the Neoproterozoic Penglai Group yielded a range in zircon ages from 2.9 to 1.8 Ga. The zircons can be broadly divided into three age populations, at: 2.0–1.8 Ga, 2.45–2.1 Ga and >2.5 Ga. Detrital zircon grains with ages >2.6 Ga are few in number and there are none with ages <1.8 Ga. These results indicate that most of the detrital material comes from a Paleoproterozoic source, most likely from the Jianshan and Fenzishan groups, with some material coming from Archean gneisses in the Jiaobei Terrane. An age of 1866 ± 4 Ma for amphibolite-facies hornblende–plagioclase gneiss, forming part of a supracrustal sequence within the Archean TTG gneiss, indicates Late Paleoproterozoic metamorphism. Both the Archean gneiss complex and Penglai metasedimentary rocks resemble previously described components of the Jiao-Liao-Ji orogenic belt and suggest that the Jiaobei Terrane has a North China Craton affinity; they also suggest that the time of collision along the Jiao-Liao-Ji Belt was at 1865 Ma.  相似文献   

4.
The paper is a first attempt to unravel the Archean multi-stage metaplutonic assemblage of the Meso/Neoarchean terrane of the State of Goiás, Central Brazil, by means of the U–Pb SHRIMP zircon and Sm–Nd techniques. Two stages of granitic plutonism, spanning ca. 140 m.y., were precisely established for the accretion of the gneiss protoliths. The earliest stage embraces tonalitic to granodioritic and minor granitic orthogneisses with Nd juvenile signature, emplaced from ca. 2845 to ca. 2785 Ma, interpreted as the roots of an early arc. Inherited zircon xenocrysts and Nd isotopic data indicate that the juvenile magmas underwent contamination from a sialic crust as old as 3.3 Ga, from which there are, so far, no recognizable exposures. The second stage comprises granodioritic to granitic gneisses and lasted from ca. 2711 to 2707 Ma. Based on their Nd isotopic signatures and on inherited zircon crystals, their protoliths are interpreted as dominantly crustal-derived. The SHRIMP data from zircon crystals did not depict a Paleoproterozoic overprinting on the Archean gneisses, which is due to geological processes with prevailing temperatures below the isotopic stability of the U/Pb/Th system in the mineral. These processes comprise crustal extension and intrusion of a mafic dike swarm at ca. 2.3 Ga, followed by low grade events mostly related to shear zones between ca. 2.15 and 2.0 Ga. The study also revealed the extent of the Pan- African tectono-thermal overprinting on the Archean orthogneisses. Most of the zircon populations show morphological evidence of metamorphic peripheral recrystallization dated between ca. 750 and 550 Ma. One of the banded gneisses with a crystallization age of ca. 2700 Ma (2σ) has a more complex zircon population including magmatic new grains, which yielded a precise 206Pb/238U crystallization age of 590 ± 10 Ma (2σ). These new grains are interpreted to have grown in anatectic veins injected within strongly sheared gneiss.The data characterize a widespread Pan-African-aged metamorphic overprinting, culminating with localized anatexis of the Archean orthogneisses.  相似文献   

5.
冀北太古代花岗质片麻岩的成因   总被引：6，自引：6，他引：0  
冀北是华北克拉通早前寒武纪变质基底的主要出露地区之一。分布于承德-滦平和赤城-张家口地区的新太古代花岗质片麻岩主要由英云闪长岩、奥长花岗岩、花岗闪长岩和二长花岗岩(TTGM)组成,构成了低钾、中钾和高钾钙碱性三个岩石化学系列。二长花岗质片麻岩的LA-ICP-MS锆石U-Pb和Lu-Hf同位素特征揭示其岩浆结晶年龄为2509±10Ma。全岩岩石化学、Sm-Nd同位素和锆石Lu-Hf同位素研究表明:(1)低钾钙碱性系列的岩石形成于拉班玄武质岩石的低度部分熔融;(2)中钾钙碱性系列岩石主要形成于玄武质岩石和杂砂岩的部分熔融,局部存在英云闪长质片麻岩的部分熔融;(3)高钾钙碱性系列的岩石形成于以高钾中酸性火山岩为主要成分的杂砂岩的部分熔融。结合近年来冀北早前寒武纪地质研究成果,这些太古代花岗质片麻岩全岩Sm-Nd同位素和锆石Lu-Hf同位素特征揭示~2.7Ga是本区太古代地壳的主要生长期。在新太古代发生了大规模的火山喷发,火山物质形成后不久发生部分熔融形成花岗质岩浆,接着发生变质、变形作用。这些花岗质片麻岩的形成与南美洲西海岸的构造-岩浆活动特征有类似之处,可能反映了太古代末期冀北地区从活动大陆边缘地壳增生、加厚到弧后伸展转化的动力学背景。  相似文献   

6.
U–Pb zircon geochronology of two Permo-Triassic granites (samples OT-52 and OT-272 with ages of 229 ± 8 Ma and 256 ± 2 Ma, respectively) in the Unazuki area, Hida Metamorphic Belt, southwest Japan, revealed the presence of Eoarchean to Paleoproterozoic inheritance. Inheritance is consistent with both samples showing low zircon saturation temperatures for their bulk compositions. In OT-52, dark in CL, low Th/U zircon domains have a mean 207Pb/206Pb age of 1940 ± 17 Ma, which is consistent with an age of 1937 ± 6 Ma for anatexis in the Precambrian Busan gneiss complex in Korea. Eoarchaean inherited zircons with 207Pb/206Pb ages from ca. 3750 to 3550 Ma are common in OT-272 but are few in OT-52, suggesting a source from rocks with affinities to those in the Anshan area in the northeast China part of the North China Craton. On the other hand, a Hida Metamorphic Belt metasedimentary gneiss into which the granites were intruded contains ca. 1840, 1130, 580, 360, 285 and 250 Ma zircons (Sano et al., 2000). These ages suggest that the Unazuki Mesozoic granites did not originate from proximal Hida Metamorphic Complex rocks, but instead from unrelated rocks obscured at depth. The predominance of Eoarchean to Paleoproterozoic age components, and the marked lack of 900–700 Ma components suggest that the source was the (extended?) fringe of the North China Craton, rather than from Yangtze Craton crust. The Mesozoic evolution of Japan and its linkages to northeast Asia are discussed in the context of these results.  相似文献   

7.
塔里木盆地东南缘的阿克塔什塔格地区,保存有较为完好的早前寒武纪基底变质岩——阿克塔什塔格杂岩,主要由米兰岩群、新太古代TTG花岗片麻岩和侵入其中的各类古元古代花岗片麻岩构成。其中米兰岩群和TTG片麻岩发育塑性流变褶皱和高角闪岩相-麻粒岩相变质,具有强烈的混合岩化,并遭受后期的角闪岩相变质改造。米兰岩群中的长英质片麻岩和TTG岩系的锆石SHRIMP U-Pb年龄分别为(2 567±32)Ma和(2 592±15)Ma,二者普遍低Si高Al、富Na贫K、富Sr贫Mg、富集LILE和LREE,亏损HSFE和HREE、轻重稀土分馏强烈、Eu异常不明显,具有类似于埃达克岩的岩石地球化学特征,表明它们形成于俯冲带的岛弧环境,为岛弧玄武岩俯冲至下地壳部分熔融的产物,指示了塔里木盆地东南缘新太古代晚期古老克拉通的大陆地壳水平增生。在此基础上,文章还探讨了塔里木盆地周缘早前寒武纪基底岩系的年代格架问题,认为塔里木盆地具有统一的早前寒武纪变质基底。  相似文献   

8.
王盟  钱加慧  张进江  张波 《地学前缘》2019,26(3):171-182
恒山五台阜平地区地处华北克拉通中部造山带的中段,早前寒武纪地体出露较好,是解析华北克拉通早前寒武纪演化过程的最佳地段。龙泉关剪切带位于五台杂岩和阜平杂岩的交接部位,其主要岩石类型为眼球状花岗质片麻岩、变粒岩和斜长角闪岩。本文利用LA-ICP-MS方法对龙泉关剪切带中的眼球状花岗质片麻岩进行了锆石U-Pb年龄和Lu-Hf同位素的测试。结果显示,龙泉关眼球状花岗质片麻岩原岩结晶年龄为(2 547±7) Ma,其εHf(t)值介于+2.7~+9.2,表明其源区主要为相对年轻的地壳物质,与五台地区新太古代花岗岩非常相似。龙泉关花岗质眼球状片麻岩中锆石两阶段Hf模式年龄(TDM2)为2 477~2 872 Ma,具有2.58 Ga、2.71 Ga和2.81 Ga三个峰值,记录了华北克拉通新太古代初始地壳生长事件。  相似文献   

9.
对胶东谭格庄地区一地质剖面进行了详细野外观察和锆石SHRIMP年代学研究.剖面主要是由奥长花岗质片麻岩和斜长角闪岩组成,两者互层产出并一起发生褶皱变形,在强变形域两者完全平行化.所以,在强变形地区,不同成分岩性的薄层状互层产出不能作为变质沉积岩的识别标志.奥长花岗质片麻岩(S1105)形成于2496±10Ma,遭受了古元古代早期和晚期变质.两个斜长角闪岩样品(S1238和S0812)都只发育变质锆石,年龄分别为1842±3Ma和1833±13Ma.该剖面岩石记录的两期变质作用在胶东地区具有普遍意义.结合前人研究,作者指出该地区太古宙末存在区域性变质深熔作用,早期地壳受到了强烈的再造;古元古代末的变质作用具有带状分布特点,与造山作用有关.  相似文献   

10.
The Spessart Crystalline Complex, north-west Bavaria contains two orthogneiss units of granitic to granodioritic composition, known as the Rotgneiss and Haibach gneiss, respectively, which are structurally conformable with associated metasediments. The igneous origin of the Rotgneiss is apparent from field and textural evidence, whereas strong deformation and recrystallization in the Haibach gneiss has obscured most primary textures. New geochemical data as well as zircon morphology prove the Haibach gneiss to be derived from a granitoid precursor, which was chemically similar to the Rotgneiss protolith, thus suggesting a genetic link between those two rock units. Both gneiss types have chemical compositions typical of anatectic two-mica leucogranites. They show characteristics of both I- and S-type granites. Rb-Sr whole rock data on the Haibach gneiss provide an isochron age of 407±14 Ma (IR = 0.7077±0.0007; MSWD 2.2), which is slightly younger than the published date for the Rotgneiss (439±15 Ma; IR=0.7048±0.0026; MSWD 4.9). Single zircon dating of six idiomorphic grains, using the evaporation method, yielded a mean 207Pb/206Pb age of 410±18 Ma for the Haibach gneiss and 418±18 Ma for the Rotgneiss. Both zircon ages are within analytical error of the Rb-Sr isochron dates and are interpreted to reflect the time of protolith emplacement in Silurian times. Three xenocrystic zircon grains from the Rotgneiss yielded 207Pb/206Pb ages of 2278±12, 2490±13 and 2734±10 Ma, respectively, suggesting that late Archaean to early Proterozoic crust was involved in the generation of the granite from which the Rotgneiss is derived. Although it is assumed that the granitic protoliths of the two gneisses were formed through anatexis of older continental crust, the relatively low 87Sr/86Sr initial ratios of both gneisses may also indicate the admixture of a mantle component. The Rotgneiss and the Haibach gneiss thus document granitic magmatism at an active continental margin during late Silurian times.  相似文献   

11.
全吉群是不整合于柴达木北缘全吉(欧龙布鲁克)地块变质基底之上的第一套盖层型沉积,目前多数研究者认为其时代大体为新元古代晚期。本文在柴北缘全吉山的全吉群红藻山组建组剖面上,对位于该组下部的两层凝灰岩开展了LA-MC-ICPMS锆石U-Pb同位素定年研究,获得其岩浆锆石的U-Pb同位素年龄分别为(1 640±15) Ma和(1 646±20) Ma,标定了红藻山组的形成年龄,为柴北缘中新元古界年代地层的重新厘定与划分,提供了新的、直接的年代学约束,具有重要的科学意义。首先,这一新进展表明,在柴北缘地区,传统的全吉群红藻山组以下各组的时代,并非一直以来的“新元古代晚期南华纪-震旦纪(埃迪卡拉纪)”,而应属于国内的中元古代长城纪、国际上的古元古代固结纪(Statherian,1 800~1 600 Ma)晚期;其次,根据当前定年并结合新近在红藻山组与上覆地层之间发现有古风化壳及区域性不整合、全吉群上部发育有埃迪卡拉纪化石及冰碛砾岩等证据,本文进一步确认,柴北缘全吉群在红藻山组与其上覆黑土山组等地层单位之间其实存在着相当长时间的地层缺失,传统的“全吉群”应予解体。由此并参考已有资料,本文认为,早先代表柴达木地块形成的“全吉运动”并不能与华南的“晋宁运动”对比,而应与华北的“吕梁运动”相当,全吉地块乃至整个柴达木及周边地区的中、新元古代地质演化历史及其大地构造亲缘属性等,都应重新予以考虑。  相似文献   

12.
赵磊  张艳斌  杨正赫  韩龙渊  金正男 《岩石学报》2016,32(10):2948-2964
朝鲜狼林地体是中朝克拉通的重要组成部分,其早前寒武纪基底岩系的岩石组成、形成和变质作用时代,直接影响到人们对古元古代辽吉活动带乃至整个华北克拉通地质演化历史的理解和认识。本文选取位于狼林地块东南缘,狼林群变质杂岩中的3个花岗片麻岩样品进行锆石U-Pb定年分析。锆石的LA-ICPMS和SIMS定年结果显示,3个片麻岩原岩的岩浆结晶时代为2521~2567Ma,并且它们都经历了古元古代1.87~1.89Ga的构造热事件改造,表现为原始岩浆锆石不同程度的铅丢失以及变质锆石的形成。基于这一认识,并结合其他学者的最新研究结果可知,太古宙片麻岩存在于狼林地块平南盆地的南缘和东缘地区,由此推测狼林地块太古宙基底岩系的规模可能远大于目前所识别的这几个地区。这些太古宙片麻岩普遍遭受了古元古代强烈变质作用(深熔作用)改造,并且变质作用的时代可以与华北克拉通三条古元古代活动带的变质-深熔作用时代相对比,表明狼林地块可能至少在古元古代之前,就已经与华北克拉通组成了统一的大陆。  相似文献   

13.
吕梁地区古元古代岩浆作用非常强烈,其成因与构造环境对客观恢复华北克拉通早期地质演化具有重要意义。本文选择了杜家沟长石斑岩、吕梁群近周峪组中基性火山岩和恶虎滩片麻岩进行了较系统研究。2个杜家沟长石斑岩的SHRIMP锆石U-Pb年龄分别为2189±6Ma和2186±3Ma,恶虎滩闪长质片麻岩的SHRIMP锆石U-Pb年龄为2182±5Ma。杜家沟长石斑岩为A型花岗岩,具有造山后花岗岩特征;吕梁群近周峪组中基性火山岩具有岛弧火山岩的地球化学特征;恶虎滩片麻岩也具有岛弧岩浆岩特征。综合分析华北克拉通2.2～2.0Ga的岩浆岩构造性质认为,吕梁地区岩浆性质的复杂性可能与所处的构造位置有关。古元古代吕梁地区可能位于华北克拉通古陆块边缘,其2.2～2.1Ga的岩浆岩可能经历岛弧和裂谷两种体制的共同制约。  相似文献   

14.
We present the first precise age for Precambrian basement rocks in Bangladesh. These lie within the Dinajpur block, located between the Indian Craton to the southwest and the Shillong Massif to the east. There are no surface outcrops and the rocks were intersected by drill holes. They consist of a suite of tonalitic and granodioritic rocks, variously deformed to granitic gneiss and intruded by younger monzogranite. A tonalite obtained at a depth of 227.48 m in drill hole BH-2 at Maddhapara, in northwestern Bangladesh, records a SHRIMP zircon 207Pb/206Pb magmatic age of 1722 ± 6 Ma. Paleoproterozoic rocks with similar magmatic ages are unknown in the adjacent Indian blocks of the Chotanagpur Plateau (Indian Craton) or Shillong Massif. This lack of comparable ages may be due to the paucity of precise radiometric ages from the Indian terrains or, more likely, because there are real age differences, with the buried rocks at Maddhapara representing a separate and discrete microcontinental fragment (the Dinajpur block) that was trapped by the northward migration of India during Gondwana dispersal.  相似文献   

15.
《International Geology Review》2012,54(13):1772-1790
The Quanji Massif (QM), in the northeast part of Tibet, consists of Palaeoproterozoic metamorphic rocks, granitoids, and mafic dikes. U–Pb dating of a diorite gneiss and a mafic dike in the QM yielded a crystallization age of 2272 ± 15 Ma and a metamorphic age of 1928 ± 11 Ma, respectively. Although some post-emplacement alteration has occurred, the mafic dikes display a sub-alkaline signature with slight light rare earth element-enrichment, depletion in Th, Nb, Ta, and Ti, and have a rare earth element pattern consistent with volcanic arc basalts. Based on the geochronology and field relationships, we conclude that the mafic dikes formed in an extensional setting within either a fore-arc or back-arc environment. We argue that the metamorphism that affected the dikes occurred shortly after intrusion. Our diorite gneiss and monzodiorite samples are characterized by relatively high Mg# (47–56) and Sr contents (367–1070 ppm), low-to-moderate Sr/Y (10–90), low Rb/Sr (0.03–0.28) and high K/Rb (179–775). These felsic melts likely originated from partial melting of a mafic source. Our new data, combined with results from previous studies, indicate that the QM formed between 2.50 and 2.30 Ga and underwent metamorphism around 1.95–1.75 Ga that may relate to the dispersal of Neoarchaean ‘Kenorland’ and the formation of the Columbia supercontinent. The similarity between the Palaeoproterozoic events in the Tiekelik, North Altyn–Dunhuang, Alashan blocks, and QM suggests that QM was part of either the Tarim or the North China Craton in the late Archaean and Palaeoproterozoic. If the model is correct, then there was a single ‘North China–Quanji–Tarim Craton’ that was later disrupted by Neoproterozoic to Phanerozoic tectonic events.  相似文献   

16.
原划分的阿拉善群包含了时代、成因各异、构造样式有别、变质程度不同的变质地层和变形深成片麻岩体。作为一个岩石地层单元已不合适,应予解体并重新厘定。根据目前的研究,原阿拉善群可划分成3套变质地层单元和两个变形深成片麻岩杂岩。本区最古老的岩石地层单元为迭布斯格岩群,由深变质的变质表壳岩系组成,是区内的基底岩系。初步的锆石SHRIMPU-Pb年代学研究表明,它们形成于27亿年左右,属于新太古代。新建立的巴彦乌拉山岩组主要由变质火山岩组成,是区内的火山岩系。目前获得的年龄为2264Ma和2271Ma,属于古元古代。重新厘定的阿拉善岩群主要由变质碎屑岩和变质碳酸盐岩组成,是陆缘沉积岩系,形成于古元古代晚期或更晚。波罗斯坦庙英云闪长质-花岗质片麻岩杂岩,由早期的英云闪长质片麻岩和晚期的花岗质片麻岩组成。早期英云闪长质片麻岩形成于18亿年左右,晚期的花岗质片麻岩大致形成于古生代期间。毕及格台花岗闪长质片麻岩杂岩主要由角闪斜长片麻岩、黑云斜长片麻岩等组成,初步的同位素年龄测定结果显示它们的主期形成于8￣10亿年的晋宁期。  相似文献   

17.
《Gondwana Research》2008,13(4):380-387
We present the first precise age for Precambrian basement rocks in Bangladesh. These lie within the Dinajpur block, located between the Indian Craton to the southwest and the Shillong Massif to the east. There are no surface outcrops and the rocks were intersected by drill holes. They consist of a suite of tonalitic and granodioritic rocks, variously deformed to granitic gneiss and intruded by younger monzogranite. A tonalite obtained at a depth of 227.48 m in drill hole BH-2 at Maddhapara, in northwestern Bangladesh, records a SHRIMP zircon 207Pb/206Pb magmatic age of 1722 ± 6 Ma. Paleoproterozoic rocks with similar magmatic ages are unknown in the adjacent Indian blocks of the Chotanagpur Plateau (Indian Craton) or Shillong Massif. This lack of comparable ages may be due to the paucity of precise radiometric ages from the Indian terrains or, more likely, because there are real age differences, with the buried rocks at Maddhapara representing a separate and discrete microcontinental fragment (the Dinajpur block) that was trapped by the northward migration of India during Gondwana dispersal.  相似文献   

18.
新疆大陆基底分区模式和主要地质事件的划分   总被引：38，自引：5，他引：38  
在同位素年代学和地球化学研究的基础上，概括了1982～2000年间的同位素年代学和地球化学研究成果，特别是1987～2000年"305"项目的研究成果。展现了近年来对新疆大陆前寒武纪基底同位素年代学研究的新成果塔里木北缘灰色片麻岩锆石U-Pb年龄为2600Ma，西昆仑基底石榴黑云母片麻岩中的锆石U-Pb一致年龄则为 （2048±20）Ma，阿尔金灰色片麻岩锆石U-Pb上交点年龄为（1820±277）Ma，测定了东天山星星峡群片麻岩的锆石U-Pb上交点年龄为（1404±18）Ma，与1986年的结果一致（1400±42）Ma，获得西天山温泉群、那拉提群和木扎尔特群混合岩化片麻岩中锆石U-Pb年龄分别为（821±11）～（798±8）Ma，（882±83）Ma和（707±7）Ma；基于Sm-Nd模式年龄统计结果，将新疆大陆基底分为5个区域，即塔里木大陆太古宙－古元古代（3.2～2.2）Ga基底区，昆仑-阿尔金造山带古元古代基底区（2.0～1.8）Ga，天山古中元古代基底区（2.1～1.7）Ga，准噶尔为年轻地壳基底区（1.4～0.7）Ga和阿尔泰古元古代、中新元古代复合基底区（≤2.6～2.4Ga,1.5～0.9Ga）；基于多年研究的积累，并综合了国内外一些可以应用的同位素年代学研究结果，提出以塔里木太古宙大陆地核向南、北逐步增生的新疆大陆地壳基底演化模式；确定了新疆大陆地壳构造演化中15次主要地质事件的时限为（3000～3200）Ma；2800Ma；2600～2500Ma；2200Ma；2000～1700Ma；1400Ma；1000Ma;；800Ma；700～>500Ma；520～480Ma；450Ma；360～300Ma；300～250Ma；210～135Ma；65～5Ma。这些同位素年代学和Nd同位素示踪研究结果无疑将成为进一步探讨新疆大陆地壳构造演化的一些重要依据。  相似文献   

19.
Two apparently distinct, sub-parallel, paleo-subduction zonescan be recognized along the northern margin of the Tibetan Plateau:the North Qilian Suture Zone (oceanic-type) with ophioliticmélanges and high-pressure eclogites and blueschistsin the north, and the North Qaidam Belt (continental-type) inthe south, an ultrahigh-pressure (UHP) metamorphic terrane comprisingpelitic and granitic gneisses, eclogites and garnet peridotites.Eclogites from both belts have protoliths broadly similar tomid-ocean ridge basalts (MORB) or oceanic island basalts (OIB)in composition with overlapping metamorphic ages (480–440Ma, with weighted mean ages of 464 ± 6 Ma for North Qilianand 457 ± 7 Ma for North Qaidam), determined by zirconU–Pb sensitive high-resolution ion microprobe dating.Coesite-bearing zircon grains in pelitic gneisses from the NorthQaidam UHP Belt yield a peak metamorphic age of 423 ±6 Ma, 40 Myr younger than the age of eclogite formation, anda retrograde age of 403 ± 9 Ma. These data, combinedwith regional relationships, allow us to infer that these twoparallel belts may represent an evolutionary sequence from oceanicsubduction to continental collision, and continental underthrusting,to final exhumation. The Qilian–Qaidam Craton was probablya fragment of the Rodinia supercontinent with a passive marginand extended oceanic lithosphere in the north, which was subductedbeneath the North China Craton to depths >100 km at c. 423Ma and exhumed at c. 403 Ma (zircon rim ages in pelitic gneiss). KEY WORDS: HP and UHP rocks; subduction belts; zircon SHRIMP ages; Northern Tibetan Plateau  相似文献   

20.
The Epupa Metamorphic Complex constitutes the southwestern margin of the Congo Craton and is exposed in a hilly to mountainous terrain of northwestern Namibia, bordering the Kunene River and extending into southern Angola. It consists predominantly of granitoid gneisses which are migmatized over large areas. This migmatization locally led to anatexis and produced crustal-melt granites such as the Otjitanda Granite. We have undertaken reconnaissance geochemical studies and single zircon U–Pb SHRIMP and Pb–Pb evaporation dating of rocks of the Epupa Complex. The granitoid gneisses, migmatites and anatectic melts are similar in composition and constitute a suite of metaluminous to peraluminous, calc-alkaline granitoids, predominantly with volcanic arc geochemical signatures. The zircon protolith ages for the orthogneisses range from 1861 ± 3 to 1758 ± 3 Ma. Anatexis in the migmatitic Epupa gneisses was dated from a melt patch at 1762 ± 4 Ma, and the anatectic Otjitanda Granite has a zircon age of 1757 ± 4 Ma. Migmatization and anatexis therefore occurred almost immediately after granitoid emplacement and date a widespread high-temperature Palaeoproterozoic event at ∼1760 Ma which has not been recorded elswhere in northern Namibia. The Nd isotopic systematics of all dated samples are surprisingly similar and suggest formation of the protolith from a source region that probably separated from the depleted mantle about 2.4–2.0 Ga ago. A major Archaean component in the source area is unlikely.  相似文献   

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