青海拉鸡山构造带是裂谷还是构造窗——与王二七研究员商榷

拉鸡山构造带东西长逾650km,展布于由元古宇组成的结晶地块内部，是一条早古生代火山岩极为发育的构造带。长期以来众多学者对拉鸡山进行大地构造学、岩石学研究。作者曾在拉鸡山从事多年野外调查工作，研究认为该地区无论在区域地质学、岩石学、构造变形学还是在大地构造演化方面其早古生代构造演化史皆具典型的裂谷带特征。晚古生代以来，经历了陆内多阶段造山过程，而为后期多阶段抬升的构造窗观点值得商榷。     

拉鸡山裂谷带特征及演化

拉鸡山构造带东西长逾 6 50km ,展布于由元古宇组成的结晶地块内部 ,是一条早古生代火山岩极为发育的构造带。通过作者在拉鸡山多年的野外调查工作和研究 ,认为该地区无论在区域地质学、岩石学、构造变形学还是大地构造演化方面 ,其早古生代构造演化史皆具典型的裂谷带特征。晚古生代以来 ,区内经历了陆内多阶段造山过程。新生代早更新世 ,拉鸡山巨型断裂带发生左旋走滑运动 ,导致断裂带弧形转折部位强烈崛起 ,形成雄伟高山。     

北祁连山及其邻区古生代以来的大地构造演化初探

北祁连山出露有中寒武世中晚期和早中奥陶世两期蛇绿岩。同位素年龄分别为335.5 Ma和440-460 Ma的两期高压变质带赋存于中寒武统和下中奥陶统中。大量地质记录揭示北祁连山是介于阿拉善地块和中祁连地块间的一个早古生代缝合带。北祁连山及其邻区古生代以来大地构造演化是复杂的。中寒武世早期统一的中国古陆经陆内裂谷作用发生裂解,于中寒武世中晚期形成北祁连洋。到晚寒武世洋盆转化成残留海盆。早奥陶世北祁连地区再次拉张,遭受第二次大洋化,到中奥陶世形成具沟弧盆体系的成熟大洋。晚奥陶世洋盆转化成残留海盆。于晚奥陶世末碰撞成山,志留纪在新生褶皱山系前形成前陆盆地,盆地底部的下志留统下部层位的磨拉斯可视为碰撞造山的标志。到泥盆纪进入碰撞期后造山阶段,泥盆纪磨拉斯则作为碰撞期后造山作用的标志。北祁连山及其邻区经历了石炭-三叠纪上叠盆地发展阶段,到侏罗纪进入陆内造山阶段。陆内造山作用的主要特征是山脉抬升、盆地沉降,形成盆-岭构造。这个作用一直持续到现在。笔者还对碰撞作用和造山作用的类型进行了讨论。认为软碰撞(soft collision)引起洋盆闭合,但不造山,硬碰撞(rigid collision)使残留海盆闭合并形成新生褶皱山系。在北祁连可以辨认出碰撞造山、碰撞期后造山及陆内造山这3种类型各具特征的造山作用。     

北祁连早古生代洋盆是裂陷槽还是大洋盆——与葛肖虹讨论

北祁连早古生代洋盆是裂陷槽还是大洋盆,是一个有争论的问题。文中讨论了蛇绿岩中保存的代表大洋盆存在的印记。认为被夹持于华北地块和柴达木地块之间的北祁连早古生代造山带属于板块增生杂岩带,是由海洋岩石圈残片、消减杂岩、岛弧增生楔等组成的(或许还包括一部分残留陆块).指出北祁连蛇绿岩属于科迪勒拉型,暗示北祁连在早古生代时可能曾经是一个规模较大的洋盆,而非裂陷槽。华北地块和柴达木地块的规模很小,只不过是浩瀚海洋中散布的微小陆块而已。     

北祁连山俯冲杂岩带的构造演化

北祁连位位于华北克拉西部阿拉善地块与中祁连－柴达木泛地块之间是我国最具特色的大陆造山带之一。带内发育有震旦纪－中寒武世的裂谷火山岩，晚寒武世－奥陶纪蛇绿岩，中晚奥陶世岛弧火山岩，晚奥陶世弧后拉张盆地火山－沉积岩，志留纪残余海盆相复理石和泥盆纪山间磨粒石等，中间夹两条变质和变形特征不同的加里东期俯冲杂岩带；南带为深层俯冲，北带为浅层俯冲杂岩；这两条杂岩石可能形成于同一俯冲带的不同深度，俯冲杂岩带中岩     

北补连蛇绿岩的特征,形成环境及其构造意义

文中总结了北祁连蛇绿岩的特征，指出北祁连蛇绿岩大多具有ＭＯＲＢ的性质，有玻安岩产生，形成在弧后和岛弧环境，北祁连蛇绿岩大多侵位在岛弧增生楔或活动陆缘地体之上，蛇绿岩属于科迪勒拉型，早古生代的北祁连造山带属于科迪勒拉型造山带，部分蛇绿岩之上整合产出一套沉积一火山岩系，称为蛇绿岩的上覆岩系，指出蛇绿岩及其上覆岩系的枕状熔岩分别来自不同的源区，具有不同的构造意义，还讨论了北祁连早古生代板块构造格局，认为     

东亚原特提斯洋(Ⅴ):北界西段陆缘属性及微陆块拼合

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。     

关于祁连山东段地块的构造叠加和折返的讨论——答左国朝研究员等的质疑

左国朝等人对我们的“青海拉鸡山:一个多阶段抬升的构造窗”一文(王二七等,2000)中的观点提出质疑。这种学术争鸣有利于促进祁连山甚至广义的造山带的研究,我们表示欢迎并感谢《地质论评》给我们以答辩的机会。 一些人包括左国朝等人用裂谷的模式来解释拉鸡山的成因,认为其构造演化与中祁连地块的开合构造有关。粗略地看,拉鸡山确实像裂谷:早古生代的蛇绿岩夹在两个非常     

西秦岭北缘早古生代天水—武山构造带及其构造演化

西秦岭北缘早古生代天水-武山构造带位于甘肃省东部天水地区,主要由寒武纪关子镇-武山蛇绿岩带、晚寒武世-早奥陶世李子园群浅变质活动陆缘沉积-火山岩系、奥陶纪草滩沟群岛弧型火山-沉积岩系以及加里东期岛弧型深成侵入岩体、俯冲-碰撞型花岗岩体等组成.关子镇蛇绿岩中变质基性火山岩属于N-MORB型玄武岩,武山蛇绿岩中变质基性火山岩属于E-MORB型玄武岩,是洋脊型蛇绿岩的重要组成部分,形成时代大致在534～489Ma之间的寒武纪.李子园群火山岩主要形成于岛弧或与岛弧相关的弧前盆地构造环境,草滩沟群火山岩形成于与俯冲作用相关的岛弧环境.关子镇流水沟和百花中基性岩浆杂岩总体形成于中晚奥陶世(471～440Ma)古岛弧构造环境,同时发育加里东期俯冲型(450～456Ma)花岗岩类和碰撞型(438～400Ma)花岗岩类岩浆活动.西秦岭北缘早古生代古洋陆构造格局经历了从洋盆形成-洋壳俯冲消减直至陆-陆碰撞造山的板块构造演化过程.总体构造演化可划分为四个阶段:①晚寒武世古洋盆初始形成阶段;②早奥陶世洋盆初始俯冲阶段;③中晚奥陶世洋壳大规模俯冲与古岛弧发育阶段;④志留纪陆-陆或陆-弧碰撞造山阶段.     

北祁连造山带低温榴辉岩的变质作用演化

北祁连造山带是在早古生代时期由洋壳俯冲形成的俯冲-碰撞造山带,主要岩石单元包括蛇绿岩、低温高压变质岩、岛弧火山岩、花岗岩体、志留纪复理石和泥盆纪磨拉石组合等.     

华南前泥盆纪构造演化：从华夏地块到加里东期造山带

在全球Rodinia超大陆的构造框架中,华夏地块占有突出的地位。然而,华夏地块在国内一直存在不同认识,其核心一是年龄,二是范围。根据出露在研究区的中－高级变质岩、韧滑流变形迹和近年大批高质量测年数据,认为华南曾经存在过一个前成冰纪的古老陆块,由元古代片岩、片麻岩、混合岩等组成,原岩为碎屑岩、火山岩和深成侵入岩,最老年龄达2 Ga,习称华夏地块,但范围比Grabau描述的要小。在8～9亿年间,伴随古华南洋的闭合,华夏地块与扬子陆块碰撞聚合,成为Rodinia超大陆的一部分。聚合不久,受成冰纪Rodinia超大陆裂解事件的影响,原华夏地块被肢解成浙南-闽北、赣中-赣南和云开大山三个古陆残块,中间是裂谷或海槽。其裂解残块集中分布在绍兴-江山-萍乡断裂和政和-大埔断裂之间的地带内,结束了其完整古陆块的历史。震旦纪-早古生代,这些海槽被进一步扩张变宽,其内被巨厚的碎屑岩（含灰岩）、浊积岩层所充填,厚达1∽2万m,但缺少同期蛇绿岩和火山岩,暗示拉张强度没有深达上地幔,为一被动陆缘环境。最新年代学结果表明,原定早古生代的蛇绿岩和火山岩均为前震旦纪的年龄,8∽9亿年居多,原先的早古生代构造格架需要再研究。到志留纪,华南发生了强烈的构造-热事件,导致震旦纪-早古生代海槽关闭,巨厚沉积物褶皱隆升,在元古代变质基底上形成了加里东期褶皱造山带。其造山的驱动力目前尚不清楚。此期褶皱变形、韧滑流变非常普遍,有推覆与走滑两种,变形峰期在420∽400Ma。同时,还发生了强烈的花岗岩浆活动,岩浆峰期为430∽400　Ma,但绝大多数是过铝质的S型花岗岩,I型花岗岩少见。之后,晚泥盆世砂砾岩层呈角度不整合大规模地覆盖在整个华南前泥盆纪岩层之上;至此,研究区和江南等邻区的沉积环境与古地理才得以真正统一     

Precise Timing of Caledonian Structural Deformation Chronology and Its Implications in Southeast Qilian Mountains, China

The middle Qilian orogenic belt and Lajishan orogenic belt, both of which were formed in the Caledonian, strike NW-SE direction across southeast Qilian Mountains and their basement consists of pre-Caledonian metamorphic rocks with lozenge-shaped ductile shear zones in the crystalline base- ment. The blunt angle between the conjugated ductile shear zones ranges from 104° to 114°, indicating approximate 210° of the maximum principal stress. The plateau ages of muscovite 40Ar/39Ar obtained from the mylonitized rocks in the ductile shear zones of Jinshaxia-Hualong-Keque massif within the middle Qilian massif are (405.1±2.4) Ma and (418.3±2.8) Ma, respectively. The chronology data confirm the formation of ductile shear zones in the Caledonian basement metamorphic rocks during the Cale- donian orogeny. Furthermore, on the basis of basement rock study, precise timing for the closing of the Late Paleozoic volcanic basin (or island-arc basin) and Lajishan ocean basin is determined. This pro- vides us a new insight into the closing of ocean basin in the structural evolution of orogenic belt.     

Tectonic evolution of Early Paleozoic island-arc systems and continental crust formation in the Caledonides of Kazakhstan and the North Tien Shan

The extended Saryarka and Shyngyz-North Tien Shan volcanic belts that underwent secondary deformation are traced in the Caledonides of Kazakhstan and the North Tien Shan. These belts are composed of igneous rocks pertaining to Early Paleozoic island-arc systems of various types and the conjugated basins with oceanic crust. The Saryarka volcanic belt has a complex fold-nappe structure formed in the middle Arenigian-middle Llanvirnian as a result of the tectonic juxtaposition of Early-Middle Cambrian and Late Cambrian-Early Ordovician complexes of ensimatic island arcs and basins with oceanic crust. The Shyngyz-North Tien Shan volcanic belt is characterized by a rather simple fold structure and consists of Middle-Late Ordovician volcanic and plutonic associations of ensialic island arcs developing on heterogeneous basement, which is composed of complexes belonging to the Saryarka belt and Precambrian sialic massifs. The structure and isotopic composition of the Paleozoic igneous complexes provide evidence for the heterogeneous structure of the continental crust in various segments of the Kazakh Caledonides. The upper crust of the Shyngyz segment consists of Early Paleozoic island-arc complexes and basins with oceanic crust related to the Saryarka and Shyngyz-North Tien Shan volcanic belts in combination with Middle and Late Paleozoic continental igneous rocks. The deep crustal units of this segment are dominated by mafic rocks of Early Paleozoic suprasubduction complexes. The upper continental crust of the Stepnyak segment is composed of Middle-Late Ordovician island-arc complexes of the Shyngyz-North Tien Shan volcanic belt and Early Ordovician rift-related volcanics. The middle crustal units are composed of Riphean, Paleoproterozoic, and probably Archean sialic rocks, whereas the lower crustal units are composed of Neoproterozoic mafic rocks.     

北祁连东段柏木峡—门岗峡地区蛇绿岩的识别及其区域构造意义

为加强对北祁连早古生代多岛弧盆系蛇绿混杂岩的调查,选取柏木峡—门岗峡蛇绿岩开展岩相学、年代学和地球化学研究。柏木峡—门岗峡蛇绿岩位于青海省海东市互助县,构造上处于北祁连造山带中东段。由橄榄岩、辉长岩和基性火山岩组成较为完整的蛇绿岩单元。对辉长岩进行单颗粒锆石LA-ICP-MS U-Pb同位素测年,获得²⁰⁶Pb/²³⁸U加权平均年龄为(525.2±1.1) Ma(MSWD=0.06),代表了蛇绿岩的形成年龄,相当于早寒武世。岩石地球化学研究表明,该蛇绿岩中的基性火山岩属于拉斑系列,具有洋岛玄武岩的地球化学特征;玄武岩Th/Yb-Nb/Yb和TiO₂/Yb-Nb/Yb等构造环境判别图显示,该套蛇绿岩的形成环境与俯冲作用无关。结合详细的野外调查和区域对比,认为该蛇绿岩代表早古生代北祁连洋壳,与玉石沟—川刺沟等蛇绿岩共同构成了达坂山—玉石沟蛇绿岩带。     

安徽北淮阳构造带基底变质岩的构造属性

安徽北淮阳构造带的基底由一套变火山沉积岩建造 (即原称庐镇关群 )所组成。岩类学、岩石地球化学、年代学的研究表明 ,这套火山岩属碱性玄武岩系列、拉斑玄武岩系列和钙碱性玄武岩系列 ,分别形成于中元古宙陆内裂解 (扩张 )带和晚元古宙岛弧两种构造环境 ,其中以岛弧环境火山岩为主要部分 ,由此可以证明北淮阳构造带是在中元古宙陆内裂解带基础上发展起来的古弧系 ,具有大陆型基底性质     

Tectonics of the Longmen Shan and Adjacent Regions,Central China

The Longmen Shan region includes, from west to east, the northeastern part of the Tibetan Plateau, the Sichuan Basin, and the eastern part of the eastern Sichuan fold-and-thrust belt. In the northeast, it merges with the Micang Shan, a part of the Qinling Mountains. The Longmen Shan region can be divided into two major tectonic elements: (1) an autochthon/parautochthon, which underlies the easternmost part of the Tibetan Plateau, the Sichuan Basin, and the eastern Sichuan fold-and-thrust belt; and (2) a complex allochthon, which underlies the eastern part of the Tibetan Plateau. The allochthon was emplaced toward the southeast during Late Triassic time, and it and the western part of the autochthon/parautochthon were modified by Cenozoic deformation.

The autochthon/parautochthon was formed from the western part of the Yangtze platform and consists of a Proterozoic basement covered by a thin, incomplete succession of Late Proterozoic to Middle Triassic shallow-marine and nonmarine sedimentary rocks interrupted by Permian extension and basic magmatism in the southwest. The platform is bounded by continental margins that formed in Silurian time to the west and in Late Proterozoic time to the north. Within the southwestern part of the platform is the narrow N-trending Kungdian high, a paleogeographic unit that was positive during part of Paleozoic time and whose crest is characterized by nonmarine Upper Triassic rocks unconformably overlying Proterozoic basement.

In the western part of the Longmen Shan region, the allochthon is composed mainly of a very thick succession of strongly folded Middle and Upper Triassic Songpan Ganzi flysch. Along the eastern side and at the base of the allochthon, pre-Upper Triassic rocks crop out, forming the only exposures of the western margin of the Yangtze platform. Here, Upper Proterozoic to Ordovician, mainly shallow-marine rocks unconformably overlie Yangtze-type Proterozic basement rocks, but in Silurian time a thick section of fine-grained clastic and carbonate rocks were deposited, marking the initial subsidence of the western Yangtze platform and formation of a continental margin. Similar deep-water rocks were deposited throughout Devonian to Middle Triassic time, when Songpan Ganzi flysch deposition began. Permian conglomerate and basic volcanic rocks in the southeastern part of the allochthon indicate a second period of extension along the continental margin. Evidence suggests that the deep-water region along and west of the Yangtze continental margin was underlain mostly by thin continental crust, but its westernmost part may have contained areas underlain by oceanic crust. In the northern part of the Longmen Shan allochthon, thick Devonian to Upper Triassic shallow-water deposits of the Xue Shan platform are flanked by deep-marine rocks and the platform is interpreted to be a fragment of the Qinling continental margin transported westward during early Mesozoic transpressive tectonism.

In the Longmen Shan region, the allochthon, carrying the western part of the Yangtze continental margin and Songpan Ganzi flysch, was emplaced to the southeast above rocks of the Yangtze platform autochthon. The eastern margin of the allochthon in the northern Longmen Shan is unconformably overlapped by both Lower and Middle Jurassic strata that are continuous with rocks of the autochthon. Folded rocks of the allochthon are unconformably overlapped by Lower and Middle Jurassic rocks in rare outcrops in the northern part of the region. They also are extensively intruded by a poorly dated, generally undeformed belt, of plutons whose ages (mostly K/Ar ages) range from Late Triassic to early Cenozoic, but most of the reliable ages are early Mesozoic. All evidence indicates that the major deformation within the allochthon is Late Triassic/Early Jurassic in age (Indosinian). The eastern front of the allochthon trends southwest across the present mountain front, so it lies along the mountain front in the northeast, but is located well to the west of the present mountain front on the south.

The Late Triassic deformation is characterized by upright to overturned folded and refolded Triassic flysch, with generally NW-trending axial traces in the western part of the region. Folds and thrust faults curve to the north when traced to the east, so that along the eastern front of the allochthon structures trend northeast, involve pre-Triassic rocks, and parallel the eastern boundary of the allochthon. The curvature of structural trends is interpreted as forming part of a left-lateral transpressive boundary developed during emplacement of the allochthon. Regionally, the Longmen Shan lies along a NE-trending transpressive margin of the Yangtze platform within a broad zone of generally N-S shortening. North of the Longmen Shan region, northward subduction led to collision of the South and North China continental fragments along the Qinling Mountains, but northwest of the Longmen Shan region, subduction led to shortening within the Songpan Ganzi flysch basin, forming a detached fold-and-thrust belt. South of the Longmen Shan region, the flysch basin is bounded by the Shaluli Shan/Chola Shan arc—an originally Sfacing arc that reversed polarity in Late Triassic time, leading to shortening along the southern margin of the Songpan Ganzi flysch belt. Shortening within the flysch belt was oblique to the Yangtze continental margin such that the allochthon in the Longmen Shan region was emplaced within a left-lateral transpressive environment. Possible clockwise rotation of the Yangtze platform (part of the South China continental fragment) also may have contributed to left-lateral transpression with SE-directed shortening. During left-lateral transpression, the Xue Shan platform was displaced southwestward from the Qinling orogen and incorporated into the Longmen Shan allochthon. Westward movement of the platform caused complex refolding in the northern part of the Longmen Shan region.

Emplacement of the allochthon flexurally loaded the western part of the Yangtze platform autochthon, forming a Late Triassic foredeep. Foredeep deposition, often involving thick conglomerate units derived from the west, continued from Middle Jurassic into Cretaceous time, although evidence for deformation of this age in the allochthon is generally lacking.

Folding in the eastern Sichuan fold-and-thrust belt along the eastern side of the Sichuan Basin can be dated as Late Jurassic or Early Cretaceous in age, but only in areas 100 km east of the westernmost folds. Folding and thrusting was related to convergent activity far to the east along the eastern margin of South China. The westernmost folds trend southwest and merge to the south with folds and locally form refolded folds that involve Upper Cretaceous and lower Cenozoic rocks. The boundary between Cenozoic and late Mesozoic folding on the eastern and southern margins of the Sichuan Basin remains poorly determined.

The present mountainous eastern margin of the Tibetan Plateau in the Longmen Shan region is a consequence of Cenozoic deformation. It rises within 100 km from 500–600 m in the Sichuan Basin to peaks in the west reaching 5500 m and 7500 m in the north and south, respectively. West of these high peaks is the eastern part of the Tibetan Plateau, an area of low relief at an elevations of about 4000 m.

Cenozoic deformation can be demonstrated in the autochthon of the southern Longmen Shan, where the stratigraphic sequence is without an angular unconformity from Paleozoic to Eocene or Oligocene time. During Cenozoic deformation, the western part of the Yangtze platform (part of the autochthon for Late Triassic deformation) was deformed into a N- to NE-trending foldandthrust belt. In its eastern part the fold-thrust belt is detached near the base of the platform succession and affects rocks within and along the western and southern margin of the Sichuan Basin, but to the west and south the detachment is within Proterozoic basement rocks. The westernmost structures of the fold-thrust belt form a belt of exposed basement massifs. During the middle and later part of the Cenozoic deformation, strike-slip faulting became important; the fold-thrust belt became partly right-lateral transpressive in the central and northeastern Longmen Shan. The southern part of the fold-thrust belt has a more complex evolution. Early Nto NE-trending folds and thrust faults are deformed by NW-trending basementinvolved folds and thrust faults that intersect with the NE-trending right-lateral strike-slip faults. Youngest structures in this southern area are dominated by left-lateral transpression related to movement on the Xianshuihe fault system.

The extent of Cenozoic deformation within the area underlain by the early Mesozoic allochthon remains unknown, because of the absence of rocks of the appropriate age to date Cenozoic deformation. Klippen of the allochthon were emplaced above the Cenozoic fold-andthrust belt in the central part of the eastern Longmen Shan, indicating that the allochthon was at least partly reactivated during Cenozoic time. Only in the Min Shan in the northern part of the allochthon is Cenozoic deformation demonstrated along two active zones of E-W shortening and associated left-slip. These structures trend obliquely across early Mesozoic structures and are probably related to shortening transferred from a major zone of active left-slip faulting that trends through the western Qinling Mountains. Active deformation is along the left-slip transpressive NW-trending Xianshuihe fault zone in the south, right-slip transpression along several major NE-trending faults in the central and northeastern Longmen Shan, and E-W shortening with minor left-slip movement along the Min Jiang and Huya fault zones in the north.

Our estimates of Cenozoic shortening along the eastern margin of the Tibetan Plateau appear to be inadequate to account for the thick crust and high elevation of the plateau. We suggest here that the thick crust and high elevation is caused by lateral flow of the middle and lower crust eastward from the central part of the plateau and only minor crustal shortening in the upper crust. Upper crustal structure is largely controlled in the Longmen Shan region by older crustal anisotropics; thus shortening and eastward movement of upper crustal material is characterized by irregular deformation localized along older structural boundaries.     

Jurassic Subduction of the Paleo-Pacific Ocean in NE China: Zircon U–Pb and Sr-Nd-Hf isotopic Evidence from the Huashan and Taili Monzogranites

The Qilian orogen along the NE edge of the Tibet‐Qinghai Plateau records the evolution of Proto‐Tethyan Ocean that closed through subduction along the southern margin of the North China block during the Early Paleozoic. The South Qilian belt is the southern unit of this orogen and dominated by Cambrian‐Ordovician volcano‐sedimentary rocks and Neoproteozoic Hualong complex that contains similar rock assemblages of the Central Qilian block. Our recent geological mapping and petrologic results demonstrate that volcano‐sedimentary rocks show typical rock assembles of a Cambrian‐early Ordovician arc‐trench system in Lajishan Mts. along the northern margin of the Hualong Complex. Island arc rocks including basalt, andesite, dacite, rhyolite, and breccia is in fault contact with ophiolite complex consisting of mantle peridotite, serpentinite, gabbro, dolerite, plagiogranite, and basalt. Accretionary complexes are tectonically separated from the ophiolite‐arc rocks, with various rock assemblages spatially. They consist of pillow basalt, basalt breccia, tuff, chert, and limestone blocks with a seamount origin within the scaly shale in Dingmaoshan and Donggoumeikuang areas, and basalt, chert, and sandstone blocks within muddy shale matrix and mélange at Lajishankou area. Abundant radiolarians occur in red chert, and trilobite, brachiopod, and coral fossils occur within Dingmaoshan limestone blocks. Although partial basalt or chert blocks are highly disrupted, duplex, thrust fault, rootless intrafolial fold, tight fold, and penetrative foliation are well‐developed at Donggoumeikuang area. Spatially, accretionary complexes lie structurally beneath ophiolite complex and above the turbidites of the Central Qilian block. Ophiolite and accretionary complexes are also overlapped by late Ordovician molasse deposits sourced from Cambrian arc‐trench system and the Central Qilian block. These observations demonstrate that a Cambrian‐early Ordovician trench‐arc system within the South Qilian belt formed during the early Paleozoic southward subduction of the South Qilian Ocean collided with the Central Qilian block prior to the late Ordovician.     

The origin and emplacement of the Lajishankou ophiolite complex in the South Qilian belt: evidences from geological mapping

Many ophiolite complexes like those of Oman and New Caledonia represent fragments of ancient oceanic crust and upper mantle generated at supra‐subduction zone environments and have been obducted onto the adjacent rifted continental margin together with the accretionary complexes and intra‐oceanic arcs. The Lajishan ophiolite complexes in the Qilian orogenic belt along the NE edge of the Tibet‐Qinghai Plateau are one of several ophiolites situated to the south of the Central Qilian block. Our geological mapping and petrological investigations suggest that the Lajishankou ophiolite complex consists of serpentinite, wehrlite, pyroxenite, gabbro, dolerite, and pillow and massive basalts that occur in a series of elongate fault‐bounded slices. An accretionary complex composed mainly of basalt, radiolarian chert, sandstone, mudstone, and mélange lies structurally beneath the ophiolite complex. The Lajishankou ophiolite complex and accretionary complex were emplaced onto the Qingshipo Formation of the Central Qilian block which shows features typical of turbidites deposited in a deep‐water environment of passive continental margin. Our geochemical and geochronological studies indicate that the mafic rocks in the Lajishankou ophiolite complex can be categorized into three distinct groups: massive island arc tholeiites, 509 Ma back‐arc dolerite dykes, and 491 Ma pillow basaltic and dolerite slices that are of seamount origin in a back‐arc basin. The ophiolite and accretionary complex constitute a Cambrian‐early Ordovician trench‐arc system within the South Qilian belt during the early Paleozoic southward subduction of the South Qilian Ocean prior to Early Ordovician obduction of this system onto the Central Qilian block.     

祁连山东南段加里东造山期构造变形年代的精确限定及其意义

祁连山东南段呈北西-南东向展布着加里东期中祁连造山带和拉脊山造山带, 其基底为前加里东变质岩系, 在该变质结晶基底岩系中发育着菱形网格状韧性剪切带, 共轭韧性剪切带面对缩短方向的夹角为104°~114°, 其最大主应力方位为SW210°左右.在中祁连地块金沙峡和化隆地块科却两处韧性剪切带中的糜棱岩化岩石, 获取变质矿物白云母40Ar-39Ar坪年龄分别为(405.1±2.4) Ma和(418.3±2.8) Ma.这一年代学结果不仅确定了加里东基底变质岩系中韧性剪切带是加里东造山作用过程中形成, 更重要的是通过对基底韧性剪切带中变质变形岩石的年代学研究, 精确地限定了祁连山东南段的早古生代火山盆地(或岛弧盆地)、拉脊山小洋盆关闭的构造年代.这为造山带构造演化过程中盆地关闭时间的确定开辟了新的途径.