Geodynamic settings and formation conditions of crystalline complexes in the south Altai and south Gobi metamorphic belts

The Hercynian mobile belts in Central Asia comprise the Hercynian proper and the Late Hercynian (Indosinian) belts separated by the South Gobi microcontinent, the origin of which is related to the evolution of the South Mongolian and Inner Mongolian basins with the oceanic crust. Crystalline complexes within these belts occur as tectonic sheets of a variety of sizes. At the early stages, the metamorphic grade of these complexes reached conditions of high-temperature subfacies of amphibolite and locally developed granulite facies. In tectonic terms, the Hercynian belt of metamorphic rocks is situated at the margin of the North Asian Caledonian continent and extends from the southeast to the northwest along the southern slope of the Gobi, Mongolian, and Chinese Altai to East Kazakhstan, where metamorphic rocks are localized in the Irtysh Shear Zone. All these rocks are combined into the South Altai metamorphic belt of more than 1500 km in extent. Another belt of isolated outcrops of crystalline rocks conventionally combined into the Indosinian South Gobi metamorphic belt is traced along the junction of the Hercynides with the South Gobi microcontinent. The high-grade metamorphic rocks within both belts are not fragments of an ensialic Caledonian or older basement. These rocks were formed 390–360 and 230–220 Ma ago as a result of the closure of the Tethian South Mongolian and Inner Mongolian oceanic basins (Paleotethys I and Paleotethys II). The spatial position of the South Altai and South Gobi metamorphic belts is caused by the asymmetric structure of the Tethian basins, where active continental margins are expressed most distinctly along their northern parts, while passive margins extend along the southern parts (in present-day coordinates).     

Vendian stage in formation of the Early Caledonian superterrane in Central Asia

Granitoids and metamorphic rocks of the Baidarik basement block of the Dzabkhan microcontinent are studied in terms of geology, geochronology (U-Pb dating of zircon microfractions and individual grains) and Nd isotopic-geochemical systematics. As is established, the formation history of metamorphic belt (disthene-sillimanite facies) in junction zone of the Baidarik block and Bayankhongor zone of the Late Riphean (～665 Ma) ophiolite association characterizes development of the Vendian (～560–570 Ma) active continental margin. The high-P metamorphic rocks of that time span evidence formation of structures with the Earth’s crust of considerable thickness. In Central Asia, events of the Vendian low-gradient metamorphism are established also in the Tuva-Mongolian massif, Kan block of the East Sayan Mountains, and South Chuya inlier of the Caledonides in the Altai Mountains. Based on these data, it is possible to distinguish the Late Baikalian stage in development of the Early Caledonian superterrane of Central Asia, which antedated the subsequent evolution of this structure during the Late Cambrian-Ordovician. The high-gradient metamorphism that affected most intensively the southeastern part of the Baidarik block can be correlated with the Early Paleozoic (525–540 Ma) evolution of active continental margin and associated development of the Vendian oceanic basins and island arcs of the Ozernaya zone.     

Tectonics and geodynamics of Gorny Altai and adjacent structures of the Altai–Sayan folded area

Packages of Late Paleozoic tectonic nappes and associated major NE-trending strike-slip faults are widely developed in the Altai–Sayan folded area. Fragments of early deformational phases are preserved within the Late Paleozoic allochthons and autochthons. Caledonian fold-nappe and strike-slip structures, as well as accompanying metamorphism and granitization in the region, are typical of the EW-trending suture-shear zone separating the composite Kazakhstan–Baikal continent and Siberia. In the Gorny Altai region, the Late Paleozoic nappes envelop the autochthon, which contains a fragment of the Vendian–Cambrian Kuznetsk–Altai island arc with accretionary wedges of the Biya–Katun’ and Kurai zones. The fold-nappe deformations within the latter zones occurred during the Late Cambrian (Salairian) and can thus be considered Salairian orogenic phases. The Salairian fold-nappe structure is stratigraphically overlain by a thick (up to 15 km) well-stratified rock unit of the Anyui–Chuya zone, which is composed of Middle Cambrian–Early Ordovician fore-arc basin rocks unconformably overlain by Ordovician–Early Devonian carbonate-terrigenous passive-margin sequences. These rocks are crosscut by intrusions and overlain by a volcanosedimentary unit of the Devonian active margin. The top of the section is marked by Famennian–Visean molasse deposits onlapping onto Devonian rocks. The molasse deposits accumulated above a major unconformity reflects a major Late Paleozoic phase of folding, which is most pronounced in deformations at the edges of the autochthon, nearby the Kaim, Charysh–Terekta, and Teletskoe–Kurai fault nappe zones. Upper Carboniferous coal-bearing molasse deposits are preserved as tectonic wedges within the Charysh–Terekta and Teletskoe–Kurai fault nappe zones.Detrital zircon ages from Middle Cambrian–Early Ordovician rocks of the Anyui–Chuya fore-arc zone indicate that they were primarily derived from Upper Neoproterozoic–Cambrian igneous rocks of the Kuznetsk–Altai island arc or, to a lesser extent, from an Ordovician–Early Devonian passive margin. A minor age population is represented by Paleoproterozoic grains, which was probably sourced from the Siberian craton. Zircons from the Late Carboniferous molasse deposits have much wider age spectra, ranging from Middle Devonian–Early Carboniferous to Late Ordovician–Early Silurian, Cambrian–Early Ordovician, Mesoproterozoic, Early–Middle Proterozoic, and early Paleoproterozoic. These ages are consistent with the ages of igneous and metamorphic rocks of the composite Kazakhstan–Baikal continent, which includes the Tuva-Mongolian island arc with accreted Gondwanan blocks, and a Caledonian suture-shear zone in the north. Our results suggest that the Altai–Sayan region is represented by a complex aggregate of units of different geodynamic affinity. On the one hand, these are continental margin rocks of western Siberia, containing only remnants of oceanic crust embedded in accretionary structures. On the other hand, they are represented by the Kazakhstan–Baikal continent composed of fragments of Gondwanan continental blocks. In the Early–Middle Paleozoic, they were separated by the Ob’–Zaisan oceanic basin, whose fragments are preserved in the Caledonian suture-shear zone. The movements during the Late Paleozoic occurred along older, reactivated structures and produced the large intracontinental Central Asian orogen, which is interpreted to be a far-field effect of the colliding East European, Siberian, and Kazakhstan–Baikal continents.     

Geological framework and Paleozoic tectonic history of the Chinese Altai, NW China: a review

The Chinese Altai, as a key portion of the Central Asian Orogenic Belt (CAOB), is dominated by variably deformed and metamorphosed sedimentary rocks, volcanic rocks and granitic intrusions. Its Early Paleozoic tectonic setting has been variously considered as a passive continental margin, a subduction-accretion complex, or a Precambrian microcontinent, and two representative competing tectonic models have been proposed, i.e., open-closure versus subduction-accretion. Recent studies demonstrate that the high-grade metamorphic rocks previously considered as fragments of a Precambrian basement have zircon U-Pb ages (predominantly 528 to 466 Ma) similar to those of the widely distributed low-grade metasedimentary rocks named as Habahe Group in the region, and all these meta-sedimentary rocks were dominantly deposited in the Early Paleozoic. Petrological evidence and geochemical compositions further suggest that these meta-sedimentary rocks were probably deposited in an active margin, not a passive continental margin as previously proposed. The detrital zircons of sediments and igneous zircons from granitoids including the inherited ones (mainly 543–421 Ma) mostly give positive ?_Hf(t) values, suggesting significant contributions from mantle-derived juvenile materials to the lower crust. A modeling calculation based on zircon Hf isotopic compositions suggests that as much as 84% of the Chinese Altai is possibly made up of “juvenile” Paleozoic materials. Thus, available data do not support the existence of a Precambrian basement, but rather indicate that the Chinese Altai represented a huge subduction-accretion complex in the Paleozoic. Zircon U-Pb dating results for granitoids indicate that magmatism was active continuously from the Early to Middle Paleozoic, and the strongest magmatic activity took place in the Devonian, coeval with a significant change in zircon Hf isotopic composition. These findings, together with the occurrence of chemically distinctive igneous rocks and the high-T metamorphism, can be collectively accounted for by ridge-trench interaction during the accretionary orogenic process.     

柴北缘北部中吾农山构造带及邻区中吾农山群物源和构造环境:来自地球化学与锆石年代学的证据

中吾农山构造带位于南祁连地块和柴北缘欧龙布鲁克地块之间,其构造特征研究对邻区的构造演化有重要的启示意义。目前关于中吾农山构造带加里东期构造-地层-热事件的研究很少,且对于该构造带及邻区中吾农山群的物质来源和其形成的构造环境仍存在不同观点。本文对中吾农山构造带及邻区中吾农山群开展岩石学、地球化学、碎屑锆石U-Pb年代学及其微区原位Hf同位素分析,结果显示中吾农山群碎屑岩具有较低的化学蚀变指数(CIA72)和斜长石蚀变指数(PIA=50.26~68.12),较高的成分变异指数(ICV=1.21~1.58),表明成熟度较低,经历了弱-中等的化学风化、不具"沉积再循环"特征。地球化学指标(La/Sc,Th/Sc,Zr/Sc,Cr/Th,Th/Co等)和判别图解(Zr/Sc-Th/Sc和La/Sc-Co/Th)显示源岩主要为长英质岩石。锆石U-Pb年龄谱存在新元古代(0.8~1.0 Ga)和早古生代(0.4~0.45 Ga)两个主要峰值,对应ε_(Hf)(t)分别为-9.6~-3.09和-15.87~9.5,而最年轻碎屑锆石年龄为410 Ma。结合前人研究,推测中吾农山群可能形成于晚奥陶世-早泥盆世早期,而柴北缘鱼卡-沙柳河高压-超高压变质带内新元古代花岗片麻岩和早古生代滩间山蛇绿杂岩带内大陆弧型花岗岩为其主要物源,其沉积形成于早古生代活动大陆边缘-大陆岛弧环境下的弧后盆地。     

The early Paleozoic tectonic evolution of the Russian Altai: Implications from geochemical and detrital zircon U–Pb and Hf isotopic studies of meta-sedimentary complexes in the Charysh–Terekta–Ulagan–Sayan suture zone

The Charysh–Terekta–Ulagan–Sayan suture zone was regarded as a tectonic boundary separating two distinct subduction–accretion systems in the Central Asian Orogenic Belt (CAOB). In the north, magmatic arcs, such as the Gorny Altai terrane, formed in the southwestern periphery of the Siberian continent, whereas in the south, arc-prism systems, such as the Altai–Mongolian terrane, formed around the so-called Kazakhstan–Baikal composite continent with Gondwana affinity. When did these two systems amalgamate and whether the metamorphic complexes in the suture zone represent Precambrian micro-continental slivers are critical for our understanding of the accretionary orogenesis and crustal growth rate in the CAOB. A combined geochemical and detrital zircon U–Pb–Hf isotopic study was conducted on the meta-sedimentary rocks from the Ulagan (also referred to Bashkaus) and Teletsk Complexes in the suture zone. The results indicate that the protoliths of these rocks were dominated by immature sediments deposited in a time period between 500 and 420 Ma. Thus, Precambrian micro-continental slivers may not exist in the suture zone and even in the whole Altai Orogen.The meta-sedimentary rocks from the Ulagan Complex yield geochemical compositions between those of common intermediate and felsic igneous rocks, implying that these kinds of rocks possibly served as dominant sources. Detrital zircons from this complex consist of a major population of ca. 620–500 Ma, a subordinate one of ca. 931–671 Ma and rare grains of ca. 2899–1428 Ma. This age spectrum is compatible with the magmatic records of the western Mongolia. We propose that the Ulagan Complex possibly represents part of a subduction–accretion complex built upon an active continental margin of the western Mongolia in the early Paleozoic. The remarkable similarities in source nature, provenance, and depositional setting to the early Paleozoic meta-sedimentary rocks from the northern Altai–Mongolian terrane imply that the Ulagan Complex was possibly fragmented from this terrane.The meta-sedimentary rocks from the Teletsk Complex show similar detrital zircon populations but contain higher proportions of mafic sediments and have more depleted whole-rock Nd isotopic compositions. Our data suggest that the detritus mostly came from the same source as that for the Ulagan Complex but those from the Gorny Altai terrane also contributed. This implies that the Gorny Altai and Altai-Mongolian terranes possibly amalgamated prior to the early Devonian rather than in the middle Devonian to early Carboniferous as previously thought. Thus, the widespread Devonian to early Carboniferous magmatism within these two terranes was possibly generated in a similar tectonic setting. Moreover, the dominant Neoproterozoic to early Paleozoic detrital zircons from the Teletsk Complex yield largely varied ɛ_Hf(t) values of − 23.8 to 12.4, indicating that crustal growth and reworking are both important in the accretionary orogenesis.     

Convergent boundaries and related igneous and metamorphic complexes in caledonides of Central Asia

Fragments of the crystalline complexes where Vendian metamorphism of moderate and elevated pressure predated Early Paleozoic metamorphism have been established in the accretionary-collisional domain of the eastern segment of the Central Asian Foldbelt (Early Caledonian superterrane of Central Asia). The geodynamic setting of the Vendian (??560?C570 Ma) South Hangay metamorphic belt located in the junction zone of the Baydrag Block and the Late Riphean (??665 Ma) ophiolite complex of the Bayanhongor Zone is considered. The origination of this belt was related to the formation of the convergent boundary in the framework of the Zabhan microcontinent about 570 Ma ago. At the same time, an island-arc complex was formed in the paleo-oceanic domain. Metamorphism of elevated pressure indicates that Vendian structures with sufficiently thick continental crust were formed in the framework of the continental blocks. Vendian metamorphism is also established in the Tuva-Mongolia Massif and the Kan Block of the Eastern Sayan. These data show that the Late Baikalian stage predated the evolution of the Early Caledonian superterrane of Central Asia. The development of its accretionary-collisional structure was accompanied by Late Cambrian-Early Ordovician low-pressure regional metamorphism. Granulite-facies conditions were reached only at the deep levels of the accretionary-collisional edifice. The outcrops of crystalline complexes in the southern framework of the Caledonian paleocontinent are regarded as fragments of the Early Paleozoic Central Mongolian metamorphic belt.     

华北地块北缘泥盆纪岩浆活动及其构造背景

晚古生代—早中生代岩浆岩广泛分布在华北地块北缘,构成了沿华北地块北缘呈近东—西向分布的上千公里的岩浆岩带。泥盆纪岩浆岩作为其中的组成部分,虽然分布范围不及石炭—二叠纪岩浆岩广泛,但近十多年来随着研究工作的深入,越来越多的泥盆纪岩浆岩被发现,其构造背景及意义也逐渐引起重视。对华北地块北缘近年来发现的泥盆纪岩浆活动进行总结分析表明,泥盆纪岩浆活动时限在400~360 Ma左右,其中碱性侵入岩和基性—超基性侵入岩时代主要集中在400~380 Ma之间,沿华北地块北缘自东向西均有分布;火山岩则主要集中在400 Ma和360 Ma,主要分布在华北北缘东段的赤峰地区。泥盆纪侵入岩在岩石组合上以碱性杂岩及碱性花岗岩为主,其次为二长闪长岩、基性—超基性杂岩。泥盆纪火山岩则以变质流纹质火山岩—次火山岩—火山碎屑岩及变质玄武安山岩、玄武岩（斜长角闪岩）为主,总体上具有双峰式岩石组合特征。综合研究该地区构造演化历史及泥盆纪岩石组合、弱构造变形特征、岩石地球化学与同位素地球化学组成等,认为泥盆纪岩浆活动的形成与白乃庙岛弧和华北地块在晚志留世发生弧—陆碰撞后的伸展背景有关。华北北缘泥盆纪岩浆岩带的形成对于认识古生代期间地壳增生过程、方式及古亚洲洋最终闭合时间有重要科学意义。      

南秦岭佛坪地区地层格架与物源分析:变质沉积岩中碎屑锆石LA-ICP-MS U-Pb定年提供的制约

南秦岭佛坪地区位于东西秦岭交汇部位,以前寒武纪基底穹状隆升为主要特征,因其特殊的构造位置和变质变形特征而受到广泛关注。为了更好地揭示南秦岭构造带的性质及其在秦岭造山带构造演化中的作用,本文对佛坪地区的前寒武纪基底和沉积盖层进行了系统的碎屑锆石LA-ICP-MS U-Pb年代学研究。佛坪穹隆核部前寒武纪基底中碎屑锆石主要年龄峰值为600~820Ma、2021Ma和2467Ma,其中3个样品给出最年轻的年龄区间约为615~728Ma,沉积时代不早于新元古代,另外1个样品给出的最年轻的年龄峰值为1113Ma,沉积时代不早于中元古代,否定了关于认为其为太古代或古元古代的变质结晶基底的认识,其碎屑物质可能来源于南秦岭构造带和扬子陆块北缘。佛坪穹隆上覆盖层和外围南侧盖层给出了相似的年龄图谱,主要年龄峰值为410～450Ma、650～880Ma和910～950Ma,其中最年轻的年龄区间为344～416Ma,表明其沉积时代晚于泥盆纪,碎屑物质来源为北秦岭构造带和南秦岭构造带。上述研究结果表明,南秦岭构造带(前寒武纪基底)于新元古代已增生为扬子陆块北缘的重要组成部分,于晚古生代(泥盆纪)紧邻华北-北秦岭构造带南缘,并共同为南秦岭大面积的泥盆纪盆地提供碎屑物质。     

内蒙古贺根山蛇绿岩形成时代及构造启示

贺根山蛇绿岩位于兴蒙造山带北缘,发育完整的地幔橄榄岩、堆晶岩和基性熔岩组合,伴生有放射虫硅质岩,但贺根山蛇绿岩的形成时代一直存在争议,给兴蒙造山带北部构造演化阶段划分造成了很大障碍。锆石U-Pb年代学研究表明,贺根山蛇绿岩中辉长闪长岩(341±3Ma)和玄武岩(359±5Ma)结晶年龄为早石炭世早期,同时玄武岩继承锆石峰值年龄为晚泥盆世早期(375±2Ma),这些继承锆石呈短柱状、棱角状,生长环带宽缓,多为补丁状、平坦状,为典型的基性岩浆锆石,表明最迟在晚泥盆世早期洋壳物质已经开始形成。上石炭统格根敖包组火山岩与蛇绿岩局部呈喷发不整合接触,该组的晶屑凝灰岩夹层时代为晚石炭世(323±3Ma),提供了蛇绿岩构造侵位年龄的上限。因此,将贺根山蛇绿岩形成时代定为晚泥盆世-早石炭世,侵位时代为晚石炭世。侵入地幔橄榄岩中的部分基性岩脉时代为早白垩世(132±1Ma、139±3Ma和120±1Ma),它们含有大量继承锆石(144±1Ma~2698±25Ma),继承锆石峰值年龄密切响应了兴蒙造山带北部早白垩世之前复杂的岩浆及构造事件,这些基性岩脉是燕山期伸展环境下的岩浆产物,并非早白垩世蛇绿岩。结合前人的工作成果和区域岩浆岩、地层时空分布特征,建立了兴蒙造山带北部晚古生代构造演化历程:二连贺根山一线早泥盆世处于剥蚀阶段,中泥盆世陆壳拉张出现新生洋盆,晚泥盆世早期洋盆持续扩张形成新生洋壳,早石炭世晚期洋壳开始向北俯冲消减,并持续增生至西伯利亚活动陆缘,晚石炭世洋盆陆续闭合,部分已经构造侵位的蛇绿岩被晚石炭世火山岩不整合覆盖,贺根山蛇绿岩正是该洋盆的残余产物。     

Palaeomagnetism of Caledonian intrusive suites in the Northern Highlands of Scotland: Constraints to tectonic movements within the Caledonian orogenic belt

To evaluate the scale of tectonic movements within the northern sector of the 500-400 Ma Caledonian orogenic belt and its Precambrian foreland zone between the Great Glen Fault (GGF) zone to the southeast and the Laurentian Block to the northwest, we have studied the palaeomagnetism of minor intrusive rocks within the Northern Highlands terrain. These rocks include

1. (1) amphibolites and other metamorphic rocks predating deformation,

2. (2) microdiorities, dolentes and related suites emplaced, and probably magnetised, between 450 and 420 Ma, and

3. (3) Lower-Middle Devonian lamprophyres.

A range of predominantly NNE negative and SSW positive components are resolved by cleaning treatment with a dispersion of declinations towards a minority WNW-ESE axis; isolated southerly negative directed hematite-held components suggests limited, but no widespread, remagnetisation in Devonian-Carboniferous times.Comparison is made with data from other tectonic divisions in the Caledonian orogenic belt and the bordering forelands. Palaeopoles from the Northern Highlands closely conform in part with North American Ordovician poles and in part with the post-Ordovician palaeopoles from Britain south of the GGF. The definitive motions of the British Caledonides to emerge from the palaeomagnetic analysis are an anticlockwise rotation of the Caledonian terrain in early Ordovician times, small relative motions during the remainder of Ordovician times followed by large clockwise and then anticlockwise rotations during late Ordovician to early Silurian times contemporary with the last major movements on the Moine Thrust (ca. 430 Ma). Late Silurian-Devonian movements along the GGF were probably below the limits of palaeomagnetic detectability. The collective data require that apparent polar wander movements and concomitant continental movements have currently been incompletely recovered by North American studies and the path for Lower Palaeozoic times is more complex than recognised hitherto.     

武夷地块中南平-宁化构造带南华纪弧火山岩属性及其对华夏板块多微地体拼合的启示

武夷地块中各地体的活动解析是认识华夏板块构造活动演化的窗口.南平宁化构造带作为划分南武夷地体和北武夷地体的重要边界活动带,其构造属性对我们认识武夷地块中南、北武夷地体的构造演化具有重要意义.前人对南平-宁化构造带中变质火山岩类的研究和认识存在争议,限制了我们对南平-宁化构造带的构造属性的理解.因此,本文利用野外路线地质调查、年代学测定、同位素地球化学测试等手段,对该套变质火山岩做了重点研究,结合前人的研究成果,我们得到以下认识:南平-宁化构造带中的变质火山岩类主要形成于756～720 Ma,主要形成于活动大陆边缘或岛弧环境,表明南平-宁化构造带可能代表了南武夷地体和北武夷地体之间的一条南华纪的近东西向的岛弧火山活动带,结合带中构造变形反映的南北向对冲的汇聚极性,我们认为南武夷地体和北武夷地体之间在南华纪可能还存在洋-陆碰撞乃至陆-陆拼合过程,这也反映了多微地块、多期拼合过程可能更符合华夏板块大地构造演化的主旋律.     

秦岭造山带是印支碰撞造山带吗?

国内外一些学者认为秦岭是一个印支碰撞造山带.但迄今为止,秦岭尚未发现三叠纪或古生代延续到三叠纪的洋盆存在的任何痕迹.秦岭泥盆系-三叠系为滨、浅海相沉积,没有远洋沉积,更没有镁铁质和超镁铁质岩石及与之密切相关的放射虫硅质岩组成的蛇绿岩套.泥盆系与下伏地质体之间有一个清楚的区域性角度不整合.商丹断裂并不是印支期,而是加里东期的板块缝合带;其两侧,中朝板块南缘和扬子板块北缘均有十分清楚的加里东造山作用的记录.沉积于扬子板块北缘的中上泥盆统刘岭群的放射性铅同位素组成与北秦岭相近,碎屑锆石年龄谱系亦证明其物质主要来自中朝板块南缘的北秦岭造山带.所谓勉略印支缝合带中的勉略和三里岗蛇绿混杂岩中的镁铁质岩,同位素测年均为元古代之产物,后者又被南华系-震旦系沉积覆盖.所谓勉略缝合带,实为一区域性大断裂带.早古生代,其北侧属扬子板块北部被动边缘;南侧为扬子板块核心部分的扬子准地台(小克拉通).所以,秦岭的印支造山作用,并不是洋盆消失后的陆陆碰撞造山作用,而是海盆消失后的中朝与扬子2个小陆块间逆冲-叠覆造山作用.作为秦岭东延的大别山超高压变质带被认为是秦岭印支碰撞造山的重要证据之一,但大别山超高压变质岩是在造山作用过程中动态超高压条件下形成的,仅用简单的静岩压力来计算其形成深度,显然是不符合实际情况的.野外地质观察、构造地质学、变质岩石学、同位素地质学、地球化学、地球物理学以及物理实验等方面的实际资料和研究结果均说明超高压变质作用并不是在上地幔而是在地壳内进行的.南秦岭-大别山的地壳构造层次,上地壳自上而下依次为:未变质的沉积岩层、绿帘-蓝片岩层、高压变质岩层、超高压变质岩层;下地壳为未卷入超高压变质作用的麻粒岩相-高角闪岩相变质杂岩.含柯石英的超高压单位只是位于上地壳下部的厚约10～12km的席状构造岩片.初步认为上地壳这一从低压到高压再到超高压的构造系统,是印支造山期间,南秦岭-大别山的上地壳以下地壳顶部为主剪切滑动面,多层次剪切作用造成的.上地壳下部的超高压变质岩,则可能是强烈剪切引起的频繁地震的震源区瞬时超高压作用的结果.     

Geochemistry of the Caledonian basic volcanic rocks at the south margin of the Qinling orogenic belt, and its tectonic implications

The geochemistry of the basic volcanic rocks at the south margin of the Qinling orogenic belt(SMQOB) suggests that they were formed in an intraplate tectonic setting.The REE distribution patterns show these rocks are strongly enriched in LREE with high ∑REE, and their trace elements geochemistry is similar to that of contimental flood basalt.All the above evidence suggests that the Caledonian basic volcanic rocks in the SMQOB were tholeiitic basalts formed in an intraplate spreading-initial rift tectonic setting.The characteristics of regional geology and geochemistry indicate that there was an intraplate spreading-rift tectonic setting between the South Qingling block and the Yangtze block in the Caledonian epoch.The dynamic spreading in this district began in the Early Caledonian and then the intraplate spreadinginitial rifts were formed in the Late Caledonian.As a result of spreading of the Tethys and geodynamic processes in deep mantle ,the Mianlue-Huashan oceanic basin was formed between the Qinling block and the Yangtze block in Devonian,and the Qinling microplate was separated from the northern part of the Yangtze plate.     

Metamorphism and tectonic evolution of the Lhasa terrane,Central Tibet

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.     

新疆南天山克孜尔河沉积物碎屑锆石U‐Pb年代学研究及其地质意义

新疆克孜尔河流经南天山造山带南缘,其河流沉积物中记录了流域内地质体的重要信息。为进一步约束南天山造山带的构造演化历史,探讨该造山带古生代地壳生长与演化,对克孜尔河沉积物中的碎屑锆石进行U‐Pb定年。结果表明锆石年龄主要集中分布在460~390 Ma和310~260 Ma,少量分布在前寒武纪,暗示南天山造山带在古生代期间发生了强烈的岩浆活动。物源分析表明克孜尔河沉积物中的碎屑锆石主要源于南天山造山带和塔里木克拉通北部,年龄为460~390 Ma的碎屑锆石很可能记录了南天山洋在晚奥陶—早泥盆世期间向南俯冲到塔里木克拉通之下的弧岩浆作用。南天山洋闭合以及塔里木克拉通与伊犁—中天山地块的最终碰撞可能发生在晚石炭世,随后发生同碰撞和后碰撞岩浆作用,以样品中大量310~260 Ma的碎屑锆石为代表。结合南天山造山带内已有的古生代岩浆岩锆石的Hf同位素数据分析表明,晚奥陶—早泥盆世南天山造山带的大陆地壳演化主要以古老地壳的再造和部分新生地幔物质的加入为主,晚石炭—早二叠世该造山带地壳演化则以前寒武纪古老基底岩石的改造为主,仅有限的新生组分加入到岩浆的形成过程中。     

辽北地区早-中二叠世变质火山-碎屑岩年代学和地球化学特征:对华北板块北缘东段构造演化的启示SCIEI北大核心CSCD

新识别的“下二台”构造杂岩作为华北板块北缘东段分布的构造混杂岩带重要组成部分,其物质组成、形成时代和构造属性仍需进一步研究,这将为探讨华北板块北缘东段晚古生代构造演化提供重要依据。作者在“下二台”构造杂岩中识别出一套早-中二叠世变质火山-碎屑岩,其以变质碎屑岩为主,并夹变质火山岩,二者在野外产出上混杂在一起。变质火山岩原岩类型包括流纹岩、英安岩、安山岩、玄武安山岩,为一套钙碱性火山岩,属于准铝质-弱过铝质岩石。根据岩相学和地球化学特征,将其分为变质酸性火山岩和变质中-基性火山岩;二者均相对富集轻稀土元素,亏损重稀土元素,轻重稀土元素分馏明显,Eu负异常不明显,但变质酸性火山岩明显亏损P、Ti元素,结合高场强元素相关性特征,认为二者不是同一基性岩浆分异的产物。变质火山岩锆石LA-ICP-MS U-Pb同位素年龄为272~288Ma,代表其原岩结晶年龄,时代为早二叠世;变质酸性火山岩原始岩浆来源于地壳物质的部分熔融,变质中-基性火山岩原始岩浆来源于岩石圈地幔(俯冲带附近),并遭受了地壳物质的混染,二者均形成于活动大陆边缘火山弧环境。变质碎屑岩原岩恢复为泥砂质沉积岩和砂泥质沉积岩,相对亏损轻稀土元素,富集重稀土元素,轻重稀土元素分馏较明显,Eu异常不明显。两件碎屑岩样品锆石LA-ICP-MS U-Pb同位素年龄主要介于267~347Ma,推断其沉积下限为267Ma和269Ma,均为中二叠世;泥砂质沉积岩可能来源于再旋回的以长英质岩石为母岩的沉积岩,砂泥质沉积岩可能来源于再旋回的以长英质和镁铁质岩石为母岩的沉积岩,二者分别形成于活动大陆边缘大陆岛弧和大洋岛弧环境。下二台地区早-中二叠世变质火山-碎屑岩为“下二台”构造杂岩重要组成部分,它表明二叠纪时期华北板块北缘东段经历了三个构造演化阶段:早二叠世古亚洲洋加速俯冲,形成新的大陆弧阶段;中二叠世古亚洲洋持续俯冲,大陆弧和大洋弧碰撞阶段;晚二叠世陆-陆碰撞前阶段。     

Structural Seismology: Exploring the Correspondence between Surface Geological Features and Heterogeneities in the Earth’s Crust and Mantle

The North Qaidam is an Early Paleozoic UHP metamorphic belt located at the north margin of the Tibet plateau. Eclogites in this belt contain both continental‐and oceanic‐type ones. In which, the continental‐type eclogites have protolith ages of 750–850 Ma and WPB or CFB geochemical signatures and are believed to have formed in a continental rift or an incipient oceanic basin setting related to the breakup of the Rodinia supercontinent, their metamorphic ages (421–458 Ma) and P–T paths are comparable to their host gneisses; oceanic‐type eclogites have cumulate gabbro or E‐MORB geochemical signatures, their protolith and metamorphic ages are 510–516 Ma and 425–450 Ma, respectively(Zhang et al., 2008). Therefore, the North Qaidam UHP belt was thought to record the whole Neoprotoerozoic–Paleozoic Wilson cycle (Song et al., 2014). In this study, we reported three new kinds of eclogites: kyanite‐bearing eclogite, lawsonite pseudomorph‐bearing eclogite and double mineral eclogite. They occur as big lentoid blocks in regional granitic gneiss in the western part of the belt. Phase equilibrium modelling and zircon LA‐ICPMS U‐Pb dating show that all these three eclogites experienced a clockwise P–T path with peak metamorphic conditions close to or fall in the coesite stability field, and their peak metamorphic age were around 436‐439 Ma, similar to those continental‐type eclogites in this belt. But their protolith ages are between 1273 and 1070 Ma, and some of them recorded an amphibolite facies metamorphic age of 927 Ma, and geochemical data and zircon Lu‐Hf and O isotope analysis indicate these eclogites have features of present day N‐MORB. Combined with the existing results, we propose that the North Qaidam is a polycyclic composite orogenwhich recorded tectonic evolution of Mesoproterozoic ocean floor spreading, assembly and breakup of Rodinia supercontinent, Early Paleozoic oceanic deep subduction and subsequently continental deep subduction.     

阿尔泰南缘冲乎尔盆地酸性火山岩地球化学及年代学研究

阿尔泰南缘冲乎尔盆地发育一套浅变质长英质岩系,火山岩原岩为酸性凝灰质-流纹质岩。盆地岩石化学分析显示,该套火山岩为钙碱性系列,微量元素显示出与大洋板块俯冲有关的陆缘岛弧构造环境。晶屑凝灰岩的SHRIMP锆石U-Pb年代学研究表明,该套岩石的喷发年龄为436±4Ma,与盆地北缘花岗质岩石侵入时代相近(413±4Ma),属早古生代志留纪喷发而成,说明冲乎尔盆地是一个早古生代火山沉积盆地。盆地火山岩地球化学和年代学揭示,志留纪区域构造性质已经从被动大陆边缘转变为活动大陆边缘,古亚洲洋在志留纪(甚至奥陶纪)逐渐俯冲、消减,形成早古生代弧盆体系和晚奥陶世-志留纪磨拉石建造,即加里东增生陆壳(陈毓川等,2003),这种俯冲可能一直持续到泥盆纪,甚至石炭纪。从冲乎尔盆地酸性火山岩岩石学、构造环境和火山活动时代可以推断其成矿效应,盆地内产出的矿床(点)可能属于加里东期与变质-岩浆热液作用有关的有色、稀有金属-白云母成矿系列。     

北秦岭高压-超高压岩石的时空分布、P-T-t演化及其形成机制

北秦岭构造带早古生代的构造格局和演化过程一直是地学界比较关注也是存在较大争议的问题之一.在已有研究基础上,系统总结了本课题组近年来在北秦岭早古生代高压-超高压变质作用研究方面的进展,从变质作用角度对北秦岭早古生代的构造演化提供重要限定.丹凤斜长角闪岩中柯石英的发现为区内超高压变质作用的存在提供了最直接的矿物学证据;东秦岭秦岭杂岩中的斜长角闪岩普遍经历了高压-超高压榴辉岩相变质,具面状分布的特征,是陆壳俯冲/深俯冲作用的产物;高压-超高压榴辉岩和围岩片麻岩都记录了顺时针的P-T-t轨迹,峰期变质时代为500~490 Ma,之后主体又经历约470~450 Ma和约420~400 Ma两期抬升退变质叠加和部分熔融作用;高压-超高压岩石两期退变质和部分熔融发生的时代与北秦岭460~440Ma和~420Ma的两期岩浆事件的时代一致,说明北秦岭早古生代岩浆作用是深俯冲陆壳板片断离和碰撞造山结束后地壳伸展作用的岩浆响应;高压-超高压榴辉岩原岩形成时代约800 Ma,具有与南秦岭新元古代中晚期岩浆岩一致的地球化学特征,北秦岭超高压岩石的形成可能是商丹洋关闭后洋壳拖曳着南秦岭陆壳物质向北发生大陆深俯冲的结果,商丹洋在500 Ma主体应该已经关闭;秦岭岩群是部分而不是整体经历了大陆的深俯冲,现今的秦岭岩群是一个俯冲碰撞杂岩带而不是一个岩石地层单元或微陆块;北秦岭早古生代造山作用在中泥盆世已经结束,整体处于构造隆升后的剥蚀阶段,是南秦岭刘岭群碎屑岩的主要蚀源区,刘岭群沉积盆地形成于碰撞造山后的伸展构造背景而非弧前环境.