Geochemistry,geochronology and Hf isotope of granitoids in the Chinese Altai: Implications for Paleozoic tectonic evolution of the Central Asian Orogenic Belt

The Chinese Altai in northwestern Xinjiang has numerous outcrops of granitoids which provide critical information on accretionary orogenic processes and crustal growth of the Central Asian Orogenic Belt.Zircon U-Pb ages, Hf-isotopic compositions and whole-rock geochemistry of monzogranite and granodiorites in the Qinghe County are employed to elucidate Paleozoic tectonics of the Chinese Altai. Granodiorites have crystallization ages of 424.6 ± 3.1 Ma(MSWD = 0.23) and 404.0 ± 3.4 Ma(MSWD = 0.18);monzogranite was emplaced in the early Permian with a crystallization age of 293.7 ± 4.6 Ma(MSWD = 1.06). Both granodiorites and monzogranite are I-type granites with A/CNK ratios of 0.92 -0.97 and 1.03 -1.06, respectively. They also show similar geochemical features of high HREE and Y contents, low Sr contents and Sr/Y ratios, as well as enrichment of Cs, Rb, Th and U, and depletion of Nb, Ta, P and Ti.These geochemical features indicate that the monzogranite and granodiorites were formed in an arc setting related to subduction. The gneissic monzogranites display high SiO_2 and K_2 O contents, and belong to the high-K calc-alkaline series. In the chondrite normalized REE distribution pattern, the monzogranite samples exhibit enrichment of LREE with strong negative Eu anomalies(σE u =0.44 -0.53), zircon εHf(t) values from +7.24 to +12.63 and two-stage Hf model ages of 463 -740 Ma. This suggests that the monzogranite was generated from the mixing of pelitic and mantle material. The granodiorite samples are calc-alkaline granites with lower contents of Si O_2 and Na_2 O + K_2 O, higher contents of TiO_2, Fe_2O_3~t, MgO and CaO compared to the monzogranite samples. They also show enrichment of LREE and moderate negative Eu anomalies(σE u= 0.54 =0.81), as well as slightly higher differentiation of LREE than that of HREE. The425 Ma granodiorite has zircon εHf(t) values from -0.51 to +1.98 and two-stage Hf model ages of 1133 -1240 Ma, whereas the 404 Ma granodiorite displays those of +2.52 to +7.50 and 816 -1071 Ma.Geochemistry and zircon Hf isotopic compositions indicate that granodiorites were formed by partial melting of juvenile lower crust. Together with regional geology and previous data, the geochemical and geochronological data of the monzogranite and granodiorites from this study suggest long-lived subduction and accretion along the Altai Orogen during ca. 425 -294 Ma.     

Geochronology and provenance of detrital zircons from late Palaeozoic strata of central Jilin Province,Northeast China: implications for the tectonic evolution of the eastern Central Asian Orogenic Belt

Here we present new U–Pb and Hf isotopic data for detrital zircons obtained from six samples of late Palaeozoic units from central Jilin Province, Northeast China, and use these data and sedimentary formations to constrain the late Palaeozoic tectonic evolution of the eastern segment of the southern margin of the Central Asian Orogenic Belt. The majority of the detrital zircons from the six samples are euhedral–subhedral and exhibit oscillatory zoning, indicating a magmatic origin. Zircons from sandstones in the Devonian Wangjiajie and Xiaosuihe formations yield seven main age populations (399, 440, 921, 1648, 1864, 1911, and 2066 Ma) and two minor age populations (384 and 432 Ma), respectively. Zircons from a quartz sandstone in the Carboniferous Luquantun Formation yield four age populations (~332, 363, 402, and 428 Ma), and zircons from quartz sandstones of the Permian Shoushangou, Fanjiatun, and Yangjiagou formations yield age populations of 265, 369, 463, 503, and 963 Ma; 264, 310, 337, 486, and 529 Ma; and 262, 282, 312, 338, 380, 465, and 492 Ma, respectively. These data, together with the ages of magmatic zircons from interbedded volcanics and biostratigraphic evidence, as well as analysis of formations, give rise to the following conclusions. (1) The Wangjiajie and Xiaosuihe formations were deposited in an extensional environment during Middle and Middle–Late Devonian time, respectively. The former was sourced mainly from ancient continental material of the North China Craton with minor contributions from newly accreted crust, while the latter was sourced mainly from newly accreted crust. (2) The Luquantun Formation formed in an extensional environment during early–late Carboniferous time from material sourced mainly from newly accreted crust. (3) The Shoushangou, Fanjiatun, and Yangjiagou formations formed during a period of rapid uplift in the late Permian, from material sourced mainly from newly accreted crust.     

Geochemistry of late Palaeozoic granitoids of the Balkhash metallogenic belt,Kazakhstan: implications for crustal growth and tectonic evolution of the Central Asian Orogenic Belt

ABSTRACT

The Balkhash metallogenic belt (BMB) in Kazakhstan is a famous porphyry Cu–Mo metallogenic belt in the Central Asian Orogenic Belt (CAOB). The late Palaeozoic granitoids in the BMB are mainly high-K calc-alkaline and I-type granites, with shoshonite that formed during a late stage. Geochemical analyses and tectonic discrimination reveal a change in the tectonic environment from syn-collision and volcanic arcs during the Carboniferous to post-collision during the Permian. The late Palaeozoic granitoids from the Borly porphyry Cu deposit formed in a classical island-arc environment, and those from the Kounrad and Aktogai porphyry Cu deposits and the Sayak skarn Cu deposit are adakitic. The ε_Nd(t) values for the late Palaeozoic granitoids are between ?5.87 and +5.94, and the ε_Sr(t) values range from ?17.16 to +51.10. The continental crustal growth histories are different on either side of the Central Balkhash fault. On the eastern side, the ε_Nd(t) values of the granitoids from the Aktogai and Sayak deposits are very high, which are characteristic of depleted mantle and suggest that crustal growth occurred during the late Palaeozoic. On the western side, the ε_Nd(t) values of the granitoids from the Borly and Kounrad deposits are slightly low, which suggests the presence of a Neoproterozoic basement and the mixing of crust and mantle during magmatism. The granitoids have initial ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb values of 18.335–20.993, 15.521–15.732, and 38.287–40.021, respectively, which demonstrate an affinity between the late Palaeozoic magmatism in the BMB and that in the Tianshan, Altai, and Junggar orogens.     

Age assessments for siliciclastic metasediments of the Bodonchin tectonic sheet, the South Altai metamorphic belt

In South Mongolia, the Hercynian structures of a linear collisional thrust-and-fold zone formed in the Carboniferous are bounded by the Caledonides of Central and North Mongolia on the north, being truncated on the south by the Indosinides of the Inner Mongolia. Tectonic sheets of the Caledonides-Hercynides junction zone confined to southern flank of the Mongolian-Gobi Altai are composed of high-gradient metamorphites of the South Altai metamorphic belt. The belt of these rocks traceable northwestward in China and eastern Kazakhstan delineates margin of the North Asian Caledonian paleocontinent. According to results of the previous geochronological study, the high- and low-gradient metamorphic rocks of the belt originated respectively 385 and 360–370 Ma ago. However, tectonic position of crystalline rock sequences, which have not been dated, remains unclear. Geochronological interval postulated for these rocks is very broad, ranging from the Early Precambrian to the Devonian. Dating results obtained in this work for detrital zircons from siliciclastic metasediments of the Bodonchin tectonic sheet of the belt show that their protoliths accumulated during the time span of 460–390 Ma (Late Ordovician-Early Devonian) on a passive continental margin. Transformation of the latter into active continental margin took place in the Early Devonian, when development of the Siberian subduction zone resulted in formation of the South Altai metamorphic belt at deep crustal levels of the Caledonian paleocontinent.     

龟山杂岩的形成时代、构造属性及变质条件:对秦岭-桐柏造山带构造演化的指示

秦岭-桐柏-大别造山带是华北板块和扬子板块碰撞拼合的产物.桐柏造山带作为古生代秦岭增生造山带与中生代大别碰撞造山带之间的纽带,记录了两大板块之间从洋壳俯冲、地体增生到最终的碰撞造山过程.本文选取桐柏造山带龟山杂岩为研究对象,对其中变沉积岩、斜长角闪岩和花岗质脉体进行了岩相学、矿物学和锆石U-Pb年代学研究.结果指示变沉...     

Geochronology of the Duguer range metamorphic rocks,Central Tibet: implications for the changing tectonic setting of the South Qiangtang subterrane

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.     

中亚造山带东段西拉木伦构造带的性质与演化：来自变形和低温热年代学的约束

中亚造山带东段何时与何地关闭,从俯冲到关闭的过程以及随后的陆内演化又经历了什么主要事件,目前还存在不同认识。中亚造山带东段林西地区的蛇绿混杂岩及其周围地区的区域地质调查表明,以杏树洼蛇绿混杂岩和双井片岩为代表的西拉木伦河构造带是一个晚古生代的增生楔,在该混杂岩带中发育了典型的岩块被包裹在基质中的构造。该楔体被中、晚二叠世克德河砾岩所覆盖。增生楔中最早的近东西向构造代表了向南俯冲阶段的变形,随后继续经历向北的逆冲推覆,卷入了中、晚二叠世地层,形成了碰撞期的变形;在晚二叠世末期—三叠纪早期,蛇绿混杂岩以及上覆的克德河砾岩又经历了区域性的强烈的右行韧性剪切,并发生应变分解。晚二叠世区域性的右行韧性剪切在中亚造山带南缘普遍发育,代表了中亚造山带已经全部进入陆内环境。双井片岩也经历了与蛇绿混杂岩类似的变形事件,在增生楔下部经历变质作用,并在碰撞期抬升至地表,晚期为区域性的右行剪切。同时,结合锆石与磷灰石低温热年代学测试表明,双井片岩和蛇绿混杂岩共同经历了中、晚侏罗世源自北侧蒙古-阿霍茨克大洋关闭导致的近南北向挤压、早白垩世期间遍及东亚的区域性伸展以及晚白垩世短暂的构造反转事件。     

Transpressional deformation,strain partitioning and fold superimposition in the southern Chinese Altai,Central Asian Orogenic Belt

Transpressional deformation has played an important role in the late Paleozoic evolution of the western Central Asian Orogenic Belt (CAOB), and understanding the structural evolution of such transpressional zones is crucial for tectonic reconstructions. Here we focus on the transpressional Irtysh Shear Zone with an aim at understanding amalgamation processes between the Chinese Altai and the West/East Junggar. We mapped macroscopic fold structures in the southern Chinese Altai and analyzed their relationships with the development of the adjacent Irtysh Shear Zone. Structural observations from these macroscopic folds show evidence for four generations of folding and associated fabrics. The earlier fabric (S₁), is locally recognized in low strain areas, and is commonly isoclinally folded by F₂ folds that have an axial plane orientation parallel to the dominant fabric (S₂). S₂ is associated with a shallowly plunging stretching lineation (L₂), and defines ∼NW-SE tight-close upright macroscopic folds (F₃) with the doubly plunging geometry. F₃ folds are superimposed by ∼NNW-SSE gentle F₄ folds. The F₃ and F₄ folds are kinematically compatible with sinistral transpressional deformation along the Irtysh Shear Zone and may represent strain partitioning during deformation. The sub-parallelism of F₃ fold axis with the Irtysh Shear Zone may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear-dominant deformation (F₃) in fold zones. The strain partitioning may have become less efficient in the later stage of transpressional deformation, so that a fraction of transcurrent components was partitioned into F₄ folds.     

Age and tectonic significance of the Louth Volcanics: implications for the evolution of the Tasmanides of eastern Australia

Abstract

Re-evaluation of geochemical and geophysical datasets, and analysis of magmatic and detrital zircons from drill-core samples extracted from the Louth region of the southern Thomson Orogen (STO), augmented by limited field samples, has shown that two temporally and compositionally distinct igneous groups exist. The older Lower Devonian, calc-alkaline group corresponds to complexly folded, high-intensity curvilinear magnetic anomalies in the Louth region (Louth Volcanics) and are probable equivalents to Lower Devonian volcanics in the northern Lachlan Orogen. A younger Permo-Triassic alkaline assemblage forms part of an E–W corridor of diatremes that appears to relate to focussed lithospheric extension associated with the later stages of the Hunter–Bowen Orogeny in the New England Orogen. The alkaline group includes gabbros previously considered as Neoproterozoic, but all magmatic rocks, including alkaline basalts, contain an unusual number of xenocrystic zircons. The age spectra of the xenocrystic zircons mimic detrital zircons from Cobar Basin sedimentary rocks and/or underlying Ordovician turbidites, suggesting incorporation of upper crustal zircons into the alkaline basaltic magmas. A distinct difference of detrital zircon age spectra from central Thomson Orogen metasediments indicates the STO metasediments have greater affinities to the Lachlan Orogen, but both orogens probably began in the Early Ordovician during widespread backarc extension and deposition of turbidites in the Tasmanides. A surprising result is that Ordovician, Devonian and Permo-Triassic basaltic rocks from the STO and elsewhere in the Tasmanides, all yield the same Nd-model ages of ca 960–830 Ma, suggesting that Neoproterozoic subcontinental lithospheric mantle persisted throughout the evolution of the Tasmanide orogenic system.     

Early Palaeozoic arc-related gabbro-diorite suite in East Junggar,southern Central Asian Orogenic Belt: petrogenesis and tectonic implications

ABSTRACT

The Armantai and Kalamaili oceans were both important branches of the Palaeo-Asian Ocean between the Altai microcontinent and the Junggar juvenile crust; however, their geological evolution is not completely clear. This paper reports detailed petrological, zircon geochronological, whole-rock elemental and Sr-Nd-Pb isotopic data for a newly discovered gabbro-diorite suite (Akputo pluton) in East Junggar to constrain this issue. Quartz diorite and hornblende gabbro yield intrusion ²⁰⁶Pb/²³⁸U ages of ca. 444–440 Ma, which indicates that mafic-intermediate magmatism occurred in the earliest Silurian. The gabbroic-dioritic rocks have tholeiite to calc-alkaline affinities and are characterised by moderately fractionated rare earth element patterns with flat heavy rare earth elements, enrichment in Rb, Ba, K, Sr, and depletion in Nb, Ta, Ti. Low initial Sr and Pb isotopic compositions and positive ε_Nd(t) indicate a derivation from the isotopically depleted mantle sources. The geochemical compositions and melting calculations suggest that the gabbro-diorite suite originated from partial melting of spinel-bearing mantle sources that had been metasomatised by subduction-derived fluids and sediments. The Akputo intrusions formed in an arc setting related to the southward subduction of the Armantai Ocean. Taking the regional angular unconformity between the Ordovician sediments and Silurian volcanics into account, we further suggest that the Early to Middle Silurian was a transition period of the closure of Armantai Ocean and the initial spreading of Kalamaili Ocean.     

阿尔泰构造带喀纳斯群时代的厘定及其意义

喀纳斯群分布于阿尔泰构造带西北部阿勒泰-青河韧性剪切带以北地区,是一套巨厚的浅变质陆源碎屑岩.由于喀纳斯群时代争议较大,该套地层所具有的大地构造意义一直存在不同的认识.本文通过对采自喀纳斯群的碎屑锆石及侵入到喀纳斯群的变质花岗岩体锆石U-Pb定年,获得了其沉积的下限和上限年龄,基本上限定了喀纳斯群的时代范围.喀纳斯群碎屑岩最年轻的碎屑锆石年龄为550±18Ma,而侵入喀纳斯群的变质花岗岩体的锆石岩浆结晶年龄为523±19Ma.因此,确定喀纳斯群沉积时代为晚震旦世-早寒武世(523～550Ma).喀纳斯群是一套形成于被动大陆边缘环境的复理石沉积,本文获得的喀纳斯群时代,暗示着阿尔泰构造带存在前寒武纪大陆地壳基底.     

Geochemistry,geochronology and Hf isotope of granitoids in the northern Alxa region:Implications for the Late Paleozoic tectonic evolution of the Central Asian Orogenic Belt

The tectonic setting of the northern Alxa region during the Late Paleozoic is highly controversial.The key to resolve this controversy is to recognize the Late Paleozoic magmatic processes in the northern Alxa.In this paper,we present new zircon U-Pb ages,Hf-isotopic compositions and whole-rock geochemical data of four granitoids along the Zhusileng-Hangwula Tectonic Belt in the northern Alxa region that could provide critical information about the tectonic evolution of this region.The zircon U-Pb data could be grouped as two phases:Late Devonian granite and diorite(ca.373-360 Ma),and Late Carboniferous granodiorite(ca.318 Ma).The Late Devonian granites and diorites are metaluminous to slightly peraluminous,with A/CNK and A/NK ratios of 0.90-1.11 and0.95-2.19,respectively.The Late Devonian diorites are characterized by high MgO,Cr and Ni contents and MgO#values,together with variable ε_(Hf)(t) values from-1.0 to+1.3 and old T_(DM2) ages varied from 1283 Ma to 1426 Ma,indicating the primary magma was potentially derived from magma mixing of depleted mantle with Mesoproterozoic continental crust.Even though the Late Devonian granites yielded most positive and minor negative e_(Hf)(t) values between-1.1 to+5.7(three grains are negative) with two-stage model ages(T_(DM2)) of 1003-1438 Ma,they display low MgO,Cr and Ni contents and MgO#values,suggesting that they were mainly derived from juvenile crustal materials,mixed with a small amount of ancient crust.The Late Carboniferous granitoids are metaluminous and medium-K calc-alkaline series,with A/CNK and A/NK ratios ranging from 0.88 to 0.95 and1.75 to 1.90,respectively.These rocks were potentially derived from juvenile crustal materials mixed with depleted mantle,as evidenced by their high ε_(Hf)(t) values(+11.6 to+14.1) and young TDM2 ages(427 Ma to 586 Ma),as well as high Mg#values,and MgO,Ni and Cr contents.Our data,along with available sedimentary evidence and previous researches,indicate that the Late Devonian and Late Carboniferous rocks are arc-related granitoids under the subduction setting.The identification of arc-related granitoids in the northern Alxa region not only reveals the Late Paleozoic magmatic process in response to the subduction of Paleo Asian Ocean,but also provide significant constrains to the tectonic evolution of the Central Asian Orogenic Belt.     

Age,nature, and origin of early cretaceous plutons in the Dabie Mo Mineralization belt: the Xinxian example

The Mesozoic granitoids in the Dabie Orogen are of particular geological interest as indicators for Mesozoic lithospheric evolution and because of their close association with porphyry Mo mineralization. Here, we present a study using zircon laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb dating, petrogeochemistry, and Hf isotopic data to constrain the timing of the magmatism and petrogenesis of the Xinxian granites in the Dabie Mo mineralization belt (DMB), Henan Province, China. Field investigations combined with previously published data show that the Xinxian pluton mainly consists of four phases. Zircon LA-ICP-MS U–Pb dating yielded ages from 153.4 ± 1.1 Ma for Phase 1 to 146.4 ± 1.6 Ma for Phase 2, 131.6 ± 1.8 Ma for Phase 3, and 125.5 ± 1.5 Ma for Phase 4. The Xinxian granites have high SiO₂ contents of 74.94–78.70 wt.% (average: 76.63 wt.%), Al₂O₃ contents of 11.59–13.68 wt.% (average: 13.01 wt.%), and K₂O contents of 3.85–4.86 wt.% (average: 4.36 wt.%) with Na₂O/K₂O ratios of 0.78–1.03 (average: 0.92) and low MgO (0.04–0.15 wt.%), TiO₂ (0.03–0.13 wt.%), and P₂O₅ (0.006–0.07 wt.%) contents. They are enriched in Rb, U, K, and Hf, but depleted in Ba, Nb, Ta, Sr, P, and Ti. The zircon ε_Hf(t) values for Phases 1, 2, 3, and 4 vary as follows: from – 22.8 to – 20.3 with T_DM2 values from 2682 to 2869 Ma, from – 24.2 to – 21.2 with T_DM2 values from 2738 to 2925 Ma, from ?24.5 to ?21.5 with T_DM2 values from 2722 to 2915 Ma, and from ?22.9 to ?19.4 with T_DM2 values from 2421 to 2643 Ma, respectively. By integrating previous geological, geochronological, and geochemical data for the DMB, we propose that the Xinxian pluton was dominantly sourced from the crust. The granites were most likely derived from the partial melting of the Northern Dabie Complex (NDC) with some Yangtze lower crust and Southern Dabie Complex (SDC). The Xinxian pluton may have formed in a post-collision extensional setting.     

青河后造山岩体成因及其对阿尔泰造山带晚古生代构造演化的启示

阿尔泰造山带位于西伯利亚板块与哈萨克斯坦-准噶尔板块之间,是中亚造山带重要组成部分。长期以来,阿尔泰何时结束造山一直存在争议,阻碍了对中亚造山带晚古生代构造演化的认识。中国阿尔泰造山带南缘发育早二叠世花岗岩,具有碱性或A型花岗岩特征,能够反映碰撞后伸展的构造环境。青河岩体位于阿尔泰造山带东南部,主要由二长花岗岩和少量闪长岩组成,具有研究阿尔泰造山带晚期构造演化的条件。本文以此为切入点,对青河岩体开展年代学和地球化学工作。新的测年数据表明似斑状二长花岗岩(283±3Ma)、中细粒二长花岗岩(280±2Ma)、糜棱岩化二长花岗岩(286±2Ma)和辉长闪长玢岩(269±1)均形成于早二叠世。岩体高硅(SiO_2=61. 98%～73. 35%),富碱(K_2O+Na_2O=5. 84%～8. 72%,碱度率AR=2. 12～3. 65),低钙(CaO=1. 29～3. 76%),里特曼指数σ=2. 38～2. 54,K_2O/Na_2O=0. 78～1. 06,属于高钾钙碱性岩石系列。微量元素显示Ba、Sr、P、Ti、Nb、Ta亏损,Eu明显负异常(δ_(Eu)=0. 46～0. 78),10000×Ga/Al=2. 85～2. 47,反映具有A型花岗岩特征,可作为阿尔泰碰撞造山作用结束的标志。另外,这些岩体ε_(Hf)(t)值介于+4. 04～+11. 78之间,二阶段模式年龄(t_(DM2))分别变化于880～694Ma、923～633Ma、875～555Ma、1030～635Ma,揭示其源区主要由新元古代幔源物质或新生地壳组成。结合区域上同时代、同构造位置富碱性(A型)花岗岩研究结果,认为青河中酸性岩体成因与地幔岩浆底侵早期下地壳有关,是新元古代玄武质物质再熔,并发生结晶分异的结果。因此,阿尔泰造山带于早二叠世(286～280Ma)已经结束了碰撞造山作用,处于伸展的构造背景。     

Early Carboniferous volcanic rocks of West Junggar in the western Central Asian Orogenic Belt: implications for a supra-subduction system

The paper presents new U–Pb zircon ages and geochemical data from early Carboniferous volcanic rocks of the Wuerkashier Mountains in the northern West Junggar region, NW China, and of the Char suture–shear zone in East Kazakhstan. The study included analysis of geological setting, major and trace elements, and rock petrogenesis. Both localities host early Carboniferous volcanic units dominated by plagioclase-porphyry andesites and dacites. A West Junggar dacite yielded a ²⁰⁶Pb/²³⁸U age of 331 ± 3 Ma. The Junggar volcanic rocks are tholeiitic, and the Char samples are intermediate between tholeiitic and calc-alkaline. Both the Junggar and Char volcanic units are characterized by LREE enriched rare-earth spectra (La/Sm_n = 1.1–2.4) with Eu negative anomalies (Eu/Eu* = 0.12–1.0) and Nb-Ta minimums (Nb/Th_pm = 0.15–0.35; Nb/La_pm = 0.3–0.7) on multi-element spectra. The Junggar andesites and dacites have higher REE and HFSE (Ti, Nb, Zr, Y, and Th) compared with the Char rocks, suggesting their derivation from a different mantle source. The melting modelling in the Nb-Yb system showed that the Junggar volcanic rocks formed by low- to medium- (2–5%) degree melting of depleted mantle harzburgite and spinel lherzolite. The Char volcanic rocks formed by high-degree melting (15–20%) of spinel lherzolite and garnet-bearing peridotite. The regional geology of West Junggar and East Kazakhstan and the geochemical features of the rocks under study (i.e. depletion in Nb, Ta, and Ti and enrichment in Th, and combination of LREE enrichment and HFSE depletion) all suggest a subduction-related origin of both Junggar and Char volcanic rocks. The early Carboniferous volcanic rocks of West Junggar possibly formed by subduction of the Junggar-Balkhash ocean beneath an active margin of the Kazakhstan continent, whereas those of East Kazakhstan formed by subduction of the Irtysh-Zaisan Ocean beneath an intra-oceanic arc at the active margin of the Siberian continent.     

中国阿尔泰北部山区早泥盆世花岗岩的年龄、成因及构造意义

对阿尔泰北部山区喀纳斯及琼库尔花岗岩体进行锆石LA-ICPMS U-Pb定年分别获得398±5Ma（MSWD=2.3）和399±4Ma（MSWD=1.2）的生成年龄,确定其形成于早泥盆世。结合近期研究成果,进一步表明,阿尔泰造山带花岗岩主要形成于早一中古生代,峰期在400Ma左右。这两个花岗岩同阿尔泰其它400Ma左右的花岗岩具有相同或相似的野外产状、岩石学、地球化学特征和同位素组成,如高硅、富钾、铝,轻稀土富集,具有Eu负异常,富集大离子亲石元素（Rb、Ba、K）,具有明显的Nb、Ta、Sr、P、Ti负异常,具有负的БNd（t）值（-1.5～-0.1）和相对于区内其它花岗岩较老的Nd同位素模式年龄tDM（1.15～1.35Ga）,表明这些花岗岩具有钙碱性-高钾钙碱性特征,其物源源除了地幔物质的贡献,可能有更多地壳物质的加入。推断这些花岗岩可能是在洋壳俯冲过程中,可能是熔自深部地壳（可能是弧物质）的岩浆与来源于亏损地幔的镁铁质岩浆发生混合形成的。结合区域地质背景分析,显示喀纳斯、琼库尔岩体形成于陆弧环境,与其它同期花岗岩一起揭示了阿尔泰造山带最为强烈的一次俯冲、裂解和拼合事件。为我们已经提出的构造演化模式提供了新的证据。     

Zircon ages and Hf isotopic compositions of Permian and Triassic A-type granites from central Inner Mongolia and their significance for late Palaeozoic and early Mesozoic evolution of the Central Asian Orogenic Belt

ABSTRACT

This work presents zircon ages and Hf-in-zircon isotopic data for Permian and Triassic A-type granitoids and reviews the evolution of central Inner Mongolia, China, during the early Permian and Late Triassic. SHRIMP U–Pb dating of zircons of peralkaline granites yielded ²⁰⁶Pb/²³⁸U ages of 294 ± 4 Ma and 293 ± 9 Ma that reflect the time of Permian magmatism. Zircon ages were also obtained for Late Triassic granites (226 ± 4 Ma, 224 ± 4 Ma). Our results, in combination with published zircon ages and geochemical data, document distinct magmatic episodes in central Inner Mongolia.

The Permian peralkaline granites show typical geochemical features of A-type granites, which also have highly positive zircon ε_Hf(t) values (+4.9 – +17.1), indicating a significant contribution of an isotopically depleted source, likely formed from mantle-derived magmas. Late Triassic A-type granitoids, however, in central Inner Mongolia show large variations and mostly positive in zircon ε_Hf(t) values (?1.3 – +13.5), suggesting derivation from a mixture of crust and mantle or metasomatized lithospheric mantle with crustal contamination. The geochemical characteristics of the Permian peralkaline granites and Late Triassic A-type granitoids are consistent with a post-collisional setting and were likely related to asthenosphere upwelling during the evolution of the Northern Block and Central Asian Orogenic Belt (CAOB).     

苏鲁仰口超高压岩石SHRIMP锆石U/Pb定年与部分熔融时限

在大型碰撞造山带中,在陆壳物质深俯冲或快速折返早期,在超高压-高压条件下,易熔组分可能发生水致或脱水部分熔融,形成花岗质熔体。在超高压-高压条件下,苏鲁超高压岩石发生过部分熔融作用,形成长英质多晶体包裹体和不同尺度的花岗质岩石, 导致可观的地球化学效应。为确定苏鲁超高压岩石部分熔融的时限,对山东仰口超高压副片麻岩和其中平行片麻理的同构造钾质花岗岩脉进行了SHRIMP锆石U/Pb地质年代学、全岩地球化学和锆石内矿物包裹体的研究。副片麻岩的锆石具有典型的核-幔-边结构。核部锆石为碎屑锆石,²⁰⁶Pb/²³⁸U年龄大于282Ma,可能反映了副片麻岩的原岩包含不同成因的物质;幔部和边部的Th/U比都小于0.1,分别给出233±3Ma和214±4Ma的²⁰⁶Pb/²³⁸U 年龄,分别对应于超高压变质和角闪岩相退变质年龄。同构造花岗岩脉是富钾过铝质花岗岩(A/CNK=1.2),锆石也具有核-幔-边结构;核部锆石年龄与副片麻岩的核部锆石年龄相当,反映了该花岗岩脉的源区可能是变沉积岩;除幔部锆石的一个点具有²⁰⁶Pb/²³⁸U年龄为234.6±3.9Ma之外,其它幔部锆石位于谐和线附近,给出²⁰⁶Pb/²³⁸U年龄为220.8±2.9Ma, 该年龄代表着该花岗岩脉的形成年龄。上述数据表明,在仰口地区,超高压岩石的部分熔融作用早于角闪岩相退变质作用。     

Thermal evolution of the Tseel terrane,SW Mongolia and its relation to granitoid intrusions in the Central Asian Orogenic Belt

The timing and thermal effects of granitoid intrusions into accreted sedimentary rocks are important for understanding the growth process of continental crust. In this study, the petrology and geochronology of pelitic gneisses in the Tseel area of the Tseel terrane, SW Mongolia, are examined to understand the relationship between igneous activity and metamorphism during crustal evolution in the Central Asian Orogenic Belt (CAOB). Four mineral zones are recognized on the basis of progressive changes in the mineral assemblages in the pelitic gneisses, namely: the garnet, staurolite, sillimanite and cordierite zones. The gneisses with high metamorphic grades (i.e. sillimanite and cordierite zones) occur in the central part of the Tseel area, where granitoids are abundant. To the north and south of these granitoids, the metamorphic grade shows a gradual decrease. The composition of garnet in the pelitic gneisses varies systematically across the mineral zones, from grossular‐rich garnet in the garnet zone to zoned garnet with grossular‐rich cores and pyrope‐rich rims in the staurolite zone, and pyrope‐rich garnet in the sillimanite and cordierite zones. Thermobarometric analyses of individual garnet crystals reveal two main stages of metamorphism: (i) a high‐P and low‐T stage (as recorded by garnet in the garnet zone and garnet cores in the staurolite zone) at 520–580 °C and 4.5–7 kbar in the kyanite stability field and (ii) a low‐P and high‐T stage (garnet rims in the staurolite zone and garnet in the sillimanite and cordierite zones) at 570–680 °C and 3.0–6.0 kbar in the sillimanite stability field. The earlier high‐P metamorphism resulted in the growth of kyanite in quartz veins within the staurolite and sillimanite zones. The U–Pb zircon ages of pelitic gneisses and granitoids reveal that (i) the protolith (igneous) age of the pelitic gneisses is c. 510 Ma; (ii) the low‐P and high‐T metamorphism occurred at 377 ± 30 Ma; and (iii) this metamorphic stage was coeval with granitoid intrusion at 385 ± 7 Ma. The age of the earlier low‐T and high‐P metamorphism is not clearly recorded in the zircon, but probably corresponds to small age peaks at 450–400 Ma. The low‐P and high‐T metamorphism continued for c. 100 Ma, which is longer than the active period of a single granitoid body. These findings indicate that an elevation of geotherm and a transition from high‐P and low‐T to low‐P and high‐T metamorphism occurred, associated with continuous emplacement of several granitoids, during the crustal evolution in the Devonian CAOB.     

Structural evolution of the Irtysh Shear Zone (northwestern China) and implications for the amalgamation of arc systems in the Central Asian Orogenic Belt

The NW–SE Irtysh Shear Zone is a major tectonic boundary in the Central Asian Orogenic Belt (CAOB), which supposedly records the amalgamation history between the peri-Siberian orogenic system and the Kazakhstan/south Mongolia orogenic system. However, the tectonic evolution of the Irtysh Shear Zone is not fully understood. Here we present new structural and geochronological data, which together with other constraints on the timing of deformation suggests that the Irtysh Shear Zone was subjected to three phases of deformation in the late Paleozoic. D₁ is locally recognized as folded foliations in low strain areas and as an internal fabric within garnet porphyroblasts. D₂ is represented by a shallowly dipping fabric and related ∼ NW–SE stretching lineations oriented sub-parallel to the strike of the orogen. D₂ foliations are folded by ∼ NW–SE folds (F₃) that are bounded by a series of mylonite zones with evidence for sinistral/reverse kinematics. These fold and shear structures are kinematically compatible, and thus interpreted to result from a transpressional deformation phase (D₃). Two samples of mica schists yielded youngest detrital zircon peaks at ∼322 Ma, placing a maximum constraint on the timing of D₁–D₃ deformation. A ∼ NE–SW granitic dyke swarm (∼252 Ma) crosscuts D₃ fold structures and mylonitic fabrics in the central part of the shear zone, but is displaced by a mylonite zone that represents the southern boundary of the Irtysh Shear Zone. This observation indicates that the major phase of D₃ transpressional deformation took place prior to ∼252 Ma, although later phases of reactivation in the Mesozoic and Cenozoic are likely. The late Paleozoic deformation (D₁–D₃ at ∼322–252 Ma) overlaps in time with the collision between the Chinese Altai and the intra-oceanic arc system of the East Junggar. We therefore interpret that three episodes of late Paleozoic deformation represent orogenic thickening (D₁), collapse (D₂), and transpressional deformation (D₃) during the convergence between the Chinese Altai and the East Junggar. On a larger scale, late Paleozoic sinistral shearing (D₃), together with dextral shearing farther south, accommodated the eastward migration of internal segments of the western CAOB, possibly associated with the amalgamation of multiple arc systems and continental blocks during the late Paleozoic.