中亚造山带东段古生代山弯构造

中亚造山带东段位于西伯利亚和华北克拉通之间，经历了多构造体系叠加和多旋回洋陆转换的复杂演化过程，目前大量研究均以构造带为核心来限定区域构造格局，但一直争议较大。本文以构造单元的构造属性及其形成过程为主线，结合区域构造带演化，重新厘定了中国东北地区基本构造格局，建立了中国东北山弯构造演化模型。研究表明，古生代时期中国东北地区的主要构造单元由两个具前寒武纪基底的古老地块——额尔古纳地块和佳木斯地块及其张广才岭陆缘弧与两个古生代增生地体——兴安增生地体和松辽增生地体组成，其间由古亚洲洋分支新林- 喜桂图洋、贺根山- 嫩江洋、龙凤山洋和索伦洋分割。早古生代，西部额尔古纳地块东南部为西太平洋型活动陆缘，发育有嘎仙- 吉峰- 环宇洋内弧和头道桥等洋岛，~500 Ma随着新林- 喜桂图洋的关闭，这些洋内弧和洋岛拼贴增生至额尔古纳地块东南缘。随后贺根山- 嫩江洋的俯冲和后撤形成了一系列沟- 弧- 盆体系，持续的俯冲导致弧陆碰撞和陆缘增生，形成兴安增生地体的主体。同时，东部佳木斯地块西侧发育有龙凤山洋的安第斯型俯冲活动陆缘，形成了张广才岭陆缘弧。伴随着各大洋的俯冲和陆缘增生，额尔古纳地块和佳木斯地块以及它们的陆缘增生带构成了一个早古生代近东西向展布的地块链。南部以锡林浩特- 龙江微地块为核心发生陆缘俯冲，形成松辽增生地体雏形。索伦洋发生双向俯冲，并通过弧陆碰撞产生陆缘增生。晚古生代，伴随着古亚洲洋的北向俯冲和后撤，早期形成的地块链逐渐发生向南弯曲。二叠纪末期—中三叠世古亚洲洋俯冲消减闭合以及西北部蒙古- 鄂霍茨克洋和东部泛大洋的俯冲挤压，导致地块链进一步弯曲，同时，早期的古老地块、增生地体、弧岩浆岩、沉积建造等发生汇聚，最终形成一个以额尔古纳地块和兴安增生地体为西翼，佳木斯地块和张广才岭陆缘弧为东翼，松辽增生地体为核心的大规模山弯构造——中国东北山弯构造。     

中国东北地区蛇绿岩

我国东北地区位于中亚造山带的东段,经历了复杂的增生造山过程,其所属微陆块的基底属性及拼贴位置、洋-陆转换一直是地学界研究的热点。根据近年来的研究进展,我们将东北地区微陆块划分为额尔古纳地块、兴安增生地体、松嫩-锡林浩特地块和佳木斯地块。同时综述了东北地区蛇绿岩/蛇绿混杂岩带的时空分布、年代学及地球化学的新资料,讨论了其构造背景及俯冲-增生过程。东北地区增生造山不仅涉及古亚洲洋和古太平洋,还可能与泛大洋有关,包括早奥陶世-晚三叠世古亚洲洋主洋盆及古亚洲洋分支——新元古代-晚寒武世新林-喜桂图洋、早寒武世-晚石炭世嫩江洋、新元古代-晚志留世黑龙江洋和晚二叠世-中侏罗世牡丹江洋的演化。早石炭世末-晚石炭世初,东北地区古亚洲洋分支洋盆全部闭合,所有微陆块完成聚合形成统一的东北陆块群。晚二叠世-早三叠世时期,古亚洲洋主洋盆沿索伦-西拉木伦-长春-延吉缝合带自西向东从早到晚以剪刀式最终闭合,完成东北陆块群与华北板块的拼接。晚三叠世-早侏罗世时期古太平洋板块俯冲启动,东北地区进入古太平洋俯冲增生构造体系。     

中亚造山带中段古亚洲洋北向平板俯冲过程:来自埃达克岩的证据

早三叠世是中亚造山带（CAOB）中部构造演化的关键时期,尽管该时期古亚洲洋在地表已经闭合,但残余的大洋板片仍在持续的挤压作用下继续俯冲,造山作用依然活跃。本文对中亚造山带中段林西地区的下三叠统幸福之路组火山岩地层进行了锆石U-Pb测年、岩石地球化学以及锆石原位Lu-Hf同位素研究。LA-ICP-MS锆石U-Pb定年结果显示了该地层的形成时代为247Ma,为早三叠世岩浆活动的产物。岩石学和地球化学研究表明,幸福之路组火山岩具有高SiO₂（64.10%~68.90%）、Al₂O₃（13.47%~17.50%）,低MgO（0.51%~1.42%）,轻稀土富集、重稀土亏损,无Eu异常（δEu平均值为1.09）以及高Sr（384×10^-6~956×10^-6,平均616×10^-6）,低Y（5.66×10^-6~7.63×10^-6,平均6.51×10^-6）的特点,表明其为大洋板片熔融产生的典型埃达克岩。锆石原位Lu-Hf同位素分析结果显示其ε_Hf（t）为+10.8~+15.6,平均为+13.9。样品单阶段Hf地壳模式年龄（t_DM1）为264~462Ma,表明其岩浆源区的亏损特征。结合区域资料,我们将研究区中晚二叠世至晚三叠世划分为四个构造演化阶段：1）中-晚二叠世时期,特征为钙碱性岩浆岩及碰撞杂岩的发育;2）早-中三叠世时期,特征为广泛分布的典型埃达克岩,是古亚洲洋地表闭合后大洋板片继续北向平板俯冲的产物;3）230Ma左右开始持续10Myr,该时期是岩浆活动宁静期;4）220Ma至晚三叠世末,研究区进入区域性伸展,A型花岗岩、富钾钙碱性花岗岩类和超基性岩大量侵位,变质核杂岩及韧性剪切带也在此时产生。前三个阶段代表了完整的古亚洲洋大洋板块平板俯冲过程,而最后一个阶段标志着研究区地壳进入了新的演化阶段。

     

内蒙古梅劳特乌拉蛇绿岩中早二叠世埃达克岩与古亚洲洋东段洋内俯冲

贺根山缝合带东部晚石炭世梅劳特乌拉SSZ型蛇绿岩中的埃达克岩,岩性为安山岩和英安岩。LA-ICP-MS锆石U-Pb定年结果显示,埃达克岩的形成时间为294.1±2.2 Ma,时代为早二叠世。地球化学特征显示,该埃达克岩属于低钾拉斑系列和中钾钙碱性岩石,具有高硅（SiO2=64.12%~69.12%）、高铝（Al2O3=16.05%~18.59%）、富钠贫钾（Na2O=5.08%~6.80%,K2O=0.70%~1.22%,Na2O/K2O=4.50~7.26）、高Sr（291.22×10-6~762.20×10-6）,低Yb（0.74×10-6~1.28×10-6）、低Y（7.33×10-6~12.74×10-6）等特征。相对富集大离子亲石元素（如K、Rb和Sr）,亏损高场强元素（如Nb、Ta、Zr、Ti和P）,稀土元素总量较低（40.97×10-6~108.69×10-6）,贫重稀土元素,无明显的负Eu异常,为典型的埃达克岩。梅劳特乌拉埃达克岩形成于俯冲带岛弧环境,可能为俯冲洋壳部分熔融而形成的埃达克质熔体,经俯冲带上升过程中与地幔楔橄榄岩发生相互作用而形成。埃达克岩和梅劳特乌拉蛇绿岩（308 Ma）的蛇纹石化方辉橄榄岩、层状-块状辉长岩、枕状拉斑玄武岩、玻安岩、富Nb玄武岩和高镁安山岩等构成洋内初始俯冲作用形成的较丰富且完整的岩石组合序列。研究结果表明,晚石炭世—早二叠世古亚洲洋东段开启了洋内初始俯冲作用。     

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.     

Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt in NE China

The eastern Central Asian Orogenic Belt (CAOB) in NE China is a key area for investigating continental growth. However, the complexity of its Paleozoic geological history has meant that the tectonic development of this belt is not fully understood. NE China is composed of the Erguna and Jiamusi blocks in the northern and eastern parts and the Xing’an and Songliao-Xilinhot accretionary terranes in the central and southern parts. The Erguna and Jiamusi blocks have Precambrian basements with Siberia and Gondwana affinities, respectively. In contrast, the Xing ’an and Songliao-Xilinhot accretionary terranes were formed via subduction and collision processes. These blocks and terranes were separated by the Xinlin-Xiguitu, Heilongjiang, Nenjiang, and Solonker oceans from north to south, and these oceans closed during the Cambrian (ca. 500 Ma), Late Silurian (ca. 420 Ma), early Late Carboniferous (ca. 320 Ma), and Late Permian to Middle Triassic (260 –240 Ma), respectively, forming the Xinlin-Xiguitu, Mudanjiang-Yilan, Hegenshan-Heihe, Solonker-Linxi, and Changchun-Yanji suture zones. Two oceanic tectonic cycles took place in the eastern Paleo-Asian Ocean (PAO), namely, the Early Paleozoic cycle involving the Xinlin-Xiguitu and Heilongjiang oceans and the late Paleozoic cycle involving the Nenjiang-Solonker oceans. The Paleozoic tectonic pattern of the eastern CAOB generally shows structural features that trend east-west. The timing of accretion and collision events of the eastern CAOB during the Paleozoic youngs progressively from north to south. The branch ocean basins of the eastern PAO closed from west to east in a scissor-like manner. A bi-directional subduction regime dominated during the narrowing and closure process of the eastern PAO, which led to “soft collision” of tectonic units on each side, forming huge accretionary orogenic belts in central Asia.©2022 China Geology Editorial Office.     

Petrogenesis of Permo-Triassic intrusive rocks in Northern Liaoning Province,NE China: implications for the closure of the eastern Paleo-Asian Ocean

ABSTRACT

The Changchun-Yanji belt recorded widespread Permo-Triassic magmatism, but their origins remain unclear, inhibiting a comprehensive understanding of the magmatic response to the final closure of the Paleo-Asian ocean in the eastern Central Asian Orogenic Belt (CAOB). Here, we present new geochronological, geochemical, and Hf isotopic data for the Permo-Triassic plutons from Northern Liaoning province, NE China. Combined the published ages with our new data, the Permo-Triassic magmatism in the eastern CAOB can be divided into five episodes: early Permian (293–274 Ma), middle–late Permian (270–257 Ma), latest late Permian–Middle Triassic (255–242 Ma), Late Triassic (240–215 Ma), and latest Late Triassic (209–200 Ma). The middle Permian and Late Triassic mafic plutons (i.e. ~266 Ma Mengjiagou gabbro–diorite, ~240 Ma Jiancaicun gabbro and ~224 Ma Shudetun gabbro-diorite) contain relatively high TFe₂O₃, MgO, Cr and Ni contents with positive ε_Hf(t) values (+1.2 to +7.2), suggesting a depleted mantle origin. These mafic rocks together with the coeval granitoids make up typical bimodal associations, suggesting that they were formed under an extensional environment. The conclusions are also supported by occurrence of A-type granites during 270–257 Ma and 240–215 Ma. By contrast, the granitoids of 255–242 Ma in the eastern CAOB, including the Jianshanzi (~251 Ma) and Daganhe (~242 Ma) monzogranites, show typical geochemical features of adakitic granites, with high Sr/Y ratios and negative ε_Hf(t) values (–8.6 to – 22.0), suggesting that the magmas were generated through partial melting of thickened ancient lower crust. Combined with previous studies, a four-stage tectonic evolution scenario was proposed: (1) active continental margin stage during 293–274 Ma; (2) continuing subduction resulted in the initiation collision, moderate crustal thickening, and slab break-off during 270–257 Ma; (3) final closure of the Paleo-Asian Ocean associated with continued crustal thickening occurred during 255–242 Ma; (4) lithospheric delamination in a post-collisional extensional environment occurred during 240–215 Ma.     

Lithospheric dripping in a soft collision zone: Insights from late Paleozoic magmatism suites of the eastern Central Asian Orogenic Belt

The closure of Paleo-Asian Ocean is considered to have occurred along the Solonker Suture in the southernmost segment of the Central Asian Orogenic Belt (CAOB), the largest Phanerozoic accretionary orogen on the globe. The suture branches to the east to form the northern Hegenshan–Heihe Suture and the southern Solonker–Changchun Suture. The Hegenshan–Heihe Suture is an ideal natural laboratory for studying the post-collisional geodynamic processes operating in a soft collision zone driven by divergent double-sided subduction. Here we report results from an integrated study of the petrology, geochronology, geochemistry, and Sr–Nd–Hf isotopic compositions of the Early Carboniferous–Early Permian magmatic suite in the Hailar Basin of the Xing’an–Erguna Block. The Early Carboniferous igneous rocks are represented by 356–349 Ma andesitic tuffs, exhibiting typical subduction-related features, such as enrichment in large-ion lithophile elements and depletion in high-field-strength elements. These features, together with the relatively depleted Sr–Nd–Hf isotopic compositions, constant Nb/Y values, but highly variable Rb/Y and Ba values indicate that these rocks were generated by partial melting of a depleted mantle wedge metasomatized by slab-derived fluids. The Late Carboniferous–Early Permian magmatic suite (317–295 Ma) is characterized by high Sr contents (313–1080 ppm) and low Y contents (5–13 ppm), and these can be subdivided into calc-alkaline adakitic rocks and high-K calc-alkaline adakitic rocks. The calc-alkaline adakitic rocks have higher values of Sr/Y, (Sm/Yb)_{source normalized}, and Mg#, and lower values of Y, Yb_{source normalized}, and K₂O/Na₂O than the high-K calc-alkaline adakitic rocks, which suggests that the former was generated by partial melting of foundered lower continental crust and the latter by partial melting of normal lower continental crust. Based on our new data, in conjunction with those in previous studies, we conclude that the tectonic evolution of the Hegenshan–Heihe Suture involved Early Carboniferous double-sided subduction of the Nenjiang Ocean, latest Early Carboniferous soft collision between the Xing’an–Erguna and Songliao blocks, and Late Carboniferous–Early Permian post-collisional extension. We also propose a new geodynamic scenario in which removal of the lithospheric root might have occurred in a soft collision zone during the post-collision period via repeated and localized lithospheric dripping, which results from combined effects of hydration weakening of the lithosphere caused by pre-collision subduction and asthenospheric stirring triggered by slab break-off.     

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.     

中亚造山带北山南部柳园洋晚古生代两阶段古地理演化

甘蒙北山地区位于中亚造山带中段,其间的柳园洋是古亚洲洋的重要分支之一,关于该洋盆的演化过程存在两阶段裂谷盆地和单阶段长期俯冲两种认识。柳园洋位于北山南部石板山地块和双鹰山地块之间,石板山地块的晚古生代沉积序列和物源变化完整记录了洋盆的古地理演化过程,对于重建区域构造演化有重要意义。此次研究聚焦石板山地块独山地区的上古生界火山- 沉积序列,通过砂岩碎屑颗粒组分和碎屑锆石U- Pb- Hf同位素分析,提供柳园洋古地理演化的约束证据。本文数据表明：独山地区下—中泥盆统碎屑锆石年龄为早古生代单峰式分布（~415 Ma）,来自于柳园洋向南俯冲在石板山地块边缘而形成的岩浆弧;上石炭统—下二叠统样品中出现了明显的中元古代碎屑锆石年龄（~1426 Ma）,来自于北侧的双鹰山地块和中天山地块,指示洋盆闭合事件;下二叠统碎屑锆石年龄主要为晚石炭世—早二叠世单峰式分布（301~290 Ma）,来自于裂谷火山活动。此外,本次研究通过对北山南部已发表物源学数据的统计计算,进一步证明并完善了古生代柳园洋两阶段演化模型：中奥陶世—中泥盆世,柳园洋双向俯冲至敦煌- 石板山地块和双鹰山地块之下;晚泥盆世,柳园洋闭合,直至石炭纪晚期,敦煌- 石板山- 双鹰山地块形成了统一的陆缘环境;早—中二叠世,柳园裂谷盆地逐步发育至原洋盆地。     

中亚造山带中新生代壳幔相互作用特征与过程——新疆北部幔源岩浆岩系对比研究

新疆北部中新生代幔源岩浆岩多位于大型走滑断裂带或者裂谷带附近,岩石系列均具有高的TiO2含量,富碱、高度富集不相容元素和轻稀土(LREE),εNd(t)值低正值到负值,(87Sr/86Sr)i值较高,不相容元素比值La/Nb和Nb/U显示中新生代玄武岩浆在形成过程中的壳幔作用明显不同于古生代造山过程,岩浆来源于类似OIB源的富集地幔。而新疆北部地区的二叠纪幔源镁铁-超镁铁岩来源于亏损地幔,陆壳物质参与了岩浆的形成,源区可能混入了早期的弧组分或俯冲的洋壳。由于古生代多个洋盆闭合和增生造山作用致使中亚造山带的岩石圈不断增厚,于古生代晚期发生拆沉去根作用,软流圈亏损地幔上涌形成二叠纪镁铁-超镁铁岩系,而拆沉的岩石圈以及俯冲下沉的洋壳及其沉积物,在地幔环境熔融交代改造原始地幔,使中亚地区的地幔自古生代晚期以后逐渐转换为整体富碱、富含轻稀土(LREE)和不相容元素的富集地幔。被改造了的富集地幔在中新生代沿裂谷带或大型走滑断裂薄弱带上涌,形成了中新生代幔源岩浆的喷发和侵位。     

Opening and closure history of the Mudanjiang Ocean in the eastern Central Asian Orogenic Belt: Geochronological and geochemical constraints from early Mesozoic intrusive rocks

The tectonic evolution of the ancient Mudanjiang Ocean within the Central Asian Orogenic Belt (CAOB), is strongly debated. The ocean played an important role in the amalgamation of the Songnen and Jiamusi massifs; however, the timings of its opening and closure have remained ambiguous until now. In this study, we analyzed early Mesozoic intrusive rocks from the eastern Songnen and western Jiamusi massifs in the eastern CAOB. The new zircon UPb ages, Hf isotope data, and whole-rock major and trace element data are used to reconstruct the tectonic evolution of the Mudanjiang Ocean. Zircon UPb dating indicates that early Mesozoic magmatism in the eastern Songnen Massif occurred in three stages: Early to Middle Triassic (ca. 250 Ma), Late Triassic (ca. 211 Ma), and Early Jurassic (ca. 190 Ma). The Triassic intrusive rocks typically consist of bimodal rock suites, which include gabbros, hornblende gabbros, and granitoids. The compositional information indicates an extensional environment that was probably related to the final closure of the Paleo-Asian Ocean. We integrated the results with observations from Triassic A-type granitoids and coeval sedimentary formations in the eastern Songnen Massif, as well as depositional ages of metasedimentary rocks from Heilongjiang Complex. We conclude that the opening of the Mudanjiang Ocean took place in the Early to Middle Triassic. The Early Jurassic intrusive rocks are bimodal and include olivine gabbros, hornblendites, hornblende gabbros, gabbro diorites, and granitoids. The bimodal rock suite indicates a back-arc style extensional environment. This setting formed in relation to westward subduction of the Paleo-Pacific plate beneath the Eurasia during the Early Jurassic. Following subduction, the closure of the Mudanjiang Ocean and subsequent amalgamation of the Songnen and Jiamusi massifs happened during the late Early Jurassic to Middle Jurassic. This sequence of events is further supported by ages of metamorphism and deformation acquired from the Heilongjiang Complex. Based on these observations, we conclude that the Mudanjiang Ocean existed between the Middle Triassic and Early Jurassic, making it rather short-lived.     

Permian Peri‐glacial Deposits from Central Mongolia in Central Asian Orogenic Belt: A Possible Indicator of the Capitanian Cooling Event

A dropstone‐bearing, Middle Permian to Early Triassic peri‐glacial sedimentary unit was first discovered from the Khangai–Khentei Belt in Mongolia, Central Asian Orogenic Belt. The unit, Urmegtei Formation, is assumed to cover the early Carboniferous Khangai–Khentei accretionary complex, and is an upward‐fining sequence, consisting of conglomerates, sandstones, and varved sandstone and mudstone beds with granite dropstones in ascending order. The formation was cut by a felsic dike, and was deformed and metamorphosed together with the felsic dike. An undeformed porphyritic granite batholith finally cut all the deformed and metamorphosed rocks. LA‐ICP‐MS, U–Pb zircon dating has revealed the following ²⁰⁶Pb/²³⁸U weighted mean igneous ages: (i) a granite dropstone in the Urmegtei Formation is 273 ± 5 Ma (Kungurian of Early Permian); (ii) the deformed felsic dike is 247 ± 4 Ma (Olenekian of Early Triassic); and (iii) the undeformed granite batholith is 218 ± 9 Ma (Carnian of Late Triassic). From these data, the age of sedimentation of the Urmegtei Formation is constrained between the Kungurian and the Olenekian (273–247 Ma), and the age of deformation and metamorphism is constrained between the Olenekian and the Carnian (247–218 Ma). In Permian and Triassic times, the global climate was in a warming trend from the Serpukhovian (early Late Carboniferous) to the Kungurian long and severe cool mode (328–271 Ma) to the Roadian to Bajocian (Middle Jurassic) warm mode (271–168 Ma), with an interruption with the Capitanian Kamura cooling event (266–260 Ma). The dropstone‐bearing strata of the Urmegtei Formation, together with the glacier‐related deposits in the Verkhoyansk, Kolyma, and Omolon areas of northeastern Siberia (said to be of Middle to Late Permian age), must be products of the Capitanian cooling event. Although further study is needed, the dropstone‐bearing strata we found can be explained in two ways: (i) the Urmegtei Formation is an autochthonous formation indicating a short‐term expansion of land glacier to the central part of Siberia in Capitanian age; or (ii) the Urmegtei Formation was deposited in or around a limited ice‐covered continent in northeast Siberia in the Capitanian and was displaced to the present position by the Carnian.     

西大别造山带新元古代中期双峰式火山岩的发现对扬子陆块北缘构造演化的约束

扬子陆块北缘新元古代中期的构造机制（是俯冲- 增生造山、还是后造山- 裂谷）是华南新元古代构造演化研究中长期争论的重要科学问题。本文在西大别造山带红安桃花地区新识别出双峰式火山岩,为进一步揭示扬子陆块新元古代中期岩浆- 构造演化过程和动力学机制提供了新的制约信息。SIMS和LA- ICP- MS锆石U- Pb定年结果表明,桃花双峰式火山岩形成年龄为750~740 Ma。基性火山岩为拉斑玄武质,显示由正常洋中脊型玄武岩向弧型玄武岩演化的微量元素特征,且具有变化的全岩εNd(t)值（0. 05~+5. 96）。这些基性火山岩可能是亏损的软流圈地幔在减压条件下部分熔融的产物,岩浆经历了结晶分异和围岩混染。酸性火山岩中,徐门寨样品富硅（SiO2≥75%）和碱（K2O+Na2O≥7%）,富集REE和HFSE（如Zr、Hf、Nb、Y）,其Zr+Ce+Y+Nb≥600×10-6,10000Ga/Al≥2. 6,与分异Ａ型花岗岩类相似。它们同时具有富集的Hf- Nd同位素组成,其锆石εHf(t)值主要集中在12. 32~1. 88,而全岩εNd(t)值为13. 29~9. 60,可能是区域上新元古代早期的岩浆岩在高温低压条件下部分熔融形成的产物。综合本文和前人的资料表明,南秦岭- 桐柏- 大别- 苏鲁造山带在新元古代中期（780~635 Ma）的双峰式岩浆组合形成于一个软流圈上涌、岩石圈拉张减薄的大陆裂谷环境;高温- 低压条件下的软流圈、岩石圈地幔和地壳的熔融及其相互反应,是这一时期不同性质岩浆形成的主要原因。     

内蒙中部苏左旗早石炭世火山岩年代学与地球化学研究：对中亚造山带东部石炭纪构造演化和地壳属性的制约

本文对苏尼特左旗北部沙尔塔拉地区原定早二叠世大石寨组火山岩进行了系统的岩相学、LA-ICP-MS锆石U-Pb定年、主微量元素和锆石Hf同位素研究。定年结果显示,该套地层中火山岩的形成时代可分为～3465Ma和3351Ma两期,其岩石组合分别为玄武安山岩-安山岩-英安岩-流纹岩和英安岩-流纹岩,其形成时代为早石炭世,而非前人认为的早二叠世。玄武安山岩具有高Al_2O_3( 17%),低MgO含量(243%～270%)和Cr、Co、Ni的特点,可能是高温低压和含少量水条件下母岩浆经历了橄榄石和辉石(早)以及斜长石(晚)等分离结晶作用的产物;相对富集轻稀土元素、大离子亲石元素(如Rb、Ba、Sr)以及Pb、Zr和Hf,亏损重稀土元素和Nb、Ta和Ti,具有较高的正ε_(Hf)(t)值(892～1379),同时存在大量434～490Ma的捕获锆石,表明早石炭世早期玄武安山岩的原始岩浆应起源于陆内伸展机制下受早期俯冲流体交代的亏损岩石圈地幔部分熔融,并经历了一定程度的地壳混染。英安岩和流纹岩的主微量元素特征(富硅、铝,贫铁、镁,富集LREEs和LILEs,亏损HREEs和HSFEs等)和锆石Hf同位素组成(ε_(Hf)(t)值分别为797～1214和835～1471,t_(DM2)分别为838～577Ma和812～407Ma),暗示它们的原始岩浆主体来源于新元古代末期和早古生代新增生的地壳部分熔融,同时,335Ma英安岩和流纹岩显示出高温(平均值为876℃)和A型花岗岩的特征,表明它们形成于陆壳伸展环境。安山岩的地球化学特征显示其岩浆可能是上述中基性和酸性岩浆混合的产物。另外,研究区石炭纪以新增生地壳的部分熔融为主,而且酸性火山岩锆石Hft_(DM2)整体随岩浆活动时代变新而变年轻。综上所述,早石炭世早期玄武安山岩表现出板内成因特点,安山岩具有岩浆混合成因,玄武安山岩与同时代英安岩-流纹岩共同构成了双峰式火山岩组合,从而表明研究区早石炭世早期已经处于陆内伸展环境;而早石炭世中期酸性火山岩的形成也进一步揭示了伸展作用的持续进行。结合前人沉积、变质作用等方面的研究成果,本文认为研究区石炭纪所记录的拉张环境可能与古亚洲洋在泥盆纪闭合后的伸展环境有关。     

内蒙古迪彦庙蛇绿岩带早石炭世闪长岩成因与古亚洲洋洋内俯冲作用

由于缺少对SSZ型蛇绿岩和洋内弧火成岩的系统研究,制约了古亚洲洋东段古生代洋内俯冲过程的进一步认识。本文报道了内蒙古迪彦庙SSZ型蛇绿岩带北部新发现的巴嘎哈尔早石炭世闪长岩。LA-ICP-MS锆石U-Pb定年显示,巴嘎哈尔闪长岩的侵位年龄为324.2±1.8 Ma,其形成时代为早石炭世末期。巴嘎哈尔闪长岩SiO₂含量为57.71%～61.24%;高铝(Al₂O₃含量为15.58%～16.68%);具有相对富钠(Na₂O含量为3.29%～4.15%)、低钾(K₂O含量为1.05%～1.69%)的特征,Na₂O/K₂O比值为2.18～3.95;MgO含量较高(3.30%～4.23%),Mg^#为47.20～51.82;贫TiO₂(0.65%～0.76%)和P₂O₅(0.14%～0.15%);稀土元素总量(ΣREE为85.19×10^-6     

Geochronology and geochemistry of early Paleozoic granitic and coeval mafic rocks from the Tannuola terrane (Tuva,Russia): Implications for transition from a subduction to post-collisional setting in the northern part of the Central Asian Orogenic Belt

Early Paleozoic magmatism of the Tannuola terrane located in the northern Central Asian Orogenic Belt is important to understanding the transition from subduction to post-collision settings. In this study, we report in situ zircon U-Pb ages, whole rock geochemistry, and Sr-Nd isotopic data from the mafic and granitic rocks of the eastern Tannuola terrane to better characterize their petrogenesis and to investigate changing of the tectonic setting and geodynamic evolution. Zircon U-Pb ages reveal three magmatic episodes for about 60 Ma from ∼510 to ∼450 Ma, that can be divided into the late Cambrian (∼510–490 Ma), the Early Ordovician (∼480–470 Ma) and the Middle-Late Ordovician (∼460–450 Ma) stages. The late Cambrian episode emplaced the mafic, intermediate and granitic rocks with volcanic arc affinity. The late Cambrian mafic rocks of the Tannuola terrane may originate from melting of mantle source that contain asthenosphere and subarc enriched mantle metasomatized by melts derived from sinking oceanic slab. Geochemical and isotopic compositions indicate the late Cambrian intermediate-granitic rocks are most consistent with an origin from a mixed source including fractionation of mantle-derived magmas and crustal-derived components. The Early Ordovician episode reveal bimodal intrusions containing mafic rocks and adakite-like granitic rocks implying the transition from a thinner to a thicker lower crust. The Early Ordovician mafic rocks are formed as a result of high degree melting of mantle source including dominantly depleted mantle and subordinate mantle metasomatized by fluid components while coeval granitic rocks were derived from partial melting of the high Sr/Y mafic rocks. The latest Middle-Late Ordovician magmatic episode emplaced high-K calc-alkaline ferroan granitic rocks that were formed through the partial melting the juvenile Neoproterozoic sources.These three episodes of magmatism identified in the eastern Tannuola terrane are interpreted as reflecting the transition from subduction to post-collision settings during the early Paleozoic. The emplacement of voluminous magmatic rocks was induced by several stages of asthenospheric upwelling in various geodynamic settings. The late Cambrian episode of magmatism was triggered by the slab break-off while subsequent Early Ordovician episode followed the switch to a collisional setting with thickening of the lower crust and the intrusion of mantle-induced bimodal magmatism. During the post-collisional stage, the large-scale lithospheric delamination provides the magma generation for the Middle-Late Ordovician granitic rocks.     

Petrogenesis of Early Permian Intrusive Rocks from Southeastern Inner Mongolia, China: Constraints on the Tectonic Framework of the Southeastern Central Asian Orogenic Belt

The late Paleozoic tectonic framework of the southeastern Central Asian Orogenic Belt is key to restricting the accretion orogeny between the Siberia Craton and the North China Craton. To clarify the framework, petrogenesis of early Permian intrusive rocks from southeastern Inner Mongolia was studied. Zircon U-Pb dating for bojite and syenogranite from Ar-Horqin indicate that they were emplaced at 288–285 Ma. Geochemical data reveal that the bojite is highly magnesian and low-K to middle-K calc-alkaline, with E-MORB-type REE and IAB-like trace element patterns. The syenogranite is a middle-K calc-alkaline fractionated A-type granite and shows oceanic-arc-like trace element patterns, with depleted Sr-Nd-Hf isotopes, (87Sr/86Sr)I = 0.7032–0.7042, εNd(t) = +4.0 to +6.6 and zircon εHf(t) = +11.14 to +14.99. This suggests that the bojite was derived from lithospheric mantle metasomatized by subducted slab melt, while the syenogranite originated from very juvenile arc-related lower crust. Usng data from coeval magmatic rocks from Linxi?Ar-Horqin, the Ar-Horqin intra-oceanic arc was reconstructed, i.e., initial transition in 290–280 Ma and mature after 278 Ma. Combined with regional geological and geophysical materials in southeastern Inner Mongolia, an early Permian tectonic framework as ‘one narrow ocean basin of the PAO’, ‘two continental marginal arcs on its northern and southern’ and ‘one intra-oceanic arc in its southern’ is proposed.     

Permian Tectonic Evolution in the Middle Part of Central Asian Orogenic Belt: Evidence from Newly Identified Volcanic Rocks in the Bilutu Area,Inner Mongolia

In this study, zircon U-Pb ages, geochemical and Lu-Hf isotopic data are presented for the newly identified volcanic rocks which were considered as Bainaimiao group in Bainaimiao Arc Belt(BAB), Inner Mongolia, which could provide important constraints on the evolution of the northern part of North China Block(NCB) and BAB. Basalt to basaltic andesite and andesite to dacite were collected from two sections, which showed eruption ages of 278.2±4.1 Ma and 258.3±3.0 Ma respectively. All samples are characterized by high abundances in Al_2O_3, LREEs, and LILEs, but depleted in HFSEs. Together with high Mg# ratios and low K/tholeiite to calc-alkaline series, these features indicated that basalt to andesite was likely derived from relatively low degree partial melting of the subduction-fluid related mantle in the spinel phase. And dacite was mainly from the partial melting of crust, then affected by mantle. All samples barely went through fractional crystallization process with the slight Eu anomaly. Compared with the contemporary basalt in NCB, rocks in BAB have a complex composition of zircon and a more positive ε_(Hf)(t) value(-6.6–6.4), indicating that they had different magma sources of rocks. Though with different basements, NCB and BAB have become an integrated whole before 278 Ma. Therefore, it could be concluded that NCB and BAB belonged to the active continental margin and the PAO had not closed yet until late Permian and then it disappeared gradually and the CAOB developed into a condition of syn-post collision.     

Time scale of an early to mid-Paleozoic orogenic cycle of the long-lived Central Asian Orogenic Belt, Inner Mongolia of China: Implications for continental growth

We present a detailed, new time scale for an orogenic cycle (oceanic accretion–subduction–collision) that provides significant insights into Paleozoic continental growth processes in the southeastern segment of the long-lived Central Asian Orogenic Belt (CAOB). The most prominent tectonic feature in Inner Mongolia is the association of paired orogens. A southern orogen forms a typical arc-trench complex, in which a supra-subduction zone ophiolite records successive phases during its life cycle: birth (ca. 497–477 Ma), when the ocean floor of the ophiolite was formed; (2) youth (ca. 473–470 Ma), characterized by mantle wedge magmatism; (3) shortly after maturity (ca. 461–450 Ma), high-Mg adakite and adakite were produced by slab melting and subsequent interaction of the melt with the mantle wedge; (4) death, caused by subduction of a ridge crest (ca. 451–434 Ma) and by ridge collision with the ophiolite (ca. 428–423 Ma). The evolution of the magmatic arc exhibits three major coherent phases: arc volcanism (ca. 488–444 Ma); adakite plutonism (ca. 448–438 Ma) and collision (ca. 419–415 Ma) of the arc with a passive continental margin. The northern orogen, a product of ridge-trench interaction, evolved progressively from coeval generation of near-trench plutons (ca. 498–461 Ma) and juvenile arc crust (ca. 484–469 Ma), to ridge subduction (ca. 440–434 Ma), microcontinent accretion (ca. 430–420 Ma), and finally to forearc formation. The paired orogens followed a consistent progression from ocean floor subduction/arc formation (ca. 500–438 Ma), ridge subduction (ca. 451–434 Ma) to microcontinent accretion/collision (ca. 430–415 Ma); ridge subduction records the turning point that transformed oceanic lithosphere into continental crust. The recognition of this orogenic cycle followed by Permian–early Triassic terminal collision of the CAOB provides compelling evidence for episodic continental growth.