Geochronology and geochemistry of the Cambrian (~518 Ma) Chagantaolegai ophiolite in northern West Junggar (NW China): constraints on spatiotemporal characteristics of the Chingiz–Tarbagatai megazone

The Chingiz–Tarbagatai megazone is a well-known early Palaeozoic unit in eastern Kazakhstan that extends into northern West Junggar along the Tarbagatai–Xiemisitai–Sharbuti Mountains. Here, we report new geochronological and geochemical data for the Chagantaolegai ophiolite, discovered in the south Xiemisitai Mountains, north of the Heshituoluogai Valley in West Junggar. The laser ablation inductively coupled plasma mass spectrometry zircon U–Pb ages of a gabbro in the Chagantaolegai ophiolite are 517 ± 3 and 519 ± 3 Ma, corresponding to the early to middle Cambrian. Serpentinized peridotites in the ophiolite show low rare earth element (REE) abundances (0.61–0.94 μg/g) and slight loss of middle REEs. Chagantaolegai metagabbro samples have flat to very slightly depleted patterns in light REEs, with a narrow range of REE abundances. The Chagantaolegai spilites and metagabbros display similar flat patterns, which show a geochemical signature similar to mid-ocean ridge basalt (MORB) between Yb and Zr, and diverge from MORB towards the most incompatible elements in MORB-normalized multi-element diagrams. The Th/Yb vs. Nb/Yb diagram for the spilite samples also suggests a mid-ocean ridge (MOR) setting. To summarize, all the geochemical characteristics of the Chagantaolegai ophiolite point to MOR ophiolite. The positive ε_Nd(t) values (0.78 to +9.85) suggest that the gabbros and spilites were derived from a depleted mantle source with chemical inhomogeneity. The Chagantaolegai ophiolite spatiotemporal characteristics correlate well with the Kujibai and Hongguleleng ophiolites in the northern part of West Junggar, and the Zhaur and Balkybek ophiolites in eastern Kazakhstan. These data show that early Palaeozoic oceanic crust started to develop in the early to middle Cambrian in northern West Junggar. Stratigraphic correlations also suggest that the South Saur Fault and an inferred fault buried in the Heshituoluogai depression are the northern and southern boundary of the Chingiz–Tarbagatai megazone, respectively.     

Zircon U–Pb ages and tectonic implications of Paleozoic plutons in northern West Junggar,North Xinjiang,China

North Xinjiang, Northwest China, is made up of several Paleozoic orogens. From north to south these are the Chinese Altai, Junggar, and Tian Shan. It is characterized by widespread development of Late Carboniferous–Permian granitoids, which are commonly accepted as the products of post-collisional magmatism. Except for the Chinese Altai, East Junggar, and Tian Shan, little is known about the Devonian and older granitoids in the West Junggar, leading to an incomplete understanding of its Paleozoic tectonic history. New SHRIMP and LA-ICP-MS zircon U–Pb ages were determined for seventeen plutons in northern West Junggar and these ages confirm the presence of Late Silurian–Early Devonian plutons in the West Junggar. New age data, combined with those available from the literature, help us distinguish three groups of plutons in northern West Junggar. The first is represented by Late Silurian–Early Devonian (ca. 422 to 405 Ma) plutons in the EW-striking Xiemisitai and Saier Mountains, including A-type granite with aegirine–augite and arfvedsonite, and associated diorite, K-feldspar granite, and subvolcanic rocks. The second is composed of the Early Carboniferous (ca. 346 to 321 Ma) granodiorite, diorite, and monzonitic and K-feldspar granites, which mainly occur in the EW-extending Tarbgatay and Saur (also spelled as Sawuer in Chinese) Mountains. The third is mainly characterized by the latest Late Carboniferous–Middle Permian (ca. 304 to 263 Ma) granitoids in the Wuerkashier, Tarbgatay, and Saur Mountains.As a whole, the three epochs of plutons in northern West Junggar have different implications for tectonic evolution. The volcano-sedimentary strata in the Xiemisitai and Saier Mountains may not be Middle and Late Devonian as suggested previously because they are crosscut by the Late Silurian–Early Devonian plutons. Therefore, they are probably the eastern extension of the Early Paleozoic Boshchekul–Chingiz volcanic arc of East Kazakhstan in China. It is uncertain at present if these plutons might have been generated in either a subduction or post-collisional setting. The early Carboniferous plutons in the Tarbgatay and Saur Mountains may be part of the Late Paleozoic Zharma–Saur volcanic arc of the Kazakhstan block. They occur along the active margin of the Kazakhstan block, and their generation may be related to southward subduction of the Irtysh–Zaysan Ocean between Kazakhstan in the south and Altai in the north. The latest Late Carboniferous–Middle Permian plutons occur in the Zharma–Saur volcanic arc, Hebukesaier Depression, and the West Junggar accretionary complexes and significantly postdate the closure of the Irtysh–Zaysan Ocean in the Late Carboniferous because they are concurrent with the stitching plutons crosscutting the Irtysh–Zaysan suture zone. Hence the latest Late Carboniferous–Middle Permian plutons were generated in a post-collisional setting. The oldest stitching plutons in the Irtysh–Zaysan suture zone are coeval with those in northern West Junggar, together they place an upper age bound for the final amalgamation of the Altai and Kazakhstan blocks to be earlier than 307 Ma (before the Kaslmovian stage, Late Carboniferous). This is nearly coincident with widespread post-collisional granitoid plutons in North Xinjiang.     

Two geodynamic–metallogenic events in the Balkhash (Kazakhstan) and the West Junggar (China): Carboniferous porphyry Cu and Permian greisen W-Mo mineralization

This study focuses on the geochronology and elemental and Nd isotopic geochemistry of the Baogutu Cu deposit and the newly discovered Suyunhe W-Mo deposit in the southern West Junggar ore belt (Xinjiang, China), as well as the geology of the newly discovered Hongyuan Mo deposit in the southern West Junggar ore belt and the Kounrad, Borly, and Aktogai Cu deposits and the East Kounrad, Zhanet, and Akshatau W-Mo deposits in the North Balkhash ore belt (Kazakhstan). The aim is to compare their petrogenesis, tectonic setting, and mineralization and to determine the relationship between the southern West Junggar and North Balkhash ore belts. Based on our newly acquired results, we propose that the Kounrad, Borly, Aktogai, and Baogutu deposits are typical porphyry Cu deposits associated with calc-alkaline magmas and formed in a Carboniferous (327–312 Ma) subduction-related setting. In contrast, the East Kounrad, Zhanet, Akshatau, Suyunhe, and Hongyuan deposits are quartz-vein greisen or greisen W-Mo or Mo deposits associated with alkaline magmas and formed in an early Permian (289–306 Ma) collision-related setting. Therefore, two geodynamic–metallogenic events can be distinguished in the southern West Junggar and North Balkhash ore belts: (1) Carboniferous subduction-related calc-alkaline magma – a porphyry Cu metallogenic event – and (2) early Permian collision-related alkaline magma – a greisen W-Mo metallogenic event. The North Balkhash ore belt is part of the Kazakhstan metallogenic zone, which can be extended eastward to the southern West Junggar in China.     

环巴尔喀什-西准噶尔成矿省矿床类型、成矿系统和跨境成矿带对接

环巴尔喀什-西准噶尔成矿省地处中亚成矿域核心区,古生代构造和岩浆活动强烈,成矿作用丰富多样,发育许多大型-超大型乃至世界级的金属矿床,包括斑岩型铜矿床、斑岩-石英脉-云英岩型钨钼矿床、矽卡岩型铜(多金属)矿床、火山成因块状硫化物型(VMS)多金属矿床、浅成低温热液型金矿床、石英脉-蚀变岩型中温热液金矿床、与花岗岩有关的Be-U矿床、岩浆熔离型铜镍硫化物矿床和豆荚状铬铁矿等,这些矿床集中分布,形成多处成矿带,包括哈萨克斯坦的扎尔玛-萨吾尔、波谢库尔-成吉斯和北巴尔喀什等成矿带以及新疆西准噶尔的萨吾尔、谢米斯台-沙尔布提和巴尔鲁克-克拉玛依等成矿带。哈萨克斯坦包含大型-超大型和世界级金属矿床的成矿带向东是否延入新疆西准噶尔?能否实现新疆西准噶尔找矿重大突破?都是备受关注的重大地质找矿问题。本文在前人研究并结合作者工作基础上,根据成矿带的成矿构造环境、矿床类型、成矿特点和成矿时代,总结出成矿省至少发育九类成矿系统,即(1)奥陶纪-志留纪岛弧斑岩型Cu-Au成矿系统;(2)奥陶纪岛弧VMS型多金属成矿系统;(3)泥盆纪岛弧岩浆熔离型铜镍硫化物成矿系统;(4)泥盆纪与蛇绿岩有关的豆荚状铬铁矿成矿系统;(5)早石炭世岛弧斑岩-浅成低温热液型Cu-Au成矿系统;(6)石炭纪岛弧斑岩型-矽卡岩型Cu-Mo-Au成矿系统;(7)晚石炭世弧后盆地与花岗岩有关的Be-U成矿系统;(8)早二叠世岛弧或岛弧和陆缘弧过渡弧斑岩-石英脉-云英岩型Mo-W成矿系统;(9)早二叠世岛弧石英脉-蚀变岩型中温热液金成矿系统。对比研究发现境内外相邻成矿带具有相同或相似的成矿系统,二者可以对接,新疆西准噶尔三条成矿带分别是哈萨克斯坦三条成矿带的东延部分,构成了成矿省北部的扎尔玛-萨吾尔Cu-Au成矿带、中部的波谢库尔-成吉斯-谢米斯台Cu-Au-Be-U多金属成矿带和南部的北巴尔喀什-克拉玛依Cu-Mo-W-Au-Cr成矿带。新疆西准噶尔具有形成大型-超大型矿床的成矿系统和成矿条件,有望实现找矿勘探的更大突破。     

Middle Paleozoic paleomagnetism of east Kazakhstan: post-Middle Devonian rotations in a large-scale orocline in the central Ural–Mongol belt

In order to test different hypotheses concerning the Paleozoic evolution of the Ural–Mongol belt (UMB) and the amalgamation of Eurasia, we studied Middle Devonian basalts from two localities (11 sites) and Lower Silurian volcanics, redbeds, and intra-formational conglomerates from three localities (20 sites) in the Chingiz Range of East Kazakhstan. The Devonian rocks prove to be heavily overprinted in the late Paleozoic, and a high-temperature, presumably primary, southerly, and down component is isolated at only four sites from a homoclinal section. Most Silurian redbeds are found to be remagnetized in the late Paleozoic; in contrast, a bipolar near-horizontal remanence, isolated from Silurian volcanics, is most probably primary as indicated by positive tilt and conglomerate tests. Analysis of paleomagnetic data from the Chingiz Range shows that southward-pointing directions in Ordovician, Silurian, and Devonian rocks are of normal polarity and hence indicate large-scale rotations after the Middle Devonian. The Chingiz paleomagnetic directions can be compared with Paleozoic data from the North Tien Shan and with the horseshoe-shaped distribution of subduction-related volcanic complexes in Kazakhstan. Both paleomagnetic and geological data support the idea that today's strongly curved volcanic belts of Kazakhstan are an orocline, deformed mostly before mid-Permian time. Despite the determination of nearly a dozen new Paleozoic paleopoles in this study and other recent publications by our team, significant temporal and spatial gaps remain in our knowledge of the paleomagnetic directions during the middle and late Paleozoic. However, the paleomagnetic results from the Chingiz Range and the North Tien Shan indicate that these areas show generally coherent motions with Siberia and Baltica, respectively.     

准噶尔北部蛇绿岩形成时限新证据及其东、西准噶尔蛇绿岩的对比研究

准噶尔北部出露有塔尔巴哈台库吉拜蛇绿岩、洪古勒楞-和布克赛尔蛇绿岩及扎河坝-阿尔曼太蛇绿岩。洪古勒楞蛇绿岩中的堆晶辉长岩样品进行SHRIMP U-Pb定年,结果为472±8.4Ma(MSWD=1.4),限定洪古勒楞-和布克赛尔蛇绿岩形成于早奥陶世。对扎河坝蛇绿岩中斜长花岗岩采用SHRIMP U-Pb方法测定年龄为495.9±5.5Ma(MSWD=2.7),证实扎河坝蛇绿岩形成于晚寒武世-早奥陶世。塔尔巴哈台蛇绿岩、和布克赛尔-洪古勒楞蛇绿岩以及扎河坝-阿尔曼太蛇绿岩均在早奥陶世已经形成,大部分以断层侵位的形式侵位于泥盆纪-石炭系火山-沉积地层中。准噶尔北部东西三段蛇绿岩在形成时代、区域地质以及地球物理特征等方面具有可对比性,认为它们构成一条贯穿东、西准噶尔的蛇绿岩带,这为新疆北部及邻区古生代构造格局的重建以及区域构造的对比连接提供了重要信息。     

Tectonics and geodynamics of the western Central Asian Fold Belt (Kazakhstan Paleozoides)

On the basis of stratigraphical and geological data, paleogeographical and palinspastic reconstructions of the Kazakhstan Paleozoides were done; their multistage geodynamic evolution was considered; their tectonic zoning was proposed. The main stages are described: the initiation of the Cambrian and Ordovician island arcs; the development of the Kazakhstan accretionary–collisional composite continent in the Late Ordovician as a result of continental subduction and the amalgamation of Gondwana blocks with the island arcs (a long granitoid collisional belt also formed in this period); the development of the Devonian and Carboniferous–Permian active margins of the composite continent and its tectonic destruction in the Late Paleozoic.In the Late Ordovician, compensated terrigenous and volcanosedimentary complexes formed within Kazakhstania and developed in the Silurian. The Sakmarian, Tagil, Eastern Urals, and Stepnyak volcanic arcs formed at the boundaries with the Ural, Turkestan, and Junggar–Balkhash Oceans. In the late Silurian, Kazakhstania collided with the island arcs of the Turkestan and Ob'–Zaisan Oceans, with the formation of molasse and granite belts in the northern Tien Shan and Chingiz. This was followed by the development of the Devonian and Carboniferous–Permian active margins of the composite continent and the inland formation of the Early Devonian rift-related volcanosedimentary rocks, Middle–Late Devonian volcanic molasse, Late Devonian–Early Carboniferous rift-related volcanosedimentary rocks, terrigenous–carbonate shelf sediments, and carbonaceous lake–bog sediments, and the Middle–Late Carboniferous clastic rocks of closed basins. In the Permian, plume magmatism took place on the southern margin of the Kazakhstan composite continent. It was simultaneous with the formation of red-colored molasse and the tectonic destruction of the Kazakhstan Paleozoides as a result of a collision between the East European and Kazakhstan–Baikal continents.     

Unraveling the early–middle Paleozoic paleogeography of Kazakhstan on the basis of Ordovician and Devonian paleomagnetic results

It is a common concept that different tectonic units in the western part of the Central Asian Orogenic Belt were united into the landmass of the Kazakhstania continent in the Paleozoic but many important details of its history remain enigmatic and controversial. Recently published paleomagnetic data from this region demonstrate that the ~ 2000 km long horseshoe-shaped Devonian Volcanic Belt was created by oroclinal bending of an originally rectilinear active margin of Kazakhstania. Still, the Silurian and Devonian paleomagnetic results which this interpretation is based upon are limited and unevenly spread along the belt, and additional middle Paleozoic data are highly desirable. Accordingly, we studied three mid-Paleozoic objects from different segments of this volcanic belt. Two of the three new objects yielded paleomagnetic directions that fit perfectly into the oroclinal scenario, whereas the third one provided no interpretable data. The earlier history of Kazakhstania, however, remains misty. We obtained three new Ordovician results in north–central Kazakhstan and found similar inclinations but widely dissimilar declinations. Previously published data show a large scatter of Ordovician declinations in South Kazakhstan and Kyrgyzstan as well. We analyzed all seven Middle–Late Ordovician paleolatitudes and came to the conclusion that a nearly E–W trending active margin of the Kazakhstania landmass had existed at low (~ 10°S) latitudes at that time. We hypothesize that this margin of the Kazakhstania landmass collided with another island arc, called Baydaulet–Akbastau, and with the Aktau–Junggar microcontinent by the Ordovician–Silurian boundary. As a result of this collision, subduction ceased, and regional deformation, magmatism, and rotations of crustal fragments took place in most of Kazakhstania. In Silurian time, Kazakhstania moved northward crossing the equator and rotating clockwise by ~ 45°. This changed the orientation of the Kazakhstania to NW–SE, and thereby established the (rectilinear) predecessor of the modern curved Devonian Volcanic Belt.     

新疆西准噶尔北部谢米斯台山南坡蛇绿岩带 的发现及其意义

新疆西准噶尔和什托洛盖谷地以北,沿谢米斯台山南坡出露一套蛇绿混杂岩(查干陶勒盖蛇绿岩),岩石组合自下而上包括蛇纹石化橄榄岩、变质辉长岩、玄武岩、细碧岩及伴生的硅质岩、同源火山碎屑岩等,基质为蛇纹石化橄榄岩和绿片岩相变质玄武岩。蛇绿岩中辉长岩的LA-ICP-MS锆石U-Pb定年结果为517Ma±3Ma和519Ma±3Ma,代表了蛇绿岩的形成年龄。蛇绿岩中不同组分(橄榄岩、辉长岩和细碧岩)的稀土元素和微量元素特征,以及细碧岩的Nb/Yb-Th/Yb图解均显示该蛇绿岩为洋中脊(MOR)型。查干陶勒盖蛇绿岩与西准噶尔北部的塔尔巴哈台(库吉拜)蛇绿岩、洪古勒楞蛇绿岩,邻区东哈克斯坦北部的扎乌尔-塔金蛇绿岩和巴尔克别克蛇绿岩可以很好地进行对比连接,为成吉斯-塔尔巴哈台构造带在西准境内的时空特征提供更多的约束。     

西准噶尔成矿带晚古生代花岗岩类岩浆活动及其构造意义

中亚造山带是晚古生代地壳显著生长与大规模成矿的重要地区。本文采集了中亚造山带西部的西准噶尔成矿带哈图-别鲁阿嘎希及其附近地区11个岩体共33件花岗岩类样品,对其开展了岩石地球化学与同位素示踪等研究,厘定了该地区晚古生代岩浆活动的特点与大地构造环境,并与哈萨克斯坦境内的巴尔喀什成矿带晚古生代岩浆活动进行了对比。研究表明,哈图地区晚石炭世花岗岩类主要为后碰撞伸展构造环境的A型花岗岩类,别鲁阿嘎希等地区存在洋内俯冲与岛弧环境的埃达克岩,显示了西准噶尔晚古生代构造环境时空变化的复杂性。该地区花岗岩类εNd(t)值较高(+4.62~+7.53)、εSr(t)值为(-57.61~+18.21),具有中亚造山带花岗岩类的共同特征,为古生代增生的新生陆壳,其源区与亏损地幔组分具有亲缘关系,这与巴尔喀什成矿带东段的花岗岩类具有一致性。花岗岩的~(206)Pb/~(204)Pb、~(207)Pb/~(204)Pb和~(208)Pb/~(204)Pb比值范围分别为18.2776~19.1677、15.5260~15.5796和38.2080~39.0821,为造山带花岗岩类。     

新疆准噶尔晚古生代陆壳垂向生长（Ⅰ）——后碰撞深成岩浆活动的时限

准噶尔是新疆北部古生代造山带的重要组成部分，以广泛发育晚古生代后碰撞花岗岩为特征，是中亚造山带中显生宙陆壳生长作用非常显著的地区之一。根据新近获得的SHRIMP锆石U-Pb年龄，并参考已经发表的锆石U-Pb年龄，本文重新厘定了准噶尔晚古生代后碰撞深成岩浆活动的时限。按照最新的国际地质年表中石炭纪和二叠纪划分方案（Gradstein et a1．，2004），准噶尔后碰撞深成岩浆活动是从早石炭世中-晚维宪期开始、于早二叠世末期结束的。东准噶尔后碰撞深成岩浆活动发生在330-265Ma之间，而西准噶尔后碰撞深成岩浆活动的时限在340-275Ma之间，持续时间分别约65Ma。但是，在东准噶尔，后碰撞深成岩浆活动集中在330～310Ma和305～280Ma两个时段发生，而在西准噶尔，后碰撞深成岩浆活动的高峰发生在310～295Ma之间。准噶尔晚古生代后碰撞深成岩浆活动在空间上没有受到重要地质界线（如蛇绿岩带）的分隔控制，在有的地方花岗岩还可以侵位在蛇绿岩带之中。而晚古生代后碰撞深成岩浆活动不但在准噶尔分布广泛，而且在准噶尔北邻的阿尔泰造山带和南邻的天山造山带中均有出现，具有广泛的区域性。     

The formation processes and isotopic structure of continental crust of the Chingiz Range Caledonides (Eastern Kazakhstan)

According to this paper, the juvenile crust of the Chingiz Range Caledonides (Eastern Kazakhstan) was formed due to suprasubduction magmatism within the Early Paleozoic island arcs developed on the oceanic crust during the Cambrian–Early Ordovician and on the transitional crust during the Middle–Late Ordovician, as well as to the attachment to the arcs of accretionary complexes composed of various oceanic structures. Nd isotopic compositions of the rocks in all island-arc complexes are very similar and primitive (ε_Nd(t) from +4.0 to +7.0) and point to a short crustal prehistory. Further increase in the mass and thickness of the crust of the Chingiz Range Caledonides was mainly due to reworking of island-arc complexes in the basement of the Middle and Late Paleozoic volcanoplutonic belts expressed by the emplacement of abundant granitoids. All Middle and Late Paleozoic granitoids have high positive values of ε_Nd(t) (at least +4), which are slightly different from Nd isotopic compositions of the rocks in the Lower Paleozoic island-arc complexes. Granitoids are characterized by uniform Nd isotopic compositions (<2–3 ε units for granites with a similar age), and thus we can consider the Chingiz Range as the region of the Caledonian isotope province with an isotopically uniform structure of the continental crust.     

The multi-stage tectonic evolution of the Xitieshan terrane,North Qaidam orogen,western China: From Grenville-age orogeny to early-Paleozoic ultrahigh-pressure metamorphism

The geodynamic evolution of the early Paleozoic ultrahigh-pressure metamorphic belt in North Qaidam, western China, is controversial due to ambiguous interpretations concerning the nature and ages of the eclogitic protoliths. Within this framework, we present new LA-ICP-MS U–Pb zircon ages from eclogites and their country rock gneisses from the Xitieshan terrane, located in the central part of the North Qaidam UHP metamorphic belt. Xitieshan terrane contains clearly different protolith characteristics of eclogites and as such provides a natural laboratory to investigate the geodynamic evolution of the North Qaidam UHP metamorphic terrane. LA-ICP-MS U–Pb zircon dating of three phengite-bearing eclogites and two country rock gneiss samples from the Xitieshan terrane yielded 424–427 Ma and 917–920 Ma ages, respectively. The age of 424–427 Ma from eclogite probably reflects continental lithosphere subduction post-dating oceanic lithosphere subduction at ~ 440–460 Ma. The 0.91–0.92 Ga metamorphic ages from gneiss and associated metamorphic mineral assemblages are interpreted as evidence for the occurrence of a Grenville-age orogeny in the North Qaidam UHPM belt. Using internal microstructure, geochemistry and U–Pb ages of zircon in this study, combined with the petrological and geochemical investigations on the eclogites of previous literature’s data, three types of eclogitic protoliths are identified in the Xitieshan terrane i.e. 1) Subducted early Paleozoic oceanic crust (440–460 Ma), 2) Neoproterozoic oceanic crust material emplaced onto micro-continental fragments ahead of the main, early Paleozoic, collision event (440–420 Ma) and 3) Neoproterozoic mafic dikes intruded in continental fragments (rifted away from the former supercontinent Rodinia). These results demonstrate that the basement rocks of the North Qaidam terrane formed part of the former supercontinent Rodinia, attached to the Yangtze Craton and/or the Qinling microcontinent, and recorded a complex tectono-metamorphic evolution that involved Neoproterozoic and Early Paleozoic orogenies.     

Ophiolites of Iran: Keys to understanding the tectonic evolution of SW Asia: (I) Paleozoic ophiolites

Iran is a mosaic of Ediacaran–Cambrian (Cadomian; 520–600 Ma) blocks, stitched together by Paleozoic and Mesozoic ophiolites. In this paper we summarize the Paleozoic ophiolites of Iran for the international geoscientific audience including field, chemical and geochronological data from the literature and our own unpublished data. We focus on the five best known examples of Middle to Late Paleozoic ophiolites which are remnants of Paleotethys, aligned in two main zones in northern Iran: Aghdarband, Mashhad and Rasht in the north and Jandagh–Anarak and Takab ophiolites to the south. Paleozoic ophiolites were emplaced when N-directed subduction resulted in collision of Gondwana fragment “Cimmeria” with Eurasia in Permo-Triassic time. Paleozoic ophiolites show both SSZ- and MORB-type mineralogical and geochemical signatures, perhaps reflecting formation in a marginal basin. Paleozoic ophiolites of Iran suggest a progression from oceanic crust formation above a subduction zone in Devonian time to accretionary convergence in Permian time. The Iranian Paleozoic ophiolites along with those of the Caucausus and Turkey in the west and Afghanistan, Turkmenistan and Tibet to the east, define a series of diachronous subduction-related marginal basins active from at least Early Devonian to Late Permian time.     

Mesoproterozoic (Grenville-age) terranes in the Kyrgyz North Tianshan: Zircon ages and Nd–Hf isotopic constraints on the origin and evolution of basement blocks in the southern Central Asian Orogen

The North Tianshan orogenic belt in Kyrgyzstan consists predominantly of Neoproterozoic to early Paleozoic assemblages and tectonically interlayered older Precambrian crystalline complexes and formed during early Paleozoic accretionary and collisional events. One of the oldest continental fragments of late Mesoproterozoic (Grenvillian) age occurs within the southern part of the Kyrgyz North Tianshan. Using SHRIMP zircon ages, we document two magmatic events at ~ 1.1 and ~ 1.3 Ga. The younger event is characterized by voluminous granitoid magmatism between 1150 and 1050 Ma and is associated with deformation and metamorphism. The older event is documented by ~ 1.3 Ga felsic volcanism of uncertain tectonic significance and may reflect a rifting episode. Geochemical signatures as well as Nd and Hf isotopes of the Mesoproterozoic granitoids indicate melting of still older continental crust with model ages of ca 1.2 to 2.4 Ga.The Mesoproterozoic assemblages are intruded by Paleozoic diorites and granitoids, and Nd and Hf isotopic systematics suggest that the diorites are derived from melts that are mixtures of the above Mesoproterozoic basement and mantle-derived material; their source is thus distinct from that of the Mesoproterozoic rocks. Emplacement of these plutons into the Precambrian rocks occurred between 461 and 441 Ma. This is much younger than previously assumed and indicates that small plutons and large batholiths in North Tianshan were emplaced virtually synchronously in the late Ordovician to early Silurian.The Mesoproterozoic rocks in the North Tianshan may be remnants of a once larger continental domain, whose fragments are preserved in adjacent blocks of the Central Asian Orogenic Belt. Comparison with broadly coeval terranes in the Kokchetav area of northern Kazakhstan, the Chinese Central Tianshan and the Tarim craton point to some similarities and suggests that these may represent fragments of a single Mesoproterozoic continent characterized by a major orogenic event at ~ 1.1 Ga, known as the Tarimian orogeny.     

准噶尔—天山—北山蛇绿岩:对中亚造山带西南缘洋陆格局演化的制约

准噶尔、天山和北山52个蛇绿岩的地质特征、地球化学性质和同位素年代学资料系统集成研究表明它们可以分为14条蛇绿(混杂)岩带。绝大多数蛇绿岩呈"岩块+基质"的混杂岩型式沿重要断裂带(构造线)线状分布,少数蛇绿岩以构造岩片叠置方式面状产出。混杂岩的基质有蛇纹岩(碳酸盐化蛇纹岩)和糜棱岩化细碎屑岩两类,岩块既有地幔橄榄岩、基性杂岩和基性火山岩等蛇绿岩组分,也有其它非蛇绿岩组分岩石。堆晶岩出露局限,典型席状岩墙群没有发育。这些蛇绿岩可归类为SSZ(Supra-Subduction Zone)和MORB(Mid-Ocean Ridge)两种类型,前者玄武岩具大离子亲石元素(LILE)富集和高场强元素(HFS)亏损特征,后者不显示该特点;洋岛玄武岩(OIB)既可出现在SSZ型蛇绿混杂岩中,也可为MORB型的组成部分;SSZ型蛇绿混杂岩辉长岩和玄武岩比MORB型具有相对更富集的Sr-Nd同位素组成,但部分形成于弧后(间)盆地的SSZ型蛇绿岩与MORB型一致,具有近亏损地幔的Sr-Nd同位素组成。已确认的最老蛇绿岩为西准噶尔572 Ma玛依勒,次之为北山542~527 Ma月牙山—洗肠井和西准噶尔531 Ma唐巴勒,最年轻蛇绿岩为325 Ma北天山巴音沟和321 Ma北山芨芨台子。根据蛇绿岩证据,结合近年来中亚造山带古地磁、岩浆岩、高压—超高压变质岩和构造地质方面的进展,可以推断埃迪卡拉纪末期—早寒武世,古亚洲洋已达到一定规模宽度,发育洋岛和洋内弧;早古生代时期,多岛洋格局发育至鼎盛期,一系列弧地体分别归属哈萨克斯坦微陆块周缘的科克切塔夫—天山—北山线性弧、成吉思弧、巴尔喀什—西准噶尔弧体系和西伯利亚南部大陆边缘弧体系;晚古生代时期,古亚洲洋于石炭纪末期闭合,增生杂岩和弧地体组成哈萨克斯坦拼贴体系和蒙古拼贴体系两个巨型山弯构造。     

Tectonics,magmatism, and mineralization of Circum–Balkash–Junggar area in the Central Asian Orogenic Belt

ABSTRACT

The Circum–Balkhash–Junggar area, including mostly Kazakhstan, NW China, Russia, Kyrgyzstan, Tajikistan, Uzbekistan, and Mongolia, occupies an important tectonic position of the Central Asian Orogenic Belt (CAOB) (Figure 1). Tectonically, this vast area records the complicated geodynamic processes, among which the most prominent stages are the formation of the U-shaped Kazakhstan Orocline and its interactions with adjacent Altai (Altay), Junggar (West Junggar, Junggar Basin, and East Junggar), and Tianshan orogenic collages in the Palaeozoic, bearing large-scale mineral deposits. The formation of the Late Palaeozoic mineral deposits is related to the tectonic evolution of the Devonian and Carboniferous–Permian volcano-magmatic arcs in the region. However, the link between the metallogeny and the evolution of the volcano-magmatic arcs is not well understood and existing geodynamic models have not explained satisfactorily the mechanism of the huge metallogenic belt. Therefore, this special issue focuses on the formation of the Late Palaeozoic porphyry Cu deposits and their link to the tectonic evolution of the Devonian and Carboniferous–Permian volcano-magmatic arcs with emphasis on comparative studies across the international borders.     

Geochemistry and U–Pb detrital zircon dating of Paleozoic graywackes in East Junggar,NW China: Insights into subduction–accretion processes in the southern Central Asian Orogenic Belt

The southern Central Asian Orogenic Belt (CAOB) is characterized by multiple and linear accretionary orogenic collages, including Paleozoic arcs, ophiolites, and accretionay wedges. A complex history of subduction–accretion processes makes it difficult to distinguish the origin of these various terranes and reconstruct the tectonic evolution of the southern CAOB. In order to provide constraints on the accretionary history, we analyzed major and trace element compositions of Paleozoic graywackes from the Huangcaopo Group (HG) and Kubusu Group (KG) in East Junggar. The HG graywackes have relatively low Chemical Index of Alteration (CIA) values (50 to 66), suggesting a source that underwent relatively weak chemical weathering. The identical average Index of Compositional Variability (ICV) values (~ 1.1) for both the KG and HG samples point to an immature source for the Paleozoic graywackes in East Junggar, which is consistent with an andesitic–felsic igneous source characterized by low La/Th ratios and relatively high Hf contents. These graywackes are geochemically similar to continental island arc sediments and therefore were probably deposited at an active continental margin. U–Pb dating of detrital zircons from the lower subgroup of the HG yielded a young age peak at ~ 440 Ma, indicating a post-Early Silurian depositional age. However, the youngest populations of detrital zircons from the KG graywackes and the upper subgroup of the HG yielded ²⁰⁶Pb/²³⁸U ages of ~ 346 Ma and ~ 355 Ma, respectively, which suggest a post-Early Carboniferous depositional age. Because of similarities of rock assemblages, these two units should be incorporated into the Early Carboniferous Nanmingshui Formation. The detrital zircon age spectrum of the Early Paleozoic HG graywackes resembles that of the Habahe sediments in the Chinese Altai, which suggests that the ocean between East Junggar and the Chinese Altai was closed before the deposition of the sediments and that the Armantai ophiolite was emplaced prior to the Early Devonian. The differences in age spectra for detrital zircons from the post-Early Carboniferous graywackes in East Junggar and the Harlik arc indicate that the emplacement of the Kalamaili ophiolite postdates the Early Carboniferous. Therefore, a long-lasting northward subduction–accretion process is suggested for the formation of East Junggar and the reconstruction of the Early Paleozoic evolution of the southern CAOB.     

U–Pb detrital zircon geochronology and its implications: The early Late Triassic Yanchang Formation,south Ordos Basin,China

U–Pb detrital zircon geochronology has been used to identify provenance and document sediment delivery systems during the deposition of the early Late Triassic Yanchang Formation in the south Ordos Basin. Two outcrop samples of the Yanchang Formation were collected from the southern and southwestern basin margin respectively. U–Pb detrital zircon geochronology of 158 single grains (out of 258 analyzed grains) shows that there are six distinct age populations, 250–300 Ma, 320–380 Ma, 380–420 Ma, 420–500 Ma, 1.7–2.1 Ga, and 2.3–2.6 Ga. The majority of grains with the two oldest age populations are interpreted as recycled from previous sediments. Multiple sources match the Paleozoic age populations of 380–420 and 420–500 Ma, including the Qilian–Qaidam terranes and the North Qilian orogenic belt to the west, and the Qinling orogenic belt to the south. However, the fact that both samples do not have the Neoproterozoic age populations, which are ubiquitous in these above source areas, suggests that the Late Triassic Yanchang Formation in the south Ordos Basin was not derived from the Qilian–Qaidam terranes, the North Qilian orogenic belt, and the Qinling orogenic belt. Very similar age distribution between the Proterozoic to Paleozoic sedimentary rocks and the early Late Triassic Yanchang Formation in the south Ordos Basin suggests that it was most likely recycled from previous sedimentary rocks from the North China block instead of sediments directly from two basin marginal deformation belts.     

Provenance of Neoproterozoic and early Paleozoic siliciclastic rocks of the Teplá-Barrandian unit (Bohemian Massif): Evidence from U–Pb detrital zircon ages

New LA-ICP-MS U–Pb detrital zircon ages from Ediacaran and Paleozoic siliciclastic rocks are used to constrain provenance and paleogeographic affinities of the Teplá-Barrandian unit (TBU) in the centre of the Bohemian Massif (Central Europe, Czech Republic). The samples taken span the period from ≤ 635 Ma to ～ 385 Ma and permit recognition of provenance changes that reflect changes in geotectonic regime. Detrital zircon age spectra of two Ediacaran, one Lower Cambrian and three Upper Ordovician samples resemble the ages known from the NW African proportion of Gondwana, particularly the Trans-Saharan belt, while three rocks from higher Lower Cambrian to Lowermost Ordovician strata contain detritus that may have been derived exclusively from local sources. The age spectrum of the Devonian rock is a combination of the NW Gondwanan and local features. These new findings in combination with a wide range of published data are in agreement with a Neoproterozoic subduction-related setting at the margin of Gondwana followed by a Cambrian/Early Ordovician rifting stage and an Ordovician passive margin setting. Furthermore the data are in favour of a position of the TBU at the Gondwanan margin throughout pre-Variscan times.