Kazakhstania Research Articles

Latest Tournaisian–late Viséan foraminiferal biozonation (MFZ8–MFZ14) of the Valiabad area, northwestern Alborz (Iran): geological implications

The Mobarak Formation in the Valiabad area (northwestern Alborz, Iran) is composed of bioclastic, oolitic and sandy limestone interbedded with black shale, and is disconformably underlain and overlain by the Cambrian Lalun and Permian Dorud formations, respectively. In this study, 104 foraminiferal species belonging to 12 families and 33 genera were determined. Among them, six genera and nine species are reported for the first time in Iran. Analysis of the foraminiferal assemblages has identified eight local biozones, which can be correlated with the MFZ8 to MFZ14 zones of the Viséan stratotypes in Belgium. The Valiabad equivalents of these biozones are essentially characterized by (1) Eoparastaffella ex gr. rotunda‐‘florigena’–Lysella cf. gadukensis; (2) Eoparastaffella simplex–Lapparentidiscus bokanensis; (3) Ammarchaediscus; (4) Uralodiscus–Glomodiscus; (5) Glomodiscus–Archaediscus; (6) Pojarkovella–Mstinia fallax; (7) Mstinia bulloides–Pseudoendothyra; and (8) Howchinia gibba–Howchinia bradyana–Tubispirodiscus attenuatus. Consequently, the Valiabad section appears to be one of the most complete Viséan sections in Iran. Some taxonomic precisions are provided about the principal taxa. Biogeographically, (1) the MFZ8–MFZ11 biozones are extended to all the shelves of the Palaeotethys (from Ireland to South China) and Urals oceans; nevertheless, due to the complete evolution of archaediscoids, they seem more related to the Perigondwanan assemblages from Sinai and Taurus (including the Antalya Nappes); (2) the MFZ12 assemblage appears relatively endemic; and (3) the impoverished assemblages of the biozones MFZ13‐14 have marked affinities with the Kazakhstan Block. It is currently impossible to indicate precisely if these variations are related with a drift of the Alborz region to the north, or to a change of oceanic currents. Moreover, the double affinity highlights the narrowness of the Palaeotethys in Iran during the Viséan. Copyright © 2014 John Wiley & Sons, Ltd.

Restoration of geometry and emplacement mode of the Permian mafic dyke swarms in Keping and its adjacent areas of the Tarim Block, NW China

Abstract The Permian mafic dyke swarms are widely distributed in the Tarim Block, East Tianshan and the western Junggar Block in northwestern China. These mafic dyke swarms, together with coeval basalts, mafic–ultramafic intrusions and A-type granitic plutons, constitute the Permian Tarim large igneous province (LIP), which is considered to be mantle plume-derived. In this study, multi-source high-resolution remote sensing images are used to identify the geometry and emplacement mode of the mafic dykes, sills and flood basalts in Keping and its adjacent areas including Bachu (northwestern Tarim Block), Beishan and Kuluketag (northeastern Tarim Block) and Kelamyi (western Junggar Block). Structural analysis and paleomagnetic results are used to restore the primary geometry of the mafic dykes. The results indicate that the lengths and thicknesses among a total of 117 extracted mafic dykes in Keping follow negative exponent size distributions. The mafic dykes have a mean thickness of 3.8 m with a maximum of 21.4 m. The length of unconnected mafic dyke segments ranges from 0.127 km to 17.1 km, with an arithmetic length of 1.8 km. The restoration of geometry of the mafic dykes indicates that the Permian mafic dyke swarms in Keping may have extended as far as 61–69 km along a primary orientation of about N320°W, and the flood basalts of the Tarim LIP may have extended to Keping so that the areal extent of the Tarim LIP could be enlarged for about 12,000 km 2 compared with that originally reported (about 250,000 km 2 ). The geometric features of the mafic dyke swarms in Keping and its adjacent areas are different from those of the giant radiating dyke swarms due to radial fractures associated with domal uplift. The mafic dyke swarms and sills in the Tarim LIP make up the plumbing system of the mantle plume. Accompanied by the upwelling of mantle plume, many eruptive centers and regional dykes are generated in Beishan, East Tianshan and Tarim basin. The flood basalts in Keping are fissure-type, which may have been fed by magma conduits from the plume center; whereas in Bachu, the central volcanic eruption is dominant.

Paleomagnetic study of Late Paleozoic rocks in the Tacheng Basin of West Junggar (NW China): Implications for the tectonic evolution of the western Altaids

To better understand the tectonic evolution of West Junggar in the western Altaids, we have conducted a combined paleomagnetic and geochronological study on Late Devonian (D3), Late Carboniferous (C2) and Early Permian (P1) successions from the Tacheng Basin, northwest of China. Both primary and secondary characteristic remanent magnetization components have been successfully isolated and yield three reliable paleomagnetic poles of C2 and Late Permian (P2) ages. The C2 and P2 poles reveal a significant clockwise rotation of 35.6°±10.4° which is comparable to the rotation deduced in the western part of the Chingiz Range during the time interval C2 to P2. Similar rotation patterns for the two units suggest that the Chingiz–West Junggar has remained as a quasi-rigid block by the P2 time and constituted the northeast limb of the Kazakhstan Orocline. The differential rotation of ca. 25° to 30° deduced between the northeast and middle limbs of the Kazakhstan Orocline further suggests that at least a part of the curvature of the Balkhash Arc was acquired during the C2 to P2 interval and was a continuation of the middle Paleozoic oroclinal bending. Moreover, the C2 poles for West Junggar and the Junggar Block are significantly different while the P2 ones almost overlap supporting the view that final closure of the Balkhash–Junggar Ocean occurred during the C2 to P2 interval. On the other hand, a large-scale counterclockwise rotation of ca. 80° to 90° is observed in West Junggar and Junggar relative to the western part of the Chingiz Range since the P2 time, and this rotation probably represents an important intra-continental deformation within the western part of the Altaids. A reconstruction of the Paleo-Asian Ocean has been proposed in which the Kazakhstan Orocline was emphasized by using the paleomagnetic results and geological evidence.

Geochemical characteristics and tectonic setting of the Tuerkubantao mafic-ultramafic intrusion in West Junggar, Xinjiang, China

Mineral chemistry, whole-rock major oxide, and trace element compositions have been determined for the Tuerkubantao mafic-ultramafic intrusion, in order to understand the early Paleozoic tectonic evolution of the West Junggar orogenic belt at the southern margin of the Central Asian orogenic belt. The Tuerkubantao mafic-ultramafic intrusion is a well-differentiated complex comprising peridotite, olivine pyroxenite, gabbro, and diorite. The ultramafic rocks are mostly seen in the central part of the intrusion and surrounded by mafic rocks. The Tuerkubantao intrusive rocks are characterized by enrichment of large ion lithophile elements and depleted high field strength elements relative to N-MORB. In addition, the Tuerkubantao intrusion displays relatively low Th/U and Nb/U (1.13–2.98 and 2.53–7.02, respectively) and high La/Nb and Ba/Nb (1.15–4.19 and 37.7–79.82, respectively). These features indicate that the primary magma of the intrusion was derived from partial melting of a previously metasomatized mantle source in a subduction setting. The trace element patterns of peridotites, gabbros, and diorite in the Tuerkubantao intrusion have sub-parallel trends, suggesting that the different rock types are related to each other by differentiation of the same primary magma. The intrusive contact between peridotite and gabbro clearly suggest that the Tuerkubantao is not a fragment of an ophiolite. However, the Tuerkubantao intrusion displays many similarities with Alaskan-type mafic-ultramafic intrusions along major sutures of Phanerozoic orogenic belts. Common features include their geodynamic setting, internal lithological zoning, and geochemistry. The striking similarities indicate that the middle Devonian Tuerkubantao intrusion likely formed in a subduction-related setting similar to that of the Alaskan-type intrusions. In combination with the Devonian magmatism and porphyry mineralization, we propose that subduction of the oceanic slab has widely existed in the expansive oceans during the Devonian around the Junggar block.

Zircon U-Pb age, geochemistry and geological implications of granitoids in Tuerkubantao, Xinjiang

Three types of granitoids were recognized in Tuerkubantao(土尔库班套) district, located on the northwestern margin of the Junggar Block. The zircon U-Pb age of the three types of granitoids suggests that they formed in different orogenic stage in response to the convergence and collision between Siberian and Kazakhstan plates. Biotite granite is similar to the volcanic arc granites in composition, with zircon U-Pb age of 382.8±2.5 and 355.6±2.4 Ma. The former is crystallizing age of the volcanic arc granite, while the later record the time of hydrothermal alteration induced by a later magmatism. The granite is severe altered and its zircon grain get a lower intercept age of 324.4±7.3 Ma, which is possibly coursed by magmatism in syn-collision. The rhyolite with composition similar to A-type granites in adjacent area, crystallized at 295.9±1.4 Ma, is possibly the extrusive phase of A-type granite formed in the post-orogenic tectonic setting. The granitoids in Tuerkubantao area record the evolution of the northwestern Junggar orogen. The area has been in convergence setting from Middle to Late Devonian, and stepped into syn-collisional tectonic setting in Early Carboniferous, finally evolved into the post-orogenic tectonic stage since Early Permian.

Magma mixing in the genesis of the Kalatongke dioritic intrusion: Implications for the tectonic switch from subduction to post-collision, Chinese Altay, NW China

The Kalatongke complex, located in south Chinese Altay of the Central Asian Orogenic Belt (CAOB), consists of the ~308Ma dioritic suite intruded by the 287Ma noritic suite. The noritic suite is composed of olivine norite and hornblende norite and hosts sulfide mineralization. The dioritic suite is composed of ferrodiorite, diorite and quartz diorite.Ferrodiorites have 43.6 to 51.8wt.% SiO2, 8.2 to 12.1wt.% Fe2O3T, 2.87 to 4.86wt.% MgO, with Mg# ranging from 42 to 53. They have highly variable Al2O3 (17.4–20.7wt.%), Na2O (2.78–4.50%), K2O (0.58–3.70%), Sr (280–1300ppm) and Ba (219–3100ppm). Diorites and quartz diorites have 15.2–19.6wt.% Al2O3, 239–890ppm Sr and170–1100ppm Ba, 11.2–25.5ppm Y and1.2–2.2ppm Yb. All rocks have LREE enriched REE patterns and do not display Eu anomalies. They have primitive mantle-normalized trace element patterns enriched in large ion lithosphile elements (LILE) (e.g. Ba and Sr) and depleted in Ti, Nb and Ta.Both ferrodiorites and diorites have relatively constant initial 87Sr/86Sr ratios of 0.7037 to 0.7040 but highly variable εNd(t) values of +4.5 to +11.6, whereas quartz diorites have higher initial 87Sr/86Sr ratios (0.7056) and lower εNd(t) values (+0.5). Plagioclase from ferrodiorites, diorites and quartz diorites has similar initial 87Sr/86Sr ratios of 0.7015 to 0.7055.The dioritic suite formed from a mixed magma of an evolved mantle-derived basaltic magma by partial melting of subduction-modified mantle source in response to slab breakoff and a crustal-derived granitic magma at ~308Ma. Before mixing with granitic magmas, basaltic magmas underwent fractional crystallization of olivine to form ferrodiorite. The mixed magma also formed diorite and quartz diorite combining fractionation clinopyroxene, amphibole and plagioclase. The noritic suite formed from a mantle-derived high-Mg magma in a post-collisional environment. Both the dioritic and noritic suites in Kalatongke suggest the switch from subduction to extension and the final collision between the Junggar Block and Chinese Altay at ~308Ma.

Architecture and evolution of accretionary orogens in the Altaids collage: The early Paleozoic West Junggar (NW China)

Understanding the development and evolution of accretionary orogens is crucial for characterizing continental crust growth in time and space. In the Altaids tectonic collage of Central Asia, conflicting geodynamic models have been proposed about the Paleozoic evolution, during which juvenile continental crust has been extensively formed. This study focuses on West Junggar (NW China), a key region that has not been extensively studied yet. A multidisciplinary approach, including detrital zircon provenance study, geochemistry, and field structural analysis, provides new constraints on the evolution of three Early Paleozoic units of West Junggar. Oceanic subduction predominates in Early Paleozoic time, as indicated by ophiolitic melanges, fore-arc volcaniclastic turbidites and magmatic arc suites. However, the development of an olistostrome supplied by a sub-contemporaneous carbonate platform, the occurrence of continental molasse, and the chronology and geochemistry of magmatic events reveal interruptions of subduction. Discrete collisions of volcanic arcs and micro-continent are inferred from the tectonic structure of the belt, which displays allochthonous units rooted in the suture zone. Early Paleozoic magmatic and sedimentary rocks of West Junggar bear some resemblance to contemporaneous analogues in Eastern Kazakhstan, and suggest a lateral connection. On the basis of these new results, we propose a geodynamic evolution in four steps: 1) Early to Middle Ordovician intra-oceanic subduction forming island arcs, 2) Late Ordovician tectonic accretion of island arcs against the Kazakhstan continental margin, 3) Early to Middle Silurian resumption of subduction and active continental margin magmatism, and 4) Late Silurian-Early Devonian micro-continent collision. This scenario, consistent with the formation of the Kazakhstan Block, supports a model of multiple episodic accretions for the Altaids tectonic collage rather than a continuous continental margin evolution. These results also suggest that, in Central Asia, continental crust growth results from alternating vertical and horizontal growth episodes throughout the Paleozoic.

Palaeozoic tectonic evolution of the Tianshan belt, NW China

The Chinese Tianshan belt is a major part of the southern Central Asian Orogenic Belt, extending westward to Kyrgyzstan and Kazakhstan. Its Paleozoic tectonic evolution, crucial for understanding the amalgamation of Central Asia, comprises two stages of subduction-collision. The first collisional stage built the Eo-Tianshan Mountains, before a Visean unconformity, in which all structures are verging north. It implied a southward subduction of the Central Tianshan Ocean beneath the Tarim ac-tive margin, that induced the Ordovician-Early Devonian Central Tianshan arc, to the south of which the South Tianshan back-arc basin opened. During the Late Devonian, the closure of this ocean led to a collision between Central Tianshan arc and the Kazakhstan-Yili-North Tianshan Block, and subsequently closure of the South Tianhan back-arc basin, producing two su-ture zones, namely the Central Tianshan and South Tianshan suture zones where ophiolitic melanges and HP metamorphic rocks were emplaced northward. The second stage included the Late Devonian-Carboniferous southward subduction of North Tianshan Ocean beneath the Eo-Tianshan active margin, underlined by the Yili-North Tianshan arc, leading to the collision between the Kazakhstan-Yili-NTS plate and an inferred Junggar Block at Late Carboniferous-Early Permian time. The North Tianshan Suture Zone underlines likely the last oceanic closure of Central Asia Orogenic Belt; all the oceanic domains were consumed before the Middle Permian. The amalgamated units were affected by a Permian major wrenching, dextral in the Tianshan. The correlation with the Kazakh and Kyrgyz Tianshan is clarified. The Kyrgyz South Tianshan is equivalent to the whole part of Chinese Tianshan (CTS and STS) located to the south of Narat Fault and Main Tianshan Shear Zone; the so-called Middle Tianshan thins out toward the east. The South Tianshan Suture of Kyrgyzstan correlates with the Central Tianshan Suture of Chinese Tianshan. The evolution of this southern domain remains similar from east (Gangou area) to west until the Talas-Ferghana Fault, which reflects the convergence history between the Kazakhstan and Tarim blocks.

Early Paleozoic (ca. 465 Ma) eclogites from Beishan (NW China) and their bearing on the tectonic evolution of the southern Central Asian Orogenic Belt

The Beishan orogen is situated in the southern margin of the Central Asian Orogenic Belt (CAOB). It is connected with the Tianshan orogen to the west and the Mongolia-Xing’anling orogen to the east. This orogen exhibits a well-preserved Neoproterozoic to Late Paleozoic sequence containing ophiolites, accretionary complexes and eclogites. In this paper we provide new geochronological data for eclogites and their country paragneiss to constrain the tectonic evolution of the Beishan orogen. U–Pb zircon dating of eclogites yielded two concordant ages, 886 ± 4 Ma for the cores and 465 ± 10 Ma for the rims. The zircon cores display bright luminescence with homogeneous or planar/sector zoning, intermediate Th/U ratios (0.17–0.47), and HREE-enriched patterns with negative Eu anomalies (Eu/Eu * = 0.16–0.58). These features, coupled with their formation temperatures of 680–710 °C, suggest that the core age represents the time of an earlier upper amphibolite facies metamorphic episode. By contrast, the zircon rims have dark luminescence with homogeneous or fir-tree zoning, low Th/U ratios (<0.02), and flat HREE distributions with small or no negative Eu anomalies (Eu/Eu * = 0.55–1.16). Thus, the rim age reflects the time of the eclogite facies metamorphism. The occurrence of tiny inclusions of garnet, omphacite and rutile in the zircon rims is consistent with the above age interpretation. In addition, the mineral inclusions and host zircons yielded P–T conditions of 660 °C and >12 kbar for the eclogite facies metamorphism. On the other hand, zircon dating of a paragneiss near the eclogite produced two major groups of ages, ca. 1450 Ma for a magmatic event and ca. 900 Ma for a metamorphic event. It seems that eclogites and their country rocks have suffered the same earlier metamorphic episode. Based on the available age data obtained for eclogites, ophiolites and granitoids from the Beishan area, a tectonic scenario can be constructed as follows. The ca. 1000 Ma oceanic crust representing by protoliths of the eclogites was involved in Early Neoproterozoic orogenesis to have formed a microcontinental block. During the Early Paleozoic, island arcs and microcontinental blocks were progressively accreted to the northern margin of the Tarim craton (Dunhuang block). This was followed by the closure of Paleoasian Ocean and final amalgamation between the Tarim craton and Kazakhstan block in the Late Paleozoic.

Late Paleozoic paleogeographic reconstruction of Western Central Asia based upon paleomagnetic data and its geodynamic implications

Carboniferous to Permian volcanoclastic rocks have been collected from South Junggar and West Junggar. Primary magnetizations have been observed from the characteristic components of 10 sites of Early Permian (P 1) and Late Permian (P 2) red beds of South Junggar area. The 14 Early Carboniferous sites from West Junggar Mountains expose post-folding secondary magnetizations and according to their spatial distribution, 9 remagnetized sites are related to Late Carboniferous–Early Permian granite emplacement whereas 5 sites are located at the vicinity of Late Permian mafic dykes. Two new paleomagnetic poles have been consequently calculated for the periods of P 1 at 79.5°N, 36.6°E and of P 2 at 60.4°N, 4.7°E, with A 95 of 6.8° and 5.4°, respectively. They yield two paleomagnetic poles at 65.3°N, 329.7°E with A 95 of 6.3 and 64.8°N, 179.5°E with A 95 of 6.9°, respectively. Compilation of available data shows stationary and consistent poles for South Junggar area during the Carboniferous and Permian whereas NW Junggar underwent a significant anticlockwise rotation between the Late Carboniferous–Early Permian and the Late Permian, indicating that Junggar was not a rigid block up to the end of the Paleozoic. West Junggar and South Junggar may have experienced contrasting tectonic evolutions. Comparisons of Late Paleozoic poles of Central Asia blocks show: (1) counterclockwise rotation of West Junggar with respect to Siberia, contrasting with the clockwise rotation of North Kazakhstan with respect to Siberia, (2) no significant movements between West Junggar, North Kazakhstan and Siberia since Late Permian, indicating that they were rigidly welded since that time, and (3) anticlockwise rotations of Tarim, Yili and South Junggar with respect to the welded Siberia–Kazakhstan–West Junggar block. Such rotations may have been accommodated by Late Permian to Early Triassic strike-slip faults with an estimation of the displacements of 1570 ± 280 km along the Irtysh–Gornotsaev Shear Zone, 410 ± 380 km along the Nikolaiev-Nalati Tectonic Line and 490 ± 250 km along the Chingiz-Alakol-North Tian Shan Fault since Late Permian time.

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.

Late Paleozoic volcanic record of the Eastern Junggar terrane, Xinjiang, Northwestern China: Major and trace element characteristics, Sr–Nd isotopic systematics and implications for tectonic evolution

The Eastern Junggar terrane of the Central Asian Orogenic Belt includes a Late Paleozoic assemblage of volcanic rocks of mixed oceanic and arc affinity, located in a structurally complex belt between the Siberian plate, the Kazakhstan block, and the Tianshan Range. The early history of these rocks is not well constrained, but the Junggar terrane was part of a Cordilleran-style accreted arc assemblage by the Late Carboniferous. Late Paleozoic volcanic rocks of the northern part of the east Junggar terrane are divided, from base to top, into the Early Devonian Tuoranggekuduke Formation (Fm.), Middle Devonian Beitashan Fm., Middle Devonian Yundukala Fm., Late Devonian Jiangzierkuduke Fm., Early Carboniferous Nanmingshui Fm. and Late Carboniferous Batamayineishan Fm. We present major element, trace element and Sr–Nd isotopic analyses of 64 (ultra)mafic to intermediate volcanic rock samples of these formations. All Devonian volcanic rocks exhibit remarkably negative Nb, Ta and Ti anomalies on the primitive mantle-normalized trace element diagrams, and are enriched in more highly incompatible elements relative to moderately incompatible ones. Furthermore, they have subchondritic Nb/Ta ratios, and their Zr/Nb and Sm/Nd ratios resemble those of MORBs, characteristics of arc-related volcanic rocks. The Early Devonian Tuoranggekuduke Fm., Middle Devonian Beitashan Fm., and Middle Devonian Yundukala Fm. are characterized by tholeiitic and calc-alkaline affinities. In contrast, the Late Devonian Jiangzierkuduke Fm. contains a large amount of tuff and sandstone, and its volcanic rocks have dominantly calc-alkaline affinities. We therefore propose that the Jiangzierkuduke Fm. formed in a mature island arc setting, and other Devonian Fms. formed in an immature island arc setting. The basalts from the Nanmingshui Fm. have geochemical signatures between N-MORB and island arcs, indicating that they formed in a back-arc setting. In contrast, the volcanic rocks from the Batamayineishan Fm. display geochemical characteristics of continental intraplate volcanic rocks formed in an extensional setting after collision. Thus, we propose a model that involves a volcanic arc formed by northward subduction of the ancient Junggar ocean and amalgamation of different terranes during the Late Paleozoic to interpret the formation of the Late Paleozoic volcanic rocks in the Eastern Junggar terrane, and the Altai and Junggar terranes fully amalgamated into a Cordilleran-type orogen during the end of Early Carboniferous to the Middle–Late Carboniferous.

Geochemistry of picrites and associated lavas of a Devonian island arc in the northern Junggar terrane, Xinjiang (NW China): Implications for petrogenesis, arc mantle sources and tectonic setting

The Junggar terrane in China's Xinjiang Province sits adjacent to the Siberian craton and Kazakhstan block. A nearly 100-m thick series of high-Mg lava flows is found in a Devonian arc in the Junggar terrane. The highly porphyritic high-Mg lavas are stratigraphically in the lower part of the Middle Devonian Beitashan Formation, and are overlain by basalts and andesites. Based on the chemistry and phenocryst contents, the high-Mg lavas are classified into picrites and ankaramites. The former are characterized by a large amount of olivine phenocrysts (Fo 79–82) with minor clinopyroxene (En 44–50Fs 1–10Wo 44–48) and rare chrome spinel with Cr# (Cr/Cr + Al) values between 0.63 and 0.86, whereas clinopyroxene phenocrysts are dominant in ankaramite. The groundmass in the picrites and ankaramites consists of plagioclase, clinopyroxene, and Ti-magnetite. Glass is not preserved, but minor metal sulfides are present. Based on olivine-melt equilibrium, the primitive magmas had approximately 9 wt.% MgO, and the high-Mg lavas contain accumulated olivine (picrites) and clinopyroxene + olivine (ankaramites). All lavas have similar primitive mantle normalized trace element patterns, characterized by negative Nb, Ta and Ti anomalies, as typical for island arc volcanic rocks. These relationships suggest that the rock suites are co-magmatic. The Zr/Nb ratios (23–66) of the picrites and basalts resemble MORBs (10–66), suggesting MORB-like sources for the picrites. The picritic rocks have slightly LREE-enriched patterns with (La/Yb) n ranging from 1.6 to 3.5 and (La/Sm) n from 1.2 to 2.1. In contrast, basaltic rocks have flat to slightly LREE-enriched patterns with (La/Yb) n ranging from 1.2 to 5.7 and (La/Sm) n from 0.8 to 2.5, whereas andesitic rocks have modestly LREE-enriched patterns with (La/Yb) n ranging from 2.8 to 9.1 and (La/Sm) n from 1.7 to 2.9. The REE patterns for all rocks do not exhibit Eu anomalies. The rocks have similar ( 87Sr/ 86Sr) t (0.7033–0.7043, t = 385 Ma) and ε Nd( t) values (6.4–7.3), overlapping with the present day island arc field. Consequently, primary magmas were most likely generated in N-MORB-type mantle, which was modified by the addition of a fluid component derived from altered subducted oceanic crust as indicated by elevated Sr/Nd and low Th/Yb ratios. REE modelling suggests that the primary magmas were derived from garnet–spinel transition zone mantle.

Middle Cambrian to Permian subduction-related accretionary orogenesis of Northern Xinjiang, NW China: Implications for the tectonic evolution of central Asia

Middle-Cambrian to Permian subduction-related records are widely distributed in Northern Xinjiang which can be grouped into the Chinese Altay–East Junggar–Eastern Tien Shan, West Junggar, Yili, and Tarim domains. By integrating paleogeographic and geological data, we suppose that the Chinese Altay–East Junggar–Eastern Tien Shan domain was more closely located to Siberia, while the West Junggar and Yili domains occupied an intermediate position near the Kazakhstan block in the early Paleozoic Paleoasian Ocean. Distribution of Andean-type magmatic arcs, island arcs, accretionary wedges, ophiolitic slices, and/or microcontinents shows an archipelago paleogeography forming a huge accretionary active margin sequences. The Tarim domain was on the opposite side of the early Paleozoic Paleoasian Ocean remaining passive margin. These tectonic units drifted northwards and approached the southern active margin of the Siberian craton in the late Paleozoic, leading to termination of the Paleoasian Ocean and formation of a complicated orogenic collage between Siberian craton and the Tarim block between the end-Permian and Triassic. These multiple accretion processes significantly contributed to the lateral growth of central Asia.

Seismic Tomography of the Crust and Upper Mantle Structure Underneath the Chinese Tianshan

From the travel time data of the P-arrivals recorded by the passive seismic array across the Chinese Tianshan and regional seismic network, the P wave velocity structure of the crust and upper mantle down to 400km along Kuqa-Kuytun profile is reconstructed by using seismic tomography technique. Our results demonstrate that the crust along the profile has an obvious blocked structure and manifest strong lateral crustal distortion. These demonstrate that the Tianshan crust is compressed strongly by the Tarim block. There are high-velocity anomalies with a thickness of 60~90 km on the top of the upper mantle beneath the Tarim and Junggar basins. The high-velocity anomaly below the Tarim-south Tianshan has a clear curved distortion, and that below the Junggar-northern Tianshan thrusts down to the depth of 300 km at the south side of the central Tianshan. Both of them construct an asymmetric bilateral thrusting system. Beneath the Tarim and Junggar block, at the depth ranging from 150 km to 400 km, exist low-velocity anomalies, one of which on the side of Tarim up-wells to bellow the south Tianshan. In the upper mantle, at the depth of 200~300 km beneath the Tarim-south Tianshan, exists a high-velocity anomaly, which could be the detachment of the lithosphere of the Tarim melted by the up-welling hot material of the upper mantle. All of our observations mentioned above illustrate that the lithosphere of the Tarim block is involved in the northward subduction, but its front is bounded merely to the north border of the south Tianshan. The long-range effect caused by the uplift of the Tibetan plateau drives not only the subduction of the Tarim lithosphere, but also the upwelling of the upper mantle on the south side of the Tianshan orogenic belt. The low-velocity anomalies widely existed in the Tianshan upper mantle should facilitate the mantle deformation, and the large difference between the high and low velocity anomalies in the upper mantle could prompt the small-scale mantle convection. According to the crustal and upper mantle velocity structure beneath the Tianshan orogenic belt, it could be inferred that the main source driving the quick uplift of the Tianshan since Miocene comes from the long-range effect caused by the uplift of the Tibetan plateau, and the relatively soft upper mantle beneath the Tianshan creates the necessary condition for accelerating the uplift and distortion of the crust.

Comparison of Permian palynological assemblages from the Junggar and Tarim Basins and their phytoprovincial significance

Comparison of Permian palynofloras between the Junggar and Tarim Basins has resulted in stratigraphic correlation and new knowledge about the phytoprovincial change of Tarim. The Junggar regime was part of the Subangara area since the early Late Carboniferous through the Permian, whereas the Tarim block belonged to the Euramerican Province prior to the Early Permian (Asselian–Sakmarian). A dramatic change from the Euramerican Province to the Subangara area occurred not later than the onset of the Middle Permian. Palynological data supports the assumptions that the collision between the Junggar block and the Kazakhstan–Siberia blocks was during the late Hercynian (not later than in the Early Carboniferous) and that between Tarim with Junggar was during the Early Permian (possibly prior to the Artinskian). Both regions were basically under the control of a semiarid climate during the Permian but Tarim was somewhat drier than Junggar, with both possibly straddling subtropical to warm temperate latitudes (ca. 25°–35° N.).

Shear wave splitting observations in the Chinese Tianshan orogenic belt

A linear array of 26 3‐component broadband digital stations across the Chinese Tianshan Orogenic Belt (CTOB) was deployed for 13 months to measure the shear wave splitting. Fast direction of teleseismic shear waves beneath the southern CTOB is NWW‐SEE and gradually turns into NEE‐SWW in the north. Delay times are 0.65∼1.48s with average value of 0.97 ± 0.15s and the maximum delay time is observed in the middle mountain and decrease in the south and the north. Fast directions are generally parallel to the strike of the orogenic belt, indicating that more than one hundred kilometers of upper mantle is involved in collisional deformation. Compared to other stations, change in fast direction for stations in the south and north ends of the array is probably resulted from subduction of the Tarim block and the Junggar block into the Tianshan range, respectively.

Structures on Lithospheric Density and Geomagnetism of the Tianshan Orogenic Belt and Their Geodynamic Implications

AbstractThe geoscience transect from Korla to Jimsar runs through the northern margin of the Tarim basin, the Tianshan orogenic belt and the southern margin of the Junggar basin. Along the profile, a joint inversion of gravity with geomagnetism has been carried out and 2‐D density structure and 2‐D geomagnetic structure have been obtained. The result shows that both the Tarim basin and the Junggar basin under thrust to the Tianshan orogenic belt in opposite directions. In the crust, the average density of the northern margin of the Tarim basin and the southern margin of the Junggar basin is relatively high, while that of the Tianshan orogenic belt is relatively low. However, the Tianshan orogenic belt bears a high geomagnetic intensity, especially in the entire crust from the southern margin of the Junggar basin to the middle of the Tiansha orogenic belt. This indicates that the southern margin and the northern margin of the Tianshan orogenic belt may possess different tectonic evolution histories, different tectonic movement patterns and different tectonic intensities. Some lowdensity bodies located at the top part of the upper mantle are found on the boundaries between the Tarim basin and the Tianshan orogenic belt and that between the Junggar basin and the Tianshan orogenic belt. This may be inferred as a result of subduction of the Tarim block northward and that of the Junggar block southward to the Tianshan orogenic belt. During this process, the mass of the lower crust on the northern margin of the Tarim basin and the southern margin of the Junggar basin has been carried down to the top part of the upper mantle beneath the Tianshan orogenic belt. The 2‐D density structure and the 2‐D geomagnetic structure of the geoscience transect from Korla to Jimsar provide some lithospheric evidence for the tectonic segmentation of the Tianshan orogenic belt.

ORDOVICIAN—PERMIAN PALAEOGEOGRAPHY OF CENTRAL EURASIA: DEVELOPMENT OF PALAEOZOIC PETROLEUM‐BEARING BASINS

In this paper, we discuss three petroleum‐bearing basins of Palaeozoic age in Central Eurasia—the Precaspian, Tarim and Chu‐Sarysu Basins. We make use of recently‐published palaeogeographic maps of the Central Eurasian region, six of which are presented here (Late Ordovician, Early‐Middle Devonian, Late Devonian, Early Carboniferous, Early Permian and Late Permian). The maps illustrate the development through the Palaeozoic of the Palaeoasian and Palaeotethys Oceans; of the East European, Siberian and Tarim cratons; and of the Kazakhstan and other microcontinental blocks.The Kazakhstan block formed during the Late Ordovician and is a collage of Precambrian and Early Palaeozoic microcontinents and island arcs. It is surrounded by collisional foldbelts (Ob‐Zaisan, Ural‐Tianshan and Junggar‐Balkhash) which formed in the Late Carboniferous — Permian. We believe that the formation of a stable Kazakhstan block is not consistent with the existence of the previously‐identified “Kipchak arc” within the Palaeoasian ocean, or (as has previously been proposed) with activity on this arc up to the end of the Palaeozoic.The oil and gas potential of the Precaspian, Tarim and Chu‐Sarysu Basins depends to a large extent on their tectonic stability during the Palaeozoic and subsequent time. The Precaspian Basin has been stable since the Cadomian orogeny (Early Cambrian) and is known to have major hydrocarbon potential. The Tarim Basin (NW China) has somewhat lower potential because the margins of the Tarim continental block have been affected by a series of collisional events; that margin with the Palaeotethys Ocean, for example, was active during the Late Palaeozoic. The Chu‐Sarysu Basin on the Kazakhstan block is the least stable of the three and contains only minor gas accumulations.

Paleozoic Continental Accretions in Central Asia Around Junggar Block: New Structural and Geochronological Data

Paleozoic Continental Accretions in Central Asia Around Junggar Block: New Structural and Geochronological Data