SW Central Asian Orogenic Belt Research Articles

Permian post-orogenic terrestrial successions in the western and northern peripheral orogens of the Junggar Basin: Records of sedimentary provenance, paleogeographic and tectonic changes

The Permian terrestrial successions were deposited in peripheral orogens of the main Junggar Basin, including the West Junggar and Chinese Altai orogens. These successions probably marked the termination of the Paleozoic orogeny in the SW Central Asian Orogenic Belt and document paleogeographic and tectonic changes during that time. However, comprehensive research of these successions is still lacking. This study investigated representative sections and outcrops of Permian successions in West Junggar and Chinese Altai, and we present a detailed sedimentological comparison in the specific chronological framework according to new zircon U-Pb ages of syn-depositional volcanics. This comparison shows that the Lower Permian successions consist of almost equal amounts of sedimentary rocks (46–56%), characterized by fluvial (meandering river-dominated)-deltaic and lacustrine facies, and volcanic rocks (44–54%). By contrast, the Middle–Upper Permian is dominated by alluvial facies (>79%) with highly reduced fluvial-deltaic and lacustrine facies. Furthermore, detrital modes and detrital zircon dating were applied to sandstones from four selected areas for provenance analysis. As a result, two major provenances were detected by detrital zircon ages, including the syn-depositional volcanics and granitoids (40–65%) and the Devonian–Early Carboniferous subduction-related plutons and arc volcano-sedimentary rocks (31–51%). Samples in Tarbagatay received a certain contribution from the Early Paleozoic arcs, and materials from the metamorphic basement in Chinese Altai localized in the vicinity and northernmost West Junggar. In summary, the Early Permian paleogeographic pattern of West Junggar is proposed as a series of small-scale fluvial plains and lacustrine basins that were separated by syn-depositional volcanoes and uplifted Paleozoic orogenic units under a tectonic setting of active rifting. The great shrinkage of sedimentation and coarsening of deposits during the Middle–Late Permian was triggered by intracontinental deformation rather than regional climate change.

Heterogeneous mush-dominated plumbing system and mantle sources of alkaline lamprophyres in Tuoyun Basin, SW Central Asian Orogenic Belt

Lamprophyres are spatially and temporally widespread in continental belts, but their magmatic differentiation process and melting dynamic mechanisms remain subjects of debate. In this study, we report mineral chemistry, bulk-rock compositions, and Sr-Nd-O isotopes of the alkaline lamprophyres (including monchiquite and camptonite) from the Tuoyun Basin of the southwestern Central Asian Orogenic Belt. Based on the P-T estimates, transcrustal magmatic reservoirs from the lower crust to upper crust are revealed for monchiquite and camptonite. These magmatic reservoirs were potentially connected, and the magmatic recharge and mixing processes that occurred were recorded by zoned clinopyroxene and amphibole. The diverse crystal populations are suggested to have originated from multiple batches of magmas, which were then involved in forming the subsequent magmas. The Sr-Nd isotopic compositions are relatively depleted, with (87Sr/86Sr)i = 0.70390−0.70452 and εNd(t) = 5.46−6.49 for the monchiquite, while the values for camptonite are 0.70391−0.70463 and 4.71−5.02. Moreover, the olivine in situ O isotopes in the monchiquite show δ18OV-SMOW values of 4.81‰−5.40‰, which are comparable to those of the mantle peridotite. Compared with the experimental melts, a hybridized mantle source of hydrous peridotite + silica-depleted pyroxenite is proposed, and partial melting of this component was induced by the upwelling asthenosphere. Ultimately, our study highlights the critical role of the magmatic plumbing system and hydrous mantle source in the formation of Tuoyun lamprophyres.

New constraints on volcanism during Ordovician-Silurian transition: Insights from marine bentonites in northern Yili Block (NW China)

The Yili Block is located at the Chinese western Tianshan and is a constituent of the Kazakhstan microcontinent. It played an important role in the evolution of the SW Central Asian Orogenic Belt (CAOB). The Yili Block underwent the Paleozoic orogeny during the subduction and closure of the Junggar Ocean. Late Paleozoic arc-type magmatic rocks have been widely recorded in the Yili Block; however, it remains poorly constrained when and how the Junggar Ocean started to subduct beneath the Yili Block. In this study, we report new results of zircon LA-ICP-MS UPb dating and LuHf isotopic analysis on a series of bentonite beds in tightly folded upper Ordovician siliceous black shales of the Guozigou Section in the northern Yili Block. The occurrence of bentonite layers and associated sulfides within graptolite-bearing siliceous black shales indicates multiple marine volcanic eruptions. Magmatic zircons separated from seven representative bentonite samples yielded consistent UPb ages ranging from 449.5 ± 2.4 Ma to 444.8 ± 2.0 Ma (Katian). In situ zircon LuHf isotopic analyses show mostly positive εHf(t) values (−0.01 and 1.31 to 11.7) and Neoproterozoic single-stage Hf model ages (TDM1 = 0.6– 1.0 Ga). A linear correlation between the εHf(t) and TDM1 suggests that these zircons likely crystallized from a common magma originated from a depleted mantle source with variable involvement of Precambrian continental crust. Our new zircon ages and Hf isotopic data are comparable with those of the Middle-Late Ordovician continental arc-type magmatic rocks in the nearby Wenquan area, both indicating that the subduction of the Junggar Ocean beneath the northern Yili Block started in the Middle-Late Ordovician, and are therefore important for early Paleozoic paleogeographic and tectonic reconstruction of the Paleo-Asian Ocean domain. In addition, the Late Ordovician marine bentonites and black shales coincide with the worldwide intensive volcanic activities and related climate/environment changes during late Katian to Hirnantian. Thus, these new data also provide further arguments for possible causes of the global climate change and mass extinction during the Ordovician-Silurian transition.

Late Devonian transition from advancing to retreating subduction in the SW Central Asian Orogenic Belt: Insights from multiple deformation and magmatic events in the southern Yili Block, NW China

In order to better constrain the Late Paleozoic tectonic evolution of the SW Central Asian Orogenic Belt, we conducted field structural investigations, zircon U-Pb dating, whole-rock geochemical and Sr-Nd isotopic studies on the meta-sedimentary rocks and intruding granites and mafic dykes in the Dahalajunshan area, southern Yili Block of NW China. Our results allow us to recognize five tectonic/magmatic events. Field-scale and microscopic structures of the low-grade meta-sedimentary rocks showed a regional top-to-the-SE thrusting (Event 1). Undeformed granites that contain dioritic enclaves (Event 2) crosscut the regional foliations and thus post-date Event 1 after ca. 379–369 Ma. Mafic dykes intruded all these rocks after ∼328 Ma (Event 4–1), posterior to brittle-ductile thrusting (N-S contraction) (Event 3) and prior to normal faulting (N-S extension) (Event 4–2). All the magmatic rocks formed in a subduction-related setting. The granites and dioritic enclaves derived from a mixing of crustal- and mantle-derived sources variously modified by slab-derived hydrous melts. The primary magma of the diabase dykes derived from a mantle source modified by the slab fluids with 5% crustal contamination. Variation from regional compression (ductile and brittle-ductile thrusting) to extension (normal faulting) and increasing mantle input in arc-related magmatism reveals a transition from Early Paleozoic advancing subduction (events 1 to 3) to Late Paleozoic retreating subduction (events 4–1 and 4–2) of the Junggar oceanic plate beneath the Yili Block.

Precambrian and Early Palaeozoic metamorphic complexes in the SW part of the Central Asian Orogenic Belt: Ages, compositions, regional correlations and tectonic affinities

Precambrian metamagmatic complexes of the Zhingeldy and Kendyktas blocks of the Zheltau and Chu-Kendyktas terranes of the SW part of the Central Asian Orogenic Belt (CAOB) consist of predominant Neoproterozoic orthogneisses with the estimated protolith ages of ca. 790–800 Ma, and subordinate epidote and garnet amphibolites, metabasalts and metadolerites of the lower metamorphic grades. The orthogneisses are characterized by negative εNd(t) values of −12 to −4 with Nd model ages of 2.3–1.7 Ga, indicating their protoliths could have formed by a reworking of the Palaeoproterozoic crustal materials. Precambrian metavolcanic rocks in the studied region are rhyolites with the estimated crystallization ages of ca. 800–830 Ma, and are associated with metabasalts. Metasedimentary complexes in the studied region consist of intensively retrogressed garnet-mica schists interlayered with marmorized limestones, metasilicic rocks, epidote and garnet amphibolites and strongly altered eclogites. The inferred protoliths of the garnet-mica schists are terrigenous rocks accumulated during the latest Ediacaran – middle Cambrian and formed from Mesoproterozoic (ca. 1.0 Ga) and to a lesser extent Palaeoproterozoic felsic source rocks (ca. 1.6–1.7 Ga and ca. 2.5 Ga). The garnet-mica schists of the Kendyktas block have negative εNd(t) of −13.4 and a model age tNd(DM) of 2.2 Ga, implying formation of their protolith from Palaeoproterozoic crustal source. The studied Zheltau and Chu-Kendyktas terranes from the SW part of the CAOB have much in common in terms of Precambrian and Early Palaeozoic evolution and chemical compositions of the key rock types. The presence of the Early Precambrian rocks (Zheltau terrane), an abundance of the Late Mesoproterozoic (1000–1100 Ma) detrital zircons in the Ediacaran-Cambrian metasedimentary complexes and a wide distribution of the Late Neoproterozoic magmatism, are distinctive features of the studied terranes. We refer such Precambrian terranes of the western CAOB to the Ulutau-Moyunkum group where tectono-magmatic evolution is consistent with that observed from the northern part of the Tarim craton. Our results also aid comprehensive correlation of metamorphic complexes of the Precambrian terranes from the SW CAOB.

Age and tectonic setting of Neoproterozoic gneissic granites in the southern Yili Block (NW China) and implications for the origins of the continental blocks in SW Central Asian Orogenic Belt

Several ribbon‐like continental fragments with Precambrian basement were wrapped within Palaeozoic accretionary complexes in the southwest Central Asian Orogenic Belt (CAOB). Their origins and tectonic evolution are essential for understanding the accretionary orogeny of the CAOB and crustal growth versus continental reworking in Phanerozoic. The southern Yili Block in NW of China is one of the major continental fragments in SW CAOB. In order to better constrain the tectonic settings of the Precambrian basement of the southern Yili Block, we present new data of field structures, whole‐rock geochemical compositions, zircon U–Pb ages and Lu‐Hf isotopic ratios of the Neoproterozoic gneissic granites. Ductile deformation characterized by mylonitic foliations and stretching/mineral lineation is commonly recognized in the gneissic granites, which were cross‐cut by undeformed granitoids and mafic dykes. The gneissic granites yielded zircon U–Pb ages of 968 ± 5 Ma, 948 ± 8 Ma, and 873 ± 9 Ma. These gneissic granites show geochemical features of high potassium, peraluminous, calc‐alkaline series, enrichment of Th, K, Rb and depletion of Ba, Nd, Sr. They show zircon εHf(t) values from −5.5 to 2.0 and two‐stage zircon Hf model ages between 1.7 and 2.1 Ga. These new results indicate that the Neoproterozoic gneissic granites in the southern Yili Block derived from partial melting of meta‐greywackes that most likely had formed in a continental arc setting. A comparison of stratigraphy, magmatism and detrital zircon ages suggests that the southern Yili Block is similar with the northern Yili Block and Chinese Central Tianshan Block in term of Precambrian basement, and these continental fragments might have originated from Neoproterozoic orogenic belts.

Structural and kinematic evolution of strike-slip shear zones around and in the Central Tianshan: Insights for eastward tectonic wedging in the southwest Central Asian Orogenic Belt

In order to better understand the late Paleozoic tectonic evolution of the southwestern Central Asian Orogenic Belt (CAOB), we carried out structural and geochronological studies on the poorly investigated Xiaergou and Wulasitai shear zones around and in the Chinese Central Tianshan. The Xiaergou shear zone is the connecting segment between the North Tianshan Fault and Main Tianshan Shear Zone along the northern margin of the Yili - Central Tianshan blocks, it strikes NW-SE with a width of ~3–5 km and shows predominant dextral kinematics. Zircon U–Pb ages of pre- and syn-kinematic granitic dykes within the Xiaergou shear zone indicate that the dextral shearing was active at ~312-295 Ma. The Wulasitai shear zone is a high-strain belt occurring in the interior of the Central Tianshan block, it extends NW-SE for more than 40 km with variable widths of ~1–5 km, steep mylonitic foliations and sub-horizontal stretching lineation are well developed and various kinematic indicators suggest prevailing sinistral shearing. New biotite 40Ar/39Ar ages of two meta-sedimentary rocks, together with the published metamorphic zircon ages constrain the timing of the sinistral shearing at ~312-301 Ma. Our new results combined with the previous studies reveal that the dextral strike-slip shear zones framing the Central Tianshan formed almost simultaneously in the latest Carboniferous (~310 Ma) and lasted until the middle to late Permian. They resulted from the eastward tectonic wedging and relative rotations between continental blocks in the SW CAOB. The sinistral shearing of the Wulasitai shear zone within the Central Tianshan was likely generated due to differential eastward motions of the northern and southern parts of the Central Tianshan.

Detrital zircon ages of the Mesoproterozoic metasedimentary rocks in the southern Yili Block: Implications for tectonic affinities of the microcontinents in SW Central Asian Orogenic Belt

The origins and Paleogeographic reconstruction of the Precambrian microcontinents in the Central Asian Orogenic Belt (CAOB) are essential for understanding the continental crustal evolution and mechanism of accretionary orogeny. The Yili Block is one of the Precambrian microcontinents in SW CAOB and is a key component of the Kazakhstan microcontinent whose amalgamation is not yet well understood. In this study, we present detrital zircon U-Pb ages of the metasedimentary rocks from the Tekesi Group in the southern Yili Block to constrain its Precambrian history and tectonic affinity. A total of 540 concordant U-Pb zircon ages from the metasedimentary rocks defined dominantly Meso- to Paleoproterozoic detritus (1.1–1.8 Ga) with minor early Paleoproterozoic to Neoarchean and Neoproterozoic components. These results are comparable to those obtained from the northern Yili, Chinese Central Tianshan and Kyrgyz North Tianshan blocks. On the basis of careful analysis on the inner textures of dated zircons, and comparing these new results with the previous studies on magmatic rocks, it is suggested that (1) the Tekesi Group most likely deposited during the latest Mesoproterozoic-earliest Neoproterozoic (1079–963 Ma); (2) the provenances of the Meso- to late Paleoproterozoic detritus might be in situ basement rocks of the Yili Block, or the Chinese Central Tianshan, Kyrgyz North Tianshan and even exotic cratonic plates (e.g., Central Australia, Laurentia); and (3) the early Paleoproterozoic to Neoarchean components may have been derived from old cratonic plates (e.g., Tarim, Siberia). In combination with the available detrital zircon ages and regional stratigraphic data, we propose that the southern Yili Block has close tectonic affinities with the northern Yili Block, Chinese Central Tianshan, and Kyrgyz North Tianshan, and these ribbon-like microcontinents once situated on the continental margins of the constituent cratonic plates (e.g., Tarim, Siberia, Central Australia, Laurentia) during the assembly and break-up of the Columbia and Rodinia supercontinents, therefore they may offer important clues for global reconstructions of supercontinent cycles.

Constraining the Intracontinental Tectonics of the SW Central Asian Orogenic Belt by the Early Permian Paleomagnetic Pole for the Turfan‐Hami Block

AbstractIn order to better understand the kinematics and geodynamics of the late Paleozoic to early Mesozoic transition from accretionary to intracontinental tectonics in the southwest Central Asian Orogenic Belt, we performed a paleomagnetic study on Lower Permian strata in the Turpan‐Hami Basin, NW China. Magnetite and hematite are shown to be the principal magnetic remanence carriers. A typical sedimentary fabric that has not been modified by postdepositional deformation is recognized via magnetic fabric study. Stable characteristic remanent magnetizations, all of reverse magnetic polarity, are revealed by step demagnetizations. Remanence age is determined to be of Early Permian affinity, based on a fold test, remanence behavior of minerals, and geochronologic data. With inclination shallowing corrections, the first Early Permian paleomagnetic pole for the Turpan‐Hami Block is acquired as λ = 75.7°N, φ = 276.3°E, A95 = 5.7°, and N = 10 sites. Comparisons of this new pole with published ones from neighboring cratonic blocks provide kinematic constraints on the tectonic transition and subsequent intracontinental tectonics of this region. The Turpan‐Hami and Yili blocks have kept their relative positions since the Early Permian. Significant latitudinal movement of 14.7° ± 7.2° occurred between the Turpan‐Hami and South Junggar blocks and could be mainly accommodated by late Paleozoic crustal shortening of at least ~530 km across an intra‐arc basin in the Bogda belt. The Turpan‐Hami Block has experienced large‐magnitude relative rotations of −41.1° ± 7.1° and 21.2° ± 9.4° with respect to Tarim and South Junggar since the Early Permian, respectively, corresponding to dominant intracontinental tectonics characterized by large‐magnitude strike‐slip displacements along megashear zones.

Early Neoproterozoic gneissic granitoids in the southern Yili Block (NW China): Constraints on microcontinent provenance and assembly in the SW Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB) was built by amalgamation of diverse geologic units including several microcontinents. The nature and geodynamic evolution of the Precambrian basement of these units are essential to constrain their origin, and thus the geodynamic and paleogeographic reconstruction of the accretionary system. The Yili Block, located in the west of the Chinese Tianshan Orogen, is one of these Precambrian-based continental constituents. Field investigations, zircon U-Pb dating and Lu-Hf isotopic analysis, whole-rock geochemical and Sr-Nd isotopic studies were conducted on the gneissic granitoids from the Giergelang area, southern Yili Block. These rocks yielded early Neoproterozoic zircon U-Pb ages (962–934 Ma), containing substantial number of Paleo- to Mesoproterozoic inherited zircons (2403–1268 Ma); their protoliths belong to peraluminous high-K granite series displaying right-declining REE patterns and Eu negative anomalies together with LILE (K, Rb, U, Th) enrichment and depletions in Ba, Nb, Sr, P, Ti; they have negative εNd(t) values ranging from −2.5 to −4.7, and old Nd model ages varying from 1650 Ma to 1910 Ma; they also exhibit highly variable zircon εHf(t) values (−12.1 to 4.4), and Hf-in-zircon model ages (1558–2574 Ma). Integrating all these results, it is suggested that the primary magma of gneissic granitoids was generated by partial melting of supracrustal rocks with involvement of old crustal materials.A comparison of data from the Yili Block and its neighboring continental units reveals that the Neoproterozoic magmatic event is an important episode of crustal reworking for the major continental blocks within the CAOB. Similar Neoproterozoic magmatic events and comparable upper Neoproterozoic to lower Paleozoic stratigraphic sequences of the Yili, Chinese Central Tianshan and Tarim blocks, suggest that they had a similar crustal evolution and close tectonic and paleogeographic relation during Neoproterozoic, and were likely derived from one single continental plate.

Contrasting deep crustal compositions between the Altai and East Junggar orogens, SW Central Asian Orogenic Belt: Evidence from zircon Hf isotopic mapping

There are long–standing uncertainties over the deep crustal composition and basement nature of the Chinese Altai and Junggar orogens of the southwestern Central Asian Orogenic Belt (CAOB). Zircon LuHf isotopic tracer techniques applied to granitoids are helpful in distinguishing old from young deep crustal compositions. This study aims to characterize the basement nature (ancient & juvenile) of the Chinese Altai and East Junggar orogens using Hf isotopic mapping of granitoids (510–200 Ma). Zircon Hf isotopic data (18 new and 79 published samples) indicate that the study area can be divided into six Hf isotopic provinces on the basis of TDMC ages: Province I, >1.4 Ga; II, 1.4–1.2 Ga; III, 1.2–1.0 Ga; IV, 1.0–0.8 Ga; V, 0.8–0.6 Ga; and VI, 0.6–0.4 Ga. Provinces I, II, and III occur mainly in the central (units 2 and 3) and southern (unit 4) Altai orogen, and have slightly positive zircon εHf(t) values of +0.5 to +9.1 with slightly old TDMC ages of 1.53–0.81 Ga. Provinces IV and V are mainly distributed in the northern (unit 1) and southernmost (unit 5) Altai orogen, with zircon εHf(t) values of +7.5 to +11.1 and young TDMC ages of 0.84–0.60 Ga. In contrast, Province VI (East Junggar orogen) has zircon εHf(t) values of +11.6 to +14.9, with much younger TDMC ages of 0.59–0.35 Ga, except for the small Taheir area with a negative εHf(t) value of −2.5 and an older TDMC age of 2.5 Ga. There is a sharp contrast between the deep crustal compositions of the Altai and Junggar orogens, which are ancient in central Altai and relatively juvenile in East Junggar. This confirms that significant Neoproterozoic–Phanerozoic continental crustal growth occurred in the southern Altai and Junggar orogens, with juvenile crust occupying ~78% of the study area, and that the CAOB is a site of significant Phanerozoic continental growth. The distribution of juvenile and ancient crust in the Altai and East Junggar orogens provides new evidence for the tectonic division of the two orogens by the Erqis fault zone, and explains the heterogeneity of crustal growth in the southwestern CAOB.

First Early Permian Paleomagnetic Pole for the Yili Block and its Implications for Late Paleozoic Postorogenic Kinematic Evolution of the SW Central Asian Orogenic Belt

AbstractWe conducted a paleomagnetic study on the Early Permian volcanic and sedimentary rocks, and the Neoproterozoic mafic dikes in the Yili Block, NW China. Magnetite and hematite were proven to be the principal magnetic remanence carriers. Demagnetizations revealed stable characteristic remanence magnetizations with a sole reversed magnetic polarity. The magnetic remanence of only the Early Permian strata turned out to be primary based on positive fold tests; meanwhile, the magnetic remanence age of the mafic dikes is ambiguous. Accordingly, the first Early Permian paleomagnetic pole for the Yili Block is calculated at λ = 81.5°N, φ = 256.5°E, N = 11, and A95 = 10.9°. Comparisons of this new pole with published ones from the Yili, Tarim, and South Junggar blocks provide new quantitative constraints on late Paleozoic kinematic evolution of the SW Central Asian Orogenic Belt: (1) Between the Yili and Tarim blocks, significant relative movement took place along major strike‐slip faults during the Late Carboniferous to Early Permian (580 ± 290 km) and the Early to Late Permian (585 ± 340 km), and the displacement rate increased from the Late Carboniferous to Early Permian (~19.3 ± 9.7 mm/yr) to the Early to Late Permian (~29.3 ± 17.0 mm/yr); (2) a significant relative rotation of 28.3° ± 18.3° in the Late Permian, and a lateral displacement of 630 ± 295 km after the Late Permian occurred between the Yili and South Junggar blocks. The significant strike‐slip movements played an important role in the formation of the Central Asian Orogenic Belt and should be considered with great attention in tectonic and paleogeographic reconstructions.

Mineralogy and geochemistry of triassic carbonatites in the Matcha alkaline intrusive complex (Turkestan-Alai Ridge, Kyrgyz Southern Tien Shan), SW Central Asian orogenic belt

Postorogenic intrusions of essexites and alkaline and nepheline syenites in the Turkestan-Alai segment of the Kyrgyz Southern Tien Shan coexist with dikes and veins of carbonatites dated at ∼220 Ma by the Ar–Ar and Rb–Sr age methods. They are mainly composed of calcite and dolomite (60–85%), as well as sodic amphibole, phlogopite, clinopyroxene, microcline, albite, apatite, and magnetite, with accessory niobate, ilmenite, Nb-rutile, titanite, zircon, baddeleyite, monazite-(Ce), barite, and sulfides. The rocks share mineralogical and geochemical similarity with carbonatites that originated by liquid immiscibility at high temperatures above 500 °C. Alkaline silicate and salt-carbonate melts are derived from sources with mainly negative bulk εNd(t) ∼ from −11 to 0 and high initial 87Sr/86Sr ratios (∼0.7061–0.7095) which may be due to mixing of PREMA and EM−type mantle material. Pb isotopic ratios in accessory pyrrhotite (206Pb/204Pb = 18.38; 207Pb/204Pb = 15.64; 208Pb/204Pb = 38.41) exhibit an EM2 trend. The intrusions bear signatures of significant crustal contamination as a result of magma genesis by syntexis and hybridism. Concordant isotope composition changes of δ13C (−6.5 to −1.9‰), δ18O (9.2–23‰), δD (−58 to −41‰), and δ34S (12.6–12.8‰) in minerals and rocks indicate inputs of crustal material at the stage of melting and effect of hot fluids released during dehydration of metamorphosed oceanic basalts or sediments. The observed HFSE patterns of the oldest alkaline gabbro may be due to interaction of the primary mafic magma with IAB-type material. The isotope similarity of alkaline rocks with spatially proximal basalts of the Tarim large igneous province does not contradict the evolution of the Turkestan-Alai Triassic magmatism as the “last echo” of the Tarim mantle plume.

Carboniferous – Early Permian magmatic evolution of the Bogda Range (Xinjiang, NW China): Implications for the Late Paleozoic accretionary tectonics of the SW Central Asian Orogenic Belt

The Late Paleozoic magmatic evolution of the Bogda Range (Chinese North Tianshan) is important for understanding the accretionary history of the Central Asian Orogenic Belt. We investigated the Carboniferous and Lower Permian volcanic and sedimentary sequences of the Daheyan section, southern Bogda Range, and present new zircon U-Pb ages and whole-rock geochemical data for the volcanic rocks. One Carboniferous rhyolite is dated at 298±8Ma; a Permian basalt yielded many Proterozoic zircon xenocrysts, and its maximum age (∼297Ma) is constrained by the detrital zircon ages of the sandstone that stratigraphically underlies it. These volcanic rocks belong to calc-alkaline series. We further synthesize previous geochronological, geochemical and isotopic data of magmatic and sedimentary rocks in the Bogda Range. The available data indicate that the magmatism occurred continuously from 350Ma to 280Ma. A comprehensive analysis allows us to propose that: (1) the Carboniferous to Early Permian magmatic rocks of the Bogda Range generally show consistent arc-type features; (2) increasing mantle input through time suggests intra-arc extension in a supra-subduction zone; (3) the localized occurrence of Early Permian alkaline pillow basalts and deep water sediments close to the major shear zone advocate a transtensional crustal thinning during the transition from Carboniferous convergence to Early Permian transcurrent tectonics; (4) occurrence of a large number of Proterozoic zircon xenocrysts in the Late Paleozoic magmatic rocks, and Proterozoic detrital zircons in the coeval clastic sediments suggest a continental or transitional basement of the Bogda Arc; (5) subduction in the Bogda area terminated prior to the deposition of Middle Permian terrestrial sediments.

Late Carboniferous high-pressure metamorphism of the Kassan Metamorphic Complex (Kyrgyz Tianshan) and assembly of the SW Central Asian Orogenic Belt

High-pressure (HP) metamorphism of the Kassan Metamorphic Complex (KMC) in the western Kyrgyz Tianshan has been related to either late Ordovician or late Carboniferous-Permian subduction processes. We report Sm-Nd ages for retrogressed eclogite samples and 40Ar/39Ar cooling ages for enclosing garnet-muscovite samples from the KMC as new age constraints on HP metamorphism and rock exhumation. These data will be used for an upgraded paleogeographic model for late Paleozoic crustal consolidation in the southwestern Central Asian Orogenic Belt. The retrogressed eclogite samples have transitional alkaline to tholeiitic affinity and trace-element patterns consistent with protoliths derived from garnet-bearing mantle sources at rifting plate margins. Geothermobarometric data for a retrogressed eclogite sample indicate peak-metamorphic conditions of 540 ± 30 °C at 1.6 ± 0.1 GPa. Samples from different lithotectonic units of the KMC provide coherent Sm-Nd garnet-whole rock ages of 317 ± 4 Ma and 316 ± 3 Ma (2σ). The prograde major-element zoning in the mm-sized garnets in combination with the moderate peak-metamorphic temperature, support our interpretation of the Sm-Nd garnet ages as unambiguous evidence for late Carboniferous HP metamorphism. The Sm-Nd garnet growth ages overlap within-error with the 40Ar/39Ar mica cooling ages of 314 ± 2 Ma and 313 ± 2 Ma (2σ) indicating rapid uplift of the subduction complex after peak metamorphism. The ca. 317-313 Ma HP-exhumation event of the KMC is contemporaneous with those of the Atbashi and Akeyazi (ca. 500 km east in NW China) HP complexes and implies similar collision histories at the South Tianshan Suture to the east and west of the Talas-Fergana Fault (TFF). The exhumation of the KMC and Atbashi HP complexes overlaps with the initiation of the TFF (Rolland et al., 2013) suggesting incipient separation of the Chatkal and Atbashi complexes during rock exhumation and early plate collision.

First evidence for ca. 780 Ma intra-plate magmatism and its implications for Neoproterozoic rifting of the North Yili Block and tectonic origin of the continental blocks in SW of Central Asia

The North Yili Block is one of the major continental constituents of the Central Asian Orogenic Belt (CAOB), study of its basement evolution and tectonic origin is essential for the understanding of the accretionary history of the CAOB and the reconstruction of global supercontinent as well. We conducted field geological, geochronological and geochemical investigations on the mafic dikes intruding in the highly deformed Proterozoic carbonate in the North Yili Block, NW Chinese Tianshan. Zircon U–Pb dating results suggest that the gabbro and gabbroic diorite dikes and associated granitic vein formed in 776–778Ma. Element and isotope geochemical data reveal that these intrusive rocks derived from an OIB-like mantle source, and were variably influenced by fraction crystallization and important crustal contamination, thus, a continental rifting setting is proposed for these dikes. Our new data provide the first evidence for a Neoproterozoic continental intra-plate magmatism occurred in the North Yili Block, this event can be linked with the coeval continental rifting-related magmatism in northern Tarim and in other microcontinents within the SW CAOB (e.g., Central Kazakhstan, Kyrgyzstan Middle Tianshan). Based on the comparison on the Neoproterozoic to early Paleozoic stratigraphic sequences between the North Yili and Tarim blocks, it is suggested that the North Yili Block most likely originated from the Tarim Block, and is potentially one additional constituent member of the Rodinia supercontinent.

Late Paleozoic–Cenozoic intra-plate continental basaltic magmatism of the Tienshan–Junggar region in the SW Central Asian Orogenic Belt

This paper reviews previously published and presents new geochemical (bulk rock), petrologic (phenocrysts and their hosted melt inclusions) and geochronological (Ar–Ar) data from Late Paleozoic to Cenozoic volcanic rocks from the Tienshan–Junggar region cropping out in the territories of Kyrgyzstan, Kazakhstan, and China. The mafic volcanic rocks are surrounded by continental sedimentary rocks. They are mainly alkaline basalts. The Late Cretaceous–Paleocene basalts of the Central Tienshan (Group 1), and the Early Jurassic (Group 2) and Early Permian (Group 3) volcanic rocks of the Junggar Basin are geochemically similar to intra-plate basalts of the Hawaiian hotspot, Siberian and Ontong-Java LIPs, and Central Mongolia, origin of which is commonly attributed to mantle plumes. The Late Carboniferous basalts (Group 4) of NW Junggar are less enriched in the LREE and moderately depleted in the HFSE (Ti, Nb, Y, Zr) resembling crustally contaminated lavas of the Kerguelen hot spot. Melt inclusions hosted by phenocrysts from Group 1 basalts homogenized at relatively high temperatures, from 1175 to 1250°С. The composition of the melt inclusions suggests that the basalts were derived from Ti-Nb-LREE enriched mafic melts. Based on geological, geochronological, petrologic and geochemical data we recognize four periods of intra-plate volcanism: Late Carboniferous, Early Permian, Early Jurassic, and Late Cretaceous–Paleocene. The data from the plume-type intra-plate basalts suggest their relation to the action of three mantle plumes: i) Tarim (Early Permian), ii) Junggar (Early Jurassic), and iii) Tienshan (Late Cretaceous–Paleocene).