North Tianshan Research Articles

Geochemistry and geochronology of mafic rocks from the Jinghe ophiolitic mélange, northwest China: Implications for plume-related magmatism and accretionary processes within the North Tianshan Ocean

We present geological, whole-rock geochemical and Sr–Nd isotopic, and zircon U–Pb geochronological data for mafic rocks found as angular fragments in the Jinghe ophiolitic mélange of the Northern Tianshan orogenic belt, northwest China. The investigated exotic blocks comprise layered and massive gabbros and pillow basalts that are embedded in a serpentinite-rich matrix. The 380.5 ± 4.9 Ma layered gabbros have chondrite-normalized rare earth element (REE) patterns characterized by significant depletions in the light rare earth elements (LREE) and positive Eu anomalies, suggesting that the petrogenesis of these rocks was controlled by cumulus processes. The 381.5 ± 3.7 Ma massive gabbros have near-flat normal-type mid-ocean ridge basalt (MORB)-normalized multi-element patterns with moderate to strong negative Nb–Ti anomalies. These rocks have initial 87Sr/86Sr ratios (87Sr/86Sr(i)) of 0.703825–0.704758 and high εNd(t) values (+6.7 to +9.0). Two compositional groups of basalts can be recognized: ocean island basalt (OIB)- and enriched MORB-type rocks. Both groups display variable enrichments in large-ion lithophile elements (LILE) in N-MORB-normalized trace element diagrams. Semi-quantitative modelling demonstrates that the parental magmas of the massive gabbros were derived by 5%–10% partial melting of a spinel-bearing lherzolite source influenced by minor inputs of subducted crustal components. The parental magmas of the subalkaline and alkaline basalts were derived by 10%–20% and 1%–5% polybaric melting, respectively, of a lherzolite source containing spinel and garnet in equal amounts. The petrogenesis of the Jinghe basalts can be linked to continental rifting followed by seafloor spreading caused by upwelling of a mantle plume. The petrogenesis of the gabbros is linked to plate re-organization that provided the geodynamic impetus for southwestward subduction in the North Tianshan Ocean, and the onset of incorporation of oceanic materials with different origins into a growing accretionary prism during the Late Devonian. Accretion resulted in chaotic mixing of dismembered pieces of variably deformed oceanic rocks, which are now preserved as an extensive block-in-matrix ophiolitic mélange.

Composition, Provenance, and Tectonic Setting of the Southern Kangurtag Accretionary Complex in the Eastern Tianshan, NW China: Implications for the Late Paleozoic Evolution of the North Tianshan Ocean

AbstractThe Kangurtag belt in the Eastern Tianshan, connecting the Dananhu Arc with the Central Tianshan Arc, contains diagnostic rocks of accretionary origin, and thus provides key information about the evolution of the North Tianshan Ocean. The Southern Kangurtag belt is composed of two types of mélange. Type I mélange consists of Enriched Mid‐Ocean Ridge Basalt‐type pillow basalts, draped by biohermal limestones and carbonate‐siliceous sediments of a slope facies, and siliceous argillites from a hemipelagic‐pelagic environment that together make up a seamount assemblage. In Type II mélange, Normal Mid‐Ocean Ridge Basalt and ribbon cherts were dismembered and entrained in a clastic matrix, showing a “block‐in‐matrix” structure. Detrital zircons of four sandstones from Devonian and Carboniferous strata within the mélanges have a predominant age population of 410–430 Ma and a distinct Proterozoic cluster around 1.4–1.6 Ga. The εHf(t) values of Phanerozoic zircons range from −25.1 to +8.6. Such age patterns, typical of the Central Tianshan Arc, and the Hf isotopic data indicate that these sedimentary successions were deposited on the northern margin of the Central Tianshan Arc. The youngest detrital zircon age of 317 Ma provides an upper limit for the time of formation of the Southern Kangurtag accretionary complex. Therefore, we suggest that the Southern Kangurtag belt comprises an accretionary complex that developed during southward subduction of the North Tianshan Ocean beneath the Central Tianshan Arc. This subduction began in the Early Ordovician and may have lasted until the Late Carboniferous–Permian.

Topographic evolution of the Tianshan Mountains and their relation to the Junggar and Turpan Basins, Central Asia, from the Permian to the Neogene

The modern Tianshan Mountains and their surrounding basins have mainly been shaped by the far field effects of the Cenozoic India-Asia collision. However, precollision topographic evolution of the Tianshan Mountains and its impacts on the Junggar and Turpan Basins remain unclear due to the scarcity of data. Detrital zircon U-Pb dating of 14 new and 23 published samples from Permian to Neogene strata in the northern Western Tianshan Mountains, northern and southern Bogda Mountains and Central Turpan Basin, are combined with sedimentary characteristics (lithofacies, petrofacies and paleocurrent data) to investigate the temporal and spatial changes in sediment provenances. Based on the age characteristics of the source rocks in the Tianshan Mountains, the detrital zircons are divided into three groups: pre-Carboniferous zircons, mainly from the Central Tianshan Mountains; Carboniferous to Permian zircons, mainly from the North Tianshan and Bogda Mountains; and Mesozoic zircons, mainly from syn-depositional volcanic activity. The topographic evolution of the Tianshan Mountains and their relation to the Junggar and Turpan Basins can be generally divided into six stages. (1) Positive-relief Tianshan and Bogda Mountains and a rifted marine basin formed during the Early Permian to early Middle Permian following late Carboniferous orogenesis, as evidenced by interbedded alluvial fan conglomerates and postcollisional extension-related volcanic rocks along the basin margins, by marine deposits far from the basin margins and by the predominance of Carboniferous to Permian detrital zircons. (2) Fluvial to lacustrine deposits in the modern southern Junggar and Turpan Basins are characterized by abundant pre-Carboniferous zircons and consistently northward-flowing paleocurrents, indicating the submergence of the Bogda Mountains and a contiguous Junggar-Turpan continental depression basin during the late Middle Permian to the Triassic. (3) The Bogda Mountains began to uplift in the Early Jurassic, resulting in opposing paleocurrent directions, a sudden increase in sedimentary lithic detritus and the dominance of Carboniferous to Permian detrital zircons along the southern and northern margins of this range. (4) In contrast to the uplift of the Bogda Mountains, the other parts of the Tianshan Mountains experienced gradual peneplanation from the Early Jurassic to the Middle Jurassic, as confirmed by widespread fluvial to lacustrine deposits, even inside the modern Tianshan Mountains, and by the dominance of pre-Carboniferous detrital zircons. (5) The dominance of Carboniferous to Permian zircons in the southern Junggar Basin suggests the West Tianshan Mountains were uplifted during the Late Jurassic, while the dominance of pre-Carboniferous zircons in the Central Turpan Basin indicates continuous peneplanation in the Eastern Tianshan Mountains. (6) The initial shape of the Tianshan Mountains-Junggar Basin-Turpan Basin system was constructed in the Late Jurassic but was modified in the Cenozoic by the India-Asia collision, resulting in much higher Western Tianshan and Bogda Mountains, low Eastern Tianshan Mountains and well-developed foreland basins. These Cenozoic changes were recorded by the rapid cooling of apatites, the dominance of Carboniferous to Permian zircons in the southern Junggar Basin and northern Turpan Basin, and the dominance of pre-Carboniferous zircons in the Central Turpan Basin.

Ediacaran to Paleozoic magmatism in West Junggar Orogenic Belt, NW China, and implications for evolution of Central Asian Orogenic Belt

The formation of the Central Asian Orogenic Belt is thought to be related to the evolution of the Paleo-Asian Ocean surrounded by the Siberian, European, and Tarim-North China cratons. The West Junggar Orogenic Belt (WJOB) in the southern Central Asian Orogenic Belt is part of the Kazakhstan-Junggar terrane between the Altay Orogenic Belt to the north and the Tianshan Orogenic Belt to the south. It is characterized by the development of the latest Ediacaran to Paleozoic intra-oceanic island arcs and accretionary complexes, including the latest Ediacaran to Ordovician Tangbale-Mayile arc-accretionary system in the south and Ordovician to Early Carboniferous Boshchekul-Chingiz arc and Zharma-Saur arc in the north, and a remnant ocean between them. Our work reveals that the Shinaizha, Kekesayi, and Barleik dioritic plutons within the Tangbale-Mayile arc are typical of arc-related magmatism and were formed between 572 and 505 Ma, which indicate the Ediacaran to Middle Cambrian subduction of the southern Paleo-Asian Ocean. However, these arc plutons were strongly deformed and surrounded by younger rocks, implying that the magmatic arc might be dismembered and involved into the Ordovician to Devonian accretionary complexes. The Boshchekul-Chingiz arc was recorded from Late Ordovician and its arc magmatism peaked in Late Silurian to Early Devonian. The arc moved northward due to the retreat of the south-facing Irtysh-Zaysan subduction zone, which resulted in the formation of contemporaneous A-type or peralkaline magmatic rocks or bimodal volcanics in response to the within-arc extension. The northward migration of the subduction zone led to the formation of the Devonian to Early Carboniferous Zharma-Saur arc. The earliest Late Carboniferous I-type granitoids occurred sporadically in WJOB, followed by pervasive occurrences of Late Carboniferous to Early Permian A- and I-type granitoids in WJOB and adjacent areas. The Late Carboniferous granitoids, combined with the Late Carboniferous stitching plutons within the Irtysh-Zaysan suture zone to the north and the North Tianshan suture zone to the south of WJOB, suggest the Kazakhstan-Junggar terrane collided with Altay and Tianshan orogenic belts in the latest Early Carboniferous (ca. 320 Ma).

An early Devonian intra-plate bimodal volcanic suite in the Kyrgyz North Tianshan belt, the central Asian orogenic belt

A suite of bimodal volcanic rocks in the Kyrgyz North Tianshan belt is composed of totally ∼100-m-thick basalt and basaltic andesite layers of the Karakastek Formation and totally ∼700-m-thick rhyolite layers of the Kastek Formation, with minor amounts of interlayered felsic to intermediate tuff/ignimbrite. Zircon grains from a rhyolite sample have a weighted mean 206Pb/238U age of 416 ± 8 Ma, which is considered to be the crystallization age of the rhyolite. The zircon grains have δ18O ranging from +6.5 to +7.3‰. The calculated zircon saturation temperatures (TZr) range from 850 to 900 °C. Rhyolites have high total alkaline, Zr, Y and Nb contents and high Ga/Al similar to A1-type granitoids formed in an intra-plate setting. Mafic volcanic rocks have SiO2 ranging from 49.4 to 55.3 wt% and FeOt/MgO from 1.4 to 2.4, showing a tholeiitic trend. They have negative Nb and Ta anomalies on the primitive mantle-normalized trace element patterns and have εNd(t) of −1.5 to 0. There is a positive correlation between εNd(t) and ΔεHf(t) of the mafic volcanic rocks, indicative of crustal contamination. However, one least evolved basalt sample has Mg# of 57 with low Zr and Hf concentrations, indicating insignificant crustal contamination. Modeling results indicate that the mafic volcanic rocks may have been derived from a dry, garnet-bearing lherzolite mantle source. They have εHf(t) and εNd(t) varying along the “Island arc array”, which is interpreted as the mantle source was metasomatized by slab-derived melts rather than fluids. In addition, the rhyolites have εNd(t) of −2.4 to −2.9, much lower than the mafic volcanic rocks, and were thus unlikely produced by the fractionation of basaltic magmas. The rhyolites show Nd-Hf isotope decoupling, which is interpreted as a result of the anataxis of Precambrian lower crust associated with the breakdown of garnet. Therefore, the early Devonian bimodal volcanic suite in the Kyrgyz North Tianshan belt was considered to have formed in an intra-plate setting.

Implication of Mesoproterozoic (∼1.4 Ga) magmatism within microcontinents along the southern Central Asian Orogenic Belt

The southern Central Asian Orogenic Belt contains numerous microcontinents including the Kazakhstan-Yili block, Central Tianshan Arc and Junggar block in the west, the Erguna, Xing'an, Songliao and Jiamusi-Khanka blocks in the east, bridging by the Tuva-Mongolia block, Beishan Orogenic Belt and Alax block in the central region. New geochemical and geochronological data from the microcontinental blocks within the southern Beishan Orogenic Belt provide new insight into the Precambrian crustal evolution of this region. Five granitoids from the Jiujing region yielded igneous zircon 206Pb/238U ages of 1450–1401 Ma, and zircon xenocrysts from late Paleozoic plutons in the Jiujing, Qiaowan, and Shibandun regions have 206Pb/238U ages that also cluster at ∼1.4 Ga. Those data suggest that crust of this age is widespread beneath the southern Beishan Orogenic Belt. The ∼1.4 Ga zircons display depleted Hf isotopic signatures (εHf(t) = 2.0–11.1) and show relatively young two-stage Hf model ages (1.9–1.4 Ga), indicating rapid reworking of juvenile mantle-derived materials. Integrating these results with similar geochronological data from the Kyrgyz North Tianshan, Yili block, Central Tianshan Arc, northern Alxa block and Xilinhot block, we propose that the CAOB incorporates an extensive assemblage of Mesoproterozoic (1450–1360 Ma) magmatism that can be traced over ca. 3000-km long a west-east trending belt. These Mesoproterozoic granitoids display distinct geochemical signatures (e.g., Ga/Al*10,000 ranging from 1.46 to 3.88 and Zr + Nb + Ce + Y varying largely from 101 to 1066 ppm) and show a temporal progression from orogenic to anorogenic over a relatively short period. This suggests a tectonic setting that likely evolved from compressional to extensional environments during 1450–1360 Ma, perhaps resulting from slab roll back. We suggest that the 1450–1360 Ma magmatism along the southern CAOB corresponds with a transition from a compressional to extensional environment on the margin of the Nuna supercontinent.

Detrital zircon ages and Hf isotopic compositions of metasedimentary rocks in the Wuqia area of Southwest Tianshan, NW China: implications for the early Paleozoic tectonic evolution of the Tianshan orogenic belt

ABSTRACTThe Chinese Southwest Tianshan Orogenic Belt is located along the boundary between the Central Asian Orogenic Belt (CAOB) and the Tarim Block (TB), NW China. It records the convergence of the Tarim Block and the Middle Tianshan, and is, therefore, a crucial region for understanding the Eurasia continental growth and evolution. The Wulagen (geographical name) metasedimentary rocks of the Wuqia area (mainly metamorphic sandstones and mica schists) form one of the metamorphic terranes in the Southwestern Tianshan Orogenic Belt. The geochronology of these rocks is poorly known, which hampers our understanding of the tectonic evolution of the belt. We analyzed 517 zircon grains for detrital zircon U–Pb dating and 93 zircon grains for in situ Lu–Hf isotopic compositions from the Wulagen metasedimentary rocks. The analyzed zircon grains yield Neoarchean to late Paleozoic U–Pb ages with major age peaks at ~2543 Ma, 1814 Ma, 830 Ma, 460 Ma, and the youngest cluster of zircon (magmatogene) ages is 395 Ma. The zircon U–Pb data show that the late Paleozoic (Early Devonian) is the maximum depositional age of the Wulagen metasedimentary rocks, rather than the previously considered Precambrian period. The zircons with Paleozoic ages yield εHf(t) values of −22.0 to +11.3 and two-stage model ages (TDM2) of 3.95 to 1.30 Ga, suggesting that the parental magmas were formed from partial melting of pre-existing crustal rocks. Our zircon U–Pb geochronology and Hf isotopic data indicate the major source regions for the Wulagen metasedimentary rocks was the Kyrgyzstan North Tianshan. The zircon age population of 600–400 Ma (peak at ~460 Ma) has negative εHf(t) values (−15.0 to −0.6) and Mesoproterozoic two-stage model ages, suggesting that the early Paleozoic magmatism resulted mainly from the melting of ancient crust, which played an important role in crustal evolution in the southern CAOB.

The Ediacaran to Early Palaeozoic evolution of the Junggar–Balkhash Ocean: A synthesis of the ophiolitic mélanges in the southern West Junggar terrane, NW China

The southern West Junggar terrane in the south‐western Central Asian Orogenic Belt (CAOB) is mainly composed of accretionary complexes and characterized by widespread outcrops of ophiolitic mélanges in the Barleik, Mayile, and Tangbale (BMT) areas, representing the site of consumption of the Junggar–Balkhash Ocean. However, due to a lack of comprehensive synthesis of the ophiolitic mélanges, the early evolution of the Junggar–Balkhash Ocean remains poorly understood. This paper first focuses on the little‐known Barleik ophiolitic blocks and associated pillow basalts and then integrates the Barleik, Mayile, and Tangbale ophiolitic mélanges into one model. Mineral and whole‐rock chemistry suggests that the Barleik ophiolitic blocks, including serpentinized dunite, clinopyroxenite, and gabbro, show supra‐subduction zone (SSZ) affinities, as indicated by high Cr# values (76–78) in spinels and strong depletion in Nb, Ta, Zr, and Hf, whereas the Barleik pillow basalts are alkaline and have high TiO2 contents (2.82–3.42 wt.%) and Nb–Ta enrichment, similar to ocean‐island basalt (OIB) lavas. Combined with published chronological and chemical data from ophiolites, metamorphic rocks, and arc plutons, we propose that the BMT Ediacaran to middle Cambrian ophiolitic gabbros were formed in a SSZ environment, implying a single Mariana‐type arc system within the Junggar–Balkhash Ocean, which was coeval with an Andean‐type arc system in the Kyrgyz North Tianshan, supporting an archipelagic model for the south‐western CAOB. By contrast, the BMT OIB‐type basalts are early Silurian in age, much younger than the SSZ‐type ophiolites, and can be interpreted as relicts of seamounts. Thus, a model involving the Ediacaran to Ordovician intra‐oceanic subduction and late Silurian to Early Devonian seamount accretion is helpful for clarifying the different components of the accretionary complexes in the southern West Junggar terrane.

Clematismae (Ranunculaceae), a new species of C.sect.Meclatis from Xinjiang, China.

Clematismae Z.Z.Yang & L.Xie, a new species of Ranunculaceae from Xinjiang, China, is described and illustrated. The new species is morphologically similar to C.orientalis and C.glauca but can be distinguished for being a less hairy plant (hairy in C.orientalis), often 2-ternate leaves (1–2-pinnate for C.orientalis and C.glauca), lanceolate to linear-lanceolate leaflets (elliptic or ovate in C.glauca), larger flowers (smaller flower in C.orientalis) and narrowly lanceolate sepals with acute to slightly attenuate apex (narrowly oblong sepals in C.orientalis and ovate to broadly lanceolate sepals in C.glauca). The new species is endemic to the southern slope of North Tianshan Mountain in Central Xinjiang. The conservation status of the species is also discussed.

Clematis mae (Ranunculaceae), a new species of C. sect. Meclatis from Xinjiang, China

Clematismae Z.Z.Yang & L.Xie, a new species of Ranunculaceae from Xinjiang, China, is described and illustrated. The new species is morphologically similar to C.orientalis and C.glauca but can be distinguished for being a less hairy plant (hairy in C.orientalis), often 2-ternate leaves (1–2-pinnate for C.orientalis and C.glauca), lanceolate to linear-lanceolate leaflets (elliptic or ovate in C.glauca), larger flowers (smaller flower in C.orientalis) and narrowly lanceolate sepals with acute to slightly attenuate apex (narrowly oblong sepals in C.orientalis and ovate to broadly lanceolate sepals in C.glauca). The new species is endemic to the southern slope of North Tianshan Mountain in Central Xinjiang. The conservation status of the species is also discussed.

New insights into multiple provenances evolution of the Jurassic from heavy minerals characteristics in southern Junggar Basin, NW China

Abstract The method of random forest was used to classify the heavy mineral assemblages of 2 418 Jurassic samples in the southern Junggar Basin, and determine the distribution of the heavy mineral assemblages from the same provenance systems. Based on the analysis of heavy minerals assemblages, ZTR index, sedimentary characteristics, U-Pb zircon ages, whole-rock geochemical and paleocurrent direction analysis, the study reveals that five important provenances were providing sediments to the southern Junggar Basin in the Jurassic period: The North Tianshan (NTS), Central Tianshan (CTS), Bogda Mountains, Zhayier Mountains and Kalamaili Mountains. During the Early Jurassic, NTS-CTS, Kalamaili Mountains and Zhayier Mountains are primary provenances, Bogda Mountains started to uplift and supply clastic materials in the Middle Jurassic. There are three sedimentary area in the Jurassic of southern Junggar Basin: the western part, the central part and the eastern part. In the western part, the clastic materials of the Early Jurassic was mainly from NTS blocks and Zhayier Mountains, and the sediments were dominantly derived from the Zhayier Mountains during the Middle–Late Jurassic. In the central part, the main provenance of the Early Jurassic switched from NTS to CTS. In the Xishanyao Formation, the main source went back to NTS again. The NTS was the primary provenance during the sedimentary periods of Toutunhe Formation and Qigu Formation. In the eastern part, the contribution of CTS and Kalamaili Mountains were considered as major provenances in the Early Jurassic−Xishanyao Formation, small proportion of sediments were from NTS. The Bogda mountains uplifted and started to provide sediments to the Junggar Basin in the sedimentary period of Xishanyao Formation, and became the major source during the Toutunhe Formation period, with small amount of sediments from CTS. The provenance from CTS was hindered during the sedimentary period of Qigu Formation owing to the uplifting of the Bogda mountains, and the sediments were mainly from the Bogda mountains and NTS.

Detrital zircon U–Pb geochronology of the Permian strata in the Turpan–Hami Basin in North Xinjiang, NW China: Depositional age, provenance, and tectonic implications

The Permian sedimentary rocks in the Turpan–Hami Basin are key records of the tectonic evolution in the Eastern Tianshan area, although their depositional ages and provenances remain relatively less studied. Here, we conducted detrital zircon U–Pb geochronology on subsurface samples collected from Well LN1 in the Turpan–Hami Basin to understand the depositional ages and provenances of the Permian rocks. In this study, detrital zircon U–Pb ages from the Late Carboniferous Sumuke Formation yielded a notable Permian age population with the youngest single‐grain age at 282.6 Ma. Compiling this result with a published dataset from the surrounding regions of the Turpan–Hami Basin, it shows that the Sumuke Formation was actually deposited in the Early Permian and the Late Carboniferous palynological assemblages within it are of recycled origin. The detrital zircon age spectra of the three Early–Middle Permian samples from Well LN1 define similar unimodal distribution, with prominent late Carboniferous age peaks and scarce Precambrian ages, further indicating that the zircon grains in these three samples should be ultimately sourced from the Jueluotag, the southern branch of the North Tianshan. Regionally, the Bogda, the northern branch of the North Tianshan, as catchment areas recorded interaction between southern and northern sources, and thus, there was a single greater Junggar–Turpan Basin during the Early–Middle Permian. During the latest Middle Permian to Late Permian, provenance shifts occurred at the Jueluotag and the southern piedmont of the Bogda, reflecting the uplift of the Central Tianshan and the Bogda and the isolation of the Junggar Basin and Turpan–Hami Basin. Integrating regional geological studies and provenance evolution trends, we suggest that the Turpan–Hami Basin and its surrounding regions were in a rift setting controlled by post‐collision extension during Early–Middle Permian. By contrast, tectonic contraction and inversion occurred during the latest Middle Permian to Late Permian as a part of the intraplate orogenic process in the Tianshan, which responded to the convergence between the Songpan–Ganzi terrane and palaeo‐Eurasian Plate.

Evolution of Cambrian and Early Ordovician arcs in the Kyrgyz North Tianshan: Insights from U-Pb zircon ages and geochemical data

Geochronological, geochemical, and structural studies of magmatic and metamorphic complexes within the Kyrgyz North Tianshan (NTS) revealed an extensive area of early Palaeozoic magmatism with an age range of 540–475 Ma. During the first episode at 540–510 Ma, magmatism likely occurred in an intraplate setting within the NTS microcontinent and in an oceanic arc setting within the Kyrgyz-Terskey zone in the south. During the second episode at 500–475 Ma, the entire NTS represented an arc system. These two phases of magmatism were separated by an episode of accretionary tectonics of uncertain nature, which led to obduction of ophiolites from the Kyrgyz-Terskey zone onto the microcontinent. The occurrence of zircon xenocrysts and predominantly negative whole-rock ɛNd(t) values and ɛHf(t) values of magmatic zircons suggest a continental setting and melting of Precambrian continental sources with minor contributions of Palaeozoic juvenile melts in the generation of the magmatic rocks. The late Cambrian to Early Ordovician 500–475 Ma arc evolved mainly on Mesoproterozoic continental crust in the north and partly on oceanic crust in the south. Arc magmatism was accompanied by spreading in a back-arc basin in the south, where supra-subduction ophiolitic gabbros yielded ages of 496 to 479 Ma. The relative position of the arc and active back-arc basin implies that the subduction zone was located north of the arc, dipping to the south. Variably intense metamorphism and deformation in the NTS reflect an Early Ordovician orogenic event at 480–475 Ma, resulting from closure of the Djalair-Naiman ophiolite trough and collision of the Djel'tau microcontinent with the northern margin of NTS. Comparison of geological patterns and episodes of arc magmatism in the NTS and Chinese Central Tianshan indicate that these crustal units constituted a single early Palaeozoic arc and were separated from the Tarim Craton by an oceanic basin since the Neoproterozoic.

Late Paleozoic post-collisional setting of the North Tianshan, NW China: New insights from geochronology, geochemistry and Sr–Nd isotopic compositions of the Permian Nileke volcanic rocks

The northern margin of the Yili Block, NW China, contains widespread Permian volcanic rocks that provide insight into the late Paleozoic evolution of the North Tianshan Orogenic Belt in the southwestern Central Asian Orogenic Belt. This paper presents geochronological, geochemical and Sr–Nd isotopic data for Permian volcanic rocks of the Hamisite and Wulang formations in the Nileke area of the western Awulale Mountains, and provides new constraints on their ages, petrogenesis and tectonic settings. The whole-rock 40Ar/39Ar plateau age of 281.6 ± 1.0 Ma for basalt in the Hamisite Formation suggests that these volcanic rocks were erupted in the early Permian. The studied samples comprise mainly basalt and basaltic andesite, with positive εNd(t) values, enriched light rare earth and large-ion lithophile elements, and depleted high-field-strength elements, indicating an origin from a subcontinental lithospheric mantle wedge that was probably metasomatised by slab-derived fluid and sediment-derived melt beneath the Yili Block. Based on geochemistry and the regional geological records, we suggest that the Permian Nileke volcanic rocks of the Yili Block were formed in a post-collisional setting, following slab break-off after closure of the North Tianshan Ocean. However, the Hamisite volcanic rocks were derived from a deeper mantle source, probably through thickening of the lithospheric mantle and crust of the Yili Block.

Petrology and geochemistry of mafic and ultramafic rocks in the north Tianshan ophiolite: Implications for petrogenesis and tectonic setting

New petrological and geochemical data for the mafic (pillow basalt, massive basalt, gabbro) and ultramafic rocks (serpentinite, serpentinized harzburgite) in the north Tianshan ophiolite cropping out along the Duku road are combined to constrain the geological evolution of the north Tianshan ophiolite, which marks the boundary between the Yili-central Tianshan and the Junggar terranes. Pillow basalt, characterized by significant enrichments of light rare earth elements (REE) as well as high Nb/Yb and TiO2/Yb ratios, probably represents the seamount fragments accreting in the north Tianshan accretionary prism. Massive basalt and gabbro, displaying flatten REE patterns with low Ti/V ratios, are similar to forearc basalts. Serpentinite and serpentinized harzburgite, belonging to the mantle section of the ophiolite, are depleted in incompatible elements compared to the primitive mantle. The compositions of primary chromian spinel, olivine, and pyroxene in these mantle rocks are comparable to those in forearc peridotites. Such characteristics indicate that the protoliths of serpentinite and serpentinized harzburgite have undergone high degrees of partial melting. They were modified by island arc tholeiitic to boninitic melts during and/or after melt extraction as suggested by the enrichment of Ti in chromian spinel coupled with the occurrence of amphibole inclusions in chromian spinel. These new petrological and geochemical data suggest that the north Tianshan ophiolite was generated in a forearc setting. Retreat and rollback of the north Tianshan subducted slab triggered the upwelling of asthenosphere with minor mass transfer from the slab, leading to the formation of massive basalt and gabbro. Increasing flux of slab-derived fluids/melts largely enhanced partial melting degree of the mantle peridotites, producing island arc tholeiitic to boninitic melts. These melts interacted with refractory mantle residues and formed the mantle rocks (i.e., the protoliths of serpentinite and serpentinized harzburgite) in the north Tianshan ophiolite.

Age assignment of the upper Carboniferous Arbasay Formation in Shichang Region, North Tianshan (NW China)

The North Tianshan Orogenic Belt contains the youngest ophiolites in the Tianshan and provides some information on timing of the last closure of the Junggar-Balkhash Ocean. LA–ICP–MS zircon U–Pb dating was conducted to define the formation age of the Arbasay Formation in the Shichang Region of North Tianshan, which is exposed near the suture zone but its age remains debated. The Arbasay Formation is mainly composed of volcanic and volcaniclastic rocks with tuff interlayers. The zircons from the tuffs yield two age populations of 315 ± 3 Ma and 304 ± 2 Ma, constraining the commencement and demise timings of volcanism, respectively. Furthermore, zircon U–Pb age spectra of the tuffaceous sandstones display the youngest peak age at 308 Ma, indicating a < 308 Ma age for the depositional age of volcaniclastic rocks. The volcaniclastic rocks therefore were likely to deposit together with the syn-sedimentary volcanism during Late Carboniferous. This means that the Arbasay Formation in Shichang Region should be re-assigned to Late Carboniferous in age. Given that the Arbasay Formation was likely to be formed during the tectonic transition from compression to extension, the Junggar-Balkhash Ocean possibly closed during Late Carboniferous.

Episodic Meso-Cenozoic denudation of Chinese Tianshan: evidence from detrital apatite fission track and zircon U–Pb data, southern Junggar Basin margin, NW China

It is generally accepted that the Tianshan was reactivated repeatedly throughout Meso-Cenozoic time. However, the extent of these episodes has not been fully understood. Using a previous magnetostratigraphic baseline, this study reports a detrital zircon U–Pb and apatite fission track dating study along the ~4 km thick Cenozoic Jingou River section at the southern margin of the Junggar Basin. Our results provide direct insight into the denudation history of the bordering Tianshan Mountains and suggest that they experienced two phases of rapid cooling in the Jurassic and Cretaceous. These cooling phases are attributed to tectonic events at distant plate margins such as the Late Triassic-Cretaceous accretion of peri-Gondwanan blocks (e.g. Qiangtang and Lhasa). The detrital zircon U–Pb ages range widely from ∼58 to 3055 Ma with a majority falling between ∼58 and 600 Ma. They can be statistically separated into two primary age peaks at 300–290 Ma and 450–410 Ma, as well as two secondary peaks at 170 Ma and 850 Ma. Detrital zircon U–Pb dating results suggest that the Central Tianshan may have still shed detritus into the Junggar Basin during the Early Palaeogene. In the Late Oligocene-Early Miocene, the modern North Tianshan started to form a topographic barrier that blocked all northward rivers flowing from the Central Tianshan to the Junggar Basin. Progressive northward indentation of India into Eurasia, caused a final intense phase of cooling and denudation of the Tianshan between ∼11 and 3 Ma, leading to deposition of the Xiyu Formation, a widely distributed thick conglomerate.

Alternating Trench Advance and Retreat: Insights From Paleozoic Magmatism in the Eastern Tianshan, Central Asian Orogenic Belt

AbstractAccretionary orogens generally involve trench advance and retreat, recognition of which is critical to understand the thermal evolution and geodynamics of the lithosphere. A systematic study has been conducted on the Silurian to Carboniferous volcanic rocks from the Dananhu‐Harlik arc in the Chinese eastern Tianshan. Three distinct episodes of volcanism have been recognized: Late Silurian normal andesite, high Sr/Y andesite and sanukitoid, Late Devonian bimodal high‐alumina basalt and dacite, and Early Carboniferous high‐alumina basalt. These magmatism experienced variable degrees of interaction between a depleted mantle wedge and subducting slab‐/sediment‐derived fluid/melt. Combining with previous geological data, we propose that the Dananhu‐Harlik arc was formed by the northward subduction of the North Tianshan Ocean (NTO), different from the traditional southward subduction of the Junggar Ocean, which was likely subducted northward to account for the East Junggar arc system. Temporal changes in Ce/Y of basalts and Ho/Yb of felsic rocks in the Dananhu‐Harlik arc imply that the arc crust underwent episodic thickening and thinning, which probably reflect the trench advance/retreat processes. The high Ce/Y and Ho/Yb ratios in the Late Ordovician to Devonian and Late Carboniferous correspond to two phases of trench advance. Conversely, the low Ce/Y and Ho/Yb ratios in the Early Carboniferous record a period of trench retreat, which caused the splitting of the Dananhu‐Harlik arc and formation of the Bogda back‐arc basin. Meanwhile, a southward subduction initiated along the south margin of the NTO, producing the Yamansu arc. Late Carboniferous bipolar subduction of the NTO led to its final closure.

Granite zircon U-Pb geochronology and geochemistry and the geological significance of the Saibo copper deposit in the western Tianshan Mountains, Xinjiang Province, China

The discovery of the Saibo copper deposit was a significant breakthrough in prospecting in the western Tianshan Mountains, Xinjiang Province. This deposit has great significance for studying the transboundary metallogenic belt because the deposit is associated with granitoids. Zircon dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yield 206Pb/238U ages of 385.9 ± 1.3 Ma and 387.2 ± 1.4 Ma for the granodiorite and granite porphyries from the deposit, respectively, indicating that the granitoids are the products of Middle Devonian magmatism. In terms of their geochemistry, these porphyries have high SiO2 contents (between 62.60% and 67.47%) and low TiO2 contents (between 0.32% and 0.52%), with aluminum saturation indexes (ASI) between 0.76 and 0.90; these characteristics indicate a metaluminous medium-potassium (K) calc-alkaline I-type granite that is enriched in large-ion lithophile elements such as K and Sr, as well as in incompatible elements such as Th and U and light rare earth elements (LREE); is relatively depleted in high-field strength elements such as Na, Ta, P, and Ti; and lacks an obvious Eu anomaly. The zircon Lu-Hf isotope ratios are relatively high, with 176Hf/177Hf values varying between 0.28253 and 0.28272 and εHf (t) that varies between −0.4 and +6.5 (mostly positive). These features demonstrate that the granites were the products of co-magmatic evolution originating from the depleted mantle and the partially melting of juvenile crustal material, which occurred due to the North Tianshan Ocean subduction towards the southern Yili block that led to magma ascent from the depleted mantle.

Geochemical and Petrological Studies on the Early Carboniferous Sidingheishan Mafic–Ultramafic Iintrusion in the Southern Margin of the Central Asian Orogenic Belt, NW China

The Sidingheishan mafic–ultramafic intrusion is located in the eastern part of the Northern Tianshan Mountain, along the southern margin of the Central Asian Orogenic Belt in northern Xinjiang autonomous region of China. The Sidingheishan intrusion is mainly composed of wehrlite, olivine websterite, olivine gabbro, gabbro and hornblende gabbro. At least two pulses of magma were involved in the formation of the intrusion. The first pulse of magma produced an olivine‐free unit and the second pulse produced an olivine–bearing unit. The magmas intruded the Devonian granites and granodiorites. An age of 351.4±5.8 Ma (Early Carboniferous) for the Sidingheishan intrusion has been determined by U‐Pb SHRIMP analysis of zircon grains separated from the olivine gabbro unit. A U‐Pb age of 359.2±6.4 Ma from the gabbro unit has been obtained by LA‐ICP‐MS. Olivine of the Sidingheishan intrusion reaches 82.52 mole% Fo and 1414 ppm Ni. On the basis of olivine‐liquid equilibria, it has been calculated that the MgO and FeO included in the parental magma of a wehrlite sample were approximately 10.43wt% and 13.14wt%, respectively. The Sidingheishan intrusive rocks are characterized by moderate enrichments in Th and Sm, slight enrichments in light REE, and depletions in Nb, Ta, Zr and Hf. The εNd (t) values in the rock units vary from +6.70 to +9.64, and initial 87Sr/86Sr ratios range between 0.7035 and 0.7042. Initial 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values fall in the ranges of 17.23–17.91, 15.45–15.54 and 37.54–38.09 respectively. These characteristics are collectively similar to the Heishan intrusion and the Early Carboniferous subduction related volcanic rocks in the Santanghu Basin, North Tianshan and Beishan area. The low (La/Gd)PM values between 0.26 and 1.77 indicate that the magma of the Sidingheishan intrusion was most likely derived from a depleted spinel–peridotite mantle. (Th/Nb)PM ratios from 0.59 to 20.25 indicate contamination of the parental magma in the upper crust. Crystallization modeling methods suggest that the parental magma of the Sidingheishan intrusion was generated by flush melting of the asthenosphere and subsequently there was about 10vol% contamination from a granitic melt. This was followed by about 5vol% assimilation of upper crustal rocks. Thus, the high‐Mg basaltic parental magma of Sidingheishan intrusion is interpreted to have formed from partial melting of the asthenosphere during the break‐off of a subducted slab.