Southern Tianshan Research Articles

Petrogenesis of Late Carboniferous-Early Permian mafic-ultramafic-felsic complexes in the eastern Central Tianshan, NW China: The result of subduction-related transtension?

Knowledge of the Late Paleozoic accretionary processes responsible for the Altaids (or southern Central Asian Orogenic Belt) is critical for a better understanding of continental growth in Central Asia. Here, we systematic study the Late Carboniferous mafic-ultramafic-felsic complexes in eastern Central Tianshan to understand the tectonic evolution of the southern Altaids. Two gabbros yield zircon crystallization ages of 301.5 ± 2.3 Ma and 300.7 ± 2.6 Ma and one granite is dated as 301.6 ± 1.5 Ma. The gabbros are tholeiitic and characterized by typical subduction-related geochemical signatures with low TiO2 contents, enrichments in Rb, Ba, U, K, Pb and Sr, negative Nb and Ta anomalies, and negative to positive Eu and Ti anomalies. The granites are the A2-type granites with high contents of SiO2, K2O + Na2O, Ga, Zr, Nb, Y, HFSE, REE, and high ratios of Fe/Mg and Y/Nb (>1.2). The gabbros and granites have high εNd(t) values of +1.64 to +5.03, zircon εHf(t) values of −2.8 to +9.1, and low (87Sr/86Sr)i (0.704007–0.705437) values. These geochemical characters demonstrate that the rocks in the complexes are fractionation genesis and that the A-type granitic rocks are the highest fractionation facies of tholeiitic mafic rocks. These results suggest that they were originated from the asthenospheric garnet-free mantle in a supra-subduction zone. Combined with regional data, we suggest that the oblique subduction with slab break-off of the Southern Tianshan oceanic plate plausibly induced asthenospheric upwelling to form the tholeiitic magma, and which intrude along the strike-faults the Central Tianshan-Yamansu arc. Accordingly, these results reveal the Paleo-Asian Ocean continued to subduct at the southern Altaids in the Late Carboniferous.

New timing of the Indosinian intracontinental deformation from the Triassic growth strata in the Kuqa Depression, Southern Tianshan, China

New timing of the Indosinian intracontinental deformation from the Triassic growth strata in the Kuqa Depression, Southern Tianshan, China

Response of glacier area variation to climate change in the Kaidu-Kongque river basin, Southern Tianshan Mountains during the last 20 years

Glaciers are crucial water resources for arid inland rivers in Northwest China. In recent decades, glaciers are largely experiencing shrinkage under the climate-warming scenario, thereby exerting tremendous influences on regional water resources. The primary role of understudying watershed scale glacier changes under changing climatic conditions is to ensure sustainable utilization of regional water resources, to prevent and mitigate glacier-related disasters. This study maps the current (2020) distribution of glacier boundaries across the Kaidu-Kongque river basin, south slope of Tianshan Mountains, and monitors the spatial evolution of glaciers over five time periods from 2000–2020 through thresholded band ratios approach, using 25 Landsat images at 30 m resolution. In addition, this study attempts to understand the role of climate characteristics for variable response of glacier area. The results show that the total area of glaciers was 398.21 km2 in 2020. The glaciers retreated by about 1.17 km2/a (0.26%/a) from 2000 to 2020. The glaciers were reducing at a significantly rapid rate between 2000 and 2005, a slow rate from 2005 to 2015, and an accelerated rate during 2015–2020. The meteorological data shows slight increasing trends of mean annual temperature (0.02°C/a) and annual precipitation (2.07 mm/a). The correlation analysis demonstrates that the role of temperature presents more significant correlation with glacier recession than precipitation. There is a temporal hysteresis in the response of glacier change to climate change. Increasing trend of temperature in summer proves to be the driving force behind the Kaidu-Kongque basin glacier recession during the recent 20 years.© 2021 China Geology Editorial Office.

Spatial Distribution of Rn, CO2, Hg, and H2 Concentrations in Soil Gas Across a Thrust Fault in Xinjiang, China

Spatial distribution characteristics and origin of soil gases were discussed in this study. We also examine the correlation between the spatial variation of soil gases and fault activity, based on the measurements of Rn, Hg, H2, and CO2 concentrations in the eastern segment of the BLT fault zone in the southern Tianshan Mountains, Xinjiang, China. The results show that the Rn and CO2 concentrations on the hanging wall were higher than those near the fault zone and on the footwall of the fault. Moreover, the Hg and H2 concentrations on the footwall were also higher than those near the fault and on the hanging wall of the fault. The main factors affecting the variations in soil gas spatial distribution were gaseous origin structure of the crust, fault activity, fault fracture degree, stratigraphic lithology, and the cover layer. The results of active structural fault mapping show that the BLT fault has been active since the Late Quaternary and the thrust has broken the fault surface. Under the influence of tectonic compressive stress, the tension zone was formed on the hanging wall of the fault and fractures were developed. This result is consistent with the characteristics of soil gas concentrations measured in this study, indicating that the concentrations of Rn, Hg, H2, and CO2 are closely related to the fault activity. These findings can be applied to identify seismic precursors from gas monitoring data of the studied area.

Initiation timing of Cenozoic compression deformation of the southern Tianshan mountains: Implications from growth strata in the Kuqa Depression

Influenced by the far‐field effect of the India–Eurasia collision during the Cenozoic, the Palaeozoic orogenic belt of the southern Tianshan mountains underwent tectonic activation. This resulted in the transmission of intense compressive deformation to the northern margin of the Tarim Basin, Kuqa Depression, where multiple east–west trending fold–thrust belts are formed. Exactly when the Cenozoic compressive deformation in the southern Tianshan orogenic belt was initiated is controversial. Growth strata are sediments of the syn‐tectonic events, and the age of the bottom boundary of the growth strata effectively constrains the initiation timing of the structural deformation. The Kuqa Depression currently receives sediments from the southern Tianshan mountains, and it is natural to use these Cenozoic strata to decipher the southern Tianshan's uplift history. Based on the detailed structural mapping and structural profile survey of the Kuqa Depression area, the Palaeogene Kumugelimu Formation growth strata in the Kuqa Depression were identified. Based on the magnetic stratigraphic sequence of the Cenozoic established by predecessors, the age of the bottom boundary of the Palaeogene Kumgelimu Formation is assumed to be 42–60.5 Ma. Combined with the analysis of the apatite fission‐track ages of the Mesozoic–Cenozoic clastic sediments in the Kuqa Depression by previous workers, there was an evident uplift and cooling event (~50 Ma) during the Palaeogene in the Kuqa Depression. The Cenozoic compression deformation in the south Tianshan and Kuqa Depression was then concluded to be initiated at approximately 50 Ma.

Segmentation of the Kepingtage thrust fault based on paleoearthquake ruptures, southwestern Tianshan, China

Decreasing deformation rates across the southern Tianshan have led to different seismogenic mechanisms and different proposed models to explain the orogen-scale fault kinematics. In this study, we focus on the segmentation of the Kepingtage fault by studying variations in the total offset and shortening rates of the Kepingtage fault along the southern front of the Tianshan. We used fault scarp mapping and trench excavations to assess fault segmentation and deformation on the Kepingtage fault. Our results indicate there are different shortening rates on the western (2.5–2.7 mm/year) versus the eastern segments (~ 0.3 mm/year), which are separated by the Piqiang tear fault. The decrease in shortening rates is not gradual; instead, it decreases sharply from west to east at the Piqiang fault. These segmentation boundaries are also supported by geodetic data and balanced structural restorations. Our data support a model where strike-slip faults accommodate step-changes in the deformation rates and the earthquake risks from west to east across the Tianshan.

Stable Isotope Signatures and Moisture Transport of a Typical Heavy Precipitation Case in the Southern Tianshan Mountains

Stable oxygen isotopes in precipitation contain meaningful environmental information on a synoptic scale and can be applied to diagnose hydrometeorological processes. A series of rainstorms occurred at the southern Tianshan Mountains during the period from May to June 2013, and the event-based precipitation was sampled along the mountain range from west to east. Based on δ18O values in precipitation samples as well as the corresponding meteorological parameters, the moisture transport paths during the sampling period were identified. In late-May (stage 1), isotopes in precipitation collected generally showed a depleting trend. In mid-June (stage 2), there was no coherent trend of isotopes in precipitation for these stations, and only isotope values in Aksu showed a continually depleting trend. Checking other meteorological proxies during the sampling period, the event-based precipitation isotopes sensitively reflected the moisture process. In central Asia, both the westerly and monsoon moisture can be delivered to cause extreme precipitation events, and the isotopic information provides an alternative tool to investigate the atmospheric processes.

Tectonic evolution of Tianshan-Bogda-Kelameili mountains, clastic wedge basin infill and chronostratigraphic divisions in the source-to-sink systems of Permian-Jurassic, southern Junggar Basin

Continuous Paleozoic-Mesozoic sediment records within Junggar Basin, Northwestern China, provide details on tectonic evolution of the surrounding Tianshan, Bogda, and Kelameili orogenic belts. We compiled 5250 zircon U-Pb ages from 58 samples (602 new and 4648 published ages) and integrated with published paleo-current directions, paleo-geography, and well core data to reconstruct the uplift timing and provenance for source-to-sink systems of southern Junggar Basin from Permian to Jurassic. This reconstruction strongly suggests that the Kelameili area, middle Southern Tianshan, and western Northern Tianshan had been uplifted before the Permian. However, the uplift of southern Central Tianshan, eastern Bogda, and western Bogda initiated in middle Permian, late Permian, and early Jurassic, respectively. In addition, maximum depositional age calculated from youngest zircon U-Pb ages modes provides new constrains on the chronostratigraphy of southern Junggar Basin, including the Jingjingzigou Fm., Karamay Fm., Haojiagou Fm., Shirenzigou Fm., Lucaogou Fm., Toutunhe Fm., and Qigu Fm. This study provides a basis for a larger reconstruction of the stratigraphic and tectono-sedimentary evolution in response to the tectonic interactions between southern Junggar Basin and surrounding ranges during Permian through Jurassic times.

Late Miocene to Quaternary slip history across the Qiulitag anticline in the southern Tianshan piedmont

AbstractIn this study, we reconstruct the Miocene to Quaternary shortening history across the Qiulitag anticline, a complex fault‐bend fold located in southern Tianshan. We studied the Yaha and Kuche sections, where we combined surface structural measurements and seismic imaging to model the stratigraphic horizons. The history of folding was reconstructed based on magnetostratigraphic analyses and eight cosmogenic burial ages in Kuche. Pleistocene deformation rates were also quantified in Yaha based on a deformed fluvial terrace that we dated to ~67 ka using a cosmogenic depth profile. Our results suggest that the fold grew at a mean slip rate of 0.9–1.3 mm/a in both sections but accelerated to ~2.5 mm/a during the Pleistocene in Kuche. These results support a migration of the deformation towards the basin during the Pleistocene and suggest that most of the present deformation in the Tianshan is accommodated across the external structures of the range.

Geochemistry of mafic volcanics in the Bayingou ophiolitic mélange, Western Tianshan, NW China: Implications for magma genesis and tectonic setting

This paper reports on the occurrence of mid-ocean-ridge basalts (MORB), boninites, and mafic volcanics with affinities to ocean island basalts (OIB) in the Bayingou Palaeozoic ophiolites, Western Tianshan, NW China, which is one of the best preserved ophiolitic sequences, and are the youngest ophiolites reported in the Western Tianshan during the Late Palaeozoic. The MORB samples have comparatively low TiO2 concentrations (0.84–1.20 wt%) and total rare earth element (REE) contents (25.30–55.97 ppm). The ISr values and εNd(t) values of MORBs range from 0.70480 to 0.70540 and + 3.58 to +6.90, respectively. The boninite sample has high SiO2 (53.79 wt%), very low TiO2 (0.22 wt%), high MgO (14.71 wt%), and a Mg# (100 × Mg2+/(Mg2++Fe2+)) value of 75. The boninite sample also has higher Cr (983 ppm) and Ni (230 ppm) contents and lower high field strength element content (e.g. Zr, Y, V, and heavy REEs) in comparison to the MORBs. The light REE (LREE) content of the boninite is slightly enriched, whereas most MORB samples display a depletion of LREEs ((La/Yb)N = 0.63–0.86), thus indicating that there are normal-type MORBs in this area. The OIB samples are characterized by a high concentration of TiO2 (1.89–3.15 wt%) and total REE content (100.68–151.07 ppm) with enriched LREEs ((La/Yb)N = 5.62–6.80). The ISr values and εNd(t) values of OIBs range from 0.70520 to 0.70541 and − 0.24 to +2.21, respectively. The whole rock Sr, Nd, and Pb isotopic compositions of OIB samples are plotted in the field between enriched mantle II (EMII) and PREMA. Such geochemical variation implies two mantle sources at the spreading zone: possibly, one source from the MORB and several sources from EMII-type mantle (mantle plum) in combination with characteristics of trace elements (Zr, Hf, Nb, Ta, Pb, and Ce).Petrogenetic modelling suggests that MORB may have derived from partial melting (ca. 10–20%) of a mantle peridotite that had previously experienced MORB extraction. For boninites, petrogenetic modelling suggests that these rocks are a product of about 20% partial melting of a source that previously experienced about 12% melt extraction from the source that formed the normal-type MORB. A hot plume may have been the thermal factor influencing the generation of the boninites. Volcanic rocks with OIB affinities in the Bayingou ophiolites are generated by 4.4–5.4% partial melting of the deeper mantle (EMII) and has a garnet lherzolite composition. The source of OIBs is affected by contamination of ancient oceanic lithosphere and terrigenous sediments, and is the same as other OIBs that occur in Palaeozoic ophiolites in Western Tianshan, indicating that these ophiolites may have formed in the same subduction event during the Palaeozoic. Our findings indicate that basaltic rocks in the Bayingou ophiolites formed in the supra-subduction zone (SSZ) environment in response to the subduction of Southern Tianshan Ocean during the late Early Carboniferous. The northward subduction of Southern Tianshan Ocean occurred until at least the late Early Carboniferous. In addition, the Northern Tianshan Ocean developed into a relatively mature ocean basin at the same time as the occurrence of MORB in the Bayingou ophiolites.

Magnetic fabric study of the Keziletage pluton in east part of southern TianShan and its tectonic implications

The Keziletage pluton which plays an important role in studying the nature of continental lithosphere and restraining the tectonic evolution of the Southern TianShan orogenic belts is located in the middle of the Keziletage synclinorium, a secondary tectonic unit of the South TianShan syncline fold belt. Emplacement mechanism of the Keziletage pluton in Xinjiang is studied through detailed analyses of magnetic fabrics. Data from 28 sampling sites within the pluton show that the value of the average susceptibility (Km) is high. The corrected anisotropy degree (Pj) is less than 1.2, which can be an indicator of flow magnetic fabrics. The shape parameter (T) of magnetic susceptibility ellipsoid displays that the Keziletage pluton is dominated by oblate compression fabric. Although the magnetic fabrics show NE-SW compression as a whole, the magnetic lineations are little over magnetic foliations. We suggest that the collision of Tarim blocks to YiLi-mid TianShan micro-plate became much weaker during the emplacement when the regional tectonic setting was changing to extension environment.

Geochronology and Chemistry of the Ophiolite in the Southern Tianshan, Uzbekistan

AbstractThe ophiolite in the southern Tianshan, Uzbekistan, is interrupted distributed along the northern border (NW‐trending) of the Nuratau Mountain. It's the suture zone between kyzylkum‐Alai and Kazakhstan paleo‐continents. In order to study the tectonic evolution of the Turkestan ocean during the Paleozoic, we present zircon U‐Pb ages, major element, trace element data for rocks from the ophiolite in the Nuratau segment. The ophiolite comprises the rock blocks of serpentinite, altered basalt, allalinite, altered diabase, pillowed basalt, limestone, etc and the matrix of quartz schist, forming the grid structure. The matrix suffers strong deformation and deterioration, while the mylonitization occurred in the western outcrop. New LA‐ICP‐MS zircon U‐Pb ages were determined for six magmatic rock blocks and two quartz schist matrix and these ages confirm the presence of Late Devonian‐Early Permian (ca. 411 to 294 Ma). Five zircon grains from greenschist facies metamorphic basic rock yielded a group minimum weighted mean age of 226 Ma, interpreted as the result of Late Triassic thermal event. The apex of zircon ages for 2 quartz schist is about 450 Ma (Late Ordovician). That indicates sedimentary material from the Late Ordovician. The basalt (SiO2=48.15‐49.93%) are Na‐enriched (average Na2O=2.32‐4.02%), with high HREE/LREE ratios (average=4.93), Weak positive anomalies (average Eu*/Eu=1.02) and convex‐type mantle‐normalized immobile trace elements patterns, which are similar to the geochemical characteristics of Alkaline Ocean island basalt (OIB) and subalkaline Oceanic plateau basalt (OPB). This ophiolite is characterized by Mantle (serpentinite) and Ocean crust (altered basalt, allalinite, pillowed basalt), formed in Early Devonian at the latest. During the evolution of the Turkestan ocean, the growth of OIB and OPB, Ordovician provided a large amount of sedimentary material. The earliest start time of the ocean closed and paleo‐continents assembly is the Early Permian. Afterward, a Late Triassic thermal event occurred on a region scale and is recorded by metamorphic zircon.

Geological, geochronological and geochemical characteristics and genesis of the Arqiale skarn Zn-Pb deposit, Western Tianshan, Northwest China

The Arqiale deposit, located in the southern Yili Block of Western Tianshan, Xinjiang, China, is a newly discovered large Zn-Pb deposit with a resource of approximately 10 million tons ore grading 9.65% Zn+Pb, and a certain amount of Cu and Ag. The Zn-Pb orebodies are tabular and lentoid in shape, developed along SW-striking and NW-dipping interlayer structures between the Lower Carboniferous Akeshake (mainly carbonate rocks) and Dahalajunshan (mainly volcanic rocks) formations.Garnet and pyroxene-rich skarn, formed in the early (prograde) stage of skarnization, occurs only locally as lenses within skarn bodies enriched in actinolite, tremolite, ilvaite, biotite, epidote, chlorite, calcite, quartz and fluorite, which were formed in the retrograde stage. Metallic minerals comprise sphalerite, galena, chalcopyrite, pyrite, and hematite. They mainly occur as disseminations, patches and massive ores, replacing retrograde skarn minerals and crosscut by later stage quartz and calcite veinlets. There is a general trend of increasing Cu contents and decreasing Zn/Pb ratios from the upper and south portion to the lower and north portion of the orebody.Rb-Sr dating of seven sphalerite samples yielded an isochron age of 339.5 ± 2.7 Ma, which is almost identical to the 341.8 ± 2.5 Ma age obtained from Sm-Nd dating of eight actinolite samples. Furthermore, zircon U-Pb dating yielded an age of 343 ± 6 Ma for a gabbro-diorite stock that is located about 600 m south of the Zn-Pb orebody. The age data indicate that the mineralization and retrograde skarnization were contemporaneous, and they are likely related to a magmatic intrusion of similar age which may be linked to the gabbro-diorite stock. Thus, although the direct causative intrusion has not been identified, the Arqiale deposit can be classified as a distal skarn deposit. The δ34SV-CDT values of 26 ore sulfide samples range from −7.1‰ to +1.3‰ with an average of −1.5‰. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of 25 ore sulfide samples range from 18.266 to 18.880, 15.577 to 15.668 and 38.068 to 38.720, respectively, which are similar to the Early Carboniferous intrusions of the Wusun Range and partly overlap with the volcanic rocks of the Dahalajunshan Formation, and are different from the carbonate rocks of the Akeshake Formation. Based on these data, we propose that the Arqiale deposit formed from magmatic-hydrothermal fluids emanated from a concealed magmatic intrusion which may be located beneath the northern and deep part of the deposit. Much of the metals and sulfur may have been derived from the magmatic intrusions, although some of them may have also been extracted from the volcanic rocks underlying the deposit. The results of this study indicate that there is a great potential of finding more skarn Zn-Pb mineralization in Western Tianshan, where numerous Early Carboniferous arc-related intrusions with similar ages as the Arqiale deposit, in relation to the subduction of the Southern Tianshan Oceanic plate underneath the Yili-Central Tianshan Block, were emplaced into or beneath carbonate rocks.

Petrogenesis and geochemical characteristics of Early Carboniferous sanukitic high‐Mg andesite from Atengtao Mountain, Yili Block: Implications for the tectonic setting during Late Palaeozoic in Chinese West Tianshan

As an important part of the Central Asian Orogenic Belt (CAOB), Chinese western Tianshan contains voluminous Late Palaeozoic volcanic rocks. Here, we present geochronological, geochemical, and in situ Lu–Hf isotopic data from the Atengtao sanukitic high‐Mg andesites (HMAs) of the western Yili Block. LA‐ICP‐MS zircon U–Pb analyses indicate that the Atengtao high‐Mg andesites were generated in the Early Carboniferous (346 ± 2 Ma). All the studied rock samples are characterized by high MgO (3.30–5.45 wt.%), SiO2 (53.25–60.10 wt.%), Cr (90.3–157 ppm), and Ni (31.0–46.7 ppm) contents, with geochemical characteristics analogous to those of sanukitoids of the Setouchi Volcanic Belt (SVB), SW Japan, and Bieluagaxi area, West Junggar. The Atengtao sanukitic HMAs are enriched in LILE and LREE but depleted in HFSE, similar to those of typical arc‐related magmatic rocks. They are also characterized by high Nb (11.4–15.8 ppm) concentrations, high Th/La (0.3) and Th/Yb (2.6–4.0) ratios, and varying zircon εHf(t) values (−2.3 to +4.3). These features are suggested to result from the partial melting of a relatively enriched mantle wedge metasomatized by subducting oceanic slab‐derived sediments. Considering the occurrence of sanukitic HMAs, combined with the contemporaneous special rock assemblages (such as slab‐derived adakite, Nb‐enrich basalt, A‐type granite/rhyolite, and bimodal volcanic rocks) found in the western part of Yili Block, we propose that the generation of the Carboniferous sanukitic HMAs in Atengtao Mountain were most likely related to the slab break‐off following the southern Tianshan Ocean slab northward subduction beneath the Yili–Central Tianshan Block. This mechanism not only produces intense magmatism with compositional diversity but also gives rise to a local extension in the western part of Yili Block. The slab break‐off model may play an important role in the crustal growth of the CAOB in Phanerozoic time.

Sources of quartz grains influencing quartz cementation and reservoir quality in ultra-deeply buried sandstones in Keshen-2 gas field, north-west China

Quartz cementation is a critical factor in the reservoir quality of ultra-deeply buried sandstones because of the high temperature and high-pressure at great depths. Therefore, determining the main influences retarding the growth of the quartz overgrowths is important for predicting the sweet spots of tight gas sandstones. The vast Keshen-2 gas field in Kuqa Depression is typical of such ultra-deep gas fields, despite the porosity and permeability of the target sandstones in the Lower Cretaceous Bashijiqike Formation being less than 10% and 0.5 md, respectively. The main gas reservoirs had been buried previously to a depth of 7000 m, with the maximum fluid temperature approaching 160 °C, in which authigenic quartz cements are extremely common. The heterogeneity of the physical properties and quartz cementation was investigated using core analysis, log interpretation, thin sections and scanning electron microscopy (SEM). The findings indicated that the Bashijiqike Formation could be stratified into upper porous and lower tight zones. The porosity of the sandstones in the upper zone is generally higher than 5% and abundant polycrystalline quartz grains are present. However, the reservoirs in the lower zone with low overall porosity values below 5%, contain high contents of monocrystalline quartz grains. The polycrystalline quartz grains show brown cathodoluminescence that indicates mainly a metamorphic source from the southern Tianshan provenance which is located to the north of Keshen-2. The monocrystalline quartz grains display primarily blue to violet luminescence colours, suggesting an origin from volcanic or plutonic mother rocks of the Wensu and Kuluketage provenance areas, situated to the southwest and southeast, respectively. The monocrystalline quartz grains are surrounded commonly by quartz overgrowths, whereas the polycrystalline quartz grains are cemented slightly. The presence of polycrystalline quartz grains hampers potential quartz overgrowths—the critical reason preserving the intergranular pores in the ultra-deeply buried sandstones of this gas field. Accordingly, we propose that the metamorphic quartz grain content is the main factor inhibiting quartz cementation, which preserves reservoir quality. This knowledge is conducive to determining the growing mechanism of high-quality reservoirs and predicting the sweet spots of gas production in the ultra-deeply buried sandstones.

New Ar‐Ar Isotopic Ages of the Southern Tianshan Mountains, Kyrgyzstan and their Geological Implications

New Ar‐Ar Isotopic Ages of the Southern Tianshan Mountains, Kyrgyzstan and their Geological Implications

Carboniferous tectonic configuration of the Yining Massif in Western Tianshan, NW China

The tectonic evolution and configuration of the Carboniferous trench‐arc‐basin system (TABS) in the Yining Massif (Western Tianshan, NW China) have long been controversial. The question of the existence of an ancient ocean basin and its subduction polarity is crucial to address this issue. In this study, we suggest that the Early Carboniferous calc‐alkaline volcano‐sedimentary rocks were formed in a fore‐arc/back‐arc basin system, whereas the Late Carboniferous bimodal volcanic rocks and their associated sedimentary rocks were formed under a continental rift setting after the Junggar‐Tarim collision. We propose that the Southern Tianshan ocean basin may have closed in the Late Early Carboniferous, terminating the TABS at Yining. Spatial–temporal distribution of volcanic‐sedimentary rock assemblages suggests that the Early Carboniferous Southern Tianshan ancient ocean basin was similar to the TABS system of the present‐day Japanese island arc.

Characterization of braided river-delta facies in the Tarim Basin Lower Cretaceous: Application of borehole image logs with comparative outcrops and cores

Abstract Borehole electrical image log with core-like presentation is an efficient way to conduct sedimentary description in the subsurface. The key is to set up a series of geological diagnostic criteria. The subsurface Lower Cretaceous Bashijiqike Formation in the Kelasu belt, Kuqa Depression, described as a sedimentary facies of braided river-delta front, has measured a mass of borehole image logs and open-hole logs at depths more than 6000 m. Combined with comparative outcrops, limited cores and thin sections, high-quality borehole images were fine calibrated to reveal their sedimentary details, with the aim of subtle sedimentary interpretation in the uncored intervals. The image elements all have corresponding geological indications. The lithologies are interpreted including mudstones, siltstones, fine- and medium sandstones and conglomerates, and the sedimentary structures are interpreted in terms of various bedding types, scoured surface and stacking modes. Thus, interdistributary bay, sandy or conglomerate subaqueous distributary channel and channel mouth bar are interpreted based on the lithologies, sedimentary structures and textures, basal and top contacts and succession variations. Furthermore, determination of paleo-current direction using borehole image logs is proposed for some time, but is far from mature and need to consider tectonic deformation settings of studied stratigraphy. Based on this, the paleo-current in the Kelasu belt is interpreted to flow southward from the Southern Tianshan to the depression during the Early Cretaceous which is consistent with the results derived from the heavy mineral analysis. However, the subaqueous distributary channels could frequently migrate locally and the paleo-current direction changed at localities with an offset angle varied between 30° and 150°. Regionally, the scale of single sand-body decreases lakeward and the sand-body may stretch at a maximum distance of 685.1 m along the flow direction. The diagnostic criteria and the workflow for borehole image interpretation in this study will have potential application to subsurface sedimentary description and guide sedimentary architecture where cores are not available.

The Central European, Tarim and Siberian Large Igneous Provinces, Late Palaeozoic orogeny and coeval metallogeny

The formation of the Central European and Tarim Large Igneous Provinces (LIPs) in the Early Permian coincided with the demise of the Variscan and the Southern Tianshan orogens, respectively. The Early Triassic Si- berian LIP was formed in the wake of the Western Altaid orogeny in the Late Permian. These processes coincided with the development of the majority of known Late Palaeozoic and Early Mesozoic hydrothermal and magmatic ore deposits in the corresponding orogenic domains. Nickel-copper(-PGE) deposits followed directly from the evo- lution of the (ultra-)mafi c melts which make up the LIPs. In Western Siberia, the diverse assemblage of associated noble and base metals in the Noril'sk-Talnakh Ni-Cu(-PGE) deposits suggests these metals also had their source in the mantle domain from which the (ultra-)mafi c melts were generated. The same metals variably found their way into the hydrothermal ore deposits in the defunct Variscan and Southern Tianshan orogenic domains. These ore de- posits have traditionally been viewed as a result of orogenic processes. However, their ages, together with the timing and nature of their by then intracontinental tectonic control cause uncertainty concerning the role of the orogens. In view of their mantle sources in their association with the Siberian LIP, the mantle contribution to these Late Palaeo- zoic hydrothermal ore deposits in the orogenic belts may have been more signifi cant than previously thought. An orogenic contribution to melting of mantle complexes and to mineralisation may have resided in the earlier modifi - cation of subcontinental mantle domains by subduction of oceanic lithosphere. In all three cases, the controlling tectonic setting of orogenic cessation, LIP formation and mineralisation was dominated by translithospheric strike- slip deformation, possibly in combination with orogenic collapse and lithosphere delamination. In view of their re- currence, the orogen-lip sequences were probably not fortuitous. The controlling strike-slip faults were principal elements of the lithosphere-scale dynamic framework that led to the amalgamation of Pangaea. At this scale, the exceptionally large volume of the Siberian LIP may, in addition to the strike-slip dissection of the lithosphere, have been related to extension in the continental lithosphere of the margin of the Supercontinent (cf. Gutierrez-Alonso et al. 2008). The peripheral extension was associated with compression in its centre. The explanation of these Large Igneous Provinces does not require the concept of an active mantle plume because deep-reaching strike-slip defor- mation, orogenic collapse and lithosphere delamination involved in the destruction of the orogenic edifi ces can have caused decompression melting in large domains of the subcontinental mantle.

A ductile to brittle shear zone–hosted Cu mineralization: Geological, geochronological, geochemical and fluid inclusion studies of the Lingyun Cu deposit, southern Tianshan, NW China

The Lingyun Cu deposit is located at the northern margin of the South Tianshan Accretionary Complex, NW China. In the Lingyun mine area, the rocks are cut by a NW–trending Yanxingshan ductile to brittle shear zone. All orebodies are hosted in crystal tuff. The sulfide mineralization in the Lingyun Cu deposit can be divided into early (E) and late (L) paragenetic stages. The E-stage is characteristic of fault-controlled sulfide assemblages (chalcopyrite + bornite+chalcocite)–quartz veins in the foliation fabric of the crystal tuff with albite + sericite ± epidote ± biotite assemblages. In the L-stage, the assemblages of ankerite + chalcopyrite + bornite + chalcocite are present in veinlets that cut the E-stage assemblages. The sulfide assemblages in the Lingyun deposit are characterized by striations in the metamorphosed crystal tuff. Ribbon-like silicate minerals in the crystal tuff share common deformation features with the sulfide assemblages.The host rock of the Lingyun Cu deposit yields a concordia age of ca. 405 Ma, indicating that the Aerbishibulake Formation formed during southward subduction of the South Tianshan Ocean beneath the Tarim Block. Moreover, crystal tuff samples show well developed negative Sr anomaly and positive Rb, Th, U, Nd and P anomalies, which are common in arc-related volcanic rocks. The ca. 297 Ma Re–Os isochron age yielded by sulfides is much younger than the host rock, which implies that the Lingyun Cu deposit is a typical epigenetic deposit. Both similar REE fractionated patterns between sulfide ores and crystal tuff and the high initial 187Os/188Os ratio (18.54 ± 0.67) indicate significant crustal materials input. The 40Ar/39Ar plateau age of ca. 292 Ma from the ribbon-like sericite in crystal tuff is coeval with the mineralization occurrence in the Lingyun Cu deposit. By contrast, diorite dikes that crosscut the crystal tuff formed at ca. 221 Ma, which is much younger than the sulfide mineralization. Thus, the mineralization in the Lingyun deposit is likely to have been generated by shear zone activity rather than the intrusion of diorite dikes.Three types of fluid inclusions have been identified in the Lingyun Cu deposit: Aqueous (W-type), mixed aqueous–carbonic (M-type) and pure carbonic (C-type) inclusions. These three types of fluid inclusions are all found in the E-stage quartz whereas only the W-type fluid inclusions have been observed in L-stage ankerite. Fluid inclusions from the E-stage commonly contain CO2 and H2O, locally with N2 with low salinity, which are common in orogenic deposits. By contrast, L-stage fluid inclusions show low CO2 concentrations and salinity, indicating a significant inflow of meteoric water. Given the regional geology, ore geology, coeval ore-forming and deformation and fluid inclusion features, the Lingyun Cu deposit may be associated with the orogenic class of mineral systems.