Yili-Central Tianshan Block Research Articles

A late Carboniferous transition from subduction to collision: Tectono-sedimentary evidence from southern Junggar, NW China

The Carboniferous to Permian tectono-sedimentary evolution of the southern Junggar area brings new insights into understanding the subduction-collision processes in the northern Tianshan region. Integrating geophysics, geochemistry, and geochronology approaches, this study investigates the Carboniferous–Permian strata in the southern Junggar Basin. The results have revealed three distinct tectono-stratigraphic evolutionary stages, each marked by a distinctive volcano-sedimentary sequence. The Early Carboniferous strata suggest intense volcanic activities in the southern Junggar area. During the Late Carboniferous, the southern Junggar Basin was controlled by normal faulting in an extensional setting, receiving sedimentary inputs from the Junggar terrane. The Lower Permian, unconformably overlying the Upper Carboniferous, was shaped by an extensional regime and is comprised by volcano-clastic sequences that received detritus from the Yili-Central Tianshan block. These findings indicate that a Late Carboniferous forearc basin developed in the southern Junggar area, and it evolved into a post-collisional rift in the Early Permian. This period marked a dynamic shift from bidirectional subduction (rollback) to the detachment of the North Tianshan oceanic slab. We propose that the collision between the Yili-Central Tianshan block and the Junggar terrane, along with the closure of the North Tianshan Ocean, likely occurred in the Late Carboniferous (ca.306–303 Ma).

Basin filling and magmatic response to the migration of two-stage rifts: New insights into the late Neoproterozoic tectonics of the northern Tarim Craton

The spatiotemporal evolution of late Neoproterozoic sedimentary basins and their tectono-magmatic context provide essential information regarding the continental rift process of the Tarim Craton during the fragmentation of the Rodinia supercontinent. However, the incomplete exposure of strata and magmatism in the distal continental margin hinders the understanding of different rift phases that trigger continental breakup. The Sawapuqi region is currently located at the northernmost margin of the Tarim Craton and presents an uninterrupted sedimentary sequence and volcanic rocks from the latest Precambrian to the Cambrian period. Herein, we perform an integrated analysis of the Sawapuqi sub-basin and provide critical insights into the Ediacaran stratigraphic and tectono-magmatic evolution that shed light on the final rifting phase. The Sawapuqi sub-basin preserves regressive and transgressive successions, ranging from thick-bedded siltstones and siliceous rocks at the lower unit of the Sugetbrak Formation, through shore gravel-bearing laminated sandstone accumulation (upper unit), and restricted platform stromatolitic carbonates (Qigebrak Formation), to Cambrian mature passive margin sedimentation. An alkaline basaltic layer is interbedded within the coarsening-upward sedimentary packages of the early Ediacaran, recording the final rift-related magmatic pulse in the northwestern Tarim margin. The geochemical signatures of these basalts suggest a relatively shallow-depth derivation from the enriched mantle reservoir in an intraplate setting. The detrital zircon provenance of the coarse-grained siliciclastic to pebbly feldspathic debris sandstone sequence of the upper Sugetbrak Formation indicates the hybrid sediment infills from local metamorphic basements and Neoproterozoic igneous rocks, with progressively increased contribution from the coeval granites in the northern periphery of the Tarim Craton. The stratigraphic evolution from syn-rift clastic rocks to post-rift carbonates and the transition of detrital provenance observed in the Sawapuqi sub-basin suggest the development of a breakup succession at the end of the early Ediacaran period. The margin-localized continental extension and mantle-derived volcanism during the early Ediacaran differed from the distributed extension and bimodal magmatism during the Cryogenian, indicating two-stage extensional modes and subsidences prior to the lithospheric breakup. The rift system of the latter stage, which migrated oceanwards, eventually led to the continental breakup in the northwestern margin of the Tarim Craton. Comparable late Neoproterozoic stratigraphy and magmatic pulses between the Tarim Craton and Yili-Central Tianshan Block (YCTB) support their reconstructions as two conjugate rifted margins. This study indicates that the rift-drift tectonics triggered by this prolonged episodic rifting between the Tarim Craton and the YCTB was closely associated with the final fragmentation of Rodinia.

Carboniferous back-arc extension in the southern Yili-Central Tianshan Block and its significance to the formation of the Kazakhstan Orocline: insights from the Wusun Mountain volcanic belt

Carboniferous back-arc extension in the southern Yili-Central Tianshan Block and its significance to the formation of the Kazakhstan Orocline: insights from the Wusun Mountain volcanic belt

Tracing tectonic processes from oceanic subduction to continental collision through detrital zircon U-Pb and Lu-Hf isotope data: An example from the Chinese West Tianshan Orogen

Combining detrital zircon (DZ) U-Pb age and Lu-Hf isotope data is important in revealing the tectonic evolution of ancient orogens. Here, we conducted DZ U-Pb and Lu-Hf isotope research on 7 modern river sand samples collected from the northern Chinese West Tianshan Orogen (CWTO). The DZs from modern river sands yield ages 340 to 320 Ma and reflect Mesozoic magmatism in the northern Yili Block (YB). In addition, the DZs reflected disproportionate contributions from the source area, which might be affected by the zircon fertility and erodibility of the source rocks. These newly obtained data, together with compiled DZ U-Pb and Lu-Hf isotope data from other modern river sands and ancient sedimentary rocks in the CWTO, are used to constrain the subduction and closure history of the southwestern part of the Paleo-Asian Ocean. The εHf(t) values of 500 to 450 Ma zircons from the southern YB and Central Tianshan Block display a fanning trend (changing from a narrow to a wide range over time), indicating a crustal thickening event at ∼ 450 Ma. The closure of the North Tianshan Ocean was completed when the YB and an immature/nascent island arc collided in the latest Carboniferous. The South Tianshan Ocean (STO) subducted bidirectionally beneath the Yili–Central Tianshan Block (YCTB) and the Tarim Craton starting at 460 Ma. Slab rollback or retreat of the STO in the northern Tarim Craton starting at 440 Ma caused the opening of several small diachronous back-arc basins. The fanning trend of εHf(t) values indicates that collision between the YCTB and the Tarim Craton occurred at 320 to 310 Ma.

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.

Geochemistry and zircon U–Pb geochronology of the oxidaban intrusive complex: Implication for Paleozoic tectonic evolution of the South Tianshan Orogenic Belt, China

The South Tianshan Orogenic Belt (STOB) occupies the southwestern margin of the Central Asian Orogenic Belt (CAOB), and is the key region to understanding the Paleozoic evolution of the southern part of the Palo-Asian Ocean. Here we present an integrated study of zircon U–Pb ages, whole-rock major and trace elements and Sr–Nd isotopes on Silurian and Late Carboniferous intermediate-felsic intrusions exposed in the Oxidaban area located in the South Tianshan region in Xinjiang, NW China. The Silurian intrusion is composed of quartz diorite, granodiorite and monzogranite, whereas the Late Carboniferous rocks include granite porphyry and granite porphyry enclaves. LA–ICP–MS zircon U–Pb dating yields ages of 426.8 ± 4 Ma for quartz diorite and 424.5 ± 4 Ma for monzogranite, respectively. The quartz diorite shows middle–K calcic and metaluminous affinity, and the granodiorite exhibits similar features, but peraluminous. They display enrichment in large-ion lithophile elements (LILEs) and depletion of high field strength elements (HFSEs) together with negative Eu anomalies, broadly comparable with typical arc-type rocks. They have similar Sr–Nd isotopic compositions, with a narrow range in (87Sr/86Sr)i of 0.70578 to 0.70644 and εNd(t) of −0.95 to −2.16. Based on geochemical and isotopic characteristics, we infer that the granodiorite is produced by fractional crystallization of quartz diorite rocks, which were dominantly produced by partial melting of medium to high–K basaltic protoliths. Besides, the monzogranite also exhibits arc-type geochemical signatures, which we correlate to partial melting of middle-lower juvenile crust and subsequent contamination by ancient crust during ascent and emplacement. Zircon LA–ICP–MS U–Pb analyses yielded crystallization ages of 300.4 ± 1.5 Ma for the granite porphyry and 304.8 ± 2.3 Ma for the granite porphyry enclave. These rocks are dominantly shoshonite and peraluminous, and are likely derived from partial melting of the greywacke mixed with minor pelitic components. The low Al2O3/TiO2 (<100) ratios suggest a high temperature condition, which may be attributed to heat input from underplated mantle-derived magmas in the post-collision extension setting. Base on these scenarios, we propose that the bi-directional subduction of the South Tianshan Ocean occurred during the Early Paleozoic and the subduction polarity changed to northward during Middle Devonian, followed by the collision between Yili-Central Tianshan Block and Tarim Craton in the Late Carboniferous.

Late Paleozoic magmatism and metallogenesis in the Aqishan-Yamansu belt, Eastern Tianshan: Constraints from the Bailingshan intrusive complex

The Aqishan-Yamansu belt in the Eastern Tianshan (NW China) contains many intermediate to felsic intrusive rocks and spatially and temporally associated Fe (-Cu) deposits. Zircon U-Pb dating of the Bailingshan granitoids, including diorite enclaves (in granodiorite), diorite, monzogranite and granodiorite, and andesitic tuff from the Shuanglong Fe-Cu deposit area yielded ages of 329.3 ± 2.1 Ma, 323.4 ± 2.6 Ma, 313.0 ± 2.0 Ma, 307.5 ± 1.7 Ma and 318.0 ± 2.0 Ma, respectively. These new ages, in combination with published data can be used to subdivide magmatism of the Bailingshan intrusive complex into three phases at ca. 329–323 Ma, ca. 318–313 Ma and ca. 308–297 Ma. Of the analyzed rocks of this study, the Shuanglong diorite enclave, diorite and andesitic tuff show calc-alkaline affinities, exhibiting LILE enrichment and HFSE depletion, with negative Nb and Ta anomalies. They have high MgO contents and Mg# values, with depleted εHf(t) and positive εNd(t) values, similar crustal-derived Nb/Ta and Y/Nb ratios, low Th/Yb and Th/Nb, and high Ba/La ratios, which are consistent with them being sourced from a depleted mantle wedge metasomatized by slab-derived fluids and crustal contamination. However, the monzogranite and granodiorite are metaluminous with characteristics of low- to high-K calc-alkaline I-type granites. The granitic rocks are enriched in LILE, depleted in HFSE and have significant Eu anomalies, with high Y contents and low Sr/Y ratios, resembling typical of normal arc magmas. Depleted εHf(t) and positive εNd(t) values with corresponding young TDMC ages of zircons, as well as Nb/Ta, Y/Nb, Th/U and La/Yb ratios suggest that the granitic rocks were probably formed by re-melting of juvenile lower crust or pre-existing mantle-derived mafic–intermediate igneous rocks. Integrating published data, we conclude that the Bailingshan granitoids (excluding the Shuanglong diorite and diorite enclave) were derived from re-melting of juvenile lower crust and mantle-derived mafic–intermediate igneous rocks, with mantle components playing a more prominent role in the formation of the younger and more felsic rocks. A comprehensive review, including our new data, suggests that the Aqishan-Yamansu belt formed as a fore-arc basin during the Carboniferous (ca. 350–300 Ma) when the Kangguer oceanic slab subducted beneath the Yili-Central Tianshan block. The ongoing southward subduction of the slab resulted in the closure of the Aqishan-Yamansu fore-arc basin (ca. 320–300 Ma), due to slab steepening and rollback followed by slab breakoff and rebound. During the Aqishan-Yamansu fore-arc basin inversion, the main phase of the Bailingshan granitoids emplaced in the Aqishan-Yamansu belt, accompanied by contemporary Fe and Fe-Cu mineralization.

Carboniferous volcanic rocks associated with back-arc extension in the western Chinese Tianshan, NW China: Insight from temporal-spatial character, petrogenesis and tectonic significance

The Yili-Central Tianshan Block, as a Late Paleozoic major continental silver of the Central Asian Orogenic Belt, holds a massive volume of Carboniferous volcanic rocks, occurring as subparallel magmatic belts. However, the petrogenesis and tectonic implications of these volcanic rocks remain enigmatic. This study compiled isotopic age data for mapping their temporal-spatial character, and conducted petrogenetic study of these magmatic belts, aiming to understand their tectonic implications. Our compiled dataset reveals four magmatic belts in the Yili-Central Tianshan Block, including the Keguqinshan-Tulasu belt and the Awulale belt in the north, and the Wusun Mountain belt and the Haerk-Nalati belt in the south. In addition, our new zircon U-Pb dating results define two significant Early Carboniferous eruptive events (ca. 355–350 Ma and 325 Ma) in the Wusun Mountain belt. Volcanic rocks of the early significant eruptive event (ca. 355–350 Ma) in the Wusun Mountain comprise basalt, trachy-andesite, andesite, dacite and rhyolite, which are similar to the typical rock assemblage of a continental arc. Their positive εNd(t) values (+0.3 to +1.5) and relatively high Th/Yb and Nb/Yb ratios suggest the derivation from a mantle source with additions of slab-derived components. The gabbroic dykes and rhyolites of the late volcanic event (ca. 325 Ma) form a bimodal rock association, and they show alkaline features, with relatively low Th/Yb and Th/Nb ratios, and higher positive εNd(t) values (εNd(t) = +3.3–+5.0). It is interpreted that the gabbroic dykes and rhyolites may have been derived from mantle and juvenile crustal sources, respectively.The isotopic and trace elemental variations with time elapse of the Wusun Mountain magmatic belt show an important clue for strengthening depletion of the magma sources. Considering the distinctive temporal-spatial character of the Carboniferous volcanic rocks, two separate subduction systems in the southern and northern margins of the Yili-Central Tianshan Block were suggested to be the causes for extensive emplacements of the igneous products, which may be in an association with synchronous subduction of the South Tianshan and the North Tianshan oceanic plates, respectively. In this tectonic context, the Carboniferous magmatic rocks of the Wusun Mountain may be a tectonic response to the change in magma sources due to back-arc propagation in the western Chinese Tianshan.

A slab break-off model for the submarine volcanic-hosted iron mineralization in the Chinese Western Tianshan: Insights from Paleozoic subduction-related to post-collisional magmatism

The distinct submarine volcanic-hosted iron ore deposits in the Awulale Iron Metallogenic Belt are the most important sources for iron in the Chinese Western Tianshan metallogenetic province, although, the exact tectonic setting for their formation remains poorly constrained. A compilation of published and new geochronological and geochemical data suggests a protracted magmatism from ca. 450 to 290 Ma was in close spatial and temporal proximity to the iron deposits. The different magmatic rocks have broadly similar arc-like geochemical signatures such as LREE and LILE enrichments, HREE and HFSE depletions, Nb–Ta–Ti negative anomalies and flat HREE patterns, all of which are attributed to magmatic differentiation and/or partial melting in relatively thin crust due to long-term extension. The contrasting whole-rock Nd isotopic patterns of the igneous rocks and the well-defined peak metamorphism (ca. 320–310 Ma) of the subduction-zone high-pressure rocks indicate that ca. 450–321 Ma magmatic rocks with an increasingly positive εNd(t) shift were formed during the northward subduction of the South Tianshan Ocean, where slab rollback resulted in an extensional setting in the southern Yili-Central Tianshan Block. In contrast, the synchronicity of the ca. 320–310 Ma subduction-zone peak metamorphism and magmatic flare-up with uniformly positive εNd(t) values is interpreted as the consequence of slab break-off subsequent to the oceanic subduction and the continental collision. By inference, the late-stage (ca. 309–290 Ma) magmatism was likely constructed in a post-collisional setting. The submarine volcanic-hosted iron mineralization exclusively occurred coeval with the peak metamorphism and intensive magmatism, probably suggesting slab break-off as the exact tectonic setting for iron mineralization. Consequently, the western extension of the Awulale Iron Metallogenic Belt occurring roughly parallel to the South Tianshan Orogen may have significant potential for prospecting the submarine volcanic-hosted iron ore deposits.

The Late Paleozoic magmatic evolution of the Aqishan-Yamansu belt, Eastern Tianshan: Constraints from geochronology, geochemistry and Sr–Nd–Pb–Hf isotopes of igneous rocks

The Aqishan-Yamansu belt in the Eastern Tianshan (Xinjiang, NW China) is an important mineralization belt. The belt mainly comprises Carboniferous volcanic, volcaniclastic and clastic rocks, and hosts many intermediate–felsic intrusions and Fe (–Cu) deposits. The biotite diorite, felsic brecciated tuff, granodiorite and syenite from the western Aqishan-Yamansu belt are newly zircon U–Pb dated to be 316.7±1.4Ma, 315.6±2.6Ma, 305.8±1.9Ma and 252.5±1.4Ma, respectively. The mafic rocks (mafic brecciated tuff and diabase porphyry) are tholeiitic to calc-alkaline series, LILE-rich (e.g., Rb, Ba and Pb), HFSE-depleted (e.g., Nb and Ta), and have high Mg#(44–60), Nb/Ta (15.0–20.0), Ba/La (>30) and Ba/Nb (>57) values/ratios, and low Th/Yb ratios (<1), probably originating from mantle wedge metasomatized by slab-derived fluids. The intermediate–felsic igneous rocks are LILE-rich, HFSE-depleted, with high Sr and Y contents showing typical of normal arc magma affinity. Moreover, the depleted εHf(t) (>2.10) and positive εNd(t) (>5.7), combined with variable Nb/Ta ratios (9.52–21.4), Y/Nb ratios (1.47–39.7) and Pb isotopes (206Pb/204Pb=16.225–17.640, 207Pb/204Pb=15.454–15.520, 208Pb/204Pb=37.097–38.025) suggest that these rocks were magma mixing products between juvenile crustal-derived magmas and minor mantle-derived magmas. Combined published works with our new ages, geochemical and isotopic data, we propose that the Aqishan-Yamansu belt was an Early Carboniferous fore-arc basin during the southward subduction of the Kangguer oceanic slab beneath the Yili-Central Tianshan block. With the continuing southward subduction, the Aqishan-Yamansu fore-arc basin initiated to close, which generated the mafic and intensive intermediate–felsic magmatism associated with regional Fe (–Cu) mineralization.

Accreted seamounts in North Tianshan, NW China: Implications for the evolution of the Central Asian Orogenic Belt

The Carboniferous Bayingou ophiolitic mélange is exposed in the North Tianshan accretionary complex in the southwestern part of the Central Asian Orogenic Belt (CAOB). The mélange is mainly composed of serpentinised ultramafic rocks (including harzburgite, lherzolite, pyroxenite, dunite and peridotite), pillowed and massive basalts, layered gabbros, radiolarian cherts, pelagic limestones, breccias and tuffs, and displays block-in-matrix structures. The blocks of ultramafic rocks, gabbros, basalts, cherts, and limestones are set in a matrix of serpentinised ultramafic rocks, massive basalts and tuffs. The basaltic rocks in the mélange show significant geochemical heterogeneity, and two compositional groups, one ocean island basalt-like, and the other mid-ocean ridge-like, can be distinguished on the basis of their isotopic compositions and immobile trace element contents (such as light rare earth element enrichment in the former, but depletion in the latter). The more-enriched basaltic rocks are interpreted as remnants/fragments of seamounts, derived from a deep mantle reservoir with low degrees (2–3%) of garnet lherzolite mantle melting. The depleted basalts most likely formed by melting of a shallower spinel lherzolite mantle source with ∼15% partial melting. It is probable that both groups owe their origin to melting of a mixture between plume and depleted MORB mantle. The results from this study, when integrated with previous work, indicate that the Junggar Ocean crust (comprising a significant number of seamounts) was likely to have been subducted southward beneath the Yili-Central Tianshan block in the Late Devonian-Early Carboniferous. The seamounts were scraped-off and accreted along with the oceanic crust in an accretionary wedge to form the Bayingou ophiolitic mélange. We present a model for the tectonomagmatic evolution of this portion of the CAOB involving prolonged intra-oceanic subduction with seamount accretion.

Geochronology and geochemistry of igneous rocks in the Bailingshan area: Implications for the tectonic setting of late Paleozoic magmatism and iron skarn mineralization in the eastern Tianshan, NW China

The late Paleozoic Bailingshan intrusions and volcanic rocks are located in the Aqishan–Yamansu arc belt in the southern part of the eastern Tianshan and are associated with an important group of iron skarn deposits. The exposed intrusive rocks are mainly granodiorite, monzonitic granite, and granite. Zircon U–Pb dating of the Tugutublak Formation tuffaceous dacitic lava yields an age of 324Ma, whereas dates of the Bailingshan granodiorite, monzonitic granite, and granite yields ages of 317Ma, 313Ma, and 307Ma, respectively. The results indicate that the Bailingshan granitoids were emplaced soon after the eruption of the Tugutublak dacite. All these rocks studied show calc-alkaline to high-K calc-alkaline and metaluminous affinities, with A/CNK values ranging 0.83–1.10. They are enriched in Rb, K, and Pb, depleted in Nb, Ta, Ti, and P, and contain low Sr/Y (4.16–23.7) and Sr (109.0–347.0 ppm) values, displaying typical arc geochemical affinities. The tuffaceous dacitic lava has low Nb/Ta (10.3–14.1) values, a wide range of Mg# (6–64), positive zircon εHf(t) (3.2–7.5) values, and elevated whole-rock εNd(t) (2.03–4.41), but low ISr values (0.70427–0.70530), indicating that the source magma may have been derived from the juvenile lower crust with minor mantle input. The Bailingshan I-type intrusions also exhibit a mixed source signal, as constrained by Nb/Ta ratios, Mg#, and isotopes characteristics. Because the granodiorite, monzonitic granite, and granite intrusions have higher zircon εHf(t) (3.3–7.5, 11.8–13.5, and 10.2–14.4, respectively) and εNd(t) (3.90, 5.78, and 5.94, respectively) values than those of the tuffaceous dacitic lava, it is suggested that mantle-derived materials may have played a more prominent role with their petrogenetic evolution. Integrating our new geological, age, geochemical and isotopic data we propose that the Aqishan–Yamansu iron skarn belt may have formed in a back-arc position or within an intra-arc basin generated by the southward subduction of the Kanggur oceanic plate beneath the Yili–Central Tianshan block during the late Paleozoic, with felsic-intermediate magmatism occurring during the basin inversion.

Geochronology and petrogenesis of granitoids and associated mafic enclaves from Xiate in Chinese Southwest Tianshan: Implications for early Paleozoic tectonic evolution

Mineral compositions, in-situ zircon U–Pb and Hf isotopes, and whole-rock major-trace elements and Sr–Nd isotopes are reported for the host granitoids and associated mafic microgranular enclaves (MMEs) in Dongdeguli valley, and the Changawuzi mylonitic pluton, in the Southwest Tianshan, NW China. The Dongdeguli host granitoids have LA-ICP-MS zircon U–Pb ages of 446±2Ma and 443±3Ma, which are comparable with those (436±6Ma and 446±23Ma) of their enclaves. Both the host granitoids and MMEs have relatively high initial 87Sr/86Sr ratios and negative ɛNd(t) values. The MMEs are enriched in large ion lithophile and light rare earth elements but depleted in high field strength elements, and they are interpreted as melts derived from the fluids/melts metasomatized mantle wedge. The emplacement of mafic arc magma is inferred to cause the partial melting of ancient crust of Yili–Central Tianshan Block, which formed the granitic magma. Magma mixing model is proposed for the origin of the host granitoids and their mafic enclaves in the Dongdeguli valley. The Changawuzi mylonitic pluton has a SHRIMP zircon U–Pb age of 453±4Ma. It has relatively low SiO2 content, high MgO content, and negative whole-rock εNd(t) values and zircon ɛHf(t) values, with two-stage Nd model ages of 1.67–1.7Ga and two-stage Hf model ages of 1635–1760Ma, respectively. These results indicate that the Changawuzi mylonitic pluton was derived from partial melting of Paleoproterozoic crustal rocks, with involvement of some mantle-derived magma. The Dongdeguli granitoids and associated MMEs, and the Changawuzi mylonitic pluton, were formed by the northward subduction of paleo-South Tianshan oceanic crust in the Late Ordovician–Early Silurian period.

Petrology and geochemistry of late Carboniferous hornblende gabbro from the Awulale Mountains, western Tianshan (NW China): Implication for an arc–nascent back-arc environment

The Awulale Mountains, the conjunction point of the Tarim Plate, Junggar Plate and Yili-Central Tianshan Block, are the crucial place to decipher the tectonic evolution of the western Tianshan. We report results of petrologic, whole-rock geochemical, Sr–Nd isotopic data and in-situ zircon U–Pb and Hf isotopes for the Wuling (poikilitic) hornblende gabbro from the Awulale Mountains, western Tianshan. Two stage crystallization processes are applied to interpret its mineral composition. Zircon U–Pb ages indicate that the Wuling hornblende gabbro was emplaced during the late Carboniferous (313Ma±3Ma). The Wuling hornblende gabbro is characterized by tholeiitic affinity with low 87Sr/86Sr ratios, positive εNd(t) and zircon εHf(t) values, resembling its coeval mafic rocks in the Awulale Mountains. They are enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs), indicating a subduction-related setting for their generation and their mantle source were mainly fluxed by fluids released by the down-going slab. The flat REE and depleted Sr–Nd–Hf isotopic signatures imply a shallow and heterogeneous mantle source with the involvement of a deep MORB-like component. We propose that an arc–nascent back-arc system had been developed between the Nalati Mountains and the Awulale Mountains during the late Carboniferous, where the magma derived from decompression melting of MORB-like mantle caused by the slab roll-back and flux-induced melting of mantle wedge were both contributed to their generation. The arc-type features of them were inherited from the strong subduction input at the initiation of back-arc rifting. However, the back-arc rifting ceased at the end of late Carboniferous because of the compressional stress from the collision between the Tarim Plate and the Yili-Central Tianshan Block.

Continuous denudation and pediplanation of the Chinese Western Tianshan orogen during Triassic to Middle Jurassic: Integrated evidence from detrital zircon age and heavy mineral chemical data

The Mesozoic tectonic framework of the Chinese Tianshan orogen is ambiguous. However, thick Mesozoic sediments both inside and in front of the orogen can provide clues to solve this problem. LA-ICPMS U–Pb dating on detrital zircons and electron microprobe analysis on detrital heavy minerals are carried out on the Triassic to Middle Jurassic sediments along the Kuqa River section in the northern Tarim basin and in two intermountain basins (Bayanbulak basin and Gongnaisi valley), to reveal the change in provenance through time and space. Zircons from the Kuqa River section are mainly of magmatic origin and their ages have a wide range from 2800 to 234Ma, 70.5% of which are Palaeozoic. This age signature indicates a main provenance from the Tianshan orogen. Detrital heavy minerals (garnet and tourmaline) from the Kuqa River section suggest a complex provenance from granitic and metamorphic rocks of the South Tianshan belt, the South Tianshan suture and the Yili-Central Tianshan block. Detrital zircon U–Pb ages of the Early-Middle Jurassic sandstones from the Gongnaisi valley and Bayanbulak basin are predominantly Palaeozoic, without Precambrian, indicating a simple source from the Yili-Central Tianshan block. The different features of detrital zircon ages and heavy mineral chemical compositions in individual sample reflect provenance changes due to continuous denudation and pediplanation of the Chinese West Tianshan orogen in an extensional setting during Triassic to Middle Jurassic: (1) during Early to Middle Triassic, the South Tianshan belt towered to the north of Tarim basin, acting as a watershed and being the main source area for northern Tarim basin; (2) since Late Triassic to Early Jurassic, material from southern Yili-Central Tianshan block was largely transported to Tarim basin because of the continuous denudation of the South Tianshan belt; (3) in Middle Jurassic, the Chinese Western Tianshan orogen reached a peneplain state, resulting in the direct deposition in intermountain basins and the decrease of sediment supplying from Yili-Central Tianshan block to Tarim.

Geochronology and fluid inclusion studies of the Lailisigaoer and Lamasu porphyry–skarn Cu–Mo deposits in Northwestern Tianshan, China

The Lailisigaoer porphyry Mo (Cu) and Lamasu porphyry–skarn Cu deposits are located in the Northwestern Tianshan Orogenic Belt, which is in the northwestern China. According to Re–Os analysis of molybdenite, the time of mineralization of the Lailisigaoer deposit is 377Ma, and SHRIMP zircon U–Pb analysis dates the ore-hosted intrusion of the Lamasu deposit to 378Ma. Five types of fluid inclusions (FIs) have been identified in both deposits: H2O-rich inclusions, CO2-bearing inclusions, halite-bearing inclusions, methane-rich inclusions and pure CO2 inclusions. In the Lailisigaoer deposit, the early-stage fluid of the quartz phenocryst belongs to a NaCl–H2O system with total homogenization temperatures (Th) ranging from 300 to 395°C with a mean salinity of 9.0wt.% NaCl. In the ore-forming stage, the fluid develops into a H2O–NaCl–CO2 system due to boiling and decarbonation. The Th values and salinities cluster between 220 and 370°C and between 0 and 33.0wt.% NaCl, respectively. The hydrothermal characteristics of the late quartz–calcite stage are similar to those of the meteoric fluid. In the Lamasu deposit, the early-stage fluid of the quartz phenocryst belongs to a NaCl–H2O system with Th values ranging from 245 to 459°C and a mean salinity of 6.0wt.% NaCl. In the ore-forming stage, due to the contact metamorphism between the hydrothermal fluid and wall rock, the fluid evolves to a NaCl–H2O–CH4 system. The Th values and salinities cluster between 220 and 360°C and between 0 and 17.3wt.% NaCl, respectively. The fluid of the late quartz–sulfide stage coincides with the meteoric fluid. In the mineralizing fluid of the Lailisigaoer deposit, the captured temperatures and pressures are between 210 and 350°C and between 170 and 600 bars, respectively, which are consistent with fluctuation between hydrostatic and lithostatic pressure corresponding to the estimated paleodepths of 1.7–2.2km. Boiling is the dominant mineralizing mechanism in the Lailisigaoer deposit, whereas the contact metamorphism between the magmatic hydrothermal fluid and country rocks is the dominant mechanism in the Lamasu deposit. Both deposits are formed in the continental arc setting related to the southward subduction of the North Tianshan Oceanic slab beneath the Yili-Central Tianshan block.

Geochemistry of the ~ 430-Ma Jingbulake mafic–ultramafic intrusion in Western Xinjiang, NW China: Implications for subduction related magmatism in the South Tianshan orogenic belt

The South Tianshan orogenic belt, the southwestern extension of the Central Asian orogenic belt, separates the Yili–Central Tianshan block to the north from the Tarim block to the south. This belt is marked by a Proterozoic metamorphic complex, ophiolites, granitoids, mafic intrusions and younger high-pressure and low-temperature metamorphic rocks. The Jingbulake mafic intrusion in this belt is a zoned body composed of pyroxene diorite, olivine gabbro and wehrlite. Pyroxenite locally intrudes all of these rocks but is most abundant along the boundaries between pyroxene diorite and olivine gabbro. Both the olivine gabbro and wehrlite contain disseminated sulfide mineralization, and a sulfide rich orebody intrudes the pyroxenite. The pyroxene diorite has a SHRIMP zircon U–Pb isotope age of 431 ± 6 Ma. All the rocks have nearly flat chondrite-normalized rare earth element patterns and show negative Nb–Ta and P–Ti anomalies on mantle-normalized trace element diagrams. They have positive εNd values (+ 1.6 to + 4.0), and low ( 87Sr/ 86Sr) i ratios (0.7039–0.7048), indicative of a depleted mantle source. The rocks all have low Th/Nb ratios, narrow isotopic variations, and slightly negative Zr–Hf anomalies, suggesting insignificant crustal contamination. They are enriched in large ion lithophile elements, such as Cs, Rb, Ba, Pb and Sr, and have constant but radiogenic Pb isotopic ratios of 206Pb/ 204Pb (18.159–18.250), 207Pb/ 204Pb (15.599–15.633) and 208Pb/ 204Pb (37.978–38.250), consistent with the involvement of subduction-related fluids and sediments in the source region. The Jingbulake intrusion, together with other mafic intrusions in the belt, suggests subduction of the South Tianshan oceanic lithosphere beneath the Yili–Central Tianshan block. The subduction-related fluids and sediments not only metasomatized the mantle wedge above the subduction zone, but also triggered the melting of the mantle due to the addition of water.