Eastern Tianshan Orogenic Belt Research Articles

Episodic evolution of intracontinental orogenic deformation: Insight from the southern Bogda fold-and-thrust belt of the Tianshan Mountains, NW China

The Mesozoic-Cenozoic Tianshan orogenic belt, a key area for studying intracontinental orogenic deformation, experienced multiple uplift episodes due to the far-field effects from a series of collision events along the southern margin of Eurasia. Focusing on the fold-and-thwrust belt at the southern margin of the Bogda Mountain, a branch of the eastern Tianshan orogenic belt, we integrated 2D seismic data, borehole and outcrop data to construct a balanced and sequentially restored cross-section across the central part of this belt. The activities and mechanical evolution of the major faults were analyzed by using the discrete-element modeling (DEM) method, offering new insights into the tectonic transformation of the Turpan-Hami Basin influenced by episodic uplifts of the Bogda Mountain in an intracontinental setting. The studied fold-and-thrust belt can be divided into two deformation systems: (i) the piedmont deformation system, comprising a southward basement thrust system and a thin-skinned thrust system on Jurassic detachment, and (ii) the thrust front belt, exhibiting an en-echelon pattern in the basin center and featured by fault-propagation folds along the lower Jurassic coal unit. Stratigraphic and reported thermochronological analyses pinpoint four key contractional deformation periods: Late Jurassic, Late Cretaceous, Paleocene-Pliocene, and Pleistocene-present. The DEM numerical simulation results indicate a critical conical Coulomb wedge model governed by the episodic activities of the southern Bogda fold-and-thrust belts from the Mesozoic to the Cenozoic. Surface processes (syntectonic erosion/deposition) significantly impacted differential activities between the orogenic belt and the basin's thrust structures. We propose that periodic arid climates in the Tianshan Mountains reduced erosion rates, promoting the expansion of the southern Bogda fold-and-thrust belts toward the basin and the widening of the orogenic belt.

Petrogenesis and geodynamic implications of the Late Devonian dioritic and granitic intrusive rocks in the Dananhu Belt, Eastern Tianshan Orogenic Belt

The Late Devonian magmatism of the Dananhu arc belt in the Central Asian Orogenic Belt provides a critical geological record. This study provides new comprehensive geochronological, geochemical, and Sr–Nd isotopic data of granodioritic and dioritic intrusions in the Dananhu belt. These findings contribute to unraveling the regional tectonic history and constraining the geodynamic processes involved. Zircon U–Pb dating indicates that the granodiorites and diorites were formed at 382.7 ± 3.8Ma and 363.1 ± 4.3Ma, respectively. The granodiorites show characteristics similar to I-type granites with high SiO2, low MgO and Mg# and aluminium saturation index (<1.1), negligible Eu anomalies, low (K2O + Na2O)/CaO ratios and zircon saturation temperatures (average = 696 °C). Granodiorites also show depleted isotope signatures (εNd(t) = +5.85 to +6.27 and low (87Sr/86Sr)i = 0.704082–0.704583) and juvenile TDM ages (741–793Ma), indicating their origin from a juvenile crust. The diorites are characterized by enrichments in large ion lithophile elements, U and Pb, but depleted in Nb and Ta displaying typical geochemical features of a subduction-related origin, together with high εNd(t) (+6.10 to +6.84) and low initial Sr isotopes (0.703745–0.704601), suggesting that they originated from a subduction fluid modified depleted mantle. The petrogenesis of both granodiorites and diorites in the Dananhu arc provides evidence that they formed through magmatic processes in a subduction tectonic setting. Considering the adakites associated with slab melting from previous studies in the Dananhu arc, it is plausible that the north-dipping subduction of the North Tianshan oceanic lithosphere have contributed to the Dananhu arc magmatism during the Late Devonian.

Identifying Multi-Scale Gravity and Magnetic Anomalies Using Statistical Empirical Mode Decomposition: A Case Study from the Eastern Tianshan Orogenic Belt

Identifying multi-scale anomalies that have simple forms and geological significance is critical for enhancing the interpretability of gravity and magnetic survey data. In recent years, empirical mode decomposition (EMD), which was developed as a significant data-driven approach for analyzing complex signals, has been widely used in identifying gravity and magnetic anomalies due to its advantages of adaptability to nonlinear and nonstationary data. Nevertheless, the traditional EMD method is usually sensitive to outliers and irregularly spaced data because of the interpolation process in the construction of envelopes. In this regard, an extended algorithm called statistical EMD (SEMD) has been proposed based on the smoothing technique. In this study, for validation purposes, the novel SEMD method has been employed to identify multi-scale gravity and magnetic anomalies. The sensitivities of local polynomial and cubic spline smoothing methods in SEMD to combination and arrangement patterns of field sources including the size, depth, and distance in gravity and magnetic anomaly identification were investigated and compared by forward modeling under the same conditions. The results demonstrated that the local polynomial smoothing method performed better than the cubic spline smoothing method. Thus, in the case study, the SEMD method using the local polynomial smoothing technique was employed for identifying multi-scale gravity and magnetic anomalies in the eastern Tianshan orogenic belt, northwestern China. It has illustrated that the SEMD method provides a novel and useful data-driven method for extracting gravity and magnetic anomalies.

Tracking multi-stage uplift history of the Eastern Tianshan Orogenic Belt during the Late Carboniferous and Early Cretaceous: Evidence from apatite U-Pb and fission track data

本文利用LA-ICP-MS磷灰石裂变径迹（FT）和U-Pb双定年技术，结合锆石原位U-Pb测年结果，对出露于东天山南部雅满苏-彩霞山地区的侵入岩样品进行了系统分析。结果显示，这些样品形成于357~309Ma和~252Ma，裂变径迹年龄变化于304~118Ma。根据裂变径迹年龄和径迹长度分布对其进行热史反演，得到晚石炭世-早二叠世（320~280Ma）、中-晚三叠世（240~210Ma）和早白垩世（120~100Ma）三个主要冷却时期。综合分析认为，研究区的石炭纪和三叠纪侵入体在晚石炭世-早二叠世、中-晚三叠世和早白垩世期间经历过构造隆升。其中，晚石炭世-早二叠世的抬升是由康古尔洋闭合后区域进入碰撞造山阶段导致，中-晚三叠世的抬升主要与古特提斯洋闭合后松潘-甘孜地体、羌塘地体先后与亚欧板块南缘的碰撞增生有关，早白垩世的抬升事件主要受到班公湖-怒江洋俯冲闭合以及拉萨地体与羌塘地体的碰撞增生影响。此外，东天山不同部位经历了显著的差异性隆升，这一现象与区内各构造单元的非均匀性、分块性及其地质演化历史的差异性密切相关，同时还受到外部驱动力以及早期先存断裂构造活化的制约。

The Changshagou gold deposit, Eastern Tianshan, NW China: orogenic gold mineralization overprinting a porphyry gold occurrence

Abstract The temporal–spatial relationships of porphyry and orogenic gold mineralization in the Eastern Tianshan Orogenic Belt are ambiguous. The newly discovered Changshagou deposit in this belt contains both porphyry and orogenic gold mineralization, which are characterized by polymetallic–sulfide veinlets and quartz–pyrite veins, respectively. Fluid inclusions in the porphyry mineralization episode were trapped at 290–340°C with salinities of 3.0–8.0 wt% NaCl equiv . The homogenization temperatures and salinities in the orogenic mineralization episode range from 240 to 300°C and from 1.0 to 5.0 wt% NaCl equiv . Coexisting V- and L-type fluid inclusions with similar homogenization temperatures are indicative of fluid immiscibility. The δ 18 O w and δ D w values are in the ranges 7.6–9.1 and −70.9 to −84.0‰ in the porphyry mineralization episode, and 6.4–7.1 and −65.7 to −72.1‰ in the orogenic mineralization episode, overlapping magmatic and metamorphic ranges, respectively. The pyrite δ 34 S values range from 3.5 to 4.9‰, falling into the magmatic range. Pyrite in porphyry and orogenic mineralization episodes yield Re–Os isotope ages of 269.1 ± 2.9 and 257.4 ± 2.4 Ma. The porphyry and orogenic gold mineralizations are genetically associated with the quartz syenite porphyry and Kanggur strike-slip shear activity, respectively.

Petrogenesis and sulfide saturation in the Yueyawan Cu-Ni sulfide deposit in Eastern Tianshan, NW China

The Yueyawan mafic intrusion hosted Cu-Ni mineralization is located at the Dananhu island arc in Eastern Tianshan Orogenic belt, NW China, and is unique in its paucity of ultramafic rocks. It formed by two intrusive phases: the phase I is composed of gabbro, troctolite and olivine gabbro, and the phase II is composed of fine-grained olivine gabbro and fine-grained troctolite. The phase I troctolite yielded SMIS U-Pb age of 283.4 ± 2.0 Ma, broadly coeval with the regional Cu-Ni sulfide mineralization. The mineral components and geochemistry characteristics suggest that the phase I rocks mainly occurred plagioclase fractionation whereas the phase II rocks primarily occurred olivine fractionation. Some plagioclase grains show reverse zoning with 63.4 to 52.1 from rim to core indicating the intrusion underwent magma mixing. The olivine is FeO-rich (16.61–32.58%), and its forsterite (61.8–80.7 mol%) and Ni (358–1218 ppm) contents are relatively low. The olivine and clinopyroxene thermobarometry show that the later pulse magma has higher temperature and higher pressure than the early pulse magma. The estimated equilibrated magma was likely much highly-evolved with 8.49 wt% MgO and 11.02 wt% FeO. The sulfide δ 34S values (–0.61 to 2.87‰) are consistent with those of the Cu polymetallic deposits in Kalatag and the Se/S ration of most samples range from 125 to 180×10−6 suggest that crustal assimilation caused prominent addition of external sulfur into the magma. The olivine crystallization, crustal sulfur addition and pressure rising were likely the triggers of sulfide saturation at Yueyawan deposit.

Paleozoic Magmatism and Mineralization Potential of the Sanchakou Copper Deposit, Eastern Tianshan, Northwest China: Insights from Geochronology, Mineral Chemistry, and Isotopes

Abstract The Sanchakou Cu deposit is located in the eastern section of the Dananhu magmatic arc in the Eastern Tianshan orogenic belt, northwest China. Sanchakou is hosted by quartz diorite and granodiorite intrusions. Chalcopyrite and bornite are the dominant ore minerals and occur as disseminations, patches, veins, and veinlets. Secondary ion mass spectrometry (SIMS) U-Pb dating of zircons shows that the ore-bearing intrusions were emplaced at ca. 435–432 Ma, recording the early subduction of the Paleo-Tianshan oceanic plate. The enrichment in large ion lithophile elements (LILEs), depletion in high field strength elements (HFSEs), and moderate Mg# values, together with mantle-like bulk Sr-Nd and zircon Hf-O isotope signatures (δ18O = 4.0–5.3‰), suggest that they were generated from partial melting of metasomatized mantle materials by subducted slab fluids. In situ S and whole-rock Pb isotope results imply that the Sanchakou diorite magmas provided ore-forming components (S and metals), with additional minor metals (e.g., Cu and Pb) sourced from crustal components beneath the Dananhu arc. The redox state of diorite magmas ranges from initial high fO2 (&amp;gt;FMQ + 2, where FMQ is the fayalite-magnetite-quartz buffer) to relatively low fO2 (&amp;lt;FMQ + 2) upon magma ascent and cooling. The late-stage less oxidized magma compositions are consistent with the presence of magmatic sulfides in primary plagioclase and magnetite. Estimates of water-sulfur-chlorine contents in magma using plagioclase, amphibole, and apatite compositions reveal that the diorite magmas had high water (&amp;gt;7 wt %), normal S (8–393 ppm), and systematically low Cl (38–1,100 ppm) contents. A constant and favorable elevated magma oxidation state appears critical for generating an economic porphyry Cu deposit. Additionally, Cl melt concentrations may be a key factor that controlled metal fertility of the deposits in the Eastern Tianshan, although the mineralization potential may also relate to depth of emplacement of the hydrothermal system. The anomalous presence of stellerite with chalcopyrite in late-stage veins indicates that original porphyry-style mineralization at Sanchakou underwent deformation-related modification after its formation.

Genesis and fluid evolution of the Huangtan Au-Cu deposit in the Kalatag district, Eastern Tianshan, NW China: Constraints from geology, geochronology, fluid inclusions, and H-O-S-Pb isotope geochemistry

The newly discovered Huangtan Au-Cu deposit is located in the central Dananhu - Tousuquan arc of Eastern Tianshan, southern Central Asian Orogenic Belt (CAOB). It is the first Au-dominated volcanogenic massive sulfide (VMS) polymetallic deposit in the Eastern Tianshan. Veinlet-disseminated and massive orebodies are hosted within Early Silurian pyritic phyllic tuff and volcanic breccia and controlled by a secondary fracture zone of the Kalatag fault with extensive hydrothermal alteration. Four primary alteration/mineralization stages have been recognized as follows: (1) Early ore stage (S1), forming mainly ore-barren fine quartz veins with minor gold-bearing sulfides; (2) main ore stage (S2), forming mainly thick quartz veins with abundant coarse-grained subhedral pyrite (S2-1), and plentiful chalcopyrite, sphalerite, barite and Au-bearing sulfide veins (S2-2); (3) late ore stage (S3), which is characterized by plenty of barren quartz–calcite veins with few sulfides; and (4) supergene stage (S4), accompanied by abundant oxide mineralization, including malachite, jarosite and other supergene minerals. From S1 to S3, microthermometric data of fluid inclusions show homogenization temperatures of 263–379 °C (mean = 308 °C), 188-292 °C (mean = 240 °C), and 118-198 °C (mean = 158 °C), respectively, and salinities of 5.3–14.2 (mean = 10.8) wt.% NaCl equiv., 2.8–10.7 (mean = 7.6) wt.% NaCl equiv., and 0.3–14.1 (mean = 2.6) wt.% NaCl equiv., respectively. The ore-forming fluids are characterized by middle-low temperature, low salinity, relatively reduced condition, and an H2O-NaCl ± CO2 ± CH4 system.The δ34S values (−5.25‰ to 0.50‰) and Pb isotopic ratios (206Pb/204Pb = 17.868–19.495, 207Pb/204Pb = 15.446–15.575, and 208Pb/204Pb = 37.350–38.491) suggest that the ore-forming materials came predominantly from a deep-seated magma source with a minor contribution of lower continental crust. The δ18OH2O and δDV-SMOW values of fluid inclusions in each metallogenic stage range from −6.1 to 5.6‰ and −66.8 to −53.9‰, respectively, suggesting the dominant role of magmatic water mixed with convectively circulating heated seawater during fluid evolution. Fluid cooling dilution, local boiling, and fluid mixing were considered as the main mechanisms of metal precipitation. The 40Ar-39Ar plateau age of hydrothermal muscovite from the late ore stage (S3) is 414.4 ± 0.4 Ma, which represents the upper limit age of shallow hydrothermal alteration and Au-Cu mineralization. It is also consistent with the Early Silurian polymetallic metallogenic event in the Kalatag district. The auriferous Huangtan and adjacent Cu-Zn-rich Huangtupo VMS deposits show obvious ore-forming element differences, and constitute a unique VMS metallogenic system in the Eastern Tianshan Orogenic Belt (ETOB), which provides an important research object and new insight for ore prospecting in the peripheral Gobi Desert area.

Petrogenesis and Tectonic Significance of the ~276 Ma Baixintan Ni-Cu Ore-Bearing Mafic-Ultramafic Intrusion in the Eastern Tianshan Orogenic Belt, NW China

The Baixintan mafic-ultramafic intrusion in the Dananhu-Tousuquan arc of the Eastern Tianshan orogenic belt is composed of lherzolite, olivine gabbro, and gabbro. Olivine gabbros contain zircon grains with a U-Pb age of 276.8 ± 1.1 Ma, similar to the ages of other Early Permian Ni-Cu ore-bearing intrusions in the region. The alkaline-silica diagrams, AFM diagram, together with the Ni/Cu-Pd/Ir diagram, indicate that the parental magmas for the Baixintan intrusion were likely high-Mg tholeiitic basaltic in composition. The Cu/Pd ratios, the relatively depleted PGEs and the correlations between them demonstrate that the parental magmas had already experienced sulfide segregation. The lower CaO content in pyroxenites compared with the Duke Island Alaskan-type intrusion and the composition of spinels imply that Baixintan is not an Alaskan-type intrusion. By comparing the Baixintan intrusion with other specific mafic-ultramafic intrusions, this paper considers that the mantle source of the Baixintan intrusion is metasomatized by subduction slab-derived fluids’ components, which gives rise to the negative anomalies of Nb, Ti, and Ta elements. Nb/Yb-Th/Yb, Nb/Yb-TiO2/Yb, and ThN-NbN plots show that the Baixintan intrusion was emplaced in a back-arc spreading environment and may be related to a mantle plume.

Dating the Xiaobaishitou skarn W–(Mo) deposit, Eastern Tianshan, NW China: Constraints from zircon U–Pb, muscovite 40Ar–39Ar, and molybdenite Re–Os system

The Xiaobaishitou deposit (36,264 tonnes @ 0.78 wt% WO3) in the Eastern Tianshan orogenic belt is a medium-sized skarn deposit characterized by W–(Mo) ore bodies that developed along contacts between Triassic biotite granite and Mesoproterozoic carbonaceous rocks. Scheelite and molybdenite are the dominant metallic minerals in this deposit and mostly precipitated during the retrograde stage and/or quartz–sulfide stage, respectively. Laser ablation multi-collector inductively coupled plasma mass spectrometry (LA–MC–ICP–MS) zircon U–Pb dating of the mineralization-related biotite granites from underground openings and outcrops yields ages of 247.9 ± 1.7 Ma (1σ; N = 22; MSWD = 0.69) and 252.2 ± 3.0 Ma (1σ; N = 12; MSWD = 1.60), implying they were emplaced in the Early Triassic. A muscovite 40Ar/39Ar weighted plateau age of 247.6 ± 2.3 Ma (2σ; MSWD = 2.03) and molybdenite Re–Os model ages ranging from 235.8 ± 5.7 Ma to 249.6 ± 3.6 Ma (weighted mean age = 251.1 ± 1.6 Ma; 2σ; MSWD = 0.70) are interpreted as the time of the hydrothermal activity mineralization. The excellent agreement among the U–Pb, 40Ar–39Ar, and Re–Os chronometers confirms that the Xiaobaishitou deposit formed in the Early Triassic. The Re contents of the molybdenites range from 18,477 to 130,447 ng/g, suggesting that the ore-forming materials were derived from continental crust together with the involvement of abundant mantle components. As one of the representative Triassic deposit in Eastern Tianshan, the Xiaobaishitou provides important chronological evidence for metallogenic regularity in the Eastern Tianshan and the Central Asian Orogenic Belt. The new ages reported here confirm that the Xiaobaishitou deposit represents one of the magmatic–hydrothermal mineralization events that occurred in the Eastern Tianshan orogenic belt in lithospheric extension during the Triassic. It is suggested that mantle material played a role in producing the Triassic mineralization and associated magmatism.

Geology and genesis of the Cihai mafic intrusions in Beishan Terrane, Xinjiang, Northwest China: Implication for iron mineralization and tectonic setting

The Cihai Fe deposit (100 Mt @ 48% Fe) represents an economically important iron deposit in Beishan Terrane, on the southern margin of the Central Asian Orogenic Belt, Xinjiang, northwest China. Many Late Carboniferous to Permian Fe(-Cu) deposits (e.g., Yamansu, Shaquanzi, and Weiya) are also present in adjacent Eastern Tianshan orogenic belt. The Cihai deposit is hosted in mafic dykes (i.e., diabase) and in the contact zone between the mafic intrusion and Mesoproterozoic schist. New zircon U–Pb isotope dating in this study gives a crystallization age of 306.9 ± 3.2 Ma for the ore-bearing diabase, which is the first age-constrained Late Carboniferous mafic magmatic activity at Cihai district. In combination with previous Ar–Ar amphibole dating (281.9 ± 2.2 Ma) for the massive ores, it is suggested that the main iron mineralization and associated mafic magmatic activities occurred at ca. 306–282 Ma, coeval with the large-scale iron mineralization epoch (ca. 324–280 Ma) in Eastern Tianshan. The Cihai diabase samples are LILEs-rich (e.g., K, Rb, and Sr) and HFSEs-depleted (e.g., Nb, Ta, and Ti) calc-alkaline series, and exhibit high Mg# values (51–63), as well as low compatible element concentrations (Ni = 14–53 ppm; Co = 18–43 ppm; and Cr = 26–219 ppm), indicating a mantle wedge source. They have elevated Ba/La and Sr/Nd values with limited Th/Yb ratios (<1), and display slightly Eu anomalies (0.90–1.19). Isotopically, they are characterized by positive εHf(t) values (+13.0 to + 16.1) and young Hf model ages (300 to 420 Ma), which are broadly similar to the isotopic compositions of the depleted mantle and within the ranges of most other mafic–ultramafic intrusions in Beishan-Tianshan orogen. These features suggest that the Cihai diabase intrusions were most likely formed by partial melting of a mantle wedge with some degree of metasomatism by slab-derived fluids. Combined our new geochemical and isotopic results with previous regional investigations, we suggest that the earliest generation of mafic dykes formed in an arc setting related to southward subduction of the paleo-Tianshan Ocean at ca. 306 Ma, and the main iron mineralization and intensive mafic magmatism occurred in a subsequent post-collisional regime during the Early Permian.

Two generations of garnets and their relevance for the hydrothermal fluid evolution of the Hongyuntan deposit, NW China

The Hongyuntan deposit, which is one of the typical volcanic-hosted Fe deposits in the Aqishan–Yamansu metallogenic belt of the Eastern Tianshan Orogenic Belt, primarily consists of Fe and skarn minerals (especially garnet). Two generations (three types) of garnets were identified in the Hongyuntan deposit based on field and optical characteristics. Hongyuntan garnets are Fe-rich and belong to the grossular–andradite solid solution series (And = 50.99%–100.00%, average: 79.06%). Garnets of the first generation (Grt1) are massive aggregates and are fine-grained with well-developed oscillatory zonings. They have Type I (two-phase liquid and vapor) and Type II (multi-phase daughter mineral-bearing) fluid inclusions and experienced high temperature (540–560 °C) and salinity (9.2–63.2 wt% NaCl eqv) conditions. The Grt1 have high ΣREE, LfREE/HREE, δEu, U, Y, and high field strength element (HFSE: sum of Nb, Ta, Zr, and Hf) contents. Garnets of the second generation (Grt2) are vefin types and are characterized by irregular shapes with irregular zonings. Type A garnets of the second generation (Grt2-A) usually coexist with mafgnetite and have Type I and Type II fluid inclusions with relatively high temperature (480–500 °C) and salinity (9.5–36.6 wt% NaCl eqv) values. Compared to Grt1, Grt2-A have low And, U, and LREE/HREE together with medium ΣREE, δEu, HFSE, and Y contents. Type B garnets of the second generation (Grt2-B) usually coexist with epidote and quartz. They merely have Type I fluid inclusions with medium–low temperature (260–280 °C) and salinity (6.6–12.7 wt% NaCl eqv) values. Grt2-B have high And, δEu, and LREE/HREE contents but very low ΣREE, HFSE, and Y contents.The fluid inclusions and geochemical properties of the garnets indicate that the temperature, salinity, oxygen fugacity (fO2), and pH of the ore-forming fluid fluctuated, indicating Hongyuntan deposit likely experienced superimposed mineralization. Grt1 originated in a magmatic–hydrothermal fluid and formed through diffusive metasomatism under relatively high fO2, neutral pH, and low water/rock (W/R) ratio conditions. The reduced Fe and other siderophile elements in the ore-forming fluid, which formed Grt1, are believed to be the material basis for iron ores in the later main metallogenic stage. Grt2-A demonstrate the characteristics associated with transitional fluid, which most likely originated from the episodic hydrothermal supplies. However, the ore-forming fluid system gradually opened up during the crystallization of Grt2-B, and the decreasing temperatures and depletion of ore-forming materials were ultimately unfavorable for the deposition of magnetite.

Petrogenesis and Geodynamic Implications of the Carboniferous Granitoids in the Dananhu Belt, Eastern Tianshan Orogenic Belt

This paper presents new LA-ICP-MS zircon U-Pb geochronology, whole-rock major and trace element geochemistry, and Sr-Nd isotopes systematically on porphyritic granitic and K-feldspar granitic intrusions from the Dananhu belt, eastern Tianshan orogenic belt (ETOB). Zircon U-Pb dating indicates that the porphyritic granitic and K-feldspar granitic plutons were formed at 357±3 and 311±3 Ma, respectively. The porphyritic granites show geochemical and isotopic characteristics (high SiO2, low MgO and Mg#, depleted Sr-Nd isotopic values (about 0.703 4 and 6.13, respectively), with Nb/Ta (13.3–14.7) and Zr/Hf (31.0–33.9) ratios) similar to those of the crustal-derived magmas. The above characteristics suggest they were probably originated from juvenile lower crustal materials. The K-feldspar granites also have high SiO2, low MgO and Mg#, depleted Sr-Nd isotopic values (0.703 3–0.704 6 and 4.41–5.67, respectively). But some trace elements contents vary widely, with variable Nb/Ta (12.7–22.7), Zr/Hf (21.3–36.1) and Nb/La (0.38–1.07) ratios, indicating that the K-feldspar granites were formed by partial melting of juvenile lower crustal materials with old crustal materials. Combined with previous data on Carboniferous granitoids in the Dananhu belt, we infer that all the Carboniferous granitic plutons in the Dananhu belt were most likely emplaced in an island arc environment (Dananhu arc). Subsequently, a tectonic transition from oceanic subduction to post-collisional extension probably occurred in the ETOB.

Geochronology, petrogenesis and tectonic implications of the newly discovered Cu–Ni sulfide-mineralized Yueyawan gabbroic complex, Kalatag district, northwestern Eastern Tianshan, NW China

The early Permian Cu–Ni sulfide-bearing mafic-ultramafic intrusions from North Xinjiang in NW China help to provide constraints on the late Paleozoic crustal growth in the Central Asian Orogenic Belt (CAOB) and improve the understanding of the genesis of magmatic sulfide mineralization in orogenic settings and the identification of future prospecting targets. The Cu–Ni sulfide-mineralized Yueyawan gabbroic complex in Kalatag represents a new breakthrough for Cu–Ni prospecting in covered areas in the northwestern part of the Eastern Tianshan orogenic belt, North Xinjiang. Rocks from this intrusion have similar LA-ICP-MS zircon U–Pb ages (282.4 ± 1.1–281.3 ± 1.2 Ma) to other mafic-ultramafic intrusions containing magmatic sulfide mineralization in North Xinjiang. These early Permian gabbroic intrusions are thus important targets for Cu–Ni mineralization exploration in this region. The rocks of the Yueyawan gabbroic complex are a ca. 1 km2 elongated NW-trending crescent-shaped at the junction of two E-W- and NW-SE-trending faults. The intrusion has the morphology of a conduit tube. The rocks include troctolite, olivine gabbro, hornblende gabbro, and gabbro, and the Cu–Ni sulfides occur as steeply dipping (65–70°) lenses with disseminated textures in the contact zone between the olivine gabbro and hornblende gabbro, indicating that sulfide precipitation was coeval with the crystallization of the magmas giving rise to these rocks. The rocks investigated in this study are gabbroic in composition; they are characterized by moderate concentrations of SiO2 (44.8–48.8 wt%) and low to moderate concentrations of MgO (1.69–13.4 wt%, Mg# values of 39.6–73.3), Ni, and Cr. Furthermore, the rocks have high positive εHf(t) values (+13.1 to +15.9) and flat rare earth element patterns ((La/Yb)N = 2.01–2.89), with marked positive Eu anomalies, high Ti/Zr ratios (70–164), and low Ce/Pb (3.65–14.83) and Nb/Ta values (9–16), indicating very restricted amounts (5–10%) of crustal assimilation. This complex was likely derived from 10 to 20% partial melting (as evidenced by the low Sm/Yb values of 1.48–2.02 and La/Sm values of 1.61–2.23) of weakly slab melt-metasomatized (as evidenced by the low Ba/Nb and Ba/Zr ratios) subcontinental lithospheric mantle (with 2% garnet and 2% spinel lherzolite) at depths greater than 85 km in a post-collisional extensional source that contained sufficient Ni and Cu for the saturation and segregation of immiscible sulfides to concentrate the metals. The magma underwent sulfide saturation and segregation together with olivine fractionation (as evidenced by Ni and TFe2O3 increase and SiO2, CaO, and Zr decrease with increasing MgO content) in the conduit-type intrusion. Therefore, regional fault structures, ore-bearing rock types, and geochemical ratios that reflect high degrees of partial melting are features of the Cu–Ni-mineralized gabbroic rocks in adjacent areas that help support exploration.

Prospectivity Mapping for Porphyry Cu–Mo Mineralization in the Eastern Tianshan, Xinjiang, Northwestern China

In order to comprehensively utilize regional-scale geological, geochemical and geophysical datasets for future exploration of undiscovered porphyry Cu–Mo polymetallic deposits (PCMPDs) in the Chinese Eastern Tianshan orogenic belt, three data-driven mineral prospectivity mapping (MPM) methods, namely ordinary weights of evidence (WofE), fuzzy weights of evidence (FWofE) and logistic regression (LR), were employed to integrate these datasets for mapping prospectivity of undiscovered PCMPDs. Firstly, the geological setting and mineralization of PCMPDs in the Eastern Tianshan district are reviewed. Then, spatial datasets based on geological maps, stream sediment geochemical data, and Bouguer gravity and aeromagnetic data are introduced, and on the basis of the prospecting model for PCMPDs, layers of structural, lithological, geophysical and geochemical evidences are constructed using the spatial datasets by means of GIS-based techniques. Finally, these evidential layers were integrated by using the WofE, FWofE and LR methods to obtain posterior probability maps of PCMPDs and the results are critically compared. The main conclusions are that: (1) the porphyry Cu–Mo mineralization in the Eastern Tianshan was occurred in the subduction boundary of the Late Paleozoic Dananhu-Dacaotan arc system of Kanguertag-Huangshan deep fault belt. This geological inference is supported by all the data-driven MPM methods; (2) the conditional independence assumption for both WofE and FWofE can be easily violated in practical applications. This issue seems very difficult to be circumvented due to geological correlations of evidence layers; (3) the uncertainty of the LR modeling approach particularly with respect to models using multiclass response variables mainly arises from over-fitting of the (ln-transformed) linear relationship; and (4) if there is no need for estimation of the number of undiscovered PCMPDs, the prospectivity map biasedly estimated by either WofE or FWofE modeling can be recommended for targeting new exploration areas with more detailed reconnaissance of potential undiscovered PCMPDs. Otherwise, the prospectivity map unbiasedly estimated by LR modeling with binary evidence modeling approach can be priority of use.

Ore Genesis and Tectonic Significance of the Xiaojiashan Tungsten Deposit, Eastern Tianshan, Xinjiang, China: Evidence from Geology, Fluid Inclusions, and Stable Isotope Geochemistry

AbstractThe Xiaojiashan tungsten deposit is located about 200 km northwest of Hami City, the Eastern Tianshan orogenic belt, Xinjiang, northwestern China, and is a quartz vein‐type tungsten deposit. Combined fluid inclusion microthermometry, host rock geochemistry, and H–O isotopic compositions are used to constrain the ore genesis and tectonic setting of the Xiaojiashan tungsten deposit. The orebodies occur in granite intrusions adjacent to the metamorphic crystal tuff, which consists of the second lithological section of the first Sub‐Formation of the Dananhu Formation (D2d 12). Biotite granite is the most widely distributed intrusive bodies in the Xiaojiashan tungsten deposit. Altered diorite and metamorphic crystal tuff are the main surrounding rocks. The granite belongs to peraluminous A‐type granite with high potassic calc‐alkaline series, and all rocks show light Rare Earth Element (REE)‐enriched patterns. The trace element characters suggest that crystallization differentiation might even occur in the diagenetic process. The granite belongs to postcollisional extension granite, and the rocks formed in an extensional tectonic environment, which might result from magma activity in such an extensional tectonic environment. Tungsten‐bearing quartz veins are divided into gray quartz vein and white quartz veins. Based on petrography observation, fluid inclusions in both kinds of vein quartz are mainly aqueous inclusions. Microthermometry shows that gray quartz veins have 143–354°C of Th, and white quartz veins have 154–312°C of Th. The laser‐Raman test shows that CO2 is found in fluid inclusions of the tungsten‐bearing quartz veins. Quadrupole mass spectrometry reveals that fluid inclusions contain major vapor‐phase contents of CO2, H2O. Meanwhile, fluid inclusions contain major liquid‐phase contents of Cl−, Na+. It can be speculated that the ore‐forming fluid of the Xiaojiashan tungsten deposit is characterized by an H2O–CO2, low salinity, and H2O–CO2–NaCl system. The range of hydrogen and oxygen isotope compositions indicated that the ore‐forming fluids of the tungsten deposit were mainly magmatic water. The ore‐forming age of the Xiaojiashan deposit should to be ~227 Ma. During the ore‐forming process, the magmatic water had separated from magmatic intrusions, and the ore‐bearing complex was taken to a portion where tungsten‐bearing ores could be mineralized. The magmatic fluid was mixed by meteoric water in the late stage.

Geology, Re-Os and U-Pb geochronology and sulfur isotope of the the Donggebi porphyry Mo deposit, Xinjiang, NW China, Central Asian Orogenic Belt

The the Donggebi porphyry Mo deposit in the eastern section of the Eastern Tianshan Orogenic Belt in the Central Asian Orogenic Belt contains Mo metal reserves of 0.5 Mt. The deposit is hosted in Early Carboniferous metasedimentary rocks, namely; metasandstone, meta-sandy mudstone . Multiple hydrothermal activities have resulted in propylitic, phyllic, and argillic alteration in this deposit. Four stages (I-IV) of hydrothermal activity are identified. Stage I is represented by a mineral assemblage of K-feldspar, quartz and wolframite. Stage II consists of quartz + magnetite+pyrite ± chalcopyrite veinlets/veins with phyllic halos. Stage III consists of quartz + molybdenite + pyrite ± galena ± sphalerite ± chalcopyrite veins that are commonly related to phyllic alteration in the altered rocks. Stage IV has an assemblage of calcite + gypsum. Molybdenite mainly occurs in Stages III. Re-Os dating results for molybdenite samples from these two stages yielded an isochron age of 234.2 ± 1.6 Ma (2σ, MSWD = 0.25, n = 8). Porphyritic granites have a SIMS U-Pb zircon age of ∼236 Ma and it was probably related to the Triassic felsic magmatism in this area. Values of δ34S of sulfides range from 1.5‰ to 3.8‰, with an average value of 2.81 ± 2.24‰ (n = 22), reflecting a deep sulfur source. Most molybdenite samples have high δ34S values (≥3.36‰) relative to other sulfide minerals (i.e., pyrite and chalcopyrite) of Stages I to III (δ34S = 1.5–3.8‰, n = 18). Based on the geological history and spatial-temporal distribution of the granitoids, it is proposed that the Mo deposits in the eastern part of the East Tianshan Orogenic Belt formed in a post-collision extensional setting in the Early Mesozoic.

Ductile shearing and LA‐ICP MS zircon U–Pb dating of Sa'erqiaoke ductile shear belt in the Bogda orogen: Constraints on the process of mountain building of eastern Tianshan

The relationship between collision orogeny and post‐orogeny tectonism remains to be a weakness of the present orogenic belt research. An E—W‐trending ductile shear zone exposed along the Bogda Mountain of the Eastern Tianshan Orogenic Belt (ETOB) has spatiotemporal correlation with the Kelameili ophiolite belt and offers important evidence for research on the evolution of the ETOB. These ductile shear belts exposed in the eastern Sa'erqiaoke display that the low greenschist facies metamorphism was followed by north‐eastward thrusting and sinistral strike‐slip shearing. LA‐ICP MS zircon U–Pb ages of the mylonitic quartz schist in Sa'erqiaoke structure section range from 226 to 1458 Ma, four old zircons with ages of 832.2, 1029.2, 1240.5, and 1458.2 Ma possess round‐shaped grains and core–mantle texture, which suggest that its protolith has a detritic and metamorphic origin. The major age peak ranges from 443.3 to 466.8 Ma, with weighted age of 452.9 ± 5.4 (MSWD = 0.62, N = 13), corresponding to the early Palaeozoic Harlic island arc magmatism. Two young metamorphic zircons with ages of 247.2 and 226 Ma could represent the time of ductile shearing, which are consistent with the final termination of the ETOB. Two weighted ages of 303.8 ± 3.4 (MSWD = 0.12, N = 16) and 305.2 ± 2.4 (MSWD = 0.46, N = 32) from Nawakuduke mylonitic belts are coeval with post‐collision A‐type granite of the Kelameili suture, which suggest that the ductile shearing should be later than the closure of the Kelameili Ocean. Based on the synthesis of voluminous evidences, we propose that the Kelameili Ocean, as part of the paleo‐Tianshan Ocean, obliquely subducted beneath the Junggar plate, bringing about the Bogda subduction‐torn‐type rift during the Carboniferous period. Although the final closure of the Kelameili Ocean has ended at late Carboniferous, the subsequent continental collision and large‐scale ductile shearing lasted until late Triassic.

Origin of the volcanic-hosted Yamansu Fe deposit, Eastern Tianshan, NW China: constraints from pyrite Re-Os isotopes, stable isotopes, and in situ magnetite trace elements

The Yamansu Fe deposit (32 Mt at 51% Fe) in the Eastern Tianshan Orogenic Belt of NW China is hosted in early Carboniferous volcano-sedimentary rocks and spatially associated with skarn. The paragenetic sequence includes garnet-diopside (I), magnetite (II), hydrous silicate-sulfide (III), and calcite-quartz (IV) stages. Pyrite associated with magnetite has a Re-Os isochron age of 322 ± 7 Ma, which represents the timing of pyrite and, by inference, magnetite mineralization. Pyrite has δ34SVCDT values of − 2.2 to + 2.9‰, yielding δ34SH2S values of − 3.1 to 2‰, indicating the derivation of sulfur from a magmatic source. Calcite from stages II and IV has δ13CVPDB values from − 2.5 to − 1.2‰, and − 1.1 to 1.1‰, and δ18OVSMOW values from 11.8 to 12.0‰ and − 7.7 to − 5.2‰, respectively. Calculated δ13C values of fluid CO2 and water δ18O values indicate that stage II hydrothermal fluids were derived from magmatic rocks and that meteoric water mixed with the hydrothermal fluids in stage IV. Some ores contain magnetite with obvious chemical zoning composed of dark and light domains in BSE images. Dark domains have higher Mg, Al, Ca, Mn, and Ti but lower Fe and Cr contents than light domains. The chemical zoning resulted from a fluctuating fluid composition and/or physicochemical conditions (oscillatory zoning), or dissolution-precipitation (irregular zoning) via infiltration of magmatic-hydrothermal fluids diluted by late meteoric water. Iron was mainly derived from fluids similar to that in skarn deposits.

Early Paleozoic dioritic and granitic plutons in the Eastern Tianshan Orogenic Belt, NW China: Constraints on the initiation of a magmatic arc in the southern Central Asian Orogenic Belt

Early Paleozoic dioritic and granitic plutons in the Eastern Tianshan Orogenic Belt (ETOB) have been studied in order to constraint the initiation of a magmatic arc formed in this region. Zircon U–Pb dating indicates that two dioritic plutons in the northern ETOB were generated in the Late Ordovician (452±4Ma) and the Early Silurian (442±3Ma), respectively. Diorites from the two plutons are characterized by enrichments in large ion lithophile elements (LILE) and highly incompatible elements, with depletions in high field strength elements (HSFE) displaying typical geochemical features of a subduction-related origin. They have positive εNd(t) values (+5.08–+6.58), relatively young Nd model ages (TDM=0.71–1.08Ga), with Ta/Yb (0.05–0.09) and Nb/Ta ratios (12.06–15.19) similar to those of depleted mantle, suggesting a juvenile mantle origin. Their high Ba/La (13.3–35.9), low Th/Yb (0.72–2.02), and relatively low Ce/Th (4.57–14.7) and Ba/Th (47.8–235) ratios indicate that these diorites were probably produced by partial melting of a depleted mantle wedge metasomatized by both subducted sediment-derived melts and slab-derived aqueous fluids. Zircon U–Pb dating of a granitic pluton in the northern ETOB yielded a Late Ordovician intrusion age of 447±5Ma. Granites from this pluton show calc-alkaline compositions with geochemical characteristics of I-type granites. They also show positive εNd(t) values (+6.49–+6.95) and young Nd model ages (TDM=0.69–0.87Ga), indicating that the granites were most likely derived from juvenile lower crust. Our new dating results on the dioritic and granitic plutons suggest that arc-type magmatism in the northern ETOB began prior to or at the Late Ordovician (452–442Ma). In addition, north-dipping subduction of the Kangguertage oceanic lithosphere may account for the arc-type magmatism and the geodynamic process of the ETOB in the Early Paleozoic.