West Junggar Research Articles

Geochemistry and Sr–Nd–Hf–Pb isotope systematics of late Carboniferous sanukitoids in northern West Junggar, NW China: Implications for initiation of ridge-subduction

The study of Carboniferous magmatism in northern West Junggar plays an important role in understanding the tectonic evolution of that part of the Central Asian Orogenic Belt. In this study, we present petrology, zircon U–Pb geochronology, mineral and whole-rock geochemistry, and the Sr–Nd–Hf–Pb isotope compositions of volcanic rocks from the Hamutusi area of northern West Junggar. LA–ICP–MS zircon U–Pb analysis of a representative andesite yielded an early to late Carboniferous age of 324.4 ± 6.9 Ma. The volcanic rocks are calc-alkaline, with high SiO2 (54.2–57.8 wt%), MgO (5.97–8.33 wt%), Mg# (60–69), Cr (147–448 ppm), and Ni (29.1–120 ppm) contents, and are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), but depleted in high field strength elements (HFSE), these characteristics are similar to those of typical sanukitoids within the Setouchi volcanic belt in Japan. All samples have radiogenic initial Sr and Pb isotopic compositions, and low εNd(t) and εHf(t) values, indicating the sanukitoids were generated by partial melting of subducting sediments in which the melts interacted with the mantle. Geochemical modeling calculations indicate a proportion of 3–10% sediment melt and slab-derived fluids were mixed with the depleted mantle to produce the bulk of the Hamutusi rocks. We conclude that the studied rocks from north West Junggar record the initiation of ridge subduction related to a transition from normal subduction to subduction of young and hot oceanic lithosphere between the early and late Carboniferous.

Permian rifting processes in the NW Junggar Basin, China: Implications for the post-accretionary successor basins

The Junggar Basin, bounded by the West Junggar along late Carboniferous strike-slip faults in the west, is a post- orogenic successor basin within the Neoproterozoic to Paleozoic Central Asian Orogenic Belt (CAOB). The Junggar Basin is a key region for understanding the Permian rifting processes since it is located in an extensional step-over zone between the Irtysh and North Tianshan strike-slip fault zones that were most active in the Permian. Based on new seismic reflection profiles, detrital petrology and zircon U-Pb ages, together with regional geological data and a refraction seismic profile, this study investigates the tectonic evolution and the geodynamic mechanics of the basin. The early Permian phase of syn-rift mechanical subsidence in the western Junggar Basin formed half grabens. Volcanic detritus is abundant, and detrital zircon U-Pb ages show sediment supply was entirely local. During the middle Permian post-rift phase with thermal subsidence, sediment supply exceeded subsidence, and there are increasingly distant sources of sediment. The Permian regional extensional stress field between major post- accretionary strike-slip fault zones triggered the reactivation of the pre-existing late Carboniferous strike-slip faults in West Junggar and produced thinning of the crust beneath the Junggar Basin, with a β crustal stretching factor of 1.74. Seismic refraction data reveals asthenospheric upwelling resulted in underplated mafic lower crust (6–10 km). The location of initial rift axis in the Junggar Basin was controlled by the juxtaposition of thin and weak crust beneath the basin and relatively strengthened thick crust in the West Junggar attributed to the cooling of A-type granites, resulting from localized strain. Similar location of post-orogenic rifting in southwestern CAOB, in areas of high heat flow following collision, suggests that Junggar Basin is an example of a more general process.

Geochemical Characteristics of Early Permian Pyroclastic Rocks in the Jimunai Basin, West Junggar, Xinjiang (NW China): Implications for Provenance and Tectonic Setting

AbstractDuring the Late Paleozoic, West Junggar (Xinjiang, NW China) experienced a shift in tectonic setting from compression to extension. Ha'erjiao is an important area for investigating collisional structures, post–collisional structures, and magmatic activities. Based on the petrological and geochemical characteristics of pyroclastic and other volcanic rocks in the Permian Kalagang Formation from the borehole ZKH1205 in the Jimunai Basin, the main types of source rock for the pyroclastic rocks deposited in the basin are identified and their implications for the Early Permian tectonic setting examined. The abundance of basalt and andesite lithic fragments in the pyroclastic rocks, together with the REE characteristics and the contents of transition and high field strength elements show that the source rocks were chiefly intermediate–basic volcanic rocks. High ICV values, low CIA values, low Rb/Sr ratios, low Th/U ratios and the mineralogical features suggest weak chemical weathering of the source rocks; the geochemical patterns of the pyroclastic rocks might not only have been impacted by crustal contamination but also might be related to the nature of the magma from the source area. The geochemical properties of the pyroclastic rocks distinguish them from arc‐related ones, and such samples plot in the within‐plate basalt (WPB) field in some diagrams. This is consistent with the formation background of the Early Permian volcanic rocks in this region.

Middle Paleozoic archipelago amalgamation and tectonic transform in the northern West Junggar, NW China: Constraints from magmatism and deformation

The West Junggar (WJ) is generally accepted as a Paleozoic archipelago-type orogenic system and records a multiple arcs amalgamation history; however, some of the associated processes require further examination. In this paper, we conducted detailed field investigations and large-scale mapping in the Saier Mountains, northern WJ, and identified several episodes of magmatism and deformation events. The felsic rocks (dioritic porphyrite and granitic porphyry), characterized by a dense cleavage, were emplaced at the Early–Middle Silurian (433–423 Ma). These rocks exhibit geochemical characteristics comparable to those of the coeval adakite-like rocks and A2-type granites in the Xiemisitai Mountains. The undeformed mafic dike swarms yielded SHRIMP zircon age of 382 ± 6 Ma and intruded the Hebukesaier ophiolitic mélange (HOM) as well as the surrounding rocks. The HOM, Silurian felsic intrusions and pyroclastic rocks (422 ± 9 Ma) in the Saier Mountains commonly display cleavage attributed north–south shortening during 422–382 Ma. Deep-sea bedded cherts and turbidites in the area also experienced deformation and exhibited cleavage, with subduction-related fold deformation partially preserved in the HOM. The fold deformation was greatly related to a subduction-accretion process of the Hongguleleng–Balkybey Ocean during the Ordovician. Considering the geochronology and geochemistry data, the extensive Silurian–Early Devonian magmatism in the northern WJ indicates a prolonged (~40 Ma) and episodic post-collisional extension in this orogen and records an obvious trend of crustal temperature increasing and thickness thinning from 435 to 410 Ma. The regional extrusion deformation was attributed to the southward subduction of the Irtysh–Zaysan Ocean during the Middle Devonian, and then the slab roll-back offered an extensional setting which favored the emplacement of mantle-derived mafic dike swarms. We propose that the northern WJ probably experienced a complicated evolution involving archipelago amalgamation and tectonic transform in the middle Paleozoic, providing a new sight to reveal the orogenic process of the Central Asian Orogenic Belt (CAOB).

Ediacaran initial subduction and Cambrian slab rollback of the Junggar Ocean: New evidence from igneous tectonic blocks and gabbro enclave in Early Palaeozoic accretionary complexes, southern West Junggar, NW China

AbstractNew zircon U–Pb ages and whole-rock chemical data from four adakitic and two non-adakitic igneous rocks as tectonic blocks in the southern West Junggar accretionary complexes, northwestern China and one gabbro enclave in adakitic block provide further constraints on the initial subduction and following rollback process of the Junggar Ocean as part of southern Palaeo-Asian Ocean. The oldest adakitic monzonite in Tangbale is intruded by the non-adakitic quartz monzonite at 549 Ma, and the youngest adakitic diorite in Tierekehuola formed at 520 Ma. The Ediacaran–Cambrian magmatism show a N-wards younger trend. The high-SiO2 adakitic rocks have high Sr (300–663 ppm) and low Y (6.68–12.2 ppm), with Sr/Y = 40–84 and Mg no. = 46–60, whereas the non-adakitic rocks have high Y (13.2–22.7 ppm) and Yb (2.32–2.92 ppm), with Mg no. = 36–40. The gabbro has high MgO (14.81–15.11 wt%), Co (45–48 ppm), Cr (1120–1360 ppm) and Ni (231–288 ppm), with Mg no. = 72–73. All the samples show similar large-ion lithophile element (LILE) and light rare earth element (LREE) enrichment and Nb, Ta, Ti and varying Zr and Hf depletion, suggesting that they were formed in a subduction-related setting. The adakitic rocks were produced by partial melting of subducted oceanic slab, but the melts were modified by mantle wedge and slab-derived fluids; the non-adakitic rocks were likely derived from partial melts of the middle-lower arc crust; and the gabbro originated from the mantle wedge modified by slab-derived fluids. The magmatism could have been generated during the Ediacaran initial subduction and Cambrian slab rollback of the Junggar Ocean.

Tracking the Tectonic Evolution of the Junggar-Balkhash Ocean: A Case Study from the Post-Collisional Takergan Pluton in the West Junggar, Xinjiang

The Late Carboniferous and Early Permian igneous rocks are widely developed in the West Junggar, Xinjiang, which are considered to be related to the evolution of the Junggar-Balkhash Ocean. However, their tectonic settings have been controversial for a long time. With the aim of providing new evidence for the Late Paleozoic tectonic evolution of the West Junggar, we present petrology, zircon U-Pb chronology, whole-rock major and trace elemental and Sr-Nd isotopic data, to discuss the petrogenesis and tectonic setting of Takergan pluton from the Barleik Mountains in the West Junggar. The Takergan pluton is mainly composed of quartz diorite porphyry and quartz monzonite. The quartz diorite porphyry has low SiO2 (57.76 wt.%–57.81 wt.%), high total alkali contents (Na2O+K2O=6.29 wt.%–6.56 wt.%), and high Mg# values (45–46), with a zircon U-Pb age of 304±5 Ma. The quartz monzonite shows relatively high SiO2 (58.71 wt.%–64.71 wt.%), total alkali contents (7.73 wt.%–9.70 wt.%), and Mg# values (34–47), with the A/CNK values of 0.91–0.98, which belongs to shoshonitic and metaluminous I-type granite series. The quartz monzonite yields zircon U-Pb ages of 302±2 and 296±3 Ma, and is characterized by low initial Sr ratios of 0.703 97–0.704 09, high eNd(t) values of +6.8–+7.0, and young Nd model ages of 551–587 Ma. Both the quartz diorite porphyry and quartz monzonite are enriched in light rare earth elements and Rb, Th, U, K, and depleted in Nb, Ta, Ti, with different degrees of negative Eu anomalies. These features indicate that the Takergan pluton was most likely formed in a post-collisional setting by partial melting of a depleted mantle source that had been metasomatized by subduction-related fluids, with significant fractional crystallization and slightly contaminated by crustal materials. Combined with the widespread distribution of the coeval stitching plutons, the occurrences of terrestrial Late Carboniferous to Permian volcano-sedimentary formations, and the absence of subduction-related rocks later than Early Carboniferous, it is believed that the Junggar-Balkhash Ocean was closed at about 320 Ma, and the central West Junggar has transformed to a post-collisional environment during the Late Carboniferous and Early Permian.

Geochemistry and genesis of the Hongguleleng Manto-type Cu deposit, West Junggar, Xinjiang, China

The Hongguleleng Cu deposit is a Manto-type deposit in the Central Asian Orogenic Belt, located in the east section of West Junggar, Xinjiang, China. It is hosted by Middle Ordovician volcano-sedimentary sequence, especially for porous units. Specifically, the orebodies of this Manto-type deposit are hosted in basalt, andesite, and andesitic breccia host rocks (the particular volcano-sedimentary sequence). The occurrence of these orebodies is controlled by normal faults. Chalcopyrite, pyrite, calcite, chlorite, and epidote are most common minerals of the Hongguleleng deposit. Three alternation belts and six ore related stages have been recognised. The epidote-chalcopyrite stage, quartz-calcite-chalcopyrite stage, and chlorite stage are ore forming stage. The δ34S of chalcopyrite in ore depositing stages varies from 0.94‰ to 2.59‰, indicating that the origin of sulfur is either magma or host volcanic rock. Further, the 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of the chlorite-chalcopyrite and quartz-calcite-chalcopyrite stages exist in the range of 17.443–17.847 and 17.855–17.973; 15.472–15.495 and 15.493–15.620; and 37.616–37.666 and 37.689–38.114, respectively, indicating that the metal originates from the Middle Ocean Ridge Basalt (MORB). The δ13CPDB value of calcite for the epidote-chalcopyrite stage ores, quartz-calcite-chalcopyrite ores with veinlet characteristics, quartz-calcite-chalcopyrite ores with brecciated characteristics, and chlorite-chalcopyrite ores range from −1.40 to −1.50‰, −2.01 to 0.41‰, −1.15 to 0.75‰, and −0.80 to 1.58‰, respectively, while the 18OSMOW ranges from 8.94–10.42‰, 11.83–13.17‰, 13.59–14.04‰, and 12.34–14.18‰, respectively. The CO isotope of calcite of the Hongguleleng deposit indicates that seawater is an essential fluid for ore formation and that modified seawater interacts with the host rock extensively. The hydrothermal magnetite in the quartz-magnetite stage and chlorite in the ore forming stages indicate that temperature decreases from its early to late stages. High Zn and Pb concentrations over four stages of pyrite, with a relatively stable contents, indicate that the content of Zn and Pb in fluid is also high. The concentration of Cu in these four stages of pyrite of the Hongguleleng deposit follow a decreasing trend, indicating that the concentration of Cu in the ore forming fluid decreases from the early to late stage. Based on information mentioned above, we argue that this deposit is a Manto-type deposit with modified seawater driving forces.

The Nature of the West Junggar Basement: Evidence from Magmatic and Detrital Zircon U‐Pb Ages

The Nature of the West Junggar Basement: Evidence from Magmatic and Detrital Zircon U‐Pb Ages

Source-to-sink system of Late Permian post-collisional stream-dominated alluvial fans under humid paleoclimate along the Irtysh-Zaisan suture zone, Central Asian Orogenic Belt

Abstract Because of few reports on the Upper Permian in the NW Junggar terrane, Northwest China, the recently identified Upper Permian Zaya conglomerate in the Saur area can provide insights into tectonics and paleoclimate at that time. The succession consists of predominant composite cobble-pebble conglomerate (~80%) and subordinate (gravelly) sandstone (~20%), showing sedimentary structures of stream-dominated alluvial fans and the unique lithofacies in the Saur area. This conglomerate was originally classified as a part of Lower Carboniferous, but its youngest detrital zircon U-Pb age is 265.3 ± 1.7 Ma and the succession shows the deformational patterns different from those of the Triassic sequences in the West Junggar terrane. Thus the conglomerate was deposited during the Late Permian. The Zaya conglomerate was unconformably deposited upon the Lower Permian volcano-sedimentary strata over the Devonian–Carboniferous Saur Arc (SA) and Saur Forearc Basin (SFB) on the northern margin of the Kazakhstan terrane, which currently juxtaposes with the Devonian Buerjin Accretionary Complex (BAC) on the southern margin of the Altai terrane to north. The contributions of the SFB + BAC and SA to the Zaya conglomerate are 89.6–96.7% and 3.3–10.4%, respectively. The conglomerate clasts are derived from Permian–Carboniferous volcano-sedimentary rocks in the SA, but the sandstones contain not only volcanic materials from the SA and SFB but also metamorphic and ultramafic detritus from the BAC. This is in support of the pre-Permian amalgamation of the Altai-Kazakhstan terranes. Plant fossils and organization of conglomerate would suggest a relatively humid paleoclimate with abundant rainfall.

The timing of Barleik Formation and its implication for the Devonian tectonic evolution of Western Junggar, NW China

Abstract The timing of Barleik Formation in Xinjiang, NW China, has not been constrained by accurate geochronology yet, while this work is of great significance to help reconstruct the geological tectonic evolution of Western Junggar. Based on the LA-ICP-MS U–Pb geochronology study, the weighted average age of magmatic zircons collected from the tuff in Barleik Formation, which reflects the formation age of the tuff, is 372 ± 2 Ma (N = 57, mean square of weighted deviates (MSWD) = 1.15). The first report in this study of the zircon U–Pb dating result indicates that Barleik Formation along the West Junggar tectonic belt occurred in the Late Devonian rather than in the middle Devonian period as previously claimed. Meanwhile, chronology data and the geochemical features comparing with the Island Arc-related rocks in the adjacent area, as well as stratigraphic structural relationship, suggest that volcanic activities may exist in the Late Devonian, and the relevant volcanic ash deposited in the Barleik forearc basin may be derived from the adjacent island arc (current geographic coordinate). In addition, the fossil assemblage dominated by bathyal-abyssal invertebrate fossils and bathyal-abyssal facies indicates that the Barleik Formation is a bathyal-abyssal sedimentary environment.

A-type granites induced by a breaking-off and delamination of the subducted Junggar oceanic plate, West Junggar, Northwest China

The A-type granites with highly positive εNd(t) values in the West Junggar, Central Asian Orogenic Belt (CAOB), have long been perceived as a group formed under the same tectonic and geodynamic setting, magmatic sourceq and petrogenetic model. Geological evidence shows that these granites occurred at two different tectonic units related to the southeastern subduction of Junggar oceanic plate: the Hongshan and Karamay granites emplaced in the southeast of West Junggar in the Baogutu continental arc; whereas the Akebasitao and Miaoergou granites formed in the accretionary prism. Here the authors present new bulk-rock geochemistry and Sr-Nd isotopes, zircon U-Pb ages and Hf-O isotopes data on these granites. The granites in the Baogutu continental arc and accretionary prism contain similar zircon εHf(t) values (+10.9 to +16.2) and bulk-rock geochemical characteristics (high SiO2 and K2O contents, enriched LILEs (except Sr), depleted Sr, Ta and Ti, and negative anomalies in Ce and Eu). The Hongshan and Karamay granites in the Baogutu continental arc have older zircon U-Pb ages (315–305 Ma) and moderate 18O enrichments (δ18Ozircon=+6.41‰–+7.96‰); whereas the Akebasitao and Miaoergou granites in the accretionary prism have younger zircon U-Pb ages (305–301 Ma) with higher 18O enrichments (δ18Ozircon=+8.72‰–+9.89‰). The authors deduce that the elevated 18O enrichments of the Akebasitao and Miaoergou granites were probably inherited from low-temperature altered oceanic crusts. The Akebasitao and Miaoergou granites were originated from partial melting of low-temperature altered oceanic crusts with juvenile oceanic sediments below the accretionary prism. The Hongshan and Karamay granites were mainly derived from partial melting of basaltic juvenile lower crust with mixtures of potentially chemical weathered ancient crustal residues and mantle basaltic melt (induced by hot intruding mantle basaltic magma at the bottom of the Baogutu continental arc). On the other hand, the Miaoergou charnockite might be sourced from a deeper partial melting reservoir under the accretionary prism, consisting of the low-temperature altered oceanic crust, juvenile oceanic sediments, and mantle basaltic melt. These granites could be related to the asthenosphere’s counterflow and upwelling, caused by the break-off and delamination of the subducted oceanic plate beneath the accretionary prism Baogutu continental arc in a post-collisional tectonic setting.©2022 China Geology Editorial Office.

Exhumation and preservation conditions of Suyunhe porphyry molybdenum deposit in the West Junggar, Xinjiang (NW China): Constraints from the fission track and (U-Th)/He thermochronology

斑岩型矿床多形成于汇聚型板块边界。由于其较浅的就位深度，大部分古老的斑岩型矿床很容易受到后期的剥蚀而消失殆尽。研究斑岩型矿床成矿后的埋藏和去顶过程对于深入理解矿床的保存条件和区域找矿前景至关重要。新疆西准噶尔西部的苏云河斑岩型钼矿床形成于晚石炭世，是一处保存良好的斑岩型矿床，为我们研究前中生代斑岩成矿系统的保存条件提供了天然的实验室。本文首次针对该矿床开展了锆石和磷灰石裂变径迹与锆石（U-Th）/He低温热年代学分析，结合热历史反演模拟以及前人的年代学数据显示，苏云河斑岩钼矿的蚀变过程至少持续了55Myr。在早二叠世到中三叠世，矿区接受5.2~8.1km厚的沉积物覆盖。中三叠世至早白垩世（240~120Ma），矿床经历了快速剥露作用，剥露速率为49.0~56.7m/Myr，去顶量为7.4~9.2km。早白垩世（120Ma）至今为缓慢剥露阶段，剥露速率为6.7~21.7m/Myr，去顶量为0.8~2.6km。中三叠世至早白垩世的快速冷却事件可能并不是特定构造事件（比如：南部羌塘和昆仑-柴达木碰撞或者羌塘和拉萨碰撞）远程效应的产物，而与区域内走滑断层的活化密切相关。而矿床早期沉积的巨厚盖层以及早白垩世以来干旱气候和缓慢剥露，为石炭-二叠纪斑岩型钼矿得以保存提供了条件。;Porphyry deposit is a kind of epithermal deposit, which is mostly formed in the convergent plate boundary. However, ancient porphyry deposit was often destroyed and disappeared due to later tectonic or exhumation. Therefore, it is vital to reveal the exhumation and uplift process after mineralization for further understanding the preservation conditions and regional prospecting prospects of the deposits. Suyunhe porphyry molybdenum deposit in the west part of the West Junggar, Xinjiang was formed in Late Carboniferous. It is a well-preserved porphyry deposit, which provides a natural laboratory for us to study the preservation conditions of ancient porphyry metallogenic system. In this paper, we present new zircon and apatite fission track (ZFT and AFT) and zircon (U-Th)/He (ZHe) data for the Suyunhe porphyry molybdenum deposit. Combined with thermal modeling inversion, we suggest that the hydrothermal alteration process of the Suyunhe porphyry molybdenum deposit lasted at least 55Myr. It underwent a thick burial (5.2~8.1km) sediments from Early Permian to Middle Triassic. The deposit experienced a rapid exhumation stage from Middle Triassic to Early Cretaceous (240~120Ma). The exhumation rate was 49.0~56.7m/Myr, and the corresponding denudation amount was 7.4~9.2km. After the Cretaceous (120Ma), it entered a slow exhumation stage with an exhumation rate of 6.7~21.7m/Myr and exhumation amount of 0.8~2.6km. Among them, the Middle Triassic to Early Cretaceous rapid exhumation was not the product of far-field effects (e.g., the collision between Qiangtang and Kunlun-Qaidam or the collision between Qiangtang and Lhasa), but related to the activation of strike slip faults in the region. Therefore, early thick burial and later dry climate and slow exhumation provided key conditions for the preservation of the Carboniferous Suyunhe porphyry molybdenum deposit.

Petrogenesis and geodynamic setting of the Late Carboniferous granodiorite porphyry in Miaoergou pluton, southern West Junggar

位于西准噶尔南部的庙尔沟岩体主体由碱长花岗岩和少量紫苏花岗岩组成。本文在前人工作基础上，以岩体东南边缘新发现的花岗闪长斑岩为研究对象，开展岩石学、年代学和Hf同位素以及全岩地球化学研究，确定花岗闪长斑岩形成时代、揭示岩石成因类型及源区属性、探讨其与碱长花岗岩和紫苏花岗岩岩浆演化成因联系及其形成的深部动力学过程。锆石U-Pb定年结果显示，花岗闪长斑岩形成于317.4±1.9Ma，为晚石炭世早期岩浆活动的产物，明显早于紫苏花岗岩（~307Ma）和碱长花岗岩（~303Ma）。岩石地球化学数据表明，花岗闪长斑岩具有较高硅、中等铝，贫钙、铁、镁，富集Rb、K、Th、U，强烈亏损Nb、Ta、Ti的特征，为钙碱性弱过铝质I型花岗岩；紫苏花岗岩更多的表现出钙碱性-高钾钙碱性镁质I型紫苏花岗岩特征；碱长花岗岩为碱性准铝质-弱过铝质A型花岗岩。锆石Hf同位素分析结果表明，花岗闪长斑岩、紫苏花岗岩和碱长花岗岩均具有高正的ε_Hf（t）值（+11.6~+15.8）和年轻的二阶段模式年龄（325~600Ma），表明其原始岩浆主要起源于亏损地幔新衍生的年轻地壳物质。综合分析认为，庙尔沟岩体花岗闪长斑岩形成于晚石炭世早期洋壳俯冲背景，由底侵的、受流体交代的幔源基性岩浆与其诱发的年轻下地壳酸性岩浆在深部混合而成。紫苏花岗岩和碱长花岗岩形成于弧后伸展背景，前者是伸展初期继续底侵于下地壳的幔源玄武质岩浆降温释放大量的水和热诱使早期侵位于下地壳的镁铁质岩石再次发生部分熔融的产物，后者是伸展后期大规模软流圈地幔上涌底垫加热年轻中下地壳使其部分熔融而成。;The Miaoergou pluton, located in southern West Junggar, is mainly composed of alkali-feldspar granites with small amount of charnockites. On the basis of previous researches, this paper mainly focuses on the newly discovered granodiorite porphyry in southwestern margin of the Miaoergou pluton. A comprehensive study of petrographical, zircon U-Pb geochronological and in-situ Hf isotopic, whole-rock geochemical analyses were carried out to determine its diagenetic age and reveal the petrogenesis, the nature and composition of the magma source of this porphyry, and furthermore, to discuss the genetic relationships among the granodiorite porphyries, alkali-feldspar granites and charnockites and their geodynamic processes. LA-ICP-MS zircon U-Pb dating for the granodiorite porphyry yielded crystallization age of 317.4±1.9Ma, defining an early Late Carboniferous magmatic event in southern West Junggar, which is significantly earlier than the ages of charnockites (~307Ma) and alkali-feldspar granites (~303Ma). The geochemical characteristics indicate that the studied granodiorite porphyries belong to cal-alkaline weakly peraluminous I-type granite, and are characterized by slightly high silicon, medium aluminum, but low ferrum, calcium and magnesium, and also enrichment in Rb, K, Th, U with pronounced negative Nb, Ta and Ti anomalies. The charnockites possess calc-alkaline to high-K calc-alkaline magnesian I-type charnockite affinities, which were previously considered A-type granites by some geologists, while alkali-feldspar granites belong to alkaline metaluminous to weakly peraluminous A-type granites. In-situ zircon Hf isotope analyses show that these granites have high positive ε_Hf(t) values (+11.6~+15.8) and very young two-stage Hf model ages of 325~600Ma, indicating that the magma was sourced from juvenile crust originated from depleted mantle during the Late Neoproterozoic to Early Carboniferous period. In association with previous analysis, it is proposed that the Miaoergou granodiorite porphyries were formed in an oceanic crust subduction setting during early Late Carboniferous. They are probably the products of the acidic magma formed by partial melting of the juvenile lower crust and mixed with the underplated basaltic magma derived from the fluid metasomatic mantle wedge in the deep crust to some extent. Both the charnockites and alkali-feldspar granites were formed in a back-arc extensional setting related to the northwestward subduction of the Junggar oceanic crust. The charnockites may be generated by re-melting of juvenile lower crust mafic rocks, probably triggered by addition of water and heat released from cooling of underplated depleted mantle-derived basaltic magma during the incipient rifting stage of the back-arc basin, and the alkali-feldspar granites may be generated by partial melting of the juvenile middle-lower crust induced by the large-scale upwelling of asthenosphere during the later stage of the back-arc extension.

Textural, trace elemental and sulfur isotopic signatures of arsenopyrite and pyrite from the Mandongshan gold deposit (west Junggar, NW China): Implications for the conditions of gold mineralization

The Mandongshan gold deposit, located in the west Junggar region (Xinjiang Province, NW China), is hosted by Carboniferous basalt, tuff, and diabase. The gold ores at Mandongshan gold deposit take the form of hydrothermal veins cutting and disseminations within the altered rocks that host the deposit. A characteristic feature of the Mandongshan is the presence of abundant arsenopyrite. Two gold events have been recorded at Mandongshan: (I) an initial quartz-sulfide stage, and (II) a subsequent calcite-sulfide stage. In situ LA-ICP-MS analyses suggest that arsenopyrite of stage I has higher Te and lower Pb, Bi, Au, Ag, and Cu concentrations than arsenopyrite of stage II. Nano-SIMS δ34S values of pyrite associated with arsenopyrite of stage I and II fall within two ranges of −7.6 to −1.5‰ and −7.8 to −1.6‰. The presence of titanite and lack of rutile in the gold ores were taken to imply that the Mandongshan gold deposit formed from reduced fluids. Titanite crystals recorded in the gold-bearing quartz-sulfide veins show strongly positive Eu anomalies (Eu* = 2.15–3.99) and weakly positive Ce anomalies (Ce* = 1.04–1.58). The δ34S values of pyrite provide evidence for a magmatic sulfur source, which is consistent with the geochemical data obtained from other nearby gold deposits as well as the spatial relationship in the west Junggar region between gold mineralization and widespread felsic intrusions. In addition, our analyses suggest that the native gold of stage I was deposited due to phase separation of the ore-forming fluid. Native gold deposition of stage II can be further subdivided into stages IIa and IIb. The former stage is represented by coarse-grained gold that co-precipitated with arsenopyrite, while stage IIb is represented by fine-grained gold that precipitated as a result of dissolution and re-precipitation process occurring in earlier-formed stage IIa arsenopyrite grains. This model is consistent with our phase equilibrium calculations for the As-Au-Cu-Fe-S-O system of the Mandongshan gold deposit.

Data for : The role and significance of juvenile sediments in the formation of A-type granites, West Junggar oceanic arc (NW China): zircon Hf-O isotopic perspectives

Data for : The role and significance of juvenile sediments in the formation of A-type granites, West Junggar oceanic arc (NW China): zircon Hf-O isotopic perspectives

The role and significance of juvenile sediments in the formation of A-type granites, West Junggar oceanic arc (NW China): Zircon Hf-O isotopic perspectives

Oceanic arc subduction systems are the loci of substantial recycling of oceanic crust and production of juvenile arc crust that differentiates to more evolved felsic crust. Inevitably, some juvenile sediments are subducted with the oceanic crust. However, distinguishing the incorporation of juvenile sediments in oceanic arcs is not always straightforward, because they may not measurably shift many geochemical signatures, such as Sr and Nd isotopes, of oceanic arcs. Nevertheless, combined zircon U-Pb, Hf, and O isotope data can provide a powerful tool to decipher sedimentary flux into oceanic arc magmas, and here we report a case study for the late Paleozoic A-type granites from the West Junggar oceanic arc in the southern Central Asian Orogenic Belt. These plutons contain hastingsite and iron biotite diagnostic minerals and have high alkali, FeOT/MgO, Zr, and Ga/Al, but possess low CaO contents, and strongly negative Eu, Sr, and Ba anomalies, demonstrating their close affinity with A-type granites. Zircon U-Pb analyses indicate that these A-type granites emplaced in the Late Carboniferous to Early Permian (ca. 307−298 Ma). Their high zircon εHf(t) values (+12.4 to +15.5), suggest that the magmas were derived from a mantle or juvenile crustal source. However, their δ18Ozrn (+7.2‰ to +11.9‰) values are significantly higher than that of the mantle, and modeling using Hf-O isotope and rare earth element data indicate the assimilation of sedimentary materials at a proportion of ∼50%. Our data suggest that juvenile sediments (e.g., greywacke) played an important role in the formation of the studied A-type granites. The re-melting of sedimentary material induced by the late Carboniferous ridge subduction can promote the transition from an intra-oceanic arc to continental crust. Our results show that the subduction and re-melting of juvenile sediments in oceanic arc systems could be an important mechanism for the maturation of oceanic arc crust.

Genesis of the volcanic-related Be-U-Mo Baiyanghe deposit, West Junggar (NW China), constrained by mineralogical, trace element and U-Pb isotope signatures of the primary U mineralisation

The Volcanic-related Be-U-Mo Baiyanghe deposit is located in the Paleozoic Xuemisitan Volcanic Belt in West Junggar (NW China). It is the largest Be deposit in Asia which is dominantly hosted in late Devonian rhyolitic tuff and in the late Carboniferous Yangzhuang granite (313 ± 2 Ma) that were emplaced after the closure of the Junggar oceanic domain. The late Devonian rhyolitic tuff is a strongly fractionated peralkaline A1-type volcanic rock with a major magmatic enrichment in Nb, Y, Th, Be, Ta and U. Highly fractionated REE patterns and significant concentrations of Pb, Nb, Zr, Y, Th and Ta in the primary U mineralisation identified these tuffs as the dominant U and probably Be source for Be-U ore genesis in West Junggar. The primary high U concentrations (6.5 < Umean < 15.0 ppm) were released through volcanic glass devitrification and mobilised by oxidising fluids for the formation of hydrothermal mineralisation. The Yangzhuang granite is to the contrary a moderately fractionated moderate- to high-K calc-alkaline A2-type granite characterised by low magmatic U and Be enrichment (Umean = 2.9 ppm; Bemean = 4.7 ppm), and thus constituted a minor U source for the mineralisation as demonstrated by U release and remobilisation from metamict U-bearing accessory minerals.In the Baiyanghe deposit, structurally-controlled primary U mineralisation was temporally constrained by an in situ U-Pb isotopic minimum crystallisation age of 240 ± 7 Ma on pitchblende. This event represents the hydrothermal stage of U mineralisation that occurred during the early Permian–middle Triassic post-accretion event in West Junggar and paragenetically postdates the Be ore stage (ca. 303–265 Ma). The hydrothermal U mineralisation is mainly characterised by Pb-Nb-Ti-Zr-Mo-rich pitchblende veins associated with calcite and quartz, which crosscut Be-stage fluorite veins. During this episode of crustal extension, the Be-U-Mo ore therefore formed in a two-stage hydrothermal model within the same mineralising event: (i) oxidising meteoric and possibly basinal waters percolated downward into the basement through structures and mixed with CO2-bearing magmatic fluids derived from early generations of mafic dykes that intruded the host rocks in the Baiyanghe area. During the circulation of these thermal solutions along faults and fractured zones, Be was leached mainly from the Devonian rhyolitic tuff during hydrothermal alteration and dominantly transported as fluoride complexes in the fluids. These low-temperature (130–150 °C) ore solutions then reacted with Ca-rich minerals of the host rocks, which resulted in a pH increase that triggered fluorite precipitation in turn promoting Be deposition as bertrandite; and (ii) in a second stage, additional oxidising surface-derived fluids including meteoric waters and possibly basinal brines infiltrated the basement. U was also leached mainly from the late Devonian rhyolitic tuff and likely transported as chlorine and/or uranyl carbonate complexes in the fluids. U precipitated as pitchblende due to fO2 decrease when these low-temperature (120–125 °C) oxidising U-bearing solutions encountered favourable reducing environments mainly represented by dolerite and diabase dykes crosscutting the host rocks and/or carbonaceous shales at the contact with the Devonian rhyolitic tuff. Pitchblende then locally experienced dyke-emplacement related post-ore hydrothermal potassic alteration characterised by abundant illite crystallisation, significant Si gain and in situ relative enrichment of Nb, Ti, Th, and Ta due to strong U and Pb depletion. The primary U mineralisation was then largely oxidised into uranophane due to supergene alteration related to further meteoric water infiltration during late Mesozoic–Cenozoic crustal extension in West Junggar.The characteristics of the volcanic-related hydrothermal Be-U mineralisation in the Baiyanghe deposit show key similarities with Streltsovka (U-Mo-F) and Spor Mountain (Be-U) world-class districts, although Baiyanghe has much smaller resources than the ones in Streltsovka.

Late Paleozoic Element Migration and Accumulation under Intracontinental Sinistral Strike‐slip Faulting in the West Junggar Orogenic Belt, NW China

AbstractThe migration, accumulation and dispersion of elements caused by tectonic dynamics have always been a focus of attention, and become the basis of tectono‐geochemistry. However, the effects of faulting, especially strike‐slip faulting, on the adjustment of geochemical element distribution, are still not clear. In this paper, we select the West Junggar Orogenic Belt (WJOB), NW China, as a case study to test the migration behavior of elements under tectonic dynamics. The WJOB is dominated by NE‐trending large‐scale sinistral strike‐slip faults such as the Darabut Fault, the Mayile Fault, and the Baerluke Fault, which formed during the intracontinental adjustment under N–S compression during ocean‐continental conversion in the Late Paleozoic. Geochemical maps of 13 elements, Al, W, Sn, Mo, Cu, Pb, Zn, As, Sb, Hg, Fe, Ni, and Au, are analyzed for the effects of faulting and folding on element distribution at the regional scale. The results show that the element distribution in the WJOB is controlled mainly by two mechanisms during tectonic deformation: first is the material transporting mechanism, where the movement of geological units is consistent with the direction of tectonic movement; second is the diffusion mechanism, especially by tectonic pressure dissolution driven by tectonic dynamics, where the migration of elements is approximately perpendicular or opposite to the direction of tectonic movement. We conclude that the adjustment of element distributions has been determined by the combined actions of transporting and diffusion mechanisms, and that the diffusion mechanism plays an important role in the formation of geochemical Au blocks in the WJOB.

Tectonic‐magmatic framework identified by remote sensing analyses and geochronology in West Junggar, Central Asian Orogenic Belt

The temporal and spatial evolutions of the Darbut Fault deformation patterns were analysed for multiple deformation stages in this study by using a regional geological survey, quantitative remote sensing image analyses, and structural analyses. Remote sensing images indicating that the Karamay granitoid batholith is greenish‐grey, the Hongshan granitoid batholith is yellowish‐green along the Darbut Fault, and the Miaoergou, Akebasitao, and Hatu granitoid batholiths are round in shape and luminous yellow colour. The remote sensing images of the Darbut Fault, Hongshan granitoid batholith, and data from previous research suggest that the upright folds (F1) in the Carboniferous strata near the Darbut Fault and northeast‐southwest (NE‐SW) striking sub‐vertical dextral shear zones in the mélanges represent the regional dextral strike‐slip shear zones. The NE‐SW striking transcurrent sinistral strike‐slip faults and ‘S‐shaped’ drag folds (F2) overprinted the dextral shear zones. The Hongshan batholith is a complex pluton, which was composed of alkaline feldspar granite, syenite granite, and monzogranite. The LA‐ICP‐MS U–Pb 311–307 Ma of zircons from the ductilely deformed Hongshan alkaline feldspar granite and monzogranites are formed during the dextral shear zone of the Darbut Fault. The 302 Ma Hongshan alkaline feldspar granite is contemporarily formed during the sinistral strike‐slip stage of the Darbut Fault. The Hongshan complex pluton has a prolonged formation age and witnesses the whole process of ocean‐land conversion from the capture of island arc‐related rocks, granite formed in the post‐collision stage, and dyke swarms formed in the intracontinental extension stage.

Insights into Carboniferous subduction-related petroleum systems in the Central Asian Orogenic Belt (CAOB) from hydrocarbons in vein calcite cements, West Junggar, northwest China

Large Carboniferous petroleum systems exist in the Central Asian Orogenic Belt (CAOB), including the South Caspian and Junggar basins. In this study, we investigated one such petroleum system in the range belt by analyzing hydrocarbons in vein cements in the Daerbute fault zone that cuts Carboniferous strata in West Junggar, northwest China. Our multiple analytical methods included conventional petrographic descriptions, fourier transform infrared (FTIR) spectroscopy, and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The results show that light hydrocarbons (C1–C7) are present in the vein calcite cements, which are characterized by bright yellow–green fluorescence, FTIR functional groups (i.e., ν [CH3], ν[CC]arom, and ν [C–H]arom), aliphatic hydrocarbon-related ion segments (i.e., [CnH2n+1]+, [CnH2n-1]+, [CnH2n-2]+, and [CnH2n-3]+; n < 7), and aromatic hydrocarbon-related compounds (i.e., C2-benzenes, C3-benzenes, and naphthalene). These hydrocarbons were sourced mainly from highly mature Carboniferous source rocks, and the subduction-related heat flow promoted hydrocarbon generation. This study provides the first insights into a subduction-related Carboniferous petroleum system in the CAOB range systems, which has significance for further petroleum exploration in orogenic belts.