Southern West Junggar Research Articles

Silurian–Devonian construction of active continental margin in the southwestern Paleo-Asian Ocean witnesses the modification of fluvial system by early land plants

In accretionary orogeny, the intra-oceanic subduction-accretion could generate juvenile continental crustal materials and form new continental blocks by their lateral tectonic accretion, and the juvenile continental crust would provide a sialic basement for continental margin arc during the subsequent oceanic-continental subduction-accretion. These two distinct subduction-accretion processes occurred successively in the southwestern part of the Paleo-Asian Ocean during the Neoproterozoic–Late Paleozoic and created a multi-stage evolution of continental crusts in West Junggar. In this study, a Late Silurian landmass in West Junggar is revealed by the detailed geological mapping and comparison of the Late Silurian–Early Devonian angular unconformity in southern and northern West Junggar. This crucial unconformity confines the Middle Silurian termination of Early Paleozoic intra-oceanic subduction-accretion, as evidenced by the pre-Late Silurian emplacement of Cambrian–Ordovician ophiolitic mélanges, the Middle Silurian cessation of Early Paleozoic arc magmatism and the Late Silurian–Early Devonian sedimentary hiatus. The Late Silurian landmass was consequently constructed by amalgamations of island arcs, accretionary complexes and seamounts, and its break-up was marked by the Middle Devonian opening of the Karamay back-arc Ocean in central West Junggar. In contrast with the Early Paleozoic intra-oceanic arc systems, continental margin arcs on the sialic basement of rifted landmass emerged since the Middle Devonian, as suggested by our new chronological data and field observations. We further unravel that the increase in mudstone of Devonian successions on the active continental margin is contemporaneous with the emergence and boom of lycopsid forest in West Junggar, which is in agreement with a global trend in alteration of fluvial systems due to evolution of early land plants. In summary, this paper presents a typical record for two distinguished styles of subduction-accretion systems that developed successively in an accretionary orogeny, and sedimentary successions of the later active continental margin documented the interaction between terrestrial sedimentation and early land plants.

Construction of an Early Cambrian intra-oceanic arc within the West Junggar, NW China: Magmatic records from proto- to mature-arc

The Junggar orogenic collage in Central Asia is characterized by the accretion of multiple Paleozoic intra-oceanic arc terranes developed within the Junggar Ocean, yet magmatic records relating to the construction of these ancient oceanic arcs are largely disrupted. Here, we report new petrological and geochemical data of mafic-ultramafic rocks documented from the Early Cambrian Saleinuohai ophiolitic mélange in southern West Junggar. The mafic rocks include dolerites and basalts, which show geochemical features (e.g., lower TiO2 and Zr relative to MORB, depleted LREE, and high εNd(t)) resembling those of forearc basalts from the Izu-Bonin-Mariana arc. Thus, they may represent the upper section of newly-accreted oceanic crust formed during subduction initiation, upon which a nascent arc is soon built. In contrast, the ultramafic rocks include dunite, wehrlite, and clinopyroxenite, which display cumulate textures with a crystallization order of olivine-clinopyroxene-plagioclase. Similar cumulate sequences are commonly observed in island-arc environments, presumably crystallized under a relatively hydrous and high-pressure condition. The calculated equilibrium melts for these cumulates further resemble those adakitic rocks previously reported from the Saleinuohai ophiolitic mélange. Thus, they may represent the basal section of mature island arc crust that formed during intracrustal fractionation. Our data, together with previous results, provide a complete magmatic record of the development of an Early Cambrian intra-oceanic arc within the Junggar Ocean.

Multiple ridge subduction processes in the southern Altaids: Implications from clinopyroxene chemistry and Sr–Nd–Hf isotopes of late carboniferous Nb-enriched, magnesian diorite-andesites in West Junggar, NW China

Magnesian (Mg# > 45) and/or Nb-rich (Nb > 6 ppm) dioritic-andesitic rocks have been identified in the Mayile area of southern West Junggar, Northwest China. A systematic petrological, geochronological, whole-rock geochemical, Sr–Nd–Hf isotope, and clinopyroxene chemistry study was conducted to constrain the petrogenesis, mantle source, and P–T conditions of magmas as well as the tectonomagmatic processes in the southern Paleo-Asian Ocean subduction zone. Mayile dioritic-andesitic rocks generated in the Late Carboniferous (306–312 Ma) have a high SiO2 content (52.3–63.2 wt%) and high Mg# (46–66) as well as sanukitic-like low LaN/YbN and Sr/Y ratios. They have high Nb contents (6.27–11.6 ppm) and Nb/U (9–18) ratios and are Nb-enriched sanukitic high-Mg andesites (HMAs). Clinopyroxene thermobarometry indicates that the parental magma of the Mayile HMAs experienced a high-temperature (1137–1199 °C) and shallow-level (0.8–0.17 GPa) partial melting in a mantle wedge. They have εNd(t) (+5.14 to +6.42), εHf(t) (+8.8 to +14.4), and zircon εHf(t) (+9.4 to +15.8) values and a slightly high (87Sr/86Sr)t ratio ranging from 0.703214 to 0.704140 relative to normal mid-ocean ridge basalt (N-MORB). The results suggest that West Junggar was dominated by continuous records of sanukitic rocks hinting at multiple ridge subduction processes in the southern Altaids. We reconstructed the subduction history of the Junggar Ocean in the Carboniferous-Permian from these particular observations in West Junggar, and gained insight into the gradual evolution of the tectonic setting, which transitioned from normal intraoceanic subduction to hot subduction of the young oceanic lithosphere from the Early to Late Carboniferous, with subsequent ridge subduction occurring mainly in the Late Carboniferous.

Late Palaeozoic final subduction records of the southwestern Paleo-Asian Ocean revealed by ca. 286–283 Ma basaltic rocks in the West Junggar, NW China

ABSTRACT The late Early Permian basaltic rocks in the West Junggar of northwest China provide a critical geological record for revealing the termination of the Late Palaeozoic final-stage subduction history of the southwestern Palaeo-Asian Ocean. With the aim of providing constraints on the late stage of the Late Palaeozoic geodynamic evolution of the southwestern Altaids, an integrated study of detailed field observation, geochronology and geochemistry has been carried out on the Early Permian basaltic rocks from the Hala’alate Mountain of the southern West Junggar. Zircon LA-ICP-MS U-Pb dating of four basaltic andesites suggests that they were erupted during the late Early Permian with ages of ca. 286–283 Ma, coeval with the neighbouring mafic diabase stock and gabbroic dyke swarms. The basaltic rocks have low SiO2 and moderate MgO contents, with low (87Sr/86Sr)i ratios (0.70397–0.70447) and high positive εNd(t) values (+5.94–+7.19), and also show remarkable enrichment in large ion lithophile elements (Ba, U, K, Pb, Sr) and prominent depletion in high field strength elements (Nb, Ta, Zr, Hf, Ti), indicating that they were originated from the partial melting of a garnet-dominant with minor spinel lherzolitic depleted mantle source previously metasomatized by slab-derived fluids and subducted sediments. These basaltic rocks were most likely formed in a fore-arc extensional setting in response to the slab retreat of the Junggar oceanic slab during the mid-late stage of Early Permian. Combined with previously published regional achievements, our new results support that the down-going of the remnant oceanic slab and the resultant arc magmatic activity of the southern West Junggar probably continued until the ca. 283 Ma, and the closure of the West Junggar branch of the Palaeo-Asian Ocean should be later than the late Early Permian.

Petrochemical and geochronological data of Permian-Lower Triassic clastic sedimentary rocks in the northwestern Junggar basin, NW China: Implications for provenance, tectonism and paleoclimate

The Permian clastic sediments in the Mahu sag offer outstanding conditions to test the control of tectonism and paleoclimate on sedimentation in a rift basin. A synthesis of petrological and geochemical analyses of sandstones, zircon U–Pb geochronology of granitic conglomerates, and principal component analysis have been used to investigate the source composition and chemical weathering indices. The rare earth and trace element compositions of 16 sandstone samples are consistent with first cycle sources from the Carboniferous–Lower Permian mafic and felsic igneous rocks in West Junggar. Lower Permian sandstones contain abundant volcanic lithic clasts and plagioclase grains, and have high Na2O, CaO, Sr and V, suggesting a coeval proximal volcanic source in Central West Junggar during the syn-rift stage. Middle Permian sandstones consist of trachyte and rhyolite clasts, and quartz and albite grains, and show a gradual increase of Th, U and SiO2, indicating an expansion of source areas to more distant Southern and Northern West Junggar during the post-rift stage. In the Lower Triassic, an abrupt increase in the percentages of quartz and K-feldspar, as well as the zircon U–Pb geochronology of granite-clast conglomerates (290–330 Ma), all indicate regional uplift and unroofing of Carboniferous–Permian granitic plutons in West Junggar during the tectonic inversion stage at the Permian-Triassic boundary. Various chemical weathering indices indicate cold and arid climate conditions during the early Permian, followed by a transition to warm and humid climatic conditions. Comprehensive analysis of petrography and geochemistry sheds a new light for studying the source-to-sink evolution of a rift basin, and has further implication for paleoclimate fluctuation and change.

Early-Middle Devonian adakitic magmatism generated by slab retreat in southern West Junggar, NW China: implications for tectonic correlation with central and East Kazakhstan

ABSTRACT The West Junggar terrane (WJT) is known to tectonically correlate with the Central and East Kazakhstan terrane; however, the Middle Palaeozoic magmatism has rarely been reported in the southern WJT, leading to significant uncertainties on the tectonic evolution and correlation of the southwestern Central Asian Orogenic Belt (CAOB). Here, we present the adakitic quartz diorites with zircon U-Pb ages of 390–394 Ma in the Alashankou region to demonstrate the presence of Early-Middle Devonian magmatism in the southern WJT for the first time. They are characterized by low Yb (1.00–1.41 ppm) and Y (8.9–14.4 ppm) but high Sr (301.5–475.9 ppm), Sr/Y (23–41), and (La/Yb)N (8.1–11) values, typical of modern adakites. As suggested by the relatively high SiO2 (64.96–67.71 wt.%), moderate K2O (1.53–3.19 wt.%), but low MgO (1.56–2.31 wt.%), TiO2 (0.28–0.42 wt.%), Cr (≤13.7 ppm), Co (2.7–9.9 ppm), Ni (8.0–18.7 ppm), and Th (3.7–4.8 ppm) concentrations as well as the high positive ɛHf(t) values (+5.6 to +10.3) and low Nb/Ta ratios, the adakitic quartz diorites were probably generated by the partial melting of a slightly thickened lower crust, which consisted mainly of the Ediacaran-Middle Palaeozoic intra-oceanic arc rocks in the southern WJT. Combined with regional data, our new results support that the southern WJT probably evolved as the eastern extension of the Kazakhstan Early-Middle Devonian outer volcanic belt during the Middle Palaeozoic. The Early-Middle Devonian tectonic extension and resultant lower crust melting may thus be related to the slab retreat of the Junggar-Balkhash Ocean.

The Evolution of Permian Source-to-Sink Systems and Tectonics Implications in the NW Junggar Basin, China: Evidence from Detrital Zircon Geochronology

The basin type of the Junggar Basin changed during the Permian, but the time constraint of the tectonic evolution remains unclear. Besides, the fan deltas developed in the Permian in the Mahu Sag in the northwestern of the oil-rich basin. However, the provenances of the sedimentary systems remain unclear. Based on petrology and detrital zircon U-Pb ages, this study investigates the source-to-sink systems evolution and tectonics implications. Abundant lithic clasts in sandstones with low compositional and textural maturity imply proximal sources. The dating results showed a dominant peak (310–330 Ma) and a secondary peak (400–440 Ma) in the northern Mahu Sag, only one peak at 295–325 Ma in the central Mahu Sag, several peaks at 270–350 Ma in the southern Mahu Sag, and multiple peaks at 370–450 Ma in the Zhongguai Uplift. Thus, the north-western Junggar Basin was divided into four major source-to-sink systems, with adjacent central West Junggar as the main provenance and northern and southern West Junggar as the secondary provenance. The proportion of sediment supply from the southern and northern West Junggar is higher during the Middle-Late Permian. It suggests that the source-to-sink systems show inheritance and evolve from a single provenance into a complex provenance, indicating the uplift of West Junggar. The tectonic inversion may occur early in the Middle Permian and the response to tectonic activity is stronger in the southern West Junggar than in the northern West Junggar.

Source-to-sink evolution of syn-rift alkaline lake sediments in the Lower Permian Fengcheng Formation, Junggar Basin, NW China: Evidence from petrology, detrital zircon geochronology and geochemistry

• Chemical weathering intensity increased upwards in the ∼ 280 Ma Fengcheng Formation. • Detrital material of the FC Formation was supplied by the West Junggar, due to active normal faults. • Syn-rift sediments were deposited in an alkaline lake under arid climatic conditions. Source-to-sink evolution of intracontinental rift basins is an effective tool to evaluate syn -rifting tectonic processes and sediment dispersal. Provenance analysis, using petrological and geochemical analysis and detrital zircons U-Pb dating of the Lower Permian Fengcheng Formation, provides new insights into the provenance of alkaline lake deposits in the Mahu sag, Junggar Basin. Volcanic detritus and detrital feldspar grains are abundant, indicative of proximal sources. The rare earth and trace element characteristics of three sandstone samples are consistent with a dominant source from the Carboniferous – early Permian mafic and felsic rocks in West Junggar. Sandstone geochemistry suggests that chemical weathering intensity increased upwards, probably in response to reduced aridity. The detrital zircon U-Pb age patterns show a prominent Carboniferous peak (340–300 Ma), minor early Permian (∼280 Ma) and Silurian peaks (440–420 Ma). Quantitative unmixing modelling suggests that most detrital zircons were derived laterally from the Central West Junggar (CWJ), and a few zircons were derived axially from the Zharma-Saur and Boshchekul-Chingiz arcs in Northern and Southern West Junggar. The lateral source was more important in the Middle Member than in the Lower Member of the Fengcheng Formation. The average age of the youngest five detrital zircons and the low weathering indices indicate that the maximum slip of the bounding normal fault occurred at 284–?279 Ma. Seismic and hydrothermal activity caused by the normal fault displacement, as well as the increase in the supply of volcanic composition from the lateral CWJ, played an important role in the alkaline lake deposition.

Geochronology, geochemistry, and Sr-Nd isotopes of Early Carboniferous magmatism in southern West Junggar, northwestern China: Implications for Junggar oceanic plate subduction

Geochronology, geochemistry, and Sr-Nd isotopes of Early Carboniferous magmatism in southern West Junggar, northwestern China: Implications for Junggar oceanic plate subduction

Ediacaran‐Cambrian intra‐oceanic arc volcanic rocks in southern West Junggar, NW China: New constraints on the initial subduction of the Junggar‐Balkhash Ocean and migration of arc magmatism

As numerous blocks of the Ediacaran‐Cambrian supra‐subduction zone ophiolites, seamounts, subduction‐related metamorphic rocks, and arc granitoids have been identified in the ophiolitic mélanges in the contiguous West Junggar (China) and Balkhash (Kazakhstan) area, the initial subduction of the Junggar‐Balkhash Ocean, a major branch of the southern Palaeo‐Asian Ocean is emerging as an interesting topic. In this study, we present new data from diabase and basalt xenoliths in an Ediacaran‐Early Cambrian quartz‐monzonite block within the Tangbale ophiolitic mélange and an isolated dacitic tufflava block within the Saleinuohai ophiolitic mélange in southern West Junggar, NW China, as another line of evidence for the Ediacaran initial subduction of the Junggar‐Balkhash Ocean and the Cambrian migration of arc magmatism in West Junggar. The diabase and dacitic tufflava yield concordant zircon U–Pb ages of 568 and 530 Ma, respectively. The diabase and basalt exhibit high Al2O3 (17.38–17.75 wt%) and low TiO2 (0.69–0.90 wt%), with Mg# = 47–55, whereas the dacitic tufflava is characterized by high Na2O (7.87–8.43 wt%), with Mg# = 46–49. All the samples display similar LREE and large‐ion lithophile element (LILE) enrichment and Nb, Ta, Ti, and varying Zr and Hf depletion, indicating that they were formed in an intra‐oceanic arc setting. An integration of the new and published data shows that the subduction of the Junggar‐Balkhash Ocean was initiated in the Late Ediacaran at least, followed by slab rollback and retreat of subduction zone, which would be responsible for the northward migration of arc magmatism.

Late Paleozoic multi-stage subduction accretion of the southwestern Central Asian Orogenic Belt: insights from the Late Carboniferous-Early Permian granites in the southern West Junggar, NW China

ABSTRACT The Late Carboniferous to Early Permian tectonic evolution of the South West Junggar (SWJ), northwest China, a crucial constituent in the southwestern domain of the Central Asian Orogenic Belt (CAOB), has long been the subject of debate. Here, detailed petrographic studies, new nine LA-ICP-MS zircon U-Pb ages and forty-nine whole-rock geochemical results from six granitic plutons in the SWJ were combined with previously available data to elucidate their petrogenesis and re-decipher Late Palaeozoic tectonic evolution and accretionary process of the southwestern CAOB. The studied granites can be geochemically classified as I- and A-type granites, and the former is further subdivided into I-type and highly fractionated I-type granites. The calc-alkaline I-type amphibole-bearing granites (Group I) yield crystallization ages of ca. 317–315 Ma, and exhibit depletions in HFSEs and enrichments in LILEs with no to slight Eu anomalies and low zircon saturation temperatures, indicating that the formation of these granites may be closely related to the subduction of the Junggar oceanic slab. The Group II granite porphyries (ca. 312 Ma) contain negligible amphibole, and are characterized by evolved SiO2 but low zircon saturation temperatures with sharply negative Eu, Ba, Nb, Ta, Sr, P and Ti anomalies, belonging to highly fractionated I-type granites, and were probably produced in a tectonic transition from compressional to extensional setting. In contrast, the latest Carboniferous (ca. 304–298 Ma) alkaline A-type granites (Group III) are amphibole-absent, and are similar to Group II granites in major element compositions, but exhibit higher zircon saturation temperatures, indicating that they probably resulted in a back-arc extensional setting triggered by roll-back of the subducting Junggar oceanic slab since ca. 310 Ma. In combination with regional geology and re-interpretation of previous data, a multi-stage subduction-accretion model is suggested to account for the Late Palaeozoic tectonic evolution of the southwestern CAOB. This model can plausibly interpret the differences in duration of subduction between the northern and southern domains of West Junggar and the coexistence of A-type granites with subduction-related sanukites in the SWJ during Carboniferous to Permian.

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.

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.

Detrital Zircon U‐Pb Geochronology and Provenance of the Hebukesaier Formation in the Shaerbuerti Mountains, Northern West Junggar: Implication for Devonian Subduction of the Junggar–Balkhash Ocean

AbstractLimited Devonian magmatic record in northern West Junggar leads to contrasting models on its tectonic evolution. In this study, we conducted LA‐ICP‐MS U‐Pb dating on detrital zircons of two sandstones from the Hebukesaier Formation in the Shaerbuerti Mountains. Detrital zircons with oscillatory zoning are characterized by high Th/U (> 0.3) and low La/Yb (< 0.15), indicating their magmatic origin. The youngest zircon ages of two samples are 402 ± 2 Ma and 406 ± 2 Ma, respectively, suggesting that the Hebukesaier Formation was deposited at the Early Devonian. Detrital zircon age patterns show single peaks (at ca. 424 Ma, n = 157), which indicates that these clastics were likely proximal accumulation after short distance transportation. Provenance of the Hebukesaier Formation was the Xiemisitai and Shaerbuerti Mountains. Detrital zircon ages range from 481 Ma to 395 Ma, which indicates that there was relatively continuous Early Paleozoic magmatism in the Xiemisitai and Shaerbuerti Mountains since the Early Ordovician. Age spectrums of sampled detrital zircons are distinct from those of Lower Devonian strata either in southern West Junggar or in East Junggar, which implies for individual tectonic evolution of northern West Junggar. We favor that Lower Devonian Hebukesaier Formation was developed in a fore–arc setting due to the northward subduction of the Junggar–Balkhash Ocean.

Paleozoic convergence processes in the southwestern Central Asian Orogenic Belt: Insights from U–Pb dating of detrital zircons from West Junggar, northwestern China

The West Junggar orogen, located in the southwestern Central Asian Orogenic Belt (CAOB), preserves an abundant record of tectonic processes associated with the evolution of the Junggar Ocean. In this study, we use detrital zircon U–Pb age data from Ordovician to Carboniferous sandstones in the southern and central West Junggar domains, complemented by literature data, to better constrain the tectonic evolution of the southwestern CAOB. The Kekeshayi, Qiargaye, and Laba formations in the southern West Junggar domain were deposited during the Darriwilian-Sandbian, Katian-Aeronian, and Homerian-Emsian, respectively. Detrital zircon provenances of these formations display a marked shift from the southern West Junggar domain to the Paleo-Kazakhstan Continent (PKC). This suggests that the southern West Junggar intra-oceanic arc might have gradually accreted to the northern margin of the PKC prior to the Emsian, which has significantly contributed to the lateral growth of the PKC. The Carboniferous strata, Xibeikulasi, Baogutu, and Tailegula formations, in the central West Junggar domain represent a coherent sequence of volcaniclastic turbidites and were deposited in a progressively shrinking remnant oceanic basin during the Visean to Moscovian. They contain unimodal detrital zircon distributions and are derived from the local and coeval magmatic rocks in the central West Junggar domain. We propose that the final closure of the Junggar Ocean likely occurred in the end of the Late Carboniferous in response to regional amalgamation events in the southwestern CAOB, which marks the final assembly of the Kazakhstan Orocline. The central and southern West Junggar domains underwent individual evolution in the Paleozoic, and were recombined by the significant intra-continental reworking along the large-scale strike-slip faults.

Geochronology and geochemistry of early Carboniferous basalts from Baogutu Formation in West Junggar, Northwest China: evidence for a back-arc extension

ABSTRACT Early Carboniferous magmatism of the southern West Junggar (SWJ) region is essential for understanding the accretionary processes and crustal growth in the southwestern segment of the Central Asian Orogenic Belt. A comprehensive study of geochronological, geochemical, and Sr-Nd-Pb-Hf isotopic compositions was carried out in this study for basalts from the Baogutu Formation in Sulushuoke region, aiming to provide constraints on the Early Carboniferous tectonic evolution of the SWJ, NW China. LA-ICP-MS zircon U-Pb dating of four basalt samples yielded crystallization ages ranging from 339 Ma to 347 Ma, indicating the presence of early Carboniferous volcanic rocks in West Junggar. The studied tholeiitic basalts are geochemically characterized by nearly flat REE patterns, enriched large iron lithosphere elements (e.g. K) and depleted high field strength elements (e.g. Nb, Ta, Zr, Hf) relative to those in N-MORB, resembling those of modern Mariana Trough back-arc basin basalts. In addition, they have high positive εNd(t) values (+5.86 to +6.59) with initial 87Sr/86Sr ratios in the range of 0.704783–0.705315 and variable εHf(t) values (+3.99 to +15.6). The rocks cover restricted ranges in initial Pb isotope ratios with (206Pb/204Pb)i from 17.529 to 18.289, (207Pb/204Pb)i from 15.509 to 15.627, and (208Pb/204Pb)i from 37.628 to 38.397. These signatures indicate that the tholeiitic basalts were derived from 10% to 20% partial melting of a spinel-lherzolitic N-MORB-like depleted mantle source modified by fluids released from the subducted lithosphere in a back-arc extensional setting. From our new data presented in this study and in conjunction with those from previous works, we, therefore, suggest that a back-arc basin related to the northwestward subduction of the Junggar oceanic crust was developed in the Hatu-Baogutu region during the early Carboniferous time.

Geochemical, Sr–Nd–Pb and zircon U–Pb–Hf isotopic constraints on the Late Carboniferous back-arc basin basalts from the Chengjisihanshan Formation in West Junggar, NW China

AbstractWest Junggar in the southwestern Central Asian Orogenic Belt is a critical area for the study of the Junggar oceanic basin and may also reveal tectonic evolutionary events before the final closure of the Palaeo-Asian Ocean. The sedimentary formations and paragenetic associations of the Upper Carboniferous Chengjisihanshan Formation in southern West Junggar jointly reveal a back-arc basin setting with zircon U–Pb ages of 313–310 Ma for the basaltic rocks. Geochemically, the basaltic rocks are tholeiitic with low SiO2 (47.76–52.06 wt %) and K2O (0.05–0.74 wt %) but high MgO (6.55–7.68 wt %) contents and Mg no. (52.9–58.9) values. They display slightly flat rare earth element patterns with weak positive Eu anomalies, and show enrichments in large ion lithophile elements relative to high field strength elements with negative Nb and Ta anomalies, exhibiting both N-MORB-like and arc-like signatures, similar to the back-arc basin basalt from the Mariana Trough. The high positive zircon εHf(t) and bulk εNd(t) values as well as high initial Pb isotopes, together with relatively high Sm/Yb and slightly low Th/Ta ratios imply a depleted spinel lherzolitic mantle source metasomatized by slab-derived fluids. The field and geochemical data jointly suggest that the volcanic rocks within the Chengjisihanshan Formation were formed in an intra-oceanic back-arc basin above the northwestward subduction of the Junggar oceanic lithosphere in southern West Junggar. The confirmation of the Late Carboniferous back-arc basin basalts, together with other geological observations, indicate that an arc-basin evolutionary system still existed in southern West Junggar at c. 310 Ma, and the Junggar Ocean closed after Late Carboniferous time.

Late Carboniferous to Early Permian adakitic rocks and fractionated I‐type granites in the southern West Junggar terrane, NW China: Implications for the final closure of the Junggar–Balkhash Ocean

The final closure of the Junggar–Balkhash Ocean resulted in the amalgamation of the circum‐Balkash–Junggar segments of the south‐western Central Asian Orogenic Belt, but the exact timing of its closure remains uncertain. Here, we use zircon U–Pb and whole‐rock chemical data for the adakitic rocks and fractionated I‐type granitoids in the Alashankou area of the southern West Junggar terrane (WJT) to investigate their petrogenesis and tectonic implications for the closure of the Junggar–Balkhash Ocean. The adakitic rocks were emplaced at 313–295 Ma and exhibit high Sr/Y ratios of 121–229. Their low MgO contents (0.91–2.09 wt%), Co, Cr, and Ni concentrations, initial 87Sr/86Sr ratios, and high εNd(t) values, indicate an origin from partial melting of juvenile lower crust under garnet‐amphibolite‐facies conditions triggered by basaltic underplating. By comparison, the fractionated I‐type granitic plutons (283–273 Ma) are weakly peraluminous and distinctive in terms of their low MgO contents (0.31–0.39 wt%) and strongly negative Nb, Ta, P, Eu, and Ti anomalies. These features, along with their higher initial 87Sr/86Sr ratios and lower εNd(t) values, suggest derivation by partial melting of a depleted mantle source and/or of newly underplated lower crust with crustal contamination. Given the Late Carboniferous depositional hiatus and post‐orogenic strike‐slip faulting and absence of post‐Devonian ophiolites and post‐Palaeozoic magmatic events in the WJT, the widespread occurrence of Late Carboniferous to Permian compositionally diverse granitoids (including the adakitic rocks, high‐Mg dioritic rocks, and I‐ and A‐type granitoids) and coeval mafic dykes can be best explained by a post‐collisional slab breakoff model. Together with other lines of geological evidence in the circum‐Balkash–Junggar area, we propose that the Junggar–Balkhash Ocean should have been closed at the beginning of the Late Carboniferous (~320 Ma), consistent with the closure time (~325–300 Ma) for regional major ocean basins.

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

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

Petrogenesis and geodynamic implications of Late Carboniferous sanukitic dikes from the Bieluagaxi area of West Junggar, NW China

Sanukitic dikes, mainly consisting of pyroxene diorite porphyrite, diorite porphyry, quartz diorite porphyrite and quartz diorite, were widely distributed in the Bieluagaxi area, southern West Junggar. In this contribution, we present new LA-ICP-MS zircon U-Pb ages, major and trace elements, and Sr-Nd-Hf isotopes for these dioritic dikes. These rocks formed in the early Late Carboniferous (ca. 319–315 Ma), coeval with abundant magmatic rocks in the region, and exhibit high MgO, Cr and Ni concentrations and K/Na ratios, and also show strong enrichment in large ion lithophile elements relative to high field strength elements, with pronounced negative Nb, Ta, and Ti anomalies, analogues to those of Cenozoic sanukitoids of the Setouchi Volcanic Belt. Compositionally, the sanukitic dikes are divided into two groups. Group 1 have MgO contents greater than 6 wt% (6.18–9.34 wt%), with Mg# values ranging from 58.2 to 73.7. They also have relatively low SiO2 (51.7–55.7 wt%) and Sr (356 ppm in average) contents, and low Sr/Y (15.1–25.4) ratios, but high concentrations of Cr (6.2–490 ppm) and Ni (12.8–163 ppm). Compared to the Group 1, the Group 2 samples have slightly lower MgO (2.12–5.58 wt%; Mg# = 45.8–68.5), Cr (10.1–329 ppm) and Ni (2.84–136 ppm), but relatively higher SiO2 (58.6–65.7 wt%) and Sr (304–521 ppm) concentrations and Sr/Y (19.2–41.0) ratios, making them akin to adakitic rocks elsewhere. These dikes also exhibit depleted Sr-Nd-Hf isotopic signatures, with (87Sr/86Sr)i = 0.70353–0.70480, εNd(t) = +5.03 to +7.61, εHf(t) = +10.9 to +15.3. Geochemical and Sr-Nd-Hf isotopic data suggest that the Group 1 rocks are similar to the typical sanukitoids of the Setouchi Volcanic Belt, and were likely derived from partial melting of a depleted mantle source with subducted sediment melts. While the Group 2 rocks show a remarkable geochemical affinity with the Northwestern Karamay sanukitoids, and were derived from partial melting of a depleted mantle source metasomatized by slab-derived adakitic melts and fluids. Combined with the identified and reported subduction-related magmatism, i.e., adakites, sanukitoids, Nb-enriched basalts, lagged arc volcanic rocks, and I-type granitoids in the Western Junggar region and taking into consideration the previous achievements, therefore, we consider a multi-stage subduction-accretionary orogeny model may account for the difference of subduction duration between the northern segment and southern segment of Western Junggar.