Southern Central Asian Orogenic Belt Research Articles

Lithospheric electrical structure of the Alxa Block, NW China, southern central Asian Orogenic Belt, revealed by 2D inversion of magnetotelluric data

The Alxa Block, situated between the Tarim and North China cratons, is essential to understand the tectonics of northwest China. It has undergone several tectonic cycles, including rifting from northern Gondwana in Neoproterozoic, subduction-accretion of the Paleo-Asian Ocean during Paleozoic, and intracontinental deformation in Mesozoic. Broadband magnetotelluric data covering a frequency range of 320 Hz to ∼ 10000 s was collected along two NW-SE trending lines across major tectonic units of the Alxa region to investigate the deep structures of the Alxa Block. By applying the nonlinear conjugate gradient (NLCG) inversion approach, two-dimensional (2D) electrical resistivity models were obtained at a 100 km depth. These findings indicate that the Zhusileng-Hangwula and Zongnaishan-Shalazhashan tectonic zones have high resistivity in their lower crust and upper mantle, with an overall layered high-low–high resistivity pattern. The upper crust of the Nuru-Langshan tectonic zone is primarily characterized by high resistivity values, whereas the lower crust and upper mantle contain a large-scale low-resistivity (or conductive) zone that is likely the result of partial melting. The large-scale Early–Middle Jurassic nappe structures may be connected to sub-horizontal low-resistivity anomalies in the shallow strata, which are located up to 20 km below the Yagan and Zhusileng–Hangwula tectonic zones. Below the Engger Us fault, two low-resistivity bands are interpreted as the product of bidirectional subduction and final closure of the Paleo-Asian Ocean. The Alxa Block may have subducted southwards beneath the Ordos Block, as suggested by a clear low-resistivity anomaly between the two blocks. Furthermore, partial melting may have happened for high-conductivity bodies in the upper mantle based on the modified Archie formula’s evaluation of their melting degree.

Formation of deep arc root cumulates and implications for crustal growth in the southern Central Asian Orogenic Belt

Formation of ultramafic cumulate rocks at the arc root serves as a key process to understanding the deep evolution of arc magma and crustal growth. However, the arc root cumulates are easy to delaminate due to their high density, making limited field exposure. Here, we conducted comprehensive petrographic observations and geochemical analysis of typical ultramafic rocks (peridotites and pyroxenites) in an ophiolitic complex from the central Beishan orogenic belt in the southern Central Asian Orogenic Belt (CAOB). Petrographic evidence shows that these rocks exhibit characteristic adcumulate and mesocumulate textures with intercumulus minerals such as olivine, orthopyroxene, and clinopyroxene. The mineral characteristics of ultramafic rocks, including olivine and orthopyroxene with lower NiO (0.10–0.33 wt% and 0.03–0.09 wt%, respectively) and Mg# (81–87 and 86–88, respectively), clinopyroxene with higher CaO (19.0–24.7 wt%), and spinel with lower Mg# (0–49), are distinct from those of mantle rocks but resemble cumulates. Both whole-rock and calculated equilibrium melt exhibit the compositions of island arc magma characterized by enrichment in large ion lithophile elements (e.g., Cs, U, Pb, and Sr) and depletion in high field strength elements (e.g., Nb, Ta, Zr, Hf, and Ti). The covariation of high Mg# and low SiO2 in whole rocks is akin to typical arc root cumulates from classic island arcs (e.g., Kohistan and Talkeetna). The positive zircon εHf(t) values (+13 to +16) and calculated crystallization pressure of 0.9–1.1 GPa (~30–40 km) support the formation of ultramafic cumulates in a juvenile island arc setting. These findings imply that most ultramafic rocks in the central Beishan orogenic belt are arc root cumulates instead of the mantle portion of an ophiolitic complex. Fractional crystallization models suggest that arc root cumulate formation may have influenced the evolution of intermediate-felsic rocks in this area, indicating that the differentiation of hydrous arc magma played a crucial role in large-scale crustal growth in the southern CAOB during the Paleozoic.

An enhanced chemical maturation of juvenile arc crust recorded by the Urad Houqi intrusive suite in Langshan arc, Central Asia

Continental arc magmatism is crucial in producing juvenile continental crust of andesitic to dacitic composition, yet its impact on the formation of the modern mature continental crust, which is more enriched in potassic and granitic components, remains poorly understood. We examined a suite comprised of norite gabbro, gabbroic diorite, high-Mg diorite, dioritic enclave, and medium-K to high-K granites (the Urad Houqi suite) from the Langshan arc in the southern Central Asian Orogenic Belt (CAOB). Zircon U-Pb geochronology indicates that the Urad Houqi suite was formed ca. 278−268 Ma by a continuous magmatic event, marking the final episode of the late Paleozoic continental arc magmatism in the southern CAOB induced by the subduction of the Paleo-Asian Ocean. The suite exhibits a more potassic and fertile composition compared to typical continental arc magmas, and shows a continuous increasing K2O/Na2O ratio and incompatible element concentrations, including K, Rb, Th, and U, with increasing SiO2 content. The geochemical evolution of the suite offers valuable insights into the maturation process of the juvenile arc crust. By combining zircon Hf and bulk-rock Sr-Nd-Pb isotope studies, we demonstrated that the various lithologies within the Urad Houqi suite recorded a three-stage enhancement of the geochemical fertility of the juvenile Langshan arc crust: (1) generation of the initially fertile primary melt from the mantle wedge metasomatized by the recycled subducted sediments; (2) trans-crustal open-system hybridization processes, including peritectic reaction and magma mixing, that further fertilized the derived melts in addition to fractional crystallization; and (3) diverse petrogenetic processes that contributed to the fertility of the granitic magmas, including polybaric fractionation of the primary magma, anatexis of ancient crust, and extraction of high-silica melts from the shallow mush system. The three-stage maturation of the juvenile Langshan arc crust was typically coupled with the accelerated subduction of the Paleo-Asian Ocean since the earliest Permian. Our study finds that the Urad Houqi suite can serve as an example of modern continental crust maturation at continental arc settings.

Permian tectonic switch of the southern Central Asian Orogenic Belt: Constraints from magmatism in the southern Alxa region, NW China

Abstract Late Paleozoic plutons are widely distributed in the Alxa region, southernmost Central Asian Orogenic Belt, and provided an important clue in constraint for the closing time of the Paleo-Asian Ocean (PAO). In this article, we present new zircon U-Pb ages and whole-rock geochemical data from the Permian Huoersen and Zongnaishan plutons in the southern Alxa region. The Huoersen gabbro (ca. 285 Ma) is enriched in large-ion lithophile elements and depleted in high-field strength elements, similar to the features of continental marginal arc. They were most likely generated by partial melting of depleted mantle that was modified by subduction metasomatic fluids. The Zongnaishan granites (ca. 267 Ma) show characteristics of I-type granites and were generated in a syn-collision setting. The Huoersen granites (ca. 259 Ma) are peralkaline and have positive Ɛ Nd(t) (+1.2 to +1.5) values, exhibiting A2-subtype granites affinities. They were formed by melting of lower crust in post-collision extension setting. Based on geodynamic mechanism, a three-stage evolution model is delineated: subduction, syn-collision to post-collisional extension for oceanic branch of the PAO during the Permian. In general, the rock assemblages indicate a tectonic switch from subduction to post-collision extension regimes and the final closure of the PAO.

High-Mg andesites and Nb-enriched basalts in the Dulate arc, East Junggar (NW China): Evidence of ocean ridge subduction

Abstract The Dulate arc, located in East Junggar (NW China) in the southern Central Asian orogenic belt, records a Devonian magmatic arc evolution, offering a window to understanding the orogenic processes of the Central Asian orogenic belt. Here we present new geochemical and isotopic data for Late Devonian high-Mg andesite (HMA) and Nb-enriched basalt (NEB) suites from the Qiakuerte area, East Junggar. The HMA samples are typical subduction-related volcanic rocks. They have SiO2 contents ranging from 53.30 to 54.59 wt%, high MgO (5.0–5.26 wt%), and high Mg# values (~55) and show enrichments in large ion lithophile elements (LILEs) and depletions in high field strength elements (HFSEs). The HMA samples have high (La/Yb)N ratios and Sr/Y (~6.5 and 50–59, respectively) with no Eu anomalies. The HMA samples have high Na2O (~3.3 wt%) and low K2O (~2.5 wt%) and Th (~2.4 ppm) contents, combined with positive εNd(t) and low (87Sr/86Sr)i values. These characteristics suggest that the samples were formed mainly through interactions between subducted oceanic melts and mantle peridotites. Compared to normal arc basalts, the NEB samples have higher concentrations of Nb (~20 ppm), higher primitive mantle–normalized Nb/La (0.50–0.58), and higher ratios of Nb/U (9.4–14.6). The NEB samples also have positive εNd(t) and low (87Sr/86Sr)i values, indicating that their source was mantle wedge that had been metasomatized by slab melt. Considering the widespread presence of A-type granites, the abnormally high heat flow, and the tectonic characteristics of East Junggar, we conclude that a slab window created by the subduction of an ocean ridge was responsible for the melting of slab and the formation of the NEB-HMA suites. These processes may have also played a key role in the tectonic evolution processes of East Junggar during the Late Devonian.

A uniform basement: Implications for the tectonics of Beishan Orogenic Belt in the southern Central Asian Orogenic Belt

The orogen is a significant window for studying the history and architecture of Earth; its architecture involves many significant issues. Currently, research on the architecture of orogenic belts mainly focuses on the process of orogeny, and few studies focus on the tectonics before the orogeny, which somewhat constrains orogen evolution. The nature of the basement(s) of microcontinents in the Beishan Orogenic Belt (BOB) in the middle Central Asian Orogenic Belt (CAOB) has been debated for a long time, which causes confusion about the tectonics of the BOB. This study conducted field surveys, geochronology, geochemistry, and Hf isotope analysis on the major microcontinents of BOB. The age information of ca. 1.4 Ga and the Paleoproterozoic-Archean with small scale were found in the northern BOB. The northern BOB (NBOB) and southern BOB (SBOB) (separated by the Hongliuhe-Niujuanzi-Xichangjing ophiolitic mélange belt) have experienced similar magmatic thermal events at ca. 1.4 Ga and ca. 0.9 Ga, which are revealed by the analogical tectonic setting, formation ages, and Hf isotopic features. The Precambrian basements of the NBOB and SBOB are analogous in terms of rock assemblages, detrital zircon ages, magmatic rock ages, εHf(t) of Precambrian rocks, and Hf crustal model ages TDMC of Paleozoic igneous rocks, which imply that they shared similar history and provenance and might originate from a uniform basement. The basement has no close tectonic affinity to the adjacent cratons or blocks, except the Baltica Craton and Laurentia Craton. And it might have been involved in the amalgamation of the Columbia Supercontinent and be situated on the periphery of the Rodinia Supercontinent, and then fragmented when the Rodinia Supercontinent rifted apart.

Arc-arc amalgamation during accretionary orogenesis: Insights from mid-Paleozoic tectono-magmatic records in eastern Junggar, NW China

Abstract Arc-arc amalgamation occurs during the evolution of composite orogens at convergent plate margins and plays a critical role in controlling accretionary patterns and processes. The eastern Junggar terrane in the southern Central Asian Orogenic Belt underwent a long-lived subduction-accretion process in the Paleozoic, but whether and how the Yemaquan and Dananhu-Harlik arcs were amalgamated remain debatable. A systematic U-Pb–Hf-O isotopic study was conducted on zircons from Silurian granitic rocks in the Yemaquan arc. The U-Pb dating results suggest that these rocks were emplaced at 433–422 Ma and inherited abundant 536–435 Ma zircons representing a predominant magmatic episode in the Yemaquan arc. Their positive εHf(t) values and young Hf model ages indicate that the Yemaquan arc is dominated by juvenile basement with significant crustal growth during the Neoproterozoic to early Paleozoic. The variations in zircon Eu/Eu*, εHf(t), and δ18O values reveal that the Yemaquan arc experienced remarkable crustal thickening and remobilization at ca. 450 Ma, similar to the northern Dananhu-Harlik arc, and this was followed by extension that initiated at ca. 420 Ma. These features support the amalgamation of these two arcs occurring ca. 450–420 Ma. Integrated with regional data, we correlated this amalgamation event in the eastern Junggar terrane with the orogenic event in the Chinese Altai terrane, and we propose a middle Paleozoic tectonic evolution model in the eastern Junggar–Altai area from arc assembly to dispersal in association with a transition in accretionary mode. This scenario probably took place as a response to plate reorganization during the breakup of the northern margin of Gondwana.

Petrogenesis of the early Permian Hongliujing granite complex in the Chinese Eastern Tianshan orogen: Evidence for crustal growth in the Central Tianshan microcontinent

The generation and modification of silicic magma systems are essential processes in resolving the differentiation of continental crust. This understanding motivated the geochronological and geochemical study of the early Permian Hongliujing granite complex, consisting of quartz monzonite, granite, and leucogranite in the Central Tianshan microcontinent of the southern Central Asian Orogenic Belt. Laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) zircon U-Pb dating of the Hongliujing complex rock units revealed almost identical ages (279 ± 2 Ma to 270 ± 2 Ma). The high-silica leucogranite and granite are characterized by positive Rb and negative Eu anomalies and Ba, Sr, P, and Ti depletions. The zircon trace elements are characterized by relatively low Ti and Th/U and high Yb/Gd. In contrast, the quartz monzonite and its mafic microgranular enclaves display minor negative Ba, Sr, P, Ti, and Eu anomalies, while the zircon trace elements are characterized by relatively high Ti and Th/U and low Yb/Gd. The complex has similar zircon Hf and whole-rock Nd isotopic compositions, with Hf and Nd model ages younger than 1.4 Ga, suggesting that their magmas were derived from an isotopically depleted mantle, with some contributions from crustal melts. The leucogranites further showed relatively large variations of εHf(t) and lower εNd(t) values, implying that their magma was affected by higher amounts of crustal contamination. We suggest that crystal-melt segregation was the major mechanism responsible for the evolution of the magmatic system, and that the early Permian magmatism represents a crust-forming episode triggered by slab rollback of the subducting South Tianshan oceanic plate beneath the eastern Central Tianshan microcontinent. Thus, our study reveals that microcontinents with Precambrian crustal basement were major sites of juvenile continental growth during the accretionary evolution of the Central Asian Orogenic Belt.

Early Permian Post-Collision Extensional Setting in the Southern Beishan Orogenic Belt: Evidence from the Zhangfangshan Granodiorite and the Baishantang Bimodal Volcanic Rocks

Outcrops of late Paleozoic magmatic rocks are common in the Southern Beishan Orogenic Belt (SBOB), Southern Central Asian Orogenic Belt (CAOB), which is a key object for the understanding of regional tectonism and defining the final closure time of the Paleo-Asian Ocean (PAO). We present zircon U-Pb chronology and whole-rock geochemistry data for late Paleozoic granodiorites and bimodal volcanic rocks from the Shuangyingshan-Huaniushan unit in the north Huitongshan-Zhangfangshan ophiolitic belt in the SBOB. The Zhangfangshan granodiorites (LA-ICP-MS, Ca. 288 Ma) are A2-type granite enriched in Rb, Th, Pb and LREEs and depleted in Nb, Ta, Ti, Sr, Ba and HREEs. They have varying MgO and TFe2O3 contents with high Mg# (38.56~48.97) values; the Lu/Yb ratios (0.14~0.15) of these granodiorites are similar to mantle-derived magma. A clear plagioclase zoning structure and acicular apatite occur in mineral assemblages derived from magma mixing between mafic and felsic magmas. The Baishantang bimodal volcanic rocks (272 Ma) consist of rhyolite and basaltic andesite. Baishantang rhyolites are A2-type felsic rock enriched in Rb, Th, Pb and LREEs and depleted in Nb, Ta, Ti, Sr, Ba and HREEs, with negative εNd(t) and εHf(t) (−5.2~−4.8 and −2.2~−1.9, respectively). Rhyolites originated from the partial melting of the crust, influenced by mantle material. Basaltic andesites belong to calc-alkaline series and have an enrichment of Rb, Ba, Th, U, Pb and LREEs, are weakly enriched in Zr-Hf, and are depleted in Nb, Ta, Ti and HREEs. The Nd-Hf isotopes of these basaltic andesites are not coupled with negative εNd(t) (−2.8~−0.4) and positive εHf(t) (1.8~5.5) values. These characteristics indicate that they originated from the partial melting of the mantle mixed with sediment-derived melts. In combination with previous studies, our findings show that the early Permian Zhangfangshan granodiorites and Baishantang bimodal volcanic rocks formed in a post-collision extensional setting, and the Huitongshan-Zhangfangshan ocean had been closed before early Permian.

Petrogenesis of the U-rich Permian Akkulen syenite intrusion, Tien Shan, Kyrgyzstan: insights into its magmatic evolution and geodynamic setting

ABSTRACT Permian alkaline granitoids are widely distributed throughout the Tien Shan Orogen and adjacent region of the southern Central Asian Orogenic Belt. However, their petrogenesis and related tectonic setting remain equivocal. A detailed mineralogical, U-Pb zircon dating, in situ elemental and Sr-Nd-Hf isotopic studies of the uranium-rich Akkulen syenite intrusion in the Northern Tien Shan of Kyrgyzstan were undertaken to better understand its magmatic processes and geodynamic evolution. Four independent oxygen barometers have shown that the oxygen fugacity of the Akkulen syenitic magma evolved from initial low fO2 (<NNO +1, where NNO is the nickel-nickel oxide buffer) to relatively high fO2 (>NNO +1) during magma ascent and cooling; Apatite geochemical characteristics also indicate that the magma is relatively high F (2.9–5.1 wt.%), with low Cl (0.053–0.13 wt.%) and anhydrous (<4 wt %) contents. According to whole-rock geochemical data, the syenite samples have abnormally high U (38.5–57.1 ppm) and Th (96.2–137 ppm) concentrations, we suggest that high F content and moderate oxygen fugacity are beneficial for uranium enrichment. Zircon grains from a syenite sample yielded a weighted 206Pb/238U age of 283.5 ± 2.6 Ma. The intrusion has an A-type granite affinity with high alkali (Na2O + K2O) contents and Fe/(Fe+Mg) ratios, high LREE/HREE (~5), and low Sr, Ba, and Eu contents. They have negative εNd(t) (−5.2 to −3.6) values with Mesoproterozoic two-stage model ages (T DM2 = 1.35–1.47 Ga), and variable εHf(t) values (−4.6 to +2.4) with Mesoproterozoic two-stage model ages (T DM2 = 1. 2–1.59 Ga), which suggest these rock derivations from the relatively old crustal with minor contribution from juvenile crustal melts. In comparison with regional available dataset, we propose that the alkaline Permian granitoids in Kyrgyz Northern Tien Shan may have been originated from large-scale partial melting of Mesoproterozoic metamorphic basement with minor juvenile crustal material in a post-extensional setting. In contrast, the Early Permian granitoid magmatism in the Tarim Craton may have been caused by the partial melting of Neoproterozoic basement rocks associated with Permian mantle plume activities.

Basement differences control granitoid compositions: insights from zircon Hf isotopic mapping of Paleozoic granitoids in the northern Beishan orogenic collage, NW China

ABSTRACT The source of granitoids in Phanerozoic accretionary orogens is debatable. Paleozoic granitoids are widespread in the southern Central Asian Orogenic Belt (CAOB) and are considered the most eloquent witness to continental crustal growth during the Phanerozoic. This study presents a comprehensive analysis of zircon U-Pb and Lu-Hf isotopic and geochemical data from six Paleozoic granitic plutons in the northern Beishan Orogenic Collage (NBOC). Zircon U-Pb data reveal that these plutons recorded a long-lasting magmatism from the Late Silurian to the Late Carboniferous. The elemental and isotopic data show a major contribution from the Precambrian crustal basement as the source of the Late Silurian to Carboniferous granitoids in the Mingshui – Hanshan massif (MHM). In contrast, the source compositions of the Carboniferous granitoids in the Heiyingshan arc (HYA) indicate a dominant role of juvenile materials. Combined with published data, three magmatic pulses from the Ordovician to Carboniferous were identified in the NBOC: Stage I magmatism (Early Ordovician to Late Silurian) formed in a subduction-related compressional setting, Stage II magmatism (Late Silurian to Early Devonian) occurred in an extensional setting, and Stage III (Carboniferous) formed in a compressional setting. For the MHM, the zircon Hf isotopic mapping results show that the eastern segment was predominantly composed of a Paleoproterozoic basement, whereas the western segment mainly consisted of a young Mesoproterozoic – Neoproterozoic basement. The Heiyingshan magmatic rocks were probably formed as a continental arc during the Mesoproterozoic – Neoproterozoic and accreted to the MHM during the closure of the Paleo-Asian Ocean. Overall, these data indicate that the NBOC had remnants of the ancient Precambrian basement during the Paleozoic. Accordingly, numerous Phanerozoic granitic rocks in the NBOC have mostly resulted from the recycling of ancient crustal materials along with the addition of minor new juvenile materials. Our study has highlighted that systematic basement differences can exert a major influence on the nature of granitoids.

Mesozoic Thermo-Tectonic Evolution of the Western Altai Orogenic Belt (NW China): Insights from Low-Temperature Thermochronology

Abstract The Meso-Cenozoic tectonic activities of the Central Asian Orogenic Belt (CAOB) played an important role in controlling the present-day topography of Central Asia. The Altai orogenic belt is a key component in the southern CAOB; so far, there is still a lack of sufficient constraints on the time and mechanism of its tectonic reactivation since the Mesozoic. In this contribution, we present new zircon and apatite (U-Th)/He and apatite fission track thermochronological data from granitoid samples in the Habahe area, western Altai orogenic belt. Therein zircon (U-Th)/He ages range from ~230 to ~238 Ma, apatite fission track central ages are ~140–157 Ma, and apatite (U-Th)/He ages vary from ~134 to ~149 Ma. Based on the associated thermal history modeling results, the Habahe area underwent a moderate cooling during the Late Triassic to Middle Jurassic (~230–170 Ma) with a cooling rate of ~0.8–1.1℃/Ma and a subsequent moderate to slightly rapid cooling stage during the Middle Jurassic to Early Cretaceous (170–130 Ma) with a cooling rate of ~1.5–2.3℃/Ma. We propose that this prolonged cooling stage occurred under a long-lasting contractional tectonism in the western Altai throughout the early Mesozoic, which was produced by multiplate convergence in East Asia during this period, mainly including the consumption of the Mongol-Okhotsk Ocean in the northeast and the Meso-Tethys Ocean in the south. The region experienced rather limited Late Cretaceous-Cenozoic cooling and exhumation due to insufficient reactivation and weak surficial erosion.

Multiple tectonic switches in the Eastern Tianshan, Central Asian Orogenic Belt: Implications for crustal growth and metallogenesis

Subduction zones are crucial sites for inputs of juvenile crust and the generation of porphyry copper deposits by arc magmatic processes that generally involve episodic trench advance and retreat. It is well documented that the generation of juvenile crust is associated with lithospheric extension during the retreat of subduction zones, whereas crustal reworking is associated with lithospheric compression in continental arcs during the advance of subduction zones. However, it remains unclear whether such models can be applied to island arc settings and how they relate to metallogenesis. To address this issue, we examine intermediate–felsic igneous rocks (∼460–300 Ma) of the Dananhu–Harlik island arc in the southern Central Asian Orogenic Belt. Our new and compiled data indicate three magmatic episodes in the Dananhu–Harlik island arc during the northward subduction of the Kangguer oceanic plate, with each involving a tectonic switch from compressional trench advance to extensional retreat. Crustal growth occurred mainly during the compressional stages of trench advance, which is in contrast with typical continental arcs. Tectonic transitions also controlled regional mineralization patterns, and the typical porphyry and related skarn-type deposits formed during subduction zone advance, whereas volcanogenic massive sulfide (VMS) deposits and overprinting mineralization were generated during subduction zone retreat. A combination of all data on tectonic settings and physical–chemical conditions (e.g., temperature, oxygen fugacity, and water content) of the pre-mineralization and ore-related intrusions led us to the conclusion that normal intra-crustal arc magmatic processes, involving sulfide pre-enrichment and reactivation, generated the typical Tuwu–Yandong porphyry Cu deposits in the Dananhu arc without the prerequisite of slab melting. Our study provides an excellent example of crustal growth in an island arc setting as well as metallogenesis during the repeated advance and retreat of a subduction zone.

Devonian continental arc magmatism in the southern Central Asian Orogenic Belt: Evidence from the Dunhuang Block, NW China

Reconstruction of periods and sources of continental arc magmatism provides critical insights for deciphering the thermal evolution and geodynamics of the continental lithosphere and unveiling subduction-related episodes of accretion and orogeny. The Dunhuang Block, located in the southern Central Asian Orogenic Belt, is built upon a Precambrian metamorphic crystalline basement that is intruded by numerous Paleozoic granitoids. In this paper, we present new zircon U-Pb ages, whole-rock geochemistry and Sr-Nd-Pb-Hf isotopes from the Hanxiakouzi and Shuangta granodiorite plutons outcropped in the central and northern Dunhuang Block, respectively. The U-Pb zircon dating indicate that the Hanxiakouzi and Shuangta granodiorites were emplaced at ca. 366–363 Ma, i.e., in the late Devonian. Geochemically, the Hanxiakouzi granodiorites are medium- to high-K, I-type granites possessing high Sr and Sr/Y, i.e., similar to adakites. The Hanxiakouzi granodiorites show negative εNd(t) values (−8.2 to −6.6), significant positive εHf(t) values (+0.7 to +8.1), and evolved Pb isotope features suggesting their derivation by melting of subducted terrigenous sediments, interaction of melts with mantle peridotite, and assimilation of ancient crust during magma ascent. The Shuangta granodiorites belong to high-K calc-alkaline I-type granites with medium Sr and low Sr/Y. The Shuangta granodiorites yield negative εNd(t) values (−2.9 to −3.1) and negative to positive zircon εHf(t) values (−1.8 to +3.6), suggesting their formation by partial melting of ancient mafic crust with addition of minor mantle-derived melt and subducted terrigenous sediments. In general, Hf-in-zircon isotopes characteristics of the late Paleozoic granitic rocks in the Dunhuang Block indicate that the southern Dunhuang area carries more ancient crust materials than the central and northern Dunhuang areas, indicating a lateral heterogeneity of the middle Paleozoic crust. Conclusively, the middle Paleozoic granitoids of the Dunhuang Block were emplaced at an active continental margin of the southern Paleo-Asian Ocean.

Predicting olivine formation environments using machine learning and implications for magmatic sulfide prospecting

Abstract Global volcanic and plutonic olivines record the compositional characteristics and physicochemical conditions of the parental magmas. Thus, they have significant potential for use as petrogenetic discriminators of the olivine formation environment and prospecting indicators for potential host rocks of magmatic sulfide deposits. Several data visualization approaches have been proposed by researchers to determine olivine origins. However, they can only discriminate specific olivine populations and require the incorporation of trace elements for which data are lacking globally. In this study, a machine-learning method consisting of the random forest algorithm and the synthetic minority oversampling technique (SMOTE) is used to discriminate the crystallization environments of olivine and predict the sulfide potential of olivine-bearing mafic-ultramafic intrusions. We employ a global data set of 24 341 olivine samples from 12 environments to determine the contents of MgO, FeO, Ni, Ca, Mn, and Cr and the Fo number [100 × Mg/(Mg+Fe)]. The results indicate that the proposed method can classify olivine into genetically distinct populations and distinguish olivine derived from mineralized intrusions from that derived from sulfide-barren intrusions with high accuracies (higher than 99% on average). We develop a dimensionality reduction algorithm to visualize the olivine classifications using low-dimensional vectors and an olivine classifier (accessible at http://101.33.204.62:8080/olivine_web/main.html, China University of Geosciences, Beijing). The model is used successfully to identify the contributions of distinct sources to regional magmatism using olivines from the late Permian picrite and basalt along the western margin of the Yangtze block (SW China) and to predict the sulfide potential of the newly discovered Qixin mafic-ultramafic complex in the southern Central Asian Orogenic Belt (NW China). The findings suggest that the proposed approach enables the accurate identification of olivine origins in different formation environments and is a reliable indicator suitable for global Ni-Cu-platinum group element (PGE) exploration.

Formation of the Permo-Triassic Ondor Sum thrust system and its constraint on Solonker Ocean closure

ABSTRACT A previously unidentified overthrust system has recently been discovered in the Ondor Sum area at the boundary between the Solonker – Xar Moron Suture Zone (SXSZ) and the Early to Mid – Paleozoic Subduction – accretion Belt to the south. The overthrusting occurred during the middle Permian and was overprinted by late Permian to Middle Triassic dextral shearing. The overthrusting (D1) was characterized by the development of WSW – ENE-trending folds and reverse faults, implying NNW – SSE shortening. The later deformation (D2) involved WSW – ENE dextral strike-slip shearing with a minor reverse component, as indicated by slightly plunging striae on slickensides. A comparison of geochronological and structural data for ophiolites in the SXSZ suggests that the overthrust system formed in response to the closure of the Solonker Ocean and the collision along the SXSZ. The earlier NNW – SSE shortening and later WSW – ENE dextral shearing indicate that a tectonic transformation took place along the SXSZ from the middle Permian to the Late Triassic. Dextral and sinistral strike-slip faults are, respectively, distributed to the south and northwest of the Songliao – Xilinhot Block to the north of the SXSZ, implying eastward extrusion of the Songliao – Xilinhot Block after the closure of Solonker Ocean. The tectonic transformation and extrusion might have been a part of the intracontinental tectonic transformation of the eastern segment of the southern Central Asian Orogenic Belt.

Continental crust delamination in a retreating subduction zone: A case study in the southern Alxa (Inner Mongolia, China), Central Asian Orogenic Belt

Carboniferous-Permian igneous rocks in the southern Alxa (Inner Mongolia, China) of the southern Central Asian Orogenic Belt exhibit an oceanward migration trend, which provides an ideal area to reconstruct the process of crust-mantle material exchange within a retreating subduction zone. This study presents new geochronological and geochemical data for the Shouji plutons from the southern Alxa to address crust-mantle material exchange process. All these diorite, granodiorite, and K-feldspar granite samples from the Shouji batholith exhibit Early Permian zircon U-Pb ages (ca. 282−270 Ma). These diorite and granodiorite are characterized by high Sr contents as well as low Y, and Yb concentrations, coupled with distinctively high Sr/Y values. They exhibit fertile Nd-Hf isotopic compositions with negative whole-rock εNd(t) values, and zircon εHf(t) values, similar to those of continental crust-related adakites, which might originate from partial melting of lower crust, and followed by melt-mantle interactions. These K-feldspar granite samples display peraluminous and high-K calc-alkaline compositions, and negative zircon εHf(t) and whole-rock εNd(t) values, and originated from partial melting of Paleoproterozoic igneous rocks, with additions of mantle-derived melts in their sources, in low-temperature, low-pressure conditions. Previously published zircon U-Pb age data show that numerous mafic igneous rocks were mostly generated during the Early Permian, marking the onset of mafic volcanism in the southern Alxa. Moreover, compiled detrital zircon U-Pb age data for Late Carboniferous−Permian sedimentary succession imply a regional topographic uplift during the Early Permian. Both of these two lines of evidence suggest an Early Permian lower crustal delamination in the southern Alxa. These Shouji adakitic plutons were most likely to be generated during the lower crustal delamination. Furthermore, Sr-Nd isotope data of Carboniferous-Permian igneous rocks show that the continental crust was recycled back into the mantle during Early Permian within the southern Alxa by means of subduction erosion and lower crustal delamination.

Intracontinental evolution of the southern Central Asian Orogenic Belt: Evidence from Late Triassic nappe structure in the northern Alxa region, NW China

• Field mapping indicated the geometry of nappe structure in the northern Alxa region, NW China. • The crustal shortening of the southern CAOB was established during the Late Triassic. • Two large-scale strike-slip systems dominated the Late Triassic tectonic framework of the southern CAOB. The Central Asian Orogenic Belt (CAOB) experienced multistage intracontinental deformation during the Mesozoic and Cenozoic. Recent studies have shown that a large-scale nappe structure developed in the northern Alxa region. Here, we present new field investigations, structural analysis, and a new balanced cross-section in order to better constrain its geometries, displacement and activity time. Three nappe stacks with distinctive lithostratigraphic assemblies, structural characteristics and displacement were recognized: (1) the structural highest China-Mongolia boundary nappe preserving as klippen has a minimum displacement of ∼65 km; (2) the Yagan thrust sheet is dominated by an E–W-trending syncline and has a minimum displacement of ∼30 km; and (3) the structural lowest Gashunzhadegai thrust sheet is characterized by S-vergent folds and imbricated faults and has a minimum displacement of ∼14 km (30%). The youngest strata involved in the nappe structure and the cross-cutting relationships indicated that the nappe structure formed during the Late Triassic (230-215 Ma). Regionally, it occurred later than the final closure of the Paleo-Asian Ocean and marked the beginning of the intracontinental evolution of the southern CAOB. Integrating previous structural, thermochronological and sedimentological data with our studies, we suggested that the Late Triassic tectonic framework of the middle segment of the southern CAOB was dominated by two large-scale en echelon strike-slip systems. In addition, the driving force for the Late Triassic intracontinental deformation was most likely to be the collision of the Bayan Har Terrane and Yangtz Craton with the Eurasian continent.

Late Palaeozoic tectonic evolution of the southern Central Asian Orogenic Belt: Constraints from the Early Permian magmatism in the northern Alxa area

The northern Alxa area is the southernmost part of the central segment of the Central Asian Orogenic Belt and is located in a crucial junction to unravel the geological evolutionary history of the Palaeo‐Asian Ocean. Here, we present an integrated study of whole‐rock geochemistry, zircon U–Pb geochronology and zircon–Hf isotope on Early Permian igneous rocks from the Zhusileng–Hangwula Belt of the northern Alxa area. Three syenogranites, two monzogranites and one granodiorite are identified as peraluminous, (high‐potassium) calc‐alkaline I‐type granites. The zircon dating results show that they were formed during the Early Permian (291–285 Ma). Geochemically, these rocks show enrichments in light rare‐earth elements (LREEs), large‐ion lithophile elements (LILEs) and negative Nb–Ta anomalies, as well as low A/CNK values. Furthermore, the syenogranites have almost positive zircon εHf(t) values (−0.4 to +4.8) and Early Palaeoproterozoic–Mesoproterozoic Hf model ages (1,768–1,296 Ma), indicating that they were probably derived from juvenile crustal components, mixed with ancient continental crust. The monzogranites have markedly positive zircon εHf(t) values (+11.2 to +13.5) and Neoproterozoic–Cambrian Hf model ages (716–513 Ma), suggesting that they were generated by rapid reworking of newly formed juvenile material. The granodiorite shows positive zircon εHf(t) values (+4.0 to +7.0) and Mesoproterozoic Hf model ages (1,368–1,090 Ma), implying that the granodiorite was probably derived from the partial melting of the juvenile crust. Our geochronological, geochemical and isotopic data, combined with published data, unravel that a crustal thinning event that was simultaneous with more input of mantle materials in the northern Alxa area, implies that a subduction‐related extensional environment occurred in the Zhusileng–Hangwula Belt which is probably due to the slab rollback of the Paleo‐Asian Ocean during the Late Carboniferous–Early Permian.

Accretion of an early Paleozoic Alaska-type arc onto northern North China: Implications for continental growth of the Central Asian orogenic belt

The accretionary orogenesis of the Central Asian orogenic belt is essential for understanding the reconstruction and growth of the Asian continent. The origin and accretion of the Bainaimiao arc along the northern margin of North China remain controversial. Here, a comprehensive study of field geology, geochemistry, and geochronology was performed on plutons and an ophiolite-bearing accretionary complex in the Ganqimaodu region, Inner Mongolia. Geochemical data indicate that ca. 445−431 Ma plutons were generated in an intra-oceanic arc and represent the western extent of the Bainaimiao arc. Field mapping and zircon U-Pb dating revealed that the ophiolitic mélange contains ca. 470−435 Ma tectonic blocks of ultramafic rocks, gabbros, plagiogranites, and metavolcanic and siliceous rocks in a matrix of intensely foliated quartz schist. The pillow basalt−limestone sequence might have originated from a seamount and then been incorporated into the accretionary complex. Northward subduction of the South Bainaimiao Ocean was responsible for the early Paleozoic tectonic evolution of the Bainaimiao arc, based on the spatial-temporal configuration of arc magmatism and accretionary complexes. Integration of new data with previous studies indicates that the Bainaimiao arc was an Alaska-type arc with various components of intra-oceanic and continental arcs. We suggest that a scissor-like arc-continent collision led to the accretion of the Bainaimiao arc onto the northern margin of North China, which played a fundamental role in the continental growth of the southern Central Asian orogenic belt.