Beishan Region Research Articles

Experimental and numerical simulation study on the relationship between cutting depth of high-pressure water jet with high traverse speed and disc cutter penetration of TBM in hard rock tunnel

The disc cutting load of a TBM is insufficient to induce crack propagation in hard rock tunnels due to the high strength of the rock. High-pressure water jets, considered an auxiliary rock-breaking method, enhance the disc cutter's capacity to penetrate cracks by creating pre-existing fissures. In order to examine the correlation between cutting depth achieved by a high-pressure water jet operating at high traverse speeds and the penetration capability of TBM disc cutters, we conducted rock cutting experiments employing varying traverse velocities for the water jet. Additionally, we performed linear disc cutting experiments using full-size disc cutters equipped with grooves. These experiments were conducted using high-strength granite from China's Bei Shan region. We utilized the Discrete Element Software PFC2D to construct a rock breaking model based on Tyson polygons, enabling us to investigate the microfine influence mechanism of grooves with symmetry. The experiments involving high-pressure water jet grooving at high traverse velocities revealed a logarithmic decrease in groove depth as the nozzle's traverse velocity increased. These findings suggest that when high-pressure water jets are employed as an auxiliary method in TBM operations, increased distance from the center of the disc hob leads to higher traverse velocities, resulting in less efficient rock breaking. Nonetheless, increasing the nozzle diameter can enhance the rock-breaking effect. Linear disc cutting experiments indicate that, at a penetration depth of 4 mm, grooves with depths less than 4 mm have minimal impact on assisting in disc cutter rock breaking. However, when the groove depth exceeds 4 mm, there is a noticeable improvement in the disc cutter's breaking performance. The numerical simulation results provide initial validation for the linear disc cutting experiment. Lastly, considering the specific conditions of the Beishan tunnel, we propose a design scheme that involves the zoned arrangement of water jets. Additionally, we recommend appropriately reducing the rotational speed when employing high-pressure water jets as an auxiliary method for rock breaking. The research findings presented in this paper can serve as valuable references for the design and implementation of high-pressure water jet-assisted TBM cutter rock breaking.

Long-lived subduction retreating led to continental rifting along the northern Gondwana: Insights from Devonian igneous rocks and ophiolite in the Beishan orogenic collage

The Beishan orogenic collage is an ideal setting for investigating the temporal and spatial relationships between extensional tectonics in the Central Asian Orogenic Belt and the rifting of Gondwana. This study presents new geochemical and geochronological data for Devonian igneous rocks and the Zhangfangshan ophiolitic mélange in the southern Beishan region. The Zhangfangshan ophiolitic mélange contains a Late Devonian gabbro block (ca. 369 Ma) with extremely depleted zircon Hf isotopic compositions (εHf[t] = +16.5 to +22.4). The Lucaogou diorites (ca. 367 Ma) have low MgO and compatible element contents, low Sr/Y and La/Yb ratios, and positive zircon εHf(t) values, indicative of derivation by dehydration melting of Mesoproterozoic mafic igneous rocks in a high-temperature and medium- to low-pressure environment. The Lucaogou K-feldspar granites (ca. 374 Ma) and Dundunshan silicic volcanic rocks (ca. 375 Ma) have A-type granite affinities, and were probably derived from magmas formed by mixing between Neoproterozoic orthogneiss-derived melts and depleted mantle-derived mafic magmas in a high-temperature shallow-crustal environment. Compiled geochronological data for subduction-related magmatism and supra-subduction zone-type ophiolites are consistent with protracted subduction–accretion until the early Carboniferous along the northern margin of the Dunhuang Block. Slab retreat in this long-lived subduction zone led to the generation of oceanic lithosphere in the southern Beishan region. Furthermore, Devonian extensional tectonics in the southern Beishan area coincided with the opening of the Paleo-Tethys Ocean, indicating that protracted subduction retreat caused the separation of the Dunhuang Block from East Gondwana.

Occurrence and geochemical characteristics of cobalt of the Poyi nickel deposit in the Beishan region, Xinjiang: Insights into cobalt mineralization

钴作为当今社会必需的一种关键矿产资源,岩浆铜镍矿床是我国钴资源供应的重要矿床种类之一,该类矿床中钴元素通常赋存于镁铁质硅酸盐矿物、硫化物、类硫化物和铁氧化物中,然而镁铁质硅酸盐和铁氧化物中的钴难以分选利用,因此钴赋存状态的厘定是铜镍矿床中钴资源评价和利用的重要前提。新疆东天山-北山地区发育大量晚古生代、普遍伴生钴的岩浆铜镍矿床,其中位于北山地区的坡一镍矿床以赋含高镍硫化物为特色,是研究岩浆铜镍矿床中钴元素的赋存状态、分布特征以及钴与镍相互关系的理想对象。坡一镍矿的含矿岩相主要为单辉橄榄岩、纯橄岩和橄长岩,以稀疏浸染状矿化为主,钴主要以类质同象形式赋存于镍黄铁矿,其次为铬尖晶石、橄榄石和磁黄铁矿,少见独立钴矿物。矿物微区原位分析结果显示原生矿物中钴含量由高到低依次为镍黄铁矿(~8404×10^-6)>>铬尖晶石(~475×10^-6)>磁黄铁矿(高Ni含量)(~299×10^-6)>橄榄石(~137×10^-6)>黄铜矿(~13.0×10^-6)>磁黄铁矿(低Ni含量)(~8.35×10^-6)。坡一岩体的蚀变橄榄岩中发育三种产状的次生磁铁矿:蚀变橄榄石裂理中细脉状磁铁矿、铬尖晶石边部环状磁铁矿边、硫化物矿边部和裂隙中次生磁铁矿边。元素面扫描结果显示次生磁铁矿中Co含量明显高于原生硫化物、氧化物和硅酸盐矿物,且显示富Co贫Ni特征。坡一矿床中硫化物和硅酸盐矿物表现出明显的Co-Ni正相关,而铬尖晶石中Co和Ni显示负相关,表明Co²⁺主要以类质同象形式替代Fe²⁺进入硫化物和硅酸盐矿物晶格,以替代Fe²⁺、Ni²⁺的形式进入铬尖晶石。坡一矿床中Ni和Co的共生行为,主要归因于岩浆演化过程硫化物熔离和硅酸盐矿物分离结晶过程中相近的配分行为,而Ni和Co的离散特征主要受控于铬尖晶石的分离结晶和R值(岩浆和硫化物质量比)的变化、热液蚀变过程中次生磁铁矿Co富集作用。由于铬尖晶石与硅酸盐矿物中的钴难以分选利用,北山地区的钴矿资源勘查应关注贫铬尖晶石、橄榄石镍亏损的富硫化物超镁铁质岩体。

Termination of the Paleo-Asian Ocean in the Beishan orogen, NW China: Constraints from detrital zircon U-Pb age and Hf isotope analysis of turbidites

The termination of the Paleo-Asian Ocean, including duration and architecture, is controversial and has led to different reconstructions of the Central Asian orogenic belt. Deep-marine turbidites are of great significance in constraining the evolution of the oceanic basin. The youngest marine turbidites in the Beishan orogen in the central part of the southern Central Asian orogenic belt are separated by the Liuyuan mélange into two sections, the South turbidites and North turbidites. Detrital zircon grains from South turbidites yielded a maximum depositional age of 255 Ma. Those from North turbidites yielded a maximum depositional age of 254 Ma. Most of the Precambrian zircons of South turbidites have concentrated εHf(t) values (−6 to −2.6), whereas those of North turbidites range from −12.6 to +10.4, indicating different provenances. The North turbidites also have more positive whole-rock εNd(t) values (−0.5 to +1.1) than the South turbidites (−4.2 to −2.4), suggesting more juvenile or less evolved crustal components in the source. Detailed comparison of the detrital zircon ages and Hf-Nd isotopic characteristics with igneous and metamorphic rocks in the south Beishan area distinguishes two sources. These findings constrain the Liuyuan suture as the latest suture in the Beishan region, occurring no earlier than 254 Ma. The youngest peak age of detrital zircons of the youngest marine sedimentary rocks in the southern Central Asian orogenic belt becomes younger eastward from 288 Ma to 247 Ma, characterizing the closure duration of the Paleo-Asian Ocean from west to east in ∼40 m.y. This study finds that the youngest turbidites in the Beishan orogen not only record sedimentary processes in response to the final episode of orogenic evolution in the southern Central Asian orogenic belt, but they also constrain how and when an archipelago-type accretionary orogen terminated.

Tectonic evolution of the Beishan orogen in central Asia: Subduction, accretion, and continent-continent collision during the closure of the Paleo-Asian Ocean

AbstractThe Beishan orogen is part of the Neo-proterozoic to early Mesozoic Central Asian Orogenic System in central Asia that exposes ophiolitic complexes, passive-margin strata, arc assemblages, and Precambrian basement rocks. To better constrain the tectonic evolution of the Beishan orogen, we conducted field mapping, U-Pb zircon dating, whole-rock geochemical analysis, and Sr-Nd isotopic analysis. The new results, when interpreted in the context of the known geological setting, show that the Beishan region had experienced five phases of arc magmatism at ca. 1450–1395 Ma, ca. 1071–867 Ma, ca. 542–395 Ma, ca. 468–212 Ma, and ca. 307–212 Ma. In order to explain the geological, geochemical, and geochronological data from the Beishan region, we present a tectonic model that involves the following five phases of deformation: (1) Proterozoic rifting that separated the North Beishan block from the Greater North China craton that led to the opening of the Beishan Ocean, (2) early Paleozoic north-dipping subduction (ca. 530–430 Ma) of the Beishan oceanic plate associated with back-arc extension followed by collision between the North and South Beishan microcontinental blocks, (3) northward slab rollback of the south-dipping subducting Paleo-Asian oceanic plate at ca. 450–440 Ma along the northern margin of the North Beishan block that led to the formation of a northward-younging extensional continental arc (ca. 470–280 Ma) associated with bimodal igneous activity, which indicates that the westward extension of the Solonker suture is located north of the Hongshishan-Pengboshan tectonic zone, (4) Late Carboniferous opening and Permian north-dipping subduction of the Liuyuan Ocean in the southern Beishan orogen, and (5) Mesozoic-Cenozoic intracontinental deformation induced by the final closure of the Paleo-Asian Ocean system in the north and the Tethyan Ocean system in the south.

Dichoephedra beishanensis gen. et sp. nov., a new ephedroid plant with unusual branching patterns from the Lower Cretaceous of northwestern China

Macrofossils of Gnetales have been widely documented in Cretaceous deposits, but most of them have been found in northeastern China. Herein, we first describe a new plant of the family Ephedraceae, Dichoephedra beishanensis gen. et sp. nov., from the Lower Cretaceous of the Zhongkouzi Basin in the Beishan region, northwestern China. It has the typical morphological characteristics of Ephedraceae, such as striate stems with nodes and internodes, opposite branches (upper part of the plant) and linear leaves, and axillary or terminal cones with one to two pairs of opposite or decussate bracts and two oval seeds. However, the Z-shaped stem and dichotomous branching in the lower part are different from those of all extant species of Ephedra and fossil ephedroids. Morphological comparison indicates that Dichoephedra beishanensis has a close affinity with the fossil taxa of Ephedraceae from the Lower Cretaceous of northeastern China. The floristic and palaeoecological differences might have accelerated the differentiation of ephedroids during the Early Cretaceous. Plants associated with both Pagiophyllum Heer and Brachyphyllum Brongniart (Cheirolepidiaceae) and the gypsum interlayers indicate an evaporation dominant arid environment in northwestern China during the Early Cretaceous.

Petrogenesis and Ni-Cu exploration potential of Devonian mafic–ultramafic intrusions in the southern part of the Central Asian Orogenic Belt, NW China: constraints from zircon O isotopes and whole-rock Sr-Nd isotopes

ABSTRACT Weakly disseminated sulphide mineralization is present in the outcrops of several mafic–ultramafic intrusions in the Beishan region of the Central Asian Orogenic Belt in western China, but the potential of these intrusions to host economic sulphide ores at depths is not known. A better understanding of the genetic control on the mineralization may help to answer this important question. With this in mind, we have selected two of them that were emplaced during the Devonian, a period when subduction-related mafic magmatism undoubtedly occurred in the region, for a geochemical study. They are the Miaomiaojing and Guaishishan mafic–ultramafic intrusions. Our new zircon U-Pb dating indicates that they were emplaced at 377.5 ± 5.3 and 358.6 ± 3.9 Ma, respectively. Lherzolite, olivine websterite, troctolite and gabbro are the main rock types for both intrusions. Disseminated sulphide mineralization is concentrated in the ultramafic units of both intrusions. The whole-rock samples are characterized by light REE enrichments, pronounced negative Nb-Ta anomalies, and variable εNd(t) values from −3.54 to +2.17. Zircons from the intrusions have elevated δ18O values (average value: 7.48 ± 0.09 ‰) compared to that of the mantle. The trace element and isotope data support the interpretation that the parental magmas for the Devonian mafic–ultramafic intrusions are contaminated arc mafic magmas. Petrological modelling reveals that fractional crystallization together with crustal assimilation may have played an important role in triggering the sulphide saturation in the magmas. The Ni tenors (in recalculated 100% sulphide) for the sulphide-mineralized samples from both intrusions are similar and up to 6.7 wt% Ni and 2.8 wt% Cu, comparable to the metal tenor of most economic Ni-Cu deposits in the CAOB, implying that samples containing higher amounts of sulphides would become Ni-Cu ores. The olivine Fo and Ni contents from sulphide-bearing ultramafic rocks show a negative correlation, consistent with sub-solid Fe-Ni exchange between olivine and sulphide melt. Together with the similar mineralization characteristics between these sulphide-bearing intrusions and the coeval Heishan Ni-Cu deposit, we suggest that these intrusions have the potential to host Ni-Cu sulphide ores at depths, which could be further tested by geophysical survey followed by drilling.

Missing adakitic granite and syn-subduction mafic dikes within Permian volcanic belts of the southern margin of the CAOB? Comment on “Permian oceanic slab subduction in the southernmost Central Asian Orogenic Belt: Evidence from adakite and high-Mg diorite in the southern Beishan”

The petrogenesis of adakite and adakitic rocks and their tectonic settings are debated. Adakites may be derived by many processes which include: partial melting of subducted oceanic slab or lower crustal material; fractional crystallization of basaltic magma; magma mixing processes associated with arc-related and non-arc tectonic settings. It may be inaccurate to assume that adakitic rocks are solely derived from subducted oceanic slabs. The development of accretionary orogens, as it relates to adakite formation, is a topic of significant interest. Geochemical data coupled with field observations are vitally important to address the interplay between tectonics and magma genesis. In this comment, we discuss the petrogenetic processes and possible tectonic setting of the “adakite” in the southern Beishan region of the Central Asian Orogenic Belt (CAOB), to provide insight on the use of geochemical data to discriminate the tectonic setting of their formation. Based on our field observations, we disagree with the model presented by Zheng et al. (2020) as it pertains to the existence of an “ophiolitic mélange” and the evidence of a Permian arc system but rather, advocate that the adakitic rocks at Beishan are generated at an extensional setting.

Quaternary crustal reactivation of the southwestern Beishan, NW China: The Liuyuan sinistral transpressional duplex

We report remote sensing evidence and reconnaissance field observations for an actively evolving transpressional duplex 150 km north of Tibet, in the southern Beishan region of western China. We analyze Landsat-8, Google Earth and SRTM-30 data to reveal active fault traces, kinematics, offset contacts and landforms, and the overall duplex geometry in an area of limited recorded seismicity compared to surrounding regions. Eight NE-striking faults transfer sinistral displacement between E-W bounding faults and accommodate N-S shortening by sinistral-reverse displacements that have tilted the internal duplex blocks. The potential moment magnitude of a surface-rupturing event on one fault, that displays a twenty km-long, 1–3 m-high surface scarp, is estimated to be 6.6. Our study adds to the growing body of evidence that suggests the Beishan is a region of subtly expressed, but widespread, continental interior, strike-slip reactivation at low strain rates - challenging the long-held view that the Beishan has anomalous crustal stability within the huge Indo-Eurasia deformation field.

A Multi-Model Ensemble Approach for Gold Mineral Prospectivity Mapping: A Case Study on the Beishan Region, Western China

Mineral prospectivity mapping (MPM) needs robust predictive techniques so that the target zones of mineral deposits can be accurately delineated at a specific location. Although an individual machine learning algorithm has been successfully applied, it remains a challenge because of the complicated non-linear relations between prospecting factors and deposits. Ensemble learning methods were efficiently applied for their excellent generalization, but their potential has not been fully explored in MPM. In this study, three well-known machine learning models, namely random forest (RF), support vector machine (SVM), and the maximum entropy model (MaxEnt), were fused into ensembles (i.e., RF–SVM, RF–MaxEnt, SVM–MaxEnt, RF–SVM–MaxEnt) to produce a final prediction. The paper aims to investigate the potential application of stacking ensemble learning methods (SELM) for MPM. In this study, 69 hydrothermal gold deposits were split into two parts: 70% for the training model and 30% for testing the model. Then, 11 mineral prospecting factors were selected as a spatial dataset constructed for MPM. Finally, the models’ performance was assessed using the receiver operating characteristic (ROC) curves and five statistical metrics. Compared with other single methods, the SELM framework showed an improved predictive performance in the model evaluation. Therefore, this finding suggests that the SELM framework is promising and should be selected as an alternative technique for MPM.

The closing of the southern branch of the Paleo-Asian Ocean: Constraints from sedimentary records in the southern Beishan Region of the Central Asian Orogenic Belt, NW China

The Carboniferous–Permian strata in the southern Beishan Orogenic Collage (BOC) have been regarded as deposits formed either in a branch of the Paleo-Asian Ocean (PAO) or in continental rift basins. This paper presents stratigraphic, sedimentological, and provenance data of a continuous Carboniferous–Permian sequence on the southern margin of the Shuangyingshan Block within the southern BOC in NW China. Carboniferous Baishan and Shibanshan formations record a shallowing-upward change from marine tidal flat to continental braided stream environments. Integrated sandstone modes and U-Pb-Hf data indicate that these formations were fed from an orogenic uplift composed of Precambrian and early Paleozoic rocks of the Shuangyingshan and Dunhuang blocks (ca. 2492, 953, and 439–400 Ma). Latest Carboniferous–Early Permian depositional environments shifted from alluvial fans (Ganquan Formation) to delta-front and prodelta environments in an open marine setting (Shuangbutang and Jushitan formations) and source areas were composed of early Paleozoic granitoids (ca. 433 Ma) and penecontemporaneous volcanoes (ca. 278 Ma). These results indicate that the closure of the branch of the PAO between the Shuangyingshan and Dunhuang blocks occurred before the Visean (ca. 347 Ma), and was followed by the development of the latest Carboniferous–Early Permian marine rift basin on the amalgamated continental block.

Early Paleozoic Ocean Plate Stratigraphy of the Beishan Orogenic Zone, NW China: Implications for Regional Tectonic Evolution

AbstractThe Beishan orogenic zone is a key area to understand evolution of the Central Asian Orogenic Belt that is an accretionary factory well‐enough preserved in the Paleozoic. In early Paleozoic, the tectonic mélange zone containing the coherent unit and mélange unit is triggered by the complicated accretionary process of the Beishan area. The early Paleozoic tectonic evolution of the Beishan orogenic zone is investigated in this study using sedimentology and stratigraphic correlations of the lowe Paleozoic deposits. From the Cambrian to the middle Ordovician, this region was characterized by geographically extensive, flat‐bedded siliceous mudstone, indicating the existence of a large ocean basin. The oceanic plate entered the convergence phase in terms of the Wilson Circle during the Middle Ordovician, when numerous magmatic arcs formed along two opposite sides of the ocean. The magmatic arcs became the widest during the Silurian, suggesting that the Hongliuhe‐Niujuanzi‐Xichangjing Ocean (HNX; a southern branch of the Paleo Asian Ocean) was reduced to a small residual ocean in the central Beishan region by that time, and probably lasted till the Carboniferous or later by newly published data.

Sulfide segregation mechanism of magmatic Ni mineralization in western Beishan region, Xinjiang, NW China: Case study of the Hongshishan mafic–ultramafic complex

A series of early Permian mafic–ultramafic complexes associated with Ni-Cu mineralization are distributed in western Beishan region, Xinjiang, NW China. The Ni mineralization in this area consists of two sub-belts, which have the same magma conduit at depth, similar rock types and similar emplacement ages, but different mineralization types. The Ni mineralization in the north (Hongshishan) sub-belt consists of low-tenor disseminated sulfide, whereas that in the south (Pobei) sub-belt consists of disseminated sulfide and vein-type sulfide with high tenor, suggesting different sulfide segregation mechanisms. The Hongshishan complex consists of dunite, wehrlite, olivine clinopyroxenite, troctolite, olivine gabbro, and gabbro (from earlier to later). The mafic rocks occur along margins of the complex, and the mineralized ultramafic rocks are confined to the center. The dunite and wehrlite host significant amounts of disseminated sulfide. The parental magma of the Hongshishan complex may have originated from 17.0% partial melting of the depleted mantle. It is characterized by high temperature (1368 ℃), high Mg (picritic affinity, 14.52 wt% MgO), oxidation and platinum group element (PGE) depletion, as constrained by the major and trace elements, olivine and chromite mineralogy, and PGE contents. Two stages of sulfide segregation have been recognized. Early sulfide segregation at depth, which is indicated by elevated Cu/Pd ratios and sulfide inclusions in chromite, produces the PGE-depleted parental magma. Later in situ segregation results in disseminated sulfide mineralization. Model of the olivine crystallization and PGE content shows that 0.01%−0.015% of the sulfide is segregated in situ, with R = 100–500. Significant crustal contamination is suggested by xenoliths of wall rock as well as negative Nb, Ta, and Ti anomalies and positive Pb and Sr anomalies in intrusive rocks. The addition of crustal Si and decrease in FeO by fractional crystallization are the main factors triggering S-oversaturation and later in situ segregation. The early segregation at depth is more likely caused by the addition of crustal S, although the δ34S values of sulfides (-0.75‰ to 1.15‰) indicate the isotopic signature of mantle sulfur. For comparison, the development of vein-type sulfide mineralization in the Poyi deposit is ascribed to assimilation of S-bearing Archean rocks. Finally, a model of “early segregation at depth + later in situ segregation + multistage pulsation” is proposed for the Ni mineralization in this area.

Quaternary Activity of the Beihewan Fault in the Southeastern Beishan Wrench Belt, Western China: Implications for Crustal Stability and Intraplate Earthquake Hazards North of Tibet

AbstractWe present evidence for the Late Quaternary activity of the Beihewan Fault (BHWF), along the southeastern margin of the Beishan block, western China. Field observations and analysis of UAV‐derived DEMs and Google Earth images reveal an ~10‐km‐long active fault trace in a region previously considered tectonically inactive. Offset landforms and trench exposures reveal that the fault is dominantly strike‐slip, with local thrust or normal displacements. Average rates of left‐lateral motion and thrusting since the Holocene are estimated to be ~2.69 and ~0.35 mm/a, respectively. The most recent surface‐rupturing event generated ~2.5 m of strike‐slip offset since 6.2–8.6 ka; with estimated M = 6.3–6.9. Modeling and two‐dimensional inversion of new magnetotelluric data indicate that the BHWF is a subvertical low‐resistivity zone that penetrates into the lower crust. Existing geological and geophysical evidence do not support subsurface linkage between the BHWF and Altyn Tagh–Jinta‐Nanshan Fault system further south in the Hexi Corridor. Close examination of high‐resolution satellite imagery west and east of the BHWF reveals other Quaternary surface fault scarps, pressure ridges, offset drainages, and truncated lithological strike belts. Collectively, these features constitute a previously unrecognized 150‐km‐long sinistral deforming belt along the southeastern Beishan and northern Hexi Corridor boundary that we call the Southeast Beishan Wrench Belt. Recognition of Quaternary activity along the BHWF within the larger Southeast Beishan Wrench Belt challenges long‐held assumptions that the Beishan region is a relatively stable and tectonically inactive block within the Late Cenozoic Indo‐Eurasian deformation field north of Tibet.

Geochronological, mineralogical and geochemical studies of sulfide mineralization in the Podong mafic-ultramafic intrusion in northern Xinjiang, western China

The Podong mafic-ultramafic intrusion is one of many Ni-Cu sulfide-mineralized intrusions in the Beishan-Tianshan Orogenic Belt in northern Xinjiang, western China. The temporal and lithological variations of the Ni-Cu mineralized intrusions in Beishan are much less constrained than those in Tianshan. This paper provides the first set of geochronological and geochemical data for the Podong magmatic sulfide deposit that is present in the Beishan region. The host intrusion is composed of lherzolite, troctolite, gabbronorite and gabbro. Ni-Cu mineralization occurs as disseminated sulfides in gabbronorite (± olivine). The zircon U-Pb age of gabbronorite is 270 ± 2.7 Ma, identical to the zircon U-Pb age of the nearby Poyi magmatic sulfide deposit that is hosted in troctolite and ultramafic rocks. These ages together define a new episode of Ni metallogeny in the Beishan region, which is ∼10 Ma younger than that in the Tianshan region to the north. The Fo contents of olivine in the Podong mafic-ultramafic intrusion are from 76 to 87 mol%, lower than the values for the nearby Poyi mafic-ultramafic intrusion. The Podong mafic-ultramafic rocks are characterized by εNd(t) values from −0.61 to +1.92, low to elevated initial 87Sr/86Sr ratios (0.7055–0.7090), and pronounced negative Nb-Ta anomalies. The Sr-Nd isotope data indicate that the parental magma of the Podong intrusion experienced 0–13 wt% crustal contamination, which cannot fully account for the observed negative Nb-Ta anomalies in the rocks. Sulfur isotope compositions of the Podong deposit are similar to those of Pre-Permian sedimentary rocks in the area as well as typical mantle values. The results support the premise that fractional crystallization and assimilation of siliceous crustal materials may have played a key role in triggering sulfide saturation in the Podong magma, although the involvement of external sulfur cannot be completely ruled out. Like other Permian magmatic sulfide deposits in the Beishan-Tianshan Orogenic Belt, the PGE tenors of bulk sulfides in the Podong deposit are extremely low, indicating a severely PGE-depleted parental magma. The Cu/Pd ratios of the Podong magmatic sulfide deposit are significantly higher than the mantle values, consistent with previous sulfide segregation at depth. The results from the Podong magmatic sulfide deposit confirm that widespread Ni metallogeny in a convergent tectonic zone is very protracted and the mineralized lithologies are highly variable, which have significant implications for regional Ni-Cu exploration.

Permian bimodal magmatism in the southern margin of the Central Asian Orogenic Belt, Beishan, Xinjiang, NW China: Petrogenesis and implication for post-subduction crustal growth

Previous studies show that Permian bimodal magmatism is widespread in the Tianshan region, the southern part of the Central Asian Orogenic Belt in northern Xinjiang, NW China. We have found that such bimodal magmatism extended farther to the south in the Beishan region. Our new zircon U-Pb age data show that the dolerite and granite dykes in the Poyi area of the region were emplaced at ~270 Ma, >20 myr after subduction ended in the region. The dated dolerite dyke is characterized by pronounced negative Nb-Ta anomalies, high εNd(t) from +4.3 to +5.1 and low initial 87Sr/86Sr ratios from 0.7051 to 0.7060 for whole rocks, plus high εHf(t) from +8.96 to +13.0 and elevated δ18O from 5.8 to 6.5‰ for zircon crystals, which indicate a subduction-modified mantle source and moderate degrees of crustal contamination (<10 wt%). Whole-rock 1000Zn/Fe, Mn/Zn, Dy/Yb and La/Yb ratios of the Poyi dolerite dyke are consistent with a peridotite-dominant mantle source. Between two possible causes of mantle partial melting, such as mantle plume impingement and lithospheric delamination coupled with asthenosphere upwelling, we favor the latter because existing data indicate that the Permian mafic-ultramafic intrusive rocks in the region were emplaced over a long period (>25 myr) and show no age progression in a single direction, which are inconsistent with the results of mantle plume magmatism. Whole rocks from the Permian granite dyke in the Poyi area are characterized by strong negative Nb-Ta anomalies, negative εNd(t) from −2.4 to −1.6 and high initial 87Sr/86Sr from 0.7080 to 0.7088. Magmatic zircon crystals from the granite dyke have εHf(t) from +0.86 to +5.64 and δ18O from 6.5 to 7.2‰. The isotope compositions of the granite dyke are within the ranges of the pre-Permian crustal rocks in the region, indicating a predominantly crustal source. Based on these data, we propose that partial melting in the crust was induced by underplating of mantle-derived mafic magmas, some of which were emplaced in the upper parts of the crust to form the broadly coeval dolerite dykes and mafic-ultramafic intrusions. The results from this study support the view that post-subduction bimodal magmatism is important for continental crustal growth and differentiation.

Unconformity-bounded Upper Paleozoic megasequences in the Beishan Region (NW China) and implications for the timing of the Paleo-Asian Ocean closure

The Upper Paleozoic sedimentary sequence in the Beishan Region, northwest China, provides a complete record of the complex amalgamation and extension history of the Central Asian Orogenic Belt (CAOB) for this part of China. Integrating newly obtained stratigraphic, paleontological and radiometric data, this paper reviews the Upper Paleozoic stratigraphic sequences of the Beishan Region. The Upper Paleozoic strata here are divided into three megasequences (MS). They are, in ascending order: Devonian, Mississippian–Guadalupian and Lopingian megasequences (MS1, MS2, and MS3), separated by two major unconformities each indicative of a huge sedimentary hiatus. MS1 is made up of continental and epicontinental marine deposits in the pre-rift stage; MS2 of transgressive marine and volcanic strata in the syn-rift stage; and MS3 of continental sediments in the inversion stage. The major unconformity between MS1 and MS2 is interpreted to represent the regional uplift and erosion caused by the final amalgamation of CAOB in the Beishan Region. The syn-rift MS2 represented by transgressive marine and volcanic strata in the Beishan Region can be well-correlated with Pennsylvanian–Guadalupian megasequences in Inner Mongolia, where they rest on a major interregional unconformity. This correlation suggests the closure of the Paleo-Asian Ocean in Beishan and Inner Mongolia before the Carboniferous, followed by the development of a synchronous marine rift megasequence in the southern part of the CAOB.

Influence of fault zone on the respect distance and margin for excavation: a case study of the F4 fault in the Jijicao rock block, China

A high-level radioactive waste repository requires a rock mass to have good retardation properties. However, because a fault zone can be a potential seepage conduit for nuclides, its influence on the hydraulic conductivity of that fault zone must be assessed. Key sets of fractures were found based on an assessment of the statistical characteristics of fracture orientations and the tectonic analysis of a representative north–east fault in the Jijicao rock block in the Beishan region of Gansu Province, China. The trace midpoint density of each set was calculated using ArcGIS, a geographic information system, and a model of the hydraulic conductivity in the fault zone was developed based on a water pressure test and calculations, such that the respect distance and margin for excavation of this fault could also be determined. The calculated results show that the fault core and host rock are less conductive when the damage zone is 10- to 100-fold more conductive due to its greater density of fractures. The density is stable at 100 m, while the key set is stable until 65 m, and the calculated hydraulic conductivity is stable until 25 m; these results are consistent with the results of water pressure analysis.

In-situ stress and fracture characterization of a candidate repository for spent nuclear fuel in Gansu, northwestern China

Hydraulic fracturing in-situ stress measurements and acoustic image logging were used to characterize the southern Beishan region of Gansu Province for China's first spent nuclear fuel repository. A total of 47 hydraulic fracturing measurements and 18 impression tests were performed in four boreholes at two sites to estimate magnitudes and orientations of horizontal principal stresses. The stress regime is mainly characterized by SHmax>Shmin≥Sv, indicating that the regional stress field is dominated by the maximum horizontal stress. This stress regime is prone to thrust and strike-slip faulting, according to Anderson's faulting theory. Measured results indicate that magnitude orders and stress gradients of horizontal principal stresses are larger than those previously obtained from other boreholes in the Beishan region, implying that the current stress field of candidate sites might be influenced by regional tectonic and geological controls. Using acoustic borehole logging data, geometric features of natural fractures of the four boreholes were mapped. We also found that wide fractures or faults penetrated in both boreholes at one site changed the orientations of the maximum horizontal principal stresses. Fractures with optimal orientations can be permeable owing to their narrow angles with in-situ principal stress orientations. Rock mass permeability is a priority for future investigation. Adopting the Coulomb frictional-failure criteria, we evaluated the possibility of reactivation of the wide fractures and faults and concluded that the crust in Jinta field should be stable.

Mineral chemistry and genesis of the Permian Cihai and Cinan magnetite deposits, Beishan, NW China

Cihai and Cinan are Permian magnetite deposits related to mafic-ultramafic intrusions in the Beishan region, Xinjiang, NW China. The Cihai mafic intrusion is dominantly composed of dolerite, gabbro and fine-grained massive magnetite ore, while gabbro, pyrrhotite+pyrite-bearing clinopyroxenite and magnetite ore comprise the major units in Cinan. Clinopyroxene occurs in both deposits as 0.1–2mm in diameter subhedral to anhedral grains in dolerite, gabbro and clinopyroxenite. High FeO contents (11.7–28.9wt%), low SiO2 (43.6–54.3wt%) and Al2O3 contents (0.15–6.08wt%), and low total REE and trace element contents of clinopyroxene in the Cinan clinopyroxenite imply crystallization early, at high pressure. This clinopyroxene is FeO-rich and Si and Ti-poor, consistent with the clinopyroxene component of large-scale Cu-Ni sulfide deposits in the Eastern Tianshan and Panxi ares, as well as Tarim mafic intrusion and basalt, implying the Cinan mafic intrusion and sulfide is related to tectonic activity in the Tarim LIP. The similar mineral chemistry of clinopyroxene, apatite and magnetite in the Cihai and Cinan gabbros (e.g., depleted LREE, negative Zr, Hf, Nb and Ta anomalies in clinopyroxene, lack of Eu anomaly in apatite and similarity of oxygen fugacity as indicated by V in magnetite), indicate similar parental magmatic characteristics. Mineral compositions suggest a crystallization sequence of clinopyroxenite/with a small amount of sulfide – gabbro – magnetite ore in the Cinan deposit, and magnetite ore – gabbro – dolerite in Cihai. The basaltic magma was emplaced at depth, with magnetite segregation (and formation of the Cinan magnetite ores) occurring in relatively low fO2 conditions, after clinopyroxenite and gabbro fractional crystallization. The evolved Fe-rich basaltic magma rapidly rose to intermediate or shallow depths, forming an immiscible Fe-Ti oxide magma as fO2 increased and leaving a Fe-poor residual magma in the chamber. The residual magmas was emplaced at different levels in the crust, forming the Cihai gabbro and dolerite, respectively. Finally, the immiscible Fe-Ti oxide magma was emplaced into the earlier formed dolerite because of late magma pulse uplift, resulting in a distinct boundary between the magnetite ores and dolerite.