Alxa Area Research Articles

Geochemistry, zircon U–Pb geochronology and Hf isotope of the early Permian gabbro and high-Mg diorites from the Zhusileng–Hangwula Belt in the northern Alxa area: Petrogenesis and tectonic implications

AbstractAs the southernmost part of the central segment of the Central Asian Orogenic Belt, the northern Alxa area is characterized by abundant Permian magmatism and records key information on the geological evolution of the Palaeo-Asian Ocean. This study reports new zircon U–Pb and Lu–Hf isotopic and whole-rock geochemical data of the early Permian (285–286 Ma) Huisentala gabbro and Huodonghaer diorites from the Zhusileng–Hangwula Belt in the northern Alxa area. The gabbro is characterized by high Al, Ca, Mg# and light rare-earth elements, and low K, P and high field strength elements (e.g., Ti, Nb and Ta). Furthermore, the gabbro shows heterogeneous zircon ϵHf(t) value (−2.5 to +2.6). The Huodonghaer diorites show high MgO (3.46–6.32 wt%), Mg# (49–58), Sr (408–617 ppm) and Ba (223–419 ppm), and low FeOT/MgO (1.27–1.83) and TiO2 (0.48–0.90 wt%), with geochemical features similar to the high-Mg andesite/diorite. They show radiogenic zircon ϵHf(t) values of +1.2 to +4.9 and high Th/Nb ratios. These features suggest that the Huisentala gabbro and the Huodonghaer diorites were derived from the partial melting of mantle peridotite that was metasomatized by subduction-related fluids and by subducted sediment-derived melts, respectively.

Revisiting the boundary between the Central Asian Orogenic Belt and North China Craton in Alxa area, China: Insights from zircon U-Pb ages and Hf isotopes of Phanerozoic granitoids

The Alxa Block, located in the southern Alxa area of Inner Mongolia, is generally accepted as the westernmost part of the North China Craton (NCC), but there is still a controversy over the boundary between the southern CAOB and the Alxa Block. Currently, either the Enger Us ophiolite belt or Quagan Qulu ophiolite belt in the mid-eastern Alxa area is a candidate for the NCC-CAOB boundary, and the extension of the two ophiolite belts in the western Alxa area is ambiguous. In this study, we present a compilation of newly obtained and published zircon U-Pb age and Hf isotopic data from the Ordovician-Triassic granitoids in the Alxa area and discuss the implications of two contrasting isotope provinces for the NCC-CAOB boundary. Specifically, the granitoids in the Zongnaishan-Shalazhashan belt between the two ophiolite belts and the North Beidashan belt in the western Alxa area share similar emplacement ages (Carboniferous to Triassic) and juvenile zircon Hf isotopic compositions (two-stage mode ages < 1.30 Ga), and both Carboniferous to Permian granitoids in the Yabulai-Honggueryulin belt to the south of Quagan Qulu ophiolite belt and Ordovician to Devonian granitoids in the South Beidashan belt of the western Alxa area are characterized by similar ancient zircon Hf isotopic features (two-stage mode ages > 1.30 Ga), but most Late Carboniferous to Permian granitoids in the South Beidashan belt are characterized by juvenile zircon Hf isotopes. The two contrasting isotope provinces (juvenile in the north and generally ancient in the south) for the Phanerozoic granitoids could not result from differential denudation, but drastic differences in compositions of deep crusts. The Yabulai-Honggueryulin and South Beidashan belts with ancient crust sources are parts of the Alxa Block, whereas the Zongnaishan-Shalazhashan and North Beidashan belts with juvenile crust sources could be parts of CAOB, and thus the boundary between the CAOB and the Alxa Block is inferred located in between.

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.

Characterization of microstructural features of Tamusu mudstone

Tamusu mudstone formation, located in the Alxa area in western Inner Mongolia, is considered a potential host formation for high-level radioactive waste (HLW) underground disposal in China. In this study, complementary analyses with X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), mercury intrusion porosimetry (MIP), and N 2 physisorption isotherm were conducted on the Tamusu mudstone to characterize its physical characteristics and microstructural features, such as mineral compositions and pore structure. Several minerals, including carbonates, feldspar, clays and analcime, were identified in Tamusu mudstone by XRD. Images from FE-SEM show that pores in the Tamusu mudstone were dominantly on nanometer scale and generally located within their mineral matrix or at the interface with non-porous minerals. The combination of the MIP and N 2 physisorption curves indicated that the Tamusu mudstone has diverse pore sizes, a porosity varying from 2.34% to 2.84%, and a total pore volume in the range of 0.0065–0.0222 cm 3 /g with the average pore diameter ranging from 9.6 nm to 19.23 nm. The specific surface area measured by MIP (2.572–5.861 m 2 /g) was generally higher than that by N 2 physisorption (1.29–3.04 m 2 /g), due to the pore network effect, pore shape (e.g. ink-bottle shape), or technique limits. The results related to pore information can be applied as an input in the future to model single- or multi-phase fluid flow and the transport of radionuclides in porous geomedium by migration and diffusion.

Geochemistry and zircon U–Pb–Hf isotopes of granodiorites in the northern Alxa area: implications for the Middle–Late Devonian tectonic evolution of the southern Central Asian Orogenic Belt

Geochemistry and zircon U–Pb–Hf isotopes of granodiorites in the northern Alxa area: implications for the Middle–Late Devonian tectonic evolution of the southern Central Asian Orogenic Belt

Tectonic affinity of the Zhusileng–Hangwula Belt in the northern Alxa area: Evidence from the zircon U–Pb ages and Hf isotopic compositions of the Mesoproterozoic (~1.4 Ga) igneous rocks

Precambrian rocks are necessary to understand the tectonic and crustal evolution of the Central Asian Orogenic Belt, especially the tectonic affinities of embedded microcontinents. We conducted a combined analysis of whole‐rock geochemistry, zircon U–Pb geochronology, and Hf isotope on the Mesoproterozoic igneous rocks from the Zhusileng–Hangwula Belt of the northern Alxa area to investigate its tectonic affinity. The zircon ages show that the gneissic granites, metagabbro, and amphibolite formed at 1,433–1,424 Ma, 1,383, and 1,373 Ma, respectively. Furthermore, zircon xenocrysts from the rhyolite in the Zhusileng area exhibit a206Pb/238U ages cluster at about 1.4 Ga. Geochemically, the gneissic granites are high‐potassium calc‐alkaline granitoids and exhibit light rare earth element‐enriched pattern, pronounced significant negative Eu anomalies, negative Nb–Ta anomalies, positiveεHf(t) values (3.2–11.3), and young two‐stage model ages (2.21–1.46 Ga), indicating that they were derived from a magma source of rapid reworking of juvenile mantle‐derived materials with a limited ancient crustal contamination. The FeOT, CaOm and MgO contents of mategabbros and amphibolite are high, but their total rare earth elements are relatively low (TREE = 31.45–32.74). They are characterized by relatively positive zirconεHf(t) values (5.3–11.0), and very young two‐stage model ages (1.98–1.49 Ga), indicating that they were generated from the depleted mantle. Our geochemical data suggest that these Mesoproterozoic rocks in the Zhusileng–Hangwula Belt are consistent with the formation period and tectonic environment of juvenile basement rocks from the Central Tianshan Arc and southern Beishan Orogenic Belt, whereas a Mesoproterozoic correlation with the Tarim Craton is highly unlikely. Thus, we believe that the Zhusileng–Hangwula Belt has an analogous tectonic evolution and a close affinity with the Central Tianshan Arc and southern Beishan Orogenic Belt during the Mesoproterozoic.

Provenance of the early Cretaceous sediments in the southern Alxa Area: Implications for tectonic evolution of the south Central Asian Orogenic Belt

The Central Asian orogenic belt is an accretionary orogenic belt that lies between the Siberian block and the Tarim and North China blocks. Since the central south of the Central Asian Orogenic Belt has been covered by desert and the Meso-Cenozoic Bayingobi Basin, its spatial configuration, evolution, closure time, and the Paleo-Asian ocean location have not been well described. Through zircon UPb dating, in-situ Hf isotope and rock geochemistry of the Lower Cretaceous Bayingobi Formation sandstones, the provenance of the Bayingobi Formation, and the central south boundary of the Central Asian orogenic belt were determined, the tectonic evolution process was discussed, and the tectonic evolution models were established. The provenance of the Bayingobi Formation mainly originated from Permian to Early Triassic granodiorite, diorite and potassium feldspar granite that cover a large area of the Zongnaishan-Shalazhashan tectonic belt. In the early Permian, this tectonic belt was mainly a continental island arc but gradually transitioned to a continental active margin, accompanied by intense tectonic magmatic activity. With the boundary as the Quagan Qulu ophiolite belt, the north is the Central Asian Orogenic Belt that mainly exhibits juvenile crustal materials, while the south is the Alxa block, which has an affinity with the North China block, and mainly exhibits older crustal materials. From the late Carboniferous to the early Permian, the Paleo-Asian Ocean plate was compressed, subducted along the Enger Us suture, and magmatic distributed along the Nuru-Langshan tectonic belt; from the late early Permian to late Permian, the Paleo-Asian Ocean plate rolled back, changing the subduction zone stress state from compression to extension. During this period, there were large-scale developments of magmatic rocks in the Zongnaishan-Shalazhashan tectonic belt. From early Early Triassic to Middle Triassic period, the Siberian active margin collided with the Alxa block, the Paleo-Asian Ocean was closed, and the overlying Alxa plate experienced lithospheric extension and thinning, accompanied by asthenospheric upwelling, partial melting and the addition of new crustal material. The Meso-Cenozoic basin sediments along the original suture belt are characterized by an enrichment of large ion lithosphere elements, a loss of high field strength elements, LREE enrichment and loss of HREE.

Zircon geochronology and geochemistry of the Permian granitoids in the northern Alxa area and its implications for an extensional setting

Zircon geochronology and geochemistry of the Permian granitoids in the northern Alxa area and its implications for an extensional setting

The Quality Monitoring of Cistanches Herba (Cistanche deserticola Ma): A Value Chain Perspective.

Cistanche deserticola Ma was used as a medicine food homology, which was mainly produced in the Alxa region of northwest China. In recent years, it has been widely used in various food items. The increasing demand for Cistanches Herba has led to problems such as overexploitation and quality deterioration. The quality and safety of herbal medicines are critical and have been shown to be affected by the value chain (VC). Using the VC framework, the study is embedded in a larger study aiming to investigate the effects of different VCs types on the quality and stakeholders of Cistanches Herba. In this study, 90 Cistanches Herba samples were collected during fieldwork. An additional 40 samples were obtained from the herbal markets and medicine purchasing stations. Semi-structured interviews and key informant interviews were performed to collect data on stakeholders in major production areas. These samples were analyzed using high performance liquid chromatography (HPLC) coupled with the k-means clustering method; a targeted quality assessment strategy based on chemical analysis was adopted to understand the quality of Cistanches Herba. Based on market research, the collected samples were divided into different grades through k-means clustering analysis. Moreover, quality differences of Cistanches Herba in Alxa region were explored through DNA barcoding and chemical analysis. Accordingly, 10 different types of VCs were determined in the production of Cistanches Herba. The results show that there is a close relationship between the quality of Cistanches Herba and stakeholder benefits. Vertical integration at different levels was found for independent farmer-based VCs, horizontal collaboration was found in the cooperative-based VCs. The vertical coordination has led to a more consistent traceability system and strict regulation of supply chains. At the same time, the Cistanches Herba were divided into three grades. Through DNA barcoding and chemical analysis, we found that the quality differences between Cistanches Herba in the Alxa area were not significant. It was found that geographical suitability and vertical integration could impact the quality and sustainable production of Cistanches Herba. At the same time, the well-developed VCs can provide products with reliable quality, and ensure adequate financial revenue for relevant stakeholders.

The Carboniferous-Permian tectonic setting for the southernmost Central Asian Orogenic Belt: Constraint from magmatic and sedimentary records in the Alxa area, NW China

The northern Alxa orogenic belt, a junction between the western and eastern Central Asian Orogenic Belt (CAOB), is not well understood, especially in its Carboniferous to Permian tectonic history. In this paper, an integrated study incorporating the geochronology, geochemistry, and isotopes of magmatic rocks and provenance analysis of clastic rocks is conducted to address the above issue. The zircon UPb age data, both in this study and in previous studies, indicates that the Permian magmatic activities in the Alxa Block were intensive and peaked at ca. 276 Ma. The magmatic zircons yield negative εHf(t) values with a wide range, indicating their dominant derivation from the crustal reworking of the Alxa Block with the addition of juvenile materials. The Permian high-K to shoshonitic, I-type granitoids combined with the A-type granitoids/rhyolites were most likely formed in extensional settings. The late Carboniferous sandstones collected along the Badain Jaran fault yield a high proportion of early Paleozoic zircons peaked at ca. 429 Ma with both positive to negative Hf(t) values and a low proportion of Precambrian zircons with positive to negative Hf(t) values. Thus, their provenance were likely the Precambrian to Paleozoic rocks in the Alxa Block and in the terranes in the CAOB. The zircon age spectra with no indicators of Carboniferous zircons approximating their depositional age supports an extensional setting. The Permian A-type granites and volcanic rocks in the Alxa area have some geochemical affinity with the Permian alkaline granitoids in the eastern Tianshan–Beishan and central-north Inner Mongolia regions. These findings support the extension of the Gobi–Tianshan belt in the Alxa area.

Adsorption of uranium(VI) from groundwater by amino-functionalized clay

A kind of amino-functionalized clay was prepared by grafting APTES from clay in the Alxa area (Inner Mongolia, China), and its application for the U(VI) removal from Alxa groundwater was studied in this work. The results indicate that the sorption strongly depended on the pH and ions. The presence of amino functional groups significantly enhances the adsorption of uranium on clay in groundwater. The sorption isotherms fitted well with the Langmuir model, whereas the sorption kinetics could be fitted by a pseudo-second-order model. These findings indicate that amino-functionalized clay can effectively adsorb U(VI) in complex groundwater samples.

Detrital zircon U–Pb–Hf isotopes for the Permo-Carboniferous sediments in the northern Alxa area, NW China: provenance and tectonic implications for the middle segment of the Central Asian Orogenic Belt

AbstractThe northern Alxa orogenic belt, located in the middle segment of the Central Asian Orogenic Belt, is a junction between the North China Craton, Tarim Craton and Mongolian microcontinents. However, its Permo-Carboniferous tectono-palaeogeographic evolution has not been well established. In this study, new zircon U–Pb–Hf isotopic data for the Permo-Carboniferous clastic rocks were used to address the above issue. Based on our dating work and fossil assemblages, we confirmed that the Amushan, Maihanhada, Aqide and Haersuhai formations were formed in the late Carboniferous to early Permian, early to middle Permian, middle Permian and late Permian periods, respectively. The Amushan Formation sandstone in the Zhusileng area contains abundant Palaeozoic zircons (with age peaks at 440 Ma, 475 Ma and 539 Ma) and some Precambrian zircons. These zircon ages and the southeast transgressive direction suggest the provenance to be the local Zhusileng–Hangwula block and the nearby Yagan continental margin. The zircon age pattern with a high proportion of detrital zircons much older than the depositional time and no detrital zircons close to the depositional time suggest an extensional basin depositional setting. The clastic rocks of the Maihanhada, Aqide and Haersuhai formations inherited the source for the Amushan Formation, with a greatly increased input of Permo-Carboniferous volcanic rocks with post-collision or intraplate chemical affinity. Accordingly, a Permo-Carboniferous extensional stage was suggested. Finally, a tectono-palaeogeographic model was reconstructed for the northern Alxa orogenic belt, evolving from a late Carboniferous transgression with crustal extension to early to middle Permian rapid basin filling and a late Permian marine regression.

Early Permian to Late Triassic tectonics of the southern Central Asian Orogenic Belt: geochronological and geochemical constraints from gabbros and granites in the northern Alxa area, NW China

AbstractSituated between the North China Craton to the east and the Tarim Craton to the west, the northern Alxa area in westernmost Inner Mongolia in China occupies a key location for interpreting the late-stage tectonic evolution of the southern Central Asian Orogenic Belt. New LA-ICP-MS zircon U–Pb dating results reveal 282.2 ± 3.9 Ma gabbros and 216.3 ± 3.2 Ma granites from the Yagan metamorphic core complex in northern Alxa, NW China. The gabbros are characterized by low contents of Si, Na, K, Ti and P and high contents of Mg, Ca, Al and Fe. These gabbros have arc geochemical signatures with relative enrichments in large ion lithophile elements and depletions in high field strength elements, as well as negative εNd(t) (−0.91 to −0.54) and positive εHf(t) (2.59 to 6.37) values. These features indicate that a depleted mantle magma source metasomatized by subduction fluids/melts and contaminated by crustal materials was involved in the processes of magma migration and emplacement. The granites show high-K calc-alkaline and metaluminous to weakly peraluminous affinities, similar to A-type granites. They have positive εNd(t) (1.55 to 1.99) and εHf(t) (5.03 to 7.64) values. These features suggest that the granites were derived from the mixing of mantle and crustal sources and formed in a postcollisional tectonic setting. Considering previous studies, we infer that the final closure of the Palaeo-Asian Ocean in the central part of the southern Central Asian Orogenic Belt occurred in late Permian to Early–Middle Triassic times.

Provenance and tectonic setting of the Palaeozoic sediments in the northern Alxa area: Implications for the Central Asian Orogenic Belt

Understanding the tectonic nature of the Alxa area is significant in tectonic reconstruction of the Central Asian Orogenic Belt, but their tectonic correlation, affinity, and implications have not been well defined. The late Palaeozoic sediments in the northern Alxa area can help to understand this question. The sedimentary sequence of the late Palaeozoic strata (Benbatu Formation) indicates that the sedimentary environment of the study area gradually changed from a stable carbonate platform to a bathyal‐abyssal environment with relatively active tectonic setting. The results of the grain size analysis of the sandstone debris show the typical characteristics of a turbidity current. The composition of clastic particles is mainly feldspar, which is characterized by short transport distance and rapid deposition of sediments. Combined with the geochemical characteristics of sandstone, the northern Alxa Block is considered to be an active continental margin in the late Palaeozoic. Through summarizing the zircon data of the Alxa Block, Tarim Craton, and North China Craton, it is concluded that the Palaeozoic clastic zircons in the study area come from the Alxa Block and the surrounding magmatic arc, the Neoproterozoic‐Middle Proterozoic zircons come from the Alxa Block, the Archean zircons come from the North China Craton. According to the age distribution of clastic zircons, the magmatic activity may exist in the early Palaeozoic, which suggests it may not be a stable passive continental margin in the margin of northern Alxa Block in the early Palaeozoic. Based on the provenance analyses, the sources block shows no tectonic affinity to the Tarim Craton, but the Hanwula area in the north of the study area might have been accreted to the Tarim Craton, which means the size of the Paleo‐Asian Ocean between the study area and the Hangwula area is large. The earliest Palaeozoic clastic zircon ages in sedimentary rocks are recorded in the late Cambrian, whereas the earliest Palaeozoic detrital zircons in the Hangwula area are recorded in the Ordovician, which means that the magmatic activity recorded in the study area is earlier than the Hangwula area and indicates that the subduction of the south side of the Paleo‐Asian Ocean began earlier than the north side. The evidence of magmatic rocks discovered so far confirms this view.

Validation of vegetation coverage and NDVI based on UAV remote sensing method and its response to hydrothermal gradient

To verify the accuracy of MODIS-NDVI data products in deserts and provide guidance for scientific management of desert grasslands in the context of climate change, we examined the responses of fractional vegetation cover (FVC) and normalized difference vegetation index (NDVI) to hydrothermal gradient in arid desert areas using unmanned aerial vehicle (UAV) remote sensing. In Alxa desert region of Inner Mongolia, GreenSeeker handheld spectrometer was used to obtain NDVI (NDVIR) of 100 sampling points. NDVI was extracted by MODIS-NDVI data products (NDVIM), and the accuracy of NDVIM was verified by NDVIR. FVC of each sampling point was obtained through unmanned aerial vehicle remote sensing (FVCU), which was used to examine the FVC that was retrieved by the pixel binary model (FVCM). In addition, combining meteorological data, we examined the responses of FVC and NDVI to hydrothermal gradient based on UAV remote sensing method. The results showed that MODIS-NDVI data products reflected the real NDVI in Alxa area with an accuracy of 84.2%, but NDVIM were 15.7% higher than the actual values. FVCM reflected the vegetation coverage of Alxa region with an accuracy of 83.1%, which were 14.8% lower than the real value. Effects of meteorological factors on NDVI was different, depending on the different acquisition methods. NDVI was affected not only by temperature and precipitation, but also by ground temperature, evaporation and the interaction between evaporation and ground temperature. Because of the different degree of atmospheric influence, NDVIM was more affected by ground temperature, evaporation and precipitation than NDVIR, while NDVIR was more affected by temperature than NDVIM. To examine the changes of vegetation coverage across hydrothermal gradient in desert area, we should consider not only precipitation and temperature, but also the interaction between evaporation, ground temperature and meteorological factors. The interaction between temperature and rainfall, evaporation and ground temperature, and between temperature and evaporation had greater impacts on FVCU.

Mobilized Mohr-Coulomb and Hoek-Brown Strength Parameters during Failure of Granite in Alxa Area in China for High-Level Radioactive Waste Disposal

Strength parameters of the host rock is of paramount importance for modelling the behaviors of underground disposal repository of high-level radioactive waste (HLW). Mobilization of strength parameters should be studied for a better understanding and modelling on the mechanical behaviors of the surrounding rock, considering the effect of temperature induced by the nuclear waste. The granite samples cored from NRG01 borehole in Alxa candidate area in China for HLW disposal are treated by different temperatures (T = 20 °C, 100 °C and 200 °C), and then are used to carry out a series of uniaxial and tri-axial compression experiments under various confining pressures (σ3 = 0, 5, 10, 20, and 30 MPa) in this study. With the recorded axial stress—axial strain and axial stress—lateral strain curves, mobilization of both Mohr-Coulomb and Hoek-Brown strength parameters are analyzed with the increasing plastic shear strain. It has been found that NRG01 granite samples show generally similar cohesion weakening and friction strengthening behaviors, as well as the non-simultaneous mobilization of Hoek-Brown strength parameters (mb and s), under the effect of various treatment temperatures. Furthermore, the samples treated by higher temperatures show lower initial values of cohesion, but their initial friction angle and mb values are relatively higher. This should be mainly owing to the thermally induced cracks in the samples. This study should be helpful for a better modelling on the mechanical behaviors of NRG01 granite samples as the host rock of a possible HLW disposal repository.

UNIAXIAL COMPRESSION TEST AND NUMERICAL STUDIES OF GRAIN SIZE EFFECT ON MECHANICAL PROPERTIES OF GRANITE

The whole world has paid more attention to the geological disposal of high-level radioactive waste. Granite is the candidate surrounding rock of high-level radioactive waste disposal project in China. It is very important to understand the strength and destructive characteristics of granite in the design and performance evaluation of disposal system. As aggregates of mineral grains, granite is a heterogeneous rock composed of minerals such as quartz, feldspar and biotite, and the macroscopic mechanical behaviors of rock is obviously affected by mineral grain size. Taking the granites in pre-selected Alxa area of China's high-level geological repository as example, the rock samples have obvious difference in grain size. The effect of grain size on granite mechanical properties is studied using uniaxial compression experiments with the combination of numerical simulation. Uniaxial compression experiments are conducted on MTS815 rock mechanics test system and numerical simulations are based on the particle flow code PFC2D. In the process of numerical simulation, the actual spatial distribution of various components within the rock ISS obtained by image processing based on the surface image of the specimen. The particle flow model of granite based on the actual distributions is thereafter established. The model is used to simulate the uniaxial compression test of granite, and the reliability of the model is demonstrated by comparison with the experimental results. Experiment and simulation results show that the failure mode of Alxa granite is brittle and tensional. Most failure is parallel to the direction of compressive stress. Numerical simulation based on digital image processing method can be used to calculate the mechanical responses of rock materials with high efficiency and accuracy. Mineral grain size has little effect on elastic modulus and Poisson's ratio of the rock, but peak strength is obviously affected by the grain size. The main results are as follows:The strength of rock with fine grained and equigranular texture is high, and the strength of rock with coarse grained and inequigranular texture is low.

Provenance of the early to mid-Paleozoic sediments in the northern Alxa area: Implications for tectonic evolution of the southwestern Central Asian Orogenic Belt

The continental fragments in Northwest China are key to revealing the tectonic and crustal evolution of the Central Asian Orogenic Belt (CAOB). However, their tectonic correlation, affinity and implications have not been well defined. The early to mid-Paleozoic sediments in the northern Alxa area can help to understand this question. These sediments were deposited in a deep to shallow marine environment during a regression. The southeast paleocurrent attributes their provenance to the northwest. Detrital zircons from the collected sandstones record peak ages of approximately 1726 Ma, 1462 Ma, 915 Ma and 438 Ma. The zircon εHf(t) values are negative to positive at 1726 Ma, 915 Ma and 438 Ma, but only positive at 1462 Ma. The detrital zircon U–Pb ages and Hf isotopes suggest the provenance to be the blocks in Central Tianshan and Southern Beishan or their analogs, rather than the Tarim Craton. The source blocks show no tectonic affinity to the Tarim Craton but might be accreted to it in the Neoproterozoic Rodinia. The provenance analyses show tectonic correlation among the northern Alxa, Tianshan and Beishan orogenic belts. The Late Devonian molasse deposits, geochemical shifting to continental margins and suddenly increased early Paleozoic zircons indicate an arc-continent collision. The discovery of more indicators for continental fragments advocates a multiterrane model and dominant crustal reworking/contamination for the tectonocrustal evolution of the CAOB at least during the early to mid-Paleozoic.

Timing and tectonic settings of the Late Paleozoic intrusions in the Zhusileng, northern Alxa: implication for the metallogeny

The Zhusileng–Hangwula tectonic zone, located in the northern Alxa area, is a key site for understanding the evolution of the Paleo-Asian Ocean (PAO) and related mineral formation. We report the major and trace elements and zircon U-Pb ages of newly discovered mafic and intermediate to felsic intrusive rocks in the Zhusileng area. LA-ICP-MS zircon U-Pb analyses of the Echenghei diabase and Haierhan and Heipingshan granites yielded crystallization ages of 305 Ma, 284 Ma, and 271 Ma, respectively. The Late Carboniferous mafic rocks have high MgO and Fe2O3T contents and low K2O and TiO2 contents, and are characterized by depletion of HREEs and notably negative Nb-Ta anomalies, with enrichment of Rb, Ba, and U, which indicate an enriched mantle source. The Early Permian granites of the Zhusileng area are highly differentiated I-type granitoids sourced from mixing of mantle and crust, are high-K calc-alkaline and enriched in LREEs, and exhibit moderately to strongly negative Eu anomalies. All the intrusive rocks formed in a post-collisional extensional environment, which indicates that the branch of the PAO in the northern Alxa area had closed before the Early Permian. The Cu-Mo/Au mineralization in the Zhusileng area formed in the Middle to Late Permian and is genetically linked to the Late Paleozoic post-collisional rifting magmatism from after the closure of the Paleo-Asian Ocean.

Failure Behavior of Granite Affected by Confinement and Water Pressure and Its Influence on the Seepage Behavior by Laboratory Experiments.

Failure behavior of granite material is paramount for host rock stability of geological repositories for high-level waste (HLW) disposal. Failure behavior also affects the seepage behavior related to transportation of radionuclide. Few of the published studies gave a consistent analysis on how confinement and water pressure affect the failure behavior, which in turn influences the seepage behavior of the rock during the damage process. Based on a series of laboratory experiments on NRG01 granite samples cored from Alxa area, a candidate area for China’s HLW disposal, this paper presents some detailed observations and analyses for a better understanding on the failure mechanism and seepage behavior of the samples under different confinements and water pressure. The main findings of this study are as follows: (1) Strength reduction properties were found for the granite under water pressure. Besides, the complete axial stress–strain curves show more obvious yielding process in the pre-peak region and a more gradual stress drop in the post-peak region; (2) Shear fracturing pattern is more likely to form in the granite samples with the effect of water pressure, even under much lower confinements, than the predictions from the conventional triaxial compressive results; (3) Four stages of inflow rate curves are divided and the seepage behaviors are found to depend on the failure behavior affected by the confinement and water pressure.