2020 Vol. 11, No. 2
Grain crushing is commonly encountered in deep foundation engineering, high rockfill dam engineering, railway engineering, mining engineering, coastal engineering, petroleum engineering, and other geoscience application. Grain crushing is affected by fundamental soil characteristics, such as their mineral strength, grain size and distribution, grain shape, density and specimen size, and also by external factors including stress magnitude and path, loading rate and duration, degree of saturation, temperature and geochemical environment. Crushable material becomes a series of different materials with the change in its grading during grain crushing, resulting in a decrease in strength and dilatancy and an increase in compressibility. Effects of grain crushing on strength, dilatancy, deformation and failure mechanisms have been extensively investigated through laboratory testing, discrete element method (DEM) modelling, Weibull statistics, and constitutive modelling within the framework of the extended crushing-dependent critical state theory or the energy-based theory. Eleven papers summarized in this review article for this special issue addressed the above issues in grain crushing through the advanced testing and modelling.
Strength and deformation behaviors of rockfill materials, key factors for determining the stability of dams, pertain strongly to the grain crushing characteristics. In this study, single-particle crushing tests were carried out on rockfill materials with nominal particle diameters of 2.5 mm, 5 mm and 10 mm to investigate the particle size effect on the single-particle strength and the relationship between the characteristic stress and probability of non-failure. Test data were found to be described by the Weibull distribution with the Weibull modulus of 3.24. Assemblies with uniform nominal grains were then subjected to one-dimensional compression tests at eight levels of vertical stress with a maximum of 100 MPa. The yield stress in one-dimensional compression tests increased with decreasing the particle size, which could be estimated from the single-particle crushing tests. The void ratio-vertical stress curve could be predicted by an exponential function. The particle size distribution curve increased obviously with applied stresses less than 16 MPa and gradually reached the ultimate fractal grading. The relative breakage index became constant with stress up to 64 MPa and was obtained from the ultimate grading at the fractal dimension (a ¼ 2:7). A hyperbolical function was also found useful for describing the relationship between the relative breakage index and input work during one-dimensional compression tests.
Particle crushing occurs near the tip of a penetrometer and influences the development of the tip resistance. To study particle crushing near a penetrometer tip, a cone penetrometer was monotonically jacked and then load-tested in medium dense and dense silica sand samples prepared in a halfcylindrical calibration chamber with viewing windows. During the tests, images of the advancing penetrometer and the surrounding soil were taken using digital cameras and analyzed to obtain the displacement and strain fields around the penetrometer using the Digital Image Correlation (DIC) technique. Subsequently, soil samples were collected near the tip of the penetrometer using a novel agarimpregnation technique and digitized using an X-ray microscope. The digitized samples were analyzed to reconstruct the three-dimensional models of individual particles, generating the gradation and relative breakage parameters of the sand around the cone penetrometer.
The Weibullian behavior of single grain crushing strength was investigated experimentally and numerically with the aim of enhancing the understanding of rock grain breakage. The morphologies of pebble grains were obtained using white light 3D laser scanning and image processing. A grain shape library was constructed for grain shape analysis with different shape descriptors. The use of the shape library and grain stability analysis is discussed for a suggested procedure to rotate a grain to its most stable configuration. Single grain crushing tests were performed for 30 pebbles to obtain force-displacement curves and fracture patterns. Each grain was compressed diametrically between flat platens. As expected, the values of the stress at bulk fracture follow a Weibull distribution. A procedure for generating crushable agglomerates with realistic particle shapes was demonstrated, which was accomplished in the discrete element modeling (DEM) of the single grain crushing test. The work presented here is novel in that both the heterogeneous micro-structures and randomly distributed flaws are considered. The DEM results demonstrate that the proposed modeling approach and calibrated parameters are reliable and can reflect the crushing behavior of rock pebbles. Finally, three parametric studies were presented evaluating the effects of micro-crack density, micro-crack disorder, and grain morphology on the Weibullian behavior of the crushing strength, none of which has previously been thoroughly considered. These three studies provide a deeper insight into the origin of the Weibullian behavior of single grain crushing strength.
Particle breakage is a common occurrence in granular systems when the external stress exceeds the individual particle strength. A large number of experimental evidences suggested that particle breakage may significantly influence the soil behavior. In the case of pile foundations, the subsoil below the pile tip experiences considerable high stress and consequently prone to break. Due to the lack of sufficient understanding on particle breakage mechanism, there is currently no consentaneous theoretical background for particle breakage analysis during the pile penetration process. This study aims to clarify the location of particle breakage and its evolving characteristics with the aid of acoustic emission (AE) source location method. The spatial distribution of AE hypocenters is interpreted to be associated with the mechanism of particle breakage. Results showed that the AE sources were not uniformly distributed, but concentrated within certain zones below the pile tip. This AE concentration zone was pushed downward with the advancing pile tip, and its distance from the real time pile tip position decreased after certain depth of pile penetration. The location of particle breakage interpreted from AE source location was verified with posttest excavations and the insights on the particle breakage evolution zone were further discussed.
There have been significant advances in the application of critical state, CS, in liquefaction potential assessment. This was done by comparing state parameter, j with estimated characteristic cyclic stress ratio, CSR due to an earthquake. A cyclic resistance ratio, CRR curve, which can be determined from cyclic liquefaction tests, separates historical liquefied and non-liquefied data points (j, CSR). On the other hand, the concepts of equivalent granular state parameter, j*, which was developed for sands with fines, can be used in lieu j to provide a unifying framework for characterizing the undrained response of sands with non/low plasticity fines, irrespective of fines content (fc). The present work combines these two propositions, and by merely substituting j* for j into the aforementioned CS approach to capture the influence of fc. A series of static and cyclic triaxial tests were conducted, separately and independently of the concept of j*, for sand with up to fc of 30%. The clean sand was collected from Sabarmati river belt at Ahmedabad city in India which was severely affected during the Bhuj earthquake, 2001. The experimental data gave a single relation for CRR and j* which was then used to assess liquefaction potential for a SPT based case study, where fc varies along the depth. The prediction matched with the field observation.
This paper experimentally explores the frictional sliding behavior of two simulated gouges: one, a series of quartz–smectite mixtures, and the other, powdered natural rocks, aiming to evaluate and codify the effect of mineralogy on gouge dilation and frictional strength, stability, and healing. Specifically, velocity-stepping and slide-hold-slide experiments were performed in a double direct shear configuration to analyze frictional constitutive parameters at room temperature, under normal stresses of 10, 20, and 40 MPa. Gouge dilation was measured based on the applied step-wise changes in shear velocity. The frictional response of the quartz–smectite mixtures and powdered natural rocks are affected by their phyllosilicate content. Frictional strength and healing rates decrease with increasing phyllosilicate content, and at 20 wt.% a transition from velocity-weakening to velocity-strengthening behavior was noted. For both suites of gouges, dilation is positively correlated with frictional strength and healing rates, and negatively correlated with frictional stability. Changes in the permeability of gouge-filled faults were estimated from changes in mean porosity, indexed through measured magnitudes of gouge dilation. This combined analysis implies that the reactivation of caprock faults filled with phyllosilicaterich gouges may have a strong influence on permeability evolution in caprock faults.
This paper presents a combined method to model grain crushing effects with discrete element method. This method combines the two most commonly used concepts to model grain crushing in DEM, i.e. the replacement method and the agglomerate method, so that it is both accurate and efficient. The method can be easily implemented. The performance is shown by several DEM simulations of biaxial tests. Particles with different crush-abilities are modeled. DEM simulation results with and without grain crushing are compared and discussed. The change of grain size distribution due to grain crushing is also investigated.
Breakage of particles will have greatly influence on mechanical behavior of granular material (GM) under external loads, such as ballast, rockfill and sand. The discrete element method (DEM) is one of the most popular methods for simulating GM as each particle is represented on its own. To study breakage mechanism of particle breakage, a cohesive contact mode is developed based on the GPU accelerated DEM code - Blaze-DEM. A database of the 3D geometry model of rock blocks is established based on the 3D scanning method. And an agglomerate describing the rock block with a series of non-overlapping spherical particles is used to build the DEM numerical model of a railway ballast sample, which is used to the DEM oedometric test to study the particles’ breakage characteristics of the sample under external load. Furthermore, to obtain the meso-mechanical parameters used in DEM, a black-analysis method is used based on the laboratory tests of the rock sample. Based on the DEM numerical tests, the particle breakage process and mechanisms of the railway ballast are studied. All results show that the developed code can better used for large scale simulation of the particle breakage analysis of granular material.
Determination of the critical state line (CSL) is important to characterize engineering properties of granular soils. Grain size distribution (GSD) has a significant influence on the location of CSL. The influence of particle breakage on the CSL is mainly attributed to the change in GSD due to particle breakage. However, GSD has not been properly considered in modeling the CSL with influence of particle breakage. This study aims to propose a quantitative model to determine the CSL considering the effect of GSD. We hypothesize that the change of critical state void ratio with respect to GSD is caused by the same mechanism that influences of the change of minimum void ratio with respect to GSD. Consequently, the particle packing model for minimum void ratio proposed by Chang et al. (2017) is extended to predict critical state void ratio. The developed model is validated by experimental results of CSLs for several types of granular materials. Then the evolution of GSD due to particle breakage is incorporated into the model. The model is further evaluated using the experimental results on rockfill material, which illustrates the applicability of the model in predicting CSL for granular material with particle breakage.
It is well known that the compressibility of crushable granular materials increases with the moisture content, due to the decrease of particle strength in a humid environment. An existing approach to take into account the effect of grain breakage in constitutive modeling consists in linking the evolution of the grain size distribution to the plastic work. But how the material humidity can affect this relationship is not clear, and experimental evidence is quite scarce. Based on compression tests on dry and saturated crushable sand recently reported by the present authors, a new non-linear relationship is proposed between the amount of particle breakage and the plastic work. The expression contains two parameters: (1) a material constant dependent on the grain characteristics and (2) a constant depending on the wetting condition (in this study, dry or saturated). A key finding is that the relationship does not depend on the stress path and, for a given wetting condition, only one set of parameters is necessary to reproduce the results of isotropic, oedometric, and triaxial compression tests. The relationship has been introduced into an elastoplastic constitutive model based on the critical state concept with a double yield surface for plastic sliding and compression. The breakage ratio is introduced into the expression of the elastic stiffness, the critical state line and the hardening compression pressure. Incremental stress-strain computations with the model allow the plastic work to be calculated and, therefore, the evolution of particle crushing can be predicted through the proposed non-linear relationship and reintroduced into the constitutive equations. Accurate predictions of the experimental results in terms of both stress-strain relationships and breakage ratio were obtained.
Crushing of grains can greatly influence the strength, dilatancy, and stress-strain relationship of rockfill materials. The critical state line (CSL) in the void ratio versus mean effective stress plane was extended to the breakage critical state plane (BCSP). A state void-ratio-pressure index that incorporated the effect of grain crushing was proposed according to the BCSP. Rowe’s stress-dilatancy equation was modified by adding the breakage voidratio- pressure index, which was also incorporated into the formulations of the bounding stress ratio and plastic modulus. A BCSP-based bounding surface plasticity model was proposed to describe the state-dependent stressstrain behaviors and the evolution of grain crushing during shearing process of rockfill materials, and was shown to sufficiently capture the breakage phenomenon.
Metamorphic provinces such as the ~1 Ga Grenvillian, ~400 Ma Caledonide and Triassic Qinling Provinces often contain rocks with high-pressure assemblages such as eclogites, which formed at mantle depths in subduction zones. These are evidence of the accretion of terranes by subduction of oceans and collision to form large tectonostratigraphic provinces. The Mesoproterozoic Namaqua-Natal Province comprises a number of terranes thought to have been assembled by plate-tectonic processes, but they have generally yielded metamorphic pressures below 5 kbar, corresponding to <20 km, crustal depths, lacking evidence for subduction processes. The Kaaien Terrane in the Namaqua Front contains two large garbenschiefer units with the unusual paragenesis garnet-hornblende-epidote-white mica-plagioclase-ilmenite-quartz. Their protoliths are graywackes influenced by andesitic volcanism during their deposition at ~1870 Ma, in a passive margin of the Rehoboth Province or Kaapvaal Craton. Prograde garnet growth dated at 1165 5 Ma culminated in peak metamorphic conditions of 645 30 C and 10.4 0.7 kbar, corresponding to 40 km depth. This is attributed to subduction of these rocks before collision between the overriding arc-related Areachap Terrane, the Kaaien Terrane and the Kaapvaal- Rehoboth cratonic block during the Namaqua orogeny. Exhumation of the garbenschiefer slabs was followed by rapid cooling, as the 1143 5 Ma argon dates of hornblende and white mica, with closure temperatures ~540 C and ~440 C respectively, are the same within error. This was probably due to tectonic juxtaposition of the garbenschiefer slab with much cooler rock units. The exhumation was accommodated along the Trooilapspan- Brakbosch Shear Zone due to ongoing transpression. Other components of the Namaqua Front have distinctly different P-T-t paths, exemplified by greenschist metamorphism in the 1300 Ma Wilgenhoutsdrift Group, and medium-pressure metamorphism in the Areachap Terrane. They were juxtaposed by late-tectonic uplift and transpressional movements. The ~40 km depth of garbenschiefer peak metamorphism is the deepest yet found in the Namaqua-Natal Province and strengthens the plate tectonic model of accretion by collision of terranes at the end of a Wilson cycle. The high pressure paragenesis of the garbenschiefer was preserved due to its location in the Namaqua Front, whereas most other parts of the Namaqua-Natal Province were overprinted by 1100–1020 Ma thermal events after the collision events.
High resolution (939 samples) total organic carbon content (TOC) analyses were conducted on the Shuanghe Section of ~152.6 m in the Changning area, Sichuan Basin. The sampling section was divided into two units considering the distinct-different deposit environment and sediments accumulation rate. The lower part (Unit 1) and the peer part (Unit 2) with high resolution sample spacing (0.08–0.4 m) enables the identification of the precession cycle in two sedimentary sequences with distinct different sedimentary accumulation rate. MTM Power spectral analyses on untuned TOC series reveals significant peaks exceeding above the 95% confidence level and shows that both Unit 1 and Unit 2 have recorded Milankovitch cycles of 405 kyr long eccentricity, short eccentricity, obliquity and precession. The floating astronomical time scale (ATS) was constructed on the Shuanghe Section in the Early Silurian (~439.673–444.681 Ma), and which was calibrated by 405 kyr long eccentricity cycles. The total duration of the Wufeng and Longmaxi shales is 5.01 Myr. The floating ATS used for estimating the duration of the graptolite zones and each stage in the study interval. Finally, we postulated two models that could verify the linkage between orbital cycle and organic accumulation. To make sure whether productivity or preservation is the main factor that under long eccentricity control, the phase correlation between the obtained filtered signal and the theoretical orbital solution should be made clear in the further research.
TheWulong lode deposit contains over 80 tonnes of gold with an average grade of 5.35 g/t. It is one of the largest deposits in Dandong City, Liaoning Province in northeast China. Previous studies on the deposit focused on its geological characteristics, geochemistry, fluid inclusions, and the timing of gold mineralization. However, controversy remains regarding the origin of the ore-forming fluids and metals, and the genesis of the gold deposit. This paper presents zircon UePb and pyrite RbeSr ages and S, Pb, He, and Ar isotopic results along with quartz H and O isotopic data for all litho-units associated with the deposit. Laser ablation inductively coupled mass spectrometry measurements yielded zircon UePb dates for samples of pre-mineralization rocks like granite porphyry dike, the Sanguliu granodiorite, fine-grained diorite, and syn-mineralization diorite, as well as post-mineralization dolerite, and lamprophyre; their emplacement ages are 126 1 Ma, 124 1 Ma, 123 1 Ma, 120 1 Ma, 119 2 Ma, and 115 2 Ma, respectively. The pyrite RbeSr isochron age is 119 1 Ma, indicating that both magmatism and mineralization occurred during the Early Cretaceous. The d18OH2O values of ore-forming hydrothermal fluids from the quartzepolymetallic sulfide vein stage vary from 4.8& to 6.5&, and the dDV-SMOW values are between 67.7& and 75.9&, indicating that the ore-forming fluids were primarily magmatic. The noble gas isotope compositions of fluid inclusions hosted in pyrite suggest that the ore-forming fluids were dominantly derived from crustal sources with minor mantle input. Sulfur isotopic values of pyrite vary between 0.2& and 3.5&, suggesting that S was derived from a homogeneous magmatic source or possibly from fluids derived from the crust. The Pb isotopic compositions of sulfides (207Pb/204Pb ¼ 15.51 e15.71, 206Pb/204Pb ¼ 17.35e18.75, 208Pb/204Pb ¼ 38.27e40.03) indicate that the Pb of the Wulong gold deposit is a mixture of crust and mantle components. Geochronological and geochemical data, together with the regional geological history, indicate that Early Cretaceous magmatism and mineralization of the Wulong gold deposit occurred during the rollback of the subducting Paleo-Pacific Plate, which resulted in lithospheric thinning and the destruction of the North China Craton (NCC), which indicates that the deposit is of magmaticehydrothermal origin.
The amount of literature on both melting and thermal conductivity of iron at Earth’s core conditions is overwhelming and the discrepancies are very large. There is a broad range of experimental techniques each of which is flawed to a certain degree, which may explain the discrepancy. In this report, we present new data using a different method for determining the phase behavior and resistivity of iron in the laser-heated diamond cell by measuring the electrical resistance of both solid and liquid iron wires. The experiment avoids some of the major flaws of previous experiments, the most important of which is the detection of the onset of melting. These measurements confirm a shallow melting curve found earlier and the resistivity data imply a trend towards low thermal conductivity in the liquid outer core.
To determine the lateral and vertical variations in crustal structure and their influence on the seismicity of the Western North China Craton, the Trans-North China Orogen, and the surrounding regions, the wavelet multi-scale structures, Moho depth, crustal density structures, and isostatic state are modelled using Bouguer gravity anomaly data, topography, and earthquake focal mechanisms. We obtained homogeneous crustal densities and deviations of <1 km between the crustal thicknesses estimated from the isostatic model and those inverted from the Bouguer gravity anomalies in the Ordos Block, the Inner Mongolia Suture Zone, the Sichuan Basin, and the Jizhong Depression. These results provide new evidence for relatively simple and stable continental crustal structures, and indicate that these regions will remain stable in both the vertical and lateral directions. The Hetao Graben, Yinchuan Graben, Weihe Basin, and Shanxi graben system have heterogeneous crustal densities and are isostatically over-compensated. In contrast, the crust beneath the Yinshan Uplift, Lvliang Uplift, and northern and central Taihang Uplift is thin and under-compensated. The heterogeneous crustal densities and non-isostatic state beneath the Tibetan Plateau and Qinling Central China Orogen indicate that these two blocks are unstable in the vertical and lateral directions. Although Cenozoic deformation of the North China Craton is thought to be driven by lithospheric stresses related to the India-Eurasia collision and Pacific slab retreat in South East Asia, we suggest that gravitational potential energy created by the heterogeneous crustal structure modulates these first-order forces. The results of this study could constrain the causes of seismicity in systems surrounding the Ordos Block.
The Precambrian basement rocks of the Eastern Granite-Rhyolite Province (EGRP) in central Illinois (midcontinent region of North America) exhibit a complex history of early volcanism, granite emplacement, and intrusion of mafic rocks. A comprehensive suite of dedicated petrographic analyses, geophysical logs, and drill core from four basement-penetrating wells, two-dimensional and three-dimensional seismic reflection data, and U-Pb age data from the Illinois Basin–Decatur Project (IBDP) and Illinois Carbon Capture Storage (ICCS) Project site provide new constraints for interpreting the Precambrian basement of the Illinois Basin. These new data reveal the basement to be compositionally and structurally complex, having typical EGRP felsic volcanic rocks intruded by the first reported gabbro in the Precambrian basement in Illinois. Zircons (n ¼ 29) from rhyolite give a U-Pb weighted mean average age of 1467 9 Ma. Zircons (n ¼ 3) from a gabbro dike that intrudes the rhyolite yield a concordia age of 1073 12 Ma, which corresponds to Grenville-age extension and represents the first Grenville-age rock in Illinois and in the EGRP. A high-resolution three-dimensional seismic reflection volume, coincident with the four wells, provides a context for interpreting the petrological data and implies a high degree of heterogeneity for basement rocks at the IBDP–ICCS site, as also shown by the drill cores. The occurrence of Grenville-age gabbro is related to a prominent bowl-like structure observed on local two-dimensional seismic reflection profiles and the three-dimensional volume that is interpreted as a deep-seated mafic sill complex. Furthermore, heterogeneities such as the brecciated EGRP rhyolite and later gabbro intrusion observed in the basement lithology at the IBDP–ICCS may reflect previously unknown distal elements of the 1.1 Ga Midcontinent Rift in the EGRP and more likely Grenville-age extension.
Phase equilibria modelling coupled with U–Pb zircon and monazite ages of garnet–cordierite gneiss from Vallikodu Kottayam in the Kerala Khondalite Belt, southern India are presented here. The results suggest that the area attained peak P–T conditions of ~900 C at 7.5–8 kbar, followed by decompression to 3.5–5 kbar and cooling to 450–480 C, preserving signatures of the partial melting event in the field of high to ultra-high temperature metamorphism. Melt reintegration models suggest that up to 35% granitic melt could have been produced during metamorphism at ~950 C. The U–Pb age data from zircons (~1.0 – ~0.7 Ga) and chemical ages from monazites (~540 Ma and ~941 Ma) reflect a complex tectonometamorphic evolution of the terrain. The ~941 Ma age reported from these monazites indicate a Tonian ultra-high temperature event, linked to juvenile magmatism/ deformation episodes reported from the Southern Granulite Terrane and associated fragments in Rodinia, which were subsequently overprinted by the Cambrian (~540 Ma) tectonothermal episode.
Catastrophic natural hazards, such as earthquake, pose serious threats to properties and human lives in urban areas. Therefore, earthquake risk assessment (ERA) is indispensable in disaster management. ERA is an integration of the extent of probability and vulnerability of assets. This study develops an integrated model by using the artificial neural network–analytic hierarchy process (ANN–AHP) model for constructing the ERA map. The aim of the study is to quantify urban population risk that may be caused by impending earthquakes. The model is applied to the city of Banda Aceh in Indonesia, a seismically active zone of Aceh province frequently affected by devastating earthquakes. ANN is used for probability mapping, whereas AHP is used to assess urban vulnerability after the hazard map is created with the aid of earthquake intensity variation thematic layering. The risk map is subsequently created by combining the probability, hazard, and vulnerability maps. Then, the risk levels of various zones are obtained. The validation process reveals that the proposed model can map the earthquake probability based on historical events with an accuracy of 84%. Furthermore, results show that the central and southeastern regions of the city have moderate to very high risk classifications, whereas the other parts of the city fall under low to very low earthquake risk classifications. The findings of this research are useful for government agencies and decision makers, particularly in estimating risk dimensions in urban areas and for the future studies to project the preparedness strategies for Banda Aceh.
High-pressure (HP) or ultrahigh-pressure (UHP) rutile-quartz veins that form at mantle depths due to fluid-rock interaction can be used to trace the properties and behavior of natural fluids in subduction zones. To explore the fluid flow and the associated element mobility during deep subduction and exhumation of the continental crust, we investigated the major and trace elements of Ti-rich minerals. Additionally, U–Pb dating, trace element contents, and Lu–Hf isotopic composition of zircon grains in the UHP eclogite and associated rutile-quartz veins were examined in the North Qaidam UHP metamorphic belt, Yuka terrane. The zircon grains in the rutile-quartz veins have unzoned or weak oscillatory zonings, and show low Th/U ratios, steep chondrite-normalized patterns of heavy rare earth elements (HREEs), and insignificant negative Eu anomalies, indicating their growth in metamorphic fluids. These zircon grains formed in 431 3 Ma, which is consistent with the 432 2 Ma age of the host eclogite. As for the zircons in the rutile-quartz veins, they showed steep HREE patterns on one hand, and were different from the zircons present in the host eclogite on the other. This demonstrates that their formation might have been related to the breakdown of the early stage of garnet, which corresponds to the abundance of fluids during the early exhumation stage. The core-rim profile analyses of rutile recorded a two-stage rutile growth across a large rutile grain; the rutile core has higher Nb, Ta, W, and Zr contents and lower Nb/Ta ratios than the rim, indicating that the rutile domains grew in different metamorphic fluids from the core towards the rim. The significant enrichment of high field strength elements (HFSEs) in the rutile core suggests that the peak fluids have high solubility and transportation capacity of these HFSEs. Furthermore, variations in the Nb vs. Cr trends in rutile indicate a connection of rutile to mafic protolith. The zircon grains from both the rutile-quartz veins and the host eclogite have similar Hf isotopic compositions, indicating that the vein-forming fluids are internally derived from the host eclogite. These fluids accumulated in the subduction channel and were triggered by local dehydration of the deeply subducted eclogite during the early exhumation conditions.
The West Junggar Orogenic Belt (WJOB) in northwestern Xinjiang, China, is located in the core of the western part of the Central Asian Orogenic Belt (CAOB). It has suffered two stage tectonic evolutions in Phanerozoic, before and after the ocean–continental conversion in Late Paleozoic. The later on intracontinental deformation, characterized by the development of the NE-trending West Junggar sinistral strike-slip fault system (WJFS) since Late Carboniferous and Early Permian, and the NW-trending Chingiz-Junggar dextral strike-slip fault (CJF) in Mesozoic and Cenozoic, has an important significance for the tectonic evolution of the WJOB and the CAOB. In this paper, we conduct geometric and kinematic analyses of the WJOB, based on field geological survey and structural interpretation of remote sensing image data. Using some piercing points such as truncated plutons and anticlines, an average magnitude of ~73 km for the left-lateral strike-slip is calculated for the Darabut Fault, a major fault of the WJFS. Some partial of the displacement should be accommodated by strike-slip fault-related folds developed during the strike-slip faulting. Circular and curved faults, asymmetrical folds, and irregular contribution of ultramafic bodies, implies potential opposite vertical rotation of the Miao’ergou and the Akebasitao batholiths, resulted from the sinistral strike-slipping along the Darabut Fault. Due to conjugate shearing set of the sinistral WJFS and the dextral CJF since Early Mesozoic, superimposed folds formed with N–S convergence in southwestern part of the WJOB.
Mantle peridotites entrained as xenoliths in the lavas of Ngao Bilta in the eastern branch of the continental Cameroon Line were examined to constrain mantle processes and the origin and nature of melts that have modified the upper mantle beneath the Cameroon Line. The xenoliths consist mainly of lherzolite with subordinate harzburgite and dunite. They commonly contain olivine, orthopyroxene, clinopyroxene and spinel although the dunite is spinel-free. Amphibole is an essential constituent in the lherzolites. Mineral chemistry differs between the three types of peridotite: olivines have usual mantle-like Mg# of around 90 in lherzolites, but follow a trend of decreasing Mg# (to 82) and NiO (to 0.06 wt.%) that is continuous in the dunites. Lherzolites also contain orthopyroxenes and/or clinopyroxenes with low-Mg#, indicating a reaction that removes Opx and introduces Cpx, olivine, amphibole and spinel. This is attributed to reaction with a silica-undersaturated silicate melt such as nephelinite or basanite, which originated as a low-degree melt from a depleted source as indicated by low Al2O3 and Na2O in Cpx and high Na2O/K2O in amphibole. Thermobarometric estimates place the xenoliths at pressures of 11–15 kbar (35–50 km) and temperatures of 863–957 C, along a dynamic rift geotherm and shallower than the region where carbonate melts may occur. The melt/rock reactions exhibited by the Ngao Bilta xenoliths are consistent with their peripheral position in the eastern branch of the Cameroon Volcanic Line in an area of thinned crust and lithosphere beneath the Adamawa Uplift.
Paleo- to Mesoproterozoic sedimentary rocks in the southern margin of the North China Craton (NCC) are represented by the Ruyang and Luoyu groups. We studied the sedimentary rocks from the Yunmengshan and Beidajian formations of the Ruyang Group and the Cuizhuang and Sanjiaotang formations of the Luoyu Group. Detrital zircon grains from these formations have U–Pb age populations of 3.64–3.31 Ga, 2.96–2.86 Ga, 2.72–2.59 Ga, 2.56–2.47 Ga, 2.45–2.0 Ga, 1.99–1.85 Ga and 1.84–1.65 Ga. The geochemical features of the sedimentary rocks suggest that some of the sediments were sourced from intermediate to felsic magmatic rocks. The age groups of the detrital zircon are roughly consistent with the tectono-thermal events in the southern margin of the NCC. The Hf isotopic compositions of detrital zircon from the sedimentary rocks in Ruyang and Luoyu groups suggest that significant crustal growth and reworking of the NCC took place during the Neoarchean and early- to mid-Paleoproterozoic, while crustal reworking at the Paleoarchean and late-Paleoproterozoic, and crustal growth at the Mesoarchean. We suggest the depositional times of the Ruyang Group and Luoyu Group are constrained to no older than 1.75–1.7 Ga and 1.7–1.65 Ga, respectively. Formation of late-Paleoproterozoic basins related to the strike slip and extrusion tectonics that cross-cut the NCC during the late Paleoproterozoic (<1.75 Ga), and the late Paleoproterozoic sedimentation once isochronous developed in the southern margin of the NCC through the Taihang region of the interior NCC and linked the Yanshan–Liaoxi regions of the northern NCC.
The volcanic rocks hosting the iron deposits in the Aqishan–Yamansu metallogenic belt are sodium-rich. The geochronology, petrography, and geochemistry of minerals and sodium-rich rocks as well as the relationship between these rocks and the iron deposits are studied. Geochemically, the ore-hosting volcanic rocks are sodiumrich (the averages of Na2O and Na2O/K2O are 4.31 wt.% and 8.56, respectively) and belong to the calc-alkaline series. They are enriched in LREEs and LILEs (Ba, U, K, and Sr), but depleted in HFSEs (Nb, Ta, and Ti). SHRIMP zircon U–Pb dating of the crystal tuff in the Aqishan Formation and the dacite in the Tugutu Bulak Formation yields ages of 337.5 2.3 Ma (n ¼ 15, MSWD ¼ 0.85) and 313.0 3.3 Ma (n ¼ 13, MSWD ¼ 0.74), respectively, indicating that the sodium-rich volcanic rocks formed from the early–late Carboniferous. Electron microprobe data from plagioclases demonstrate that albites and/or oligoclases were formed in the basic–intermediate–acid volcanic rocks. Two stages of albitization are identified, and the latter is likely attributed to the dissolution of iron in the Aqishan–Yamansu belt. The sodium-rich volcanic rocks probably formed by the interaction between volcanic lava and seawater after volcanoes erupted on the seafloor; meanwhile, the albites formed by element substitution in a low-metamorphic environment. The spatiotemporal coupling relationship between sodium-rich volcanic rocks and iron deposits in the Aqishan–Yamansu belt is favorable. Iron dissolved from the dark minerals of basic–intermediate volcanic rocks through sodium metasomatism is one of the material sources for the iron deposits.