2020 Vol. 11, No. 4
Soft computing techniques are becoming even more popular and particularly amenable to model the complex behaviors of most geotechnical engineering systems since they have demonstrated superior predictive capacity, compared to the traditional methods. This paper presents an overview of some soft computing techniques as well as their applications in underground excavations. A case study is adopted to compare the predictive performances of soft computing techniques including eXtreme Gradient Boosting (XGBoost), Multivariate Adaptive Regression Splines (MARS), Artificial Neural Networks (ANN), and Support Vector Machine (SVM) in estimating the maximum lateral wall deflection induced by braced excavation. This study also discusses the merits and the limitations of some soft computing techniques, compared with the conventional approaches available.
This study proposed a random Smoothed Particle Hydrodynamics method for analyzing the post-failure behavior of landslides, which is based on the Karhunen–Loeve (K-L) expansion, the non-Newtonian fluid model, and the OpenMP parallel framework. Then, the applicability of this method was validated by comparing the generated random field with theoretical result and by simulating the post-failure process of an actual landslide. Thereafter, an illustrative landslide example was created and simulated to obtain the spatial variability effect of internal friction angle on the post-failure behavior of landslides under different coefficients of variation (COVs) and correlation lengths (CLs). As a conclusion, the reinforcement with materials of a larger friction angle can reduce the runout distance and impact the force of a landslide. As the increase of COV, the distribution range of influence zones also increases, which indicates that the deviation of influence zones becomes large. In addition, the correlation length in Monte Carlo simulations should not be too small, otherwise the variation range of influence zones will be underestimated.
Collision between the Indian and Eurasian plates formed the ~2500 km long Yarlung Zangbo Suture Zone and produced the Himalaya mountains and Tibetan plateau. Here we offer a new explanation for tectonic events leading to this collision: that the northward flight of India was caused by an Early Cretaceous episode of subduction initiation on the southern margin of Tibet. Compiled data for ophiolites along the Yarlung Zangbo Suture Zone show restricted ages between 120 Ma and 130 Ma, and their supra-subduction zone affinities are best explained by seafloor spreading in what became the forearc of a north-dipping subduction zone on the southern margin of Tibet. The subsequent evolution of this new subduction zone is revealed by integrating data for arcrelated igneous rocks of the Lhasa terrane and Xigaze forearc basin deposits. Strong slab pull from this new subduction zone triggered the rifting of India from East Gondwana in Early Cretaceous time and pulled it northward to collide with Tibet in Early Paleogene time.
The Shatsky and Hess Rises, the Mid-Pacific Mountains and the Line Islands large igneous provinces (LIPs) present different challenges to conventional plume models. Resolving the genesis of these LIPs is important not only for a more complete understanding of mantle plumes and plume-generated magmatism, but also for establishing the role of subducted LIP conjugates in the evolution of the Laramide orogeny and other circum-Pacific orogenic events, which are related to the development of large porphyry systems. Given past difficulties in developing consistent geodynamic models for these LIPs, it is useful to consider whether viable alternative geodynamic scenarios may be provided by recent concepts such as melt channel networks and channel-associated lineaments, along with the “two mode” model of melt generation, where a deeply-sourced channel network is superimposed on the plume, evolving and adapting over millions of years. A plume may also interact with transform faults in close proximity to a mid ocean ridge, with the resultant bathymetric character strongly affected by the relative age difference of lithosphere across the fault. Our results suggest that the new two-mode melt models resolve key persistent issues associated with the Shatsky Rise and other LIPs and provide evidence for the existence of a conduit system within plumes that feed deeply-sourced material to the plume head, with flow maintained over considerable distances. The conduit system eventually breaks down during plume – ridge separation and may do so prior to the plume head being freed from the triple junction or spreading ridge. There is evidence for not only plume head capture by a triple junction but also for substantial deformation of the plume stem as the distance between the stem and anchored plume head increases. The evidence suggests that young transforms can serve as pathways for plume material migration, at least in certain plume head – transform configurations. A fortuitous similarity between the path of the Shatsky and Sio plumes, with respect to young spreading ridges and transforms, helps to clarify previously problematic bathymetric features that were not readily ascribed to fixed plumes alone. The Line Island Chain, which has been the subject of a vast number of models, is related mainly to several plumes that passed beneath the same region of oceanic crust, a relatively rare event that has resulted in LIP formation rather than a regular seamount track. Our findings have important implications for the timing and mechanism for the Laramide Orogeny in North America, demonstrating that the Hess Rise conjugate may be much smaller than traditionally thought. The Mid Pacific Mountains conjugate may not exist at all, given large parts of these LIPs were formed at an ‘off-ridge’ site. This needs to be taken into account while considering the effects of conjugate collision on mineralization and orogenic events.
The Budunhua Cu deposit is located in the Tuquan ore-concentrated area of the southern Great Xing’an Range, NE China. This deposit includes the southern Jinjiling and northern Kongqueshan ore blocks, separated by the Budunhua granitic pluton. Cu mineralization occurs mainly as stockworks or veins in the outer contact zone between tonalite porphyry and Permian metasandstone. The ore-forming process can be divided into four stages involving stage I quartz–pyrite–arsenopyrite; stage II quartz–pyrite–chalcopyrite–pyrrhotite; stage III quartz- –polymetallic sulfides; and stage IV quartz–calcite. Three types of fluid inclusions (FIs) can be distinguished in the Budunhua deposit: liquid-rich two-phase aqueous FIs (L-type), vapour-rich aqueous FIs (V-type), and daughter mineral-bearing multi-phase FIs (S-type). Quartz of stages I–III contains all types of FIs, whereas only L-type FIs are evident in stage IV veins. The coexisting V- and S-type FIs of stages I–III have similar homogenization temperatures but contrasting salinities, which indicates that fluid boiling occurred. The FIs of stages I, II, III, and IV yield homogenization temperatures of 265–396 C, 245–350 C, 200–300 C, and 90–228 C with salinities of 3.4–44.3 wt.%, 2.9–40.2 wt.%, 1.4–38.2 wt.%, and 0.9–9.2 wt.% NaCl eqv., respectively. Ore-forming fluids of the Budunhua deposit are characterized by high temperatures, moderate salinities, and relatively oxidizing conditions typical of an H2O–NaCl fluid system. Mineralization in the Budunhua deposit occurred at a depth of 0.3–1.5 km, with fluid boiling and mixing likely being responsible for ore precipitation. C–H–O–S–Pb isotope studies indicate a predominantly magmatic origin for the ore-forming fluids and materials. LA-ICP-MS zircon U–Pb analyses indicate that ore-forming tonalite porphyry and post-ore dioritic porphyrite were formed at 151.1 1.1 Ma and 129.9 1.9 Ma, respectively. Geochemical data imply that the primary magma of the tonalite porphyry formed through partial melting of Neoproterozoic lower crust. On the basis of available evidence, we suggest that the Budunhua deposit is a porphyry ore system that is spatially, temporally, and genetically associated with tonalite porphyry and formed in a post-collision extensional setting following closure of the Mongol– Okhotsk Ocean.
Impact diamond is one of perspective natural type of superhard carbon materials, forming huge resources sometimes, such as Popigai impact structure counting the largest diamond storage on the Earth. By present, there are two known types of impact diamonds – after-graphitic and after-coal varieties formed from different carbon precursors. Here we present for the first time a new impact diamond type – diamond fossils, named by “karite”, formed about 70 Ma from unmetamorphosed organics in the giant Kara impact crater (Pay-Khoy, Russia). A full complex of the diamond fossil characteristics is described proving its nature and phase state. Karite is presented with supernanocrystalline diamond aggregates, nicely preserves tiny cell morphology and relict features of lignin and cellulose. The diamond fossils are spread widely through the Kara impactites, point to possible wider distribution of impact diamonds within large impact occurrences around the world, can be used for impact modeling, astrobiological and material studies.
The Gudui geothermal field records the highest temperature at equivalent borehole depths among the mainland hydrothermal systems in mainland China. Located about 150 km southeast of Lhasa City, the capital of Tibet, the Gudui geothermal field belongs to the Sangri–Cuona rift belt, also known as the Sangri–Cuona geothermal belt, and is representative of the non-volcanic geothermal systems in the Himalayas. In this study, oxygen-18 and deuterium isotope compositions as well as 87Sr/86Sr ratios of water samples collected from the Gudui geothermal field were characterized to understand the origin and mixing processes of the geothermal fluids at Gudui. Hydrogen and oxygen isotope plots show both, deep and shallow reservoirs in the Gudui geothermal field. Deep geothermal fluids are the mixing product of magmatic and infiltrating snow-melt water. Calculations show that the magma fluid component of the deep geothermal fluids account for about 21.10%–24.04%; magma fluids may also be a contributing source of lithium. The linear relationship of the 87Sr/86Sr isotopic ratio versus the 1/Sr plot indicates that shallow geothermal fluids form from the mixing of deep geothermal fluids with cold groundwater. Using a binary mixing model with deep geothermal fluid and cold groundwater as two end-members, the mixing ratios of the latter in most surface hot springs samples were calculated to be between 5% and 10%. Combined with basic geological characteristics, hydrogen and oxygen isotope characteristics, strontium concentration, 87Sr/86Sr ratios, and the binary mixing model, we infer the 6th-Class Reservoirs Evolution Conceptual Model (6- CRECM) for the Gudui geothermal system. This model represents an idealized summary of the characteristics of the Gudui geothermal field based on our comprehensive understanding of the origin and mixing processes of the geothermal fluid in Gudui. This study may aid in identifying the geothermal and geochemical origin of the Gudui high-temperature hydrothermal systems in remote Tibet of China, whose potential for geothermal development and utilization is enormous and untapped.
Numerous lenses of garnet amphibolite occur in the garnet-bearing biotite-plagioclase gneiss belt in the Baishan area of the Beishan Orogen, which connects the Tianshan Orogen to the west and the Mongolia-Xing’anling Orogen to the east. The study of metamorphism in Beishan area is of great significance to explain the tectonic evolution of Beishan orogen. According to the microstructures, mineral relationships, and geothermobarometry, we identified four stages of mineral assemblages from the garnet amphibolite sample: (1) a pre-peak stage, which is recorded by the cores of garnet together with core-inclusions of plagioclase (Pl1); (2) a peak stage, which is recorded by the mantles of garnet together with mantle-inclusions of plagioclase (Pl2) þ amphibole (Amp1) þ Ilmenite (Ilm1) þ biotite (Bt1), developed at temperature-pressure (P-T) conditions of 818.9–836.5 C and 7.3–9.2 kbar; (3) a retrograde stage, which is recorded by garnet rims þ plagioclase (Pl3) þ amphibole (Amp2) þ orthopyroxene (Opx1) þ biotite (Bt2) þ Ilmenite (Ilm2), developed at P-T conditions of 796.1–836.9 C and 5.6–7.5 kbar; (4) a symplectitic stage, which is recorded by plagioclase (Pl4) þ orthopyroxene (Opx2) þ amphibole (Amp3) þ biotite (Bt3) symplectites, developed at P-T conditions of 732 59.6 C and 6.1 0.6 kbar. Moreover, the U-Pb dating of the Beishan garnet amphibolite indicates an age of 301.9 4.7 Ma for the protolith and 281.4 8.5 Ma for the peak metamorphic age. Therefore, the mineral assemblage, P-T conditions, and zircon U-Pb ages of the Beishan garnet amphibolite define a near-isothermal decompression of a clockwise P-T-t (Pressure-Temperature-time) path, indicating the presence of over thickened continental crust in the Huaniushan arc until the Early Permian, then the southern Beishan area underwent a process of thinning of the continental crust.
Natural hazards are often studied in isolation. However, there is a great need to examine hazards holistically to better manage the complex of threats found in any region. Many regions of the world have complex hazard landscapes wherein risk from individual and/or multiple extreme events is omnipresent. Extensive parts of Iran experience a complex array of natural hazards – floods, earthquakes, landslides, forest fires, subsidence, and drought. The effectiveness of risk mitigation is in part a function of whether the complex of hazards can be collectively considered, visualized, and evaluated. This study develops and tests individual and collective multihazard risk maps for floods, landslides, and forest fires to visualize the spatial distribution of risk in Fars Province, southern Iran. To do this, two well-known machine-learning algorithms – SVM and MARS – are used to predict the distribution of these events. Past floods, landslides, and forest fires were surveyed and mapped. The locations of occurrence of these events (individually and collectively) were randomly separated into training (70%) and testing (30%) data sets. The conditioning factors (for floods, landslides, and forest fires) employed to model the risk distributions are aspect, elevation, drainage density, distance from faults, geology, LULC, profile curvature, annual mean rainfall, plan curvature, distance from man-made residential structures, distance from nearest river, distance from nearest road, slope gradient, soil types, mean annual temperature, and TWI. The outputs of the two models were assessed using receiver-operating-characteristic (ROC) curves, true-skill statistics (TSS), and the correlation and deviance values from each models for each hazard. The areas-under-the-curves (AUC) for the MARS model prediction were 76.0%, 91.2%, and 90.1% for floods, landslides, and forest fires, respectively. Similarly, the AUCs for the SVM model were 75.5%, 89.0%, and 91.5%. The TSS reveals that the MARS model was better able to predict landslide risk, but was less able to predict flood-risk patterns and forest-fire risk. Finally, the combination of flood, forest fire, and landslide risk maps yielded a multi-hazard susceptibility map for the province. The better predictive model indicated that 52.3% of the province was at-risk for at least one of these hazards. This multi-hazard map may yield valuable insight for land-use planning, sustainable development of infrastructure, and also integrated watershed management in Fars Province.
We investigate the effect of the westerly rotation of the lithosphere on the active margins that surround the Americas and find good correlations between the inferred easterly-directed mantle counterflow and the main structural grain and kinematics of the Andes and Sandwich arc slabs. In the Andes, the subduction zone is shallow and with low dip, because the mantle flow sustains the slab; the subduction hinge converges relative to the upper plate and generates an uplifting doubly verging orogen. The Sandwich Arc is generated by a westerly-directed SAM (South American) plate subduction where the eastward mantle flow is steepening and retreating the subduction zone. In this context, the slab hinge is retreating relative to the upper plate, generating the backarc basin and a low bathymetry single-verging accretionary prism. In Central America, the Caribbean plate presents a more complex scenario: (a) To the East, the Antilles Arc is generated by westerly directed subduction of the SAM plate, where the eastward mantle flow is steepening and retreating the subduction zone. (b) To the West, the Middle America Trench and Arc are generated by the easterly-directed subduction of the Cocos plate, where the shallow subduction caused by eastward mantle flow in its northern segment gradually steepens to the southern segment as it is infered by the preexisting westerly-directed subduction of the Caribbean Plateau. In the frame of the westerly lithospheric flow, the subduction of a divergent active ridge plays the role of introducing a change in the oceanic/continental plate’s convergence angle, such as in NAM (North American) plate with the collision with the Pacific/Farallon active ridge in the Neogene (Cordilleran orogenic type scenario). The easterly mantle drift sustains strong plate coupling along NAM, showing at Juan de Fuca easterly subducting microplate that the subduction hinge advances relative to the upper plate. This lower/upper plate convergence coupling also applies along strike to the neighbor continental strike slip fault systems where subduction was terminated (San Andreas and Queen Charlotte). The lower/upper plate convergence coupling enables the capture of the continental plate ribbons of Baja California and Yakutat terrane by the Pacific oceanic plate, transporting them along the strike slip fault systems as para-autochthonous terranes. This Cordilleran orogenic type scenario, is also recorded in SAM following the collision with the Aluk/Farallon active ridge in the Paleogene, segmenting SAM margin into the eastwardly subducting Tupac Amaru microplate intercalated between the proto-Liqui~ne- Ofqui and Atacama strike slip fault systems, where subduction was terminated and para-autochthonous terranes transported. In the Neogene, the convergence of Nazca plate with respect to SAM reinstalls subduction and the present Andean orogenic type scenario.
The tectonic evolution history of the South China Sea (SCS) is important for understanding the interaction between the Pacific Tectonic Domain and the Tethyan Tectonic Domain, as well as the regional tectonics and geodynamics during the multi-plate convergence in the Cenozoic. Several Cenozoic basins formed in the northern margin of the SCS, which preserve the sedimentary tectonic records of the opening of the SCS. Due to the spatial non-uniformity among different basins, a systematic study on the various basins in the northern margin of the SCS constituting the Northern Cenozoic Basin Group (NCBG) is essential. Here we present results from a detailed evaluation of the spatial-temporal migration of the boundary faults and primary unconformities to unravel the mechanism of formation of the NCBG. The NCBG is composed of the Beibu Gulf Basin (BBGB), Qiongdongnan Basin (QDNB), Pearl River Mouth Basin (PRMB) and Taixinan Basin (TXNB). Based on seismic profiles and gravity-magnetic anomalies, we confirm that the NE-striking onshore boundary faults propagated into the northern margin of the SCS. Combining the fault slip rate, fault combination and a comparison of the unconformities in different basins, we identify NE-striking rift composed of two-stage rifting events in the NCBG: an early-stage rifting (from the Paleocene to the Early Oligocene) and a late-stage rifting (from the Late Eocene to the beginning of the Miocene). Spatially only the late-stage faults occurs in the western part of the NCBG (the BBGB, the QDNB and the western PRMB), but the early-stage rifting is distributed in the whole NCBG. Temporally, the early-stage rifting can be subdivided into three phases which show an eastward migration, resulting in the same trend of the primary unconformities and peak faulting within the NCBG. The late-stage rifting is subdivided into two phases, which took place simultaneously in different basins. The first and second phase of the early-stage rifting is related to back-arc extension of the Pacific subduction retreat system. The third phase of the earlystage rifting resulted from the joint effect of slab-pull force due to southward subduction of the proto-SCS and the back-arc extension of the Pacific subduction retreat system. In addition, the first phase of the late-stage faulting corresponds with the combined effect of the post-collision extension along the Red River Fault and slab-pull force of the proto-SCS subduction. The second phase of the late-stage faulting fits well with the sinistral faulting of the Red River Fault in response to the Indochina Block escape tectonics and the slab-pull force of the proto-SCS. *
Paleogeography can be reconstructed using various crust- or mantle-based reference frames that make fundamentally different assumptions. The various reconstruction models differ significantly in continental paleolongitude, but it has been difficult to assess which models are more valid. We suggest here a “LLSVP test”, where an assumed correlation between present-day large low velocity shear-wave provinces and the paleogeography of supercontinent Pangea at breakup ca. 200 million years ago can be used to assess the relative accuracy of published reconstructions. Closest correlations between continental paleolongitude and the African LLSVP are achieved with mantle-based reference frames (moving hotspots and true polar wander), whereas shallower crustbased reference frames are shown to be invalid. The relative success of mantle-based frames, and thus the importance of the depth of reference frame, supports the notion that mantle convection is largely vertical compared to the horizontal plate motion of tectonics.
Landslides influence the capacity for safe and sustainable development of mountainous environments. This study explores the spatial distribution of and the interactions between landslides that are mapped using global positioning system (GPS) and extensive field surveys in Mazandaran Province, Iran. Point-pattern assessment is undertaken using several univariate summary statistical functions, including pair correlation, spherical-contact distribution, nearest-neighbor analysis, and O-ring analysis, as well as bivariate summary statistics, and a markcorrelation function. The maximum entropy method was applied to prioritize the factors controlling the incidence of landslides and the landslides susceptibility map. The validation processes were considered for separated 30% data applying the ROC curves, fourfold plot, and Cohen’s kappa index. The results show that pair correlation and O-ring analyses satisfactorily predicted landslides at scales from 1 to 150 m. At smaller scales, from 150 to 400 m, landslides were randomly distributed. The nearest-neighbor distribution function show that the highest distance to the nearest landslide occurred in the 355 m. The spherical-contact distribution revealed that the patterns were random up to a spatial scale of 80 m. The bivariate correlation functions revealed that landslides were positively linked to several linear features (including faults, roads, and rivers) at all spatial scales. The mark-correlation function showed that aggregated fields of landslides were positively correlated with measures of land use, lithology, drainage density, plan curvature, and aspect, when the numbers of landslides in the groups were greater than the overall average aggregation. The results of analysis of factor importance have showed that elevation (topography map scale: 1:25,000), distance to roads, and distance to rivers are the most important factors in the occurrence of landslides. The susceptibility model of landslides indicates an excellent accuracy, i.e., the AUC value of landslides was 0.860. The susceptibility map of landslides analyzed has shown that 35% of the area is low susceptible to landslides.
The northern Xinjiang region is one of the most significant iron metallogenic provinces in China. Iron deposits are found mainly within three regions: the Altay, western Tianshan, and eastern Tianshan orogenic belts. Previous studies have elaborated on the genesis of Fe deposits in the Altay orogenic belt and western Tianshan. However, the geological characteristics and mineralization history of iron deposits in the eastern Tianshan are still poorly understood. In this paper I describe the geological characteristics of iron deposits in the eastern Tianshan, and discuss their genetic types as well as metallogenic-tectonic settings. Iron deposits are preferentially distributed in central and southern parts of the eastern Tianshan. The known iron deposits in the eastern Tianshan show characteristics of magmatic Fe–Ti–V (e.g., Weiya and Niumaoquan), sedimentary-metamorphic type (e.g., Tianhu), and iron skarn (e.g., Hongyuntan). In addition to the abovementioned iron deposits, many iron deposits in the eastern Tianshan are hosted in submarine volcanic rocks with well-developed skarn mineral assemblages. Their geological characteristics and magnetite compositions suggest that they may belong to distal skarns. SIMS zircon U–Pb analyses suggest that the Fe–Ti oxide ores from Niumaoquan and Weiya deposits were formed at 307.7 1.3 Ma and 242.7 1.9 Ma, respectively. Combined with available isotopic age data, the timing of Fe mineralization in the eastern Tianshan can be divided into four broad intervals: Early Ordovician– Early Silurian (476–438 Ma), Carboniferous (335–303 Ma), Early Permian (295–282 Ma), and Triassic (ca. 243 Ma). Each of these episodes corresponds to a period of subduction, post-collision, and intraplate tectonics during the Paleozoic and Mesozoic time.
This paper presents the results of geochronological (40Ar-39Ar, U–Pb SHRIMP II), petrological and geochemical studies of the Late Paleozoic complexes of alkaline rocks (Zimovechinsky, Tuchinsky and Koma) located within the Vitim Plateau (the western part of the Mongol-Okhotsk Orogenic Belt). The rocks were formed at 310–280 Ma. It is coeval with Late Paleozoic magmatism within the Central Asian Orogenic Belt. The εNd(T) values show large variations from 2.1 to þ3.3 as well as the initial Sr(I) isotopic ratios from 0.7042 to 0.7138, that demonstrate strong isotopic heterogeneity of the magmatic source. The geochemical characteristics of the rocks show pronounced positive Pb and negative Ti, Zr–Hf anomalies that can be explained by involvement of the subducted component in primary melts. The rocks intruded in a setting of extension at the active continental margin of the Siberian Craton during subduction of Mongol-Okhotsk oceanic crust under the Siberian Craton.
The Western Qinling Orogen (WQO) is characterized by voluminous distribution of Indosinian granitoids, the formation of which provides an important window to unravel the geochemical and geodynamic evolution and associated metallogeny. Here we investigate a group of intrusions termed “Five Golden Flowers” based on petrological, geochemical, zircon U–Pb geochronological and Lu–Hf isotopic studies on the granitoids and their mafic microgranular enclaves (MMEs). Our results show that these intrusions are genetically divided into two types, namely, magma-mixing and highly fractionated. The Jiaochangba, Lujing, Zhongchuan, and Luchuba granitoids are biotite monzogranites (220 0.8 Ma to 217 2.6 Ma) with abundant coeval MMEs (220 1.1 Ma to 217 2.7 Ma). The rocks contain moderate to high SiO2, high MgO, Rb, Sr, Ba, and Th contents, but low TiO2, P2O5, and Sc values, A/CNK of <1.1, and a range of εHf(t) values of 11.7 to þ2.23 with corresponding TDM2 values of 1967–1228 Ma. The MMEs possess K-feldspar megacrysts, abundant acicular apatites, and show lopsided textures. They have lower SiO2, Al2O3, and Th contents, but higher MgO, TiO2, and Sc, with εHf(t) values of 18.0 to þ3.18 and TDM1 of 849–720 Ma. The data indicate that the MMEs were derived from a magma sourced from the enriched lithospheric mantle. We suggest that these host granitoids were produced by partial melting of late- Paleoproterozoic to early-Mesoproterozoic lower crust with the involvement of Neoproterozoic SCLM-derived mafic magmas. The Baijiazhuang pluton is dominantly composed of leucogranite (muscovite granite and twomica monzogranite, 216 1.5 Ma) without MMEs. The rocks are peraluminous with high A/CNK (1.06–1.27). Compared with the other four granitoids, the Baijiazhuang leucogranite shows higher SiO2 content, markedly lower concentrations of TiO2, MgO, Al2O3, CaO, and Fe2O3 T, and lower LREE/HREE and (La/Yb)N values. These leucogranites are also rich in Rb, Th, and U, and display marked depletions in Ba, Sr, Ti, and Eu, indicating that they experienced significant fractionation. Zircon εHf(t) values (10.2 to 3.27) and TDM2 (1868–1424 Ma), as well as the Nb/Ta and K2O/Na2O values are similar to the other four granitoids, indicating that they are likely to have been derived from a similar source; with sediments playing only a minor role in the magma generation. The low contents of Yb and Y suggest that their partial melting was controlled by garnets and micrographic texture of K-feldspar reflects high-temperature melting through undercooling. Based on the above features, we infer that the Baijiazhuang leucogranite likely represents the product of high degree fractionation of the I-type biotite monzogranite magma which generated the other four granitoids at relatively high temperatures, within magma chambers at mid-crust depths. We propose that the granitoid suite was formed in the transitional setting from synto post-collision during the collisional orogeny between the SCB and NCB, following break-off of the subducted South China Block lithosphere during 220–216 Ma.
Low-temperature Sb (Au–Hg) deposits in South China account for more than 50% of the world’s Sb reserves, however, their genesis remains controversial. Here we report the first study that integrates U–Pb and Lu–Hf analysis by LA-(MC)-ICPMS and conventional (U–Th)/He analysis, all applied to single zircon crystals, in an attempt to constrain the origin and timing of world-class Sb (Au–Hg) deposits in Banxi (South China). Zircon separated from a quartz-stibnite ore and an altered country rock samples revealed similar U–Pb age spectra defining two major populations – Paleoproterozoic (~1900–2500 Ma) and Neoproterozoic (~770 Ma), which are characterized by variable εHf(t) values (–10.7 to 9.1 and –16.5 to 11.2, respectively) and Hf crustal model ages (TDM C) (2.48 to 3.24 Ga and 0.97 to 2.71 Ga, respectively). The U–Pb age and Hf isotopic features of the zircons are consistent with the Banxi Group in the region, indicating that the zircons involved in the low-temperature hydrothermal system were originally from the Banxi Group country rocks. Thirty-three mineralization-related zircon crystals yielded a mean (U–Th)/He age of 123.8 3.8 Ma, which is interpreted to represent the timing of the latest low-temperature mineralization stage of the Banxi Sb deposit. The combined U–Pb, Lu–Hf and (U–Th)/ He data suggest that Precambrian basement rocks were the major contributors to the low-temperature mineralization, and that Early Cretaceous (130–120 Ma) could be the most important ore-forming epoch for the Sb deposits in South China. This study also demonstrates the analytical feasibility of integrated U–Pb - Lu–Hf - (U–Th)/He “triple-dating”, all applied to single zircon crystals. This approach reveals the full evolution of zircon, from its origin of the magmatic source, through its crystallization and low-temperature cooling. Although this study demonstrates the usefulness of this integrated approach in dating low-temperature mineralization, it has great potential for zircon provenance and other studies that may benefit from the large amount of information that can be extracted from single zircon crystals.
The Quantougou (QTG) Fauna in central Lanzhou Basin is an important late Mid-Miocene fauna on the northeastern Tibetan Plateau margin, but its numerical age remains a matter of debate. Here, we present a new magnetostratigraphic record for a fluvio-lacustrine section to further constrain the age of the QTG Fauna. Results suggest that the studied section spans from polarity chrons C5Cn.2n to C5n.2n or C5An.1n, with ages of ca. 16.5 Ma to 10 Ma or 16.5 Ma to 12 Ma. The QTG Fauna is located at the top of polarity chron C5r.3r or C5Ar.2r, which corresponds to an age of 11.7 Ma or 12.8 Ma for the fauna. Accordingly, the associated Myocricetodontinae (a subfamily of Gerbillidae, Rodentia) is suggested to have appeared in the Lanzhou Basin at 11.7 Ma or 12.8 Ma, which is the oldest Myocricetodontinae in East Asia but is still much younger than the ~20 Ma appearance of this subfamily in West and South Asia. Our age data support the interpretation that East Asian Myocricetodontinae originated from South Asia. The QTG fauna further suggest a dry and open grassland environment, which is consistent with global cooling after the Mid-Miocene Climatic Optimum.
One of the clocks that record the Earth history is (quasi-) periodic astronomical cycles. These cycles influence the climate that can be ultimately stored in sedimentary rocks. By cracking these (quasi-) periodic sedimentation signals, high resolution astronomical time scale (ATS) can be obtained. Paleoclimate proxies are widely used to extract astronomical cycles. However different proxies may respond differently to astronomical signals and nonastronomical noises including tectonics, diagenesis, and measurement error among others. Astronomical time scale constructed based on a single proxy where its signal-to-noise ratio is low may have uncertainty that is difficult to evaluate but can be revealed by utilizing other proxies. Here, we test eight astronomical age models using two astrochronological methods from four paleoclimate proxies (i.e., color reflection L* and b*, natural gamma radiation, and bulk density) from the Turonian to the Coniacian of the Cretaceous Period at the Demerara Rise in the equatorial Atlantic. The two astrochronological methods are time calibration using long eccentricity bandpass filtering (E1 bandpass) and tracking the long eccentricity from evolutive harmonic analysis (tracking EHA). The statistical mean and standard deviation of four age models from the four proxies are calculated to construct one integrated age model with age uncertainty in each method. Results demonstrate that extracting astronomical signals from multiple paleoclimate proxies is a valid method to estimate age model uncertainties. Anchored at the Cenomanian/Turonian boundary with an age of 93.9 0.15 Ma from biostratigraphy, the ages for CC11/CC12 (calcareous nannofossil zones), Turonian/Coniacian (CC12/CC13), CC13/CC14, and Coniacian/ Santonian boundaries are 91.25 0.20 Ma, 89.87 0.20 Ma, 86.36 0.33 Ma, and 86.03 0.32 Ma in E1 bandpass method, compared with 91.17 0.36 Ma, 89.74 0.38 Ma, 86.13 1.31 Ma, and 85.80 1.33 Ma respectively in tracking EHA method. These results are consistent with previous studies within error and provide a reliable estimation of uncertainties of the ages.
Chronologically well-constrained loess-palaeosols (recorded glacial and inter-glacial climate) revealed pedogenesis induced ionic substitutions, caused end-member compositional deviations in illite and chlorite, linked to widespread climatic changes occurred during Late Pleistocene. Further, micro-level climatic resolution is yet to be resolved. Thus, layer-wise X-ray diffraction analyses of clay separates, followed by Rietveld refinement revealed varied cell parameters and interatomic distances. Obtained values for detrital and pedogenic illite and chlorite when plotted against stratigraphic succession show notable changes in the crystallographic axes. The illite lattices associated with inadequately pedogenized palaeosols have been altered into illite/smectite mixed layers, but, the chlorite lattices represent expansion of a-, b- and contraction of c-axes with much greater amount of distortions, suggestive of warm-humid and acidic environment. The detrital 48, 44 and 83, 74 bonded illite and chlorite with 2 sub-types each, when pedogenized retained 48, 44 and 34; and 83 and 74 bonds (in their neo-formed 3 and 2 sub-types), respectively. The Al–O bond shows expansion, but, unchanged Si–O and decreased Si–K and K–O bonds show loss of Al and retention of Si and K ions in the illite lattices. The illite with 32 atoms and 48 bonds represent contraction of K–O, Si–K, Al–O and Si–O bonds caused bond reinforcement; however, loss of Al3þ reflects all-out illite alteration. Owing to Al–O and K–O bond expansion, major Kþ and Al3þ ionic loss occurred during the LGM, however, further ionic loss depends upon the magnitude of the loess-palaeosol weathering that they have suffered. The climate sensitive Fe, Mg and Al ionic losses for Fe–O, Mg–O and Al11–O9 bond length expansions were recognized in the chlorite lattices. Such ionic losses are common, but, complete distortion is attributed to Al, Si, Fe and Mg ionic losses, followed by weakening of Al–O, Si–O, Fe–O and Mg–O bonds. Though, Si–O4 and Fe1–O4 bonds, and Si and Fe1st ions remain intact. Thus, three major glacial episodes of ~5 ka each occurred under alkaline environment, but, intervened by two successive cycles of 55 ka each, encompassing three alternate warm and cold climatic sub-cycles of 12–15 ka. But, the coldness increases with each warm-cold sub-cycle that attained the glacial maxima. Further, these events correlate well with the deep-sea records of the North Atlantic (MIS-1 to MIS-5e) and CLP loess-palaeosols (~127 ka).
This paper reports new geochronological (U–Pb) and isotope (C, O, and S) data to investigate the timing of mineralization and mode of ore genesis for the recently discovered Changtuxili Mn–Ag–Pb–Zn deposit, located on the western slopes of the southern Great Hinggan Range in NE China. The mineralization is hosted by intermediate– acidic lavas and pyroclastic rocks of the Baiyingaolao Formation. Three stages of mineralization are identified: quartz–pyrite (Stage I), galena–sphalerite–tetrahedrite–rhodochrosite (Stage II), and quartz–pyrite (Stage III). δ13C and δ18O values for carbonate from the ore vary from 8.51‰ to 4.96‰and 3.97‰to 15.90‰, respectively, which are indicative of a low-temperature alteration environment. δ34SV-CDT values of sulfides range from 1.77‰ to 4.16‰ and show a trend of equilibrium fractionation (δ34SPy > δ34SSp > δ34SGn). These features indicate that pyrite, sphalerite, and galena precipitated during the period of mineralization. The alteration mineral assemblage and isotope data indicate that the weakly acidic to weakly alkaline ore-forming fluid was derived largely from meteoric water and the ore-forming elements C and S originated from magma. During the mineralization, a geochemical barrier was formed by changes in the pH of the ore-forming fluid, leading to the precipitation of rhodochrosite. On the basis of the mineralization characteristics, new isotope data, and comparison with adjacent deposits, we propose that the Changtuxili Mn–Ag–Pb–Zn deposit is an intermediate-to lowsulfidation epithermal deposit whose formation was controlled by fractures and variability in the pH of the oreforming fluid. The surrounding volcanic rocks yield zircon U–Pb ages of 160146 Ma (Late Jurassic), indicating that the mineralization is younger than 146 Ma.
The influence of water is evaluated in this last contribution of a series aiming to study the petrological and dynamic evolution of mantle melting. Water is considered to be either a chemical component in the melt or solid assemblage but it can also be present as a pure water phase in a oversaturated environment. A three-phase-flow model was developed for this purpose. Only a limited set of conditions has been applied to the 1-D upwelling mantle column. A range of fixed temperatures (1150–1450 C) and water contents in the solid mantle (0, 0.02 wt.%, 0.2 wt.%) have been imposed at the entry point (120 km deep) for the two melting models introduced in the previous installments, dynamic equilibrium melting (DEM) and dynamic fractional melting (DFM) model. As expected, for a given temperature at the base of the mantle column, the depth of the first melt formation increases with higher water content in the mantle. After the first melt is created, very negligible amount of melt is formed over a certain depth interval which approximately ends at the depth where the first melting of the dry mantle would take place. However melt is present as a dynamic phase thorough the entire region regardless whether the DEM or DFM model has been applied. Under a quasi-steady state regime, the melt and residual mantle compositions vary significantly over depth, depending on the conditions imposed to the model (DEM, DFM, bottom temperature and water content). Several distinctions can be made at the extraction point (top of the mantle column ¼ 15 km deep). For DEM and DFM models at this lowest depth, the most influential factor affecting the melt composition after the quasi-steady state condition has been reached is the temperature at the base of the column. In general, for a high temperature model, the input water in the mantle does not seem to play a significant role on the bulk composition of the melt (except for the water content in melt). But at low temperature water does have some noticeable influence on the variation of some chemical components in melt (SiO2, Fe2O3, CaO, Na2O at T ¼ 1250 C or lower). A similar conclusion can be made also for the residual mantle composition. The presence of a dynamic free water phase is detected only in absence of melt or in coexistence with a melt phase when the mantle is relatively cold (bottom temperature 1250 C) and the input water content at the base of the model is relatively high (0.2 wt.%). Complete output data for several numerical simulations and six animations illustrating various melting models are available following the instructions in the supplementary material.
The southern regions of Madagascar have the country’s lowest water supply coverage and are highly vulnerable to drought. Access to potable drinking water is a major challenge for the local population. Chronic droughts lead to annual emergency appeals to save the lives of acute malnourished children. UNICEF’s response consisting in providing potable drinking water through the drilling of boreholes has been challenged by the complex hydrogeology, the low yield of boreholes and high-level salinity of water, the lack of reliable groundwater data and the weak capacity of the drilling sector. These constraints result in a high rate of drilling failure. To improve drilling success and provide more potable drinking water to local communities, it is vital to undertake reliable groundwater investigation. UNICEF Madagascar and the European Union delegation in Madagascar collaborated on the use of satellite imagery to improve sector knowledge and access to safe and clean water for local communities in southern Madagascar. The methodology relies on produce thematic layers of groundwater potential areas. Later, these thematic layers were overlaid with ground-based hydrogeological data to map the groundwater potential zones (GWP) and identify the most suitable sites for borehole siting and drilling. Findings of this study are very encouraging, and the integrated approach used has proven its applicability in mapping groundwater potential areas in the eight drought-affected areas of south Madagascar. The groundwater potential zone map is being used by UNICEF and partners to plan water supply projects and identify the best sites for positioning new boreholes and reduce the likelihood of drilling failure. Additionally, the project developed a database of groundwater resources, which will improve knowledge of the regional hydrogeological context and strengthen the capacity of the water sector. Lessons learnt from this study show that an integration of the groundwater potential zone map with demographics and water demand information will help identifying priority areas for detailed studies. Moreover, a capacity building activity is required for knowledge/technology transfer to the Ministry of Energy, Water and Hydrocarbons (MEEH), allowing the possibility of scaling-up this integrated approach to the rest of Madagascar. Finally, strengthening the capacity of the MEEH and refining this approach as suggested above will certainly help in the pursuit to improve equitable access to safe and clean water for households located in the drought-affected areas of southern Madagascar, allowing them to be more resilient to the effects of climate change.
As the southernmost segment of the Central Asian Orogenic Belt (CAOB), the northern Alxa orogenic belt (NAOB) connects the southeastern and southwestern segments of the CAOB. The NAOB amalgamated with the closure of the Paleo-Asian Ocean; however, the closure time of the Paleo-Asian Ocean is still on great debate. In this study, we reported new detrital zircon U–Pb geochronology and Hf–O isotopes for the Permo–Carboniferous sediments in the northern Alxa to constrain the provenance and its tectonic implications. The Permo–Carbonifereous Amushan Formation is composed of volcanic-carbonite-clastic rocks and was deposited in a shallow marine environment. Based on the zircon U–Pb geochronology, the Amushan Formation was deposited in the late Carboniferous to early Permian, but some outcrops of volcanic and clastic rocks in the Quaganqulu area were likely formed in the middle to late Permian. The integrated zircon age spectrum for the clastic rocks shows a wide range from late Archean to Paleoproterozoic, Mesoproterozoic (with a peak age at 1458 Ma), early Neoproterozoic (with peak ages of 988 Ma and 929 Ma), early Paleozoic (with a peak age at 447 Ma) and late Paleozoic (with a peak age at 294 Ma). Combined with the zircon Hf–O isotopes, the provenance was considered to be the Alxa Block, the Shalazhashan terrane and the Zhusileng–Hangwula block (and the southern Beishan orogenic belt). The multiple source regions to the south and north of the Paleo-Asian Ocean indicate the closure of this ocean before the late Carboniferous. The absence or small proportion of depositional age-approximated zircons in most samples makes their age spectra similar to extensional basins. Combined with the intra-plate volcanism, the deposits were considered to be formed in extensional settings. Accordingly, after the closure of the Paleo-Asian Ocean, the NAOB stepped into an extensional stage.
The Mahanadi delta, deposited on a series of horst and graben basement structures, is considered an extension of the East Lambert Rift of Antarctica. Current study is based on the hydrogeochemical assessment of this deltaic aquifer system and geospatial analysis thereof, to appreciate the basement structure influence on groundwater chemistry. Major ion chemistry of subsurface waters portrays a distinct saline contamination across the terrain and varied regimes of water types, specifically with respect to southern and northern parts of this aquifer system. Findings of the study indicate a general near surface saline horizon and significant fragmentation of the hydrostatic units. This, in turn, implies noteworthy influence of formational water to salinity regimes and basin structural changes for the escape of these waters to surroundings. A plot of recent low intensity earthquakes displays proximity of epicenters to the faults as well as striking similarity to the trend of terrestrial faults indicating multiple reactivations of the faults. To further corroborate the above findings, spatial pattern analysis of individual hydrochemical variables is carried out which reveals specific clusters of sources (groundwater mixing) and sinks (groundwater dispersion) in proximity to basement fault dispositions. While the faults can be disregarded as conduits or barriers owing to their great depth, the overlying sedimentary mass, particularly, the horizons with significant clayey content have been distorted due to post rift subsidence and fault reactivations. A proximity analysis of ionic clusters points towards a greater influence of longitudinal faults to that of the transverse ones on groundwater mixing or dispersion.
Ordovician diorite-quartz diorite mylonite (Saganoseki quartz diorite) was discovered in Sambagawa metamorphic terrane at the northern margin of Saganoseki Peninsula, Kyushu Island, Japan. The LA-ICP-MS zircon U–Pb geochronology revealed that the intrusion age of Saganoseki quartz diorite was 473.3 3.6 Ma. These rocks show the volcanic arc affinity based on the trace element composition. On the basis of geochronological and geochemical results, Saganoseki quartz diorite is considered to be a member of the Early Paleozoic igneous rocks of the Kurosegawa tectonic zone. Saganoseki quartz diorite is located just south of the Median Tectonic Line (MTL) and is in close contact with the pelitic and psammitic schist without any brittle shear zone. U–Pb ages of detrital zircon grains from two psammitic schists show the estimated sedimentation age of early Late Cretaceous, indicate that these psammitic schists are the member of Sambagawa metamorphic rocks. Together with these results and the mode of occurrence in the field, we argue that the Early Paleozoic igneous rocks of the Kurosegawa tectonic zone existed as an upper structural unit of the Sambagawa terrane. This relationship is the key to reconstruct the Mesozoic tectonics of Japan as a part of East Asia, and its evolution through time.