Tectonic-magmatic-metallogenic system, Tongling ore cluster region, Anhui Province, China
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The Tongling ore district of the Middle-Lower Yangtze metallogenic belt is a famous Cu-Au-Fe-S polymetal region, and its tectonic deformation, magmatic evolution, and metallogenic processes have been studied for decades. In this article, we propose a comprehensive tectonic-magmatic-metallogenic model of the ore-forming mechanism constrained by magmatism and regional deformation. In the Tongling district, the tectonic regime underwent two transitions. (1) In the Middle Triassic, the tectonic regime transitioned from quiescence to intense compression. During contraction, the lithosphere thickened and a series of NE-trending folds developed in the cover sequence; because of the multi-layered structure of this caprock, bedding faults, typically cut by steeply dipping faults, developed widely. (2) From 134 to 150 Ma, the tectonic regime changed from compression to extension. During this transition, mantle–crust interaction was prominent; ore-bearing magma was generated by the mixing of crust-derived and mantle-derived melts triggered by delamination of the thickened lithosphere. Meanwhile, detachment faults developed along the interfaces, for example between the lower and upper crust, serving as emplacement sites for several magma chambers. Ore-bearing magma dikes containing large amounts of volatiles derived from a shallow chamber at about –10 km depth migrated into the cover sequence along the pre-existing steeply dipping faults. Melt injection reworked the structural framework, facilitating further development of steeply dipping faults, as well as the vertical transport of ore-bearing fluids. Hydrothermal fluids derived from the emplaced magmas not only formed a range of deposits, including skarns, porphyries, and cryptobreccias around the intrusions but also widely replaced carbonates along bedding-parallel faults and formed so-called stratabound skarn ore bodies, as well as superimposing synsedimentary orebodies developed in the quiescence stage to form several large polymetallic hydrothermal ore deposits. Various types of ore deposits at different depths are clustered in a single orefield, composing a multi-layered mineralization network. In the network, skarn deposits dominate and are characterized by fluid immiscibility processes and diverse element enrichments. The intense mineralization in the Tongling region was caused by the abundance of metals derived from the mantle, favourable ore-controlling structures, and widespread fluid boiling of magmatic hydrothermal fluids, which facilitated metal deposition during the Mesozoic, as well as the superposition of Mesozoic hydrothermal reworking of earlier Palaeozoic sedimentary ore bodies.The block structure of the Earth crust and tectonic zones are displayed at the dynamic seismic section in the amplitudes of the scattered waves. Tectonic zones are identified with thrusts.<br>The effective density model of the crust was created at the base on the complex interpretation of seismic and gravity data.<br>Revealed tectonic elements of the Earth crust can be use for prognosis of oil perspective areas.<br>
Earth crust
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In this article, we study the possibility that Ceres has, or had in the past, a crust heavier than a pure or muddy ice mantle, in principle gravitationally unstable. Such a structure is not unusual in the Solar system: Callisto is an example. In this work, we test how the composition (i.e. the volumetric quantity of ice) and the size of the crust can affect its survival during thermo-physical evolution after differentiation. We have considered two different configurations: the first characterized by a dehydrated silicate core and a mantle made of pure ice, the second with a hydrated silicate core and a muddy mantle (ice with silicate impurities). In both cases, the crust is composed of a mixture of ice and silicates. These structures are constrained by a recent measurement of the mean density by Park et al. The Rayleigh–Taylor instability, which operates in such an unstable structure, could reverse all or part of the crust. The whole unstable crust (or part of it) can interact chemically with the underlying mantle and what is currently observed could be a partially/totally new crust. Our results suggest that, in the case of a pure ice mantle, the primordial crust has not survived until today, with a stability timespan always less than 3 Gyr. Conversely, in the case of a muddy mantle, with some 'favourable' conditions (low volumetric ice percentage in the crust and small crustal thickness), the primordial crust could be characterized by a stability timespan compatible with the lifetime of the Solar system.
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[1] Although presence of weak layers due to hydration and/or metasomatism in the lithospheric mantle of cratons has been detected by both geophysical and geochemical studies, its influence on craton evolution remains elusive. Using a 2‒D thermomechanical viscoelastoplastic numerical model, we studied the craton extension of a heterogeneous lithospheric mantle with a rheologically weak layer. Our results demonstrate that the effect of the weak mantle layer is twofold: (1) enhances deformation of the overlying lithosphere and (2) inhibits deformation of the underlying lithospheric mantle. Depending on the weak‒layer depth, the Moho temperature and extension rate, three extension patterns are found (1) localized mantle necking with exposed weak layer, (2) widespread mantle necking with exposed weak layer, and (3) widespread mantle necking without exposed weak layer. The presence of the weak mantle layer reduces long‒term acting boundary forces required to sustain extensional deformation of the lithosphere.
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Many geologists have been interested in the Okinawa Trough due to its unique tectonic environment.The magmatism is regarded as one of the key questions in the Okinawa Trough.The advances associated with the magmatism in the trough have been overviewed,including the studies about the characters of the magmatic sources,the processes of the magma melting and evolution and the regularities of melting.Based on these,the difficult questions on this field have been presented and a new idea that using the method of U-series disequilibria to study the magmatism in Okinawa Trough has been brought forward.Meanwhile,some research directions about the study of magmatism in the Okinawa Trough should be paid much attention to in the future,e.g.① the influence of the Philippine subduction slab on the magmatism processes,②the implications of the subducting sediment for the magma,③the controlling factors of the magma melting,④ the link between the magmatism and the seafloor hydrothermal activity.
Trough (economics)
Seafloor Spreading
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Abstract The area of the Danube Basin is interesting in the light of the evaluation both of the lithosphere structure and of various theories of Carpathian-Pannonian region tectonic evolution. The aim of this paper is to analyse both the thermal conditions in the Danube Basin and the mutual relations to geological structure and tectonic development of the region under study. First the improved distributions of the terrestrial heat flow density and of the lithosphere thickness were constructed using recently gained geophysical and geological knowledge. Then the critical analysis of existing models of the tectonic development of the region under study was carried out. The tectono-thermal interpretation activities were accomplished by new geothermal modelling approach for transient regime which utilizes also the backstriped sedimentology data as a control parameter of model. Finally the McKenzie’s “pure-shear” model of the Danube basin was constructed as acceptable conception for used geothermal and tectonic data. The determined stretching parameter has an inhomogeneous horizontal distribution and the thinning factors express the depth dependency for separate lithospheric layers.
Tectonic phase
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The Malaysian Peninsula belongs to the Southeast Asian tin belt. It is divided by a sutUre into two different magmatic provinces of s- and I-type characteristics. Three areas within the S-type granitoid province of the Main Range have been studied. New K Ar mica and fission track zircon and apatite ages were established, and Rb-Sr whole rock data from literature have been reinterpreted in the light of these new data. The results were compared with existing Rb-Sr isochrons of the I-type granitoid province and with the one reported from the northern extension of the belt stretching into Thailand. This integrated study led to the better understanding of: a) the behavior of the different isotopic systems in different geological environments, and b) the complex magmatic, tectonic and cooling history of Southeast Asian granitoids. Applying the intrusion extrapolation method, based on the evolutionary trend of decreasing Rb-Sr ages and increasing initial (87186)Sr (Kwan et al., paper submitted), periods of 300, 250, 210, and 90 Ma for the Main Range and of 260, 240, 210 and 90 Ma for the I-type granite province became evident. The best evaluated initial (87186)Sr for the source regions of the granitoids are 0.708 to 0.709 and 0.704 respectively. the major magmatic event in the Main Range dates of the late-Permian. The close time range of individual magmatic periods and their spatial distribution can best be explained in the context of plate tectonics by northward motion of compositionally different arcs; by their collision to one another and to the East Asian Continent; subsequent deformation; subduction reversal; following the model of Hamilton (1988). The most crucial event for the Main Range granitoids was the intrusion of the highly evolved, late-Triassic post-collision granites. They induced a regional hydrothermal convection system, which is considered to have lasted for a maximum of 40 Ma dependent upon the crustal level. This hydrothermal system was responsible for the major tin mineralization processes and the entire granite alteration. Biotite of the crystallized granitoids is considered to be the main Sn supplier. The K-Ar and fission track ages indicate a slow post-orogenic cooling in the range of 1- 5°ClMa. With FT zircon ages different crustal levels can be distinguished within the Main Range, and the discordant K-Ar mica and the differently reset Rb-Sr whole-rock ages can be correlated with depth. After the early-Cretaceous the Main Range has been dominated by tectonic activities, such as differential vertical uplift of the Main Rage combined with thejuxtapositioning of blocks, thrust faulting close to the Bentong-Raub suture and Tertiary left-lateral displacement, which is inferred from isotopic and petrological evidences.
Fission track dating
Overprinting
Peninsula
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Large igneous provinces (LIPs) are formed by enormous (i.e., frequently >106 km3) but short-lived magmatic events that have profound effects upon global geodynamic, tectonic, and environmental processes. Lithospheric structure is known to modulate mantle melting, yet its evolution during and after such dramatic periods of magmatism is poorly constrained. Using geochemical and seismological observations, we find that magmatism is associated with thin (i.e., ≲80 km) lithosphere and we reveal a striking positive correlation between the thickness of modern-day lithosphere beneath LIPs and time since eruption. Oceanic lithosphere rethickens to 125 km, while continental regions reach >190 km. Our results point to systematic destruction and subsequent regrowth of lithospheric mantle during and after LIP emplacement and recratonization of the continents following eruption. These insights have implications for the stability, age, and composition of ancient, thick, and chemically distinct lithospheric roots, the distribution of economic resources, and emissions of chemical species that force catastrophic environmental change.
Hotspot (geology)
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Tectonics and magmatism are inextricably interconnected phenomena of the single tectonic-magmatic process. Their scales must be a priori commensurate. However, in practice of geological work, small magmatic complexes often do not correspond to extended magma-controlling structures, due to the notorious complex creation that distorts the geological history, leads to genetic misconceptions and errors in forecasting and searching for mineralization. The urgency of the problem of correct identification of magmatic complexes is illustrated by examples on the materials of large tectonic structures of the Urals, Siberia and the Far East of Russia.
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Deviation from isostasy is commonly believed to be caused by the strength of the Earth's lithosphere. An analysis of crustal compensation dynamics suggests that the deviation may have a dynamic origin. The analysis is based on analytic models that assume that (1) the medium is incompressible and has a layered and linear viscoelastic rheology and (2) the amplitude of topography is small compared with its wavelength. The models can describe topographic relaxation of different density interfaces at both small (e.g., postglacial rebound) and large time‐scales. The models show that for a simple crust‐mantle system with topography at the Earth's surface and Moho representing the only mass anomalies, while the crust always approaches the isostatic state at long wavelengths (>800 km), crustal isostasy may not be an asymptotic limit at short wavelengths, depending on crustal and lithospheric rheology. For a crust with viscosity smaller than lithospheric viscosity, at wavelengths comparable with widths of orogenic belts (i.e., <300 km), the crust tends to approach a state with significant overcompensation (i.e., excess topography at the Moho) within a timescale of about 10 7 years, and this characteristic time depends on wavelengths and crustal viscosity. This overcompensation is greater for weaker crust and stronger lithosphere. A thicker crust or lithosphere also enhances this overcompensation. If crustal and lithospheric viscosities are both large and comparable, the asymptotic state for the crust displays a slight undercompensation. For an elastic and rigid upper crust, the crust eventually becomes undercompensated after a characteristic decay time of topography at the Moho. The characteristic time is dependent on viscosity and thickness of the lower crust. The deviation from isostasy arises because these viscosity structures result in a ratio of vertical velocity at the surface to vertical velocity at the Moho which in the asymptotic state for short wavelengths differs from the ratio of density contrast at the Moho to that at the surface.
Isostasy
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