Mantle Diapir Research Articles

Neoarchean clockwise P-T trajectories with evolution sequence from HP to UHT metamorphism during decompression: Evidence from petrology, mineralogy, and phase equilibria modeling of mafic granulites in the Sittampundi Complex, southern India

The dispute of tectonic environments such as subduction-collision setting versus extension tectonic setting, is a common phenomenon in the Precambrian. Despite the ongoing controversy, the pressure–temperature (P–T) paths recorded by metamorphic rocks are often used as an indicator for distinguishing between subduction-collision and extension tectonic settings in the early Precambrian. It is widely acknowledged that high-pressure (HP) granulites may have formed in crustal-thickening-dominated collision/subduction environment. Conversely, ultra-high temperature (UHT) granulites may have formed in the elevated mantle heat flux environment. The Sittampundi Complex (SC), where HP granulites and UHT granulites coexist, plays a crucial role in unraveling the late-Neoarchean to early-Paleoproterozoic tectonic evolution of northern South Granulite-facies Terrane (NSGT) of India. In this study, we present new P–T paths from three HP mafic granulites overprinted by UHT granulite facies metamorphism in the SC and provide more data to unravel metamorphic evolution of NSGT. Phase equilibria modeling and traditional thermobarometry calculations illustrated that clockwise metamorphic pressure–temperature (P–T) paths were yielded, passing from 3.3 to 9.3 kbar/631–669 °C (prograde stage) through 11.9–18.5 kbar/828–986 °C (pressure-peak stage), and through 6.6–11.1 kbar/1019–1085 °C (temperature-peak stage) to 4.4–8.1 kbar/845–930 °C (retrograde stage). The HP mafic granulites exhibit clockwise P–T paths with a considerable heating process (ca. 150–180 °C) from the pressure-peak stage (HP-granulite sub-facies) to the temperature-peak stage (UHT-granulite sub-facies). Zircon U-Pb dating yields an age of 2513 ± 29 indicates that the metamorphism took place in the Neoarchean. The uncommon P–T paths from Neoarchean HP granulite in the NSGT suggest that during late Neoarchean, the NSGT had experienced the crustal-thickening dominated collision/subduction (assembly of micro-blocks of Dharwar Craton) and subsequently mantle diapirism process.

Geoelectric Interpretation of the Magnetic Monitoring Data in the Southwest Part of Baikal

Abstract—The long-term gradient magnetic measurements performed in the southwest part of Baikal as supplementing the experiment on deep-water monitoring of the electric field vertical component, can be applied to study the deep geoelectric section. Baikal cannot be approximated by a two-dimensional model in the region under study. Therefore, a three-dimensional geoelectric model of the lake and adjacent territories was constructed based on the existing geological and geophysical data. However, these data are insufficient for the region where our observations are carried out. Refinement of the section upper part over the magnetic field gradients on long bases has been performed using the neural network inversion method. Then, proceeding from the obtained basic model, alternative models were developed, which reflect the known hypotheses about the structure of the deep part of the Baikal rift section: mantle diapir and asthenospheric upwelling. The phase differences between the base-averaged field gradient and the field itself at the base point, as well as the phase differences on the ends of this base, as the most noise-proof gradient functions, are calculated for them. Based on the comparison of the experimental and model frequency curves, it is concluded that the mantle diapir fits best the experimental data, at least, in the southwest part of the Baikal rift zone.

Geodynamic aspects of the metallogeny-magmatic zonality relationship of orogenic volcanic-plutonic belts

Research subject. The orogenic marginal-continental and collisional volcanic-plutonic belts, and the zonality of magmatism and metallogeny within them, as well as the relationship of endogenous metallogeny with magmatic zonality and geodynamics of belt formation.Materials and methods include the long-term studies of the Devonian and Upper-Paleozoic belts of Kazakhstan and the Cretaceous-Paleogenic belt of Sikhote-Alin; an analysis of the alterations in the material parameters of magmatites; the identification of the front and rear parts of the belts and the arrangement features of metallogenic zones within their limits; and the data on other belts of the world.Results. For the marginal continental belts, an asymmetric change of the tholeiitic and low-potassium magmatites of the calcareous-alkaline petrochemical series of the frontal zone of the belt into the magmatites of the high-potassium branch of the calcareous-alkaline and shoshonite series of the rear zone (Devonian and Upper Paleozoic belts of Kazakhstan) has been established. In the same direction, the elemental, as well as the genetic type of deposits changes: pyrite copper-polymetallic frontal zones are replaced by molybdenum-copper-porphyry rear zones (near the border with the frontal zone) and then by rare metal ones. This is also characteristic of the Andean and Okhotsk-Chukchi belts. The collision belts are characterised by symmetrical zonality with a change from rare metal deposits in the centre to copper-polymetallic deposits at the edges (Mongolia). In some cases (Sikhote-Alin and Japan), the continuity of the marginal-continental belt is disrupted as a result of the mantle diapir (or plume), symmetrical magmatic and metallogenic zonality occurs with the change of polymetallic manifestations (near the ocean–continent boundary) to rare-metal ones in the distance. Conclusions. The change in the material parameters of the magmatites of the marginal-continental belts correlates with the removal of their ranges from the ocean–continent transition zone to the interior of the latter. In this case, copper-polymetallic pyrite ore formations are replaced by copperporphyry and then by rare metal ones in this direction. This is due to subduction processes. The collision situation is characterised by a symmetrical zonality of material magmatic parameters with respect to the crowding zone and a symmetrical arrangement of metallogenic zones with a change of rare metal deposits in the crowding zone to copper and polymetallic along the frame. These data are important for metallogenic prediction, as well as for palaeotectonic reconstructions.

Geodynamics of Alpine Belt and Caribbean Region: Plate - Tectonics and Plume - Tectonics

The origin and evolution of geological structures reflect lithosphere-asthenosphere interaction in the process of lithospheric plate movement. Mantle diapirs contribute significantly to the sedimentary basins formation in Alpine belt and Caribbean region. Mantle diapirs are the result of density inversion in the asthenosphere–lithosphere system in the periods of tectonomagmatic activations. Increasing heat flow and mantle diapirs on the phone of convergence of Africa and Eurasia in Alpine belt and North and South Americas in Caribbean region produce intercontinental seas in the Cenozoic. The analytical solution of the problem give possibility to find the critical parameters connecting the mantle flow dynamics with surface relief evolution. In Alpine belt, the mantle diapirs form new basins at the final stage of Africa–Eurasia collision in the Cenozoic. In the Caribbean region, great mantle diapir separates the North and South Americas in the Mesozoic, and then the diapir is the source for different smaller diapirs during the convergence of these continents in the Cenozoic. The Gulf of Mexico and Pre-Caspian Depression are connected with mantle diapirs upwelling and have common geological-geophysical features as very rich oil-gas and salt bearing structures. Geodynamics of Alpine belt and the Caribbean region is determined by plume - tectonics on background of plate - tectonics in these regions.

Heat Flow variations in Siberia and neighboring regions: A new look

The present work deals with an updated evaluation of heat flow in Siberia (Asian part of Russia) and neighboring regions. It is based on results of geothermal measurements in more than 1680 boreholes and shallow measurements of bottom sediments in lakes localities in Siberia. Also considered are results of heat flow measurements in about 1500 sites in neighboring areas of China, Mongolia and Kazakhstan. Features of geological structures and development of adjacent regions have been factors considered in the evaluation of thermal regime of Siberia. Most of this territory is occupied by two broad platforms (West-Siberian and East-Siberian), markedly different in their thermal regimes. Heat flow in the West-Siberian plate averages 55–60 mW/m2, sometimes rising to 70–80 mW/m2. The East-Siberian platform is characterized by a predominance of low heat flow values in the range of 30–40 mW/m2. The southern and eastern segments of these platforms are framed by mountain-folded areas (Sayano-Baikal, Transbaikal, Verkhoyano-Kolymskaya areas) with heat flow values (in the range of 70–80 mW/m2). These are formed mainly under the influence of mantle heat sources (asthenosphere upwellings and mantle diapirs). Analysis of temperature distributions shows that thermal conditions in the five-kilometer upper layer of rocks are determined by heat flow values. Nevertheless, the thermal field of the Asian part of Siberia has been insufficiently studied. The work also considers heat flow variations of adjacent regions located south of Siberia: Kazakhstan, Northern China and Mongolia.

New data on the trace element composition and Lu-Hf isotopic system of zircons from the Early Cambrian granites of the Ufaley block (Middle Urals): a step to the correction of geodynamic concepts

Research subject. Composition and isotopic parameters of zircons and their host granites of the Bitim complex, localised in the eastern part of the Ufaley block (Middle Urals).Materials and methods. The determination of trace element composition and Lu-Hf isotope system in zircons was carried out by LA-ICP-MS (the “Geoanalitik” shared research facilities of the IGG UB RAS, Ekaterinburg).Results and conclusions. The images and geochemical features of the main zircon population from granites confirm their magmatic genesis and minimal alteration, which proves previously defined Vendian- Cambrian (520 ± 9 Ma) age for the granites. Younger zircons (401–459 Ma) differing in composition from the older ones, were formed during subsequent tectonic-thermal activity, possibly under the fluid action. The mixed mantle-crustal nature of substrate for the granite melting is indicated by specific isotopic parameters (87Sr/86Sr = 0.703389, εHf(t)aver = +6.3) and a number of characteristic ratios for zircons and granites. The geodynamic concepts of the granite formation settings of the Bitim complex are corrected: in the Vendian–Early Cambrian, a transform-type margin has developed in this sector, the divergent movements on which have been accompanied by a break in the continuity of the crust and the intrusion of a deep mantle diapir; the participation of the slab substance in the magma generation might also be assumed.Conclusions. The results obtained can be used for the geological mapping and correction of a general geodynamic scheme for the junction zone between the Ural Mobile Belt and the East European platform.

Geochemistry of Mafic Rocks from the Birimian Basement of Doropo (Northeast of Côte d’Ivoire): Petrogenetic and Geodynamic Implications

The northeastern region of Côte d’Ivoire is characterised by a granitic basement mainly composed of biotite granite rocks. According to mapping work in the Gbabédjou and Doropo areas, these Birimian granitoids are cut by gabbro dykes and amphibolite enclaves, which are the subject of this study. In order to better understand the role and the implication of mafic rocks in the Doropo basement emplacement, a multidisciplinary methodology integrating microscopic observations and geochemical analyses of major and trace elements was carried out on 4 samples considered representative of the outcrops studied. Green hornblende, clinopyroxene, and accessory sphene minerals are found in mafic mineral phases, according to petrographic research. Whole-rock analyses reveal that mafic samples with TiO2 contents 1.1 ratios giving them an orogenic granite nature (I-type). Their REE patterns are moderately fractionated (La/Sm)N = 2.66 - 6.13 and (La/Yb)N = 11.17 - 43.70) with a very negative Eu anomaly (Eu/Eu* = 0.75 - 0.97). The multi-element diagrams are characterized by negative Nb-Ta anomalies and geotectonic studies have identified them as volcanic arc formations. All these characteristics allowed us to distinguish the Doropo mafic rocks as formations originating from the juvenile continental crust, emplaced under the Archean tectonics model with significant crustal contamination in the source. Magma driven by mantle diapir has been injected at the base of the continental crust and the heat induces the partial melting of the overlying crust giving rise to mixed liquids. This magma now enriched in LILE was immediately drained to the upper crust to form the mafic rocks from the studied area.

ВУЛКАН БОЛЬШОЙ ПАЯЛПАН (СРЕДИННЫЙ ХРЕБЕТ, КАМЧАТКА). К ПРОБЛЕМЕ КОНВЕРГЕНТНОСТИ «ОСТРОВОДУЖНЫХ» И «ВНУТРИПЛИТНЫХ» ПЕТРОЛОГО-ГЕОХИМИЧЕСКИХ ПРИЗНАКОВ В МАГМАТИЧЕСКОЙ СИСТЕМЕ

The paper presents the data on age, mineralogy, geochemistry, and isotope composition of rocks from the Bolshoi Payalpan Volcano (Sredinny Range, Kamchatka). We compared these data with those on the Nosichan and Belogolovsky volcanoes, located within the Belogolovsky volcanic center. The basalts of the neck and the upper lava complex of Bolshoi Payalpan are compositionally similar to the intraplate-type trachybasalts of the Belogolovsky Volcano, and the basaltic andesites of the lower lava and the cone complex are similar to the island arc rocks of the Nosichan Volcano. Analysis of the data obtained evidences that spatial and temporal manifestations of intraplate and island-arc volcanism at the Bolshoi Payalpan Volcano are not accidental, but may be a consequence of a change in the degree and depth of melting of the same deep source with the involvement of a mantle diapir. The Belogolovsky volcanic center formed in a setting of the Late Miocene-Early Pliocene rifting. Its evolution, right up to its extinction, proceeded in the same geodynamic setting with an increase in depth of the mantle source and a decrease in the degree of its melting. Rock compositions of the Lower-Middle Pliocene Nosichan Volcano remain of the island-arc type under conditions of rifting, since they are associated with the mantle reservoir located at a shallower depth, which has experienced a higher degree of melting. There is good reason for considering large volcanic centers as spontaneously-developing geological entities. As the endogenous activity dies down, the degree of melting decreases and the depth of melting increases with the replacement of island-arc volcanism by intraplate volcanism. The volcanic center becomes extinct.

Upper Mantle Dynamic and Evolution beneath Lake Nyos Volcano (Cameroon Volcanic Line), Documented by Peridotite Xenoliths Enclosed in Alkali Basalts

Volcanic deposits from the lake Nyos contain ultramafic xenoliths: lherzolites, harzburgites and wehrlites, sometimes containing amphiboles and phlogopites. The lithospheric mantle beneath Nyos, as inferred from chemical diagrams, has experienced partial melting and variably cryptic and modal metasomatism of the two groups of samples that have been distinguished: Group 1 samples are characterized by spoon-shaped REE patterns, and Group 2 samples show light (L) REE-enriched patterns. Metasomatic events were associated with pervasive infiltration of volatile (Ti, CO2, H2O) or alkali-rich small melts fractions and fluids. Later on, hydrous phases, Ti-rich Cpx, CaO-rich Ol, Ti-rich Ol, Cr poor and low values of NiO and FO (%) in wehrlite compared to other xenoliths, precipitated from alkali enrichments due to the percolation of the mantle by basaltic magmas. The metasomatic liquid which percolates the Nyos mantle column was a dense alkaline silicate rich in volatile, displaying low HFSE abundances in the metasomatic hydrous melts compared to the LILE. It is suggested that 1) cryptic metasomatism affected Group 1 samples, 2) the spinel-free wehrlite is a Group 2 sample corresponding to a cumulate of a similar melt and 3) amphibole may be a potassium-bearing mineral in addition to phlogopite at shallower levels of Nyos upper mantle. P-T estimated indicates that xenoliths were initially equilibrated in the garnet stability field, at depth of 85 Km, and then they were re-equilibrated in the spinel field owing to isobaric heating up to 1000℃. Adiabatic decompressions occur from 85 to 50 Km materialized by sample NK14 showing transitional porphyroclastic to equigranular texture and displaying pyroxene-Cr spinel symplectites, and from 50 to 30 Km corresponding to 8 - 18 Kbar, pressures in which most xenoliths were incorporated in the host lavas. Therefore, the presence of rising mantle plumes from 85 to 50 Km (sample NK14) and from 50 to 30 Km (all studied samples) is probably related to the evolution of heterogeneous translithospheric mantle diapirs beneath this section of the continental Cameroon Volcanic Line. Local diapirs here may be interpreted as rift-zone initiators.

Evolution of the Late Mesozoic Magmatism of the Omulevka Terrane of the North Part of the Verkhoyansk–Kolyma Orogenic Region

This article presents the results of a study of Late Mesozoic intrusive formations of the Omulevka terrane of the Verkhoyansk–Kolyma orogenic region. The research area covers the Selennyakh block of the Omulevka terrane and the territory adjacent to the south. The compositions of rock-forming, accessory and restitic minerals and geochemical features of intrusive rocks are considered. The methods of optical microscopy, microprobe, silicate and spectral analyses were used. There are the following several stages in the evolution of magmatism: (1) the Late Jurassic supra-subduction (gabbro, dolerites), (2) the beginning of the Early Cretaceous-transitional from supra-subduction to marginal-continental (gabbro-diorites, diorites, granodiorites), (3) the Early Cretaceous of active continental margin (granodiorites, granites), (4) the Late Cretaceous postorogenic or continental-riftogenic (alkali-feldspar granites of A-type), (5) the Late Cretaceous continental riftogenic (subalkaline gabbroids and basaltoids). In the process of evolution from stage one to stage four, there was an increase in the silicic acid content, total alkalinity and ferruginousity of rocks with the movement of magmogeneration levels to higher and higher horizons of the lithosphere (calculated pressure from 1.6–1.4 GPa to 0.6–0.9 GPa). At the same time, the preservation of high temperatures of magmogeneration (1000–1150 °C) and crystallization implies the supply of additional heat from an external (deep) source during the formation of granitoid melts. The magmatic activity is completed by the intrusion of subalkaline derivatives of a deep hearth, formed by metasomatized lherzolites. All the studied igneous rocks are either direct mantle fusions, or bear signs of the participation of mantle matter in the generation of parent melts in crustal substrates: the presence of tschermakite in gabbroids, nonequilibrium structures, the composition of early generations of biotites corresponding to biotites of mantle and crust-mantle derivatives, the presence of pyroxenes and accessory minerals characteristic of mantle magmas in granitoids. In the diagram Al-Na-K-2Ca–Fe + Ti + Mg, the composition points of the studied intrusive rocks tend to the mixing trend. In general, the research results suggest that the evolution of the Late Mesozoic intrusive magmatism of the studied territory and the specific matter of rock compositions were caused by the crust-mantle interaction as a result of the rise of mantle diapirs in the crust from a long-existing deep hearth of the main melt.

Chromian spinel neomineralisations and the microstructure of plastically deformed ophiolitic peridotites (Kraka massifs, Southern Urals, Russia)

Results of a microstructural study of spinel peridotite samples obtained from the Kraka massif in the Southern Urals, involving findings of Cr-spinel neomineralisations within intensive ductile deformed silicates (olivine and orthopyroxene), are presented. The new-formed Cr-spinel grains show different stages of syn-deformation growth as evidenced by investigations combining petrography, decorated dislocation structure analysis, scanning electron microscopy and electron backscatter diffraction (EBSD). Initial precipitations appearing as rods or lamellae are observed to form around structural defects of host silicate grains (olivine and orthopyroxene) by means of impurity segregation or heterogeneous nucleation in the most distorted lattice regions (i.e., in the predominant recrystallisation zones). Syn-deformational crystal growth leads to a complication and coarsening of the grain morphology by coalescence due to a reduction in grain boundary (interfacial) energy. While in the process of growing, the Cr-spinel grains capture fragments of silicate matrix in the solid-state process. The final stage of Cr-spinel growth involves a change in morphology resulting in their characteristic crystallographic forms (spheroidisation). The presence of euhedral Cr-spinel grains, typical for ophiolitic dunite bodies, is a result of interfacial energy reduction in areas of grain boundaries of the hardest phase. The general trend of the observed stages relates closely with the localisation of deformation zones in the upwelling upper mantle (diapir), which are composed by the weakest phase of olivine (dunites). The concentration of the weakest olivine phase in the mobile zones, which is energetically beneficial, explains why dunite bodies having euhedral chromite grains comprise the dynamic equilibrium rocks in the plastic flow localisation zones in upper mantle diapirs. Conversely, assemblages having pyroxene phases, which are stronger and larger in size compared to olivine, are not stable in these zones.

The Chicken and Egg Dilemma Linking Dunites and Chromitites in the Mantle–Crust Transition Zone beneath Oceanic Spreading Centres: a Case Study of Chromite-hosted Silicate Inclusions in Dunites Formed at the Top of a Mantle Diapir (Oman Ophiolite)

AbstractThe mantle–crust boundary beneath oceanic spreading centres is a major chemical and thermal interface on Earth. Observations in ophiolites reveal that it is underlined by a dunitic transition zone (DTZ) that can reach a few hundred meters in thickness and host abundant chromitite ore bodies. The dunites have been deciphered as essentially mantle-derived in most ophiolitic massifs; that is, reactional residues of interactions between peridotite and percolating melt(s). Although both dunite and chromitite in ophiolites have been the focus of many studies, the reasons for their systematic association remain unclear. In this study we have explored the inclusion content of the chromite grains disseminated in the dunites from the DTZ exposed in the Maqsad area of the Oman ophiolite where a former asthenospheric diapir is exposed. Similarly to chromite in chromitite ore bodies, disseminated chromite grains in dunites contain a great diversity of silicate inclusions. Based on the major and minor element composition of 1794 single silicate inclusions in chromites from 285 samples of dunite and associated rocks in the DTZ, we infer that the disseminated chromites formed by a similar ‘metallogenic’ process to the chromitites, and that, as a whole, dunites from the DTZ actually represent the low-grade end-member of a single, giant ore body. The nature of the silicate inclusions (amphibole and mica among others) enclosed in chromite grains in dunites from the Maqsad DTZ precludes their crystallization from an anhydrous primitive basaltic melt, and rather calls for a crystallization from a melt hybrid between common mafic melts and more exotic Si-, Na- and volatile-rich fluids. The hybrid parent medium of both dunites and chromitites results from the interaction between an asthenospheric diapir (the mid-ocean ridge basalt source), and a colder, altered lithospheric lid and hydrothermal fluids responsible for this alteration. The excess silica in the hybrid melt is provided by the incongruent dissolution of enstatite from mantle harzburgite and/or from moderate degree of partial melting of the altered gabbroic crust. The chemical composition of the silicate inclusions is more variable when enclosed in the disseminated chromites than in the chromitites, suggesting a greater variability of melt and/or fluid fractions involved in the genesis of dunites than of chromite ores. Finally, the DTZ can be viewed as a metamorphic contact aureole between episodically rising asthenospheric diapirs and formerly accreted axial lithospheric lids. Our conclusion about the chicken and egg dilemma linking dunites and chromitites beneath oceanic spreading centres (i.e. do the chromitites form in response to the formation of dunites or conversely?) is that the mantle dunitization itself is a potential process for the release of Cr and its re-concentration as chromite ores, and that in turn the competition between orthopyroxene (± plagioclase) and chromite fractionation during this fluid–melt–peridotite reaction process is responsible for the great mineralogical and chemical variability of the DTZ dunites.

Petrogenetic implications and geochronology of middle Miocene Tannayama igneous rocks, Goto Islands, Japan Sea southern margin, northwestern Kyushu, Japan

AbstractThe subduction of “hot” Shikoku Basin and the mantle upwelling related to the Japan Sea opening have induced extensive magmatism during the middle Miocene on both the back‐arc and island‐arc sides of southwest Japan. The Goto Islands are located on the back‐arc side of northwestern Kyushu, and middle Miocene granitic rocks and associated volcanic, hypabyssal, and gabbroic rocks are exposed. The igneous rocks at Tannayama on Nakadori‐jima in the Goto Islands consist of gabbronorite, granite, granite porphyry, diorite porphyry, andesite, and rhyolite. We performed detailed geological mapping at a 1:10 000 scale, as well as petrographical and geochemical analyses. We also determined the zircon U–Pb age dating of the igneous rocks from Tannayama together with a granitic rock in Yagatamesaki. The zircon U–Pb ages of the Tannayama igneous rocks show the crystallization ages of 14.7 Ma ± 0.3 Ma (gabbronorite), 15.9 Ma ± 0.5 Ma (granite), 15.4 Ma ± 0.9 Ma (granite porphyry), and 15.1 Ma ± 2.1 Ma (rhyolite). Zircons from the Yagatamesaki granitic rock yield 14.5 Ma ± 0.7 Ma. Considering field relationships, new zircon data indicate that the Tannayama granite formed at ~16–15 Ma, and the gabbronorite, granite porphyry, diorite porphyry, andesite, and subsequently rhyolite formed at 15–14 Ma, which overlaps a plutonic activity of the Yagatamesaki. The geochemical characteristics of the Tannayama igneous rocks are similar to those of the tholeiitic basalts and dacites of Hirado, and the granitic rocks of Tsushima in northwestern Kyushu. This suggests that the Tannayama igneous rocks can be correlated petrogenetically with the igneous rocks in those areas, with all of them generated by the upwelling of hot mantle diapirs during crustal thinning in an extensional environment during the middle Miocene.

Composition gradients in silicate inclusions in chromites from the dunitic mantle-crust transition (Oman ophiolite) reveal high temperature fluid- melt-rock interaction controlled by faulting

The transition between the mantle section and the oceanic crust in the Maqsad area (Oman ophiolite) is mainly made of variably impregnated dunites locally associated with chromitite ore bodies. There, the dunitic transition zone (DTZ) developed above a mantle diapir that fed with MORB the former oceanic spreading centre. However, orthopyroxene and amphibole impregnations in dunites from the DTZ are witnesses of a hydrated magmatism that looks restricted to this interface. The main other piece of evidence is the nature of silicate minerals included in chromite grains scattered in dunite (e.g., amphibole, orthopyroxene, mica), which are mostly issued from a hydrated and silica-rich melt or fluid. Here, we report on a study of such inclusions along a section sampled in detail in the Maqsad DTZ. It brings critical information on the processes involved in the fluid-melt-peridotite reaction below oceanic spreading centres, complementary to the one provided by the interstitial silicates forming the matrix of the dunite. We first show that both the nature and the composition of the inclusions are well-correlated to those of the impregnations in the host dunites, then that the chemical evolution along the cross-section for all materials correlate to the presence of faults that developed at an early, syn-magmatic stage. This confirms that the early tectonics in the deep oceanic lithosphere primarily controls the fluid-melt-rock reactions and can condition chemical cycling, including for halogens (Cl, F), in oceanic spreading centre setting

Structural Elements of the Sea of Japan’s Basin and Adjacent Continental and Island Dry Land, According to Data from Space-Based Geological Investigations

A representative system of fault and block dislocations in the Sea of Japan segment of the continent–ocean transition zone, identified using space-based geological data and a set of available geomorphological, geological, and geophysical materials, allow the eponymous Late Paleozoic mega-arch to be constructed. The ring form, duration, and scales of basite, acidic magmatism, and the presence of geological and geophysical signs of mantle diapirism associate the main features of its structure and development with plume activity. The geological prehistory, plume tectonics, and periodic inversions of the geodynamic regime determined the direction and character of destructive transformations of the continental margin in the Early Mesozoic and Middle and Late Cenozoic, and the specific character of the region evolution in general. Features of the inherited development of the structural plan include the preservation of the granite core of the mega-arch, associated with the underwater Yamato rise having a continental type of crust, in the geometric center of the Sea of Japan basin; the presence of radial-concentric hypsometric and structural-magmatic zonation; and the relict nature of many other underwater rises. The absence of compensatory collision–accretion dislocations in the east of Honshu Island and on the island arc slope of the adjacent deep-sea trench, the weak variability of strikes of dry land–sea tectonic marks through faults in satellite images, the preservation of the paleoarch structural elements, and other data contradict ideas about large horizontal movements of lithospheric blocks within the Sea of Japan segment of the Eurasia–Pacific Ocean transition zone.

Structures of Coronae on Venus: Results of Topographic and Geologic Analysis

From the analysis of the topography and geology of coronae on Venus, the most common topographic profiles of coronae were obtained, their relative age was estimated by the characteristic annulus type and the relation with the surrounding volcano−tectonic units, and the corona evolution stages consistent with numerical modeling results for the evolution of mantle diapirs on Venus were determined. It has been found that most coronae is the manifestation of a late regressive stage of the evolution of mantle diapirs. These coronae are spatially associated with groove belts and characterize late episodes of the tectonic resurfacing regime. According to our results, the number of coronae that may appear at the early and late phases of the regressive stage is almost the same. This allows us to suppose that the parent corona-forming diapirs were mostly formed at the end of the tectonic regime, but this activity had terminated before extensive lava plains (shield and regional plains) were formed during the volcanic regime. A minor portion of the entire corona population (18%) represents the initial, progressive, stage in the evolution of diapirs, which is characterized by growing a dome-shaped structure above a diapir.

Deep structure and geodynamic conditions of granitoid magmatism in the Eastern Russia

We investigated the deep structure of the lithosphere and the geodynamic conditions of granitoid magmatism in the Eastern Russia within the borders of the Far Eastern Federal District. The relevance of the work is determined by the need to establish the geotectonic and geodynamic conditions of the granitoids petrogenesis and ore genesis in the Russian sector of the Pacific Ore Belt. The purpose of the article is to study the deep structure of the lithosphere and determine the geodynamic conditions of granitoid magmatism in the East of Russia. The author's data on the magmatism of ore regions, regional granitoids correlations, archive and published State Geological Map data, survey mapping, deep seismic sounding of the earth's crust, gravimetric survey, geothermal exploration, and other geophysical data obtained along geotraverses. The magma-controlling concentric geostructures of the region are distinguished and their deep structure is studied. The connection of plume magmatism with deep structures is traced. The chain of concentric geostructures of Eastern Russia controls the trans-regional zone of leucocratization of the earth's crust with a width of more than 1000 km, which includes the Far Eastern zone of Li-F granites. Magmacontrolling concentric geostructures are concentrated in three granitoid provinces: Novosibirsk-Chukotka, Yano-Kolyma, and Sikhote-Alin. The driving force of geodynamic processes and granitoid magmatism was mantle heat fluxes in the reduced zones of the lithospheric slab. The distribution of slab windows along the Pacific mobile belt's strike determines the location of concentric geostructures and the magnitude of granitoid magmatism in the regional provinces. Mantle diapirs are the cores of granitoid ore-magmatic systems. The location of the most important ore regions of the Eastern Russia in concentric geostructures surrounded by annuli of negative gravity anomalies is the most important regional metallogenic pattern reflecting the correlation between ore content and deep structure of the earth's crust.

A Strong Seismic Reflector within the Mantle Wedge above the Ryukyu Subduction of Northern Taiwan

AbstractMajor structures within the mantle wedge are often revealed from seismic velocity anomalies, such as low‐velocity zones at magma reservoirs, partial melting regions, or the upwelling asthenosphere. However, no significant seismic boundaries have been reported in the shallow mantle wedge beneath volcanic arcs. Here, we present evidence for a strong seismic reflector dipping in the opposite direction of the subducting slab in the mantle wedge beneath northern Taiwan in the western end of the Ryukyu subduction system. We find that two unambiguous P waves generated by a deep earthquake (ML 5.1) at a depth of 132.5 km were clearly recorded by the dense seismic array (Formosa Array), composed of 140 broadband seismic stations with a station spacing of approximately 5 km in northern Taiwan. Forward modeling using both raytracing and travel times shows that a seismic reflector exists beneath the Tatun volcano group (TVG) around depths of 80–110 km. The reflector dips in the opposite direction of the subducting slab and is unlikely to be associated with mantle wedge corner flow. Instead, it probably belonged to parts of possible structures such as the asthenospheric flow, the mantle diapir, or other undiscovered structures above the subducting slab. No matter what the seismic boundary is exactly, it might be associated with the active volcanism in the TVG. The detailed geometry and mechanism of the seismic boundary in the mantle wedge will be obtained as the Formosa Array collects more seismic data in the near future.

Multi-scale development of a stratiform chromite ore body at the base of the dunitic mantle-crust transition zone (Maqsad diapir, Oman ophiolite): The role of repeated melt and fluid influxes

A stratiform chromite ore body crops out in the lower part of the dunitic mantle-crust transition zone (DTZ) that developed at the top of a mantle diapir in the Maqsad area in the Oman ophiolite. It is made of layers ranging in thickness from a few mm to a maximum of 3 m, and in modal composition from massive to antinodular and disseminated ore. The ore body is about 50 m thick and its lateral extent does not exceed several hundred meters. The layering dips gently to the southeast, parallel to that of the overlying gabbroic cumulates. The chromite composition is typical of a MORB kindred - moderate XCr (100 × Cr/(Cr + Al) atomic ratio), ranging from 48 to 60, and relatively high TiO2 content, ranging from about 0.3 to 0.5 wt% -, a characteristic shared by most lithologies issued from the igneous activity of the Maqsad diapir. The silicate matrix is essentially made of slightly serpentinized olivine with minor clinopyroxene and rare pargasitic amphibole, orthopyroxene and garnet. This strongly contrasts with the nature of the mineral inclusions mostly made of the assemblage amphibole-orthopyroxene-mica, enclosed in the chromite grains and represented in abundance all along the ore body whatever the ore grade. The inclusions demonstrate the involvement of a silica- and water-rich melt and/or fluid, in addition to MORB, in the early stages of chromite crystallization. The chemical composition of chromite, silicate matrix, together with the one of silicate inclusions display well-defined evolutions vertically along the stratiform chromitite. At the scale of the ore body, the compositional trends are independent of the ore concentration but the major kinks in these trends are well-correlated with levels of magmatic breccias. This shows that abrupt chemical changes can be attributed to sudden melt ± fluids injection events followed mainly by melt-fluid-rock interaction and in a lesser extent by quieter evolution by fractional crystallization. At the thin section scale, second order chemical variations, essentially in the Mg# (100 × Mg/(Mg + Fe2+) atomic ratio) of chromite and Fo of olivine, are clearly attributable to re-equilibration between these two solid phases, possibly in the presence of an interstitial melt/fluid.

Melt hybridization and metasomatism triggered by syn-magmatic faults within the Oman ophiolite: A clue to understand the genesis of the dunitic mantle-crust transition zone

On Earth, most of the critical processes happen at the frontiers between envelopes and especially at the Moho between the mantle and the crust. Beneath oceanic spreading centers, the dunitic transition zone (DTZ) appears as a major interface between the upwelling and partially molten peridotitic mantle and the accreting gabbroic lower crust. Better constraints on the processes taking part in the DTZ allows improved understanding of the interactions between silicate melts and hydrated fluids, which act competitively to generate the petrological Moho. Here we combine mineral and whole rock major and trace element data with a structural approach along three cross-sections up to 300 m thick above the fossil Maqsad mantle diapir (Oman ophiolite) in order to understand the vertical organization of the DTZ with depth. Our results highlight that most of the faults or fractures cross-cutting the DTZ were ridge-related and active at an early, high temperature magmatic stage. Chemical variations along the cross-sections define trends with a characteristic vertical scale of few tens of meters. There is a clear correlation between the chemical variation pattern and the distribution of fault zones, not only for fluid-mobile elements but also for immobile elements such as REE and HFSE. Faults, despite displaying very limited displacements, enhanced both melt migration and extraction up to the crust and deep hydrothermal fluids introduction down to the Moho level. We propose that these faults are a vector for upwelling melt modification by hybridization, with hydrothermal fluids and/or silicic hydrous melts, and crystallization. Infiltration of these melts or fluids in the country rock governs part of the gradational evolutions recorded in composition of both the olivine matrix and interstitial phases away from faults. Finally, these faults likely control the thermal structure of the mantle-crust transition as evidenced by the spatial distribution of the crystallization products from percolating melts, organizing the transition zone into pure dunites to impregnated dunites horizons. In this context, the DTZ appears as a reactive interface that developed by the combination of three primary processes: tectonics, magmatism and deep, high temperature hydrothermal circulations. Accordingly, these features fundamentally contribute to the variable petrological and geochemical organization of the DTZ and possibly of the lower crust below oceanic spreading centers, and may be a clue to interpret part the heterogeneity observed in MORB signatures worldwide.