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Highlights • The crustal thickness of the Zhongsha Block ranging from ~6 to ~25 km. • Rapid transition from the Zhongsha Block to the adjacent oceanic basins was revealed. • Different oceanic structures were observed in the adjacent oceanic basins. • The pre-rift lithospheric configuration may affect the process of rifting and seafloor spreading. Abstract Continental rifting, break-up, and onset of seafloor spreading are inherently controlled by the segmentation and structure of the continental domain suffering from extension. Today, the Zhongsha Atoll (ZS) is wedged between the Northwest Sub-basin (NWSB) and the Southwest Sub-basin (SWSB), two oceanic abyssal basins of the South China Sea (SCS). The nature of the crust and the structure of the transition from continental to oceanic domain are key to revealing the processes and dynamics during the rifting and break-up of the Zhongsha block. In this paper, we present a P-wave velocity model obtained from both forward modeling and tomographic inversion of wide-angle seismic line OBS2017-2. The results support the continental nature of the Zhongsha Block with a thickness of up to ~25 km. However, the transition from the thick continental domain of the ZS into both adjacent abyssal basins shows clear differences. To the north, a ~120 km wide domain of extended continental crust was observed. Farther north, the NWSB is characterized as a narrow basin with typical oceanic crust. The transitional domain between the continental and oceanic crust shows a ~30–40 km wide region with a high-velocity lower crust reflecting excessive magmatism. In contrast, the SWSB is characterized by a sharp transition from the thick continental crust of the ZS to thin oceanic crust which is probably underlain by serpentinized mantle. The strong rheological properties of the pre-rift crust in the western part of the SCS margin may be the reason that rifting concentrated on narrow rifts and thinning focused on necking domains, while the ZS avoided any intense extension. The configuration of rigid blocks thereafter affected the break-up position and the style of oceanic crust.

A three-dimensional tomographic image of the seismic velocity structure in the crust and upper mantle of northwestern Nicaragua was performed using the back-projection technique. The study is based on ~ 15,000 earthquakes located between 1975 and 1982 by the network installed by the USGS and the Institute of Seismic Research of Nicaragua. This data represents the longest continuous recording of seismicity by a homogeneous seismic network in Nicaragua. Data from this network has not been used before. Initially, a one-dimensional model of the seismic velocity structure (1-D) was determined in order to relocate the seismicity and to obtain a first estimate of the seismic velocity model of the continental crust. This 1-D crustal model was complemented with the upper mantle structure of the IASPEI91 Earth model. This initial 1-D model is used to iterate for a 3-D tomographic imaging of Nicaragua. The results show three distinct features. There is clear evidence of a low seismic velocity plume rising from the subducted slab at depths of between 60 and 120 km. This low-velocity zone may be interpreted as the high temperature melts originating in the slab and feeding the volcanic arc. In northern Nicaragua, a high-velocity region is identified beneath the coast that presumably reflects the thickness of the subducting oceanic plate, estimated to be approximately 50 to 60 km thick. Beneath the volcanic arc, there is a low seismic velocity zone extending from 10 to 20 km. This low-velocity zone is interpreted as high-temperature material in the crust feeding the magma chambers of the volcanic arc.

There is increasing evidence indicating that melts derived from subducted oceanic crust and sediments may have played a key role in building continental crust. This mechanism predicts that juvenile arc crust should have oxygen isotope characteristics ranging from mantle-like to supracrustal, but consistent mantle-like radiogenic (Nd-Hf) isotopic signatures. Here we present in-situ zircon U-Pb dating, Hf-O isotope analyses, and whole rock major-trace element and Nd isotope analyses of a granitoid from NW India. In-situ secondary ion mass spectrometry (SIMS) zircon U-Pb dating yields a weighted mean 207Pb/206Pb age of 873±6 Ma for the granitoid. It displays mantle-like zircon eHf(eHf(873 Ma)=+9.3 to +10.9) and whole-rock Nd (eNd(873 Ma)=+3.5) values but supracrustal δ18O values, the latter mostly varying between 9‰ and 10‰. The calculated whole-rock δ18O value of 11.3‰±0.6‰ matches well with those of hydrothermally-altered pillow lavas and sheeted dykes from ophiolites. The major and trace element composition of the granitoid is similar to petrological experimental melts derived from a mixture of MORB+sediments. Thus, the granitoid most likely represents the product of partial melting of the uppermost oceanic crust (MORB+sediments). We propose that the decoupling between Hf-Nd and O isotopes as observed in this granitoid can be used as a powerful tool for the identification of slab melting contributing to juvenile continental crustal growth. Such isotopic decoupling can also account for high δ18O values observed in ancient juvenile continental crust, such as Archean tonalite-trondhjemite-granodiorite suites.

Earth’s magnetic field is recorded as oceanic crust cools, generating lineated magnetic anomalies that provide the pattern of polarity reversals for the past 160 million years1. In the lower (gabbroic) crust, polarity interval boundaries are proxies for isotherms that constrain cooling and hence crustal accretion. Seismic observations2–4, geospeedometry5–7 and thermal modelling8–10 of fast-spread crust yield conflicting interpretations of where and how heat is lost near the ridge, a sensitive indicator of processes of melt transport and crystallization within the crust. Here we show that the magnetic structure of magmatically robust fast-spread crust requires that crustal temperatures near the dike–gabbro transition remain at approximately 500 degrees Celsius for 0.1 million years. Near-bottom magnetization solutions over two areas, separated by approximately 8 kilometres, highlight subhorizontal polarity boundaries within 200 metres of the dike–gabbro transition that extend 7–8 kilometres off-axis. Oriented samples with multiple polarity components provide direct confirmation of a corresponding horizontal polarity boundary across an area approximately one kilometre wide, and indicate slow cooling over three polarity intervals. Our results are incompatible with deep hydrothermal cooling within a few kilometres of the axis2,7 and instead suggest a broad, hot axial zone that extends roughly 8 kilometres off-axis in magmatically robust fast-spread ocean crust. A record of Earth’s magnetic field constructed from near-bottom magnetization observations and oriented samples provides three-dimensional imaging of magnetic stripes in fast-spread crust, and suggests slow cooling off-axis, as opposed to deep hydrothermal cooling close to the spreading ridge.

The subduction of an oceanic plate is studied as the motion of a high-viscosity Newtonian fluid. The subducting plate spreads along the 670-km depth boundary under the influence of oppositely directed horizontal forces. These forces are due to oppositely directed horizontal temperature gradients. We consider the flow structure and heat transfer in the layer that includes both the oceanic lithosphere and the crust and moves underneath a continent. The heat flow is estimated at the contact between the subducting plate and the surrounding mantle in the continental limb of the subduction zone. Our study results show that the crustal layer of the subducting plate can melt and a thermochemical plume can form at the 670-km boundary. Our model of a thermochemical plume in the subduction zone shows the following: (1) formation of a plume conduit in the crustal layer of the subducting plate; (2) formation of a primary magmatic chamber in the area wherein the melting rate equals the rate of subduction; (3) origination of a vertical plume conduit from the primary chamber melting through the continent; (4) plume eruption through the crustal layer to the surface, i.e. formation of a volcano. Our experiments are aimed to model the plume conduit melting in an inclined flat layer above a local heat source. The melt flow structure in the plume conduit is described. Laboratory modeling have revealed that the mechanisms of melt eruption from the plume conduit differ depending on whether a gas cushion is present or absent at the plume roof.

Using the example of the conjugate Mozambique-Antarctica rifted margins, we investigate how mantle plume magmatism interacts with extensional processes during lithospheric rifting and breakup. The presence of extensive plume related magmatic additions formed during rifting masks the edges of the continental margins, making it difficult to determine a tight conjugate margin restoration and hindering the evaluation of their pre-breakup evolution. Based on rift domain mapping from seismic reflection interpretation, crustal thickness mapping using gravity inversion, lithosphere thinning from subsidence analysis, and the testing of different plate kinematic scenario, we develop a tight initial fit of the conjugate Mozambique-Antarctica margins and describe their evolution by a three stage model. These three stages consist of: i) during early rifting, the filling of accommodation space created by continental crust thinning with Karoo related magmatism resulting in a hybrid crust that maintains bulk crustal thickness producing shallow bathymetry or emergence and allowing SDR formation; ii) the decrease of magmatic budget relative to extension rates leading to bulk crustal thinning and increased bathymetries; and iii) the transition to seafloor spreading with a steady state magmatic budget resulting in normal thickness (6,5 ± 1 km) oceanic crust. This study not only adds new constraints on the regional breakup of Gondwana but also presents a new multidisciplinary methodology for restoring magma rich rifted margins formed in the presence of mantle plumes and for improving the understanding of the nature of such hybrid magma-rich crust.

The 60 km-long Pan-African Bape massif intruded in the Adamawa-Yade Archaean block is an excellent case for the study of subduction–collision-related magmatism, its origin by interaction among magmas from various sources, and its bearing on the evolution of the Pan-African orogenic belt in Cameroon. An integrated study including mapping, petrography and geochemistry was carried out on these Ba–Sr rich quartz monzonites, quartz monzodiorites, monzonites, granodiorites (adakites), syenites and micromonzogabbros. The rocks are dominated by plagioclase + alkali feldspars + amphibole + biotite + sphene + opaque minerals + apatite + zircon ± pyroxene ± quartz assemblage. They are I-type, ferroan, metaluminous and belong to high-K calc-alkaline, shoshonitic series. REE patterns form two sets of tight closed and of highly differentiated patterns. The spider diagrams display Th, U, Nb, Ta, Zr and Hf negative and Ba, K, Pb and Sr positive anomalies. High K2O, Ba–Sr and moderate SiO2 contents and the variation of canonical ratios such as Zr/Sm, Zr/Hf, La/Sm, and Sm/Yb, point to multiple origins for the Bape granitoids such as (i) melting of a dehydrated subducting crust (adakites) and/or the metasomatized heterogeneous mantle wedge (spinel– and phlogopite–amphibole–garnet lherzolite), or (ii) the mixing of these liquids (other granitoids). The U–Pb emplacement age (640–603 Ma), coeval with the subduction–collision between the West-African and the Congo cratons, the occurrence of adakites, combined with geophysical data, support that these granitoids are link to the southeast–northwest subduction of the hot Yaounde oceanic crust under the Adamawa-Yade craton, during the Neoproterozoic time.

Transformation of the oceanic crust into the continental one in orogenic belts is an important problem in petrological studies. In the paleocontinental sector of the Urals, a key object for tracing the stages of metamorphism and investigating the origin of anatectic granites is the Murzinka-Adui metamorphic complex. We have analyzed trace elements in zircons and established their genesis, sources, crystallization conditions, and stages of metamorphic events and granite generation in this complex. Zircons compositions were determined by the LA-ICP-MS method. Temperatures were calculated from Ti contents in the zircons. We distinguish three geochemical types of zircons, which differ in the ratios of light and heavy REE, U, Th, Ti, Y and show different values of Ce- and Eu-anomalies and Zr/Hf ratios, which are indicative of different crystallization conditions, as follows. Type I: minimal total LREE content; clear negative Eu- and Ce- anomalies; features of magmatic genesis; crystallization temperatures from 629 to 782 °C. Type II: higher contents of Ti, La, and LREE; low Ce-anomaly; assumed crystallization from highly fluidized melts or solutions. Type III: low positive Eu-anomaly; high REE content; low Th/U-ratio; zircons are assumed to originate from a specific fluidized melt with a high Eu-concentration. Ancient relict zircons (2300–330 Ma) in gneisses and granites show features of magma genesis and belong to types I and II. Such grains were possibly inherited from granitoid sources with different SiO2 contents and different degrees of metamorphism. Based on the geological and petrogeochemical features and zircon geochemistry of the Murzinka-Adui complex, there are grounds to conclude that the material composing this complex was generated from the sialic crust. The main stages of metamorphism and/or granite generation, which are traceable from the changes in types and compositions of the zircons, are dated at 1639, 380–370, 330, and 276–246 Ma. Thus, transformation of the oceanic crust into the continental one was a long-term and complicated process, and, as a result, the thickness of the sialic crust is increased in the study area.

To investigate the thermo-rheological structure and passive continental margin rifting in the Qiongdongnan Basin (QDNB), thermo-rheological models of two profiles across the western and eastern QDNB are presented. The continental shelf of western QDNB, having the lowest crustal extension factor, is recognized as the initial non-uniform extension crust model. This regime is referred to as the jelly sandwich-1 (JS-1) regime, having a lower crustal ductile layer. The oceanward part of the western QDNB changes from the relatively strong JS-1 to the weak creme brulee-1 (CB-1) regime with a significantly thinned lower crust. However, the crustal extension in the eastern QDNB is significantly higher than that in the western QDNB, with conjugate faults extending deep into the lower crust. The central depression zone of the eastern QDNB is defined as the much stronger JS-2 regime, having a brittle deformation across the entire crust and upper mantle and characteristics of a cold and rigid oceanic crust. Unlike the widespread lower crustal high-velocity layers (HVLs) in the northern margin of the South China Sea, the HVLs are confined to the lower crustal base of the central depression zone of the QDNB. The HVLs of QDNB are the results of non-uniform extension with mantle underplating during the lower crustal-necking stage, which is facilitated by the lower crustal ductile layer and derived by mantle lateral flowing. The gigantic mantle low-velocity zone related to the Red River Fault should be a necessary factor for the east-west differential margin rifting process of QDNB, which may drive the lateral flowing in the mantle.

Adding newly processed old marine and land-based wide-angle seismic data to modern geological and geophysical information about the region provides the correct localization and understanding of interaction of the Sinai, Anatolian, and African plates in the Eastern Mediterranean. Wide-angle seismic profile data acquired by ocean bottom and land-based seismometers in 1989–1990 by USSR Academy of Sciences expeditions were reprocessed. Land-based shots and a marine airgun shooting were used as seismic signal sources. As a result, the northern and western boundaries of the Sinai Plate were determined; absence of subduction in the southwestern part of the Cyprus Arc was proved; the structure of the Earth’s crust and upper mantle of oceanic crust of the African and Sinai plates and the continental crust of the island of Cyprus has been refined.

The results of magnetotelluric soundings obtained in 2020 along the Rzhev–Velikie Luki profile in the suture zone of three segments of the Precambrian crust of the East European Platform: Fennoscandia, Sarmatia, and Volga-Ural are presented. An anomaly zone of increased conductivity in the crust was revealed in the central part of the profile. The crust conductor is weakly expressed on the components of the telluric tensor due to the screening effect of the conducting sedimentary cover; therefore, the main emphasis in the qualitative analysis of the data and inversion is placed on magnetovariation parameters. According to the results of 2D inversion carried out in A.E. Kaminsky’s ZONDMY2D program, a subvertical zone of low resistivity has been identified on the geoelectric model in the crust, which is assumed to be associated with a paleosubduction zone of the Belarusian Oceanic Plate.

We present new major and trace element and Sr, Nd, Pb, Hf, and Os isotope data for Quaternary basaltic lavas and tephra from the Itasy volcanic field, in order to assess the origin and sources of recent volcanism in the central highlands of Madagascar. Our age determinations provide further evidence for Pleistocene volcanism on the island, with ages between ∼31 and 104 ka. Mafic magmas from Itasy exhibit significant geochemical heterogeneity, and they are chemically distinct from those of the neighboring Ankaratra volcanic complex. Trace element signatures of the Itasy volcanic field are very similar to those of ocean island basalts (OIB), with significant enrichment in high field strength elements (e.g. Nb and Ta) and depletion in large ion lithophile elements (e.g. Rb, Cs, and K), and relatively high Nb/U (>45) and Ce/Pb (>25) ratios. Significant intra-volcanic field isotopic heterogeneity is observed, with well-defined negative correlations of 87Sr/86Sr with 206Pb/204Pb and 143Nd/144Nd. These correlations, along with highly radiogenic Os isotopic signatures in many Itasy samples, could be attributed to assimilation of the Precambrian basement. However, with the exception of Os, the elemental and isotopic characteristics of the Itasy magmas are inconsistent with significant crustal assimilation, and instead are interpreted to represent features of their mantle sources. The Itasy ΔeHf and Δ7/4 values fall within the range of MORB and OIB, and indicate derivation primarily from an asthenospheric mantle source. The absence of significantly negative ΔeHf in the Itasy magmas argues against the involvement of ancient recycled oceanic crust or stranded lower crust in their sources, but the presence of a low µ (LOMU) component in some Itasy magmas can be attributed to pollution of the asthenosphere by delaminated subcontinental lithospheric mantle. A simple two-stage, three-component mixing model indicates that a mixture of mantle containing 90% depleted asthenosphere (DMM) and 10% foundered SCLM, which is further mixed with 30 to 50% of a plume-type mantle component, can reproduce the isotopic compositions of the Itasy magmas. Based on our model, it is possible that easily fusible superplume materials might be widely distributed in the upper mantle underneath the EARS, from the Indian Ocean to the Red Sea-Gulf of Aden, and mixed with shallow asthenosphere that has itself incorporated eroded SCLM. These components are heterogeneously distributed at a small spatial scale, similar to that observed recently in other mafic monogenetic volcanic fields in both intraplate and subduction settings.

For more than half a century, exploring a complete sequence of the oceanic crust from the seafloor through the Mohorovičić discontinuity (Moho) and into the uppermost mantle has been one of the most challenging missions of scientific ocean drilling. Such a scientific and technological achievement would provide humankind with profound insights into the largest realm of our planet and expand our fundamental understanding of Earth's deep interior and its geodynamic behavior. The formation of new oceanic crust at mid-ocean ridges and its subsequent aging over millions of years, leading to subduction, arc volcanism, and recycling of some components into the mantle, comprise the dominant geological cycle of matter and energy on Earth. Although previous scientific ocean drilling has cored some drill holes into old (> 110 Ma) and young (< 20 Ma) ocean crust, our sampling remains relatively shallow (< 2 km into intact crust) and unrepresentative of average oceanic crust. To date, no hole penetrates more than 100 m into intact average-aged oceanic crust that records the long-term history of seawater–basalt exchange (60 to 90 Myr). In addition, the nature, extent, and evolution of the deep subseafloor biosphere within oceanic crust remains poorly unknown. To address these fundamentally significant scientific issues, an international workshop “Exploring Deep Oceanic Crust off Hawai`i” brought together 106 scientists and engineers from 16 countries that represented the entire spectrum of disciplines, including petrologists, geophysicists, geochemists, microbiologists, geodynamic modelers, and drilling/logging engineers. The aim of the workshop was to develop a full International Ocean Discovery Program (IODP) proposal to drill a 2.5 km deep hole into oceanic crust on the North Arch off Hawai`i with the drilling research vessel Chikyu. This drill hole would provide samples down to cumulate gabbros of mature (∼ 80 Ma) oceanic crust formed at a half spreading rate of ∼ 3.5 cm a−1. A Moho reflection has been observed at ∼ 5.5 km below the seafloor at this site, and the workshop concluded that the proposed 2.5 km deep scientific drilling on the North Arch off Hawai`i would provide an essential “pilot hole” to inform the design of future mantle drilling.

The formation of continental crust in the Mongolia-Transbaikalia region is researched to identify the mechanisms of interactions between the crust and the mantle in the development of the Neoarchean, Proterozoic and Paleozoic magmatic and sedimentary complexes in the study area. Using the results of his own studies conducted for many years and other published data on this vast region of Central Asia, the author have analysed compositions, ages and conditions for the formation of Karelian, Baikalian, Caledonian and Hercynian structure-formational complexes in a variety of geodynamic settings. Based on the geostructural, petrological, geochemical, geochronological and Sm-Nd isotope data, he determines the crustal and mantle sources of magmatism, conducts the identification and mapping of isotopic provinces, and reveals the role of island-arc oceanic, accretion-collision and intraplate magmatism in the formation of continental crust. Considering the formation of the bulk continental crust, three main stages are distinguished: (1) Neoarchean and Paleoproterozoic (Karelian) (almost 30% of the crust volume), (2) Meso-Neoproterozoic (Baikalian) (50%), and (3) Paleozoic (Caledonian and Hercynian) (over 20%). This sequence of the evolution stages shows the predominance of the ancient crustal material in igneous rocks sources at the early stage. During the subsequent stages, tectonic structures created earlier were repeatedly reworked, and mixed crustal-mantle and juvenile sources were widely involved in the formation of the bulk continental crust in the study area.

The record of the Li-isotopic composition of ancient-seawater has the potential to provide important insight into the Earth system. However, we still have an incomplete understanding of the controls on the magnitude and isotopic compositions of the fluxes of Li into and out of the ocean. An important sink for Li from seawater is through low-temperature hydrothermal alteration of the upper oceanic crust. Here we present a detailed study of three adjacent drill cores in 120 Myr old Atlantic crust (Holes 417A, 417D and 418A) that had very different sedimentation histories. Samples from Hole 417A, drilled on a topographic high and exposed to seawater for >20 Myr before sediment started to accumulate, have much higher Li contents (up to ∼70 ppm) than samples from the other sites where sediment began accumulating immediately after crustal accretion (up to ∼20 ppm). Despite this difference in Li content, the δ7Li of the altered basalts in all cores increases with depth over the upper ∼40 m from around −1‰ at the lava-sediment interface to 6 ± 2‰ at ∼40 m depth, remaining roughly constant with depth after this. If the uppermost lavas equilibrated with seawater, with a bulk fractionation factor of ∼0.980 ± 0.003, then the isotopic composition of seawater 120 Myr ago was ∼19 ± 3‰, within the range of other estimates for Cretaceous seawater. Fluid-rock reaction models, with the hydrological regime varying as a function of depth and sediment accumulation history, can explain the observed rock compositions. However, modelling shows that complexities related to the time-varying hydrological regime, along with the possibility of time-varying δ7Li of seawater, mean that the Li-isotopic composition of altered oceanic lavas cannot be uniquely interpreted. The bulk Li content and isotopic composition of the well-sampled 120 Myr crust in Holes 417A, 417D and 418A suggest that Cretaceous upper oceanic crust was a sink for ∼4×109 mol yr−1 of Li. Surprisingly, the bulk Li-isotopic composition of the lavas sampled at these sites (∼6‰) is not much higher than that of fresh MORB, and within uncertainty of that of high-temperature hydrothermal vent fluids; i.e., high- and low-temperature alteration processes approximately balanced one another for Li-isotopes in the late Cretaceous. This could be fortuitous; alternatively, if the ocean was at steady state the main source of Li to the ocean at this time may have had a δ7Li similar to that of high-temperature hydrothermal fluids.

Subduction of atmospheric noble gases has been considered to play an important role in altering the primordial isotopes of Earth’s mantle over geological time. Analysis of natural samples and experiments indicate that large quantities of noble gases can be dissolved in volatile-bearing hydrous minerals in the subduction slabs. To quantitatively investigate the recycling efficiency of noble gases and relevant consequences on the mantle noble gas isotopic evolution, the diffusivities of noble gases in these minerals are needed. In this study, diffusion of He, Ne, Ar, Kr and Xe in lizardite, antigorite and tremolite have been calculated by first-principles methods based on density functional theory. Our results disclose that diffusion is slower with increasing radius of the noble gas atom (DHe > DNe > DAr > DKr > DXe) as expected. The common ring-structures in hydrous silicate minerals provide incorporation sites for the noble gas atoms and control their mobility. The diffusion activation energies are 84.9, 157.3, 287.5, 347.4, 414.9 kJ/mol from He to Xe in lizardite, and despite the very similar lattice structure between lizardite and antigorite, the activation energies are found to be significantly higher in antigorite, which are 120.6, 267.3, 449.6, 497.9 and 550.0 kJ/mol, respectively. In tremolite, the energy barriers are 93.6, 158.2, 266.3, 322.2 and 385.0 kJ/mol, which are also found to be in very good agreement with available experimental values and similar to those in lizardite. We also calculated diffusion activation energies at higher pressures (1 GPa for liazardite, 3 GPa for antigorite and tremolite) to better understand how much noble gases can be preserved against diffusive loss during subduction. Our result show that the oceanic crust and the lithospheric mantle of the subduction slab play different roles in delivering noble gases into the mantle. We find that all Ar, Kr, Xe and possibly part of the Ne can be entrained by the serpentine-dominated lithospheric mantle into the deep mantle due to the high diffusive energy barriers in antigorite. In contrast, noble gases in the amphibole-enriched oceanic crust would be characterized by fractionated noble gas signature, with the concentrations of retained noble gases in the crust following their respective ionic radius (Ne

Gravimetric data obtained by the GOCE spacecraft contributes to the development of global models of the deep structure of the Earth’s crust and upper mantle (referred to below as global models), improving the understanding of the metallogenic specialization of ore regions. Several examples of using these models in metallogenic analysis are considered here. It is shown that the concept of the global metallogenic homogeneity and zonality of the Pacific Ore Belt (POB) and its boundaries, is well supported by modern maps based on global models. Disputed POB boundaries also receive additional support in the thermal and density model of the upper mantle. Based on the analysis of global models, it is noted that the ore deposits of central and northern Chile clearly gravitate to a region with a denser and colder mantle at depths of 60 km, which corresponds to a flat segment of the subduction zone at the time of ore formation (Miocene), where the dehydration of the submerging oceanic plate and hydration of the upper mantle occurred. The distribution of polymetallic deposits in the Balkans toward the northwest corresponds to the spread of the mean values of density and temperature of the upper mantle. A similar direction is also observed in the structure of the Moho’s surface. Analysis of global models shows that tectonic–magmatic activation during the formation of magmatic arcs due to erosion unearths deposits associated with ophiolites and rifting conditions (Cu–Ni, SEDEX) and the forming of new porphyry Cu, skarn, and/or vein (Pb–Zn) deposits. These results are of great importance in predicting new deposits.

Deju basalts is a characteristic volcanic association and was first founded in the 1 : 50 000 regional geological survey. There is no previous study. The basalts’ characteristics and genesis are systematically studied from the geochemistry. It is showed that the basalts’s w(SiO2) = 50.61‒54.54%, w(K2O + Na2O) = 0.15‒5.96%, w(P2O5) = 0.18‒0.45%, w(TiO2) = 1.82‒3.58%, belong to sub alkaline basalt series. Chondrite-normalized REE patterns indicate the basalts are relatively rich in LREE but deficient in HREE (LREE/HREE = 5.81‒6.51), and have a similar mantle source with WPB and OIB. The mantle source have a characteristics of GP. The discrimination diagrams of basalt tectonic setting reflect the basalts in Deju belongs to continental intraplate basalt, formed in a continental rift stretching environment, and during the diagenesis process, the magma source was contaminated by the crust and oceanic crust subducted materials.

Adakitic rocks are intermediate-acid magmatic rocks characterized by enrichment in light rare-earth elements, depletion in heavy rare-earth elements, positive to negligible Eu and Sr anomalies, and high La/Yb and Sr/Y ratios. Cenozoic adakitic rocks generated by partial melting of subducted oceanic crust (slab) under eclogite-facies conditions (i.e., the original definition of “adakite”) occur mainly in Pacific Rim volcanic arcs (intra-oceanic, continental, and continental-margin island arcs), whereas those generated by partial melting of thickened lower crust occur mainly in Tethyan Tibetan collisional orogens. In volcanic arcs, adakitic melts derived from the melting of subducted oceanic crust metasomatize the mantle wedge to form a unique rock suite comprising adakite-adakite-type high-Mg andesite-Piip-type high-Mg andesite-Nb-rich basalt-boninite. This suite differs from the basalt-andesite-dacite-rhyolite suite formed from mantle wedge metasomatized by fluids derived from subducted oceanic crust. Previously published data indicate that partial melting of mafic rocks can generate adakitic magmas under pressure, temperature, and hydrous conditions of 1.2–3.0 GPa, 800–1000°C, and 1.5–6.0 wt.% H2O, respectively, leaving residual minerals of garnet and rutile with little or no plagioclase. Cenozoic Au and Cu deposits occur proximally to adakitic rocks, with host rocks of some deposits actually being adakitic rocks. Adakitic rocks thus have important implications for both deep-Earth dynamics and Cu-Au mineralization/exploration. Although studies of Cenozoic adakitic rocks have made many important advances, there remain weaknesses in some important areas such as their tectonic settings, petrogenesis, magma sources, melt-mantle interactions of pre-Cenozoic adakitic rocks, and their relationship with the onset of plate tectonics and crustal growth. Future research directions are likely to involve (1) the generation of adakitic magmas by experimental simulations of partial melting of different types of rock (including intermediate-acid rocks) and magma fractional crystallization at different temperatures and pressures, (2) the relationship between magma reservoir evolution and the formation of adakitic rocks, (3) the tectonic setting and petrogenesis of pre-Cenozoic adakitic rocks and related geodynamic processes, (4) interactions between slab melts and the mantle wedge, (5) the formation of Archean adakitic tonalite-trondhjemite-granodiorite and its link to the onset of plate tectonics and crustal growth, and (6) the relationship between the formation of adakitic rocks and metal mineralization in different tectonic settings.

Dehydration of the subducting slab is a crucial process in the generation of hydrous convergent margin magmas, yet the exact processes of how and where the slab dehydrates and how these fluids are transported to the mantle wedge remain obscure. Strontium is a “fluid-mobile” element and as such well suited to investigate the source of slab-derived fluids. We employ mass-dependent Sr isotope systematics (δ88/86Sr; the deviation in 88Sr/86Sr of a sample relative to NIST SRM 987) of primitive arc lavas, in tandem with conventional radiogenic 87Sr/86Sr measurements, as a novel tracer of slab dehydration. To characterise the δ88/86Sr composition of subduction zone inputs, we present new δ88/86Sr data for subducting sediments, altered oceanic crust and MORB. Calcareous sediments are isotopically lighter and carbonate-free sediments are isotopically heavier than mid-ocean ridge basalts (MORB). Samples of the altered oceanic crust display elevated 87Sr/86Sr but only the most intensely altered sample has significantly higher δ88/86Sr than pristine MORB. Mafic arc lavas from the Aegean and Mariana arc invariably have a mass-dependent Sr isotope composition that is indistinguishable from MORB and lower 87Sr/86Sr than upper altered oceanic crust. This δ88/86Sr-87Sr/86Sr signature of the arc lavas, in combination with their high but variable Sr/Nd, can only be explained if it is provided by a fluid that acquired its Sr isotope signature in the deeper, less altered part of the subducted oceanic crust. We propose a model where the breakdown of serpentinite in the slab mantle releases a pulse of fluid at sub-arc depths. These fluids travel through and equilibrate with the overlying oceanic crust and induce wet partial melting of the upper altered crust and sediments. This hydrous melt is then delivered to the mantle source of arc magmas as a single metasomatic component. From mass balance it follows that the slab-derived fluid contributes >70% of the Sr budget of both Mariana and Aegean arc lavas. Whereas this fluid-dominated character is unsurprising for the sediment-poor Mariana arc, the Aegean arc sees the subduction of 3–6 km of calcareous sediments that were found to exert very little control on the Sr budget of the arc magmas and are overwhelmed by the fluid contribution.

Mid-ocean ridges are the oceanic counterparts of continental graben structures. Both are zones of extension although mid-ocean ridges have substantially higher spreading rates and also mark plate boundaries where new oceanic crust and lithosphere are formed. The presence of normal faulting and development of a central graben document that as new crust is formed, it is still under extension as it drifts and spreads.

The use of ocean bottom seismometer (OBS) is of great significance to explore the tectonic evolution of the East China Sea. To study the characteristics of the deep geological structure from the East China Sea shelf basin to the Ryukyu trench uplifting fold belt, the NW-SE active source deep seismic experiment (OBS-2015) was carried out in the East China Sea. By using the delayed fire source of multilayer gun array, wide-angle seismic reflection and refraction signals up to the Moho can be obtained in the continental shelf and Okinawa Trough, and various seismic phases such as Ps, Pg and PmP can be recognized. The data acquisition results of OBS deep geoscience seismic exploration in the East China Sea show that the effective seismic phase up to Moho surface can also be obtained by using the three-dimensional array source composed of medium and small capacity air guns, which provides a new way for the selection of excitation source for OBS deep exploration. After data preprocessing, identifying and picking up seismic phases, a 2D velocity structure profile was obtained by ray tracing and travel time forward and inversion modeling. Geological interpretation and comprehensive study show that the East China Sea shelf basin is a Mesozoic-Cenozoic sedimentary basin with the maximum superimposed Mesozoic-Cenozoic thickness of 13 km. The crust of the East China Sea shelf area is the extension of the continental crust to the sea. The Moho surface has small fluctuation and the buried depth is about 30 km. From the Diaoyu Island fold to the Ryukyu Island Arc, the Moho Surface fluctuates sharply, rising rapidly from west to East. The crustal thickness of the Okinawa Trough area is only about half the thickness of in the East China Sea shelf basin. It is a high velocity geological body with the thinnest central axis of approximately 13 km and rapid thickening to both sides (approximately 19 km). The high velocity (up to 7.1 km/s) abnormal body develops above the Moho surface in the Okinawa Trough area. The Moho surface in the eastern depression has uplifted significantly. This shows an obvious mirror image relationship with the extensional basin. The vertical velocity structure of the Okinawa Trough is quite different from that of the continental crust and is close to that of the oceanic crust. It is speculated that the thick high-speed body developed on the Moho surface is the manifestation of a large amount of upwelling of mantle derived materials during the back arc stretching process. Combined with the phenomena of low-speed abnormal magma chamber in the shallow part and the penetration of magmatic rocks into the sedimentary basement layer, it is believed that the crust in the southern Okinawa Trough along the survey line has ruptured and entered the initial stage of sea-floor spreading, and the initial oceanic crust has appeared in the southern Okinawa Trough and the crust has atypical oceanic crust characteristics. This experiment provides important basic data for studying the deep crustal structure and regional geological evolution of the East China Sea.

The deep crustal structure of the Northwest Sub-basin (NWSB) of the South China Sea (SCS) is of great importance for understanding the tectonic nature of the continent-ocean transition (COT) and magmatism in this oceanic basin. The 2-D wide-angle reflection/refraction seismic profile OBS2006-2 is almost parallel to the extinct spreading ridge (ESR) of the NWSB. In addition to the original data, we added the data of two reprocessed OBS stations, and carried out seismic phase re-picking and travel-time imaging to obtain the crustal velocity structure along this profile. Resolution tests demonstrate that the newly acquired velocity structure is more reliable than the prior interpretation. The depth of the Moho (23.5–11.8 km) and crustal thickness (20.5–6.5 km) systematically changes from continental crust of the Xisha Block to the oceanic crust within the NWSB. The COT zone has a width of ∼20 km and the depth of the Moho decreases from 15.0 to 11.0 km, corresponding to a ∼4 km decrease in crustal thickness (6–10 km). A high velocity layer (HVL, 7.2–7.4 km s−1) exists at the bottom of the crust at the location where the sharp lateral transition of the continental crust to the oceanic crust occurs. Age dating shows that the Double-peak Seamount was formed at ∼23 Ma, after the cessation of the NWSB seafloor spreading (∼32-25 Ma). The crust beneath the Double-peak Seamount is oceanic with a thickness of 9 km. We infer that this oceanic crust was formed by magmatic upwelling and decompression melting along a pre-existing zone of weakness.

SUMMARYThe Philippine Sea plate (PHS) is subducting beneath southwest Japan from the Nankai Trough. In this region, deep low-frequency earthquakes/tremors (LFEs) are active and their epicentres are distributed along the downdip of the source region of M8-class earthquakes that have occurred every 100–150 yr. The LFE activity may potentially be strongly related to the occurrence of great earthquakes between tectonic plates. To investigate the structural features around the LFEs, we applied teleseismic receiver function (RF) analysis to the seismograms observed at permanent and temporal seismograph stations in western Shikoku and we detected seismological evidence of a slab dehydration reaction linked to LFE activity. Based on the first-order harmonic decomposition of RFs, we first estimated the average plunge azimuth of the PHS beneath western Shikoku. Considering the backazimuth dependence of converted phase amplitude, we constructed the cross-section of the radial component RFs, excluding the incoming waveform data from the updip directions of the dipping slab. In this profile, the parallel negative and positive P-to-S converted phase alignment within a distance of 10 km were imaged. These phase alignments corresponded to the top of the slab and the slab Moho discontinuity, respectively. At the northern side of the profile, the landward (continental) Moho was also detected. In the region where LFEs were actively distributed, both the landward Moho and slab surface were unclear. The second-order harmonic decomposition of RFs for several kilometres above the slab Moho indicated that the anisotropic symmetry axes suddenly changed at the southern limit of the LFE active region; the fast axes in the region were normal to the trench though axes in the southern area were parallel. According to the thermal and pressure condition, a phase transition with a dehydration reaction can occur in the oceanic crust near the southern edge of the LFE active region. Once the dehydration process advances, released water causes the serpentinization of the mantle material near the slab surface and the corresponding seismic velocity decreases. The impedance contrast decreases at the boundary between the lower crust and the mantle wedge, as well as that between the mantle wedge and oceanic crust; therefore, the amplitudes of the converted phases at these boundaries also become small. Considering that serpentinite ordinarily has strong anisotropy with a seismic fast axis direct to the shear direction, all features observed are evidence of the dehydration process in the flat slab.

—Homogenized melt inclusions in olivine were studied in Archean komatiites from the Barberton Greenstone Belt, Weltevreden Formation in South Africa (3.3 Ga), Abitibi Greenstone Belt in Canada (2.72 Ga), and the Belingwe Greenstone Belt in Zimbabwe (2.69 Ga). Contamination of the komatiite melts with crustal material enriched in Rb, Cl, and H2O during the crystallization of olivine is demonstrated. Uncontaminated melts have mantle Rb/Nb ratios but are significantly enriched in Cl and H2O relative to K and Ce, respectively, exhibiting similar incompatibility during crystallization and partial mantle melting. These observations suggest the presence of a chlorine- and water-enriched mantle source before 3.3 Ga. The excess Cl and H2O contents in the komatiites are assumed to result from the interaction of partially molten mantle plumes with the mantle transition zone. The most likely source of Cl and H2O enriching the deep mantle is the oceanic lithosphere that endured a seafloor alteration. We conclude that the recycling of the altered oceanic lithosphere into the mantle, probably via subduction, began in the first billion years of the Earth’s history. Delamination of the Archean crust could not cause transport of chlorine and water into the deep mantle.

The Ivrea Zone of northwest Italy exposes a section of the lower crust and lithospheric mantle deformed during the Variscan Orogeny, where a series of carbonated alkaline pipes was emplaced over a time span of ca. 40 Ma, following a crustal underplating event at ca. 288 Ma. We use a coupled 2D petrological-thermomechanical approach to geodynamically model the processes that led to the Variscan continental collision and subsequent emplacement of the alkaline pipes with application to craton margin metasomatic events. The following criteria are assessed to develop a generic model: 1) closure of Rheic Ocean through subduction of oceanic crust and subduction-related metasomatism of the lithospheric mantle, 2) high-temperature metamorphism of the lower crust associated with continental collision, preceding the 3) emplacement of a crustal underplating event, and 4) partial melting of metasomatized lithospheric mantle domains during gravitational collapse of the orogen to source the alkaline pipes. Our model investigates the collision process by varying thermal ages of the intervening oceanic crust (40, 60, and 80 Ma) and by changing the rheological strength of the lower crust. We employ two ideal end-member cases, wet quartzite and plagioclase An75, an intermediate dry quartz case to represent a migmatized lower crust, and two polymineralic cases of felsic granulite and mafic granulite as possible representations to the Ivrea Zone specifically. We discuss the results through three scenarios controlled by the age of the oceanic crust and rheology of the lower crust that can reproduce tectonic scale features that are observed in the Ivrea Zone. Scenario 1: the wet quartzite 40 Ma model features widespread H2O and CO2 fluxes within the continental lithospheric mantle. The model displays little melt productivity due to the presence of an overthickened crust and the continental lithospheric mantle decouples and peels off beneath the lower crust of the colliding terrane. Scenario 2: the plagioclase An75 60 Ma model shows high-temperature metamorphism in the lower crust and reproduces the metasomatism of the continental lithospheric mantle that is a precursor to the genesis of the alkaline pipes. The model is permissive for the emplacement of the crustal underplating event, but the relative timings are misplaced. Scenario 3: the mafic granulite 80 Ma model recreates the tectonic relationships between lower crustal high-temperature metamorphism followed by initiation of the magmatic underplating event, and lastly by localized melting of metasomatized domains of the lithospheric mantle. This model can potentially explain two of the prevailing hypotheses for the formation of the alkaline pipes. Whereas all models reproduced different stages of the tectono-magmatic evolution of continental collision, only scenario 3 could sufficiently recreate the relative sequence of events observed in the Ivrea Zone. Scenarios 1 and 2 may provide precious information about the nature and timing of metasomatic processes in the lithospheric mantle, with implications for the enhanced prospectivity for a wide range of ore systems in specific domains along the margins of lithospheric blocks.

In subduction zones, landward dipping regions of low shear wave velocity and elevated Poisson’s ratio, which can extend to at least 120 km depth, are interpreted to be all or part of the subducting igneous oceanic crust. This crust is considered to be overpressured, because fluids within it are trapped beneath an impermeable seal along the overlying inter-plate boundary. Here we show that during slow slip on the plate boundary beneath southern Vancouver Island, low frequency earthquakes occur immediately below both the landward dipping region of high Poisson’s ratio and a 6–10 km thick shear zone revealed by seismic reflections. The plate boundary here either corresponds to the low frequency earthquakes or to the anomalous elastic properties in the lower 3–5 km of the shear zone immediately above them. This zone of high Poisson’s ratio, which approximately coincides with an electrically conductive layer, can be explained by slab-derived fluids trapped at near-lithostatic pore pressures., Regions of the subducting oceanic crust are often considered to be overpressured, owing to fluid trapped beneath an impermeable seal along the overlying inter-plate boundary. Here, the authors show that slow slip earthquakes at the Cascadia subduction zone occur immediately below a 6-10 km-thick shear zone, in which slab-derived fluids are likely trapped at near-lithostatic pore pressures.

The Late Paleozoic magmatism in central Inner Mongolia provides important insights on the tectonic evolution and crustal growth in the Central Asian Orogenic Belt (CAOB), which formed due to the closure of the Paleo-Asian Ocean (PAO). This paper presents new zircon U Pb ages and Hf isotopic compositions as well as whole-rock geochemical data on a suite of volcanic rocks from the Late Paleozoic Baoligaomiao Formation and coeval intrusions in the Baiyinwula region of the Mongolian Arc. This study revealed that the magmatic sequences evolution includes: (1) early andesites (317–311 Ma) with enrichment in large ion lithophile elements (LILEs), depletion in high field strength elements (HSFEs), and positive zircon eHf (t) values from +9.0 to +15.5, indicating a derivation from enriched mantle; (2) felsic rocks emplaced from 306 Ma to 292 Ma, with relatively lower eHf (t) values from +6.3 to +11.3, implying juvenile crust as the primary magma source; and (3) A-type igneous rocks (280–278 Ma). The comparison of palaeontological, lithostratigraphical, and magmatic evolution in Late Paleozoic between different tectonic units in the eastern part of CAOB has displayed that the subduction of Paleo-Asian oceanic crust caused the opening of the Hegenshan Ocean along the southern margin of Mongolian Arc in Devonian; and the Baoligaomiao Formation volcanic rocks and coeval intrusions have recorded early northwards subduction and subsequent slab rollback of Hegenshan oceanic crust.

This study explores the spatial distribution of seismicity, the pattern of b-values, gravity and magnetic anomalies, heat flow data, and focal mechanism solutions to more thoroughly understand the present-day stress distribution and the nature of the crust, which characterize the rifting process in the northern Red Sea region. The region shows alternating low and high b-values in congruence with low and high cumulative seismic moment releases, respectively, and with negative and positive gravity anomalies, respectively. In general, except for larger-sized earthquake locations that characterized by high stress accumulation, low b-values, low seismic moment release, and low-gravity anomalies dominate the zones of the Gulf of Aqaba and the Gulf of Suez, thereby implying low stress accumulation consistent with an old crust of no differential development. Conversely, the rest of the Northern Red Sea region exhibited heterogeneities in the spatial distribution of b-values, cumulative seismic moment releases, and gravity anomalies, thereby implying stress heterogeneities. The stress heterogeneities may impute to the differences in material properties of the upper crust in the region. Zones of a positive Bouguer anomaly and high stress are notably associated with a relatively weak crust characterized by high rate of seismicity, while zones of the negative Bouguer anomaly and low stress characterized an older and more stable crust. However, details on the nature of crust required additional geophysical and geological data of high resolution in future studies.

Porosity is one of the most basic physical properties of a stratum. Previous studies have shown that porosity generally decreases with depth. There are five main types of empirical models available for describing such a porosity–depth relationship: the linear model, the exponential model, the power law model, the reciprocal model, and the parabolic model. For the majority of past and existing studies, each tends to focus on one study area, and thus does not present enough data to cover a large depth range and represent the general attenuation of porosity through Earth’s crust. Whether those models can deal with a larger depth scale or not remains unknown. This paper proposes a new empirical porosity–depth model that can describe the decrease of porosity through the entire Earth’s crust. Porosity–depth data are collected from the literature and the proposed model fits the data very well. The model improves the agreement between calculated and measured porosity values and can be used to describe the porosity–depth relationship for continental crust, oceanic crust, sedimentary rocks, and unconsolidated sediments. The proposed model tends to slightly overestimate the measured data for Earth’s continental crust in the depth range of 5–15 km. It is also difficult for a model to fit the near-surface porosity data well for unconsolidated sediments. This study improves the usefulness of the empirical models in estimating porosity where measured data are unavailable, in discerning abnormal strata, and in supporting regional groundwater studies.

Seismicity, heat flow, seismic tomography data, prerift and synrift magmatism are considered as intensity indicators of geodynamic processes along the Atlantic-Arctic rift system (AARS). In this rift system, several large (over 100 km ) sub-latitudinal displacements of the rift axis are due to left-lateral strike-slip faulting. The AARS segments are distinguished by the age of splitting of continental plates from each other. A dependence is revealed between the current thermal state of the mantle under the AARS and the age of spreading start. This dependence is established from both seismic tomography and heat flow data. In section δ ( Vp/Vs ), the locations of the main segmenting faults and ‘cold’ anomalies in the upper mantle are coincident. Distributions of total seismic moments are practically synchronous in the depth intervals of 0–13, 13–35, and >35 km. The maximum values above the plumes are represented by higher seismic moments in the surface layer. The main demarcation zones differ in maximum energy release values in the AARS with shearing features. Comparison of these values against the age of the start of spreading processes shows trends of heat flow and medium field tomography in the AARS segments. The trends confirm the thermal interpretation of the seismic tomography data and suggest mantle cooling with age and a decrease in the mean temperatures of the mantle. The main factor causing the sublatitudinal asymmetry of heat flow in the AARS is the impact of Coriolis forces on the magma in the asthenospheric source. Most of the synrift igneous formations seem to be related to the influence of long-lived anomalies in the mantle, which had lower rates of magma generation than those typical of the formation of magmatic provinces. In conditions for spreading and the formation of the oceanic crust, the process followed the principle of energy cost minimization, and the prerift magmatic provinces with the pre-processed crust contributed to the choice and positioning of the AARS trajectory. The plume branches are imposed in the tomographic section and thus ‘concealing’ the relationship between the age and the thermal state. However, that does not change the trend to cooling of the mantle beneath the AARS, proportionally to the time since the start of spreading.

The dynamic process of ocean-continent subduction depends on not only the properties of the subducting oceanic plate, but also the characteristics and state of the overriding continental plate. Numerical models conducted to date have mostly focused on the oceanic lithosphere in this regard; research on the properties of overriding continental lithosphere remains relatively limited, especially the influence of its thermal state on subduction dynamics. Here we explored the performance of continental lithosphere with different thermal states during the subduction process using two-dimensional thermal-mechanical modeling and systematically investigated the effects of the thermal state of overriding continental plate, the age of subducting oceanic plate, and relative convergence rate on subduction dynamics. Modeling results show that: (1) When the geothermal gradient of continental crust is low (between 10 and 15 °C km−1), the oceanic plate first subducts at a low angle. As subduction continues, the slab dip gradually increases and the slab begins to retreat rapidly driven by its negative buoyancy, opening an ocean basin ranging from 600 to 1100 km in width. This leads to the decoupling between the overriding continental plate and oceanic plate. As the trench retreat continues, the horizontal deviatoric stress inside the overriding continental crust alternates between being positive and negative in a local area. Thinning of the overriding lithosphere mainly occurs at the region adjacent to the subduction zone, where the surface experiences significant subsidence. (2) When the geothermal gradient of continental crust is higher (greater than 15 °C km−1), oceanic plate retreat causes the overriding continental plate to be strongly stretched. In this case, the trench retreat distance decreases and the width of the ocean basin also reduces by between 100 and 1000 km. The horizontal deviatoric stress inside the whole overriding continental crust first manifests as compression and then changes into extension, which causes the surface to first uplift and then slowly subside. (3) Increasing the age of oceanic lithosphere accelerates trench retreat and promotes overriding plate thinning. (4) An advancing overriding continental plate slows down trench retreat. In cases where the geothermal gradient of continental crust is greater than 17.5 °C km−1, the hot continental crust experiences gravitational collapse and is overthrusted onto oceanic lithosphere, resulting in slow trench retreat. We analyzed the subduction process of the western Paleo-Pacific Plate in the Early Cretaceous based on our modeling results and discussed its possible control on the tectonic evolution of the rift basins in east Asia. We suggest that the development of a wide rift basin system on the Amurian Superterrane in the Early Cretaceous was likely related to slow trench retreat and the collapse of the hot crust, and the formation of a series of passive rift basins in the North China Craton was likely caused by the relatively cold thermal state of the lithosphere and the rapid retreat of the Paleo-Pacific Plate.

The oceanic crust consists mainly of a lower layer of cumulate gabbroic rocks and an upper layer of differentiated basalts. The thicknesses and proportions of the gabbroic and basaltic layers in different oceans are largely controlled by spreading rate, magma supply, and magmatic differentiation processes. Evaluating the effects of complex magmatic differentiation as a function of spreading rate on Ca isotope composition is critical to understanding whether the Ca isotope compositions of oceanic crust from different oceans are homogeneous and thus whether the observed considerable variation of δ44/40Ca in basalts (up to 0.4‰) results from magmatic differentiation or mantle source heterogeneity. To address the question, we present δ44/40Ca measurements of a series of gabbroic rocks (n = 38) and mineral separates from the 810-m-long U1473A hole drilled into the gabbroic lower crust at the ultraslow-spreading Southwest Indian Ridge (SWIR), along with 12 mid-ocean ridge basalts (MORBs) from the slow-spreading South Mid-Atlantic Ridge (SMAR) and the fast-spreading East Pacific Rise (EPR). Although the gabbroic rocks of the SWIR reflect several events of magma supply and strong magmatic differentiation (bulk rock Mg# of 64–79 for each event), their δ44/40Ca values (0.85 ± 0.09‰, 2sd, n = 37) are uniform. The results are consistent with limited inter-mineral Ca isotope fractionation between plagioclase (Pl) and co-existing clinopyroxene (Cpx) in the accumulated gabbros (average Δ44/40CaPl-Cpx = −0.10‰, n = 5). This indicates that no measurable Ca isotope fractionation occurs during formation of ultraslow-spreading oceanic crust. The MORBs from the SMAR and EPR show consistent δ44/40Ca values (0.82 ± 0.08‰ (2sd, n = 4) and 0.86 ± 0.09‰ (2sd, n = 8), respectively), regardless of the degree of fractional crystallization. On the whole, the ultraslow-, slow- and fast-spreading gabbroic cumulates and MORBs display indistinguishable δ44/40Ca within analytical uncertainty, suggesting a homogenous Ca isotope composition for the global igneous oceanic crust (δ44/40Ca = 0.85 ± 0.09‰, 2sd, n = 49) even if they experience complex magmatic differentiation. Comparison with values for fertile mantle rocks (δ44/40Ca = 0.94 ± 0.10‰) reveals that partial melting triggers only slight Ca isotope fractionation (0.09 ± 0.02‰, 2se). In this light, the considerable variation of previously reported δ44/40Ca values for basalts may result from their different mantle sources, and is probably attributable to the recycling of crustal materials.

The known deep subsurface biosphere on Earth persists in diversified habitats, including deep within igneous rocks of the oceanic crust. Here, we extend the range of the deep subsurface biosphere to metamorphic ocean crust of a subduction zone. We report fossilised life in zeolite facies rocks, which formed by low grade metamorphism, from the southern Mariana trench. Dense carbonaceous spheroids, filaments, and Frutexites-like structures are preserved in these rocks, which are enriched in organic carbon but depleted in 13C. The distinct difference in the GDGT-0 vs. crenarchaeol and the branched vs. isoprenoid tetraether values between the inner and outer portions of these rocks indicate the in situ production of organic carbon. We demonstrate that these structures may result from the past activity of potential chemolithoautotrophs within the metamorphic crust, as implied by their morphologies, Raman spectra, carbon isotopes, and biomarker signatures, as well as the Fe oxidation state within whole rocks. We propose that fluid-rock reactions at temperatures within the tolerance of life during low grade metamorphism contributed to microbial subsistence within the biotope. The low grade metamorphic ocean crust of the subduction zone likely represents Earth’s deepest, and one of its largest, microbial ecosystems, which may potentially influence the deep carbon cycle.

Granulite-facies xenoliths entrained within igneous rocks shed a light on poorly understood yet critical questions about age, origin and history of the lower continental crust. Here, we present the first coupled in-situ U-Pb, Lu-Hf and O isotope data for the Precambrian zircons from fourteen deep-crustal xenoliths from the North China craton. The geochronological data demonstrates that the oldest lower crustal remnants formed at ∼3.82 Ga and related magmatism continued until ∼3.55 Ga. The Eo-Paleoarchean zircons, with exception of one analysis, have subchondritic initial Hf isotopes with negative eHf(t) and are characterized by normal mantle to elevated δ18O values (5.37–6.90‰), indicating a development of a low Lu/Hf reservoir and hydrosphere-crust interactions in the early Earth. Magmatic zircons from lower crustal xenoliths of North China define a strongly episodic distribution of ages at 3.82–3.55 Ga, ∼2.7 Ga, ∼2.5 Ga and 1.95–1.85 Ga and a non-linear Hf isotope-age array for nearly 2 Gyr, indicating episodic crustal generation and reworking through time. The lower crustal xenoliths record a change in zircon O isotope compositions from a restricted range at 3.8–2.6 Ga to more variable after ∼2.5 Ga. Both heavier (δ18O up to 10.25‰) and lighter (δ18O low to 2.16‰) O isotope compositions than the normal mantle are observed in ∼2.5 Ga magmatic zircons. The secular change in zircon O isotopes documents an increase in recycling rate of surface-derived materials (including high-δ18O sediments, weathered and altered rocks and low-δ18O altered oceanic crust) into magma sources at the end of Archean, which, in turn, is possibly linked to modern style subduction processes and maturation of the crust at that time.

This paper presents new data on rocks dredged from the Hahajima Seamount during the cruise KH03-3 of R/V Hakuho-Maru in 2003. It is shown that the seamount is a tectonically displaced block of the Earth’s crust composed of island-arc ophiolites. The basement of the ophiolite complex is made up of highly depleted mantle residue (dunites and harzburgites). Upward, they grade into cumulate lherzolites, various gabbroids, dolerites, and basalts. This complex marks the initial stages of island-arc evolution: formation of an oceanic crust of the marginal sea. Upward, the section is built up of the island-arc rock association consisting of tholeiites, boninites, and adakites. New data on mineralogy and geochemistry of the studied rocks are reported. The ophiolitic complex served as the basement for island-arc association and was affected by the powerful heat flow that resulted in repeated melting of mantle residue and metamorphism of the above rocks.

The Oman-United Arab Emirates ophiolite has been used extensively to document the geological processes that form oceanic crust. The geometry of the ophiolite, its extension into the Gulf of Oman, and the nature of the crust that underlies it are, however, unknown. Here, we show the ophiolite forms a high velocity, high density, >15 km thick east-dipping body that during emplacement flexed down a previously rifted continental margin thereby contributing to subsidence of flanking sedimentary basins. The western limit of the ophiolite is defined onshore by the Semail thrust while the eastern limit extends several km offshore, where it is defined seismically by a ~40–45°, east-dipping, normal fault. The fault is interpreted as the southwestern margin of an incipient suture zone that separates the Arabian plate from in situ Gulf of Oman oceanic crust and mantle presently subducting northwards beneath the Eurasian plate along the Makran trench., The Semail ophiolite provides evidence for geological processes that form oceanic crust, however, its deep structure remains debated. Here, the authors use geophysical imaging to determine that the ophiolite is bound by a thrust fault in the west, and a normal fault in the east, bounding a rapidly subsiding basin, implying the ophiolite may not be rooted in the Gulf of Oman crust.

The timing of the closure of the Paleo-Tethys Ocean in West China remains debated. To investigate this problem, we examined the geochemical characteristics, zircon U–Pb chronology, and Hf isotopes of monzogranites and quartz diorites from the Duocai granite in the Zhiduo region, at the intersection of the Jinshajiang and Ganze–Litangsutures. The monzogranites have the chemical characteristics of calc-alkaline I-type granites and yield a weighted mean zircon U–Pb age of 234.6 ± 0.9 Ma [mean square weighted deviation (MSWD) = 0.36]. Initial eHf(t) values are high and positive, ranging from + 7.9 to + 13.6 with a mean of + 10.7, corresponding to two-stage Hf isotope model ages (TDM2) of 762–395 Ma. Geochemical and isotopic data indicate that the source magma of the monzogranites formed from mantle-derived magmas mixed with the overlying crustal materials. The quartz diorites, which also have compositional characteristics of calc-alkaline I-type granites, yield a weighted mean zircon U–Pb age of 209.1 ± 0.7 Ma (MSWD = 0.29). Initial eHf(t) values range from − 2.5 to + 0.6 with a mean of − 1.5, with the corresponding TDM2 of 1402–1210 Ma. Geochemical and isotopic data indicate that the primary magma of the quartz diorites was derived mainly from partial melting of the mafic lower crust and small amount mantle-derived magma involved. Combining these results with regional data, the studied granites are inferred to have formed as a result of continuous subduction of plates underlying the Western Jinshajiang Ocean–Ganze–Litang Ocean from 234 to 209 Ma, and were unrelated to subduction of the South Jinshajiang oceanic plate. We suggest that the Western Jinshajiang Ocean–Ganze–Litang Ocean closed by the end of Late Triassic.

The Mesozoic magnetite-(apatite) deposits of the Coastal Cordillera of Chile are interpreted as the product of the crystallization of oxidized iron-rich melts and subsequent hydrothermal alteration produced by related magmatic-hydrothermal systems. These deposits form a regional-scale mineral system controlled by the Atacama Fault System and where the mineralization spans more than 10 km in vertical extent. Individual sub-vertical bodies of massive magnetite coexist with and evolve vertically into pegmatite-, breccia-, and vein-like apatite-actinolite-magnetite/ilmenite rock. The mineralization is always hosted by a hydrothermal aureole of alkali-calcic-iron alteration that includes stockwork-like to disseminated mineralization. The deposits cluster in two groups. Those located in the northern part are mostly vein-like, and are hosted by Jurassic diorite. They have 87Sr/86Sri and eNdi values of 0.7042–0.7062 and + 5.1 to + 7.2, respectively. The southern group includes shallowly emplaced ore lenses in broadly coeval (sub-)volcanic intermediate rocks. They show similar 87Sr/86Sri signatures (0.7033–0.7065, with one value up to 0.7097) and more variable eNdi values (+ 3.9 to + 8.6). As a whole, the Sr-Nd data do not seem to be influenced by the type of crust intruded, but rather, likely track the mixing between a MORB-like reservoir and another source with elevated 87Sr/86Sri (≥ 0.706). The genetic model proposed involves the dehydration of variably altered subducted oceanic crust, the interaction of fluids released from the mantle wedge, the separation of iron-rich melts, and their ascent along transcrustal faults. The broadly coeval intermediate host rocks show a lesser contribution of subducted crust, something that perhaps excludes a genetic relationship between these rocks and the magnetite-(apatite) mineralization.

Formation of the first continents belongs to fundamental questions regarding the evolution of the Earth. Though the growth of early crust is often debated, role of the mantle lithosphere that represents the biggest volume of continents is often overlooked, particularly in geologic interpretations of tectonic processes. This is mainly due to difficulties in its imaging and uncertainty in its rheology. Investigation of seismic anisotropy from propagation of teleseismic P and S waves in three dimensions (3D), i.e., with no limitation imposed on the symmetry axis orientation into the horizontal or vertical directions, provides a unique constraint on tectonic fabrics and character of past and present-day deformations of the continental lithosphere. In this paper, we collect independent findings from seismology, petrology and geochemistry to support our 3D anisotropic model of mantle lithosphere with tilted symmetry axes, derived from data of passive seismic experiments organised in tectonically different domains of Archean, Proterozoic and Phanerozoic provinces of Europe.Olivine preferred orientation, formed by mantle convection in the oceanic mantle lithosphere due to its spreading on both sides of the mid-ocean ridges, is a prerequisite for the tilted anisotropies that we model in the continents. We have explained the systematically oriented dipping fabrics in the continental mantle lithosphere by successive subductions of ancient oceanic plates and their accretions enlarging primordial continent cores. Consequent continental break-ups and assemblages of wandering micro-plates preserve “frozen” anisotropic fabrics and create patchwork structures of the present-day continents. Supporting arguments for such model arise from petrological and geochemical studies indicating that continental mantle peridotites formed in oceanic environments and became “continental” after significant thickening or underthrusting. Combining seismological, petrologic and geochemical findings can help to bridge the gulf between the different viewpoints and evoke further discussions on possible growth mechanisms of the continental lithosphere. Keywords: Continental growth, Mantle lithosphere, Seismic anisotropy, Subduction cycles, Dipping olivine fabrics, Relict oceanic subductions

Mantle evolution is governed by continuous depletion by partial melting and replenishment by recycling oceanic and continental crust. Several important unknowns remain, however, such as the extent of compositional variability of the residual depleted mantle, the timescale, mass flux and composition of recycled oceanic and continental crust. Here, we investigate the Ce-Nd-Hf isotope systematics in a globally representative spectrum of mid ocean ridge and ocean island basalts. Using a Monte Carlo approach for reproducing the observed Ce-Nd-Hf isotope variation shows that the type and age of depleted mantle and recycled crust have the dominant influence on the slope, scatter, and extent of the modeled Ce-Nd-Hf isotope array. The model results suggest a relatively young (

The study of the petrogenesis of some magmatic rocks with special geochemical attributes provides effective information for us to explore the deep geodynamic background of their formation. A series of granitic porphyry dykes have been found in the mélange zone of the Asa region in southern Tibet, whose genesis may be closely related to the evolution of the Meso-Tethyan Ocean. Regional geodynamic evolution is investigated by whole-rock geochemical analysis, zircon U–Pb dating and Lu–Hf isotopic analysis of two porphyritic granites. The Asa porphyritic granites have high SiO2 (74.29–78.65 wt %) and alkalis (Na2O + K2O = 6.51–9.35 wt %) contents, and low Al2O3 (11.60–14.51 wt %), CaO (0.04–0.19 wt MgO (0.01–0.10 wt %) contents. They are enriched in Zr, Nb, Ce, Y and Hf and depleted in Ti, Ba, Sr and P, consistent with A-type granites. The samples are relatively rich in LREEs, with LREE/HREE ratios of 1.73–3.04. They display negative Eu anomalies (Eu/Eu* = 0.24–0.28) and obvious Ce anomalies in some samples. Zircon U–Pb analyses show that the porphyritic granites formed in late Early Cretaceous time, 107.4 to 105.5 Ma. Zircon εHf(t) values are in the range of 6.9 to 12.0. These data indicate that the porphyritic granites were sourced from interaction between mantle-derived and juvenile lower crust-derived melts, with the addition of oceanic sediment-derived melts. This occurred when the subducting Bangong–Nujiang oceanic crust split to create a slab window. Rising asthenosphere triggered re-melting of lower crust basalts, resulting in the formation of the late Early Cretaceous A-type granites around Asa.