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The Miocene Galatian Volcanic Province (GVP) is one of the largest volcanic provinces in central-western Anatolia, with an extent of ~ 8,900 km2. The volcanic activity is extended from 22.5 to 7.5 Ma. The volcanic compositions straddle the alkaline-subalkaline fields, from basic to acid compositions and mostly transitional to sodic affinity. Major oxides show good correlation with SiO2 indicating prolonged effects of fractional crystallization. Primitive mantle-normalized multi-element patterns indicate overall similarities among the different samples of the three geographic sectors, sharing strong negative anomalies in Nb–Ta–Ti, strong positive peaks at Cs and K, coupled with a common, albeit not always present, positive anomaly at Pb. Mineral-melt geothermobarometric estimates indicates ~1070–1235°C and ~7–19 kbar for melting conditions of basaltic compositions and ~1000–1150°C and ~3–12 kbar for andesitic-dacitic rocks. The absence of correlation between radiogenic isotopes and SiO2 and MgO is here interpreted as consequence of assimilation-fractional-crystalization processes involving lower continental crust as contaminant. The GVP parental magmas are generated from ~2% to 10% partial melting of a lherzolitic mantle with high spinel/garnet ratio based on intra-REE fractionation constraints. The subduction-related metasomatism inferred for the GVP mantle sources based on their chemistry is interpreted to be linked to the northward subduction of the northern branch of the Neo-Tethys slab. Successive slab retreat resulted in extension for the critical stress distribution through the Cyprus slab, favouring magma propagation for the GVP volcanic region. The eventual break-off of the slab after the continent-continent collision of Arabian and Eurasian plates could have caused a toroidal mantle flow, favouring the widely distributed 15–16 Ma alkaline magmatism in the eastern GVP, associated with passive hot asthenospheric upwelling imaged by teleseismic P-wave tomography. [ABSTRACT FROM AUTHOR]

[Display omitted] • Early Mesozoic dyke swarms intruding phlogopite peridotite in Finero were studied. • The dykes record a transition from orogenic-like to anorogenic magmatism. • Heterogeneity of the mantle sources of Early Mesozoic magmatism in Southern Alps. The Ivrea-Verbano Zone (IVZ) in the westernmost sector of the Southern Alps is an iconic upper mantle to lower continental crust sequence of the Adriatic Plate and provides a geological window into the tectono-magmatic events that occurred at the Gondwana–Laurussia boundary from Late Paleozoic to Early Mesozoic. In this work, we document new geochemical and Nd-Sr-Hf-Pb isotopic data for Early Mesozoic alkali-rich dyke swarms which intruded the Finero Phlogopite Peridotite (northern IVZ) to provide geological constraints on the nature, origin and evolution of Early Mesozoic magmatism in the Southern Alps. The studied dykes are amphibole-phlogopite-bearing and show geochemical features varying between two end-member groups. A dyke group is characterized by HFSE-poor, Al-rich amphibole (Al 2 O 3 up to 16 wt%) with high LILE and LREE contents, high radiogenic 87Sr/86Sr (i) (0.704732 to 0.704934) and low radiogenic Nd isotopes (εNd (i) from –0.1 to –0.7), which support the occurrence of significant amounts of recycled continental crust components in the parental mantle melts and impart an overall "orogenic-like" affinity. This dyke group was largely derived from metasomatized lithospheric mantle sources. The second group is HFSE-rich with Al-poorer amphibole enriched in LILE and LREE, low radiogenic 87Sr/86Sr (i) (0.703761–0.704103) and higher radiogenic Nd isotopes (εNd (i) from +3.4 to +5.4) pointing to an "anorogenic" alkaline affinity and asthenospheric to deep lithospheric mantle sources. Some dykes show both orogenic and anorogenic affinities, providing evidence that the orogenic-like magmatism in the IVZ predates the alkaline anorogenic magmatism. The Finero dyke swarms therefore record a geochemical change of the Early Mesozoic magmatism of the Southern Alps from orogenic-like magmatism, typical of post-collisional settings, to anorogenic alkaline magmatism, common in intraplate to extensional settings, and places a temporal correlation of Early Mesozoic magmatism in the IVZ to those in the eastern and central sectors of the Southern Alps. [ABSTRACT FROM AUTHOR]

The ~ 515 Ma Kathalguri Pluton of Mikir Massif, Northeast India, presents the first evidence of Pan-African bimodal magmatism from the easternmost part of the Indian shield. It is characterized by a number of outcrop-scale features like mafic flows and mafic magmatic enclaves (MME) within granitic host rocks, which suggests that mafic magma intruded into the felsic magma chamber during its evolution. Hybrid intermediate rocks are also encountered in the granitoid body indicating proper mixing between the mafic and felsic magmas. The mafic flows and MME are observed at the lower portion of the pluton, while the hybrid intermediate rocks are distributed in the upper part. Such field observations suggest that the felsic magma chamber was vertically zoned when mafic magma intruded into it. Textural features associated with magma mixing like resorbed crystals, boxy-cellular morphology, and oscillatory zoning in plagioclase, including orthoclase–microcline transformation are preserved in the hybrid rocks. Geochemical signatures confirm that magma mixing has played an important role in the evolution of the Kathalguri Pluton. Our results also suggest that differential mobility of elements was responsible for variable rates of homogenization in the magma mixed system. From fractal analysis, it has been inferred that MME from the Kathalguri Pluton underwent different degrees of interaction with the felsic magma. From lighter to darker MME (based on color index), the complexity of the interface morphologies increases with increasing Dbox (fractal dimension) and log VR (viscosity ratio) values, which indicates decreasing rate of homogenization between the mafic and felsic end-members. Moreover, viscosity calculations also suggest that the more viscous smaller MME are more evolved than the relatively less viscous larger MME. The mafic rocks preserved in the host pluton show higher LILE/HFSE and LREE/HREE ratios, and dominantly plot in the field of 'within-plate basalts' in various tectonic discrimination diagrams. From the results presented in this work, we infer that the Kathalguri Pluton developed in a post-collisional, within plate extension setting through melting of mid- to lower-crustal rocks by mantle-derived mafic magmas. The mafic magmas during the Pan-African episode were generated in a continental rift setting, which was probably activated by asthenospheric upwelling. [ABSTRACT FROM AUTHOR]

Plate tectonics was originally established as a kinematic theory of global tectonics, in which the Earth's rigid outer layer, the lithosphere, consists of different size plates that move relative to each other along divergent, convergent or transform boundaries overlying the ductile asthenosphere. It comprises three elements: rigid lithosphere plates, ductile asthenosphere, and coupled movement systems. It operates through the interlinked processes of continental drift, seafloor spreading and lithospheric subduction, resulting in the generation, modification and demise of lithospheres throughout geological time. The system of lithospheric plates in horizontal and vertical movements forms the spatiotemporal linkages of matter and energy between the surface and interior of Earth, advancing the kinematic theory with a dynamic explanation. While top-down tectonics through lithospheric subduction plays a key role in the operation of plate tectonics, it is balanced for the conservation of both mass and momentum on the spherical Earth by bottom-up tectonics through asthenospheric upwelling to yield seafloor spreading after continental breakup. The gravity-driven subduction of cool lithosphere proceeds through convergence between two plates on one side, and rollback of the subducting slab makes the vacancy for upwelling of the hotter asthenosphere to form active rifting in backarc sites. Plate convergence is coupled with plate divergence between two plates along mid-ocean ridges on the other side, inducing passive rifting for seafloor spreading as a remote effect. Thus, plate tectonics is recognizable in rock records produced by tectonic processes along divergent and convergent plate margins. Although the asthenospheric upwelling along fossil suture zones may result in continental breakup, seafloor spreading is only induced by gravitational pull of the subducting oceanic slab on the remote side. Therefore, the onset and operation of plate tectonics are associated with a series of plate divergent-convergent coupling systems, and they are critically dependent on whether both construction and destruction of plates would have achieved and maintained the conservation of both mass and momentum on the spherical Earth. Plate margins experience different types of deformation, metamorphism and magmatism during their divergence, convergence or strike-slip, leaving various geological records in the interior of continental plates. After plate convergence, the thickened lithosphere along fossil suture zones in intracontinental regions may be thinned by foundering. This causes the asthenospheric upwelling to reactivate the thinned lithosphere, resulting in superimposition and modification of the geological record at previous plate margins. The operation of plate tectonics, likely since the Eoarchean, has led to heat loss at plate margins and secular cooling of the mantle, resulting in the decrease of geothermal gradients and the increase of rheological strength at convergent plate margins. Modern plate tectonics is characterized by the predominance of rigid plate margins for cold subduction, and it has prevailed through the Phanerozoic. In contrast, ancient plate tectonics, that prevailed in the Archean and Proterozoic, is dominated by relatively ductile plate margins for collisional thickening at forearc depths and then warm subduction to subarc depths. In either period, the plate divergence after lithospheric breakup must be coupled with the plate convergence in both time and space, otherwise it is impossible for the operation of plate tectonics. In this context, the creation and maintenance of plate divergent-convergent coupling systems are responsible for the onset and operation of plate tectonics, respectively. Although a global network of mobile belts is common between major plates on modern Earth, it is difficult to find its geological record on early Earth if microplates would prevail at that time. In either case, it is important to identify different types of the geological record on Earth in order to discriminate between the different styles of plate tectonics in different periods of geological history. [ABSTRACT FROM AUTHOR]

The tectonic evolution and deformation process of the southeastern Tibetan Plateau has been one of the focuses of the geoscience community in recent decades. However, the crustal and mantle deformation mechanism in the southeastern Tibetan Plateau is still under debate. Seismic anisotropy inferred from surface wave tomography could provide critical insights into the deformation mechanism of the Earth’s tectonosphere. Here, we constructed a 3-D azimuthal anisotropy velocity model with Rayleigh wave phase velocity dispersion data from 132 permanent stations to analyze the deformation modes in the southeastern Tibetan Plateau. In the upper crust, the azimuthal anisotropy near the main strike-slip faults exhibits strong magnitude with fast axis subparallel to the fault strike, which is consistent with the rigid block extrusion pattern. In the mid-lower crust, two low-velocity anomalies appeared beneath the Sonpan-Ganzi Terrane and Xiaojiang Fault zone with strong azimuthal anisotropy, which may indicate ductile deformation of the weak mid-lower crust. However, the two low-velocity anomalies are separated by a high-velocity barrier with weak azimuthal anisotropy in the inner zone of the Emeishan large igneous province. In the upper mantle, the anisotropy pattern is relatively simple in the rigid Yangtze Craton and mainly represents fossil anisotropy. In the southern part, the lithosphere thinned beneath the Indochina and Cathaysia Blocks, and the anisotropy is plausibly caused by the upwelling and lateral flows of upwelling hot asthenospheric materials.

The North China Craton (NCC) is renowned for cratonic reactivation and dramatic destruction. Detailed lithospheric structure is crucial for understanding the geological evolution and seismogenesis of the NCC. We imaged the lithospheric structure of the central and eastern NCC with ambient noise and teleseismic surface wave tomography. Our new model unveils remarkable N‐S variation in the lithospheric structure. An E‐W elongated lithospheric weak zone, coinciding with contemporary left‐lateral shearing revealed by geodetic observation, occupies the northern central and eastern NCC. Our model suggests that radical destruction of the eastern NCC in the late Mesozoic facilitated the late‐Cenozoic shearing induced by the continuous India‐Eurasia convergence. The late‐Cenozoic shearing progressively altered the pre‐thinned lithosphere accompanied by asthenospheric upwelling and strong seismic activity. Our results suggest a transition from the Pacific subduction dominating tectonic regime to the India‐Asia collision dominating lithospheric deformation framework in the Cenozoic and ongoing lithospheric alteration of the NCC. Plain Language Summary: The NCC is one of the oldest cratons in China and a typical example of cratonic reactivation. The NCC experienced significant destruction since the Mesozoic with conspicuous internal deformation and seismic activity within the central and eastern NCC. The fine lithospheric structure is essential for our understanding of the geological evolution of the ancient craton. We imaged the lithospheric structure of the central and eastern NCC with ambient noise and teleseismic surface wave tomography. Our results reveal complicated lithospheric structure due to the superposition of multi‐stage lithospheric modification. In addition to the predominant Mesozoic‐Cenozoic cratonic destruction, Cenozoic shearing of the lithosphere, induced by the far‐field effect of the India‐Asia collision, also has a profound influence on the present lithospheric structure and regional seismicity. These new results benefit our understanding of the evolution of the NCC in the Cenozoic and the genetic mechanism of regional seismic hazards. Key Points: High‐resolution lithospheric structure of the central and eastern North China Craton (NCC)Significant N‐S variations in lithospheric strength and seismicity within the NCCTransition of the deformation style and ongoing lithospheric destruction of the NCC [ABSTRACT FROM AUTHOR]

A 3-D lithospheric resistivity model is obtained with 699 MT stations. The upper-mantle low-resistivity anomalies beneath the eastern SCB show a spatial distribution that is gradually shallowing from south to north, probably indicating that the asthenospheric materials are upwelling from south to north, corresponding with the changing progressively of magma and metallogenic activities. [Display omitted] • The 3-D lithospheric resistivity structure of the lithosphere in South China is obtained by inverting 699 MT stations. • The resistivity structure and disposition of four metallogenic belts in the South China Block are outlined. • The upper-mantle low-resistivity anomalies show a spatial distribution of gradual shallowing, indicating the asthenospheric upwelling from south to north. The South China Block (SCB) is located at the junction of the Pacific, Eurasian, and Indo-Australian plates. Their interaction led to large-scale multi-stage mineralization in the SCB during the Mesozoic. Several regional ore-concentration areas, such as the Middle-Lower Yangtze Metallogenic Belt (MLYMB), the Qinzhou-Hangzhou Metallogenic Belt (QHMB), the Wuyishan Metallogenic Belt (WYMB), and the Nanling Metallogenic Belt (NLMB) were formed during this process. However, the dominant mineral types of these metallogenic belts are different. To study the deep mechanisms of the different metallogenic types developed in the same tectonic background at almost the same period, the magnetotelluric sounding (MT) data from 691 sites located mainly within the eastern SCB are employed to obtain a regional lithospheric 3-D resistivity model. It could be concluded from the resistivity model that deep low-resistivity anomalies and mantle upwelling channels mainly controlled almost all the Mesozoic deposits. Large-scale low-resistivity bodies extend from the crust to the upper mantle beneath the MLYMB and the QHMB, interpreted as channels of mantle-derived magma and melts/fluids containing metallogenic elements upwelling. However, the MLYMB and the QHMB have different upper crustal ore-conducting and ore-controlling structures, thus forming porphyry copper and copper-bearing tungsten deposits. Small-scale low-resistivity anomalies are invading the high-resistivity crust in the NLMB. And remelting the upper crust, mixing, and intrusion of crust-source magma, enriching tungsten-tin elements near the NLMB. Larger-scale low-resistivity anomalies exist deep in the WYMB, remelting the lower crust and resulting in the formation of porphyry copper in it. Significantly, the upper-mantle low-resistivity anomalies beneath the eastern SCB show a spatial distribution that is gradually shallowing from south to north, probably indicating that the asthenospheric materials are upwelling from south to north, corresponding with the changing progressively of magma and metallogenic activities. We propose that the lithospheric delamination and asthenospheric upwelling caused by the far-field effects of the Paleo-Pacific Plate subduction are the sources of solid magmatic activities and related metallogeny. [ABSTRACT FROM AUTHOR]

Northeast China was formed during the progressive closure of the Paleo‐Asian Ocean before the Early Mesozoic, and was then later affected by the westward subduction of the Paleo‐Pacific Plate and the west‐to‐east closure of the Mongolia‐Okhotsk Ocean. Huge volumes of granites were emplaced across this region during the Mesozoic, and widespread Cenozoic volcanism is still active in the Quaternary. To study past and present magmatic processes in this region, two parallel high‐resolution magnetotelluric profiles were collected to produce lithospheric resistivity models. Northwest of the Greater Xing'an Range, the lithosphere has a relatively low resistivity. The resistivity of the upper mantle low‐resistivity anomaly (C2) is consistent with the presence of a few percent of basaltic melt. This feature connects with a shallower low‐resistivity zone (C1) that extends into the crust. Its properties are consistent with 5–10% shoshonitic melt. The lithosphere in the southeast part is highly resistive. The upper mantle high‐resistivity zones (R1 and R2) are interpreted as regions of thickened lithosphere that may be undergoing delamination. The lower crust exhibits localized low‐resistivity zones (C3, C4, and C5), and they are interpreted as partial melt which may have originated in the ongoing upwelling. The overall pattern of magmatism appears to be driven by asthenosphere upwelling which might have been triggered by the Paleo‐Pacific Plate retreat, but the upper mantle low‐resistivity zone originates and rises from the northwest. Down‐welling in the southeast might be the combined effect of the subduction and retreat of the Paleo‐Pacific Plate and the instability of thickened lithosphere. Plain Language Summary: Northeast China is located in the southeastern Central Asian Orogenic Belt. The crust and mantle of this area experienced the multistage closure of the Paleo‐Asian Ocean in the Paleozoic, and the subduction and retreat of the Paleo‐Pacific Plate in the Mesozoic. These tectonic events have led to extensive magmatism and volcanism that continues today. Magnetotellurics (MT) is a geophysical method that uses natural radio signals to image the electrical resistivity structure of the Earth. Two high‐resolution magnetotelluric profiles were collected in Northeast China to study the deep structure of the lithosphere. The data were inverted to produce resistivity models, that were interpreted to map the distribution of partially molten rock in the crust and upper mantle. These models show that surface volcanism can be related to regions of upwelling in the asthenosphere. Key Points: Low‐resistivity bodies (C2 and C1) in the northwest part are interpreted as partial melt, and the melt supply is moving northeastLow‐resistivity bodies beneath the Greater Xing'an Range (C3 and C4) may be caused by partial melt associated with delaminationThe tectonics of this region is characterized by zones of upwelling causing volcanism, and down‐welling caused by thickened lithosphere [ABSTRACT FROM AUTHOR]

Abstract What drives the formation of basaltic melts beneath intraplate volcanoes not associated with extensive thermal anomalies or lithospheric extension? Detailed constraints on the melting conditions and source region are imperative to resolve this question. Here we model the geochemistry of alkali basalts and mantle nodules brought up by young (12–0.1 Ma) intraplate volcanoes distributed across the Carpathian‐Pannonian region and combine the results with geophysical observations. Rare earth element inversion and forward calculation of elemental concentrations show that the basalts require the mantle to have undergone less than 1% melting in the garnet‐spinel transition zone, at depths of about 63–72 km. The calculated melt distributions correspond to a mantle potential temperature of ∼1257°C, equivalent to a real temperature of 1290°C at 65 km beneath the Pannonian Basin. The composition, modal mineralogy, and clinopyroxene geochemistry of some of the entrained mantle nodules closely resemble the basalt source, though the latter equilibrated at greater depths. The gravity anomalies and topography of the Basin reveal no large‐scale features that can account for the post‐extensional volcanism. Instead, the lithospheric thickness and geotherm show that melting occurs because the base of the lithosphere, at ∼50‐km depth, is close to or at the solidus temperature over a large part of the Basin. Hence, only a small amount of upwelling is required to produce minor volumes (up to a few cubic kilometers) of melt. We conclude that the Pannonian volcanism originates from upwelling in the asthenosphere just below thinned lithosphere, which is likely to be driven by thermal buoyancy.

Post-collisional alkaline mafic rocks record significant information regarding crust–mantle interaction in the subduction channel and provide an excellent opportunity to investigate the mechanism of transition from pre-collisional to post-collisional magmas. In this contribution, we present whole-rock major and trace element and radiogenic isotope data obtained from alkaline mafic dykes in the Guzhang-Anjiang areas in the southwestern Jiangnan orogenic belt (JOB), South China. LA–ICP–MS U–Pb zircon dating of four representative samples yields a consistent age of ~780 Ma. These mafic dykes are characterized by large variations in SiO2 (42.4–54.1 wt.%) and MgO (2.2–15.0 wt.%), alkalis (K2O+Na2O = 1.96–11.19 wt.%), Nb (17.55–49.62 ppm) and Ta (1.15–3.26 ppm). They exhibit ocean island basalt (OIB)-like geochemical features, which are characterized by enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) relative to heavy rare earth elements (HREEs) and by positive zircon εHf(t) (+1.86 to +5.14) and whole-rock εNd(t) (+0.31 to +1.80) values. All these geochemical characteristics suggest these dykes were derived from a small degree (2–5%) of partial melting of an asthenospheric mantle source that had previously been modified by earlier subduction processes. Based on the previous reports and present study, we infer that three periods of Neoproterozoic basaltic magmatism occurred in the western Jiangnan Orogen, and constrain the time of the tectonic regime transformation at ~830, and at ~780 Ma. The source of these basaltic rocks varied in composition spatially and evolved with time. The ~780 Ma OIB-like rocks in this study serve as an important indicator of large-scale upwelling of the asthenospheric mantle and the transition from orogenic to post-orogenic tectonic setting. [ABSTRACT FROM AUTHOR]

What drives the formation of basaltic melts beneath intraplate volcanoes not associated with extensive thermal anomalies or lithospheric extension? Detailed constraints on the melting conditions and source region are imperative to resolve this question. Here we model the geochemistry of alkali basalts and mantle nodules brought up by young (12–0.1 Ma) intraplate volcanoes distributed across the Carpathian‐Pannonian region and combine the results with geophysical observations. Rare earth element inversion and forward calculation of elemental concentrations show that the basalts require the mantle to have undergone less than 1% melting in the garnet‐spinel transition zone, at depths of about 63–72 km. The calculated melt distributions correspond to a mantle potential temperature of ∼1257°C, equivalent to a real temperature of 1290°C at 65 km beneath the Pannonian Basin. The composition, modal mineralogy, and clinopyroxene geochemistry of some of the entrained mantle nodules closely resemble the basalt source, though the latter equilibrated at greater depths. The gravity anomalies and topography of the Basin reveal no large‐scale features that can account for the post‐extensional volcanism. Instead, the lithospheric thickness and geotherm show that melting occurs because the base of the lithosphere, at ∼50‐km depth, is close to or at the solidus temperature over a large part of the Basin. Hence, only a small amount of upwelling is required to produce minor volumes (up to a few cubic kilometers) of melt. We conclude that the Pannonian volcanism originates from upwelling in the asthenosphere just below thinned lithosphere, which is likely to be driven by thermal buoyancy. Plain Language Summary: On Earth's continents, there are surprising instances of basaltic volcanism that are not related to plate stretching or hotspots. To understand just such activity in the Carpathian‐Pannonian region, in eastern central Europe, during the last 12 million years, we studied the chemical composition of the erupted lavas. Using a computer code that takes as input the concentrations of elements in basaltic rocks, we calculated the fraction of melt and the depth at which it formed within Earth's mantle. We find that the mantle has undergone less than 1% melting in the garnet‐spinel transition zone—a region at about 63‐ to 72‐km depth where these minerals both occur. Using a model of mantle melting, we find that the temperature of the mantle at 65 km below the Pannonian Basin is about 1290°C. The plate's thickness and the temperature variations with depth tell us that the magma source region is at a temperature close to that at which melting begins (the solidus). This means that if mantle material is moved upwards just slightly, it will produce small volumes of melt that can rise to the surface. The most likely cause of such upward movement is convection in the mantle below the rigid plate. Key Points: Carpathian‐Pannonian intraplate basalts are formed by less than 1% melting in the garnet‐spinel transition zone, at depths of about 63–72 kmThe calculated melt distributions correspond to a mantle potential temperature of 1257°C beneath the Pannonian BasinMelting occurs because the base of the lithosphere (at about 1300°C) is close to the solidus temperature over a large part of the Basin [ABSTRACT FROM AUTHOR]

Southeastern Tibet, which has complex topography and strong tectonic activity, is an important area for studying the subsurface deformation of the Tibetan Plateau. Through the two-station method on 10-year teleseismic Rayleigh wave data from 132 permanent stations in the southeastern Tibetan Plateau, which incorporates ambient noise data, we obtain the interstation phase velocity dispersion data in the period range of 5–150 s. Then, we invert for the shear wave velocity of the crust and upper mantle through the direct 3-D inversion method. We find two low-velocity belts in the mid-lower crust. One belt is mainly in the SongPan-GangZi block and northwestern part of the Chuan-Dian diamond block, whereas the other belt is mainly in the Xiaojiang fault zone and its eastern part, the Yunnan-Guizhou Plateau. The low-velocity belt in the Xiaojiang fault zone is likely caused by plastic deformation or partial melting of felsic rocks due to crustal thickening. Moreover, the significant positive radial anisotropy (VSH>VSV) around the Xiaojiang fault zone further enhances the amplitude of low velocity anomaly in our VSV model. This crustal low-velocity zone also extends southward across the Red River fault and farther to northern Vietnam, which may be closely related to heat sources in the upper mantle. The two low-velocity belts are separated by a high-velocity zone near the Anninghe-Zemuhe fault system, which is exactly in the inner and intermediate zones of the Emeishan large igneous province (ELIP). We find an obvious high-velocity body situated in the crust of the inner zone of the ELIP, which may represent maficultramafic material that remained in the crust when the ELIP formed. In the upper mantle, there is a large-scale low-velocity anomaly in the Indochina and South China blocks south of the Red River fault. The low-velocity anomaly gradually extends northward along the Xiaojiang fault zone into the Yangtze Craton as depth increases. Through our velocity model, we think that southeastern Tibet is undergoing three different tectonic modes at the same time: (1) the upper crust is rigid, and as a result, the tectonic mode is mainly rigid block extrusion controlled by large strike-slip faults; (2) the viscoplastic materials in the middle-lower crust, separated by rigid materials related to the ELIP, migrate plastically southward under the control of the regional stress field and fault systems; and (3) the upper mantle south of the Red River fault is mainly controlled by large-scale asthenospheric upwelling and may be closely related to lithospheric delamination and the eastward subduction and retreat of the Indian plate beneath Burma. [ABSTRACT FROM AUTHOR]

[Display omitted] • Electrical resistivity model across the Late Cenozoic Abaga-Dalinor volcanic field (ADVF) reveals some striking features related to intraplate volcanism. • A remarkably high-conductivity zone lying in the middle-lower crust indicates the presence of saline fluids and a northwest-southeast trending fault. • A moderately highly-conductivity feature detected in the uppermost mantle indicates the presence of a low degree of partial melting. • The pre-existing tectonic weak zones such as ancient sutures, and major faults control the Late Cenozoic volcanism in the ADVF. • Late Cenozoic intraplate volcanism in the ADVF may be attributed to the decompression melting in a local asthenospheric upwelling. The Abaga-Dalinor volcanic field (ADVF) in Inner Mongolia, China, located to the west of the North-South Gravity Lineament (NSGL), is the largest and less known area of Late Cenozoic intraplate volcanism in Northeast China. Knowledge of the subsurface structure beneath the ADVF will improve our understanding of its evolution process and formation mechanisms. New broadband magnetotelluric (MT) data were collected along a profile of about 300 km that crosses the major basaltic areas exposed at the surface in the ADVF and a series of east-north-east (ENE) trending faults. The electrical resistivity structure of the crust and uppermost mantle across the ADVF was generated by 3-D inversion of full impedance tensor and tipper data. This model reveals a thick high-resistivity layer (>1000 Ωm) in the upper crust that may represent the Precambrian basement and Late Paleozoic-Mesozoic granitic plutons. In contrast, high conductivity anomalies dominate the middle-lower crust. The northward-dipping high-conductivity feature at depths of 20–40 km below sea level under the Chagan Obo fault is suggested to be the involvement of the Early Paleozoic subducted oceanic crust. A remarkably high-conductivity zone (1–20 Ωm) exists in the middle-lower crust beneath the Abaga and Dalinor volcanic areas. It is explained to be a saline fluid zone of ∼0.45%–0.48% that exsolved from the partial melts as magma ascent rapidly. The impermeable upper crust traps these fluids in the middle-lower crust. A moderately high-conductivity anomaly in the uppermost mantle below near the Solonker Suture Zone (SSZ) is attributed to a minimum partial melt of ∼3–4% associated with the localized asthenospheric upwelling, which feeds the Abaga and Dalinor volcanos as a common magma source. Integrating our resistivity model with previous geophysical and geochemical studies, we suggest that intraplate volcanism in the ADVF is mainly controlled by the pre-existing tectonic weak zones. The lithosphere within the SSZ is susceptible to thermal perturbation from the upwelling mantle, providing the preconditions for melt generation. A set of ENE trending faults and an inferred NW-SE trending fault jointly dominated the vertical transport of magma and lateral extension along the faults, which may have influenced the spatial distribution of basalts and volcanic cones at the surface in the ADVF. In addition, we tend to suggest that Late Cenozoic intraplate volcanism in the ADVF could be the result of decompressing melting within the localized small-scale asthenospheric upwelling induced by the edge-driven convection near the NSGL, which is indirectly related to the westward subduction and retreat of the (Paleo-)Pacific plate during the Late Mesozoic to Cenozoic. [ABSTRACT FROM AUTHOR]

Intracontinental orogens are major sites to better understand tectonic regime of plate interior. However, deep response to those processes remains poorly constrained due to the rare exposure of magmatic rocks. Middle-Palaeozoic magmatic rocks from the eastern Yangtze Block (EYB) and Cathaysia Block (CB) of the southeastern South China Block (SCB) have markedly unimodal age distribution with peaks at ~424 and ~438 Ma, respectively. The zircon saturation temperatures (T–Zr) range from 699 to 920°C, and increase in granitic rocks of younger age in the CB; however, most of the granitic T–Zr values in EYB are low and relatively uniform (643–855°C, most of them lower than 800°C). In CB, the Nd-Hf isotopic ratios of the granitic rocks (εNd(t) = – 13.2 to + 1.0; magmatic zircon εHf(t) = – 36 to + 12) are variable, nonetheless they clearly show the presence of a juvenile component. On the contrary, Nd isotopic ratios (εNd(t) = – 7.8 to + 1.6) of mafic rocks in EYB abruptly increase in rocks of younger age. Within the long-lived intracontinental orogenic processes of EYB and CB, consistent age peaks between metamorphism and magmatism and the abrupt increase in contributions of juvenile materials suggest that the thickened lithospheric roots were delaminated into asthenospheric mantle as many independent parts below the metamorphic domes in CB. The data suggest that the deep response to the intracontinental orogenic processes was strong non-uniform between EYB and CB in terms of magmatic period and contribution of mantle derived material to granitic magmas (related to the crustal reworking). We deduced that the combined influence of far-field stress transmission (derived from the convergence of Australia and the SCB), local delamination event in CB and different degree of reworking of the crustal sources, is the main reason to the different responses between EYB and CB. [ABSTRACT FROM AUTHOR]

We present new, high‐resolution constraints on crustal and uppermost mantle structure of the southeastern, eastern, and northeastern Tibetan plateau and adjacent regions by combining extensive data from three regional‐scale dense seismic arrays. Our results show significant differences in the crustal and uppermost mantle structure within the Songpan‐Ganzi terrane. The southern part has a thick crust and high Poisson's ratio, while the crustal thickness becomes less and Poisson's ratio is nearly normal in the northeast. Prominent low‐velocity anomalies appear beneath both the southern and northeastern Songpan‐Ganzi terrane at depths of 20–40 km, but they are not interconnected. Seismic velocities of the uppermost mantle are slow in the northeastern Songpan‐Ganzi terrane relative to the south. We further find that the crust of the central Qilian orogen is thickened but with remarkable low Poisson's ratio. Low velocities are visible in both the mid‐lower crust and uppermost mantle, similar to the northeastern Songpan‐Ganzi terrane. For comparison, the crustal low velocities are less pronounced beneath the central Qilian orogen. High Poisson's ratio and the mid‐lower crustal anomaly of the southern Songpan‐Ganzi terrane strongly indicate partial melting of the crust. Localized asthenospheric upwelling, however, can account for the coincident anomalies in both the mid‐lower crust and uppermost mantle beneath the northeastern Songpan‐Ganzi terrane and central Qilian orogen. We conclude that outward expansion of the Tibetan plateau has evolved by a combination of crustal flow on its southeastern margin and mantle upwelling likely induced by removal of thickened lithosphere on its easternmost and northeastern edges. Key Points: Variation in crustal and uppermost mantle structures within the Songpan‐Ganzi terraneThe crustal and uppermost mantle structures are correlated beneath the northeastern Songpan‐Ganzi terrane and central Qilian orogenTibet has evolved by crustal flow on its southeastern margin and localized mantle upwelling on its easternmost and northeastern edges [ABSTRACT FROM AUTHOR]

The mafic to felsic intrusive rocks of Nay (IRN) are located in the northeast of the central Iranian block. In this study, we present new major and trace element geochemistry, U-Pb zircon ages, and Sr-Nd isotopic data to discuss the origin of the IRN postcollisional units. The oldest units in the Nay area belong to Paleocene-early Eocene volcanic and pyroclastic series including basalt-andesite, latite, dacite, and tuff. These series are crosscut by subvolcanic and granitoid rocks with lithological composition varying from quartz gabbro to K-feldspar granite. The youngest igneous activity is represented by quartz monzodiorite dikes. Hornblende-biotite quartz monzonite from Nay granitoids was dated at 40 Ma (zircon U-Pb). The IRN rocks are metaluminous to peraluminous with high-K calc-alkaline and shoshonitic affinities. They display enrichment in light REEs [(La/Yb)N = 3.79-8.71] and LILEs (such as Ba, Th, Rb, U, and K), with depletion in HFSEs (such as Nb, Zr, Y, and Ti). All rocks have negative Eu anomalies [(Eu/Eu*)N = 0.17-0.88] and relatively flat heavy REE patterns [(Gd/Yb)N = 1.12-1.69]. Granitoids have initial 87Sr/86Sr values from 0.7053 to 0.7061 and eNd values from -1.65 to -0.02 calculated at 40 Ma. The geochemical composition of IRN rocks along with the low ISr and positive eNd values and mantle model ages of 0.6-0.8 Ga indicate that two end-members, enriched mantle and a continental crust, were involved in the magma generation. We argue that the Eocene IRN magmatism occurred as a postcollisional product by asthenospheric upwelling owing to the convective removal of the lithosphere during an extensional collapse of the central Iranian block. [ABSTRACT FROM AUTHOR]

Abstract The Li-O isotopes of olivine and pyroxene from the mantle sequence of the Luobusa ophiolite, a fragment of Neo-Tethyan forearc lithosphere in Tibet, reveal a series of magmatic processes and geochemical evolution in a nascent mantle wedge during subduction initiation. Olivine grains from the ophiolitic mantle sequence display δ18O values similar to those of normal mantle (+5.18 ± 0.2‰), but have large δ7Li variations, ∼+4‰ to +13‰ in harzburgites and ∼+1‰ to +11‰ in high-Cr chromitites. Clinopyroxene grains in cpx-bearing harzburgites have finger-like protrusions and LREE-depleted patterns comparable to those of clinopyroxene in abyssal peridotites, and also show high Li concentrations (∼2–8 μg/g) and negative δ7Li values (∼−15‰ to −8‰), which are beyond the ranges of normal upper mantle (<1.3 μg/g and +4 ± 2‰, respectively). Our dataset suggests that the nascent Luobusa harzburgitic mantle wedge was penetrated by limited amounts of slab-derived fluids at an early stage, leading to decoupling of high δ7Li but normal mantle-like δ18O values of the mantle rocks. The hydrated harzburgites were later refertilized by depleted mantle-derived melts. This process generated the cpx-bearing harzburgites with light-δ7Li clinopyroxene, and also imposed normal mantle-like Li isotopic signatures on some previously hydrated high-δ7Li mantle areas at deeper levels. Subsequent melting in both the high-δ7Li and normal mantle-like Li isotopic mantle regimes generated the parental magmas of the high-Cr chromitites with high-Ca boninitic affinities, accounting for the large δ7Li variation of olivine in the chromitites. Slab rollback below the nascent mantle wedge induced asthenospheric upwelling and a high geothermal gradient, which played a more direct role than slab dehydration in triggering the magmatic events of refertilization and formation of the high-Cr chromitites. Numerical modeling of inter-mineral Li diffusion in cpx-bearing harzburgites suggests an interval of <1000 years between the two magmatic events and rapid slab rollback before the generation of the high-Cr chromitites. Intra-oceanic emplacement of the Luobusa proto-forearc lithosphere was possibly finished within 106 years after the chromitite formation. [ABSTRACT FROM AUTHOR]

Highlights • A 3-D azimuthal anisotropy model of the crust and upper mantle is inferred under SE Tibet. • Layered anisotropic structures are found in the vicinity of Eastern Himalaya Syntaxis. • The presence of the local differences may be related to the upwelling asthenosphere of east-central Tibet. • Anisotropic model suggests that different blocks have different deformation mechanisms beneath SE Tibet. Abstract Knowledge about Rayleigh-wave phase-velocity azimuthal anisotropy can provide important insight into the dynamic processes and complex deformation mechanisms beneath Southeastern (SE) Tibet. In this study, we imaged the azimuthal anisotropy of the Rayleigh-wave phase-velocity in the period range of 10–60 s beneath SE Tibet. The model provides seismological evidence for the existence of asthenospheric upwelling beneath the Indo-China block (ICB) and east-central Tibet. Stratified anisotropy architectures around the Eastern Himalaya Syntaxis (EHS) are observed, and the upper boundary of the subducting Indian slab at least extended to the center Lhasa block (LSB). Additionally, our models also reveal the complex deformation pattern of different blocks in the crust and upper mantle beneath SE Tibet, and the crustal deformation is much more complicated than that of the upper mantle. The EHS and LSB are deformed in the whole crust, which is affected by the subducting Indian lithosphere. The Qiangtang block, ICB and Songpan-Ganzi block deformation was mainly controlled by the strikes of the faults and the west-east tectonic movement of the plateau materials. The deformation of the Sichuan-Yunnan diamond block is related to mid-lower crustal flow. In the upper mantle, the fast-propagation directions of most of the blocks match the fast-polarization directions of SKS and the absolute plate motion, excluding the ICB and east-central Tibet. Based on the combined results of the isotropic and anisotropic models and previous studies, two dynamic models are proposed to shed new light on the complex deformation and dynamics processes in SE Tibet. [ABSTRACT FROM AUTHOR]

The Auca Mahuida volcanic field lies on the southernmost Payenia Volcanic Province, one of the broadest retroarc volcanic plateaux in the southern Central Andes (~38°S). This voluminous basaltic flooding of Quaternary age was originated from a deep asthenospheric source, interpreted as a mantle plume product of changing slab dynamics. The geometry of this source is deduced from magnetotelluric data, but the limited spatial coverage of this array does not allow a detailed resolution of this anomaly. In order to present a detailed geometry of the conductive anomaly and related crustal magmatic bodies, we used multiple data sources. We combined Magnetic and Bouguer anomalies, Curie isotherm depth (Tc), Elastic Thickness (Te) and Moho depth derived from the Global Earth Magnetic Anomaly Grid (EMAG2) and terrestrial gravity measurements, all together in a holistic geophysical analysis. The magnetic data depict a nearly 200‐km‐in‐diameter circular anomaly that would correspond to a dense body according to the Bouguer anomaly. Geoid data from the Gravity Field Model (EIGEN‐6c4) have been filtered in order to isolate deeper mass influences and visualize the asthenospheric upwelling previously described from magnetotelluric data. Moho inversion yields a crustal attenuation at 36‐ to 32‐km depth coinciding with Te below 20‐km depth and a shallow Tc (≤15‐km depth) at the site where Geoid positive undulation was calculated. Finally, surface analysis allowed defining a topographic swell, compatible with the dimensions of the identified magnetic anomaly, where the main rivers deviated, potentially due to a recent base level change. Key Points: A magmatic intrusion due to an asthenospheric upwelling in the Southern Central Andes was constrained by magnetic anomaliesThe Geoid was used to interpret an asthenospheric upwellingWe found a correlation between drainage geometry, its diversion pattern, and magnetic anomalies [ABSTRACT FROM AUTHOR]

Abstract It is well established that the Late Jurassic magmatism in the southeastern area of the South China Block (SCB) was associated with an extensional event. However, the triggering mechanism for this event remains unclear, with models commonly invoking either back-arc extension or intra-continental extension in response to slab roll-back of the Pacific plate or asthenospheric upwelling. The key issue about these models revolves around whether the subducted Pacific slab contributed to the mantle source beneath the SCB during the Jurassic, or not. Basalts erupted during the Late Jurassic can provide significant clues for probing the nature of the lithospheric mantle and underlying convective mantle. In this study, we present detailed zircon U-Pb geochronological, whole-rock element and Sr-Nd isotope data for Late Jurassic trachybasalt and trachyandesite from the Mashan Complex, and provide new constraints on the condition of the lithospheric mantle and mantle dynamics of the SCB during that time. LA-ICP-MS zircon U-Pb dating suggests that these volcanic rocks erupted in the Late Jurassic (~158 Ma). All volcanic samples have shoshonitic geochemical affinities with high K 2 O (2.03–4.89 wt%) and K 2 O/Na 2 O (0.55–2.50). They are strongly enriched in LILE and LREE with positive K anomalies, positive ε Nd (t) values (from +0.8 to +3.0), moderate Dy/Yb (1.98–2.36) and low Ba/La (11.1–30.7), Th/Yb (2.29–5.77), U/Th (0.18–0.35), Th/Nb (0.16–0.27) and Th/Ce (0.08–0.15) ratios. Such geochemical signatures suggest that the Mashan volcanic rocks were derived from low-degree (1–5%) partial melting of a metasomatised lithospheric mantle with little evidence supporting the involvement of the Pacific slab in the genesis of the magmas. The synthesis of the available data shows that the Jurassic shoshonitic and mafic rocks have higher ε Nd (t) values than the Triassic shoshonitic and mafic rocks (ε Nd (t) < 0), reflecting source transformation from a Triassic enriched lithospheric mantle to a Jurassic depleted source. The metasomatism of the lithospheric mantle is likely to be associated with asthenospheric upwelling prior to the Late Jurassic. The asthenosphere-lithosphere interaction might have produced a newly metasomatised zone within the lowermost lithospheric mantle. It is inferred that the Jurassic magmatism in the southeast of the SCB might be associated with asthenospheric upwelling in an intra-continental extension setting. Highlights • The Mashan shoshonitic volcanic rocks erupted in the Late Jurassic. • They were derived from a newly metasomatised lithospheric mantle • This metasomatism was associated with the asthenosphere-derived melts during the Jurassic. • They formed in an intra-continental extension setting. [ABSTRACT FROM AUTHOR]

Abstract The origin and geodynamic setting of the Maden Complex, which is situated in the Bitlis-Zagros Suture Zone in the Southeast Anatolian Orogenic Belt, is still controversial due to lack of systematic geological and geochemical data. Here we present new whole rock major-trace-rare earth element and Sr–Nd isotope data from the Middle Eocene volcanic rocks exposed in Maden Complex and discuss their origin in the light of new and old data. The volcanic lithologies are represented mainly by basalt and andesite, and minor dacite that vary from low-K tholeiitic, calc-alkaline, high-K calc-alkaline, and shoshonitic in composition. They exhibit enrichments in large ion lithophile and light rare earth elements, with depletions in high field strength elements. Basaltic rocks have uniform Sr and Nd isotope ratios with high ε Nd (t) values varying from +5.5 to +6.7, in contrast to, andesitic rocks are characterized by low ε Nd (t) values ranging from −1.6 to −10. These geochemical and isotopic characteristics indicate that two end-members, a subduction-related mantle source and a continental crust, were involved in the magma genesis. Considering all geological and geochemical data, we suggest that the Eocene Maden magmatism occurred as a post-collisional product by asthenospheric upwelling owing to convective removal of the lithosphere during an extensional collapse of the Southeast Anatolian ranges. Graphical abstract Image 1 Highlights • Geochemical study of Southeast Anatolian (Turkey) Eocene magmatic rocks. • The rocks were derived from a subduction-modified, N-MORB and E-MORB-like mantle source. • The Eocene Maden magmatism can be explained by lithospheric removal and asthenospheric upwelling. [ABSTRACT FROM AUTHOR]

Abstract: The formation of large porphyry Cu deposits requires shallow intrusion of felsic stocks that are injected into the focused sites of the mineralization. The parental magmas for these igneous rocks are oxidized, with a fO2 above FMQ + 2 (2 logarithmic units above fO2 of fayalite‐magnetite‐quartz buffer), which enables the magmas to transport large amounts of S and metals. The Japanese islands contain continental and island‐arc magmatic belts of various ages, from Mesozoic to recent, that were formed in response to westward subduction of oceanic plates. Mesozoic–Paleocene magmatism formed arcs on the eastern margin of Eurasia before the opening of the Sea of Japan. The back‐arc rifting that formed the Sea of Japan separated these igneous rocks from Eurasia, with their relics exposed as volcano–plutonic belts in Japan. Among these, the San‐in belt is the only representative of an upper crustal section that includes magnetite‐bearing granitic rocks. The opening of the Sea of Japan was accompanied by the asthenospheric mantle upwelling and eruption of oceanic magma. This was followed by extensive Miocene submarine volcanic activity that formed the Green Tuff belt. The current configuration of the Japanese islands was established in Pliocene. Young arcs, such as the Kuril, Izu‐Bonin, and Ryukyu arcs, are largely composed of mafic magmas, with accompanying asthenospheric upwelling in back‐arc extension. As the asthenospheric mantle and associated magmas have a reduced fO2, below FMQ (fayalite‐magnetite‐quartz buffer), continual upwelling of asthenospheric mantle creates limited areas of potential porphyry Cu mineralization in Japanese islands. Possible prospective areas include young stratovolcanoes in Honshu and Kyushu islands. Fertile rocks that may be associated with porphyry Cu mineralization can be identified by evaluating their magmatic oxidation conditions through mineralogical and mineral chemistry studies. [ABSTRACT FROM AUTHOR]

Complete subduction of an oceanic plate results in slab-window opening. A key uncertainty in this process is whether the higher heat flux and asthenospheric upwelling conventionally associated with slab-window opening generate a detectable topographic signature in the overriding plate. We focus on the Baja California Peninsula, which incorporates the western margin of the Gulf of California rift. The topography and tectonics of the rift flank along the peninsula are strongly bimodal. North of the Puertecitos accommodation zone, the primary drainage divide attains a mean elevation of ca. 1600 m above sea level (asl), above an asthenospheric slab-window opened by Pacific-Farallon spreading ridge subduction along this section of the trench at ca. 17–15 Ma. To the south, mean topography decreases abruptly to ca. 800 m asl (excluding the structurally distinct Los Cabos block at the southern tip of the peninsula), above fragments of the oceanic Farallon slab which stalled following slab tear-off at ca. 15–14 Ma. Along the peninsula, a low-relief surface established atop Miocene subduction-related volcaniclastic units has been incised by a west-draining canyon network in response to uplift. These canyons exhibit cut-and-fill relationships with widespread post-subduction lavas. Here, we utilise LANDSAT and digital elevation model (DEM) data, integrated with previously published K-Ar and 40 Ar/ 39 Ar lava crystallisation ages, to constrain the onset of rift flank uplift to ca. 9–5 Ma later than slab-window formation in the north and ca. 11–10 Ma later in the south. These greatly exceed response time estimates of ca. 2 Ma or less for uplift triggered by slab-window opening. Instead, uplift timing of the high-elevation northern region is consistent with lower-lithospheric erosion driven by rift-related convective upwelling. To the south, stalled slab fragments likely inhibited convective return flow, preventing lithospheric erosion and limiting uplift to the isostatic response to crustal unloading during rifting. [ABSTRACT FROM AUTHOR]

One highly disputed topic about the Jurassic igneous rocks in the eastern South China Block (SCB) is whether they were products of the Pacific plate subduction or the intra-continental extension of the SCB. The early Jurassic mafic rocks, which are regarded as important tracers for the mantle nature, geodynamic processes and tectonic setting, are of great importance to solve this controversy. This paper presents the detailed zircon U-Pb geochronological, elemental geochemical and Sr-Nd-Hf isotopic data for the Xialan gabbroic intrusion from the Eastern Nanling Range. SIMS zircon U-Pb dating results for olivine gabbro and hornblende gabbro yield weighted mean 206 Pb/ 238 U ages of 190.6 ± 1.3 Ma and 196.0 ± 2.7 Ma, respectively. Olivine gabbros are high in MgO, Mg-numbers and CaO and low in SiO 2 and depleted in LILEs and LREEs. They have significant Ba, Sr and Eu positive anomalies with ε Nd (t) of +0.8 - +2.9, suggestive of their origination from an ancient lithospheric source. Hornblende gabbros have a wide compositional range of SiO 2 , MgO, Cr and Ni and are characterized by enrichment in LILEs and LREEs and insignificantly negative Nb-Ta anomalies, along with positive ε Nd (t) values ranging from +1.7 to +6.2 and zircon ε Hf (t) values from +9.7 to +12.7, indicative of their origination from the asthenosphere-lithosphere interaction. The estimated mantle potential temperature (1341–1407 °C) of hornblende gabbros is similar to that of the western Basin and Range Province (1350–1450 °C). In combination with the E-W-trending early Jurassic rock-association in the Nanling Range, it is herein proposed that the early Jurassic igneous rocks most likely formed in an intra-continental extension setting in response to the asthenospheric upwelling. [ABSTRACT FROM AUTHOR]

A petrological, geochemical and Sr–Nd–Pb isotopic study was carried out on the Tarom-Olya pluton, Iran, in the central part of the Alpine–Himalayan orogenic belt. The pluton is composed of diorite, monzonite, quartz-monzonite and monzogranite, which form part of the Western Alborz magmatic belt. LA–ICP–MS analyses of zircons yield ages from 35.7 ± 0.8 Ma to 37.7 ± 0.5 Ma, interpreted as the ages of crystallization of magmas. Rocks from the pluton have SiO 2 contents ranging from 57.0 to 69.9 wt.%, high K 2 O + Na 2 O (5.5 to 10.3 wt.%) and K 2 O/Na 2 O ratio of 0.9 to 2.0. Geochemical discrimination criteria show I-type and shoshonitic features for the studied rocks. All investigated rocks are enriched in light rare earth elements (LREEs), large ion lithophile elements (LILEs), depleted in high-field strength elements (HFSEs), and show weak or insignificant Eu anomalies (Eu/Eu* = 0.57–1.02) in chondrite-normalized trace element patterns. The Tarom-Olya pluton samples also show depletions in Nb, Ta and Ti typical of subduction-related arc magmatic signatures. The samples have relatively low ISr (0.7047–0.7051) and positive εNd (36 Ma) (+ 0.39 to + 2.10) values. The Pb isotopic ratios show a ( 206 Pb/ 204 Pb) i ratio of 18.49–18.67, ( 207 Pb/ 204 Pb) i ratio of 15.58–15.61 and ( 208 Pb/ 204 Pb) i ratio of 38.33–38.77. The εHf (t) values of the Tarom-Olya pluton zircons vary from − 5.9 to + 8.4, with a peak at + 2 to + 4. The depleted mantle Hf model ages for the Tarom-Olya samples are close to 600 Ma. These isotope evidences indicate contribution of juvenile sources in petrogenesis of the Tarom-Olya pluton. Geochemical and isotopic data suggest that the parental magma of the Tarom-Olya pluton was mainly derived from a sub-continental lithospheric mantle source, which was metasomatized by fluids and melts from the subducted Neotethyan slab with a minor crustal contribution. Subsequent hot asthenospheric upwelling and lithospheric extension caused decompression melting in the final stage of Neotethyan subduction through slab rollback or shortly after the cessation of arc magmatism. [ABSTRACT FROM AUTHOR]

To investigate the nature and origin of across-arc geochemical variations over time in mantle wedge derived magmas, we have carried out a geochemical study of basalts in the NE Japan arc spanning an age range from 35Ma to the present. Back-arc basalts erupted at 24-18 Ma, 10-8 Ma, 6-3Ma and 2-5-0Ma have higher concentrations of both high field strength elements (HFSE) and rare earth elements (REE) [particularly light REE (LREE) and middle REE (MREE)], and higher incompatible trace element ratios compared with frontal-arc basalts at any given time. Geochemical modeling of Nb/Yb versus Nb shows that the frontal-arc and back-arc compositional differences are independent of subduction modification and can, in many cases, be explained by different degrees of melting (higher degrees of melting for frontal-arc magmas and lower degrees of melting for back-arc magmas) of a nearly homogeneous depleted mid-ocean ridge basalt (MORB) mantle (DMM)-like source, although there are several exceptions. These include some Pliocene frontal-arc basalts that may originate from a source that is slightly more depleted than DMM, several 35-32Ma and 24-18Ma back-arc basalts derived from a lithospheric mantle source that is enriched in HFSE compared with DMM, and a rare 16-12Ma basalt that was erupted in the back-arc but was produced by a similar degree of melting to frontal-arc basalts erupted at the same time. Variations in ratios of fluid-mobile and -immobile elements and those of melt-mobile and -immobile elements for the 35-0Ma NE Japan basalts indicate that the principal subduction component added to the source mantle prior to generation of these basalt magmas is a sediment-derived melt. Comparison of Sr and Nd isotopic compositions for Pacific Ocean MORB, the NE Japan basalts and subducting sediments suggests that the isotopic compositions of most post-16Ma more depleted back-arc basalts can be explained by the addition of <2% bulk sediment; the most enriched isotope compositions of the subcontinental lithosphere-derived magmas can be accounted for by addition of a maximum 5-7% Japan Trench Sediment (JTS), if the original Sr and Nd compositions of the lithosphere approximated that of DMM. The Sr and Nd isotope composition of the frontal-arc basalts can be accounted for by the addition of 1-5% JTS. A depleted asthenospheric mantle (DMM-like) upwelling model with interaction between asthenospheric mantle-derived magmas and overlying lithospheric mantle can account for the geochemical characteristics of the 35-0Ma NE Japan basalts. The frontal-arc magmas were generally generated by higher degrees of melting of the shallower part of the asthenospheric mantle, whereas the back-arc magmas resulted from lower degrees of melting of the deeper part of asthenospheric mantle. These latter magmas underwent interaction with the lithospheric mantle, resulting in more enriched Sr and Nd isotopic signatures for the pre-18Ma back-arc basalts and post-22Ma frontal-arc basalts, but less interaction, resulting in more depleted Sr and Nd isotopic signatures, for most of the back-arc basalts younger than 16 Ma. [ABSTRACT FROM AUTHOR]

Zircon U-Pb ages, major and trace elements, and Sr, Nd and Hf isotope compositions of the Changboshan-Xieniqishan (CX) intrusion from the Great Xing'an Range (GXAR), northeastern China, were studied to investigate its derivation, evolution and geodynamic significance. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb dating yields an emplacement age of 161 ± 2 Ma for the CX intrusion. Bulk-rock analyses show that this intrusion is characterized by high SiO2, Na2O and K2O, but low MgO, CaO and P2O5. They are enriched in large-ion lithophile elements and light rare earth elements, with marked Eu anomalies (mostly from 0.36 to 0.65), and depleted in heavy rare earth elements and high field strength elements. Most samples have relatively low (87Sr/86Sr)i values (0.70423-0.70457), with εNd( t) fluctuating between −0.4 and 2.3. The εHf( t) for zircons varies from 5.4 to 8.7. Sr-Nd isotope modelling results, in combination with young Nd and Hf model ages (760-986 and 549-728 Ma, respectively) and the presence of relict zircons, indicate that the CX intrusion may originate from the partial melting of juvenile crust, with minor contamination of recycled crustal components, and then underwent extensive fractional crystallization of K-feldspar, plagioclase, biotite, sphene, apatite, zircon and allanite. Considering the widespread presence of granitoids with coeval volcanic rocks, we contend that the CX intrusion formed in an extensional environment related to the upwelling of asthenospheric mantle induced by the subduction of the Palaeo-Pacific plate, rather than a lithospheric delamination model. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The South Korea Plateau is a remnant of continental crust in the middle of the eastern Korean margin that was deformed by extension in association with back-arc rifting and separation of the southwestern Japan Arc in the Cenozoic. Multichannel seismic profiles show that the South Korea Plateau preserves structure of rift basins flanked by uplifted footwall blocks. Rift basins in the plateau are filled with distinct syn- and post-rift sequences separated by a prominent breakup unconformity resulting from uplift. The crustal and upper mantle structures computed by ambient noise tomography indicate that the South Korea Plateau defines the seaward limit of rifted continental crust; in addition, rifting occurred with vigorous asthenospheric upwelling induced along the Korean margin and depth-dependent stretching. Therefore, the South Korea Plateau underwent the entire sequence of tectonic events typical of a passive continental margin encompassing rifting with subsidence, uplift, erosion, and breakup prior to the separation of the southwestern Japan Arc. Breakup at the Korean margin may substantiate successive episodes of back-arc spreading which migrated toward the arc in response to trench retreat. We suggest that the southwestern Japan Arc moved to its present location from the southern part of the Korean margin with a significant amount of clockwise rotation during back-arc spreading. [ABSTRACT FROM AUTHOR]

The Ryozen Formation, which crops out on the trench side of the NE Japan arc, contains middle Miocene rhyodacite with adakite-like trace element geochemical characteristics (Ryozen adakitic rhyodacite) and spatially and temporally related basalt (Ryozen basalt) and andesite (Ryozen andesite). K–Ar age data for the basalt and a zircon U–Pb age for the adakitic rhyodacite, combined with the stratigraphy, suggest that all of these volcanic rocks were erupted at about 16–14 Ma. The primitive nature of the Ryozen basalt is shown by its high MgO (maximum 14·1 wt %), Ni (392 ppm) and Cr (1193 ppm) contents. Using the olivine maximum fractionation model, the segregation depth of the parental primary magma to this basalt is estimated at c. 50 km (about 1·5 GPa). The Ryozen andesite has slightly higher 87Sr/86Srinitial (SrI) and lower 143Nd/144Nd initial (NdI) ratios than the Ryozen basalt. This, and the characteristics of the variation trends defined by basalt and andesite samples in SiO2 versus major and trace element variation diagrams, suggests that the andesite may have resulted from fractional crystallization of basaltic magma with minor assimilation of pre-Cretaceous sedimentary rocks (i.e. an AFC process). The Ryozen rhyodacite has phenocrysts of plagioclase, amphibole, garnet and titanomagnetite, and is characterized by low Sr/Y ratios (∼30), low Y concentrations (<10 ppm), high chondrite-normalized La/Yb [(La/Yb)cn] (>25) and low chondrite-normalized Yb [(Yb)cn] values (<5 ppm). These geochemical characteristics are similar to those of a new adakite subgroup (rhyodacite lavas in eastern Jamaica; Jamaican-type adakite). Thus, we define the Ryozen rhyodacite as the Ryozen low Sr/Y adakitic rhyodacite. A potential mechanism for the generation of this rhyodacite is crystal fractionation of plagioclase, orthopyroxene, clinopyroxene, amphibole, garnet, titanomagnetite and minor apatite from an andesitic parent magma. This mechanism is consistent with mass-balance modeling, which matches the observed major and trace element chemistry, as well as SrI and NdI, for the Ryozen andesite and low Sr/Y adakitic rhyodacite. The most likely tectono-magmatic model for the production of the volcanic rocks of the Ryozen Formation involves the upwelling of depleted hot asthenosphere, which modified the thermal structure of the mantle wedge beneath the trench side of the arc during the middle Miocene. This resulted in partial melting of both mantle wedge peridotite and the relatively cool subducting Pacific plate, leading to the simultaneous production of primitive basalt, normal andesite, high-magnesium andesite and low and high Sr/Y adakitic rhyodacites. [ABSTRACT FROM PUBLISHER]

Early Eocene adakitic volcanic and granitoid rocks are widespread in the Eastern Pontides of NE Turkey, providing significant constraints for the early Cenozoic tectonomagmatic evolution of the region. These adakitic rock units exhibit relatively high Sr/Y and La/Yb ratios, but low Y and Yb values, similar to modern adakites generated by partial fusion of a subducted oceanic slab. They also have high K2O and low MgO contents, and show moderately enriched ISrand low ϵNd(t) isotopic signatures. Our trace element modelling suggests that these adakitic magmas were generated from partial melting at low pressures of a garnet-bearing amphibolitic source in the continental lower crust. This lower crustal melting resulted from slab break off-induced asthenospheric upwelling and related magmatic underplating beneath the Eastern Pontides. We interpret this melting event and the adakitic magmatic activity as a syn- to post-collisional process involving early Cenozoic collision of the Pontide and Anatolide–Tauride continental blocks. The geochemical and tectonic constraints presented here indicate that early Eocene adakitic magmatism in the Eastern Pontides did not result from partial fusion of a subducted oceanic slab, but instead represent continental-type adakite formation. [ABSTRACT FROM AUTHOR]

Late Oligocene to Middle Miocene adakitic andesites are found in the southern part of Okushiri Island, the northern Noto Peninsula and in the Toyama region in the present-day back-arc margin of the SW and NE Japan arcs. On Okushiri Island, adakitic andesite is accompanied by moderately alkaline basalt, whereas on the Noto Peninsula, adakitic andesite has been erupted along with high magnesian andesite (HMA), bronzite andesite and tholeiitic basalt. Adakitic andesites from all three locations are characterized by high Sr/Y and low Y, and have higher MgO contents than adakitic melts generated by experimental melting of metabasalt and amphibolite. They also have higher Ni and Cr contents than either Archaean tonalite–trondhjemite–granodiorite (TTG) suites or Early Cretaceous adakitic granites, which have been attributed to partial melting of subducted oceanic crust. The Noto Peninsula adakitic andesite has Sr and Nd isotopic compositions identical to normal mid-ocean ridge basalt (N-MORB), whereas the Okushiri Island and Toyama adakitic andesites are more isotopically primitive than N-MORB. The Noto Peninsula primary adakitic melt was derived from subducted oceanic N-MORB crust, whereas the Okushiri Island and Toyama primary adakites are interpreted as melts of subducted N-MORB and sediment that have subsequently interacted with the overlying mantle wedge peridotite. To explain the comagmatism of adakite, HMA and basalt, the following model is proposed. A hydrated adakitic diapir ascends from the subducting slab and is heated because it enters the overlying hot mantle wedge. The subsequent establishment of thermal and H2O gradients in the adakitic diapir and surrounding mantle wedge peridotite results in concurrent generation of adakitic andesite magma in the inner adakitic diapir region (low temperature and high H2O content), HMA and bronzite andesite magmas in the intermediate peridotite region (intermediate temperature and H2O content), and tholeiitic basalt magma in the outer peridotite region (high temperature and lower H2O content). Comagmatic adakite and mildly alkaline basalt are found in cooler and wetter adakitic diapirs and hotter and drier peridotite regions respectively. The most likely tectono-magmatic situation for the genesis of adakitic magmas in this example of a cool subduction zone involves upwelling of hot asthenosphere into the subcontinental lithosphere beneath the back-arc side of the NE Japan arc and northern end of the SW Japan arc, during the period spanning the pre-Japan Sea opening to syn-opening stages. The unusually high temperature conditions established in the mantle wedge owing to upwelling of hot asthenosphere caused partial melting of the relatively cool subducting Pacific plate, resulting in the generation of adakitic magmas. [ABSTRACT FROM AUTHOR]

According to the most common interpretation, the Apennines developed in Neogene and Quaternary times in the hanging wall of a west directed subduction zone. Seismic tomography is the most powerful tool to investigate large volume of Earth at depth, and it has been extensively applied to shed light on the geometry and shape of the subduction under the Italian peninsula. The various experiments were able to display the slab under the Southern Apennines, but even the most recent tomographic images were non-uniquely interpretable and left open questions about the characteristics of the subduction in the Northern-Central sector of the chain. We here present the results of an improved inversion experiment focused on the Northern and Central Apennines. The results do not show any pronounced subduction slab and the most evident anomaly is a low velocity body extending down to 100 km depth, located in a relatively small area under the western Tuscany. On the basis of accurate synthetic tests, we assess that, if established, a subduction like geometry should be visible in our tomographic images. We then conclude that no subduction is imaged in the Northern and Central Apennines. We thus interpret this anomaly as an asthenospheric flow. However, we cannot exclude that our result is due to intrinsic limitations of the methodology. In fact in response to the original question about the capability of local earthquake tomography to settle the matter about subduction, we underline that the absence of deep earthquakes to illuminate the model from below, the existence of seismic gaps in some sectors of the area under study even at shallow depth and the non uniqueness of interpretation of the tomographic images make local tomography unable to give alone definitive information on the deep structure of the Northern and Central Apennines. [ABSTRACT FROM AUTHOR]

Abstract: Large-scale late Mesozoic to early Cenozoic plutons in the Eastern Pontide orogenic belt mostly show calc-alkaline and I-type geochemical features. However, we identify the Sisdaği pluton that has shoshonitic affinity and I-type character in the region. The pluton was emplaced at shallow depths (<5km), revealed by Al-in hornblende barometry, in the northern margin of the orogenic belt, with SHRIMP zircon U–Pb age of 41.55±0.31Ma, interpreted as dating magma crystallization. It is composed of monzonite, monzodiorite and monzogabbro. The samples show low zircon saturation temperature (T Zr) ranging from 603 to 769°C compared to A-type rocks. They exhibit SiO2 contents of 47.6–61.4wt.%, and high K2O+Na2O (5.0–8.8wt.%) and K2O/Na2O (0.8–1.8). All the samples are characterized by low Mg# (<47) and relatively high but variable Al2O3 content (16.9–22.2wt.%). They are also enriched in light rare earth elements (LREE), large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE), with a weak negative Eu anomaly (Eu/Eu* =0.58–1.26) in mantle-normalized trace element patterns. The samples possess homogeneous initial Sr and Nd isotopic compositions, marked with low I Sr =0.70376–0.70408 and ε Nd (42Ma)=+1.3 to +2.4. T DM ages of the samples range from 0.70 to 0.85Ga. The Pb isotopic ratios are (206Pb/204Pb)=18.64–18.72, (207Pb/204Pb)=15.51–15.58 and (208Pb/204Pb)=38.31–38.65. These geochemical features imply that the parental magma resulted from melting of chemically enriched lithospheric mantle source. In such a case, a hot upwelling asthenosphere is necessary to partially melt the lithospheric mantle in order to form the parental magma. The pluton is considered to be a post-orogenic intrusion that was emplaced in an environment of lithospheric extension, triggering asthenospheric upwelling. The thermal anomaly induced by asthenospheric upwelling resulted in partial fusion of chemically enriched subcontinental lithospheric mantle beneath the region. Then, the shoshonitic melt, which subsequently underwent fractional crystallization with minor crustal contamination, ascent to shallower crustal levels to generate a monzonitic rock series ranging from monzogabbro to monzonite. All these data combined with the regional geology suggest that the crustal thickening as a consequence of regional compression during the Paleocene changed into a lithospheric extension and thinning throughout the early Cenozoic (at ∼42Ma) in the Eastern Pontides. Hence, the middle Eocene shoshonitic I-type magmatism is a unique pluton, signifying initiation of lithospheric thinning and thus of hot asthenospheric upwelling in the region. These interpretations also argue against the presence of an early Cenozoic subduction of oceanic slab in the Eastern Pontides. [Copyright &y& Elsevier]

Abstract: We present SHRIMP zircon ages and whole-rock geochemical data to identify mafic–ultramafic massifs of early Carboniferous and early Cretaceous ages from northern Inner Mongolia, China. The early Carboniferous massifs (Chongenshan–Xiaobaliang–Wusnihei) comprise predominantly lherzolite and minor gabbroic rocks. Zircons from a microgabbro (εNd (t) =9.8) and a plagiogranite (εNd (t) =9) yielded weighted mean 206Pb/238U ages of 354±7Ma and 333±4Ma, that we interpret as reflecting the time of zircon (and magma) crystallization. A volcanic breccia resting unconformably on the ultramafic rocks (serpentinites) has an eruption age of 300±2Ma and provides a younger minimum time limit for the emplacement of the early Carboniferous mafic–ultramafic massifs. In contrast, the early Cretaceous mafic–ultramafic massifs (Hegenshan–Chaogenshan) consist predominantly of harzburgite. A pegmatoid gabbro and a rodingite (εNd (t) =10.6) from the Hegenshan massif have magmatic zircon ages of 139±2Ma and 132±7Ma respectively. The Chaogenshan massif is concentrically zoned with an ultramafic core (dominantly harzburgite–dunite+subordinate lherzolite) surrounded by gabbro (with minor hornblendite) and an outermost volcano-sedimentary sequence (metabasaltic flows+metapelites). We dated zircons from a leuocogabbro (125±2Ma; εNd (t) =10.6), a coarse-grained gabbro (142±4Ma), and a metabasalt (134±2Ma). Zircon xenocrysts (ca. 152–492Ma) are ubiquitous in these and other associated rocks (a coarse-grained gabbro and a diabase), as in the Hegenshan rocks (ca. 152–468Ma). The youngest detrital zircon in outer zone garnet schist (i.e. metapelite) has an age of 132±2Ma. Accordingly, we conclude that the northern Inner Mongolia lherzolite-dominant and harzburgite-dominant massifs are temporally and genetically unrelated. Their emplacement is attributed to asthenospheric upwelling and consequent lithospheric extension during periods of orogenic quiescence in the southern Central Asian Orogenic Belt. [Copyright &y& Elsevier]

Abstract: Felsic volcanic rocks (mainly rhyolites) and basalts found in northeast Hokkido, Japan, result from intense volcanism during the Middle Miocene (14–9Ma), Late Miocene (8–6Ma) and Pliocene to Quaternary (5–2Ma). Rhyolites were examined to determine any genetic relationship to coeval basalts, high magnesian andesite (HMA), lower crustal rocks and mantle peridotite. Rhyolites have initial Sr and Nd isotopic ratios (SrI and NdI) which overlap with coeval north Hokkaido basaltic rocks and HMA. Previously published Sr and Nd isotopic data show that most Middle Miocene to Quaternary (14–0Ma) basaltic rocks from north Hokkaido have a relatively narrow SrI- and NdI- range (SrI 0.70299 to 0.70400, and NdI 0.51281 to 0.51311), with no temporal variation in either SrI or NdI. Basalt and HMA from three locations are, however, more undepleted in terms of both SrI and NdI than other north Hokkaido basaltic rocks. Some of rhyolites (termed undepleted rhyolites here) have similar SrI and NdI to some north Hokkaido basalts and HMA. Rhyolites with similar SrI and NdI to other north Hokkaido basaltic rocks are termed depleted rhyolites. Although the similarity of SrI and NdI between rhyolites and coeval basalts and HMA can be accounted for by fractional crystallization, this process is inconsistent with the REE chemistry of basalts, HMA and rhyolites, and with the results of fractional crystallization modeling. However, a few rhyolites may result from the fractional crystallization of basaltic and HMA magmas with assimilation of some metasedimentary rocks. Small degrees of partial melting of a metazomatized mantle peridotite is an unlikely mechanism to explain the genesis of rhyolites according to REE chemistry and partial melt modeling of an amphibole bearing spinel lherzolite source. Gabbros of the Hidaka metamorphic belt are a possible source for isotopically depleted rhyolites, as both the rhyolites and gabbros have similar SrI and NdI. I-type tonalite and some gabbros in the Hidaka metamorphic belt are possible source rocks for isotopically undepleted rhyolites based on similarity in SrI and NdI. These hypotheses are supported by partial melt modeling of olivine gabbro and I-type tonalite respectively. A possible tectono-magmatic model for the production of post-Middle Miocene rhyolites from NE Hokkaido involves upwelling of the asthenosphere during the Middle Miocene, associated with the spreading of the Kurile back-arc basin and Japan Sea back-arc basin. This would have resulted in thinning of the overlying lithosphere beneath north Hokkaido, and the production of asthenosphere-derived basaltic rocks with low SrI and high NdI throughout north Hokkaido since the Middle Miocene, but less common production of lithosphere-derived basaltic rocks and HMA (with high SrI and low NdI). Basaltic magmas formed since the Middle Miocene either erupted, or caused melting of the crust, resulting in the generation of rhyolitic magma. [Copyright &y& Elsevier]

Abstract: Basalts and felsic volcanic rocks (mainly dacite and rhyolite) found in southwest Hokkaido, northern part of the NE Japan arc, result from protracted volcanism during the Oligocene (34–30Ma), Early Miocene (25–17Ma), Middle Miocene (16–12Ma), Late Miocene (10–5Ma), Pliocene (4Ma) and Quaternary (2Ma), thus spanning the pre-Japan Sea opening to post-opening stages. The majority of basaltic rocks after about 16Ma show depleted Sr (SrI) and Nd (NdI) isotopic signatures compared with some Middle to Early Miocene basalts, which strongly resemble, in terms of both timing and extent, the change in SrI and NdI values for back-arc basaltic rocks of the central NE Japan arc. However, significant differences exist for younger basaltic rocks, in that basaltic rocks with depleted SrI and NdI signatures are found from the Middle Miocene onwards throughout the eastern-, transitional- and western-volcanic zones in SW Hokkaido, whereas in the central NE Japan arc, basaltic rocks with similar isotopic signatures are confined to the back-arc side. Felsic volcanic rocks in southwest Hokkaido have SrI and NdI values, which overlap with coeval southwest Hokkaido basaltic rocks. Although the relationship between mafic and felsic rocks could be attributed to fractional crystallization, this process is inconsistent with REE chemistry, as total REE do not increase systematically from basaltic rocks to felsic volcanic rocks. Alternatively, lower crustal mafic rocks, represented by gabbroic and amphibolitic xenoliths found in basaltic rocks at Itinome-gata (Oga Peninsula), are a possible source for Late Paleogene to Quaternary felsic magmas, as both felsic volcanic rocks and xenoliths have similar SrI and NdI. A possible tectono-magmatic model for the production of post-Late Paleogene volcanic rocks from SW Hokkaido commences in the Oligocene (34Ma) with asthenospheric mantle upwelling followed by partial melting to generate basalt magma (Matsue basalt) with depleted SrI and NdI, followed by interaction of asthenosphere-derived basalt magmas with overlying subcontinental lithosphere. In the Early Miocene (25–17Ma), asthenospheric upwelling triggered partial melting of the overlying lithospheric mantle from which most basalts with undepleted SrI and NdI values were derived. During the Middle Miocene (16–12Ma), thinning of the overlying lithosphere due to the opening of the Japan Sea resulted in asthenospheric upwelling which reached the region beneath the present NE Japan arc volcanic front. Partial melting of the asthenosphere led to generation of voluminous basalt magma with depleted SrI and NdI values throughout southwest Hokkaido. Most basaltic rocks that erupted since the Late Miocene are considered to have also formed from asthenospheric mantle. Basaltic magmas formed since the Oligocene have either been erupted, or fluxed and heated the lower crust from which coeval felsic magmas were generated. [Copyright &y& Elsevier]

Abstract: Volcanism occurring away from ridges and subduction zones does not have an obvious plate tectonic explanation, but instead must arise from sub-lithospheric processes that generate upwelling flow and decompression melting. Several convective processes, such as mantle plumes, convective instability, edge-driven convection, and Richter rolls, produce upwelling via the action of gravity on density heterogeneities in the mantle. Here we investigate an alternative mechanism, the shear-driven upwelling (SDU), which instead generates upwelling solely through the action of asthenospheric shear flow on viscosity heterogeneity. Using a numerical flow model, we examine the effect of viscosity heterogeneity on viscous shear flow induced within an asthenospheric layer. We demonstrate that for certain geometries and viscosity ratios, circulatory flow develops within a “cavity” or “step” embedded into the lithospheric base, or within a low-viscosity “pocket” embedded within the asthenospheric layer. For asthenosphere shearing at 5cm/yr, we estimate that SDU can produce upwelling rates of up to ∼0.2cm/yr within a continental rift, ∼0.5cm/yr along the vertical edge of a craton, or ∼1.0cm/yr within a “pocket” of low-viscosity asthenosphere. In the last case, the pocket must feature an aspect ratio of more than 5, occupy ∼20–60% of the asthenosphere''s thickness, and be at least 100 times less viscous than the surrounding asthenosphere. Such viscosity heterogeneity may be associated with thermal, chemical, melting, volatile, or grain-size anomalies, and is consistent with tomographic constraints on asthenospheric variability. We estimate that SDU may generate up to 2.5km/Myr of melt that is potentially eruptible as surface volcanism; this is faster than eruption rates observed at some locations of continental basaltic volcanism. We conclude that SDU could provide an explanation for intraplate volcanism occurring above rapidly shearing asthenosphere, for example in the Basin and Range region of North America. [Copyright &y& Elsevier]

Abstract: We determined three-dimensional (3D) P-wave velocity structure down to 600km depth beneath the North China Craton using 149,054 arrival times from 7940 local and regional earthquakes, and 193,085 data from 12,657 teleseismic events, which were recorded by 585 seismic stations in our study region. Our tomographic images show some new features. Prominent low-velocity (low-V) and high-velocity (high-V) anomalies are imaged beneath the North China Basin, Trans-North China Orogen, and the cratonic Ordos Block. A lithospheric root of over 250km thick is imaged clearly beneath the Ordos block. A high-V anomaly which may represent the subducting Pacific slab is imaged in the mantle transition zone beneath the eastern edge of the study area. A few other high-V zones are also imaged in the deep portion of the upper mantle, which may reflect the stagnant Pacific slab and detached Archaean continental lithosphere. Two prominent low-V anomalies are imaged beneath the North China Basin, which may reflect asthenospheric upwelling associated with the deep subduction of the Farallon and Pacific plates since late Mesozoic. We consider that the lithospheric thinning of the eastern part of the North China Craton was caused by the long-term replacement, metamorphism, and chemical and thermal erosion of the ancient lithosphere by the hot asthenosphere. [Copyright &y& Elsevier]

Abstract: The post-collisional Cenozoic magmatic activity in the Biga Peninsula (NW Anatolia) started in the Middle Eocene (45.3±0.9 Ma) and lasted continuously until the Late Miocene (8.32±0.19 Ma) with minor punctuations, producing widespread volcanoplutonic complexes. These magmatic suites are characterized by calc-alkaline, high-K calc-alkaline, shoshonitic, mildly alkaline, and alkaline series. The Middle Eocene to Middle Miocene lavas have high LILE/LREE, LILE/HFSE and 87Sr/86Sr(i) (0.7046–0.7087) and low ɛNd(i) (+1.2 to −6.4) values. By contrast, the Late Miocene lavas have the lowest 87Sr/86Sr(i) (0.7030–0.7033) and the highest ɛNd(i) (+2.6 to +6.7) values. Geochemical features of these rocks indicate two episodes of enrichment processes during the evolution of the Eocene–Early Miocene volcanism in the Biga Peninsula. The first stage, source enrichment metasomatism, occurred within the continental lithospheric mantle as a result of the previous subduction event(s), and produced a heterogeneously enriched source. The second stage, crustal contamination, resulted from interaction between the metasomatized mantle-derived melts and those magmas derived from the Sakarya crustal basement during their ascent to the surface. Slab breakoff-induced asthenospheric upwelling caused the melting of metasomatized mantle lithosphere beneath the Sakarya continent and produced the Middle Eocene calc-alkaline lavas. Geochemical, isotopic and age relationships suggest increasing amounts of crustal contamination and a decreasing subduction signature during the evolution of magmas from the Eocene through the Early Miocene. Crustal input was peaked during the Early Miocene volcanism, coinciding with the exhumation and rapid uplift of the Kazdag metamorphic massif. Asthenospheric upwelling beneath the Kazdag core complex contributed to the generation of hybrid magmas formed from mixing of mantle and crustal melts during this stage. The Middle Miocene (15.2 Ma) mildly alkaline rocks show less pronounced enrichment in LILE and LREE and display intermediate major and trace element compositions between the Early Miocene and Late Miocene lavas. The Cenozoic volcanism in Biga Peninsula waned briefly during 15–11 Ma, and was subsequently rejuvenated (Late Miocene, 11.0±0.4–8.32±0.19 Ma) producing mainly OIB-type alkaline lavas. Advanced extensional tectonics throughout NW Anatolia in the Late Miocene led to upwelling of the asthenospheric mantle and its decompressional melting that collectively resulted in the eruption of short-lived episodes of mainly OIB-type alkaline lavas. This tectonomagmatic evolution of the Cenozoic continental volcanism in western Anatolia presents a well-documented case study of an idealized post-collisional magmatism in most orogenic belts around the world. [Copyright &y& Elsevier]

Abstract: The mantle xenoliths included in Quaternary alkaline volcanics from the Manzaz-district (Central Hoggar) are proto-granular, anhydrous spinel lherzolites. Major and trace element analyses on bulk rocks and constituent mineral phases show that the primary compositions are widely overprinted by metasomatic processes. Trace element modelling of the metasomatised clinopyroxenes allows the inference that the metasomatic agents that enriched the lithospheric mantle were highly alkaline carbonate-rich melts such as nephelinites/melilitites (or as extreme silico-carbonatites). These metasomatic agents were characterized by a clear HIMU Sr–Nd–Pb isotopic signature, whereas there is no evidence of EM1 components recorded by the Hoggar Oligocene tholeiitic basalts. This can be interpreted as being due to replacement of the older cratonic lithospheric mantle, from which tholeiites generated, by asthenospheric upwelling dominated by the presence of an HIMU signature. Accordingly, this rejuvenated lithosphere (accreted asthenosphere without any EM influence), may represent an appropriate mantle section from which deep alkaline basic melts could have been generated and shallower mantle xenoliths sampled, respectively. The available data on lherzolite xenoliths and alkaline lavas (including He isotopes, Ra <9) indicate that there is no requirement for a deep plume anchored in the lower mantle, and that sources in the upper mantle may satisfactorily account for all the geochemical/petrological/geophysical evidence that characterizes the Hoggar swell. Therefore the Hoggar volcanism, as well as other volcanic occurrences in the Saharan belt, are likely to be related to passive asthenospheric mantle uprising and decompression melting linked to tensional stresses in the lithosphere during Cenozoic reactivation and rifting of the Pan–African basement. This can be considered a far-field foreland reaction of the Africa–Europe collisional system since the Eocene. [Copyright &y& Elsevier]

Abstract: We report trace element and Sr–Nd isotopic compositions of Early Miocene (22–18 Ma) basaltic rocks distributed along the back-arc margin of the NE Japan arc over 500 km. These rocks are divided into higher TiO2 (>1.5 wt.%; referred to as HT) and lower TiO2 (<1.5 wt.%; LT) basalts. HT basalt has higher Na2O+K2O, HFSE and LREE, Zr/Y, and La/Yb compared to LT basalt. Both suite rocks show a wide range in Sr and Nd isotopic compositions (initial 87Sr/86Sr (SrI)=0.70389 to 0.70631, initial 143Nd/144Nd(NdI)=0.51248 to 0.51285). There is no any systematic variation amongst the studied Early Miocene basaltic rocks in terms of Sr–Nd isotope or Na2O+K2O and K2O abundances, across three volcanic zones from the eastern through transitional to western volcanic zone, but we can identify gradual increases in SrI and decreases in NdI from north to south along the back-arc margin of the NE Japan arc. Based on high field strength element, REE, and Sr–Nd isotope data, Early Miocene basaltic rocks of the NE Japan back-arc margin represent mixing of the asthenospheric mantle-derived basalt magma with two types of basaltic magmas, HT and LT basaltic magmas, derived by different degrees of partial melting of the subcontinental lithospheric mantle composed of garnet-absent lherzolite, with a gradual decrease in the proportion of asthenospheric mantle-derived magma from north to south. These mantle events might have occurred in association with rifting of the Eurasian continental arc during the pre-opening stage of the Japan Sea. [Copyright &y& Elsevier]

Abstract: In the Niigata region of the NE Japan arc, basaltic and intermediate to felsic volcanic rocks (silicic andesite, dacite and rhyolite) have been produced during the Early Miocene, Middle Miocene and Pliocene ages, spanning the pre-Japan Sea opening to post-opening stages. Early Miocene basaltic rocks are characterized by enriched Sr and Nd isotopic signatures (initial 87Sr/ 86Sr (SrI)=0.70557 to 0.70592 and initial 143Nd/ 144Nd (NdI)=0.51255 to 0.51262), whereas Middle Miocene and Pliocene basaltic rocks are slightly enriched than MORB in terms of SrI (0.70314–0.70416) and NdI (0.51286–0.51310). Early Miocene basaltic rocks are also characterized by higher abundances of HFSE and LREE, and higher Zr/Y compared to Middle Miocene and Pliocene basaltic rocks. The geochemical features of Early Miocene basaltic rocks are almost identical to those of basaltic rocks found in continental rift zones, such as the Rio Grande rift. The different geochemical signatures of Early and Middle Miocene basaltic rocks can be ascribed to the geochemical differences in the mantle source. Apart from andesitic rocks of the Kakuda area, most of Early and Middle Miocene intermediate to felsic volcanic rocks from the Niigata region show significantly higher SrI values (0.70673–0.70773) and lower NdI values (0.51234–0.51254) than Early Miocene basaltic rocks, indicating a lower crustal origin for these more felsic volcanic rocks. A possible tectono-magmatic model for the production of post-Early Miocene volcanic rocks from the Niigata region includes: (1) at the Early Miocene (22–20 Ma); commencement of asthenospheric mantle upwelling followed by both partial melting of the overlying lithospheric mantle and crustal rifting, resulting in the production of continental rift zone-type basaltic magmas which have either erupted on the surface or initiated partial melting of the lower crust to produce more felsic magmas, and (2) at the Middle Miocene (after 15 Ma); thinning of the overlying lithosphere due to the opening of the Japan Sea. This was associated with asthenospheric upwelling followed by partial melting of the asthenosphere to generate extensive basaltic magmas. These were either erupted or melted the lower crust to generate more felsic magmas. Such tectono-magmatic processes may also be applicable to the generation of Early to Middle Miocene basaltic to felsic volcanic rocks in many other parts of the back-arc margin of the NE Japan arc. [Copyright &y& Elsevier]

Most middle Miocene to Quaternary basaltic rocks younger than about 12 Ma from north Hokkaido show a narrow range in initial 87Sr/86Sr (SrI) and initial 143Nd/144Nd (NdI) isotope ratios, which are slightly undepleted, compared to MORB. SrI ranges from 0.70296 to 0.70393 and NdI from 0.51288 to 0.51307, but there is no temporal variation in either SrI or NdI. An exceptional basalt from Ohmu area, which is more undepleted in terms of both SrI and NdI than other north Hokkaido (NH) basaltic rocks. Most of the NH basaltic rocks are isotopically similar to basaltic rocks found on the back-arc side of the NE Japan arc (BA basaltic rocks) younger than about 15 Ma, whereas basaltic rocks from the Ohmu area are isotopically similar to BA basaltic rocks older than 15 Ma. The generation of both NH and BA basaltic rocks appears to be closely related to mantle evolution. This involves upwelling of asthenosphere into the continental lithosphere beneath north Hokkaido and the back-arc side of the NE Japan arc, at the same time as spreading of the Kurile back-arc basin and Japan Sea back-arc basin. The magma source for the Ohmu area basalts and the BA basaltic rocks prior to 15 Ma may correspond to the subcontinental lithospheric mantle thinned by the upwelling of asthenospheric mantle material from which most of the NH basaltic magmas and the BA basaltic magmas younger than 15 Ma were derived. This asthenospheric mantle is slightly different from MORB source in terms of major- and trace-element characteristics such as TiO2/K2O and Zr/Y as well as Sr and Nd isotopes, which may be due to involvement of EM II-like component. [Copyright &y& Elsevier]

• The Mesoarchean Mayurbhanj Gabbro Anorthosite Complex (GAC), a well-preserved mafic body in the eastern margin of the Singhbhum Craton. • Mineralogy of these rocks and the mineral composition data rules out the role of a hydrous mantle source. • Geochemical and Nd isotopic signatures suggest GAC originated from heterogeneous depleted mantle source undergone crustal contamination. • This GAC was most likely emplaced in intraplate tectonic setting due to asthenospheric upwelling. Understanding the petrogenetic evolution of Archean gabbroic cumulates provides a window to the mantle geodynamics operated in the early Earth. Gabbroic cumulates are associated with various tectonic settings including convergent plate boundaries, mantle plumes, mid-ocean ridges, anorogenic, post-orogenic, continental rift settings and oceanic plateaus. The exposures of preserved gabbroic suites in the Archean are limited. The Mesoarchean Mayurbhanj Gabbro Anorthosite Complex (GAC) is a well-preserved mafic body situated along the eastern margin of the craton along with concomitant anorogenic Mayurbhanj granite. These rocks are devoid of deformational structures and/or mineral recrystallization. Based on their mineralogical characteristics, GAC rocks are mainly grouped into gabbronorite, leucogabbronorite, leucogabbro and gabbro. The minimal abundance of magmatic amphibole and the variation in the anorthite content (An 36 -An 84) rules out the role of a hydrous mantle source in the evolution of these rocks. The studied samples display tholeiitic trend transitioning to calc-alkaline field, major and trace element data indicates the fractional crystallization of olivine and pyroxene. The trace element systematics of the Mayurbhanj GAC show LILE enrichment over HFSE depletion, with negative Nb-Ta and mixed Zr-Hf and Ti anomalies. The geochemical signatures such as elevated Th/Yb, Nb/Yb, variable Dy/Yb and Dy/Dy* and varying Gd/Yb CN ratios suggest that parental magmas of the Mayurbhanj GAC originated from a heterogeneous depleted mantle source. The studied rocks' Sr-Nd isotopic systematics reveal that their parental magma has undergone assimilation by a Paleoarchean felsic crust. We argue that the 'arc-like' geochemical signatures displayed by these rocks resulted from crustal contamination and may not be due to subduction origin. Consolidating the field, geochemical and isotopic evidence suggests that the Mesoarchean Mayurbhanj GAC possibly evolved from an anhydrous heterogeneous mantle source in a non-compressive tectonic regime. [ABSTRACT FROM AUTHOR]

[Display omitted] • Contrasting lithospheric structure beneath the western and eastern Nanling. • Cu-Mo/Pb-Zn deposits in western Nanling related to a reactivated lithospheric suture. • W-Sn deposits in eastern Nanling related to lithospheric thinning. We have obtained a lithospheric electrical resistivity model of the Nanling Range through 3-D inversion of magnetotelluric data from 118 sites comprising an array covering this important metallogenic belt. Approximately divided by longitude 114°E, the western and eastern parts of this area show significantly different lithospheric electrical features, spatially coincident with its west-east differential Jurassic metallogeny. The lithosphere beneath western Nanling is mainly characterized by high-resistivity, but with an approximately north–south trending, lithosphere-scale, nearly upright, high-conductivity zone that occurs beneath the Qin-Hang magmatic belt. We interpret this high-conductively zone to be a long-lived lithospheric weak zone being located on an ancient plate-boundary that experienced multiple reactivations. The zone controlled Jurassic mafic magmatic events, e.g., 174–176 Ma Daoxian and 150–154 Ma Ningyuan, and also acted as a narrow pathway for the rapid upwelling of hot and volatile-rich asthenosphere-derived materials during the Middle-Late Jurassic Cu-Mo/Pb-Zn mineralization. In contrast, to the east of longitude 114° E, a nearly 80 km deep, clear and continuous electrical lithosphere-asthenosphere boundary is imaged, which could result from a major lithospheric delamination event, probably caused by the early Paleozoic Wuyi-Yunkai orogeny and subsequent post-orogenic collapse, or the subduction and rollback of the Pacific slab after the Triassic. The eastern Nanling region with its thinned lithosphere is suggested as allowing broader zones of upwelling asthenospheric during the intensive Jurassic W-Sn metallogenesis, causing slow upwelling and lateral spreading of asthenosphere-derived materials and subsequent strong extension of the crust and also the mixing and assimilation with melts. Consequently, metallic elements were extensively extracted from the crust, eventually giving rise to the world-class W-Sn deposits in the eastern Nanling region. [ABSTRACT FROM AUTHOR]

Most previous studies advocate that crustal weakening in the Yunnan area is predominantly the result of eastward growth of the Tibetan Plateau. Here we observe regional variations in the Yunnan crustal structure from the waveform characteristic variations of receiver functions at 342 seismic stations. Furthermore, receiver function imaging along three seismic sections reveals the response of crustal structure to different tectonic evolution. Our results show that northwestern Yunnan is characterized by 49–64 km-thick crust with multi-layer low-velocity materials and local lower crustal "doublet". These low-velocity structures may reflect partial melting caused by crustal thickening associated with the eastward growth of the plateau. However, in southern and eastern parts of Yunnan, the topography of the Moho varies relatively gently with crustal thicknesses of 26–42 km, and widespread low-velocity zones are located above the crust-mantle boundary. We speculate that these features might favor the action of partial melting of crust triggered by asthenospheric upwelling related to the subduction of the India plate. • Receiver function analysis reveals regional variations in Yunnan crustal structures. • The effect of the eastward growth of the Tibet plateau is confined in NW Yunnan. • The tectonism of upwelling related to subduction is recorded in the South. [ABSTRACT FROM AUTHOR]