Magmatic Underplating Research Articles

Deep crustal structure and compositions for tectonic and geodynamic implications of the Dharwar Craton (Southern India) inferred from 3-C wide-angle seismic data

The 3-C deep seismic study with integrated interpretation of Bouguer gravity, heat flow, geological and geochronological studies along the Perur-Chikmagalur profile of the Dharwar Craton reveals important crustal elements with compositional distinctions of different geological structures such as horst and grabens , major shear zones like BSZ, CSZ, prominent batholiths as CG , granitic and gneissic complexes, presence of LVL, plutons , magmatic intrusives, thick underplating in WDC and significant Moho upwarping towards EDC due to plume head and komatiite magmatism . • Crustal V P and V S models are derived using 3-C seismic data of the Dharwar Craton. • Rock compositions are constrained by V P / V S , Poisson’s ratio, density and heat-flow. • Imaged a major suture zone CSZ dividing EDC and WDC blocks of the Dharwar Craton. • The LVL forms a detachment zone sandwiching the supracrustals and Archean gneisses. • Moho upwarping and underplating justify plume-arc settings for the Dharwar Craton. The Dharwar Craton of southern India is an important stable cratonic province of the world with complex geology and tectonic settings. Extensive studies provide insights of crustal velocity structure for the tectonic and geodynamic evolution of this Archean craton. This region has experienced several tectonically disturbed zones like Chitradurga Shear Zone (CSZ), Bababudan Shear Zone (BSZ) and Closepet Granites (CG). We have developed a comprehensive geologically plausible tectonic model using both P- and S-wave velocity structures to image major structural elements like shear zones and decipher the compositional distinctions of different rock assemblages of Western Dharwar Craton (WDC) and Eastern Dharwar Craton (EDC) part using 3-C wide-angle seismic data acquired along the 200-km long Perur-Chikmagalur deep seismic profile. The tectonic model show large compositional changes of subsurface rocks with anomalous high V P , V S , V P / V S , Poisson’s ratio ( σ ) and density ( ρ ) forming a major tectonic divide or suture zone called CSZ between EDC and WDC blocks. Significant crustal thinning (37–41 km) is observed due to Moho upwarping towards the Neo-Archean EDC block mainly composed of felsic granites and granodiorites. The WDC block show relatively thick crust (48–50 km) due to mafic underplating and mantle plume activity below CSZ forming Meso-Archean greenschist-facies-gneisses with dominant mafic/ultra-mafic compositions. Hence, crustal velocity, density, heat-flow, geology and geochronology studies support a plume-arc model with evidence of thick magmatic underplating of the lower-crust, complex subduction and development of highly strained shear zones like CSZ as suture juxtaposing EDC and WDC blocks.

The detrital zircon record of Variscan to post-Variscan tectonosedimentary and magmatic processes in the Tauern Window (Eastern Alps)

Coupled U–Pb and Lu–Hf LA-ICP-MS detrital and igneous zircon data were obtained from metasedimentary sequences (Kaserer Formation, Schmirntal Quartzite, Seidlwinkel Formation, Bündnerschiefer Basin, Riffler Basin) of the western Tauern Window (Eastern Alps). Results show maximum deposition ages between the Late Permian and the Triassic, indicating protracted sedimentation and magmatism between the Late Paleozoic and the Mesozoic. The Lu–Hf fingerprint shows a change from subchondritic to variable subchondritic to suprachondritic compositions at ca. 290 Ma, possibly documenting the transition from Late Paleozoic Variscan post-collisional processes to intracontinental extension. Lithospheric thinning and magmatic underplating may explain the observed Hf isotopic evolution as the result of mixing of crustal and mantellic sources. From a paleogeographical perspective, results confirm that the Tauern Window was situated between Alpine basement units (South Alpine, Austroalpine and External Massifs) and the Bohemian Massif during the Permian–Triassic.

Crustal Structure and Anisotropy in the Lower Yangtze Region and its Metallogenic Implications

In this study, we performed receiver function profiling and fitted harmonic functions to the arrival time variations of Pms phases to calculate the crustal seismic anisotropy with delay time and fast polarization direction, using broadband seismic data obtained from 55 temporary stations in two linear profiles and 39 stations in the Lower Yangtze and adjacent region. Moreover, we determined the crustal thickness and Poisson’s ratio using a novel H-κ-c stacking method. Our results revealed that the Middle-Lower Yangtze Metallogenic Belt and the north east section of the Qinzhou-Hangzhou Metallogenic Belt are characterized by Moho upliftment (<32 km), a relatively high Poisson’s ratio (>0.26), local lithospheric thinning (<70 km), and a pattern of deep faults that connect the crust and asthenosphere and serve as conduits for magma upwelling. The NE-SW fast polarization direction was consistent with the SKS splitting results, and the average delay time was 0.45 s. Moreover, underplating of deep magma and upwelling along the weak zone caused local Moho uplift and ductile shear of the lower crust, resulting in the directional arrangement of amphibole and other minerals, which may be the controlling mechanism for the crustal anisotropy in the study area. The variations in crustal structure and anisotropy characteristics indicated that in the context of the northeastern Paleo-Pacific plate subduction, the existence of weak lithospheric zones and the northeastern asthenospheric flow are important conditions for metal supernormal enrichment in the Lower Yangtze region.

Juvenile crust and mantle sources for the Nasrand intrusive rocks, the central Urumieh–Dokhtar magmatic arc, Iran: Insights from elemental and isotopic geochemistry

The Oligocene Nasrand intrusive rocks (NIRs), located in the central part of the Urumieh–Dokhtar magmatic arc, are mainly composed of granite and subordinate diorite and gabbro. These rocks are characterized by the enrichment of light rare earth elements (LREEs), Nb‐Ta negative anomalies, and high LILE/HFSE, suggesting a subduction‐related origin. The felsic rocks with granite compositions have high SiO2 content (69.77–77.88 wt.%), low Mg# (0.02–0.43), low Nb/Ta, and Zr/Hf (Avg: 12.53, and 33.08, respectively). The initial 87Sr/86Sr ratio and εNd(t) values in felsic rocks range from 0.70577 to 0.70576 and 0.4 to −0.3, respectively. These data suggest that these felsic rocks originated from juvenile crust by heat rising from basaltic magmas in an orogenic setting. The mafic‐intermediate rocks and dikes are gabbro, gabbroic diorite, and monzodiorite, with low SiO2 content (48.08–56.11 wt.%) and high Mg# (0.37–0.68), with average ratios of Nb/Ta and Zr/Hf are 27.88 and 39.84, respectively. The initial 87Sr/86Sr ratio and εNd(t) values for mafic‐intermediate rocks and dikes are 0.70583 to 0.70476 and −0.1 to +2.6, respectively. These characteristics indicate magma from a mantle source that had undergone minor mixing with juvenile crust. It seems mantle‐derived juvenile magma underplating played an important role in the crustal growth in the Urumieh–Dokhtar magmatic belt during the Ediacaran to Early Cambrian, relative to other time frames.

Joint inversion of receiver functions and surface wave dispersion in the Recôncavo–Tucano basin of NE Brazil: implications for basin formation

SUMMARYThe crustal structure of the Recôncavo–Tucano basin, an aborted rift system that developed in NE Brazil during extension related to the opening of the South Atlantic Ocean, has been investigated through local constraints from receiver functions developed at 18 seismic stations in the region. Gravity modelling has proved unable to unequivocally localize crustal thinning under the basin depocentre and, together with a general lack of sediments from a putative thermal sag phase, this has led to a range of basin formation models invoking either pure or simple shear or a combination of both. In particular, the ‘flexural cantilever’ model has assumed simple shear extension in the upper crust and pure shear extension in the lower crust and mantle, enabling local erosion of the rift flanks after footwall uplift and regional erosion of the thermal sag phase after magmatic underplate of the basin's crust. Our results reveal that the crust is over 40 km thick beneath the Tucano and Recôncavo basins and that it contains a thick (5–8 km) layer of high velocity (Vs > 4.0 km s–1) material below ∼35 km depth. These observations contrast with structure immediately West (São Francisco Craton) and East (Borborema Province) of the basin, for which crustal thicknesses average 42 and 36 km, respectively, lower crustal velocities are below 4.0 km s–1, and local instances of crust as thin as 33.5 km are observed. We propose, in agreement with the ‘flexural cantilever’ model, that the fast velocity layer making the basin's lowermost crust resulted from mafic underplating after stretching and thinning during the syn-rift phase, restoring crustal thickness to pre-rift values (or larger) and providing the necessary buoyancy to trigger regional uplift. Moreover, although not pervasive, instances of thin crust along the footwall could be related to rift flank erosion. We thus conclude that, regardless of the mode of extension in the upper crust, our results favour models of basin formation invoking extension of the lower crust by pure shear.

Composition, Age, and Origin of Ordovician-Devonian Tanjianshan granitoids in the North Qaidam Orogenic Belt of northern Tibet: Implications for Tectonic Evolution

ABSTRACT The North Qaidam Orogenic Belt (NQOB) is characterized by widespread Palaeozoic magmatism and high to ultrahigh-pressure metamorphism that provides important information regarding the continental crust growth and tectonic evolution in the North Qaidam. Our work focuses on the geology, geochronology, whole-rock geochemistry, and Sr–Nd isotopy of granitoids in the Tanjianshan area in the northern NQOB. The results indicate that the 486–469 Ma adakitic and TTG-like granitoids originated from the partial melting of Middle–Late Mesoproterozoic lower crustal materials with the addition of juvenile mafic lower crust at high pressures under an active continental margin setting. The 396–392 Ma I-type granitoids originated from the partial melting of juvenile crustal materials with the addition of ancient crustal component in a post-collisional setting. Geochronological studies show that there are significant regional differences in magmatism between the northern and rest parts of the NQOB. In the northern part, extensive and prolonged magmatism was widespread during oceanic subduction (~526–460 Ma) and post-collisional extension (~420–350 Ma) but magmatism in the stage of orogenic collision was absent. The Early Palaeozoic adakitic magmatism suggest a significant continental crustal reworking in a subduction zone during 486–469 Ma along the NQOB. The initial oceanic subduction may have occurred prior to ca. 526 Ma. An intense and multiple-stage extension developed at ~420–350 Ma in the northern part of the NQOB following the orogenic collapse. Lithospheric delamination and subsequent mafic magma underplating may be the main geodynamic mechanism resulting in the uprooting of the continental crust and ore mineralization in response.

Passive magmatism on Earth and Earth-like planets

Magmatism has occurred throughout Earth's history. From the early Earth to the modern plate-tectonic Earth, the amount of magmatism has varied, but it has always occurred on multiple scales, in various tectonic environments and at various depths in the crust and mantle. Magma compositions also vary. In this paper, we argue that the mechanism of magma emplacement has generally been passive at all stages of Earth evolution. We conclude that most magmatism related to subduction, rifting, mid-oceanic spreading, flood basalts and large igneous provinces and related to mantle upwellings, magma underplating, slab windows, orogenic collisions as well as Archean TTG formation are predominantly passive from the lithosphere-scale to the crystal-scale. Our results weigh against the view that magmatism drives plate motions. Most of the magmatism on other Earth-like planets is also passive regardless of the tectonic environments.

Plume-ridge interactions in the Central Indian Ocean Basin: Insights from new wide-angle seismic and potential field modelling

The central Indian Ocean Basin (CIOB) comprises numerous tectonic elements related to the early-mid Cretaceous Antarctica-India rifting and subsequent formation of the Indian Ocean. A protracted rift-drift journey of the Indian plate was accompanied with frequent interactions between concurrent mantle plumes and spreading centres. In 2015, we acquired ~420-km long wide-angle reflection and refraction seismic profile in the CIOB south off Sri Lanka, which lies in close proximity to the Indian Ocean Geoid Low. Our primary aim is to decipher crustal and uppermost mantle structure variations with an emphasis on prior tectonics. Two dimensional travel-time tomographic inversion complemented by potential field modelling yields sub-surface velocity-interface structures of the CIOB. Results demonstrate presence of an anomalously thick crust (~14 km) underneath the Comorin ridge, which gradually thins out in the east towards the centre of the geoidal low. We observe distinct evidence for an anomolously high velocity lower crust (velocities greater than 7 km/s across a ~ 160 km wide region) west of 790 E longitude. However, crust to the east of this point appears to be normal oceanic type. We attribute the lateral diversity in the crustal type to possible magmatic underplating beneath the Comorin ridge. We interpret that the concurrent plume-ridge interaction during the late Cretaceous India-Madagascar break-up may have caused this crustal underplating. These findings have significant implications towards better understanding of plume-ridge interaction processes along various passive margins as well as precise reconstruction of India-Madagascar dispersal.

Controlling factors of prolonged REE mineralization in the Maoniuping REE deposit: Constraints from alkaline granite in the syenite–carbonatite complex

Previous studies have shown that the formation of Maoniuping giant rare-earth elements (REE) deposit is closely related to the protracted magmatic evolution (>2 Myr) of syenite–carbonatite magma. However, the reasons responsible for the protracted magmatic evolution of the Maoniuping deposit has not yet been well constrained. The REE-barren alkaline granite intruding the Cenozoic REE-mineralized syenite–carbonatite complex provides an opportunity to resolve this problem. In this study, zircon U–Pb geochronology, whole-rock geochemistry, and Sr–Nd–Hf isotope analyses of the alkaline granite and syenite were undertaken to elucidate the long timescales of magmatic evolution in the Maoniuping deposit. Zircon U–Pb dating indicates that the alkaline granite formed slightly later (25.5 ± 0.4 Ma) than the syenite (27.2 ± 0.6 Ma). The former is characterized by high SiO2 (72.8–75.8 wt%), Na2O + K2O (10.5–11.6 wt%), and FeOT (2.24–2.87 wt%) contents, low Al2O3 (10.3–11.8 wt%), MgO (0.08–0.20 wt%), and CaO (0.04–0.33 wt%) contents, and negative Eu, Ba, Sr, Ti, and P anomalies. It has characteristics typical of A-type granitoids, such as high 10000 Ga/Al (3.46–5.16) and FeOT/(FeOT + MgO) (0.93–0.97) ratios, and high (Zr + Nb + Ce + Y) contents (650–1065 ppm). The distinct Sr–Nd–Hf isotopic and compositional signatures of the alkaline granite and syenite indicate that the former is likely a product of direct high-temperature partial melting of shallow crust induced by the upwelling of asthenospheric mantle in a transtensional setting. Combining our results with those of previous studies, we suggest that shallow-seated, continuous underplating of mantle-derived magmas and stable regional thermal anomalies were responsible for the prolonged magmatic evolution and REE mineralization of the Maoniuping syenite–carbonatite complex.

Petrogenesis of Dacites in a Triassic Volcanic Arc in the South China Sea: Constraints From Whole Rock and Mineral Geochemistry

Triassic volcanic rocks, including basalts and dacites, were drilled from Meiji Atoll in the South China Sea (SCS), which represents a rifted slice from the active continental margin along the Cathaysia Block. In this study, we present apatite and whole rock geochemistry of Meiji dacites to decipher their petrogenesis. Apatite geochronology yielded U-Pb ages of 204–221 Ma, which are identical to zircon U-Pb ages within uncertainty and thus corroborate the formation of the Meiji volcanic rocks during the Late Triassic. Whole rock major elements suggest that Meiji dacites mainly belong to the high-K calc-alkaline series. They display enriched patterns in light rare earth elements (LREE) and flat patterns in heavy rare earth elements (HREE). They show enrichment in large-ion lithophile elements (LILE) and negative anomalies in Eu, Sr, P, Nb, Ta, and Ti. The dacites have initial 87Sr/86Sr ratios of 0.7094–0.7113, εNd(t) values of -5.9–-5.4 and εHf(t) values of -2.9–-1.7, whereas the apatite has relatively higher initial 87Sr/86Sr ratios (0.71289–0.71968) and similar εNd(t) (-8.13–-4.56) values. The dacites have homogeneous Pb isotopes, with initial 206Pb/204Pb of 18.73–18.87, 207Pb/204Pb of 15.75–15.80, and 208Pb/204Pb of 38.97–39.17. Modeling results suggest that Meiji dacites can be generated by <40% partial melting of amphibolites containing ∼10% garnet. Therefore, we propose that the Meiji dacites were produced by partial melting of the lower continental crust beneath the South China block, triggered by the underplating of mafic magmas as a response to Paleo-Pacific (Panthalassa) subduction during the Triassic. Meiji Atoll, together with other microblocks in the SCS, were rifted from the South China block and drifted southward due to continental extension and the opening of the SCS.

Relicts of a Cambrian oceanic arc in the Lajishan suture, NE Tibetan Plateau: Evidence for early-stage subduction within the Proto-Tethyan Ocean

Lajishan suture in the Qilian Orogen records subduction process of the southern branch of the Proto-Tethyan Ocean. However, when and where the subduction started remain controversial issues. We present a study of the Cambrian volcanic rocks from the Lajishan suture. These volcanic rocks are coherent and composed of basalt, andesite, and volcanic breccia with minor interlayers or veins of dacite and rhyolite. Zircons from the dacite and rhyolite yield concordant Early-Middle Cambrian ages ranging from 521 to 510 Ma. Basalt and andesite exhibit subduction-related geochemical signatures with enrichment in large ion lithophile elements and depletion in high field strength elements. They have evolved REE, LREE, ε Nd (t) (+2.6 to +8.2), and (La/Sm) N ratios, suggesting derivation from depleted mantle wedge with addition of hydrous fluid or various amounts of sediment-derived melts. Dacite and rhyolite have positive zircon ε Hf (t) values (+6.1 to +11.6) and whole-rock ε Nd (t) values (+1.7 to +4.2). They were generated by partial melting of juvenile mafic crust resulting from magma underplating. These Cambrian volcanic rocks in the Lajishan suture are exposed to the south of the accretionary complex and therefore are likely to be relicts of Cambrian oceanic arc built on the oceanic crust during southward subduction of the South Qilian Ocean. Considering the Cambrian-Ordovician intra-oceanic trench-arc-basin system reconstructed in the North Qilian suture, it is suggested that both southern and northern branches of the Proto-Tethyan Ocean have experienced intra-oceanic subduction process which initiated at ca. 530 Ma. • Cambrian volcanic rocks are coherent and widely exposed in the Lajishan suture. • They represent the relicts of an oceanic arc within the South Qilian Ocean. • Early-stage subduction of the Proto-Tethyan Ocean initiated at ca. 530 Ma.

Late Devonian magmatic event in the South Beishan orogenic belt, Gansu: Constraints from zircon U-Pb chronology, geochemistry and Sr-Nd-Hf isotopes

北山造山带处于中亚造山带南缘，研究其岩浆事件对于推演中亚造山带地质构造演化具有重要意义。在甘肃北山南带的双鹰山-花牛山岛弧带中，分布有大量花岗质岩体。本文选取甘肃北山南带双峰山南复式岩体中的花岗闪长岩和二长花岗岩，开展LA-ICP-MS锆石年代学、Hf同位素、全岩地球化学和Sr-Nd同位素研究。研究结果表明该复式岩体成岩时代、地球化学与同位素特征基本一致，具体如下：（1）锆石年代学结果显示二长花岗岩年龄为367.5±1.7Ma、368.0±1.8Ma，花岗闪长岩年龄为366.3±2.2Ma，均为晚泥盆世；（2）复式岩体SiO₂含量为64.39%~74.95%、碱含量为5.01%~9.21%，属准铝质（A/CNK=0.88~1.00），P₂O₅含量低（0.02%~0.17%），且与SiO₂呈负相关，具有典型的I型花岗岩特征；（3）样品总体富集轻稀土元素（（La/Yb）_N=6.23~23.0），具有较强烈的Eu负异常（δEu=0.31~0.55），富集Rb、Th、U等大离子亲石元素，亏损Nb、Ti、P等高场强元素；（4）锆石ε_Hf（t）值为-3.1~+6.0，t_DM2为928~1327Ma，全岩（⁸⁷Sr/⁸⁶Sr）_i在0.705607~0.708523之间，ε_Nd（t）在-4.1~-1.9之间。结合区域构造环境，实验数据表明，双峰山南晚泥盆世复式岩体是在活动大陆边缘的环境下，由幔源岩浆底侵加热下地壳使之部分熔融，并与之混合形成的，代表了晚泥盆世柳园洋向北俯冲的构造事件。;Beishan orogenic belt (BOB) is located on the southern margin of the Central Asian Orogenic Belt (CAOB). The study of magmatic events in BOB is of great significance for understanding the tectonic evolution of the CAOB. A large number of granitiod plutons are distributed in the south of BOB, where the Shuangyingshan and Huaniushan arcs are located. In this study, we focus on granodiorite and monzogranite from the south Shuangfengshan complex in the southern BOB. We present whole rock geochemistry, Sr-Nd isotopic composition, in situ U-Pb dating and Hf isotopic composition of zircons from this complex, from which the following results are obtained:(1) The granodiorites and monzogranites have similar zircon U-Pb chronology, whole rock geochemistry and isotopic composition. (2) Zircon LA-ICP-MS U-Pb dating yields ages of 367.5±1.7Ma and 368.0±1.8Ma for the monzogranite, and 366.3±2.2Ma for the granodiorite, suggesting they were formed in the Late Devonian. (3) The SiO₂ contents from samples collected in the complex are 64.39%~74.95%, with alkali contents of 5.01%~9.21%, suggesting it belongs to metaluminous (A/CNK=0.88~1.00); the measured samples have low P₂O₅ contents (0.02%~0.17%), which are negatively correlated to SiO₂. These features indicated they are of typical I-type granite characteristics. (4) For the REE compositions, this complex is characterized by enriched light rare earth elements ((La/Yb)_N=6.23~23.0), negative Eu anomaly (δEu=0.31~0.55), enriched large ion lithophile elements, such as Rb, Th, U, and depleted high field strength elements such as Nb, Ti, and P. (5) The complex has ε_Hf(t) values of -3.1~+6.0, two stage model ages (t_DM2) of 928~1327Ma, (⁸⁷Sr/⁸⁶Sr)_i values of 0.705607~0.708523, and ε_Nd(t) values of -4.1~-1.9. Combined with the regional geology and tectonic environment, these data reveals that the south Shuangfengshan complex was formed in an active continental margin in Late Devonian, and during its evolution, the lower crust was partially melted by the underplating of mantle-derived magma and mixing with mantle-derived materials, which represented a northward subduction of the Liuyuan Ocean plate in the Late Devonian.

Geochronology and Geochemistry of Felsite Dike from Shanmen Silver Deposit in Central Jilin Province, China, and Its Geological Significance

Shanmen silver deposit is located on the south section of Daheishan horst polymetallic mineralization belts in central Jilin province, Northeast China. The deposit supplies not only the greatest portion of silver in Jilin province, and a significant amount of gold also. The deposit exhibits a closely spatiotemporal relationship with Mesozoic ore-hosting diorite and granite intrusions and Mesozoic basic-felsic dikes. Among the dikes the felsite one occurred at the same fault zone as the silver-bearing veins in the deposit, implying that felsite dike has an important theoretical and ore-prospecting significance. Here, we mainly investigate felsite dike based on zircon LA-ICP-MS U-Pb geochronology, petrochemical compositions. Zircon U-Pb dating from felsite dike yielded a weighted mean age of 163.1± 2.3 Ma, which is slightly younger than the ore-hosted intrusions of quartz diorite (167.6± 1.9 Ma) and monzonite granite (167.0 ± 1.5 Ma), indicating that petrogenetic age of felsite dike is late Jurassic. Major element data suggest that felsite dike is of granodioritic and granitic in chemical composition, belonging to high-K calc-alkaline and shoshonite series, with SiO₂, K₂O+Na₂O, Al₂O₃, TiO₂, CaO and MgO contents ranging from 62.98-79.18%, 4.13-5.3%, 13.31-16.01%, 0.05-0.57%, 0.15-3.07%, and 0.28-1.8%, respectively. Felsite dike is enriched in Rb, U, and Pb, and depleted in Ba, Nb, Sr, Ti and other high field strength elements. REE contents of felsite dike are in the range of 28.42-214.23 ppm, showing weak negative Eu anomalies with Eu/Eu* ratios of 0.24-0.61. Except for the altered felsite sample with SiO₂ content of 79.18%, the distribution characteristics of trace elements and rare earth elements in felsite dike are similar with those of ore-hosted intrusion of granite and diorite. The geochemical features show that late Jurassic felsite dike was formed in the tectonic background of active continental margins associated with subduction in the Paleo-Pacific Plate. The underplating of mantle-derived magma caused the partial melting of the meta-sediment in lower crust materials to form the mid-late Jurassic magmatism and late Jurassic silver mineralization.

Cambro–Ordovician ferrosilicic magmatism along the northern Gondwana margin: constraints from the Cézarenque–Joyeuse gneiss complex (French Massif Central)

It is well-acknowledged that the northern margin of the Gondwana supercontinent was affected by a major magmatic event at late Cambrian (Furongian) to early Ordovician (Tremadocian–Floian) times. However, an accurate assessment of its extent, origin, and significance is partly hampered by the incomplete characterization of the numerous gneiss massifs exposed in the inner part of the Variscan belt, as some of them possibly represent dismembered and deformed Furongian–Lower Ordovician igneous bodies. In this study, we document the case of the “Cézarenque–Joyeuse” gneiss complex in the Cévennes parautochthon domain of the French Massif Central. The gneisses form decametre- to kilometre-thick concordant massifs interlayered within a pluri-kilometric sequence of mica- and quartz schists. They encompass two main petrological types: augen gneisses and albite gneisses, both typified by their blue and engulfed quartz grains with the augen facies differing by the presence of centimetre-sized pseudomorphs after K-feldspar and the local preservation of igneous textures. Whole-rock geochemistry highlights that many gneisses have magmatic ferrosilicic (acidic with anomalously high FeOt and low CaO) compositions while others are akin to greywackes. Collectively, it is inferred that the bulk of the Cézarenque–Joyeuse gneisses represents former rhyodacite lava flows or ignimbrites and associated epiclastic tuffs. Volumetrically subordinate, finer grained, and strongly silicic leucogneisses are interpreted as microgranite dykes originally intrusive within the volcanic edifices. LA–ICP–MS U–Pb dating of magmatic zircon grains extracted from an augen gneiss and a leucogneiss brackets the crystallization age of the silicic magmas between 486.1 ± 5.5 Ma and 483.0 ± 5.5 Ma which unambiguously ties the Cézarenque–Joyeuse gneisses to the Furongian–Lower Ordovician volcanic belt of SW Europe. Inherited zircon date distributions, Ti-in-zircon and zircon saturation thermometry demonstrate that they formed by melting at 750–820 °C of Ediacaran sediments. Zircon Eu/Eu* and Ce/Ce* systematics indicate that the melts were strongly reduced (fO2 probably close to the values expected for the iron–wüstite buffer), possibly because they interacted during ascent with Lower Cambrian black shales. This would have enhanced Fe solubility in the melt phase and may explain the peculiar ferrosilicic signature displayed by many Furongian–Lower Ordovician igneous rocks in the northern Gondwana realm. We infer that crustal melting resulted from a combination of mantle-derived magma underplating in an intracontinental rift setting and anomalously elevated radiogenic heat production within the Ediacaran sedimentary sequences.

Transport of mafic magma through the crust and sedimentary basins: Jameson Land, East Greenland

Igneous sheet-complexes transport magma through the crust, but most studies have focused on single segments of the magma transport system or have low resolution. In the Jameson Land Basin in East Greenland, seismic reflection data and extensive outcrops give unparalleled constraints on mafic intrusions down to 15 km. This dataset shows how sill-complexes develop and how magma is transported from the mantle through sedimentary basins. The feeder zone of the sill-complex is a narrow zone below a basin, where a magmatic underplate body impinges on thinned crust. Magma is transported through the crystalline crust through dykes. Seismic data and published geochemistry indicate that magma is supplied from a magmatic underplate without perceptible storage in crustal magma chambers and crustal assimilation. As magma enters the sedimentary basin, it forms distributed, bowl-shaped sill-complexes throughout the basin. Large magma volumes in sills (4–20 times larger than the Skaergaard Intrusion) and the presence of few dykes highlight the importance of sills in crustal magma transport. On scales smaller than 0.2 km, host-rock lithology, and particularly mudstone tensile strength anisotropy, controls sill architecture in the upper 10 km of the basin, whereas sills are bowl-shaped below the brittle–ductile transition zone. On scales of kilometres and towards basin margins, tectonic stresses and lateral lithological changes dominate architecture of sills. Supplementary material: An uninterpreted and unwarped version of the seismic line in Fig. 4 (DR1), a spreadsheet showing thickness of sills and width of dykes in the study area (DR2), and assumptions and calculations of magma volume (DR3) are available at https://doi.org/10.6084/m9.figshare.c.5670470

Basin modelling of a complex rift system: The Northern Vøring Volcanic Margin case example

AbstractExtensional processes can lead to complex crustal configuration depending on the mechanisms of lithospheric thinning and the impact of magmatic additions during rifting and breakup. In this context, we studied the Vøring volcanic passive margin offshore Norway. The evolution of the inner Vøring Margin is well explained by standard models of lithosphere extension. However, these models fail to reproduce key observations at the outer (volcanic) province such as regional uplift at the time of breakup and excess magmatism. Therefore, additional processes are required to explain these observations. Excess magmatism and uplift have been related to mantle processes such as the arrival of the hot Icelandic mantle ‘plume’ or small‐scale convection processes. Melt retention in the asthenosphere has also been proposed to explain uplift. At last, mantle phase transitions during extension may contribute to uplift. We present tectonic and thermal models of basin evolution along a seismic profile crossing the Northern Vøring Margin. The thermal and isostatic history of basins is constrained through time‐forward basin modelling based on an automated inverse basin reconstruction approach. Two scenarios are evaluated: The first one includes pronounced mantle stretching during the last late Cretaceous‐Paleocene rifting event, and the second one includes late Paleocene‐early Eocene mantle thinning, at the breakup time around 56–54 Ma. Models incorporating late Paleocene‐early Eocene mantle thinning and taking into account magmatic processes (melt retention and magmatic underplate) and mantle phase transitions satisfactorily reproduce the specific observations of the outer (volcanic) margin. This result supports the contribution of the hot Iceland plume on the evolution of the Vøring Margin. Our results also indicate that thin‐crust models can produce a partially serpentinized mantle beneath the highly extended parts of the Vøring Basin. However, this model fails to reproduce observations. This suggests that serpentinization can occur locally but could not explain the entire lower crustal body nature.

Vestiges of underplating and assembly in the central North China Craton based on S-wave velocities

The destruction of the North China Craton (NCC) is a controversial topic among researchers. In particular, the crustal structure associated with the craton’s destruction remains unclear, even though a large number of seismic studies have been carried out in this area. To investigate the crustal structure and its dynamic implications, we perform noise tomography in the central part of the NCC. In this study, continuous vertical-component waveforms spanning one year from 112 broadband seismic stations are used to obtain the group velocity dispersion curves of Rayleigh waves at different periods, and surface wave tomography is employed to extract the Rayleigh wave group velocity distributions at 9–40 s. Finally, the S-wave velocity structure at depths of 0–60 km is determined by the inversion of pure-path dispersion data. The results show obvious differences in the crustal structure among the Western Block (WB), the Trans-North China Orogen (TNCO) and the Eastern Block (EB). The lower crust of the northern part of the EB exhibits a high-velocity S-wave anomaly, which may be related to magmatic underplating in the lower crust induced by an upwelling mantle plume. The S-wave velocity of the WB is lower than that of the TNCO in the upper and middle crust and is lower than that of both the TNCO and the EB in the lower crust. The crust of the TNCO shows higher S-wave velocities than the WB and EB in the upper and middle crust, and its overall S-wave velocity structure is clearly different from those of the WB and EB, implying that the crustal structure of the TNCO may contain vestiges of the Paleoproterozoic collision between the WB and EB and their subsequent assembly. This study marks the first time these findings are identified for the NCC.

Seismic evidence for magmatic underplating and intrusion in Northeast China

Seismic evidence for magmatic underplating and intrusion in Northeast China

Bulk Crustal Properties and Layered Velocity Structure in Fujian, SE China: Constraints From P and S Receiver Functions

Multiple tectonic events since the Neoproterozoic era have framed the present-day lithosphere in the Fujian province affiliated with the eastern part of the South China Block. Comprehensive information of the crustal structure and bulk properties can aid to understand the geological features and tectonic processes of still much debate in this region. An attempt is made in this study to explore crustal thickness and internal velocities across Fujian using the teleseismic receiver functions (RFs). The H-V stacking of joint P and S RFs improves to simultaneously estimate crustal thickness, average Vp and Vs, and derived Vp/Vs ratio and bulk sound speed in three backazimuth sectors for each of 17 stations. Furthermore, a Neighborhood Algorithm nonlinear inversion of P RFs is employed to determine the layered structures of Vs and Vp/Vs beneath all the stations. Results indicate the crustal thickness varies from at most ∼35 km in northwest Fujian to 30–35 km in the inland mountains and 27–30 km in the southeastern coasts. The inferred Moho geometry is nonplanar or inclined across the Zhenghe-Dapu (ZD) and Changle-Zhaoan (CZ) fault zones, especially in the southern ZD fault area. The average Vp/Vs suggests that the crust is predominantly felsic in the Wuyi-Yunkai orogen and intermediate-to-mafic in the Cretaceous magmatic and metamorphic zones. A high-velocity upper crust along the coastline is revealed, which attributes to the Pingtan-Dongshan metamorphic belt. At the sites near the ZD fault zone, the intracrustal negative discontinuity occurs at a shallower depth of ∼15 km marking an abrupt Vs decrease into the low-velocity mid-to-lower crustal layer, probably linked to the closed paleo-rift basin remnants. The lower crust across the Fujian is generally characterized by relatively lower Vs and higher Vp/Vs (1.80–1.84) consistent with those of the mafic-ultramafic rocks, which do not support the proposed extensive magmatic underplating in the Late Mesozoic.

Petrogenesis and tectonic implications of two‐stage Cretaceous bimodal volcanism in the coast of Zhejiang Province, South‐east China

Shipu volcanic area is located at the east of the Jiangshan‐Shaoxing Fault zone, an important part of the Mesozoic volcanic belt in the coastal south‐east China, which provides an excellent opportunity to study the origin of rarely outcropped and coexisting bimodal volcanic rocks and bimodal dyke swarms. Two stages of bimodal volcanic rocks and dyke swarms yield zircon U–Pb ages of ca. 118–121 Ma and ca. 95–93 Ma, respectively. Both bimodal magmatic rocks show enrichment in LILEs and LREEs, but depletion in HFSEs. Basalts of the early‐stage bimodal volcanism have whole‐rock (87Sr/86Sr)i ratio of 0.71121, εNd(t) value of −8.0, and zircon εHf(t) values of −19.3 to −11.7, while the contemporaneous rhyolites show whole‐rock (87Sr/86Sr)i ratios of 0.71023 to 0.71106, εNd(t) values of −7.5 to −6.6 and zircon εHf(t) values of −15.3 to −7.6 with TDMC of 0.96–2.11 Ga. Basaltic dykes of the late‐stage bimodal volcanism have whole‐rock (87Sr/86Sr)i ratios of 0.70898 to 0.70908, εNd(t) values of −5.8 to −5.7 and zircon εHf(t) values of −10.5 to 2.9, while coexisting rhyolitic dykes display whole‐rock (87Sr/86Sr)i ratios of 0.69618 to 0.71172, εNd(t) values of −3.1 to −3.0 and zircon εHf(t) values of −9.2 to 11.9 with TDMC of 0.37–1.71 Ga. These features suggest that the early‐stage basalts could be derived from partial melting of an enriched lithospheric mantle metasomatized by slab‐derived fluids, and its corresponding felsic end‐member is the product of partial melting of Proterozoic middle‐lower crust due to underplating of basaltic magmas with minor addition of mantle materials. As for the late‐stage bimodal volcanism, the basaltic dykes are also derived from an enriched mantle similar to the early‐stage counterpart. However, the rhyolitic dykes mainly originated from partial melting of ancient lower crustal materials with abundant input from the depleted mantle. As a whole, two stages of bimodal magmatism in Shipu area provide direct constraints on regional tectonic transition from an extensional arc setting to a back‐arc setting.