Geodynamic Evolution Research Articles

3D basin modeling of the Lower Saxony Basin, Germany: the role of overpressure in Mesozoic claystones with implications for nuclear waste storage

Abstract Jurassic and Cretaceous organic-lean claystone formations in the Lower Saxony Basin (LSB), Germany, are considered for nuclear waste storage due to their favorable physical properties. Understanding the burial and thermal history of these formations, including overpressure generation and its impact, is crucial for site selection. Past undetected overpressures may result in erroneous estimation of present-day petrophysical properties. Therefore, this study investigates the evolution and spatial distribution of overpressure in claystones in northern Germany on a rather large scale, focusing on Jurassic and Lower Cretaceous units. Utilizing 3D numerical basin modeling, this study: (i) reconstructs the geodynamic evolution of the LSB, (ii) identifies key mechanisms driving overpressure during burial, (iii) detects areas of high pore-to-lithostatic pressure ratios susceptible to fracturing, and (iv) assesses overpressure’s influence on the evolution of petrophysical properties. Results reveal that during the fastest burial phase, overpressure generation began, primarily driven by disequilibrium compaction coupled with gas generation, peaking around 99 Ma at the basin’s depocenter. During the Late Cretaceous uplift, thousands of meters of sediment were eroded within the basin’s depocenter, and overpressure dissipated. According to the model, absolute overpressures were higher for Jurassic than Cretaceous claystones, but dissipation was slower for Cretaceous claystones leading to relatively higher pore-to-lithostatic pressure ratios during uplift. During overpressure buildup, the porosity reduction slowed due to undercompaction effects, underscoring overpressure’s influence on petrophysical properties. While Pleistocene glaciations caused localized overpressure, they did not impact the petrophysical properties of the assessed units. Glacial-induced erosion, however, is projected to reduce remaining overpressure substantially. Graphical abstract

Crustal and Upper Mantle Structure of the Assam Valley Region, NE India: A Review of Geophysical Findings

The northeastern region of India is one of the six most seismically active convergent plate tectonic areas in the world. The north–south convergence along the Indo-Tibetan Himalayan Ranges and the east–west subduction within the Indo-Burma Ranges create a complex stress regime, resulting in significant seismic activity and a history of great/large earthquakes. The region’s intricate strain patterns, active faults, and potential seismic gaps underscore the need for detailed subsurface studies to effectively assess seismic hazards and impending seismicity. Geophysical research is essential for understanding the region’s geodynamic evolution, seismotectonics, and mineral resources. This manuscript reviews the geological and tectonic settings of the region and summarizes recent geophysical studies, including seismic, gravity, magnetic, and magnetotelluric surveys conducted in the Assam Valley and adjacent areas (within latitudes 24.5–28.5° N and longitudes 89–97.5° E). The review highlights key findings on hydrocarbon-bearing sediments, the configuration of the crystalline basement, the heterogeneous structures of the crust and upper mantle, and seismic discontinuities. By synthesizing these results, the review aims to enhance the understanding of seismic hazards in Northeast India, guide mitigation strategies, and identify key knowledge gaps to direct future research efforts.

Integrating gravity data to enhance understanding of northern Türkiye’s geodynamic evolution

Integrating gravity data to enhance understanding of northern Türkiye’s geodynamic evolution

On the use of remote sensing data to study the geological structure and forecast mineral resources of the Shu-Ile suture

Purpose. This article focuses on analyzing the geodynamic processes that underline the formation of the Shu-Ile ore zone. The study is based on the principles of plume tectonics, which have gained widespread recognition in the geological community in recent decades. Methods. The article considers the main aspects of the plume-tectonic concept, including the mechanisms of mantle material flows and their impact on regional geological processes. Special attention is paid to analyzing tectonic and magmatic events associated with forming the Shu-Ile ore zone within plume-tectonic mechanisms. Findings. The study highlights the critical role of mantle plumes in influencing the geodynamic evolution of the Shu-Ile ore zone. Geochemical and geophysical evidence reveals the composition and characteristics of magmatic rocks, demonstrated a direct link between mantle plume processes and ore potential in the region. The findings underscore how mantle-derived material flows have significantly contributed to the formation of geological structures and mineral resources. Originality. The study results offer a new perspective on the geological history of the Shu-Ile ore zone, emphasizing the significance of plume tectonics as a critical factor in the geodynamic processes involved in ore deposit formation. Practical implications. This research is practical for geological studies and forecasting promising ore deposits in this region. It may serve as a basis for further research on plume tectonics and its influence on forming geological structures and mineral resources.

Geodynamic evolution of felsic magmatism in the Yellowknife greenstone belt, N.W.T., Canada: evidence for Archean magmatism in a volatile-saturated environment

Geodynamic evolution of felsic magmatism in the Yellowknife greenstone belt, N.W.T., Canada: evidence for Archean magmatism in a volatile-saturated environment

The Main Geohazards in the Russian Sector of the Arctic Ocean

The Arctic region, including vast shelf zones, has enormous resource and transport potential and is currently key to Russia’s strategic development. This region is promising and attractive for the intensification of global economic activity. When developing this region, it is very important to avoid emergency situations that could result in numerous negative environmental and socio-economic consequences. Therefore, when designing and constructing critical infrastructure facilities in the Arctic, it is necessary to conduct high-quality studies of potential geohazards. This paper reviews and summarizes the scattered information on the main geohazards in the Russian sector of the Arctic Ocean, such as earthquakes, underwater landslides, tsunamis, and focused fluid discharges (gas seeps), and discusses patterns of their spatial distribution and possible relationships with the geodynamic setting of the Arctic region. The study revealed that the main patterns of the mutual distribution of the main geohazards of the Russian sector of the Arctic seas are determined by both the modern geodynamic situation in the region and the history of the geodynamic evolution of the Arctic, namely the formation of the spreading axis and deep-sea basins of the Arctic Ocean. The high probability of the influence of seismotectonic activity on the state of subsea permafrost and massive methane release is emphasized. This review contributes toward better understanding and progress in the zoning of seismic and other geological hazards in the vast Arctic seas of Russia.

Seismic radial anisotropy in southeastern Tibetan Plateau and its implications for regional geodynamic evolution

The southeastern Tibetan Plateau exhibits intricate crustal tectonics, encompassing recent seismic megathrust events. Previous research suggested the presence of north-south-oriented channelized viscous flow within the crust. However, recent investigations have unveiled a notable northeast-southwest-oriented geological structure, potentially rigid, intersecting with the presumed crustal channelized flow. Several questions persist regarding the composition of the northeast-southwest-oriented structure, the continuity of crustal channelized flow, and the interplay between them. In this study, dispersion data from a dense seismic array are employed to significantly refine regional crustal models for shear wave velocity and radial anisotropy through ambient noise tomography. The resulting high-resolution model further reveals the style of the crustal deformation and supports the interpretation that the northeast-southwest structure, which shows higher velocity and significant negative radial anisotropy, results from mafic material at the base of crust, obstructing the crustal channelized flow. However, the northeast-southwest structure is not as rigid as the Sichuan Block and exhibits depth-dependent deformation. The interpretation proves useful in further understanding regional earthquake focal mechanisms and strain distribution. Additionally, this research identified a region of generalized negative radial anisotropy in the crust of the western Chuan-Dian fragment, suggesting a reduced horizontal channel crustal flow in this area. Drawing upon various geophysical and geological evidence, we present a geodynamic evolution model, proposing a sequence of events: Permian plume activity resulting in mafic material at the base of the crust near Anninghe-Zemuhe fault, northward advancement of the east Himalayan syntaxis inducing crustal compressional stress field, reduced lower crustal channel flow in the western Chuan-Dian fragment leading to the regional east-west extension, and initiation of the Xianshuihe fault causing shift of strain concentration and depth-dependent deformation near the Anninghe-Zemuhe fault. The geodynamic model provides valuable insights into the regional distribution of crustal strain and the underlying mechanisms of large seismic events.

Multi-stage tectonic record of the Central Sakarya Terrane Basement (NW Turkey): Implications for the Paleozoic evolution of Rheic Ocean and northern Gondwanan margin

In northwestern Turkey, within the Central Sakarya Terrane, the Söğüt Metamorphics is one of the Variscan tectonic units preserved as a pre-Jurassic basement assemblage of the Sakarya Composite Terrane (SCT). It consists of para- and orthogneisses that host bands, lenses, boudins, and tectonic slices of amphibolites, and are altogether intruded by the Sarıcakaya Granitoid. The characterization and timing of the precursor magmatic events for the protoliths of this assemblage and the timing and conditions of metamorphism have been the topics of a long-lasting debate. Here we present new geochemical and geochronological data from the Söğüt orthogneisses, amphibolites, and the crosscutting Sarıcakaya Granitoid to provide further understanding of the geodynamic evolution of the SCT Basement and the larger-scale tectonic events that controlled its formation. Available geological and geochronological data indicate that the generation of the orthogneiss protoliths began in the Late Cambrian (ca. 485 Ma) and continued until the middle Silurian (ca. 430 Ma), accompanied by the subduction of the Iapetus Ocean and its progressive roll-back beneath northern Gondwana. The slab roll-back caused mantle-upwelling and Ordovician (ca. 465 Ma) continental rift-related mafic magmatism, eventually leading to the opening of the Rheic Ocean. Prior to the Variscan orogeny, a short-lived episode of middle Carboniferous (ca. 336−328 Ma) intra-oceanic arc magmatism occurred within the Rheic Ocean north of the SCT. Accretion of all this material and continuous subduction beneath the SCT led to a two-stage metamorphism during the late Carboniferous (ca. 326−324 Ma and ca. 318 Ma) and synchronous generation of granitic (ca. 327−324) and dioritic (ca. 319−317 Ma) rocks of the Sarıcakaya Granitoid during the Variscan orogeny, terminating the life cycle of the Rheic Ocean.

Unraveling the Geodynamic Evolution of the Pre– and Early–Andean Margin: Insights From Numerical Modeling

AbstractAn outstanding question in the geological evolution of the Chilean Andes is the cause of the westward shift and relocation of magmatism from the High Andes (HA) to the Coastal Cordillera (CC) during the Late Triassic, Pre–Andean stage. The spatiotemporal distribution of Permian–Triassic–Jurassic igneous rocks in northern‐central Chile (20°S–32°S) reveals a significant westward magmatic shift of ∼120 km during the Norian time. Despite diverse proposed models, the precise geodynamic mechanism behind this shift remains unclear. To address this, we used 2D numerical modeling to investigate two contrasting scenarios: (a) subduction rollback and (b) subduction transference/jump and reinitiation by terrane accretion. Our modeling results strongly support Scenario B, where mantle density and the size of the oceanic plateau are crucial for triggering subduction jump and reinitiation. This model aligns with geological and geophysical evidence and offers new insights into unraveling the Pre– and Early–Andean evolution.

Seismotectonic map of the Sinai Triple Junction

The geodynamic evolution of the Sinai Triple Junction, a highly deformed and seismically active area, is controlled by the Red Sea rift, Gulf of Suez and Aqaba-Dead Sea conjunctions. However, the driving forces for the focusing deformation at crustal depths beneath this area are still ambiguous. Here, we provide an updated seismotectonic map of the area relying on updated seismological and geodetic datasets. A homogenized earthquake catalog has been compiled from well-located earthquakes (> Mw 2.0) by the Egyptian Seismic Network and International Seismological Center in the period between 1990 and 2020. We calculated the average b-value along three seismogenic zones including Gulf of Aqaba, northern Red Sea and Gulf of Suez that amount to 1.1, 0.99 and 0.97, respectively. Additionally, we complied and updated a comprehensive P-wave-based database for the fault plane solutions in the area for events with Mw > 3.5 till 2023. Furthermore, a unified velocity field for the region as well as slip-rate and locking-depth at the active fault segments were estimated from a consistent geodetic dataset from peer-reviewed GPS velocities between 1999 and 2018. Results indicate a dominant NNE left-lateral strike-slip fault with normal component along the Gulf of Aqaba. Pure NW-SE to WNW-ESE dip-slip normal faulting, associated with a strike-slip component in some cases, is dominating the northern and central parts of the Gulf of Suez, whereas pure normal dip-slip movement with an NNE–SSW extension in a horizontal direction is observed in the southern part of the gulf. The estimated slip-rate and locking-depths at the Aqaba fault segments falls between 4.8 and 4.9 mm/yr and 8–12 km, respectively.

The Subduction-Related Metavolcanic Rocks of Maroua, Northern Cameroon: New Insights into a Neoproterozoic Continental Arc Along the Northern Margin of the Central African Fold Belt

The metavolcanic rocks around Maroua in the Far North Region of Cameroon are located at the northern margin of the Central African Fold Belt (CAFB) and have not been studied to date. The petrographic and whole-rock geochemical data presented in this paper highlight their magma genesis and geodynamic evolution. The lavas are characterized by basaltic, andesitic, and dacitic compositions and belong to the calc-alkaline medium-K and low-K tholeiite series. The mafic samples are essentially magnesian, while the felsic samples are ferroan. On a chondrite-normalized REE diagram, mafic and felsic rocks display fractionated patterns, with light REE enrichment and heavy REE depletion (LaN/YbN = 1.41–5.38). The felsic samples display a negative Eu anomaly (Eu/Eu* = 0.59–0.87), while the mafic lavas are characterized by a positive Eu anomaly (Eu/Eu* = 1.03–1.35) or an absence thereof. On a primitive mantle-normalized trace element diagram, the majority of the samples exhibit negative Ti and Nb–Ta anomalies (0.08–0.9 and 0.54–0.74, respectively). These characteristic features exhibited by the metavolcanic rocks of Maroua are similar to those of subduction-zone melts. This subduction would have taken place after the convergence between the Congo craton (Adamawa-Yadé domain) and the Saharan craton (Western Cameroonian domain). Petrological modelling using major and trace elements suggests a derivation of the Maroua volcanics from primitive parental melts generated by the 5–10% partial melting of a source containing garnet peridotite, probably generated during the interaction between the subducted continental crust and the lithospheric mantle and evolved chemically through fractional crystallization and assimilation.

Geochemical study of Ambaji − Sendra granitoids and mafic dykes from the South Delhi fold belt, NW Indian shield: Implications for magma generation, emplacement and geodynamic evolution

This study presents a comprehensive account of geochemical data on the granitoids of Ambaji-Sendra Terrain and associated mafic dykes. These granitoids, interpreted as S–type, I-type and A-type, lack a consensual interpretation probably due to the study of only those rocks which are exposed in Sendra region. We identified that these granitoids are A-type (A2- subtype) granites. There are two petrogenetic groups of the granitoids having exclusive crustal and crust-mantle interactive origins which are supported by their mineralogy and structure. The associated dykes also display two distinct geochemical groups one with within-plate character and other with typical arc signatures. Whole body of evidence suggests that magma for the granitoids and dykes was generated during the closure of Delhi Ocean. The simultaneous emplacement of granitoids started during subduction process and culminated after the collision or/ accretion of Delhi arc with eastern crustal block, consequent to the Rodinian assembly mechanism.

Evidence of an Archean-Paleoproterozoic micro-continent remobilized during the Neoproterozoic from the western Hoggar (Tuareg Shield, Algeria): Geodynamic evolution and implication on gold mineralization

The western boundary of the Tirek terrane in the Hoggar contains the two largest gold deposits in Algeria, namely the Amesmessa and Tirek gold fields. New geological mapping of this area and results of a combined petrological, geochemical and geochronological study (U-Pb zircon geochronological (LA-)ICP-MS method) reveals a Paleoproterozoic TTG basement, dated at 1965 ± 18 Ma (inherited zircons from 2771 to 2006 Ma), overlain by a supracrustal unit made up of quartzites and metapelites. Detrital zircons from metapelites yielded ages ranging from Archaean to Paleoproterozoic (2710 Ma–2050 Ma). Furthermore, sills and dykes of sub-alkaline orthogneisses that intrude the previous lithologies have been dated at 1843 ± 19 Ma. On the other hand, the migmatitic granitic-granodioritic batholith, which covers 50 % of the studied area and intrudes all units, has a subduction magmatism affinity and is dated at 663 ± 4 Ma.The study area was affected by a HT-LP metamorphic overprint during a clockwise P-T evolution with a peak conditions around 800–850 °C at 5 kbar followed by a decompression, then a cooling reaching the P-T conditions of 600 °C and 3 kbar. Uranium-Pb dating of monazite yields an age of 578 ± 5 Ma, which is interpreted to reflect the time of peak metamorphic overprint synchronous with the emplacement of synkinematic granites along shear zones with transcurrent movement. During this event, the western border of the studied area, the East Ouzzalian Shear zone, was the site of magmatic intrusions as well as the circulation of various fluids. This promoted an important fluid/rock interaction event, leading to extraction of gold by fluid percolation through the rocks and its deposition along N-S/NE-SW quartz vein ore shoots. The comparison with the western terranes of the Tuareg Shield (In Ouzzal, Iforas, Tassendjanet, and Kidal terranes) suggests that Tirek and these terranes represent relics of a single Archean-Paleoproterozoic continent. Currently, the Archean lower crust cross out in the central part of this micro-continent. The latter is mainly affected by Paleoproterozoic granulite facies metamorphism, unlike the Paleoproterozoic middle crust, which is exposed on either side and was strongly remobilized during the Pan-African orogeny (850–570 Ma), allowing gold mineralization during the later stages of this event.

Geodynamic evolution of the Еastern part of the Baltic shield against the background of supercontinental cyclicity

Geodynamic evolution of the Еastern part of the Baltic shield against the background of supercontinental cyclicity

Phase equilibrium constraints on the pre-eruptive conditions of alkaline basalts of the Main Ethiopian Rift and their bearing on the production of peralkaline rhyolites

Abstract Bimodal magmatism is characteristic of the geodynamic evolution of the Main Ethiopian Rift (MER), which is a reference area for the study of the processes leading to continental break-up before seafloor spreading. There are abundant emissions of basalts and rhyolites, which are in possible parent-daugther relationships. However, the P-T-H2O conditions of production and storage of the basaltic end member remain unclear. Crystallization experiments have been conducted on an alkali basalt from the MER to define its pre-eruptive conditions and shed light on the compositional evolution of derivative liquids and source conditions. The experiments were performed at 100-200 MPa, 975-1080°C, varying H2O/CO2 ratios, corresponding to melt water contents of 1-5 wt%, at fO2 slightly lower than the Fayalite-Magnetite-Quartz (FMQ) solid buffer. Comparison between the petrological attributes of the starting rock and the experiments shows that the basaltic magma was stored at 150-200 MPa, 1050 ± 10°C, with 1-2 wt% H2O in melt, with fO2 near FMQ prior to eruption. Geochemical modelling shows that the corresponding mantle source contained about 0.1 wt% H2O, reflecting a metasomatized source. Extensive crystallization of such basalts produces SiO2-rich liquids, which are not yet peralkaline, however. This underscores that extreme fractionation (>90 wt%) is required in order to produce peralkaline derivatives from mildly alkaline basalts. This extreme fractionation and the water-rich nature of the starting basalt readily explain the H2O-rich condition of peralkaline rhyolites that have fueled caldera forming eruptions in the Rift.

Geochemistry and Zircon U–Pb Chronology of Jinchanshan Gold-Hosted Granitoids, Inner Mongolia: Implications for Petrogenesis and Geodynamic Evolution

Jinchanshan is a medium-sized, granitoid-hosted gold deposit located in the Kalaqin area of Inner Mongolia. Mineralization predominantly occurs in the contact zone between biotite granites and quartz porphyry rocks, associated with the Jinchanshan minor intrusion, suggesting a genetic link to the granitoid-hosted gold deposit. In this study, the petrography, geochemistry, and LA-ICP-MS zircon U–Pb chronology of these two granitoid samples were studied. The results indicate that the zircon U–Pb age of the biotite granites is 127.9 ± 3.0 Ma, while that of the quartz porphyry is 121.4 ± 1.5 Ma, both dating back to the Early Cretaceous. The average SiO2 content of the granites is 66.64%, and the rocks have high total alkali (K2O + Na2O) content, averaging 9.13%. The average K2O content is 4.39%, with a K2O/Na2O ratio of 0.93. The quartz porphyry rocks are enriched in SiO2 (74.41%–76.85%) and have high Na2O + K2O content (8.67%–9.59%), but are low in MgO (0.03%–0.09%), CaO (0.44%–1.02 %), and TiO2 (0.08%–0.12%). Most samples of the biotite granite and the quartz porphyry rocks exhibit high-K peraluminous and medium-K calc-alkaline characteristics, respectively. Both rock types are enriched in Rb, Th, U, K, Zr, Hf, and Gd and relatively depleted in Ba, Sr, P, Ti, Nb, Ta, and Eu, with a pronounced negative Eu anomaly. The biotite granites show high ∑LREE/∑HREE ratios (6.1–6.9), while the quartz porphyry rocks exhibit lower ratios (2.0–4.2). Both granitoid types have elevated FeOT content and FeOT/(FeOT + MgO) ratios, indicating that the Jinchanshan granitoids are A-type granites. The zircon U–Pb ages, combined with the regional tectonic settings, suggest that these granitoids formed during large-scale metallogenic events in the Early Cretaceous, within the Yanshanian post-orogenic extensional tectonic regime. This is consistent with the lithospheric thinning and extensional processes in Eastern China during this period.

Do gravity data justify a rifted “Liguro-Provençal Basin”?

The geodynamic evolution of the Liguro-Provençal Basin and its crust and upper mantle structure remain debated, especially regarding the role of rifting in continental break-up and seafloor spreading. Our study incorporates updated datasets, including new gravity maps from the AlpArray Gravity Working Group (complete Bouguer, free air, and isostatic anomalies) for 3D modeling and gravity field analysis, seismic data from Lobster offshore campaigns for direct comparison, and geodynamic models, supplemented by seismic profiles from previous French and Italian campaigns to constrain the interpretation. We used GFZ’s IGMAS + software for interactive 3D modeling, creating a density model extending to 300 km depth that includes crustal and upper mantle inhomogeneities based on prior geodynamic models. This hybrid approach, with polygonal structures for the crust and voxels for the upper mantle, clarifies individual contributions to the gravity field. Extending initial gravity modeling from the SPP MB4D project INTEGRATE, our work provides a consistent 3D density model for the Alps and Ligurian Basin. The constrained 3D modeling and numerical analyses (terracing, clustering, filtering, curvature), along with vertical stress and gravitational potential energy calculations, suggest that rifting has significantly influenced the basin’s geological evolution.

Basin–Orogen Coupling‐Driven Meso‐Cenozoic Deformation Along the Southern Margin of the Junggar Basin, NW China: Insights From Integrated Multidisciplinary Analysis

ABSTRACTThe southern margin of the Junggar Basin (SMJB) represents the typical intra‐continental basin–orogen coupling structure of the Central Asian Orogenic Belt and is a key area to study the deformation mechanisms and the geodynamic evolution processes of the North Tianshan Orogen. Herein, we compiled data from boreholes, gravity and magnetism, seismicity‐ and magnetotellurics‐derived geological profiles and field data to recover the sedimentary history of the SMJB and discuss the intra‐continental deformation driven by the basin–orogen coupling mechanism. The results show that the sedimentary and tectonic evolution of the SMJB were both profoundly controlled by the intra‐continental orogeny of the North Tianshan Orogen and its coupling with the Junggar Basin during the Meso‐Cenozoic period. Consequently, the SMJB is dominated by thick‐skinned thrust nappes accompanied with strike‐slip faulting and thrusting. The foreland thrust belt of the SMJB is characterised by three structural belts, from south to north, including the basement‐involved thrust belt, the cover‐detached foreland thrust‐fold belt and the thrusted foreland basement uplifts, respectively. Meanwhile, as indicated by the geometry of the basement thrusts, the thickness of décollements and the structure of the foreland basement, the stress field in the SMJB is obviously stronger in the west and weaker in the east. The sedimentation and deformation migrated northwards into the basin area in a stepwise process, that corresponded to the pace of the overthrusting of the North Tianshan Orogen onto the Junggar Block. Intense regional compression induced the rapid uplifting of the North Tianshan and the significant crustal shortening of the Junggar Block, driving the three structural belts to form accordingly during the Late Jurassic to the Neogene. There are at least one or two décollements within the SMJB, representing one of the main features of basin–orogen coupling structure in most cases. The décollement of some layers represents the decoupling of the sedimentary cover with the basement, which helps to accommodate the lateral crustal shortening of the SMJB by a translation into vertical uplift. As a result, the detached foreland thrust‐fold belts in the shallow level of the upper crust overthrusted upon the basement‐involved nappes of the mountain's side, forming the opposite thrust system. Coevally, the basement of the basin, in the deep level of the upper crust keeps underthrusting beneath the North Tianshan Orogen, forming a typical crocodile mouth‐like structure. In general, both the shallow and deep deformation in the SMJB have been formed by the intense intra‐continental compression during the Meso‐Cenozoic, which were driven by the basin–orogen coupling mechanism.

Origin of Helium and Associated Fluid in Fault‐Related Hydrothermal Systems of the Eastern Pyrenees

ABSTRACTIntense hydrothermal activity hosted by regional tectonic structures occurs in the Eastern part of the Pyrenees. Helium isotope ratios (3He/4He) in hot springs along the Têt (0.033–0.099 Ra) and the Tech (0.171–0.375 Ra) faults indicate different signatures from purely crustal to slightly contaminated by magmatic helium. 3He/4He ratio increased towards the East, consistently with the observed thinning of the continental crust. These results suggest localised and not interconnected hydrothermal systems at fault scale. The origin of magmatic helium contamination is discussed in the light of the Gulf of Lion geodynamic evolution and data from comparable orogenic hydrothermal systems in the Alps.

Unravelling different mechanisms of metasomatism and geodynamic evolution of pyroxenite-veined subcontinental lithospheric mantle beneath the Central European Variscides

Abstract Pyroxenite-veined garnet peridotites from the Gföhl Unit of the Moldanubian Zone in the Bohemian Massif provide direct constraints on diverse mechanisms of mantle metasomatism and refertilization driven by a single pulse of melt beneath the Central European Variscides. Here, we provide a detailed study on an intriguing example of this rock association where the garnet peridotites show a fertile character (high Al2O3, CaO, TiO2), corresponding to the subcontinental lithospheric mantle (SCLM). By contrast, their conspicuous LREE depletion and Sr–Nd isotopic signatures (87Sr/86Sr338 ≤ 0.7028; εNd338 ~7.3) are typical of depleted mantle residue after melt extraction. Such signatures reflect transformation of an original refractory protolith (likely harzburgite) to fertile lherzolite through percolation of primitive tholeiitic melts, parental to garnet pyroxenite in veins. The SCLM refertilization is further documented by the whole-rock positive correlation between incompatible elements (Zr, Yb, Sc, V), and trace element composition of clinopyroxene (high Ti/Eu and Ti/Nb) and garnet (elevated ∑REE, Zr, Ti). Trace element and Sr–Nd isotopic systematics of pyroxenites (87Sr/86Sr338 ~0.7025–0.7029; εNd338 ≤ 7.9) correspond to a source of melt similar to the depleted MORB mantle (DMM). Three mechanisms of metasomatism related to the interaction of this melt with the host peridotites were distinguished: (i) stealth metasomatism, reflected by extensive clinopyroxene and garnet crystallization in lherzolite adjacent to pyroxenite veins, (ii) cryptic metasomatism, recorded by lower Mg# values of orthopyroxene and olivine in lherzolite, and (iii) modal metasomatism, resulting in crystallization of amphibole and phlogopite in lherzolite close to the veins. The percolating basaltic melt was hydrous, moderately enriched in fluid-mobile elements (Cs, Rb, Ba, Pb, U, Li). Immiscible liquids, dense Ti-Mg-Fe-rich oxide melt and C-O-H fluid, trapped and crystallized as mono/multiphase solid inclusions in garnet, likely separated from a basaltic melt upon cooling. The lherzolite-pyroxenite interface reveals strong micro-scale element fractionation due to differentiation of a basaltic melt within the percolation channel. Volatile-bearing liquids that segregated from the melts migrating through wall-rock peridotites most likely caused chromatographic enrichment in highly incompatible elements (e.g. LREE) in distal peridotites relative to the LREE-depleted lherzolites adjacent to the veins. The DMM-like affinity of pyroxenites and pressure-temperature estimates for lherzolite (3.9–5.4 GPa/1010–1200 °C) and pyroxenites (2.8–4.2 GPa/860–1020 °C) point towards exhumation-driven SCLM refertilization. This was linked to decompression-induced partial melting of upwelling asthenosphere producing basaltic melts penetrating through and metasomatizing the SCLM beneath the Variscan orogenic belt in Central Europe.