Search Results (21)

The Kumishi ophiolitic mélange contains well-preserved large-scale serpentinites and their accompanying granulites in the eastern South Tianshan Accretionary Complex (STAC), southwestern Altaids. Previous studies have mainly focused on the thermodynamic conditions and tectonic setting of granulites. However, the petrogenesis of the widespread serpentinites in the Kumishi ophiolitic mélange remains largely unexplored. In this paper, petrological, geochemical, and geochronological studies were carried out on the Kumishi serpentinites, as well as the host sediment and intermediate–felsic volcanic rocks. The serpentinites show variable LOI values of 8.3–16.5 wt% and relatively consistent SiO₂/(sum oxides) ratios of 0.81, which demonstrate that the major elements of their protoliths have been preserved well during serpentinization. Multi-trace element and REE diagrams suggest that the protoliths of the Kumishi serpentinites have experienced varying degrees of refertilization, with distinct natures seen between the Yushugou, Tonghuashan, and Liuhuangshan serpentinites. Zircon U-Pb chronology of the Tonghuahsan serpentinites yields a mean age of 355.8 ± 7.3 Ma (MSWD = 1.0, N = 26). Detrital zircons from the host sediment record a maximum depositional age of 375 ± 10 Ma (MSWD = 0.4, N = 3), with a peak at ca.419 Ma. Subduction-related volcanic rocks yield ages of ca.437 Ma. Hence, clues are provided to the petrogenesis of the Kumishi serpentinites, with calls for future in-depth works from an isotopic perspective. Full article

Petrographic studies have been carried out on the Early Miocene Darnó Conglomerate Formation, which consists only of debris of ophiolitic mélange and is found today on Darnó Hill in SW Bükk, NE Hungary. The studied sediments are bounded by the Darnó line from Darnó Hill. The aim of this work was to show if it is possible to reconstruct the petrographic composition of the source area only from its debris. The rock types were determined in thin sections using a polarizing microscope, and a quantitative analysis of the different rock types was carried out using the grain counting method, the results of which were interpreted as volume ratios. The main rock types observed in the studied samples (textural varieties of basalt, dolerite/microgabbro, claystone, siltstone, and radiolarite) are similar to the rock types of the mélange assemblage of Darnó Hill. Based on the volume calculations of basaltic detrital grains with different textures characteristic for pillow basalts, it could be established that pillow basalts dominated the igneous rocks in the source area of the Darnó Conglomerate on Darnó Hill already in the Miocene. Thus, this work shows that the lithological composition of a source area can be precisely outlined by a detailed petrographic analysis of the debris eroded from the immediate vicinity. Full article

Here, we present a petrographic and microanalytical study of microinclusions in chromite from podiform chromitites hosted by the Sartohay ophiolitic mélange in west Junggar, northwestern China, to investigate the parental magma evolution and chromitite genesis. These silicate inclusions comprise olivine, enstatite, diopside, amphibole, and Na-phlogopite. Their morphological characteristics suggest that most inclusions crystallized directly from the captured melt, with a few anhydrous inclusions (olivines and pyroxenes) as solid silicates trapped during the chromite crystallization. Equilibrium pressure–temperature conditions of coexisting enstatite–diopside inclusions are 8.0–21.6 kbar, and 874–1048 °C. The high Na₂O and TiO₂ contents of hydrous minerals indicate that the parental magma of chromitites was hydrous and enriched in Mg, Na, Ca, and Ti. The calculated Al₂O₃ content and FeO/MgO ratio of the parental melts in equilibrium with chromite showed MORB affinity. However, the TiO₂ values of parental melts, TiO₂ contents of chromite, and estimated fO₂ values for chromitites (1.3–2.0 log units above the FMQ buffer) evoked parental MORB-like tholeiitic melts. The composition of olivine inclusion was determined, and it was revealed that the primary melts of the Sartohay podiform chromitites had MgO contents of ~22.7 wt %. This aligns with the observed high magnesian signature in mineral inclusions (Fo = 96–98 in olivine, Mg^# = 0.91–0.97 in diopside, and Mg^# = 0.92–0.97 in enstatite). We propose that Sartohay podiform chromitites initially formed through the mixing/mingling of primary hydrous Mg-rich melt and the evolved MORB-like melt derived from the melt–peridotite reaction in the upper mantle. In this process, the continuous crystallization of chromite captured micro-silicate mineral inclusions, finally leading to the formation of the Sartohay podiform chromitites. Full article

Beryl mineralization in the Nugrus-Sikait domain in the South Eastern Desert (SED) of Egypt occurs as disseminated crystals in granitic pegmatite and quartz, as well as pegmatite veins crosscutting mélange schist and ophiolitic rocks. When granitic pegmatite comes into contact with the ophiolitic rocks, phlogopite and amphibole schists are formed due to K metasomatism. The ophiolitic mélange is intruded by leucogranite and related pegmatite along the NNW to NW Nugrus shear zone. Beryl samples have been collected from Um Sleimat, Madinat Nugrus, Wadi Abu Rusheid, and Wadi Sikait. Major oxides and in situ trace and rare earth elements (REEs) of beryl and associated minerals were analyzed through EPMA and LA-ICP-MS, respectively. The investigated beryl, based on its color and chemical compositions, can be classified into the two following types: pegmatitic beryl (type I) and schist-related beryl (type II). The former is colorless to pale green, and is mainly restricted in pegmatite veins; it is poor in Cr₂O₃ (up to 0.03 wt%) and MgO (Nil). The latter, deep green in color, is rich in Cr₂O₃ (up to 0.27 wt%) and MgO (up to 2.71 wt%), and occurs within quartz veins, phlogopite schists, and tremolite schists. The abundant beryl mineralization in phlogopite schists and their related quartz veins suggests that granite and associated pegmatite are the source rocks for the Be-bearing fluids that migrate along the NW-SE trending deep-seated tectonic zone, such as the Nugrus shear zone. Therefore, the formation of beryl in schists is attributed to the interaction of granitic/pegmatitic-derived Be-bearing fluids with serpentinite and gabbro interlayered with mélange schists. Variations in the trace and REE contents of both beryl types (I and II) indicate their two-stage formation from different compositions of Be-rich fluids, where light REEs, Zr, Nb, Ba, and Th decrease from type I beryl to type II. These two phases of beryl could be attributed to the magmatic/hydrothermal fluids associated with the pegmatite emplacement. The early phase of the late-stage magmatic-derived fluids was closely related to magma evolution and pegmatite formation, forming euhedral type I beryl. The late phase of pegmatite-derived fluids was mixed with serpentinite/schist-derived fluids that cause high V and Cr content in type II beryl. The composition of parent magmas of felsic rocks, the high degree of magma fractionation or the late stage melts, fluid compositions (rich in Be, Li, Cs, Rb, K), and alkali metasomatism, as well as the linear NW-SE trending deep-seated shear zone, are all factors possibly influencing beryl mineralization in the SED of Egypt. Full article

In this paper, the Hellenic orogenic belt’s main geological structure and architecture of deformation are presented in an attempt to achive a better interpretation of its geotectonic evolution during Alpine orogeny. This study was based not only on recent research that I and my collaborators conducted on the deformational history of the Hellenides but also on more modern views published by other colleagues concerning the Alpine geotectonic reconstruction of the Hellenides. The structural evolution started during the Permo–Triassic time with the continental breaking of the supercontinent Pangea and the birth of the Neotethyan ocean realm. Bimodal magmatism and A-type granitoid intrusions accompanied the initial stages of continental rifting, followed by Triassic–Jurassic multiphase shallow- and deep-water sediment deposition on both formed continental margins. These margins were the Apulian margin, containing Pelagonia in the western part of the Neotethyan Ocean, and the European margin, containing continental parts of the Serbo-Macedonian and Rhodope massifs in the eastern part of the Neotethyan ocean. Deformation and metamorphism are recorded in six main deformational stages from the Early–Middle Jurassic to the present day, beginning with Early–Middle Jurassic Neotethyan intra-oceanic subduction and ensimatic island arc magmatism, as well as the formation of a suprasubduction oceanic lithosphere. Compression, nappe stacking, calc-alkaline magmatism, and high-pressure metamorphic events related to subduction processes alternated successively over time with extension, orogenic collapse, medium- to high-temperature metamorphism, adakitic and calc-alkaline magmatism, and partial migmatization related to the uplift and exhumation of deep crustal levels as tectonic windows or metamorphic core complexes. A S- to SW-ward migration of dynamic peer compression vs. extension is recognized during the Tertiary Alpine orogenic stages in the Hellenides. It is suggested that all ophiolite belts in the Hellenides originated from a single source, and this was the Neotethyan Meliata/Maliac-Axios/Vardar ocean basin, parts of which obducted during the Mid–Late Jurassic on both continental margins, Apulian (containing Pelagonia) and European (containing units of the Serbo-Macedonian/Rhodope nappe stack), W-SW-ward and E-NE-ward, respectively. In this case, the ophiolite nappes should be considered far-traveled nappes on the continental parts of the Hellenides associated with the deposition of Middle–Late Jurassic ophiolitic mélanges in basins at the front of the adjacent ophiolite thrust sheets. The upper limit of the ophiolite emplacement are the Mid–Upper Jurassic time(Callovian–Oxfordian), as shown by the deposition of the Kimmeridgian–Tithonian Upper Jurassic sedimentary carbonate series on the top of the obducted ophiolite nappes. The lowermost Rhodope Pangaion unit is regarded as a continuation of the marginal part of the Apulian Plate (External Hellenides) which was underthrust during the Paleocene–Eocene time below the unified Sidironero–Kerdylia unit and the Pelagonian nappe, following the Paleocene–Eocene subduction and closure of a small ocean basin in the west of Pelagonia (the Pindos–Cyclades ocean basin). It preceded the Late Cretaceous subduction of the Axios/Vardar ocean remnants below the European continental margin and the final closure of the Axios/Vardar ocean during the Paleocene–Eocene time, which was associated with the overthrusting of the European origins Vertiskos–Kimi nappe on the Sidironero–Kerdylia nappe and, subsequently, the final collision of the European margin and the Pelagonian fragment. Subsequently, during a synorogenic Oligocene–Miocene extension associated with compression and new subduction processes at the more external orogenic parts, the Olympos–Ossa widow and the Cyclades, together with the lower-most Rhodope Pangaion unit, were exhumed as metamorphic core complexes. Full article

The properties of ancient magmatic arcs are crucial for understanding the tectonic evolution of the Central Asian Orogenic Belt. The Middle Devonian Kulumudi Formation in the Laofengkou area of West Junggar lacks accurate chronological data constraints, which hampers the knowledge of the nature of the Late Paleozoic magmatic arcs in the West Junggar and circum-Balkhash areas. In this contribution, samples of pyroclastic rocks and sedimentary rocks were collected from the volcano–sedimentary strata of the Kulumudi Formation. Petrography, zircon U-Pb-Hf isotopic analysis and whole-rock geochemistry were carried out to constrain the age and the tectonic setting of the Kulumudi Formation. The zircon U-Pb age of the lithic crystal tuff from the Kulumudi Formation on the northeast side of the Alemale Mountains was 386 ± 2 Ma, accurately indicating that this rock unit formed during the Middle Devonian. However, the fine sandstone near the Huojierte Mongolian Township, originally assigned as the “Kulumudi Formation”, yielded a maximum depositional age of 341 ± 3 Ma. Combined with the stratigraphic contact, this rock unit was redefined to belong to the Lower Carboniferous Jiangbasitao Formation. According to the whole-rock geochemistry study, the lithic crystal tuff of the Kulumudi Formation was characterized as medium potassium–calc–alkaline series rock, which is relatively enriched in light rare earth elements and large ion lithophile elements (i.e., Rb, Ba, K) and depleted in high-field-strength elements (i.e., Nb, Ta, Ti), showing similar geochemical characteristics to the volcanic arc rocks. By contrast, the fine sandstone from the Jiangbasitao Formation had Al₂O₃/SiO₂ (0.25–0.29) and K₂O/Na₂O (1.29–1.72) ratios close to those derived from the continental arc and active continental margin and was characterized as part of the continental arc field in the La-Th-Sc and Th-Sc-Zr/10 tectonic discrimination diagrams. Zircon Hf isotope analysis showed that the ε_Hf(t) values of the Kulumudi Formation were +5.6–+12.8, and those of the Jiangbasitao Formation were +11.43–+15.48, both of which show highly positive juvenile characteristics. The above data indicate that the Kulumudi Formation and Jiangbasitao Formation both formed in a juvenile arc setting with ocean–continent subduction. Combined with the previous work, it was concluded that the southward subduction of the ocean basin represented by the Darbut–Karamay ophiolitic mélanges beneath the newly accreted arc crustal segments produced a juvenile arc with positive Hf isotope characteristics. Full article

The ophiolites in the Beishan Orogenic Belt provide important information about the evolution of the Beishan Ocean in the Paleozoic Era. We studied ophiolite petrology, geochemistry and isotopic chronology. The Shazouquan ophiolites consist of dunites, wehrlites, gabbros and anorthosites. Ophiolitic mélange belts are composed of matrixes and blocks, and different rocks are fault-bounded. Dunites and wehrlites are high in Mg^#, Cr^# and MgO, low in TiO₂, relatively depleted in large-ion lithophile elements (Ti and P) and enriched in high-strength elements (U, Zr and Hf). They have a total REE of 1.25 × 10^–6−5.39 × 10⁻⁶ and δEu of 1.12–3.54, which are similar to those of SSZ-type ophiolites, indicating that their parent magma source region may be a weakly depleted mantle source region. The anorthosite and gabbro are high in Al₂O₃, MgO and Mg^#, low in TiO₂, enriched in large-ion lithophile elements (Rb and Sr), and depleted in high-strength elements (Nb, Ta and Ti), but enriched in Zr and Hf. They have similar geochemical signatures to those of arc magmatic rocks. They are derived from the mantle peridotite formed against the tectonic background of subduction and modified by the fluid materials in the subduction zone. We collected anorthosite and gabbro, which were produced as ophiolite for U-Pb dating. The anorthosite yields a zircon U-Pb, aged 394 ± 11 Ma (MSWD = 0.84), and a gabbro zircon U-Pb, aged 466 ± 12 Ma (MSWD = 3.2), indicating that the Shazouquan ophiolite was formed in the Middle Ordovician–Early Devonian eras. Combining the above evidence, we conclude that the Beishan Ocean was in a subduction tectonic background from the Middle Ordovician to Early Devonian periods. Full article

The Fawakhir area consists of an ophiolite sequence surrounded by an ophiolitic mélange. In the mélange, serpentinized ultramafic rock, gabbro, gabbroic diorite, diabase, andesite, and basalt occur as tectonic blocks within the metasediments. The gabbro gives a zircon U–Pb age of ~816 Ma, and the trace element composition of the zircon suggests its generation under a continental-arc tectonic setting. The geochemistry of gabbro and other tectonic blocks in the ophiolitic mélange indicates their formation from a backarc basin in a continental island arc tectonic setting. The ophiolite sequence consists of serpentinized ultramafic rock, gabbro, and basaltic rocks and was intruded by felsic dikes. The gabbro from the ophiolite sequence and felsic dikes give zircon U–Pb ages of 742 Ma and 723 Ma, respectively. Trace elements composition of this zircon refers to their formation in a continental-arc tectonic setting. The geochemistry of rocks in the ophiolitic sequence indicates their formation in a forearc basin. Together with previous studies, this study suggests that the tectonic blocks in the mélange formed in a backarc during the early-stage northwards subduction event, which may have started at ~816 Ma or earlier. On the other hand, the rocks in the ophiolite sequence can be considered to have formed in a forearc by the later eastwards subduction event at ~742–723 Ma. Full article

Abundant hydrocarbon resources were discovered in the Xiazijie fan-delta in the Triassic Baikouquan Formation in Mahu sag, Junggar Basin. However, the maximum depositional age of Baikouquan Formation and provenance of this fan-delta are still unclear, which would be unfavourable for further hydrocarbon exploration. In this study, we used detrital zircon U-Pb dating and composition statistics of conglomerate clast and sandstone grain from Baikouquan Formation to constrain the maximum depositional age and provenance of the Xiazijie fan-delta. The results showed that (1) the conglomerate clast compositions of Xiazijie fan-delta mainly consisted of tuff and intermediate-felsic magmatic rocks, and sandstone samples could be classified as litharenite type with the lithic fragments were almost entirely volcanic lithic fragments; (2) the average Qt:F:L values of sandstone samples (M152-S1 and M152-S2) were 26:7:67 and 21:8:71, respectively, and they plotted in the magmatic arc domain in the Qt-F-L ternary diagram, indicating the tectonic setting of source area of Xiazijie fan-delta was magmatic arc; (3) M152-S1 yielded U-Pb ages ranging from 417 Ma to 253 Ma, with a dominant age peak at 313 Ma and two secondary age peaks at 411 Ma and 268 Ma, respectively, while M152-S2 yielded U-Pb ages ranging from 467 Ma to 256 Ma, with a dominant age peak at 307 Ma and two secondary date peaks at 405 Ma and 262 Ma; (4) the mean age of youngest two zircon grains of M152-S1 was 254.8 ± 4.7 Ma, while that of M152-S2 was 257.6 ± 3.8 Ma, suggesting the Baikouquan Formation might be deposited after the Changhsingian to Olenekian; (5) the magmatic rock ages of central West Junggar were distributed mostly between 450–260 Ma, with a dominant age peak at 307 Ma. The ages distribution between magmatic rock of central WJ and detrital zircons of M152-S1 and M152-S2 were similar, indicating the central WJ domain should be the major source area of the Xiazijie fan-delta; (6) the magmatic rock of Hakedun–Hongguleleng area in the Central WJ was characterized by a peak age at 305 Ma, which was consistent with the peak ages of M152-S1 and M152-S2, indicating the Hakedun-Hongguleleng area was likely to be their major source area; and (7) one minor peak age at 411 Ma and another at 405 Ma were obtained from M152-S1 and M152-S2, respectively, and a zircon grain with Middle Ordovician age at 467 Ma was obtained from M152-S2, indicating Late Silurian–Early Devonian Chagankule pluton in the Saier Mountain and Ordovician Honggleleng ophiolite mélange in the Sharburt mountain were the minor source areas. This research has significant implications for stratigraphic correlation in Junggar Basin and hydrocarbon exploration in the Xiazijie fan-delta conglomerate reservoir. Full article

The Darbut ophiolitic mélange is located in the central West Junggar area, southwestern Central Asian Orogenic Belt (CAOB), and rodingites are widespread within serpentinized peridotites in the mélange. Here, we conducted field, structural, mineralogical, and geochemical investigations of the Darbut rodingites for the first time to constrain their metasomatic processes. Rodingites usually occur as strongly sheared blocks surrounded by chloritic blackwall, and their preferred axial surface orientations are subparallel to the serpentinite foliations. Based on the petrology and geochemistry of these metasomatic rocks, two stages of metasomatic processes, namely rodingitization and derodingitization, were recognized: (1) rodingitization of gabbroic protolith was characterized by the input of Ca and the release of Si, K, Na, and LILE; this stage was related to the diapiric emplacement of the Darbut ophiolitic mélange in the Late Carboniferous; and (2) derodingitization of rodingites led to the replacement of Ca-rich minerals by chlorite, accompanied by Mg increase, and depletions of Ca and REE; the derodingitization stage occurred under enhanced CO₂/H₂O ratio conditions and was likely associated with regional postcollision volcanism in the Early Permian. Hence, the rodingite in the Darbut ophiolitic mélange provides important fingerprints recording the tectonic evolution. Full article

The Shiquanhe–Laguoco–Yongzhu–Jiali ophiolite mélange belt in Tibet includes the Laguoco Tso ophiolite as part of its western segment. Researchers studying the evolution of the Tibetan Plateau and the Tethys have taken a keen interest in the debated relationship between this ophiolite belt and the Bangong–Nujiang ophiolite belt. However, there is little research in this field. This study reports the rock geochemistry, SIMS (the secondary ion mass spectrometry) zircon U-Pb ages, and O isotope compositions of gabbro from the Laguoco ophiolite. Gabbro has substantial depletion in HFSEs (the high field strength elements) such as Th, Nb, Zr, and Hf, as well as exhibits comparatively high Mg^# (80.6 on average), and low K, N, P, and Ti contents. Therefore, it was possibly formed in an intra oceanic subduction island arc environment originating from ~30% partial melting of the depleted spinel lherzolite. The SIMS zircon U-Pb ages of the gabbro samples are 187 ± 1.2 Ma and 189 ± 2.1 Ma, and the weighted averages of δ¹⁸O are 5.24‰ ± 0.12‰ and 5.34‰ ± 0.13‰, which are close to mantle-derived zircon δ¹⁸O (5.3‰ ± 0.3‰). These results suggest that during the Early Jurassic, the Laguoco ophiolite recorded the intra oceanic subduction of the northern branch of the Neo-Tethys. Full article

The Alxa Block is located in the middle part of the Central Asian orogenic belt, which is the coupling belt of the North China, Tarim, and Kazakhstan Plates. The east–west-trending deep faults control stratigraphic distribution and magmatic activity in the region. To detect the EW-trending ore-controlling deep structures, a 440 km NS section of magnetotelluric sounding was conducted from Minle to Ejinaqi. The phase tensor and electrical principal axis were analyzed based on the measured data to build the exploration model. The electrical structure model along the section was obtained using nonlinear conjugate gradient (NLCG) 2D inversion. Combined with the geological, geophysical, and deposit distribution characteristics in the area, the comprehensive study of magnetotelluric sounding profile shows that the resistivity presents as zoned along the profile. The Engelwusu ophiolite mélange belt is stacked in clumps with high and low resistivity, indicating that the northern margin of the Engelwusu Belt subducts below the high-resistivity zone, representing the passive continental margin. The southern end of the Engelwusu ophiolite belt is primarily the prospecting potential area for copper–gold deposits, whereas the northern end of the Engelwusu ophiolite belt corresponds to copper polymetallic deposits. Full article

The Tianshui-Sunjiaxia area is located in the connection zone of West Qinling Orogen and North Qilian Orogen, which could provide great insights into the amalgamation processes between the northern and southern blocks of China. Three subduction- and rift-related rocks gneissic granite from North Qilian arc-interarc belt (NQAI) granite and metabasalt from North Qinling back-arc basin (NQBA) are distinguished across the connection zone. The gneissic granite was generated by melts from older crustal materials of Longshan Group with the addition of a relatively juvenile basaltic source from the lower crust during the collision process. The Liwanxincun metabasalt reflects the mixing of the partial melting of the shallow asthenospheric mantle and the metasomatized mantle in a back-arc extension setting. The LA-ICP-MS zircon U-Pb dating of gneissic granite (068, 069) yields crystallization ages of 457.0 ± 1.6 Ma and 445.9 ± 2.1 Ma. The study area is divided into six tectonic units in Early Paleozoic time involving NQAI (Yanjiadian-Xinjie) continental arc, interarc rift basin (Maojiamo-Xiwali), continental arc (Chenjiahe-Wangjiacha); NQBA back-arc rift basin (Huluhe-Hongtubao), island arc and ophiolitic melange belt (North Qinling-Shangdan). A tectonic model is proposed in which the NQAI continental arc (Yanjiadian-Xinjie) might represent the early period of subduction of North Qilian Ocean (NQO) and the interarc rift is the product of the extension triggered by southward subduction of NQO. The ongoing subduction of NQO then leads to the formation of Chenjiahe-Wangjiacha continental arc, as well as the Hongtubao back-arc spreading ridge in NQBA back-arc basin (Huluhe). The tectonic evolution of the connection zone is closely associated with the closure of the North Qilian Ocean and North Qinling-Shangdan Ocean in the context of the convergence process of micro-continental blocks, including North China block, Longshan group and North Qinling Terrane. Full article

Arranged from oldest to youngest, the main granitic rock units exposed in Khour Abalea are metagabbros, cataclastic rocks, ophiolitic melange, granitic rocks, pegmatite and lamprophyre dykes. The presence of radioactivity associated with the heavy bearing minerals in construction materials—like granite—increased interest in the extraction process. As it turns out, granitic rocks play an important economic part in the examination of an area’s surroundings. The radionuclide content is measured by using an NaI (Tl)-detector. In the mineralized pegmatites, U (326 to 2667 ppm), Th (562 to 4010 ppm), RaeU (495 to 1544 ppm) and K (1.38 to 9.12%) ranged considerably with an average of 1700 ppm, 2881.86 ppm, 1171.82 ppm and 5.04%, respectively. Relationships among radioelements clarify that radioactive mineralization in the studied pegmatites is magmatic and hydrothermal. A positive equilibrium condition confirms uranium addition to the studied rocks. This study determined ²²⁶Ra, ²³²Th and ⁴⁰K activity concentrations in pegmatites samples and assessed the radiological risks associated with these rocks. The activity concentrations of ²²⁶Ra (13,176 ± 4394 Bq kg⁻¹), ²³²Th (11,883 ± 5644 Bq kg⁻¹) and ⁴⁰K (1573 ± 607 Bq kg⁻¹) in pegmatites samples (P) are greater than the global average. The high activity of the mineralized pegmatite is mainly attributed to the presence of uranium mineral (autunite), uranophane, kasolite and carnotite, thorium minerals (thorite, thorianite and uranothorite) as well as accessories minerals—such as zircon and monazite. To assess the dangerous effects of pegmatites in the studied area, various radiological hazard factors (external, internal hazard indices, radium equivalent activity and annual effective dose) are estimated. The investigated samples almost surpassed the recommended allowable thresholds for all of the environmental factors. Full article

We report the first radiogenic Nd-Sr isotope data in the magmatic rocks island-arc ophiolite assemblage from the middle branch of the East Sayan ophiolite complexes to better constrain geodynamic processes in this segment of the CAOB in southern central Siberia. The magmatic rocks belong to the following geochemical types: (1) Ensimatic island-arc boninites; (2) island-arc assemblage; (3) enriched basalts of mid-ocean ridges; and (4) oceanic island-like basalts. The boninites have a positive value εNd (T), which is generated from a depleted mantle source (N-MORB). The island-arc assemblage has negative or slightly positive εNd (T) and was formed from an enriched mantle source due to the subduction of terrigenous rocks. The source of the terrigenous material was most likely the rocks of the Archean TTG (Trondhjemite Tonalite Granodiorite) complex of the Gargan block. Isotopic ratios for E-MOR and OIB-like basalts are characterized by positive or slightly negative values of εNd (T). The mafic dike, which crosscut ophiolite rocks, corresponds to OIB-like basalts. The values of εNd (T), measured ⁸⁷Sr/⁸⁶Sr and I (Sr), in the mafic dike correspond to the EM I mantle source. The E-MOR and OIB-like basalts appear to be formed in late-stage asthenospheric mantle melting via the decompression melting processes. The obtained isotope geochemical data for the E-MOR and OIB-like basalts probably indicate the mixing of island-arc melts with asthenospheric melts. We undertook ⁴⁰Ar/³⁹Ar dating of the mafic dike, which crosscut the ophiolite unit. The mafic dike has a whole-rock ⁴⁰Ar/³⁹Ar weighted mean plateau age of 799 ± 11 Ma. The dating constrains the minimum age of the ophiolite and island-arc magmatism in the region. Full article