Late Devonian-Early Carboniferous Research Articles

Petromagnetism and paleomagnetism of kimberlite pipesof the Verkhnemunskoe deposit (Yakutsk diamondiferous province)

The purpose of the studies of kimberlite pipes of the Verkhnemunskoe diamond field is to calculate the Middle Paleozoic paleomagnetic pole to clarify the trajectory of the apparent migration of the pole and reconstructions of the paleogeographic position of the Siberian platform at the time of the manifestation of active tectono-magmatic processes. The Verkhnemunskoye deposit is located within the Verkhnemunskoye kimberlite field of the Yakutsk diamondiferous province and includes five kimberlite pipes (Deimos, Zapolyarnaya, Komsomolskaya-Magnitnaya, Novinka and Poiskovaya), the age of which according to geological and isotopic data is estimated as Late Devonian-Early Carboniferous (372–347 Ma). For the first time scalar and vector physical parameters of kimberlites and captured xenoliths from different structural-material complexes of the Earth’s crust, as well as the host terrigenous sedimentary rocks of the Early Paleozoic were obtained, which are necessary for the development of physical-geological models of the Verkhnemunskoe field deposits. A relatively deep level of erosional shearing of the field has been established. The primary (synchronous with the formation of the field) natural residual magnetization was preserved in the kimberlite cohesive mass. The main carrier minerals of natural remanent magnetization vectors of kimberlites are unaltered magnesioferrite and magnetite, which indicates their thermo-sufficient nature. The natural remanent magnetization vectors of captured xenoliths indicate that the influence of hypergenic processes did not strongly affect the NRM vectors of kimberlites. Firing test is positive. The paleomagnetic pole with coordinates Φ = 26.5°N, Λ = 142.2°E, dp/dm = 6.2/7.8° was calculated from the obtained clusters of N = 10 vectors of the primary natural remanent magnetization of kimberlite pipes. On its basis, we reconstructed the paleogeographic position of the Siberian Platform, which at the time of the kimberlite intrusion was located in the middle latitudes of the northern hemisphere and was facing north with its southern edge.

Mineralogy, fluid regime and potential ore content of the post-accretion Ivdel gabbrodolerite-monzonite-granodiorite complex (Northern Urals)

Research subject. Hypabyssal intrusive bodies of a gabbro-dolerite-monzonite-granodiorite composition, common among the Devonian island-arc sequences of the East Tagil structural-formational zone in the Northern Urals. These bodies are dated as the Late Devonian-Early Carboniferous and attributed to the post-accretionary continuously differentiated Ivdel complex. Material and methods. The composition of rock-forming and accessory minerals of these intrusive bodies was studied using an SX-100 microanalyzer. This made it possible to obtain new data on the conditions of their formation, features of the fluid regime, and potential ore content. In addition, the geodynamic setting of this period was clarified, along with the composition, possible position of the primary mantle source, and the features of its fluid regime. Results. It was established that the mafic rocks (dolerites) in the Ivdel complex correspond to derivatives of the depleted mantle, with a primary source close to N-MORB basalts. The connection between the source and the separation of the slab and the “mantle window” located under the frontal zone of the former East Tagil island arc was substantiated. Such a position of the melting chamber determines the primary weak water saturation of dolerite magmas and their redox regime. According to mineralogical data, this feature is further manifested in the formation of the entire series of rocks of the Ivdel complex (with an increase in the iron index of clinopyroxenes and amphiboles, an increase in titanium contents in titanomagnetite, and other data). Further evolution of magmas (from dolerites to monzonites, quartz diorites, and granodiorites) occurred as they moved upward to the Earth’s surface and reflected the increasing contamination of differentiated melts by suprasubduction island-arc matter. Another important petrological feature of this complex is its formation with the participation of a chlorine-rich magmatogenic fluid. Depending on the composition of intruding melts, their contents of water, chlorine and oxygen activity, a number of autonomous degassing fluid-magmatic systems are formed. These systems are characterized by noticeable features in the processes of post-magmatic autometasomatosis, with the participation of an iron-chloride fluid. According to the regime of volatiles (halogens, water, and oxygen), the common affiliation of such autonomous fluid-magmatic systems to titanomagnetite ferrofacies (according to G.B. Fershtater), determines both their general metallogenic specialization and potential ore content. The removal of volatiles from such fluid-magmatic systems could be accompanied by extraction and transfer of chlorophyll ore elements from crystallizing melts to exocontacts of intrusive bodies. This, in the presence of migration zones favorable for fluids (faults, crushing zones, etc.) and geochemical barriers (especially in the presence of favorable enclosing sulfide-bearing island-arc strata), could have contributed to the formation of hydrothermal-metasomatic ore mineralization (Cu, Zn, Au, Mo and etc.). Such ore-bearing zones during erosion could be the primary source of gold for the known placers of this region. Large bodies of gabbrodolerites of the Ivdel complex may be promising for high-Ti magnetite-ilmenite mineralization.

Geochemistry, Chronology and Tectonic Implications of the Hadayang Schists in the Northern Great Xing’an Range, Northeast China

The Late Paleozoic tectonic evolution of the Xing’an block in the eastern Central Asian orogenic belt has long been the subject of debate. In this paper, a comprehensive study of U-Pb zircon ages, Lu-Hf isotopes and whole-rock elemental analyses was carried out on Hadayang schists. Representative samples of the epidote-biotite-albite schist and biotite-albite schist yielded the weighted mean 206Pb/238U ages of 360 ± 2 Ma and 355 ± 3 Ma, respectively. This indicated the presence of Late Devonian–Early Carboniferous intermediate-basic rocks in the eastern Xing’an block. The Hadayang schists exhibited a Na-rich, tholeiitic and calc-alkaline affinity in composition with low Mg# (35.2–53.0), Cr (23.7–86.5 ppm), Ni (21.1–40.0 ppm) and Co (12.1–30.6 ppm). They were characterized by enrichment of LILEs, depletion of HFSEs and highly positive zircon εHf(t) values (the average values were +8.93 and +9.29, respectively). The magma source of the Hadayang schists was a mantle that consisted of both spinel and garnet lherzolite, with a partial melting degree of 1%–5%, and it had undergone fractional crystallization of olivine, orthopyroxene and plagioclase. The Hadayang schists, together with other Late Devonian–Early Carboniferous intermediate-basic magmatic rocks in the eastern Xing’an block, were formed in an intracontinental extension tectonic setting similar to that of the North American Basin and Range basalt. Moreover, Late Devonian–Early Carboniferous ophiolite under a similar tectonic background in the western Xing’an block has been reported. We believe that the Xing’an block would - have been in the stage of intracontinental extension during the Late Devonian–Early Carboniferous.

The Nenjiang oceanic slab rollback in late Devonian: Constraints from Heihe granite and monzonite in the Xing’an block, NE China

Disputes regarding the subduction process, closure time, and the location of the Nenjiang Ocean have arisen due to limited exposure of late Paleozoic magmatic rocks in the Xing'an block. In this study, we present new zircon U-Pb age, zircon Hf isotope, zircon trace elements and whole rock geochemistry to reveal the evolution process of the Nenjiang Ocean in the late Paleozoic. The Heihe monzonite samples originated from the partial melting of a depleted mantle wedge metasomatized by subduction-related fluids and experienced fractional crystallization of plagioclase and amphibole. The Heihe granites show adakitic affinity, with low contents of compatible elements and U-shaped chondrite-normalized REEs patterns, indicating that the adakitic characteristics of the Heihe granites were formed by amphibole-dominated fractional crystallization. Geochemical modeling shows that the Heihe granite can be generated at a degree of 30–75 % amphibole-dominated fractional crystallization. The geochemical characteristics and genetic type of Heihe granites, associated with coeval sedimentary strata, imply extensional tectonic setting in late Devonian-early Carboniferous. By analyzing the spatiotemporal distribution and geochemical data of the 300–400 Ma magmatic rocks in the Xing'an block, and by conducting locally weighted regression fitting, we propose a geodynamic model. According to this model, the Nenjiang oceanic slab began to rollback at approximately 375 Ma, resulting in the production of the Heihe monzonite and granite. Subsequently, the closure of the Nenjiang Ocean, and collision between the Xing'an block and the Songnen block occurred at around 320 Ma.

Late Triassic initial closure of the Mongol-Okhotsk Ocean in the western segment: Constraints from sedimentological, detrital zircon ages and paleomagnetic evidence

The Mesozoic evolution of the Mongol-Okhotsk Ocean (MOO) has significantly affected the configuration of the modern Asian continent. Although a scissor-like closure of the MOO has long been proposed, when and how the MOO closed are still hotly debated, especially the timing of initial closure of the MOO in its western segment, hindering our understanding of both the evolution of the MOO and tectonics of the northern Asian continent. In order to uncover the timing of initial closure of the MOO, we performed a multidisciplinary study in sedimentology, detrital zircon U-Pb dating and paleomagnetic on the Late Triassic clastic strata from the Tarvagatay Block and the Amuria Block (AMB) on the both sides of the Mongol-Okhotsk Suture. The upper Triassic strata on both sides of the suture were dominated by plant fossil-bearing alluvial-fluvial facies sediments, which unconformably overlain pre-Triassic geological units, indicating a terrestrial setting after the closure of the MOO. Detrital zircon U-Pb dating results revealed consistent age distribution patterns for samples from both sides of the suture with a predominant peak at ∼253–251 Ma and a secondary peak at ∼359–357 Ma, representing two main arc magmatic events during the bidirectional subduction of the MOO in the Late Devonian-Early Carboniferous and Late Permian-Early Triassic. Coeval Late Triassic paleomagnetic poles were obtained from the northern AMB and Tarvagatay Block, revealing a comparable paleolatitude of the AMB (∼31–33°) and Tarvagatay Block (∼32–34°) in the Late Triassic, arguing for that the western segment of the MOO should have closed at the Late Triassic. The compilation of sedimentology, detrital zircon U-Pb dating, magmatic and paleomagnetic evidence provides integrated constraints on the Late Triassic initial closure of the MOO in its western segment.

Late Devonian-early Carboniferous bryozoans from Zhankurgan (Greater Karatau, Kazakhstan)—taxonomy and palaeobiogeographical implications

Three bryozoan species Anomalotoechus sp., Pseudobatostomella sp. and Cyphotrypa sp. are described from the upper part of the Ermaksy Formation (lower-middle Famennian) of the Zhankurgan outcrop (Greater Karatau). These descriptions are important, because it is the first finding of upper Devonian bryozoans in Southern Kazakhstan. Five more species including one new Primorella zhankurganica sp. n. were found in limestones of the lower part of Orgalysay Formation (Upper Tournaisian) in the same outcrop. The hitherto known bryozoans from the Tournaisian of Greater Karatau constitute the most important bryozoan fauna from the carbonate platform, located on southwestern passive margin of the Kazakhstan palaeocontinent. The studied fauna reveals connections between different areas of Palaeotethys and Palaeoasian oceans in late Devonian-early Carboniferous.

Detrital Zircon U-Pb Geochronology of Upper Devonian and Lower Carboniferous Strata of Western Laurentia (North America): A Record of Transition from Passive to Convergent Margin

Abstract The Late Devonian-Early Carboniferous (DC) Antler orogeny in southwestern Laurentia is contemporaneous with influx of clastic sediments, unconformities, and volcanism across much of western Laurentia (WL), suggesting the demise of the Paleozoic passive margin. However beyond the type Antler orogeny in southwestern Laurentia, the DC tectonic setting is still unclear. Westerly sediment provenance has been suggested as evidence of a convergent margin setting in a foreland basin. However, there is a gap in our understanding in central WL (Alberta and Montana) despite the fact that paleogeographic reconstructions place this area at the centre of WL. We provide detrital zircon (DZ) U-Pb geochronological data from strata in Alberta, Montana, and Nevada that are synchronous with the Antler orogeny to constrain sediment dispersal patterns and test the westerly sediment sourcing hypothesis. We show three DZ facies specific to particular geographic locations: DZ facies 1 in southern Nevada has a prominent subpopulation of early to mid-Mesoproterozoic (mode at 1430 Ma), DZ facies 2 in northeastern Nevada has a late Paleoproterozoic population (mode at 1823 Ma), and DZ facies 3 in Alberta and Montana displays Mesoproterozoic to Neoproterozoic (mode at 1036 Ma), mid-Paleozoic (mode at 411 Ma), and depositional (ca. 360-340 Ma) ages. North-south variation in DZ facies indicates that WL basins were locally sourced from various tectonic fragments having different signatures. Comparing our data with published data, we show that WL is dominated by DZ recycled from uplifted older strata with input from mid-Paleozoic arc terrane (s) to the west. Westerly sourcing is evidenced by the presence of near-depositional ages and affinities of this study’s DZ facies with strata located to the west. Our results and geological evidence from other studies suggest that the Antler orogeny triggered a depositional shift and controlled sediments dispersal in WL, signaling the demise of the Paleozoic passive margin.

Late Devonian-Early Carboniferous isolated carbonate platforms of the North of the Urals and Pay-Khoy

The article considers isolated carbonate platforms known in the Sub-Polar Urals (basin of the Bolshaya Nadota River; boreholes of the Yunyakha and Levaya Grubeyu areas) and the NW Pay-Khoy (basin of the Lymbad’yakha River and coast of the Barents Sea). The three stages of formation of the platforms (Frasnian, Famennian-Tournaisian, and Visean-Serpukhovian) are distinguished, and the sedimentological models of these platforms are developed. Subsidence curves based on the back-striping demonstrate some differences in the evolution of the studied isolated carbonate platforms. Similarities and differences in the history and structure of the platforms are observed. Formation of the intra-shelf depressions (the Kozhim Depression in the Sub-Polar Urals, and the Korotaikha Depression in the Pay-Khoy) in the Frasnian — Early Famennian caused appearance of isolated carbonate platforms. The depressions probably were formed by the regional tectonics. The following development of the carbonate platforms was controlled by eustatic fluctuations. The isolated platforms differ by stratigraphic spans (Late Frasnian — Serpukhovian for the Polar Urals Platform and Famennian-Tournaisian for the Pay-Khoy Platform), relief, facies, and size. The isolated depressions differ in size as well: the Kozhim Depression is larger than the Korotaikha Depression. Additionally, it is supposed that the Polar Urals platform was of warm-water type, but the Pay-Khoy platfrom was of cool-water type.

The biostratigraphy of the Upper Devonian and lowermost Carboniferous of the Khoshyeilagh area, northeastern Alborz, Iran

Diverse and well-preserved marine palynomorphs occur in the Padeha, Khoshyeilagh, and Mobarak formations in the Khoshyeilagh area, northeastern Alborz, northern Iran. The marine palynomorph assemblages consist of acritarchs, prasinophytes, and chitinozoans, comprising 27 genera and 46 species. In addition, the studied sediments contain 20 species of algae, foraminifera, and corals allocated to 11 genera. The vertical distribution of acritarchs and chitinozoans allows the erection within these sediments of three local assemblage zones. Biozone I occurs in the upper part of the Padeha Formation and the basal part of the Khoshyeilagh Formation, suggesting a Frasnian–early Famennian age; Biozone II is present in the Khoshyeilagh Formation and indicates a Famennian age. Biozone III, which occurs in the basal part of the Mobarak Formation, suggests a Tournaisian age for this interval. Many of the palynomorph species, such as Crassiangulina tessellita, Daillydium pentaster, Deltotosoma intonsum, Gorgonisphaeridium discissum, Helosphaeridium microclavatum, Horologinella horologia, Papulogabata annulata, Somphophragma miscellum, and Veryhachium downiei, are closely comparable with coeval assemblages recorded from Australia, Ireland, Portugal, Saudi Arabia, North Africa, South America, and North America. This indicates the close relationship of the Iranian Platform to other parts of the northern Gondwana and southern Laurentian domain during the Late Devonian–Early Carboniferous. Moreover, results of similarity indices of marine microphytoplankton indicate a relatively high degree of similarity between the assemblages of the Khoshyeilagh area and those elsewhere in the Alborz Mountains and the Kerman Basin. Palaeontological evidence indicates that the Khoshyeilagh Formation was deposited in a low-energy, somewhat offshore, normal marine environment within the Alborz Basin, along the Palaeo-Tethys Ocean during this time interval. Furthermore, based on palynological data gathered from Iran, it can be concluded that Iran was an island during this interval of time.

Dynamics of the conodont diversity in the Late Devonian–Early Carboniferous (Famennian–Serpukhovian)

Subject.The article is aimed to evaluate of the conodont diversity dynamics at the species level in the Famennian – Serpukhovian interval.Materials and methods.The database compiled from the published and original data contains information on the stratigraphic ranges of 389 Famennian-Serpukhovian conodont species (https://1drv.ms/x/s!AvPFMTPLPc7T4nFU81CaO5UJ6nlw). Conodont zones compose the geochronological basis of the database. The conodont diversity, origination, extinction, and diversification were calculated. Dynamics of these parameters in the Late Devonian–Early Carboniferous was analyzed.Results.The Famennian-Serpukhovian conodonts demonstrate four cycles in the diversity: the early Famennian (triangularis-early postera zones), the late Famennian (late postera-praesulcata zones), the Tournaisian (sulcata-anchoralis zones), and the Visean-Serpukhovian (texanus-bollandensis zones). The cycles are separated by the low-diversity episodes. The highest diversity (80 species) is detected in the early and late marginifera zones (Famennian).Conclusions.The successive decreasing in diversity comprises interval from the late Famennian through Serpukhovian. The global events gave little influence on the conodont diversity except for the Frasnian/Famennian (about 70% conodont species became extinct) and Devonian/Carboniferous extinction events. Conodont diversity demonstrates weak dependence form the global sea level fluctuations. The transition from the green-house to ice-house climate at the beginning of the Carboniferous and successive changes in the marine ecosystems are considered as main probable cause of the decline in conodont diversity in the late Tournaisian-Serpukhovian.

LATE PALEOZOIC ALKALI-GRANITOID MAGMATISM OF SOUTHERN KAZAKHSTAN AND ITS ROLE IN THE FORMATION OF THE ORE-BEARING VOLCANIC STRUCTURES OF KENDYCTAS-CHU-ILI-BETPAK-DALA URANIUM ORE PROVINCE

Areas of distribution of alkaline granitoids in southernKazakhstanhave been identified, most of which are located within the Kendyktas-Chu-Ili-Betpakdalin uranium-bearing province, traditionally considered as Late Devonian-early Carboniferous. Arguments in favor of late Paleozoic age of alkaline rocks have been given. The proximity of the petrographic composition, petrochemical and geochemical features of the alkaline granites of the Chu-Ili-West-Balkhash area and late Paleozoic alkaline granites of the Zailiysky area has been established, confirming their age and continental rift origin. Similarity in the development of magmatism and hydrothermal mineralization of ore-bearing volcanic structures of the Chui area (Kurmanshitinskoe) and North-Western Chingiz (Ulkentuz and Dostar) has been revealed. In these structures, Devonian volcanic rocks are interspersed with subvolcanic bodies of comendites and later dikes of microgabbrodiorites and microdiorites. The hydrothermal mineralization of these structures took place in multiple stages. The early stage includes secondary quartzites, which developed in Devonian volcanogenic rocks. Comendite is associated with K-feldspated and albitized rocks, as well as with zones of rare-metal (Zr, Nb, Th, U and TR) Fe-Mg metasomatites that originated in the middle stage. In the later stage, after the introduction of microgabbrodiorite dikes, berezites with U-Mo mineralization formed. Comendites in the structures of the North-Western Chingiz break through the deposits D2-3 and D3-C1 and are considered to be the comagmatic alkaline granites of the neighboring late Paleozoic massifs (Tleumbet and Kuyrektykol). The noted similarity may also indicate that the Chui area comendites are likely to be dated to the late Paleozoic age. Examples have been given to the superposition of berezites and U-Mo mineralization on the late Paleozoic granosyenites, which are widespread in the Kurdai volcanic structure, as well as on the alkaline granites of Karasai volcanic structure. Their age may also be late Paleozoic. Additional research has been recommended for the final conclusions on the age of alkaline rocks and uranium mineralization.

Polycyclic Palaeozoic evolution of accretionary orogenic wedge in the southern Chinese Altai: Evidence from structural relationships and U–Pb geochronology

Structural analysis and U–Pb geochronological study on zircons from the southern Chinese Altai (the Kalasu area, SE of the Altai city) show that the Cambro-Ordovician accretionary wedge (ca. 520–492 Ma) underwent four major geological events: 1) emplacement of Early Devonian magmas (ca. 410–400) associated with formation of a volcano-sedimentary cover, 2) major Middle Devonian (ca. 390–374 Ma) tectono-metamorphic event, 3) Late Devonian-Early Carboniferous folding without apparent metamorphism, and 4) a regional folding with localized Early Permian high- to ultrahigh-temperature reworking (ca. 300–280 Ma). The Early Devonian magmatism is characterized by emplacement of mafic rocks and granitoids in the centre of the NE-SW profile, coevally with granitoid magmatism and rhyolite volcanism in the southwest and northeast, respectively. The whole volcano-sedimentary and magmatic edifice was transposed by sub-horizontal metamorphic fabric associated with variable metamorphic degrees in different areas ranging from greenschist facies in the northeast (mu + bi±g) to amphibolite facies in the southwest (st + g ± sill) and granulite facies in the centre (g + sill+kfs). This metamorphic architecture, distribution of magmatism and character of metamorphic zircon populations allow to correlate these areas with upper, middle and lower orogenic crust that developed during important vertical shortening and horizontal flow in Middle Devonian. Subsequently, the whole edifice was affected by regional NE-SW trending upright (possibly Late Devonian-Early Carboniferous) folding. Finally, Early Permian shortening produced NW-SE trending regional upright folds in the southwest and northeast and a crustal-scale vertical, tabular deformation zone in the centre. The Permian deformation is accompanied by granulite facies (kfs + cd + sill+g) metamorphism and anatexis reworking the Devonian lower orogenic crust, with extensive resetting and growth of new zircons and with intrusions of Permian granites and gabbros. This study suggests that the Early Permian event was related to massive perturbation of thermal structure of the mantle lithosphere due to the collision of the Junggar arc with the Chinese Altai terrane.

Formation and sedimentary filling of the Dnieper- Donets depression (geodynamics and facies) in the light of new data of paleotectonic modeling

Results of numerical modeling have been presented for development of the Dnieper-Donets depression based on the thermal problem for lithosphere with selecting the surface conditions of accumulation of the cover. A series of paleotectonic sections has been plotted with facial filling characterizing northwestern, central and southeastern parts of the depression. Modeling reproduces the initial transgressive and posterior regressive directionality of sedimentation during riftogenesis, iterative marine transgression in Tournaisian — Early Visean, step out to subcontinental conditions at the end of Middle Carboniferous; wide development is forecasted in southeastern part of DDD for abyssal depression formations of Late Devonian-Early Carboniferous. Frames of lithosphere and asthenosphere have been plotted; profiles of average temperature of anomalous mantle and tectonic component of plunge have been reconstructed. They certificate differences in the course of deep process during riftogenesis in different parts of the depression, for post-rift stage they give, on the contrary, the pattern of spatially steady descent.

Middle to Late Devonian–Carboniferous collapse basins on the Finnmark Platform and in the southwesternmost Nordkapp basin, SW Barents Sea

Abstract. The SW Barents Sea margin experienced a pulse of extensional deformation in the Middle–Late Devonian through the Carboniferous, after the Caledonian Orogeny terminated. These events marked the initial stages of formation of major offshore basins such as the Hammerfest and Nordkapp basins. We mapped and analyzed three major fault complexes, (i) the Måsøy Fault Complex, (ii) the Rolvsøya fault, and (iii) the Troms–Finnmark Fault Complex. We discuss the formation of the Måsøy Fault Complex as a possible extensional splay of an overall NE–SW-trending, NW-dipping, basement-seated Caledonian shear zone, the Sørøya–Ingøya shear zone, which was partly inverted during the collapse of the Caledonides and accommodated top–NW normal displacement in Middle to Late Devonian–Carboniferous times. The Troms–Finnmark Fault Complex displays a zigzag-shaped pattern of NNE–SSW- and ENE–WSW-trending extensional faults before it terminates to the north as a WNW–ESE-trending, NE-dipping normal fault that separates the southwesternmost Nordkapp basin in the northeast from the western Finnmark Platform and the Gjesvær Low in the southwest. The WNW–ESE-trending, margin-oblique segment of the Troms–Finnmark Fault Complex is considered to represent the offshore prolongation of a major Neoproterozoic fault complex, the Trollfjorden–Komagelva Fault Zone, which is made of WNW–ESE-trending, subvertical faults that crop out on the island of Magerøya in NW Finnmark. Our results suggest that the Trollfjorden–Komagelva Fault Zone dies out to the northwest before reaching the western Finnmark Platform. We propose an alternative model for the origin of the WNW–ESE-trending segment of the Troms–Finnmark Fault Complex as a possible hard-linked, accommodation cross fault that developed along the Sørøy–Ingøya shear zone. This brittle fault decoupled the western Finnmark Platform from the southwesternmost Nordkapp basin and merged with the Måsøy Fault Complex in Carboniferous times. Seismic data over the Gjesvær Low and southwesternmost Nordkapp basin show that the low-gravity anomaly observed in these areas may result from the presence of Middle to Upper Devonian sedimentary units resembling those in Middle Devonian, spoon-shaped, late- to post-orogenic collapse basins in western and mid-Norway. We propose a model for the formation of the southwesternmost Nordkapp basin and its counterpart Devonian basin in the Gjesvær Low by exhumation of narrow, ENE–WSW- to NE–SW-trending basement ridges along a bowed portion of the Sørøya-Ingøya shear zone in the Middle to Late Devonian–early Carboniferous. Exhumation may have involved part of a large-scale metamorphic core complex that potentially included the Lofoten Ridge, the West Troms Basement Complex and the Norsel High. Finally, we argue that the Sørøya–Ingøya shear zone truncated and decapitated the Trollfjorden–Komagelva Fault Zone during the Caledonian Orogeny and that the western continuation of the Trollfjorden–Komagelva Fault Zone was mostly eroded and potentially partly preserved in basement highs in the SW Barents Sea.

Mantle sources of kimberlites through time: A U-Pb and Lu-Hf isotope study of zircon megacrysts from the Siberian diamond fields

A comprehensive, internally consistent U-Pb and Lu-Hf isotope data set for 93 mantle-derived zircons from the Yakutian kimberlite province confirms and further refines the four major episodes of kimberlite magmatism on the Siberian craton: 421–409 Ma (Late Silurian-Early Devonian), 358–353 Ma (Late Devonian-Early Carboniferous), 226–218 Ma (Late Triassic), and 161–144 Ma (Middle-Late Jurassic). The relatively narrow, constant range of εHf values between +2 and +10 for both the Paleozoic and Mesozoic mantle-derived zircons (and by inference kimberlites) suggests that the volatile-rich magmas were repeatedly sourced from the convecting upper mantle beneath the Siberian craton. This finding is in keeping with the narrow and constant range of εNd values for groundmass perovskites from the Yakutian kimberlite province between +1.8 and +5.5 between 420 and 150 Ma. Our preferred model implies that the convecting upper mantle beneath the Yakutian kimberlite province ‘recovered’ rapidly back to ambient conditions shortly after the giant plume-related flood volcanic event that produced the Siberian Traps at 250 Ma. Although close spatial relationships exist between kimberlites and flood basalts on the Siberian craton during both the Paleozoic and Mesozoic, exact timing of the igneous events and the isotopic compositions of the diverse deep-sourced melting products rule out any direct genetic links.Besides the highly economic kimberlite-hosted diamond deposits of Late Devonian age (e.g., Mir and Udachnaya), the Siberian craton also contains significant Mesozoic placer diamond deposits (e.g., along the Anabar river), for which lamproite sources have been suggested recently. Our study shows that mantle-derived zircon megacryst fragments from the Ebelyakh placer deposit have Late Triassic ages of ca. 224 Ma. Their long-term depleted Hf isotopic compositions (+8.5 εHf) suggest that the alluvial diamonds were sourced from asthenosphere-derived Triassic kimberlites rather than from lithospheric mantle derived isotopically enriched lamproites.

Tectonic implications of the lithospheric structure across the Barents and Kara shelves

AbstractThis paper considers the lithospheric structure and evolution of the wider Barents–Kara Sea region based on the compilation and integration of geophysical and geological data. Regional transects are constructed at both crustal and lithospheric scales based on the available data and a regional three-dimensional model. The transects, which extend onshore and into the deep oceanic basins, are used to link deep and shallow structures and processes, as well as to link offshore and onshore areas. The study area has been affected by numerous orogenic events in the Precambrian–Cambrian (Timanian), Silurian–Devonian (Caledonian), latest Devonian–earliest Carboniferous (Ellesmerian–Svalbardian), Carboniferous–Permian (Uralian), Late Triassic (Taimyr, Pai Khoi and Novaya Zemlya) and Palaeogene (Spitsbergen–Eurekan). It has also been affected by at least three episodes of regional-scale magmatism, the so-called large igneous provinces: the Siberian Traps (Permian–Triassic transition), the High Arctic Large Igneous Province (Early Cretaceous) and the North Atlantic (Paleocene–Eocene transition). Additional magmatic events occurred in parts of the study area in Devonian and Late Cretaceous times. Within this geological framework, we integrate basin development with regional tectonic events and summarize the stages in basin evolution. We further discuss the timing, causes and implications of basin evolution. Fault activity is related to regional stress regimes and the reactivation of pre-existing basement structures. Regional uplift/subsidence events are discussed in a source-to-sink context and are related to their regional tectonic and palaeogeographical settings.

Palaeoecology of Famennian-Tournaisian (Late Devonian-Early Carboniferous) bryozoans from central and southern regions of Russia

The palaeoecology of Famennian-Tournaisian (Late Devonian-Early Carboniferous) bryozoans from central and southern regions of Russia is analysed. Famennian-Tournaisian bryozoans associations of the Altai-Sayan Folded Area and the south-western region of the West-Siberian plate was a shallow-middle shelf according to our own and literature data. Robust branching-fenestrate-bilaminar bryozoan associations are found in locations of warm water, transitional low-high water energy, normal salinity and changing sedimentation rate. Bryozoan associations from South Urals and the central part of the Russian plate are characterised by a low number of species and specimens. Encrusting unilaminar and delicate branching growth habits are dominate in the Russian Plate and indicate an environmental setting in close proximity to strandline.

Late Paleozoic granitoids from central Qiangtang, northern Tibetan plateau: A record of Paleo-Tethys Ocean subduction

The Longmu Co-Shuanghu suture zone in central Qiangtang, northern Tibetan plateau, is a segment of the main suture zone of the Paleo-Tethys Ocean. Numerous studies have focused on ophiolite and high-pressure metamorphic rocks from this suture zone, whereas the associated subduction-related magmatic rocks have received little attention. In this study, we present new data for the Late Paleozoic granitoids from the Gangma Co, Guoganjianian and Chasum areas along the Longmu Co-Shuanghu suture zone. Zircon U-Pb dating indicates that these granitic rocks were emplaced between 364Ma and 346Ma (Late Devonian-Early Carboniferous). Three granitic plutons exhibit different geochemical compositions, but show uniform Sr-Nd and zircon Hf isotopic features with positive ƐNd(T) and zircon ƐHf(T) values. Furthermore, these rocks were all formed in a volcanic arc setting associated with an oceanic subduction zone. Together with data from coeval volcanic rocks, we suggest that the Late Paleozoic magmatic rocks were formed as a result of northward subduction of the Paleo-Tethys Ocean. Subduction of the Paleo-Tethys Ocean probably commenced in the Late Devonian, continued throughout the Late Paleozoic, and ended in the Triassic.

Structural transect through Ellesmere Island (Canadian Arctic): superimposed Palaeozoic Ellesmerian and Cenozoic Eurekan deformation

AbstractThe 400 km long transect through Ellesmere Island is located perpendicular to the North American continental margin between the Arctic Ocean in the NNW and the Greenland–Canadian Shield in the SSE. It provides an insight into the structural architecture and tectonic history of the upper parts of the continental crust. The northernmost segment of the transect is dominated by the composite Pearya Terrane, which amalgamated with the Laurentian margin during the Late Devonian–Early Carboniferous Ellesmerian Orogeny. The Neoproterozoic to Devonian Franklinian Basin is exposed south of the terrane boundary and most probably overlies the crystalline basement of the Greenland–Canadian Shield. The structures along the transect in this area are dominated by kilometre-scale Ellesmerian folding of the Franklinian Basin deposits above a deep-seated detachment, which is suggested to be located at the boundary between the basement of the Canadian Shield and the overlying >8 km thick Franklinian Basin. Following the development of the Late Mississippian–Palaeogene Sverdrup Basin, the complex Eurekan deformation reactivated Ellesmerian thrust faults and probably parts of the associated deep-seated detachment. In addition, large Eurekan strike-slip faults affected and displaced pre-Eocene deposits and tectonic structures, particularly in the northern part of the transect.

Accreted seamounts in North Tianshan, NW China: Implications for the evolution of the Central Asian Orogenic Belt

The Carboniferous Bayingou ophiolitic mélange is exposed in the North Tianshan accretionary complex in the southwestern part of the Central Asian Orogenic Belt (CAOB). The mélange is mainly composed of serpentinised ultramafic rocks (including harzburgite, lherzolite, pyroxenite, dunite and peridotite), pillowed and massive basalts, layered gabbros, radiolarian cherts, pelagic limestones, breccias and tuffs, and displays block-in-matrix structures. The blocks of ultramafic rocks, gabbros, basalts, cherts, and limestones are set in a matrix of serpentinised ultramafic rocks, massive basalts and tuffs. The basaltic rocks in the mélange show significant geochemical heterogeneity, and two compositional groups, one ocean island basalt-like, and the other mid-ocean ridge-like, can be distinguished on the basis of their isotopic compositions and immobile trace element contents (such as light rare earth element enrichment in the former, but depletion in the latter). The more-enriched basaltic rocks are interpreted as remnants/fragments of seamounts, derived from a deep mantle reservoir with low degrees (2–3%) of garnet lherzolite mantle melting. The depleted basalts most likely formed by melting of a shallower spinel lherzolite mantle source with ∼15% partial melting. It is probable that both groups owe their origin to melting of a mixture between plume and depleted MORB mantle. The results from this study, when integrated with previous work, indicate that the Junggar Ocean crust (comprising a significant number of seamounts) was likely to have been subducted southward beneath the Yili-Central Tianshan block in the Late Devonian-Early Carboniferous. The seamounts were scraped-off and accreted along with the oceanic crust in an accretionary wedge to form the Bayingou ophiolitic mélange. We present a model for the tectonomagmatic evolution of this portion of the CAOB involving prolonged intra-oceanic subduction with seamount accretion.