Early Carbonate Research Articles

Latitudes of the Eastern Ural microcontinent and Magnitogorsk island arc in the Paleozoic Ural Ocean

Research subject. Rocks of the Paleozoic Eastern Ural microcontinent and Magnitogorsk island arc occupy a significant part of the Southern Urals and some part of the Middle Urals. The Western Urals are composed of rocks of the ancient Baltic continent and overthrust oceanic rocks. In the Eastern Urals and Trans-Urals rocks of the accretion complexes, oceanic crust, island arcs, the Eastern Ural microcontinent and the Kazakhstan Paleozoic continent are widespread. Rocks are exposed in the Denisov tectonic zone. The Magnitogorsk simatic Island Arc originated in the Ural Ocean, near the Baltic continent, in the early Devonian, developing from the Emsian to the Famennian. A collision between the Magnitogorsk arc and the Baltic continent occurred in the Famennian century. In the pre-Carboniferous age, the Eastern Ural microcontinent was located in the Ural Ocean. In the Tournaisian period, the Eastern Ural microcontinent accreted with the Baltic continent. The Kazakhstan continental massif was located on the other side of the Ural Ocean. The volcanic belt above the subduction zone was active on the edge of the Kazakhstan continent in the Early–Middle Devonian and in the Early Carboniferous. A subduction under the Baltic and Kazakhstan continents consumed most of the crust of the Ural Ocean by the middle of the Bashkir century. As a result, the Baltic continent (together with the Eastern Ural microcontinent) came into contact with the Kazakhstan continent. The formation of folded orogen began in the Moscow century following the collision of sialic terrains.Materials and methods. The research was based on the relevant data obtained by several researchers in 2000–2018 on rock paleomagnetism. Results. The paleolatitudinal positions of the Eastern Ural microcontinent were determined, comprising 5.3 ± 7.4°) in the Middle Ordovician and 8.2 ± 7.2° in the Early–Middle Silurian. The respective paleolatitudinal positions for the Early–Middle Devonian comprised: the Ural margin of the Baltic paleocontinent (7.7 ± 3.7°), the Magnitogorsk island arc (3.2 ± 3.1°) and the Ural margin of the Kazakhstan paleocontinent (20.6 ± 3.8°).Conclusion. According to the analysed paleomagnetic data, in the Early–Middle Devonian, the distance between the latitudes of the margins of the Baltic and Kazakhstan continents was not less than 600 km provided they were in the same hemisphere, and more than 2,300 km provided they were in different hemispheres. The convergence of the terrains was associated with the subduction of the Ural Ocean crust before its closure, which occurred in the Tournaisian century.

Tracing proto-Rheic - Qaidam Ocean vestiges into the Western Tatra Mountains and implications for the Palaeozoic palaeogeography of Central Europe

Zircon petrochronology from amphibolites and retrogressed eclogites from the basement of the Western Tatra Mountains (Central Western Carpathians) reveals a complex rock evolution. An island-arc related basaltic amphibolite from Žiarska Valley shows three distinct zircon forming events: igneous zircon growth at ca. 498 Ma (Middle/Late Cambrian) and two phases of amphibolite-facies metamorphism at ca. 470 Ma (Early Ordovician) and at ca. 344 Ma (Early Carboniferous). A retrogressed eclogite from Baranèc Mountain records two zircon forming events: metamorphic zircon growth under eclogite-facies conditions at ca. 367 Ma (Late Devonian) and amphibolite-facies metamorphism at ca. 349 Ma (Early Carboniferous). These data contribute towards understanding and correlating major tectonothermal events that shaped the eastern margin of Gondwana in the Early Palaeozoic and its subsequent Variscan evolution. The metabasites record vestiges of two completely independent oceanic domains preserved within the Central Western Carpathians: (1) An Ediacaran to Cambrian oceanic arc related to the proto-Rheic - Qaidam oceans and metamorphosed to amphibolite-facies in the Early Ordovician subduction of the proto-Rheic - Qaidam arc during the Cenerian orogeny (ca. 470 Ma) and (2) Late Devonian oceanic crust related to a back-arc basin (Pernek-type), formed by the opening of the Paleotethys and metamorphosed to eclogite-facies during Devonian subduction (ca. 367 Ma). The common Variscan and later evolution of these oceanic remnants commenced with amphibolite-facies metamorphic overprinting in the Early Carboniferous (amphibolite: ca. 344 Ma; retrogressed eclogite: ca. 349 Ma) related to an Early Variscan consolidation and the formation of Pangea. None of the investigated rocks of the Central Western Carpathians show any evidence of being chronologically or palaeogeographically related to the Rheic Ocean, therefore any prolongation of the Rheic suture from the Sudetes into the Alpine-Carpathian realm is highly problematic. Instead, the Southern and Central Alpine Cenerian orogeny can be traced into the Central Western Carpathians.

Geochronology and geochemistry of Devonian magmatism in the Frontal cordillera (Argentina): geodynamic implications for the pre-Andean SW Gondwana margin

ABSTRACT Whereas north of 33°S Devonian calcalkaline magmatism is notably absent, a Devonian arc is well developed to the south of 33°S. The Carrizalito, Pampa de los Avestruces, and Papagayos plutons are located in the Frontal Cordillera between 34° and 35°S. U-Pb zircon SHRIMP and LA-MC-ICP-MS data from the plutons yield Early Devonian ages of 389 ± 3 and 409 ± 3 for Carrizalito, 413 ± 2 Ma for Pampa de los Avestruces, and 346 ± 3, 393 ± 5 and 412 ± 3 Ma for Papagayos. The younger zircon age from the Papagayos pluton suggests that the Devo nian magmatism continued during the Early Carboniferous. Carrizalito and Pampa de los Avestruces plutons display metaluminous to slightly peraluminous granodiorite composition, with SiO2 ranging from 65.1 to 67.8 wt.% and from 67.9 to 69.7 wt.%, respectively, and define a magnesian calc-alkalic trend. The Papagayos pluton is metaluminous to slightly peraluminous and slightly more evolved in composition (felsic granodiorite to monzogranite), with a SiO2 content that varies between 68.4 to 70.5 wt.%, defining a magnesian with ferroan enrichment and calc-alkalic to alkali-calcic trend. These geochemical characteristics are similar to those reported for the Carboniferous arc granitoids in the Frontal Cordillera and Western Sierras Pampeanas at 27°–30°S and the archetypical calc-alkaline granitoids of the Peruvian Coastal batholith, indicating an Early Devonian magmatic arc. Rare earth element patterns ([La/Yb]N = 10–20) with mostly slight to moderate negative Eu anomalies (Eu/Eu* = 0.62–0.80) and geochemical ‘spider-diagrams’ are similar to arc granitoids. Combined whole-rock Nd and Sr isotope data from the largest Carrizalito and Pampa de los Avestruces plutons (εNdt = −4.3 and −4.6; 87Sr/86Srt = 0.70704 and 0.70762; TDM = ca. 1.5 Ga) together with in situ U–Pb and Hf isotope data from magmatic zircon (εHft ranging from −16.5 to −5.4; TDM = ca. 1.7 Ga), suggest a common older continental lithosphere source. The presented data indicate tectonic segmentation in the pre-Andean margin controlling the presence or absence of Devonian subduction-related magmatism. This segmentation is best explained by changes in the tectonic plate configuration occurred in a long-lived convergent margin.

Small carbonaceous fossils (SCFs) from North Greenland: new light on metazoan diversity in early Cambrian shelf environments

AbstractThe Sirius Passet Lagerstätte of North Greenland is one of the oldest records of soft‐bodied metazoan‐dominated ecosystems from the early Cambrian. The Lagerstätte site itself is restricted to just a singlec. 1‐km‐long outcrop located offshore from the shelf margin, in an area affected by metamorphic alteration during the Ellesmerian Orogeny (Devonian – Early Carboniferous). The recent recovery of small carbonaceous fossils (SCFs) to the south, in areas that escaped the effects of this deformation, has substantially expanded the known coverage of organic preservation into shallower water depositional settings in this region. Here, we describe additionalSCFassemblages from the siliciclastic shelf succession of the Buen Formation (Cambrian Series 2, stages 3–4;c. 515 Ma), expanding the previously documentedSCFbiota. Newly recovered material indicates a rich diversity of non‐mineralizing metazoans, chiefly represented by arthropod remains. These include the filtering and grinding elements of a sophisticated crustacean feeding apparatus (the oldest crustacean remains reported to date), alongside an assortment of bradoriid sclerites, including almost complete, 3D valves, which tie together a number ofSCFs previously found in isolation. Other metazoan remains include various trilobite cuticles, diverse scalidophoran sclerites, and a range of metazoan fragments of uncertain affinity. This shallower water assemblage differs substantially from the Sirius Passet biota, which is dominated by problematic euarthropod stem‐group members and sponges. Although some of these discrepancies are attributable to taphonomic or temporal factors, these lateral variations in taxonomic composition also point to significant palaeoenvironmental and/or palaeoecological controls on early Cambrian metazoan communities.

Early Devonian tectonic conversion from contraction to extension in the Chinese Western Tianshan: A response to slab rollback

Abstract Extensive angular unconformities that occur across the Tianshan Orogen provide insight into the Paleozoic tectonic evolution of the southwestern Central Asian Orogenic Belt. This study presents new geochronological and geochemical data to unravel the age and origin of a pronounced angular unconformity in the Baluntai domain of the Chinese Western Tianshan. The angular unconformity separates a ductilely deformed metamorphic complex from overlying unmetamorphosed sequences. Detrital zircon age spectra suggest that the sedimentary samples received detritus from both the Precambrian basement and contemporaneous igneous rocks in the Central Tianshan and Yili blocks. The youngest detrital and metamorphic zircons in metasediments from the metamorphic complex reveal that their protoliths were deposited at ca. 419 Ma, and subsequently overprinted by metamorphism and ductile deformation at ca. 409 Ma. Along with ca. 532–407 Ma metavolcanics and foliated intrusions, the metamorphic complex constitutes a diachronous stratigraphic unit deposited during early Cambrian to late Silurian times, rather than a Precambrian basement as previously thought. The youngest detrital zircon age data from siliciclastic samples immediately above the angular unconformity constrain their deposition to ca. 407–397 Ma. In conjunction with a ca. 406 Ma rhyolitic tuff in the lower terrestrial siliciclastic unit and an early Carboniferous paleontological age of carbonates in the upper shallow-marine unit, it is suggested that the overlying unmetamorphosed sequences were accumulated during the early Devonian to the early Carboniferous. Thus, the angular unconformity in the Baluntai domain was formed between ca. 409 and 407 Ma. New and already published geochemical data reveal that the early Paleozoic (ca. 530–410 Ma) and late Paleozoic (ca. 410–320 Ma) igneous rocks are mainly medium- to high-K calc-alkaline I-type granitoids, which exhibit typical subduction-related geochemical signatures despite contrasting structural features. These geochronological and geochemical results are in accordance with those of similar early Devonian angular unconformities and Paleozoic igneous rocks in the southern Yili Block. The early Paleozoic igneous rocks display highly scattered zircon εHf(t) values, while the late Paleozoic rocks show a progressively increasing trend toward more positive values, attributed to slab rollback of the northward subducting South Tianshan Ocean under the Yili and Central Tianshan blocks. Thus, we propose that the formation of the angular unconformity was caused by tectonic conversion from contraction to extension due to slab rollback during the early Devonian. This study highlights the significance of geological and geochronological investigations of angular unconformities in the context of associated episodic magmatism and slab behavior in order to unravel distinct tectonic processes in the long-lived accretionary evolution of the Chinese Western Tianshan.

Viséan Fossil Plants from the Jiujialu Formation in Central Guizhou Province: Implications for Age of Bauxite and Palaeoenvironment

AbstractFossil plants from the carbonaceous shale of upper part of the Lower Carboniferous Jiujialu Formation in Lindai bauxite deposit fill a gap regarding the distribution of Early Carboniferous flora in Central Guizhou Province. Here the Lindai plant assemblage is systematically described, and its composition and geological age are discussed. Research demonstrates that 9 species in 6 genera of the fossil plants are discovered in this study, and they belong to lycopsids, sphenopsids and ferns, respectively. Based on the analysis of the floral composition, the geological age of the Lindai plant assemblage belongs to the Early Carboniferous Viséan. The discovery of Lindai plant assemblage not only provides a new fossil evidence to restrict the age of bauxite deposits in Lower Carboniferous Jiujialu Formation of Central Guizhou Province, but also has great significance to further research the palaeoecological setting and floral evolution during the Early Carboniferous.

The closing of the southern branch of the Paleo-Asian Ocean: Constraints from sedimentary records in the southern Beishan Region of the Central Asian Orogenic Belt, NW China

The Carboniferous–Permian strata in the southern Beishan Orogenic Collage (BOC) have been regarded as deposits formed either in a branch of the Paleo-Asian Ocean (PAO) or in continental rift basins. This paper presents stratigraphic, sedimentological, and provenance data of a continuous Carboniferous–Permian sequence on the southern margin of the Shuangyingshan Block within the southern BOC in NW China. Carboniferous Baishan and Shibanshan formations record a shallowing-upward change from marine tidal flat to continental braided stream environments. Integrated sandstone modes and U-Pb-Hf data indicate that these formations were fed from an orogenic uplift composed of Precambrian and early Paleozoic rocks of the Shuangyingshan and Dunhuang blocks (ca. 2492, 953, and 439–400 Ma). Latest Carboniferous–Early Permian depositional environments shifted from alluvial fans (Ganquan Formation) to delta-front and prodelta environments in an open marine setting (Shuangbutang and Jushitan formations) and source areas were composed of early Paleozoic granitoids (ca. 433 Ma) and penecontemporaneous volcanoes (ca. 278 Ma). These results indicate that the closure of the branch of the PAO between the Shuangyingshan and Dunhuang blocks occurred before the Visean (ca. 347 Ma), and was followed by the development of the latest Carboniferous–Early Permian marine rift basin on the amalgamated continental block.

Elucidating hydrological complexity and climate-influenced diagenesis during the early Carboniferous of South Wales

A revised age provided by conodonts from the Gilwern Oolite of the Clydach area of South Wales allows a clearer understanding of the palaeohydrology, palaeoclimatic history and diagenesis of previously correlated oolitic units. Earlier uncertainty over the apparent sub-regional differences in climate during the early Visean (Chadian—Arundian) lowstand is resolved. Previously a humid interval evidenced by prominent palaeo-epikarst capping the Gilwern Oolite along the northern outcrops of the South Wales synclinorium, was not recognized in what were regarded as correlative outcrops in the Chadian Gully Oolite in the southern part of the synclinorium. Conodont dating now shows that the Gilwern Oolite is much older (Courceyan) than the Chadian Gully Oolite, and during the prolonged exposure of the former there was an interval of weathering under a humid climate. This also explains the contrast between the diagenesis seen between the Gilwern and Gully oolites, which are no longer seen as correlatives.

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.

Labechia carbonaria Smith 1932 in the Early Carboniferous of England; affinity, palaeogeographic position and implications for the geological history of stromatoporoid-type sponges

Stromatoporoid sponges were very abundant during the middle Palaeozoic Era and are thought to disappear at the end of the Devonian Period in the Hangenberg Crisis. However, there are records of organisms with stromatoporoid-type structure in Carboniferous strata, the subject of this study. The Viséan fossil Labechia carbonaria Smith 1932 has been discussed previously in literature and its affinity has not been confirmed. In this study, the type material of L. carbonaria collected from the middle part of the Frizington Limestone Formation (previously called Seventh Limestone), Holkerian Substage, stored in the Natural History Museum (London, UK) and British Geological Survey (Keyworth, UK) was re-examined. The Holkerian Substage, in which L. carbonaria was found, lies between ca 335–339 Ma, and the Frizington Limestone Formation ranges from topmost Arundian to upper Holkerian, so middle Frizington Limestone Formation is likely approximately 337 Ma. L. carbonaria comprises thick long pillars connected by thin curved cyst plates consistent with the structure of the stromatoporoid genus Labechia. However, a common opinion is that L. carbonaria fossils may be mistaken for fragments of rugose corals, but there are problems with assigning it to the Rugosa. In vertical section (VS) L. carbonaria could be mistaken for a transverse section (TS) of a Carboniferous rugose coral. However, in TS L. carbonaria shows the rounded cross sections of stromatoporoid pillars. If it was a coral, septal sheets of the VS of a coral should be seen. For a rugose affinity to still apply, a coral structure would have to be composed of free trabeculae, but these are not known after the middle of the Devonian Period; there are no corals of Early Carboniferous age with the structure of L. carbonaria. Another interpretation, that L. carbonaria is a chaetetid, is discounted because it lacks calicles and is very different in structure from chaetetids. We conclude that L. carbonaria is a stromatoporoid. Because the beginning of the Carboniferous Period was ca 359 Ma, stromatoporoids thus occur approximately 22 million years after their purported disappearance at the end-Devonian Hangenberg Crisis. L. carbonaria, together with other rare occurrences in Carboniferous strata of stromatoporoid-form sponges Newellia mira (Newell) in USA, and uncertain taxa Komia Korde and Palaeoaplysina Krottow that have been attributed to stromatoporoids by some authors, supports some published views that end-Devonian stromatoporoid extinction may not have been as final as is traditionally interpreted. Thus Mesozoic stromatoporoids may represent resurgence of sponge lineages that survived the late Palaeozoic, perhaps in uncalcified form. Palaeogeographically, during the Early Carboniferous, the UK was positioned in low latitudes and in a central location of global distribution of reefal buildups during the late Palaeozoic continental assembly towards Pangaea. Thus it is curious that L. carbonaria is found in only one place; future search may determine its true palaeographic distribution, with potential reconsideration of the extinction of stromatoporoids at the end of the Devonian Period.

New Crassigyrinus-like fibula from the Tournaisian (earliest Carboniferous) of Nova Scotia

The transition between the Devonian and Carboniferous is important for tetrapod vertebrates. By the end of the Devonian the first limbs are present in aquatic animals, and by the mid Carboniferous fully terrestrial tetrapods have diversified. Knowledge of the fin-to-limb transition is sparse because few fossils from the earliest Carboniferous (Tournaisian) are known. Blue Beach Nova Scotia, in addition to a small number of sites in Scotland and Australia, is an exception to this global trend. Previous reports from Blue Beach identified fossils from a variety of Devonian-like and Carboniferous-like tetrapod body forms, making it a valuable site for studying the fin-to-limb transition. Here we report on a new left fibula from Blue Beach, which we attribute to the later occurring Visean-aged (early Carboniferous) Crassigyrinidae. Recent investigations of deposits in Scotland, similar in age to the Tournaisian exposed at Blue Beach, have found Crassigyrinus-like elements as well, reinforcing this 20-million-year lineage extension.

Ecological and Taxonomical Succession of Rhynchonellids (Brachiopoda) in the Devonian and Early Carboniferous of Transcaucasia

The Devonian and Early Carboniferous brachiopod assemblages of the order Rhynchonellida from Transcaucasia (Nakhchivan Autonomous Republic of Azerbaijan and Armenia) are described. The succession of dominating rhynchonellid superfamilies and ecological types has been identified. Rhynchonellids were numerous in the Devonian; in the Early and Middle Devonian, the representatives of the superfamily Uncinuloidea prevailed. Their relict species survived up to the beginning of Famennian. The free-lying and anchoring cavern-dwelling rhynchonellids dominated in the Early and Middle Devonian. This ecological subtype is recognized for the first time. The rhynchonellid taxonomic and ecological groups changed at the Frasnian–Famennian boundary but rhynchonellids remained a significant part of the brachiopod fauna. The variability of rhynchonellids decreased at the Famennian–Tournaisian boundary but the superfamily Rhynchotrematoidea and the rhynchonellid anchoring ecological type continued to dominate starting from the Frasnian. The role of Rhynchonellida in the Early Carboniferous brachiopod fauna is insignificant. The decrease of variability within some zones is probably connected with taphonomic conditions.

Volcanism and intrusive magmatism of the Magnitogorsk paleoarc in the epoch of its “soft” collision with a margin of the East European continent

Research subject. The article sets out to investigate the change of the geodynamic regime from the island-arc type to the accretionary-collisional type in the Late Devonian–Early Carboniferous, which occurred as a result of 1) a collision between the Western part of the Magnitogorsk island arc and the Eastern margin of the East European continent and 2) its later coupling with the heterogeneous composite East Uralian terrain.Materials and methods. The content of petrogenic elements and microelements in the rocks of the Late Paleozoic island-arc complexes of the Magnitogorsk island arc were determined using XRF and ICP MS methods at the Laboratory of Physicochemical Research Methods of the Institute of Geology and Geochemistry, Ural Branch of the Russian Academy of Sciences. In addition, available publications on the composition and formation conditions of these complexes were reviewed.Results. It was found that, in the Late Devonian–Early Carboniferous period, the process of island-arc magmatism of the Magnitogorsk paleoarc was substituted with the formation of intraplate volcano-intrusive complexes. The island-arc magmageneration and its manifestations were controlled by a latitudinal linear zoning and different depths of formation of magmatic cameras, reflecting the self-consistency and spatial isolation of these events.Conclusion. Due to the intensifying collision, melts from different mantle sources were mixing, thus contaminating the island-arc rocks by intraplate (plume-dependent) magmas. According to the composition and concentrations of high-field strength and fluid-mobile chemical elements, suprasubductional fluids played an important role in the evolution of late-island arc magmatic series.

Provenance and paleogeography of Carboniferous–Permian strata in the Bayanhot Basin: Constraints from sedimentary records and detrital zircon geochronology

The Bayanhot Basin is a superimposed basin that experienced multiple-staged tectonic movements; it is in the eastern Alxa Block, adjacent to the North China Craton (NCC) and the North Qilian Orogenic Belt (NQOB). There are well-developed Paleozoic–Cenozoic strata in this basin, and these provide a crucial window to a greater understanding of the amalgamation process and source-to-sink relationships between the Alxa Block and surrounding tectonic units. However, due to intensive post-depositional modification, and lack of subsurface data, several fundamental issues—including the distribution and evolution of the depositional systems, provenance supplies and source-to-sink relationships during the Carboniferous– Permian remain unclear and thus hinder hydrocarbon exploration and limit the geological understanding of this basin. Employing integrated outcrop surveys, new drilling data, and detrital zircon dating, this study examines the paleogeographic distribution and evolution, and provenance characteristics of the Carboniferous–Permian strata in the Bayanhot Basin. Our results show that the Bayanhot Basin experienced a long-term depositional evolution process from transgression to retrogression during the Carboniferous–late Permian. The transgression extent could reach the central basin in the early Carboniferous. The maximum regional transgression occurred in the early Permian and might connect the Qilian and North China seas with each other. Subsequently, a gradual regression followed until the end of the Permian. The northwestern NCC appeared as a paleo-uplift area and served as a sediments provenance area for the Alxa Block at that time. The NCC, Bayanwula Mountain, and NQOB jointly served as major provenances during the Carboniferous–Permian. There was no ocean separation, nor was there an orogenic belt between the Alxa Block and the NCC that provided sediments for both sides during the Carboniferous–Permian. The accretion of the Alxa and North China blocks should have been completed before the Carboniferous period.

Geochemistry and organic petrology of the permian whitehill formation, Karoo Basin (RSA) and the Devonian/Carboniferous shale of the Appalachian Basin (USA)

Globally, there is significant economic interest in shale gas as a hydrocarbon resource. The success of unconventional shale gas exploration in the United State of America (USA) has raised interests for potential shale gas development in the Karoo Basin of South Africa. The Karoo Basin technically recoverable shale gas resource is estimated to be 13 Tcf, with 19–23 Tcf recoverable free gas. Due to the potential importance of the South African resource, this study characterized the Lower Permian Ecca Group shale and compared its geochemical and organic petrology properties with those of Devonian/Carboniferous shales of the Appalachian Basin, USA. The comparison of research data will aid in the understanding of shale gas potential of the Karoo Basin. Horizons from two boreholes (KZF-01 and KWV-01) drilled in the southern and southeastern Karoo Basin were evaluated and compared with samples of the Late Devonian Ohio and Early Carboniferous Sunbury Shales from the central Appalachian Basin, USA. Total organic carbon (TOC), total sulphur (TS), and organic petrology (bitumen and vitrinite reflectance, and maceral analysis) were used to assess organic richness and thermal maturity of the shale samples. In addition, the samples were evaluated for mineralogical content using X-Ray Diffraction (XRD). Data indicates that the Whitehill Formation of the Karoo Basin has good to excellent shale source potential, with an average TOC of 4.35 wt%. However, thermal maturity data of the Whitehill Formation shows the organic matter to be overmature, with vitrinite reflectance equivalent values greater than 3% RoV. Total sulphur data for the Whitehill Formation and the Devonian/Carboniferous shales suggests that both were influenced by marine environments. Both the KWV-01 and Ohio/Sunbury shale samples contain high amounts of quartz, with clay mineral contents greater than 40% being observed in borehole KWV-01 samples. Collectively, this research study provides valuable data for shale gas exploration and development in the Karoo Basin, South Africa.

Early Carboniferous Back‐Arc Rifting‐Related Magmatism in Southern Tibet: Implications for the History of the Lhasa Terrane Separation From Gondwana

AbstractThe Lhasa terrane is widely regarded as an integral unit that separated from Gondwana in the Carboniferous or late Triassic, but this terrane may more reasonably consist of two subterranes that separated from Gondwana at different times. Here, we describe newly identified early Carboniferous (355–344 Ma) basic and intermediate intrusive rocks in the Xiongcun area, southern Tibet. The basic rocks that yield negative whole‐rock εNd(t) values (−5.04 to −2.94) and zircon εHf(t) values (−9.2 to −1.7), combined with the chemical compositions, indicate that the magmas were derived by the partial melting of an enriched lithospheric mantle in a back‐arc extensional setting. The intermediate rocks yield εNd(t) and εHf(t) values of −5.78 to −5.36 and −9.2 to −4.5, respectively, which suggests that they were likely produced via crust‐mantle mixing. By combining our results with those of previous studies on the Lhasa terrane, we consider that a broad back‐arc rift system, including northern and southern branches, developed along the northern margin of Gondwana under the southward subduction of the Paleo‐Tethys oceanic lithosphere during the early Carboniferous. The northern branch evolved into the Sumdo Paleo‐Tethys Ocean, which led to the separation of the North Lhasa terrane from northern Gondwana during the late Carboniferous. The subsequent northward subduction of the Sumdo Paleo‐Tethys oceanic lithosphere beneath the North Lhasa terrane during the early to middle Permian may have triggered the southern branch to further evolve into the Neo‐Tethys Ocean and eventually may have resulted in the South Lhasa terrane separating from Gondwana.

Discovery and Geological Significance of Early Carboniferous High‐Mg Diorite in the Balikun Area, Eastern Tianshan

AbstractThe granodiorites, monzogranites and diorites are widely developed in the Balikun area of Eastern Tianshan Orogen. Of which, LA–ICP–MS zircon U‐Pb isotopic dating from the diorites revealed that they were emplaced at 327∼333 Ma, representing an important period of magmatism in the Early Carboniferous. Geochemically, they are characterized by moderate SiO2 (51.33–62.48 wt%), high but variable MgO (2.04–11.16 wt%, average 5.35), higher Mg# (40–73) and TiO2 (0.67–1.29 wt%), Na2O/K2O (1.39–2.95) as well as variable Cr (2.49–675 ppm) and Ni (1.31–174 ppm), showing a geochemistry similar to those of high‐Mg diorites or sanukitoids. In addition, they are enriched in the LILE, poor HFSE with an evident negative Nb anomalies and a REE pattern of moderate fractionator between LREE and HREE without or weak negative Eu anomalies. Their ∊Hf(t) are positive (+3.63–+15.65), suggesting a source from the depleted mantle. In addition, they have high TiO2 and Pb, and large quantity of amphibolite and biotite, indicating that they were most likely derived from the partial melting of depleted mantle metasomatized by the slab‐derived melt under a hydrous condition. Consequently, combined with the contemporary volcanics and granitoids formed in the island arc settings, we proposed that the subduction was continued till Early Carboniferous in the Bogda‐Harlik tectonic belt. After that, wide occurrence of the post‐collisional A‐type granites and mafic‐ultramafic intrusions indicate this tectonic belt entered the post‐collisional environment from Late Carboniferous to Permian.

Carboniferous Magmatism in the Polar Urals

Dikes and sills of dolerites, essexite–dolerites, diorites, monzodiorites, and lamprophyres belonging to the Musyur (Malyi Khanmey) hypabyssal complex (Polar Urals) cut all Ordovician–Middle Devonian oceanic and island arc formations. Based on the geological data and previous isotope dating, the age of this complex is estimated from the end of the Late Devonian to the Early Triassic. The 40Ar/39Ar dating of hypabyssal magmatic bodies (various intrusive phases) of the Musyur complex was performed for the first time for the northern part of the Urals. The following 40Ar/39Ar ages were obtained: 349 ± 3, 347 ± 9, 339 ± 4, 334 ± 3, and 313 ± 10 Ma. They recorded the collision of the Polar Ural island-arc system with the continent of Arct–Laurussia (Arctida + Baltica + Laurentia), which led to the formation of the Carboniferous Early Ural Orogen. The prevailing northwestern (transversely to the supposed orogen) strike of most dikes in the Toupugol–Khanmeyshor area, predominantly the subduction zone geochemical features of the rocks (low and moderate TiO2 contents (0.7–2.1 wt %), P2O5 (0.1–0.8 wt %), enrichment in large-ion lithophile elements (Cs, Rb, Ba, Pb, Sr) compared to high-field-strength trace elements, the Ta–Nb minimum, and Pb and Th–U maxima), and the Early Carboniferous age estimates (349–334 Ma) lead to the conclusion that they probably formed in a syncollisional setting. The orthogonal northeastern (along the orogen axis) strike of some dikes (essexite–dolerites in more southerly regions), significantly higher concentrations of TiO2 (3.1–3.6 wt %), P2O5 (0.6–1.3 wt %), high-field-strength (Ta, Nb) and most rare earth elements (Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, Lu), which are more typical for rift-related and intraplate igneous rocks, and their Late Carboniferous age (313 Ma) may testify to their emplacement in a post-collision setting.

Early Carboniferous HP metamorphism in the Hida Gaien Belt, Japan: Implications for the Palaeozoic tectonic history of proto‐Japan

AbstractWe report two new eclogite localities (at Kanayamadani and Shinadani) in the high‐P(HP) metamorphic rocks of the Omi area in the western most region of Niigata Prefecture, Japan, which form part of the Hida Gaien Belt, and determine metamorphic conditions and pressure–temperature (P–T) paths. The metamorphic evolution of the eclogites is characterized by a tight hairpin‐shapedP–Tpath from prograde epidote–blueschist facies to peak eclogite facies and then retrograde blueschist facies. The prograde metamorphic stage is characterized by various amphibole (winchite, barroisite, glaucophane) inclusions in garnet, whereas the peak eclogite facies assemblage is characterized by omphacite, garnet, phengite and rutile. PeakP–Tconditions of the eclogites were estimated to be ~600°C and up to 2.0 GPa by conventional cation‐exchange thermobarometry, Ti‐in‐zircon thermometry and quartz inclusion Raman barometry respectively. However, the Raman spectra of carbonaceous material thermometry of metapelites associated with the eclogites gave lower peak temperatures, possibly due to metamorphism at different conditions before being brought together during exhumation. The blueschist facies overprint following the peak of metamorphism is recognized by the abundance of glaucophane in the matrix. Zircon grains in blueschist facies metasedimentary samples from two localities adjacent to the eclogites have distinct oscillatory‐zoned cores and overgrowth rims. Laser ablation inductively coupled plasma mass spectrometry U–Pb ages of the detrital cores yield a wide range between 3,200 and 400 Ma, with a peak at 600–400 Ma. In the early Palaeozoic, proto‐Japan was located along the continental margin of the South China craton, providing the source of the older population of detrital zircon grains (3,200–600 Ma) deposited in the trench‐fill sediments. In addition, subduction‐related magmatismc. 500–400 Ma is recorded in the crust below proto‐Japan, which might have been the source for the younger detrital zircon grains. The peak metamorphic age was constrained by SHRIMP dating of the overgrowth rims, yielding Tournaisian ages of 347 ± 4 Ma, suggesting subduction in the early Carboniferous. Our results provide clear constraints on the initiation of subduction, accretion and the development of an arc‐trench system along the active continental margin of the South China craton and help to unravel the Palaeozoic tectonic history of proto‐Japan.

Radiolarian age and geochemistry of cherts from the Atbashi accretionary complex, Kyrgyz South Tianshan

Cherts, radiolarites, and radiolarian cherts are one of the most important components of accretionary complexes, and much used in the reconstruction of imbricated sections of ocean plate stratigraphy, and of the palaeogeographic and tectonic settings of ophiolitic mélanges. There has been little research on radiolarian cherts in the Kyrgyz South Tianshan, and the tectonic setting and temporal framework of the South Tianshan ocean are controversial. Here, we report well‐preserved radiolarians in ribbon‐bedded cherts in an ophiolitic mélange of the Atbashi accretionary complex (South Tianshan, Kyrgyzstan), which provide key evidence of ambient Palaeozoic oceanic conditions. The hydrofluoric acid technique was used to extract from the cherty rocks siliceous radiolarian, which yielded an age of late Early Carboniferous. Analyses of major, trace, and rare‐earth elements of four cherts indicate they are biogenic and deposited with little hydrothermal influence in a hemipelagic to continental margin environment. Our fossil ages and geochemical results of the radiolarian cherts from the Atbashi accretionary complex indicate that the South Tianshan ocean was still open in the Late Carboniferous, implying that it was a wide, long‐lived, major ocean that closed by northward subduction.