Late Devonian-Early Carboniferous Research Articles

Позднедевонско-раннекаменноугольная изолированная карбонатная платформа на Приполярном Урале (р. Бол. Надота)

Позднедевонско-раннекаменноугольная изолированная карбонатная платформа на Приполярном Урале (р. Бол. Надота)

Phylogeny of Selaginellaceae: There is value in morphology after all!

The cosmopolitan lycophyte family Selaginellaceae, dating back to the Late Devonian-Early Carboniferous, is notorious for its many species with a seemingly undifferentiated gross morphology. This morphological stasis has for a long time hampered our understanding of the evolutionary history of the single genus Selaginella. Here we present a large-scale phylogenetic analysis of Selaginella, and based on the resulting phylogeny, we discuss morphological evolution in the group. We sampled about one-third of the approximately 750 recognized Selaginella species. Evolutionary relationships were inferred from both chloroplast (rbcL) and single-copy nuclear gene data (pgiC and SQD1) using a Bayesian inference approach. The morphology of the group was studied and important features mapped onto the phylogeny. We present an overall well-supported phylogeny of Selaginella, and the phylogenetic positions of some previously problematic taxa (i.e., S. sinensis and allies) are now resolved with strong support. We show that even though the evolution of most morphological characters involves reversals and/or parallelisms, several characters are phylogenetically informative. Seven major clades are identified, which each can be uniquely diagnosed by a suite of morphological features. There is value in morphology after all! Our hypothesis of the evolutionary relationships of Selaginella is well founded based on DNA sequence data, as well as morphology, and is in line with previous findings. It will serve as a firm basis for further studies on Selaginella with respect to, e.g., the poorly known alpha taxonomy, as well as evolutionary questions such as historical biogeographic reconstructions.

Petrogenesis of Late Devonian–Early Carboniferous volcanic rocks in northern Tibet: New constraints on the Paleozoic tectonic evolution of the Tethyan Ocean

Paleozoic arc-type volcanic rocks in northern Tibet, provide important information on the Paleozoic tectonic evolution of the Longmu Co–Shuanghu Tethyan Ocean (LSTO). In this study, we obtained zircon U–Pb and Hf isotopic data, and major and trace element data for volcanic rocks from the Riwanchaka and Laxiongco areas in northern Tibet. LA-ICP-MS U–Pb analyses of zircons show that volcanic rocks in the Riwanchaka and Laxiongco areas formed at 374–372 and 356–353 Ma, respectively. These calc-alkaline volcanic rocks consist of basalt, andesite, dacite, and rhyolite. The basalt and andesite in the Riwanchaka area exhibit low TiO2 and high MgO and Na2O concentrations, and are enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs). The basalt probably originated from partial melting of a lithospheric mantle source, metasomatized by subduction-related fluids, whereas the andesite was probably derived from hydrous basalts after fractional crystallization. The Riwanchaka and Laxiongco rhyolites are characterized by high Na2O/K2O ratios and low Al2O3 concentrations, are weakly metaluminous to peraluminous (A/CNK=0.97–1.11), and exhibit typical arc-like geochemical signatures (e.g., enrichment in LILEs and strong depletion in HFSEs and Sr). In addition, they show high zircon εHf(t) values (+7.61 to +4.45). The geochemical characteristics and zircon εHf(t) compositions of the rhyolites indicate that they were derived from partial melting of juvenile basaltic lower crust. These results, combined with previous data, indicate that the LSTO lithosphere was subducted northward beneath the North Qiangtang subterrane (NQT) during the Late Devonian–Early Carboniferous. We suggest that the NQT represents a slice of an active continental margin developed on the southern margin of the Eurasian continent during the Late Devonian–Early Carboniferous.

Geochemistry of the Late Paleozoic cherts in the Youjiang Basin: Implications for the basin evolution

We analyzed the major and rare earth element compositions of siliceous deposits from the Upper Devonian Liujiang Formation, Lower Carboniferous Luzhai Formation, Lower–Middle Permian Sidazhai Formation and Tapi Formation, which are widely distributed as bedded cherts in the interplatform basinal successions of the Youjiang Basin. The Liujiang Formation and Luzhai Formation cherts generally have high Al/(Al+Fe+Mn) values (0.38–0.94) and are non-hydrothermal cherts. These cherts are generally characterized by moderately negative Ce anomalies and high Y/Ho values relatived to PAAS, indicating that the Youjiang Basin might have evolved into an open rift basin during the Late Devonian–Early Carboniferous. The Sidazhai Formation cherts from Ziyun generally have high Al/(Al+Fe+Mn) values (0.60–0.78), suggesting negligible contribution from a hydrothermal component. The Sidazhai Formation cherts from Hechi and the Tapi Formation cherts from Malipo generally have low Al/(Al+Fe+Mn) values (0.09–0.41), indicating an intense hydrothermal input. Relatived to the Sidazhai Formation cherts, the Tapi Formation cherts have higher Ce/Ce* values (0.68±0.19) and lower Y/Ho values (41.83±13.27), which may be affected by the terrigenous input from the Vietnam Block. The Sidazhai Formation cherts from Ziyun and Hechi exhibit negative Ce anomalies (0.43±0.12, 0.33±0.17, respectively) with high Y/Ho values (57.44±16.20, 46.02±4.27, respectively), resembling the geochemical characteristics of open-ocean basin cherts. These cherts were deposited on a passive continental margin adjacent to the Babu branch ocean, which may have contributed to upwelling. Detailed spatial studies on geochemical characteristics of the Late Paleozoic cherts can unravel the evolution of the Youjiang Basin.

Geochemical variation of amphiboles in A-type granites as an indicator of complex magmatic systems: Wentworth pluton, Nova Scotia, Canada

The Wentworth pluton is the most complex intrusion among a series of A-type granitic plutons, emplaced along an active shear zone in the Cobequid Highlands of Nova Scotia during the latest Devonian–earliest Carboniferous. This pluton consists of A-type granites of different generations and a large gabbroic body. Among all late Paleozoic plutons in the Cobequid Highlands, only the Wentworth granites contain both primary calcic (edenite, hornblende) and sodic amphiboles (arfvedsonite). Whole-rock, and mineral chemical data were examined along with estimated magmatic parameters, such as temperature, emplacement pressure, oxygen fugacity and volatile contents, in order to understand the geological factors responsible for this mineralogical variation. The granites with calcic amphiboles show systematically lower εNd values, magmatic temperatures, and F-in-melt contents and appear geochemically less evolved, compared to the granites with the sodic amphiboles. All granites, however, appear geochemically homogeneous with limited evidence of fractionation. Specific chemical differences between these two types of granites indicate the presence of two coeval but distinct granitic systems in the Wentworth pluton. The granites with the calcic amphiboles were formed from a geochemically less evolved, hydrous, relatively calcic melt, whereas the granites with sodic amphiboles were emplaced at structurally deeper levels and were derived from a relatively drier sodic melt, which had a larger mantle component and was enriched in fluorine. Coexistence of these two magmas is indicated by the presence of interstitial sodic–calcic amphiboles in a late granitic dyke. This study provides evidence that in A-type granites, where fractional crystallization is limited, extreme variation in the type of amphibole can be the result of a very complex magmatic history.

Late Paleozoic intrusive rocks from the southeastern Lhasa terrane, Tibetan Plateau, and their Late Mesozoic metamorphism and tectonic implications

Abstract The Lhasa terrane in southern Tibet experienced Late Paleozoic and Mesozoic–Cenozoic composite orogenesis. This work reports a study on the petrology, geochemistry, zircon U–Pb chronology and Hf isotopes of Late Paleozoic and Late Mesozoic intrusive rocks from the southeastern Lhasa terrane. The Late Paleozoic intrusive rocks crystallized in the Late Devonian–Early Carboniferous of 371 to 355 Ma, representing a bimodal igneous association formed in the back-arc extensional setting. The mafic end-member originated from the enriched mantle and experienced contamination of crustal materials, characterized by a slight enrichment of LREE, positive anomalies of U, K and Pb and negative anomalies of Th, Nb, Ta and Ti. The felsic end-member was derived from the partial melting of the ancient continental crust, characterized by metaluminous, positive anomalies of Th, Zr and Hf, negative anomalies of Ba, Sr, Nb, Ta and Ti and negative e Hf (t) values of zircon with T DM2 ages from 1.90 to 1.40 Ga. The Late Cretaceous (ca. 107 Ma) mafic intrusions, along with the Late Paleozoic intrusive rocks, underwent nearly syn-intrusion amphibolite-facies metamorphism under P–T conditions of 0.56 to 0.69 GPa and 692 to 735 °C during the Andean-type orogeny correlated with the subduction of the Neo-Tethyan oceanic slab beneath the Lhasa terrane. This study provides a new insight into the pre-Cenozoic tectonic evolution of the Lhasa terrane.

Qiarbahete Sanukitoids and adakitic rocks in NW Tianshan: age and geochemical constraints on their petrogenesis and tectonic settings

The Qiarbahete complex in NW China consists of gabbroic diorite, granodiorite, and late-stage quartz diorite porphyry veins. Zircon sensitive high-resolution ion microprobe (SHRIMP) U–Pb analyses show that the gabbroic diorite and granodiorite formed at 368 ± 5.2 Ma and 354 ± 4.1 Ma, respectively, indicating that the complex was emplaced in the Late Devonian–Early Carboniferous. The gabbroic diorites, characteristic of Sanukitoids, exhibit high Mg# (62 average), MgO (6.84% average), Cr (195 ppm average), and Ni (61.4 ppm average) contents. The rocks show moderately fractionated rare earth element (REEs) patterns and weak negative Eu anomalies (δEu: 0.83–0.89), enrichment of large ion lithophile elements (LILEs), and depletion of high field strength elements (HFSEs), with low ϵNd(t) values (1.46–1.73). The gabbroic diorites originated from partial melting of a hydrous mantle wedge followed by assimilation of crust during ascent. The granodiorites show a geochemical affinity with adakitic rocks, e.g. SiO2 (64.95–67.87%) > 56%, Al2O3 (15.88–16.56%) > 15%, MgO (1.79–2.31%) < 3%, Sr (315–375 ppm) > 300 ppm, and Yb (1.84–2.06 ppm). They are enriched in light rare earth elements (LREEs) and LILEs and depleted in HFSEs, with weak negative Eu anomalies (δEu: 0.78–0.87). The granodiorites were mainly derived by the partial melting of a subducted oceanic slab, followed by subsequent melt–mantle interaction and crustal rocks contamination. All these indicate that the Qiarbahete complex was emplaced in a continental arc setting attending the southward subduction of the Junggar Ocean during the Late Devonian–early Carboniferous, generating the lateral accretion of continental crust in NW Tianshan.

Rhenium in porphyry copper deposits of the urals

The overwhelming majority of porphyry Mo-Au-Cu deposits in the Urals are related to the low-K quartz diorite minor intrusions of the island-arc type, which were formed from Silurian Middle-Late Carboniferous. In the South Urals, the Cu/Mo ratio of ore decreases eastward along with enrichment in Re. At the same time, molybdenite is depleted in this metal in compliance with more sialic crust and potassium content in ore-bearing dioritic rocks. Quartz diorites at the highest-Re deposits contain 1–2 wt % K2O. At most Early-Middle Devonian deposits and occurrences of the western Tagil-Magnitogorsk-West Mugodzhary femic megazone, molybdenite is sporadic. The Re content in rocks was mainly determined using the kinetic method and to a lesser extent with ICP-MS. A Cameca SX-100 microprobe was also used for study of molybdenite. The Cu/Mo ratio of ore exceeds 600; the Mo content is commonly 1–15 ppm (occasionally up to 30 ppm and higher); the Re content is up to 0.01–0.04 ppm, sporadically increasing to 0.08–0.17 ppm. At the same time, the Re content in molybdenite often reaches 0.2–0.4 wt %. The highest Re concentration was established in the ore of the largest Mikheevsky deposit formed in the Late Devonian-Early Carboniferous and localized in the easternmost part of the East Ural sialic-femic megazone. The Re content in the orebodies of this deposit often reaches 0.2–0.5 ppm (up to 1.4–2.7 ppm) and 0.21 wt % in molybdenite. The average Mo grade of ore is 80 ppm and Cu/Mo ratio is 66. These data and Sr isotopic composition of ore-bearing granitoid and metasomatic rocks [(87Sr/86Sr)t = 0.7038–0.7051; (ɛNd)t = 3–7] testify to the mantle source of matter with insignificant admixture of crustal material. The same is apparently valid for Re and Cu in contrast to Mo. This statement is corroborated by the inverse correlation between Cu/Mo and Mo/Re ratios in the ore. Fluid-crystal fractionation of ore-bearing dioritic rocks is accompanied by enrichment of ore in Mo and by decrease in Re content in molybdenite. In the Tarutino ore field, the pyrite-chalcopyrite mineralization gives way to the molybdenite mineralization in line with in-sequence intrusion of diorite with quartz-bearing groundmass and granodiorite porphyry. Because of increasing silica content in granitoids, the Re concentration in molybdenite commonly remains below 0.07 wt % as is noted at the rare deposits localized in the sialic megazones.

Problems of geodynamics, tectonics, and metallogeny of orogens

This is an overview of papers published in the present volume of Russian Geology and Geophysics (Geologiya i Geofizika), a special issue that covers presentations at the International Conference “Geodynamic Evolution, Tectonics, and Metallogeny of Orogens”, held on 28–30 June 2010 in Novosibirsk ( http://altay2010.igm.nsc.ru). The workshop concerned the general evolution of the Central Asian orogenic system, with a special focus on continental growth, history of oceans and continental margins, and role of plumes in accretionary-collisional tectonics and metallogeny. The discussed papers are grouped in three sections: 1. General issues of geodynamics and geodynamic evolution; 2. Role of mantle plumes in tectonics, magmatism, and metallogeny; 3. Regional tectonic and geodynamic problems of Asia. The synthesis of data reported at the workshop demonstrates critical importance of mantle plumes for the evolution of the Paleoasian ocean and for orogenic processes in Central Asia. In addition to three large pulses of continental growth at about 2900–2700, 1900–1700, and 900–700 Ma, three orogenic stages have been distinguished in the geological history of Eurasia: Late Cambrian–Ordovician (510–470 Ma), Late Devonian–Early Carboniferous (380–320 Ma), and Permian–Triassic (285–230 Ma). In the evolution of the Central Asian orogen, these stages were associated with events of ultramafic-mafic and bimodal plume magmatism which promoted translithospheric strike-slip faulting. Plume magmatism was an active agent in ocean opening when the Paleotethys, Ural, Ob–Zaisan, and Turkestan basins appeared while the Late Cambrian–Ordovician orogen was forming in Central Asia (North Kazakhstan, Altai–Sayan, Tuva, and Baikal areas). Closure of the Ob–Zaisan ocean and collision of the Kazakhstan–Baikal continent with Siberia in the Late Devonian–Early Carboniferous was coeval with the maximum opening of the Turkestan ocean, possibly, as a consequence of plume activity. The Tarim (285–275 Ma) and Siberian (250–230 Ma) superplume events corresponded in time to closure of the Ural ocean and opening of the Meso- and Neotethys, as well as to major metallogenic events.

The early siphonodellids (Conodonta, Late Devonian-Early Carboniferous): overview and taxonomic state

Previous conodont studies on the Devonian/Carboniferous (D/C) boundary revealed that the current definition of the D/C boundary cannot be maintained due to several problems relating to the index taxon. The base of the Carboniferous System is defined by the first occurrence of the cono- dont species Siphonodella sulcata, within the S. praesulcata - S. sulcata lineage. However, problems in discriminating S. praesulcata from S. sulcata have been evidenced by the conodont studies of a variety of authors. To provide an overview about the early siphonodellids and the taxonomic state, conodonts from the current GSSP La Serre (Montagne Noire, France), as well as all data from the literature were taken into account. Based on these data, several morphotypes could be distinguished, although the great majority of specimens show intermediate features between the two taxa. There- fore, precise identification of such specimens is very much dependent on personal interpretation, thus strongly influencing the precise position of the D/C boundary which, as a result, is not synchronous. Alternative scenarios for the definition of the D/C boundary need to be considered in the future.

Geochemical constraints on the depositional environment of Upper Devonian radiolarian cherts from Loei, north-eastern Thailand

Late Devonian radiolarian chert sequences in the Indochina block of north-eastern Thailand are exposed in a narrow belt located to the east of Loei province. The analyzed radiolarian cherts were collected from Chiang Klom, Sumnoi and Pha Samyod localities. They are characterized by high silica content (>90 wt.%), high aluminum content and low iron content (Al2O3/(Al2O3 + Fe2O3): geomeans 0.91, 0.88 and 0.92). The Al/(Al + Fe + Mn) values are high in both cherts and interbedded shales (geomean 0.89). High Si/(Si + Al + Fe) ratios are observed in cherts (geomean 0.97) and slightly low in shales (geomean 0.69), whereas Fe2O3/TiO2 values are low (geomean 5.91). For rare earth element (REE) analysis, the cherts exhibit low La abundances (geomean 4.31, 3.59 and 4.22), slightly negative Ce anomalies (Ce/Ce*: geomean 0.81, 0.76 and 0.93), intermediate ratios of North American Shale Composite (NASC) normalized Lan/Cen (geomean 1.33, 1.37 and 1.12) and intermediate Lan/Ybn values (0.79, 0.94 and 1.22). In the interbedded shales, REE characteristics are more or less equal to the cherts. The results indicate that these late Devonian radiolarian cherts were deposited in a continental margin environment. High Si/(Si + Al + Fe) values indicate a biogenic origin of the cherts, however, additional silica content in the cherts could be the result of diagenetic alteration. Intermediate positive Eu anomalies (Eu/Eu*: geomean 1.32, 1.25 and 1.44) are interpreted as the result of detrital feldspar contribution corresponding to the distinctive low content of Fe. Geological evidences from the field support volcanic activities during the Late Devonian-Early Carboniferous. Weathered materials and fragments of basalts and andesites would possibly be supplied to the basin during chert sedimentation. These geochemical constraints indicate that, in western portion of Indochina, deep marine basin was closed before Late Devonian which was followed by the incursion of Early Carboniferous orogeny.

Tectonics of Northeast Asia: An overview

The tectonic units of the Verkhoyansk-Chukotka Mesozoides and the Koryak-Kamchatka Fold Region substantially differ from each other in the structure and composition of terranes. The geodynamic settings of terrane formation are defined and the main stages of their tectonic history are reconstructed. The formation of Mesozoides was mainly controlled by collision, largely between the continent and the Kolyma-Omolon and Chukchi microcontinents. The accretionary structure of the Koryak Highland comprises various terranes transported by Pacific plates and docked to the Asian continent, periodically accreting its margin. The following evolutionary stages are established: destruction of the North Asian continent (Ordovician, Late Devonian-Early Carboniferous, Permian-Triassic); amalgamation (Middle Jurassic for Kolyma and Mid-Cretaceous for Koryak terranes); collision (terminal Early Cretaceous); and continental growth (terminal Early Cretaceous, terminal Late Cretaceous, middle Eocene).

Manifestations of the Late Carboniferous and Early Permian stages of formation of nappe-fold structures in the southern framework of the Siberian platform (East Sayany, South Siberia)

The geological structure of the Tunka Goltsy (the Tunka Range) of the East Sayany is characterized by a complex nappefold structure, composed mainly of Paleozoic terrigenous and carbonate rocks and their metamorphosed analogues (1-3). It is generally rec� ognized that the nappefold structure of the East Sa� yany, including its southeastern segment, regarded as the Tunka terrain (3) or Ilchirskaya zone (4), formed in the Ordovician as a result of collision between the Tuva-Mongolian microcontinent and the Siberian continent. As referred to in (5), the Ordovician-Mid� dle Paleozoic deformations over the entire vast terri� tory of Central Asia, from the Olkhon zone of the Pribaikalie to the North Kazakhstan, were manifested as a result of the closing of the oceanic basin and the subsequent collision between the Kazakhstan- Baikalian complex continent (including the Tuva- Mongolian microcontinent) and the Siberian conti� nent. In the Ordovician the Olkhon nappeoverthrust zone was formed along the southeastern framework of the Siberian Craton. In addition, the metamorphism was manifested over the entire vast territory of the East Sayany that could probably be connected with nappe formation. In the Late Ordovician-Silurian, the oblique slipthrust structures, magmatism, and meta� morphism were manifested in the Sangilen highlands and Tuva. Later, the deformations continued. In the Late Devonian-Early Carboniferous, the dextral strikeslip fault Charysh-Terektinskaya zone was formed; in the Late Carboniferous, the Kurayskaya and Kuznetsko-Teletsko-Bashkaus sinistral strike� slip shear zones were formed. In the Late Carboniferous-Permian, the collision between the East European, Kazakhstan-Baikalian, and Siberian continents took place (5, 6). In East Kazakhstan, the sinistral strikeslip movements took place in the band of more than 400 km width along the Chara ophiolite zone and the Irtysh and Northeastern shear zones. The age of deformation rejuvenates regu� larly to the east towards the internal part of the Sibe� rian continent; the movement amplitude along shears decreases in the same direction from several thousand to a hundred kilometers. In the late Palaeozoic in the Tunka Goltsy and the adjacent regions, a number of geological events of Late Carboniferous and Early Permian age are distin�

Tectonics of the Ural paleozoides in comparison with the Tien Shan

The main differences and similarities between the tectonic features of the Urals and the Tien Shan are considered. In the Neoproterozoic and Early and Middle Paleozoic, the Ural and Turkestan oceanic basins were parts of one oceanic domain, with several distinct regions in which tectonic events took different courses. The Baltic continental margin of the Ural paleoocean was active, whereas the Tarim-Alay margin of the Turkestan ocean, similar in position, was passive. The opposite continental margin in the Urals is known beginning from the Devonian as the Kazakh-Kyrgyz paleocontinent. In the Tien Shan, a similar margin developed until the Late Ordovician as the Syr Darya block with the ancient continental crust. In the Silurian, this block became a part of the Kazakh-Kyrgyz paleocontinent. The internal structures of the Ural and Turkestan paleooceans were different. The East Ural microcontinent occurred in the Ural paleoocean during the Early and Middle Paleozoic. No microcontinents are established in the Turkestan oceanic basin. Volcanic arcs in the Ural paleoocean were formed in the Vendian (Ediacarian), at the Ordovician-Silurian boundary, and in the Devonian largely along the Baltic margin at different distances from its edge. In the Turkestan paleoocean, a volcanic arc probably existed in the Ordovician at its Syr Darya margin, i.e., on the other side of the ocean in comparison with the Urals. The subduction of the Turkestan oceanic crust developed with interruptions always in the same direction. The evolution of subduction in the Urals was more complicated. The island arc-continent collision occurred here in the Late Devonian-Early Carboniferous; the continent-continent collision took place in the Moscovian simultaneously with the same process in the Tien Shan. The deepwater flysch basins induced by collision appeared at the Baltic margin in the Famennian and Visean, whereas in the Bashkirian and Moscovian they appeared at the Alay-Tarim margin. In the Devonian and Early Carboniferous, the Ural and Turkestan paleooceans had a common active margin along the Kazakh-Kyrgyz paleocontinent. The sudduction of the oceanic crust beneath this paleocontinent in both the Urals and the Tien Shan started, recommenced after interruptions, and finally ceased synchronously. In the South Ural segment, the Early Carboniferous subduction developed beneath both Baltica and the Kazakh-Kyrgyz paleocontinent, whereas in the Tien Shan, it occurred only beneath the latter paleocontinent. A divergent nappe-fold orogen was formed in the Urals as a result of collision of the Kazakh-Kyrgyz paleocontinent with the Baltic and Alay-Tarim paleocontinents, whereas a unilateral nappe-fold orogen arose in the Tien Shan. The growth of the high divergent orogen brought about the appearance of the Ural Foredeep filled with molasse beginning from the Kungurian. In the Tien Shan, a similar foredeep was not developed; a granitic axis similar to the main granitic axis in the Urals was not formed in the Tien Shan either.

Palaeozoic orogeneses around the Siberian craton: Structure and evolution of the Patom belt and foredeep

This paper sheds light on the evolution of the Patom belt. This mountain range draws an arc along the southeastern edge of the Siberian craton. It is supposed to be of Caledonian age and to result from the accretion of microcontinents against the craton, but up to now, its detailed stratigraphic and tectonic history was unclear. A field study allows us to clarify it. The sedimentary record is marked by a slow evolution with stable periods of more than 250 Ma. The observed sedimentary succession confirms the existence of a passive margin setting in the late Riphean (900 Ma), followed during the Vendian (650–600 Ma) by the obduction of the Baikal‐Muya ophiolites belt and a foredeep inversion. After that, a Late Cambrian extension occurred, which is first described in this paper. The second collision stage occurred after 385 Ma, in the Late Devonian–Early Carboniferous. Sedimentary and tectonic data are interpreted in the light of the geodynamic evolution of Siberia, which is dominated by continental collages against the Siberian craton. Field data reveal a homogeneous direction of compression from the inner areas to the foreland. Whereas the inner range displays metamorphosed units deformed in the ductile domain, deformation is weaker in the foreland, which developed above the cratonic crust. We relate this high deformation gradient to the presence of the stiff craton which impeded strain propagation. The irregular shape of the craton as well as preexistent basement topography can explain the different tectonic styles observed along the belt.

A review of the pre-Permian geology of the Variscan French Massif Central

The Massif Central, like the southern part of the Massif Armoricain, belongs to the north Gondwana margin. The Massif Central consists of a stack of nappes resulting from six main tectonic-metamorphic events. The first, D 0, is coeval with a Late Silurian (ca 415 Ma) high-pressure (HP) (or ultra high-pressure) metamorphism for which the associated structures are poorly documented. The Early Devonian D 1 event, responsible for top-to-the-southwest nappe displacement, is coeval with migmatization and the exhumation of HP rocks around 385–380 Ma. In the northern part of the Massif Central, metamorphic rocks with retrogressed eclogites are covered by Late Devonian undeformed sedimentary rocks. The Late Devonian-Early Carboniferous D 2 event involves top-to-the-northwest shearing, coeval with an intermediate pressure-temperature metamorphism dated around 360–350 Ma. The Visean D 3 event is a top-to-the-south ductile shearing, which is widespread in the southern Massif Central. Coevally, in the northern Massif Central, the D 3 event corresponds to the onset of synorogenic extension. The next two events, D 4 and D 5, of Early and Late Carboniferous age, correspond to the syn- and late orogenic extensional tectonic regimes, respectively. The former is controlled by NW–SE stretching whereas the latter is accommodated by NNE–SSW stretching. These structural and metamorphic events are reconsidered in a geodynamic evolution model. The possibilities of one or two cycles involving microcontinent drifting, rewelding and collision are discussed.

Crust-forming processes in the Hercynides of the Central Asian Foldbelt

The paper reports data on the evolutionary history of magmatism, its conditions, and sources in the process of the development of the Southern Mongolian Hercynides during the pre-accretion, continental-mar- gin, and rifting stages within the time span from the Silurian to Early Permian. The Hercynian continental crust in the southern Mongolian segment of the Central Asian Foldbelt (CAFB) was determined to have grown in the environment of ensimatic island arcs, backarc basins, spreading centers, and oceanic islands or plateaus, with material coming from the depleted and, perhaps, also enriched mantle sources in the open ocean that surrounded the Siberian paleocontinent on the side of the Caledonian margin. This made it possible to recognize the Early- Middle Paleozoic epoch of juvenile crustal growth in CAFB and the corresponding isotopic crustal province with a total area of more than 200 thousand km 2 . The principal differences between the composition and struc- ture of the blocks surrounding the Hercynian regions (Caledonides in the Gobi Altai and Grenwillides in the South Gobi microcontinent) testify that the southern margin of the Caledonian Siberian continent and the Gren- villides of the South Gobi microcontinent had different geological histories and were spatially separated. The structural complex of the Paleoasian ocean, including the terranes of the South Gobi microcontinent, were transformed into a continental block in the latest Devonian-earliest Carboniferous, in relation with accretion processes, folding, metamorphism, and tectonic delamination along the boundaries of structurally heteroge- neous domains. The subsequent recycling of the crust by magmatic processes was related to the development of an active continental margin (ACM). The development of an ACM in the Hercynides resulted from and was a continuation of the motions of the continental and oceanic lithospheric plates, i.e., processes that brought about the Hercynian accretion. The evolution history of the ACM was subdivided into two stages: early (a con- tinental-margin stage proper) and late (rifting stage). The rocks of the early stage were produced at 350-330 Ma and compose a differentiated basalt-andesite-rhyodacite complex and related massifs of the granodiorite-pla- giogranite and banatite (diorite-monzonite-granodiorite) associations. During the rifting stage at 320-290 Ma, a bimodal basalt-comendite-trachyrhyolite association was formed, along with accompanying alkali granite massifs. In the southern Mongolian segment of the Hercynides, the rocks of the rifting stage compose two sub- parallel rift zones: Gobi-Tien Shan, which extends along the boundaries of the South Gobi microcontinent, and the Main Mongolian lineament, which marks the boundaries between the Hercynides and Caledonides in the CAFB. The rift structures are made up of alkali granitoids and normal-alkalinity granitoids, which are atypical of rift zones. Their genesis is thought to have been related to crustal anatexis, a process that was triggered by rift-related magmas at an unusual combination of rifting and ACM tectonic setting. The basic rocks of the rift associations have geochemical signatures atypical of continental rifting. They show Ta and Nb minima and K and Pb maxima, as is typical of rocks generated at convergent plate boundaries. Nevertheless, the broad varia- tions in the concentrations and ratios of some major and incompatible trace elements and in the Sr, Nd, and O isotopic composition of the rift basaltoids allowed us to distinguish their high- and low-Ti varieties, which were produced with the participation of three mantle sources: depleted mantle similar to the source of basalts in mid- oceanic ridges, enriched mantle like the source of basalts in oceanic islands, and the mantle material of the metasomatized mantle wedge. The origin of andesites in the rift zones is explained by the contamination of mantle basaltoid melts with sialic (predominantly sedimentary) material of the continental crust or the assimi- lation of anatectic partial granite melts.

Bryozoan diversity in Southern Siberia at the Devonian–Carboniferous transition: New data confirm a resistivity to two mass extinctions

Two spectacular mass extinctions, namely the Kellwasser and the Hangenberg events, affected the Earth's biota during the Late Devonian–Early Carboniferous. Diverse assemblages of bryozoans are known from the Fasnian–Tournaisian deposits of Southern Siberia, which includes such regions as the Kuznetsk Basin, the Kolyvan'–Tom' Zone, the Rudny Altay, and the Gorny Altay. Our new data establish the stratigraphic ranges of 154 species, which belong to 56 genera and 21 families. These data have been analyzed quantitatively to measure the patterns of diversity dynamics and the changes in the taxonomic diversity structure. The species diversity increased at the Frasnian–Famennian transition. After a short-term, but dramatic decrease in the middle Famennian, the total number of species accelerated in the late Famennian and the Tournaisian. A similar pattern occurred for both genera and families, although the radiation at the Frasnian–Famennian transition was not as large, and total family diversity decreased slightly in the Tournaisian. Changes in total species diversity differed between orders of bryozoans. Changes in taxonomic diversity structure were moderately rapid. The Tournaisian species diversity was governed by the other genera comparing to the early time intervals. Thus, our regional data suggest that bryozoans generally survived both the Frasnian/Famennian and the Devonian/Carboniferous mass extinctions, confirming similar conclusions made earlier with global data. However, a mid-Famennian regional crisis (diversity drop) in the evolution of bryozoans is documented within Southern Siberia. Bryozoan radiations coincided with pulses of basin deepening that occurred during both mass extinction intervals, perhaps explaining why bryozoans were resistant to extinction. The mid-Famennian crisis might have been caused by a rapid regression coupled with basin shallowing.

Late Devonian – Early Carboniferous vertebrate microremains from the Carnic Alps, northern Italy

Vertebrate microremains from the Late Devonian–Early Carboniferous of the Carnic Alps are predominantly chondrichthyan, with minor placoderm and actinopterygian remains. The faunas are sparse and, with very few exceptions, occur only in conodont-rich pelagic limestones (Pramosio Limestone) representative of the palmatolepid-bispathodid conodont biofacies. Phoebodont and jalodont chondrichthyans, also reflecting open-ocean environments, predominated during the Famennian, and eventually symmoriids seem to predominate during the Early Carboniferous. The presence of Siamodus in this assemblage gives a new locality for this genus known from few regions in the world and allows confirming its stratigraphical range ( limpidus Zone) and its relation to deep-water environments. The Late Devonian vertebrate faunas are tropical and cosmopolitan, having much in common with coeval taxa from the North-Gondwanan margins and Asian terranes. Composition of the vertebrate faunas is consistent with the Carnic Alps terrane having occupied a position intermediate between Gondwana and Laurussia, as hypothesized by various authors, but because of sparsity of the taxa represented and the pronounced cosmopolitan nature of both the conodont and vertebrate faunas, the data are not compelling.

The Palaeozoic tectono-metallogenic evolution of the northern Tasman Fold Belt System, Australia: Interplay of subduction rollback and accretion

The Tasman Fold Belt System in eastern Australia provides a record of the Palaeozoic geological history and growth of the Australian continent along the proto-Pacific margin of Gondwana inboard of an extensive and long-lived subduction system. The Hodgkinson and Broken River provinces represent prominent geological elements of this system and together form the northern Tasman Fold Belt System. Geochronological age dating of the timing of gold formation in the Amanda Bel Goldfield in the Broken River Province and the Hodgkinson Goldfield in the Hodgkinson Province provides constraints on the occurrence of a deformation and mineralisation episode in the Late Devonian–Early Carboniferous. Integration of these newly-obtained data with petrogenetic constraints and a time–space evaluation of the geological evolution of the Hodgkinson and Broken River provinces, as well as other terranes in the northern Tasman Fold Belt System, allows for the development of a geodynamic model for the Palaeozoic evolution of the northern Tasman Fold Belt System. Our model indicates that three cycles of extension–contraction occurred during the Palaeozoic evolution of the northern Tasman Fold Belt System. Episodes of extension were controlled by rollback of the subduction system along the proto-Pacific margin of Gondwana, whereas episodes of contraction resulted from accretion following the arrival of positively buoyant segments (i.e., micro-continental blocks/oceanic plateaus) at the subducting trench. Our composite interpretative model on the geodynamic evolution of the northern Tasman Fold Belt System integrates the timing of the development of mineral deposits throughout this part of the system and provides a significant advancement in the understanding of Palaeozoic geodynamics along the margin of Gondwana in northeast Australia and allows comparison with the southern part of the Tasman Fold Belt System.