Island-arc Complexes Research Articles

AGE AND GEODYNAMIC SETTINGS FOR FORMATION OF CARBONATITE COMPLEXES AND ASSOCIATED RARE METAL DEPOSITS OF THE SOUTH URAL

The Ilmen-Vishnevogorsky and Buldym carbonatite complexes occurring in the Southern Urals represent linear deformed carbonatite complexes. Their origin, as well as the age and geodynamic conditions remain the subject of debate. The isochron methods (Rb-Sr, Sm-Nd, TIMS) and local U-Pb-dating of zircons (SHRIMP II and LA-ICP MS) of these carbonatite complexes were employed to determine the age and duration of the stages of alkali-carbonatite magmatism and associated rare-metal ore formation. The Silurian-Devonian U-Pb zircon ages of the early phases of Ilmen-Vishnevogorsky miaskites and carbonatites were determined as 420.7±11 Ma (S2) and 417±2.8 Ma (D1), respectively. In the later phases of miaskites and carbonatites, early zircons are resorbed, they have broken isotope systems, and later zircon generations form a cluster of 386±7.6 Ma (D2). The Lower Permian U-Pb age of zircon 280±8 Ma (P1) was determined in the miaskite-pegmatite and late carbonatite. The isochron dating of late ore-bearing varieties of carbonatites provided the ages from the Lower Permian to the Early Triassic (P1–T1): 254±18 Ma, Sm-Nd and 247±4 Ma, Rb-Sr, IVC; 280±53 Ma, Sm-Nd, Buldym complex. Thus, the generation and intrusion of alkaline magmas in the Urals occurred ~420 Ma (S2–D1), synchronously with the formation of island-arc complexes. They are related to rifting on the emerging continental margins. The tectonic activity and formation of alkaline rocks and carbonatites proceeded in the Middle Devonian (~380 Ma, D2); it correlates with the accretion-collision stage of the Urals development. At the stage of "hard" collision (~280 Ma, P1), the Ilmen-Vishnevogorsky and Buldym complexes were plastically deformed, underwent melting and emplaced conformably with collision-slip tectonic structures. Recrystallization of rocks and minerals, plastic and brittle deformations, processes of pegmatitic, carbonatitic and rare-metal ore formation are associated with palingenic-metasomatic transformation of rifting alkaline complexes of Silurian-Devonian age at the collision and post-collision (~250 Ma, P3–T1) stages of the Urals emplacement.

ISOTOPE COMPOSITION OF Pb AND Nd OF THE LATE NEOPROTEROZOIC–EARLY PALEOZOIC GRANITOIDS IN EASTERN TUVA (Case study of the Kaa-Khem batholith)

We present the results of Nd and Pb isotope studies of late Neoproterozoic–early Paleozoic granitoids of the Kaa-Khem batholith, which formed in different geodynamic settings within the early Caledonian structures of Eastern Tuva (Altai–Sayan sialic area). Based on the obtained isotope data, we assess the role of different source materials in the formation of melts for these granitoids and compare these rocks with coeval granitoids from the Lake Zone of Mongolia. The plagiogranitoid associations of the island arc stage of the Kaa-Khem batholith formation (572–562 Ma) are characterized by mantle Nd isotope values. In contrast, the Pb isotope characteristics of these intrusive associations correspond to an upper-crust source formed predominantly by terrigenous sediments. The granitoid associations of the accretion–collision stage of the Kaa-Khem batholith formation (512–450 Ma) have a less radiogenic primary Pb isotope composition as compared with the island arc granitoids. Evidently, during the magma formation at this stage, the relative role of terrigenous material decreased, while the role of depleted mantle material increased. At the same time, the decrease in ɛNd values in the granitoids as compared with the depleted mantle indicates that, along with the upper-mantle component, the role of the lower-crust component increased. These Nd and Pb isotope characteristics of the accretionary granitoids of the Kaa-Khem batholith indicate a significant contribution of the lower-crust source to their formation. This source is the crystalline basement of the Tuva–Mongolian microcontinent. The Pb isotope characteristics of the accretion–collision granitoids of the Kaa-Khem batholith are shifted to the upper-crust compositions relative to those of the Lake Zone granitoids. The reason is that the latter formed predominantly through the transformation of early Caledonian island arc complexes, which were generated with a higher portion of mantle component.

Plagioclase of Intrusive Rocks from the Pervomai–Ayudag Island-Arc Complex (Mountainous Crimea): from Plagiolherzolite to Plagiogranite

Plagioclase of Intrusive Rocks from the Pervomai–Ayudag Island-Arc Complex (Mountainous Crimea): from Plagiolherzolite to Plagiogranite

Geochemistry and petrogenesis of ophiolitic rocks from the Indus Suture Zone (ISZ), Ladakh Himalaya: insights for depleted mantle beneath an intra-oceanic island arc complex

ABSTRACT The Indus Suture Zone preserves the ophiolitic remnants of the eastern part of the Neo-Tethys Ocean. Parts of these ophiolitic remnants are exposed along the Khangral-Chiktan and Dras-Kargil road sections of western Ladakh. The observed rock types include ultramafic-mafic cumulates, gabbros, and volcanics emplaced as faulted blocks over the Mesozoic Dras arc complex. Geochemically, these rocks show sub-alkaline tholeiitic characteristics with basalt to basaltic-andesite compositional variation. Based on modal mineralogy, the ultramafic cumulates classify as olivine-websterite, the mafic cumulates as olivine-norite, and the gabbros as norite to hornblende-gabbro. The ultramafic cumulates depict depleted chondrite normalized REE patterns [(La/Yb)N = 0.6–1.1] while the mafic cumulates display depleted to enriched REE patterns [(La/Yb)N = 0.6–3.2]. Fractionated patterns are observed in gabbros [(La/Yb)N = 1.6–4.1], while flat and enriched chondrite normalized REE-patterns in volcanics [(La/Yb)N = 1.0–12.3] similar to NMORB, and EMORB. The multi-element patterns depict subduction-related geochemical characteristics such as enriched LILE and depleted HFSE compared to the primitive mantle. Presence of Mg-rich olivines, orthopyroxenes, and clinopyroxenes while Ti-poor clinopyroxenes in ultramafic-mafic cumulates reflect their derivation from previously depleted mantle sources at high-pressure and temperature comparable to the base of modern intra-oceanic island arc tholeiitic sequences. The studied rocks exhibit close similarity to an intra-oceanic subduction system contemporaneous to the Dras-Shergol-Suru-Thasgam ophiolitic slices of western Ladakh.

SOURCES OF LOWER PALEOZOIC SANDSTONES FROM THE NORA-SUKHOTINO TERRANE: RESULTS OF U-Th-Pb AND Lu-Hf ISOTOPE STUDIES OF THE DETRITAL ZIRCONS

This paper presents the results of comprehensive geological and geochemical studies of sandstones from the Belaya basin of the Amur fragment of the Nora-Sukhotino terrane of the northeastern flank of South Mongolia-Khingan orogenic belt, as well as the results of isotopic (U-Th-Pb and Lu-Hf) studies of detrital zircons therefrom. The sandstones from the Belaya basin are characterized by significant variations in the composition of clastic material and major rock-forming elements, and by correspondence to arkoses and litharenites. They were accumulated due to erosion of geochemically mature felsic and intermediate igneous rocks. The sandstones are dominated by Ordovician, Cambrian, and Neoproterozoic zircons. The youngest zircons in sandstones from the Belaya basin are dated back to 451±8 Ma, with the lower age boundary set at the Late Ordovician. The Ordovician zircons, which have positive ε Hf(t) values and Early Paleozoic – Neoproterozoic Hf-model ages, could have been derived from island arc complexes. The Cambrian and Neoproterozoic zircon grains with positive ε Hf(t) values and Neo – Mesoproterozoic Hf-model ages could have been derived from Early Paleozoic and Neoproterozoic igneous rocks of the Bureya or Mamyn terranes. The available geological materials provide evidence that the sandstones from the Belaya basin are a fragment of the Early Paleozoic accretionary complex, which formed in the Early Paleozoic along the southeastern margin of the Mamyn (Xing’an) massif. However, a small amount of data on the Early Paleozoic and Neoproterozoic magmatism of the region hinders drawing more definite conclusions.

SVECOFENNIAN ACREТION OROGEN: MODERN VIEWS ON ITS STRUCTURE AND EVOLUTION

The most important characteristic of the Svecofennian orogen is mainly the juvenile nature of the substances of the Proterozoic crust and the absence of geological, petrological and isotopic-geochemical evidence of the existence of rocks of the Archean complex at the base of the newly formed block of the svecofennians. All volcano-sedimentary rocks are altered up to granulite facies of metamorphism during two stages of endogenous activity. Modern configuration and the structure of the joint suture zone between Svecofennian orogeny and Karelian craton reflect the consistent development of oroclines, and not subparallel amalgamation of the island-arc complexes.

The Lower Ordovician Island-Arc Complex of Northern Kazakhstan: Substantiation of Age and Features of Composition

The Lower Ordovician Island-Arc Complex of Northern Kazakhstan: Substantiation of Age and Features of Composition

Geological setting and genesis of the Kurmansky gabbro-trondhjemite massif (Middle Urals)

This paper reports the results of petrogeochemical studies of the Kurmansky gabbro-trondhjemite massif (eastern slope of the Middle Urals), lying in the western part of the large Reftinsky allochthonous block within the accretion East Uralian megazone. The relevance of this study is determined by the uncertainty in geodynamic setting and formation conditions of the rock massif and its role in the evolution of the Ural Mobile belt. We specified the countours of the massif. It is shown that the rocks were resulted from spatiotemporal convergence of partial melting in the mantle and lower crust at the island-arc stage of the Ural Mobile belt evolution. Partial melting of mantle peridotite, under the influence of an aqueous fluid rising from the subduction zone, initiated the occurrence of basite melts. The separation of the melt and its subsequent evolution to the compositions of gabbrodiorite and diorite took place at Ptot=10 kbar. Trondhjemites were formed as a result of partial melting of amphibolites at Ptot≥8 kbar, PH2O=0.1–0.2 kbars. The crystallization of trondhjemites in the crust was accompanied by the wollastonite skarns on contact with carbonate rock and xenoliths culminated at mesoabyssal level, Ptot=PH2O=1 kbar. The comparison between the composition of Kurmansky gabbro-trondhjemite massif and the island-arc- and collision-related magmatic suites in the region allowed us to assume that the Kurmansky massif belongs to the independent Early Devonian (?) gabbro-trondhjemite complex of island arc origin. The rock metamorphism conditions were evaluated, with the transformations supposedly related to the accretion of early island arc complexes at the Murzinsky-Aduysky microcontinent, which took place in the Devonian.

Petrogenesis of Ophiolitic Rocks from Indus Suture Zone, Ladakh Himalaya: Insights for Depleted Mantle Beneath an Intra-Oceanic Island Arc Complex

Petrogenesis of Ophiolitic Rocks from Indus Suture Zone, Ladakh Himalaya: Insights for Depleted Mantle Beneath an Intra-Oceanic Island Arc Complex

U - Pb geochronology and composition of zircon mineral from granodiorite in the G18 Gold mine, Quang Nam and its significance in regional tectonics

The Tam Ky - Phuoc Son suture zone (TPSZ) is located in central Indochina and is referred to as a amalgamation site between the Truong Son and Kon Tum terranes during the early Paleozoic. The amphibolite and ultramafic rocks within the region were considered as a part of the Tam Ky - Phuoc Son ophiolitic complex. In this study, the authors present results of the U - Pb dating and trace element composition of the zircon grains derived from a granodioritic sample collected in the G18 gold mine in Quang Nam province in order to clarify the timing of magma emplacement and tectonic setting. The U - Pb dating data indicates that the granodioritic rocks formed at 447,4±2,9 Ma while the U/Yb ratio is ̴1 (average: 1.32) and the Sc/Yb ratio is ̴ 1.04, high Hf content (Average Hf: 10937 ppm) and low Yb content (Average Yb: 308 ppm). These geochemical values are comparable with the zircon, which is formed in the continental magmatic arc. Combination with the pre - existing data allowed us to confirm the existence of two contrast magma members in the North Kontum massif: The Middle Cambrian island arc complex and the Middle Ordovician continental complex.

Early Neoproterozoic crustal growth and microcontinent formation of the north–central Central Asian Orogenic Belt: New geological, geochronological, and Nd–Hf isotopic data on the Mélange Zone within the Zavkhan terrane, western Mongolia

In this study, new geological, geochronological, geochemical, and Nd–Hf isotopic data are presented for the Mélange Zone within the Zavkhan terrane, Mongolia, and the terrane structure, early Neoproterozoic continental crust growth, and microcontinent formation in the north–central part of the Central Asian Orogenic Belt (CAOB) are discussed. The Mélange Zone separates high-grade complexes of the northwestern part of the Zavkhan terrane and unmetamorphosed Neoproterozoic Zavkhan Formation covered by Cryogenian–Cambrian shelf deposits of the southwestern part. Zone consist of a lower-grade association of basalts, basaltic andesites, rarely felsic volcanic rocks, trondhjemites of the Kharuul Massif and variably metamorphosed from greenschist- to upper amphibolite-facies, and high-grade metamorphic rocks including quartzite–gneisses, hornblende schists, and amphibolites with relics of eclogite and blueschist-facies metamorphism assemblages. Emplacement of trondhjemites and gabbro dykes of the Kharuul Massif occurred at about 960–930 Ma. Geochemical, Nd whole-rock, and Hf-in-zircon isotopic data indicate an oceanic island arc setting for the lower-grade association. LA-ICP-MS dating and Hf-in-zircon data for detrital zircons from the quartzite–gneisses of the Mélange Zone indicate that the sources comprise Palaeoproterozoic (ca. 2.02 and 2.48 Ga), Neoarchean (ca. 2.59 and 2.67 Ga), and Mesoarchean (ca. 2.8 and 3.0 Ga) magmatic and metamorphic crustal rocks as well as 2.6–2.5 Ga juvenile complexes. High-grade amphibolites show geochemical similarity to continental arc and within-plate basalts, and were formed from enriched mantle sources. Obtained combined with published data demonstrate that the Zavkhan terrane represents composite terrane composed of ca. 960–930 Ma island arc complex, and ca. 860–800 Ma active continental margin complexes in the north and reworked at ca. 800–720 Ma early Precambrian continental crust in the south, which are separated by a Mélange Zone. We speculate that formation of the latest Mesoproterozoic–early Neoproterozoic island arc and active continental margin complexes in the north–central CAOB could be related to the assembly of the Rodinia supercontinent at ca. 1.1–1.0 Ga, which initiated subduction in Mirovian Ocean and led to the development of accretionary orogens around supercontinent margins.

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.

Famennian crinoids and blastoids (Echinodermata) from Mongolia

Herein we report on the most abundant and diverse fauna of Palaeozoic crinoids and blastoids collected from Mongolia to date. The fauna is from the Late Devonian (Famennian) Samnuuruul Formation in western Mongolia. The fauna consists of two genera of blastoids and twelve genera of crinoids—four genera of camerates, three genera of flexibles, one disparid genus, and four genera of cladids. The crinoids and blastoids were living on an active island arc complex in the Central Asian Orogenic Belt (CAOB) in a high physical stress environment with frequent and often voluminous pyroclastic eruptions. The Mongolian fauna is similar to coeval faunas collected from the Hongguleleng Formation in western China and supports the hypothesis that the CAOB was a biodiversity hotspot for Famennian echinoderms and a precursor to the very successful echinoderm communities that dominated Mississippian shallow-marine ecosystems globally. Three new taxa are described. Mongoliacrinus minjini, new genus and species, is the oldest member of the Acrocrinidae, previously known from the Mississippian and Pennsylvanian and the first occurrence of the family outside North America. Eutaxocrinus ariunai and Eutaxocrinus sersmaai are new species of the flexible crinoid Eutaxocrinus, a genus with a widespread distribution during the Early and Middle Devonian, which survived into the Lower Mississippian. It is restricted to the CAOB in the Late Devonian.

Lu–Hf Isotope Composition of Zircon and Magma Sources of the Vendian–Early Paleozoic Granitoids in Tuva (by the Example of the Kaa-Khem and East Tannu-Ola Batholiths)

Abstract —We present results of geochemical and Sr–Nd isotope studies of rocks and of local dating and determination of the Lu–Hf isotope composition of zircons from late Vendian–early Cambrian and Cambrian–Ordovician intrusive associations (granitoids and gabbroids) of the Kaa-Khem and East Tannu-Ola batholiths in Eastern Tuva. The wide ranges of the eNd values (6.9 to 0.5) of rocks and the εHf values of magmatic and inherited zircons reflect the diversity of the magma sources of late Vendian–early Paleozoic intrusive associations formed at the island arc and accretion–collision stages. Late Vendian (572–562 Ma, Kopto and Buren massifs) and early Cambrian (522–518 Ma, East Tannu-Ola batholith) island arc tholeiitic and calc-alkalic plagiogranitoids resulted from the melting of the Vendian–early Cambrian island arc crust without the contribution of a more ancient crustal material. The subalkalic gabbro–monzodiorite–granosyenite association of the Zubovka massif (510 Ma) formed from a mantle source depleted isotopically but enriched in incompatible elements, with the participation of an island arc crust material; this process took place in the early phase of plume activity at the accretion–collision stage. Island arc complexes were the main source of Cambrian–Ordovician accretion–collision calc-alkalic plagiogranitoids (500–450 Ma, Terektyg-Cheder, Karaos, Tapsa, Baisyut, and other massifs). Variations in their composition were due to the melting of thick crust, whose isotopic heterogeneity was caused by the different contributions of a more ancient crustal source. The crust of the Tuva–Mongolian terrane made the main contribution to the formation of the potassic granitoids of the Bren’ massif (450 Ma), marking the completion of accretion–collision processes in this region. The isotope parameters of the Vendian–early Paleozoic granitoids are indicators of the crust formation and evolution in the course of subduction and accretion–collision processes.

Volcanic Rocks in the Udyl’ Segment of the Kiselevka–Manoma Accretionary Terrane (Sikhote-Alin): Petrogeochemistry, Formation Conditions, and Tectonic Setting

New petrogeochemical data were used to determine the conditions of the formation and tectonic setting of volcanic rocks of the Lake Udyl’ area. These rocks were compared with similar rocks in the region. The volcanic rocks of the Lake Udyl’ area are subdivided into two genetic types: (1) within-plate oceanic volcanic rocks ascribed to the accretionary volcanogenic–siliceous complex (Tithonian–Valanginian–Hauterivian–Barremian) similar to accretionary complexes in other segments of the Kiselevka–Manoma Terrane, and (2) suprasubduction-zone volcanic rocks from the island-arc volcanogenic-terrigenous complex (Valanginian–Hauterivian–Aptian–Albian–Cenomanian). A synthesis of new geological and petrogeochemical data, as well as previous geochronological and paleomagnetic data indicates the co-occurrence of genetically diverse complexes in the Udyl’ segment.

Pb and Nd Isotopic Data on Granitoids from the Lake Zone, Mongolian and Gobi Altai with Implications for the Crustal Growth of the Central Asian Orogenic Belt

Pb and Nd isotopic composition of granitoids from the Lake Zone, Mongolian and Gobi Altai was analyzed to decipher the continental crustal growth of heterogeneous terranes of the Central Asian orogenic belt. The granitoids were generated in the Late Neoproterozoic–Early Paleozoic juvenile crust of the Lake Zone during island-arc, accretion, and post-accretionary stages at 535–440 Ma. Their Pb and Nd isotopic characteristics are similar and illustrate the dominance of juvenile material in the source of granitoids of all three stages. An insignificant terrigenous contribution in their source provides dispersion of the Pb isotopic composition at a weak effect on the Nd isotopic composition. The Pb and Nd isotopic study of Middle Paleozoic (∼380–355 Ma) synkinematic and Late Paleozoic (∼350–270 Ma) post-kinematic granitoids in the Mongolian and Gobi Altai revealed a significant heterogeneity of their source, which is mainly inherited from terrigenous rocks of the Altai accretionary wedge variably metamorphosed under greenschist to granulite facies conditions. The main source of Altai terrigenous sediments was volcanic rocks from the Lake Zone juvenile crust with subordinate contribution of more mature rocks from the Tuva-Mongolian and Dzabkhan microcontinents, which supplied sediments with highly radiogenic Pb and Nd isotopic composition. The Pb isotopic compositions of the granitoids from the Lake Zone, Mongolian and Gobi Altai, and also the Trans-Altai Gobi (Savatenkov et al., 2016) show that their mantle source has an elevated Th/U ratio compared to the depleted mantle (Kramers and Tolstikhin, 1997). This is a hallmark of the depleted mantle source of the Paleo-Asian province. Juvenile continental crust terrains of the Central Asian orogenic belt (Lake Zone and Trans-Altai Gobi terranes) were formed in various tectonics settings. The island-arc complexes of the Lake Zone developed near the Precambrian Tuva-Mongolian and Dzabkhan microcontinents, which supplied terrigenous material with highly radiogenic Pb to granitoid sources. The Trans-Altai Gobi was a system of ensimatic island arcs separated from the Siberian paleocontinent by a continent-dipping subduction zone, which prevented the influx of ancient terrigenous material from the continent.

Late Neoproterozoic Island-Arc Volcanic Associations in the Accretion Belt at the Southwestern Margin of the Siberian Craton (Predivinsk Terrane of the Yenisei Ridge)

Based on geological, geochemical, and isotope-geochronological studies, the geodynamic nature of the island-arc magmatic associations of the Predivinsk terrane of the Yenisei Ridge has been established. It is shown that the Predivinsk terrane consists of two blocks, which differ in age and composition of rock associations. The Eastern Block is composed of the following associations: (1) typical island-arc differentiated calc-alkaline basalt–andesite–dacite association dominated by andesites and dacites; (2) bimodal subalkaline metabasalt–trachyrhyolite association, which is enriched in incompatible elements, was formed under extensional conditions, and reflects the evolution of subduction magmatism, and (3) ophiolite fragments consisting of N-MORB metabasalts. The Western Block is composed of geochemically primitive metabasalt–plagiorhyolite and moderately differentiated calc-alkaline metabasalt–basaltic andesite–rhyodacite associations typical of oceanic island arcs. The magmatic associations of the Eastern Block are dated within 650–630 Ma, while volcanic and sedimentary associations of the Western block, within 620–600 Ma. The accretion of the island-arc complexes of the Predivinsk Terrane and their amalgamation to the Siberian margin occurred about 610–600 Ma.

Parautochthonic paleooceanic and island-arc complexes of the Southern Urals

Research objects. Paleozoic (Ordovician–Middle Devonian) volcanic-sedimentary complexes of the contiguous Sakmara and Mugodzharskaya structural zones of the Southern Urals, which have a paleo-oceanic and island-arc nature. Мaterials and methods.The work was carried out on the basis of many years of personal research of the author with the involvement of literature data on other folded zones of the Southern Urals. Main results. In the early Devonian, the first intense clustering of different facies complexes of rocks took place, which determined the tectonic face of the modern western part of the fold belt. The uniformity and set of Paleozoic formations in all structures of the Urals are maintained along its entire strike and are not repeated in neighboring zones.The drilling data showed the primary character of bedding of the rocks of the Upper Devonian Zilair Formation on the dislocated volcanogenic-sedimentary deposits of the Lower-Middle Paleozoic. Thrust contacts are observed only in the marginal parts of the zone. The almost textbook views on the cover bedding of the Kraka and Kempirsai hyperbasite massifs are in contradiction with the data on the presence of “roots” up to 4–8 km deep in them. By the beginning of the Devonian, the Kempirsai massif was located within the Sakmara zone and was “cross-linked” with the surrounding effusive rocks of the Middle Ordovician by gabbro-diabase dykes. According to the results of seismometric studies, features of similarity of the deep structure of the base of the Sakmara zone with the structure of the Magnitogorsk-Mugodzharskaya zone, which also revealed a melanocratic basement, have been established. Conclusions. The formed structural zones of the Southern Urals are distinguished by a set of rocks of the same age, their stratigraphic range, and the autonomy of feeding areas. These differences have survived to this day. All subsequent tectonic episodes, including the sharpening epochs, only complicated the appearance and structure of the Southern Urals, without changing the relative position of structures that arose in the Early Devonian. There was no transfer, swarming of rocks from one zone through another in any of the subsequent stages of compression. The Early Devonian stage was the first, but at the same time the most intense, which determined the tectonic face of the modern western part of the Urals.

Geodynamic Model of the Neoproterozoic Evolution of the Yenisei Paleosubduction Zone (Western Margin of the Siberian Craton), Russia

The article proposes a model of collisional and accretionary events of the Neoproterozoic at the western margin of the Siberian Craton based on the results of geological, petrological, and geochronological studies of Precambrian complexes of the Yenisei Ridge and comprehensive geophysical data. Mineralogical and petrological evidence of paleosubduction is presented, represented by relicts of glaucophane-bearing high-pressure mineral assemblages in metabasites (630‒620 Ma) within the tectonic melange of the suture zone and by metamorphosed ophiolites and island arc complexes of the Isakovka and Predivinsk terranes (700–600 Ma) of the Sayan–Yenisei accretionary belt. The proposed geodynamic evolutionary model of the Yenisei paleosubduction zone includes reconstructions for time intervals of 740‒700, 640‒600, 580‒540 Ma, during which a suture zone was formed as a result of collision of the Kas–Turukhansk microcontinent with the Siberian Craton. Tectonized fragments of the suture partially outcrop on the surface on the right bank of the Yenisei River. The bulk of the deformed and metamorphosed oceanic crust is buried beneath the Ediacaran and Phanerozoic sedimentary cover in the eastern part of the Kas–Turukhansk microcontinent, which is traceable by geophysical data. The history of the region’s geological development in the Late Meso‒Neoproterozoic correlates with synchronous endogenic events along the Arctic margin of the Nuna and Rodinia paleocontinents, which confirms the spatial proximity of the Siberian and North Atlantic cratons (Laurentia and Baltica) in a wide time range.

Tectono-stratigraphic correlations between Northern Evvoia, Skopelos and Alonnisos, and the postulated collision of the Pelagonian carbonate platform with the Paikon forearc basin (Pelagonian–Vardar zones, Internal Hellenides, Greece)

The Pelagonian stratigraphy of the Internal Hellenides consists of a Permo-Triassic basement and an Upper Triassic and Jurassic carbonate platform formation that has been overthrust by the Eohellenic ophiolite sheet during the Early Cretaceous. Intensive erosion, during the Cretaceous, removed most of the ophiolite and parts of the Jurassic formation. It is hypothesised that uplift and erosion of eastern Pelagonia was triggered by the break-off of the subducted oceanic leading edge of the Pelagonian plate. An investigation of the rocks that succeed the erosional unconformity shows that they constitute a shear-zone that is tectonically overlain by Cretaceous platform carbonates. Geochemical analyses of the shear-zone rocks substantiate that they are of mid-oceanic ridge and island arc provenience. Eastern Pelagonia collided with a Cretaceous carbonate platform, probably the Paikon forearc basin, as the Almopias ocean crust subducted beneath that island-arc complex. The Cretaceous platform, together with a substrate of sheared-off ocean floor mélange, overthrust eastern Pelagonia as subduction continued, and the substrate was dynamically metamorphosed into cataclastic rocks, mylonite, phyllonite and interpreted pseudotachylite. This complex of Cretaceous platform rocks and a brittle-ductile shear-zone-substrate constitute the here named Paikon-Palouki nappe, which was emplaced during Early Palaeocene. The Paikon-Palouki nappe did not reach Evvoia. Seismic tomographic models of the Aegean region apparently depict images of two broken-off ocean-plate-slabs, interpreted as Almopias-lithosphere-slabs. It is concluded that the western Almopias slab began to sink during the Early Cretaceous, while the eastern Almopias slab broke off and sank after the Paikon-Palouki nappe was emplaced in the Early Palaeocene.