Mongol Okhotsk Suture Zone Research Articles

THE ZABAIKALIAN PARAMETRIC BOREHOLE: A FIRST EVIDENCE ABOUT DEEP STRUCTURE OF THE BORSHCHOVOCHNY METAMORPHIC CORE COMPLEX

The Zabaikalian parametric borehole drilled out a metamorphic core complex on the reference geological and geophysical profile within the Central Asian fold belt, south of the Mongol-Okhotsk suture zone. In the section of the borehole drilled to a depth of 2600 m, a set of geophysical, geological and laboratory-analytical studies identified three main structural elements of metamorphic core complexes: the upper plate, detachment, and the lower plate. The upper plate (weakly metamorphosed rocks) is represented by serpentinite melange, which is separated from the underlying rocks of the lower plate by a zone of chlorite breccias (detachment). The lower plate, which is part of the crystalline basement complex, includes rocks of granite-schist and granite-plagiogneiss subcomplexes. The granite-schist subcomplex is composed of mafic and intermediate schist with amphibolites and quartzites. Biotite granite is widespread in the section; it forms layered veins with signs of synkinematic formations. According to preliminary estimates, the age of biotite granite is about 130 Ma. The schist is characterized by mylonite structure at intervals of interbedding with biotite granite veins. In accordance with data of geophysical surveys in the borehole and the petrophysical characteristics of cored rocks, the mylonite zone is the provenance of a series of inclined reflectors on the seismic reflection-CDP section, which can be traced to a depth of at least 10 km. The granite-plagiogneiss subcomplex is represented by plagiogneiss, leucogranites, and gneissic granites. In the Zabaikalian parametric borehole, a detailed section of the orshchovochny metamorphic core complex, fully characterized by core samples, of the was obtained, which should be considered as a reference for studying these structures. The investigation of this section will answer later many questions about the formation and evolution of the Central Asian fold belt.

The Trans-Baikal Parametric Borehole: First Evidence of the Deep Structure of the Borshchovochnyi Metamorphic Core Complex

The Trans-Baikal parametric borehole drilled out a metamorphic core complex in the reference geological and geophysical profile within the Central Asian fold belt, south of the Mongol–Okhotsk suture zone. In the section of the borehole drilled to a depth of 2600 m, a set of geophysical, geological, and laboratory–analytical studies made it possible to identify three main structural elements of metamorphic core complexes: the upper plate, detachment, and the lower plate. The upper plate (slightly metamorphosed rocks) is represented by a serpentinite mélange separated from the underlying rocks of the lower plate by a zone of chlorite breccias (detachment). The lower plate as a part of the crystalline basement complex comprises rocks of granite–schist and granite–plagiogneiss subcomplexes. The granite–schist subcomplex is composed of mafic and intermediate schist with amphibolites and quartzites. Biotite granite is widespread in the section as layered veins with synkinematic signs. According to the preliminary estimates, the age of biotite granite is about 130 Ma. Schist is characterized by a mylonite structure at intervals of interbedding with biotite granite veins. In accordance with the geophysical survey data on the borehole and the petrophysical characteristics of cored rocks, the mylonite zone is the provenance of a series of inclined reflectors in the seismic RSS–CDP section, which can be traced to a depth of at least 10 km. The granite–plagiogneiss subcomplex is represented by plagiogneiss, leucogranite, and gneissic granite. In the Trans-Baikal parametric borehole, a detailed section of the Borshchovochnyi metamorphic core complex, fully characterized by core samples, was obtained. It should be considered as a reference for studying these structures. The investigation of this section will make it possible later to answer many questions about the formation and evolution of the Central Asian fold belt.

Chronology and Geochemistry of Early Cretaceous Magmatism in the Northwestern Erguna Block, Northeast China

This study was conducted to define the background structure and petrogenetic significance of the Early Cretaceous magmatic rocks in the Badaguan area of northern Daxing’anling and to explore the Late Paleozoic tectonic evolution of the Mongol-Okhotsk suture zone. The Early Cretaceous magmatic rock was systematically investigated using zircon U-Pb dating and geochemical and petrological analyses. The results show that the rock: mainly consists of granites and rhyolites; has an age of 125–140 Ma; has a strong MgO, Al2O3, and total alkali content; has a SiO2 content of 61.68 wt% to 77.41 wt%; and contains Rb, Th, U, and light REEs with depleted levels of of Sr, P, Ti, and heavy REEs. When combined with the Hf isotopic characteristics of the Early Cretaceous magmatic rock from the Erguna Massif, these results suggest that the magma originated from the partial melting of basal crustal materials during the Neoproterozoic–Phanerozoic period and that various mineral forms (including hornblende, plagioclase, and apatite) underwent fractional crystallization processes during the evolution of the magma. The Early Cretaceous magmatic rock from the Badaguan area recorded the extensional environment of the lithosphere after the closure of the Mongol-Okhotsk Ocean, and this hypothesis is consistent with the results of previous studies on the tectono-magmatic activities in Northeast China during the same period.

Low‐temperaturethermochronological insights into the Mesozoic–Cenozoic exhumation history of the Taihang–Lvliangshan region: A review

The Taihang–Lvliangshan region is located in the central part of the North China Craton (NCC), which has undergone significant reactivation and reconstruction during the Mesozoic–Cenozoic period. Documenting its long‐term uplift and exhumation history is critical for accurately characterizing the reformation extent of the NCC. In order to further reveal the Mesozoic and Cenozoic exhumation stages and spatial exhumation pattern of the Taihang–Lvliangshan, we present a comprehensive low‐temperature thermochronometric datasets from the region. The compilation revealed five distinct rapid exhumation episodes within the Taihang–Lvliangshan, including the Middle‐Late Jurassic (~160–150 Ma), Early Cretaceous (~120–110 Ma), Late Cretaceous (~100–60 Ma), Eocene (~50–40 Ma), and Late Oligocene–Early Miocene (~30–20 Ma). During the Middle–Late Jurassic, the Taihang–Lvliangshan underwent a strong thrusting deformation caused by the synthesis of northwestward subduction of the Palaeo‐Pacific Plate and southward subduction/collision of the Mongol–Okhotsk suture zone. Subsequently, rapid exhumation under an extensional tectonic environment occurred during the Early Cretaceous primarily owing to the subduction of the Palaeo‐Pacific Plate. During the Late Cretaceous, the collision between the Okhotomorsk Block and East Asia resulted in the regional northwest–southeast shortening and uplift of the Taihang–Lvliangshan. During the Eocene, normal fault activations were occurring throughout the range, likely owing to the change in the convergence rate and direction between the Palaeo‐Pacific Plate and East Asia. Another important exhumation event occurred during the Late Oligocene–Early Miocene period, which was likely attributed to the result of combined effects from both the India–Eurasia collision and Pacific Plate subduction. Furthermore, differential exhumation of the Taihang–Lvliangshan were clearly revealed according to the five separate E–W elevation–distance profiles of the Taihang–Lvliangshan. They displayed greater exhumation thicknesses at the southern regions than those along the northern regions, which can be explained by the development of the Shanxi Rift that began to form following the Oligocene–Miocene period.

Three-phased Middle Permian–Mesozoic magmatism in the Great Xing'an Range: implications for episodic southern subduction of the Mongol–Okhotsk Ocean

The Erguna Massif is located in the southwestern portion of the Great Xing'an Range and is adjacent to the Mongol–Okhotsk suture zone. It has not been determined whether the tectonic evolutionary processes of the Erguna Massif belonged to the Mongol–Okhotsk tectonic regime during the Middle Permian–Mesozoic. In this study, a suite of rocks comprising Mesozoic S-type monzogranite (U–Pb zircon age of 248 ± 1.2 Ma), highly fractionated I-type rhyolite (204 ± 1.1 Ma), gabbro (196 ± 1.9 Ma), A2-type volcanic rocks (190 ± 0.9 Ma), A1-type trachydacite (167 ± 0.8 Ma) and Early Cretaceous A1-type alkaline rhyolite are newly identified and geochemically studied. The rhyolite, gabbro, trachydacite and alkaline rhyolite whole-rock Sr–Nd isotope analyses obtained initial values 87 Sr/ 86 Sr ratios ranging from 0.7044 to 0.7058 and ε Nd ( t ) values of −0.68 to +2.73. These samples show ε Hf ( t ) values ranging from +5.3 to +11.2 and T DM2 ranging from 0.48 to 0.90 Ga. The 248 Ma monzogranites were produced by the partial melting of greywackes. The 204 Ma rhyolites were derived from the partial melting of lower mafic crust. The 196 Ma gabbros originated from the partial melting of an enriched mantle metasomatized by subduction-slab-released fluids. The 190 Ma volcanic rocks, 167 Ma trachydacite and Early Cretaceous alkaline rhyolite were mainly formed by the partial melting of the basaltic rocks. They all show enrichment in the large-ion lithophile elements (e.g. Rb, Ba, and K) and depletion in the high-field-strength elements (e.g. Nb, Ta, and Ti), suggesting they formed in an active continental margin setting. The features of these igneous rocks indicate the southward subduction of the Mongol–Okhotsk oceanic plate. Based on compiled age data, three phases of middle Mesozoic magmatism were identified in the Erguna Massif at c. 275–225, 215–165, and 150–110 Ma. In addition, three similar magmatic phases were found in the Xing'an Massif. However, a hysteresis of c. 15–20 Ma exists between the two massifs. These magmatic rocks may record the three stages of the southward subduction of the Mongol–Okhotsk oceanic plate, and two periods of slab rollback occurred during the Middle Permian to Early Cretaceous. Supplementary material: Analytical details, radiometric ages and Tables S1–S5 are available at: https://doi.org/10.6084/m9.figshare.c.5459285

Segmental closure of the Mongol-Okhotsk Ocean: Insight from detrital geochronology in the East Transbaikalia Basin

The Late Paleozoic–Early Mesozoic Mongol-Okhotsk Ocean extended between the Siberian and Amur–North China continents. The timing and modalities of the oceanic closure are widely discussed. It is largely accepted that the ocean closed in a scissor-like manner from southwest to northeast (in modern coordinates), though the timing of this process remains uncertain. Recent studies have shown that both western (West Transbaikalia) and eastern (Dzhagda) parts of the ocean closed almost simultaneously at the Early–Middle Jurassic boundary. However, little information on the key central part of the oceanic suture zone is available. We performed U-Pb (LA-ICP-MS) dating of detrital zircon from well-characterized stratigraphic sections of the central part of the Mongol-Okhotsk suture zone. These include the initial marine and final continental sequences of the East Transbaikalia Basin, deposited on the northern Argun-Idemeg terrane basement. We provide new stratigraphic ages for the marine and continental deposits. This revised chronostratigraphy allows assigning an age of ~165–155 Ma, to the collision-related flexure of the northern Argun-Idemeg terrane and the development of a peripheral foreland basin. This collisional process took place 5 to10 million years later than in the western and eastern parts of the ocean. We demonstrate that the northern Argun-Idemeg terrane was the last block to collide with the Siberian continent, challenging the widely supported scissor-like model of closure of the Mongol-Okhotsk Ocean. Different segments of the ocean closed independently, depending on the initial shape of the paleo continental margins.

Postcollision Magmatism of Western Sector of the Mongol–Okhotsk Orogenic Belt: Oxidation Conditions and Metallogenic Zoning

The Transbaikal sector of the Mongol–Okhotsk orogenic belt displays a well-consistent geochemical and metallogenic zoning. The axial zone of the belt, which is controlled by the Mongol–Okhotsk suture zone from the north, is composed of reduced ilmenite-series rocks. Tin, tungsten, and rare-metal deposits and occurrences previously distinguished by S.S. Smirnov as a rare-metal belt are confined to the ilmenite-series rocks. The reduced rocks are framed from the north and south by oxidized magnetite-series rocks with chalcophile mineralization. The distribution zones of the ilmenite-series rocks correspond to anomalies with lower gravity and magnetic fields, whereas the magnetite-series oxidized rocks correspond to anomalies with higher gravity and magnetic fields.

Tectonostratigraphic evolution of the Mohe-Upper Amur Basin reflects the final closure of the Mongol-Okhotsk Ocean in the latest Jurassic–earliest Cretaceous

The Mohe-Upper Amur Basin to the south of the eastern Mongol-Okhotsk Suture Zone contains important stratigraphic records for understanding the closure of the eastern Mongol-Okhotsk Ocean. The basin is crossed by the Russia/China border, and its Chinese and Russian parts are known as the Mohe Basin and Upper Amur Basin, respectively. Using most up-to-date data on stratigraphy, sedimentology, petrography, and detrital zircon U-Pb geochronology, this study establishes the stratigraphic correlation between the two, Mohe and Upper Amur parts of the basin, and analyzes depositional ages, provenance, and paleogeography of their Jurassic–Lower Cretaceous strata. The adopted Middle–Late Jurassic ages for the Xiufeng, Ershierzhan, Emuerhe, and Kaikukang formations in the Mohe Basin, are revised and constrained to late Kimmeridgian, Tithonian, Berriasian–early Valanginian, and late Valanginian ages, respectively. During the late Kimmeridgian–Tithonian, extension occurred in the Mohe-Upper Amur Basin, and sediments were mainly sourced from areas to the south of the basin. Later, in the Berriasian–early Valanginian, the northern margin of the Mohe-Upper Amur Basin was uplifted and started furnishing sediments into the basin. In the late Valanginian, regional uplift of the northern part of the Mohe-Upper Amur Basin transformed the basin into a compressional intermountain basin, with sedimentation localized in its southern part. After the Valanginian, extension and associated volcanism occurred in the basin. We suggest that the evolution of the Mohe-Upper Amur Basin reflects the gradual closure of the eastern Mongol-Okhotsk Ocean and associated collision of the Siberia Craton and the Amuria Block, that occurred from the Kimmeridgian–Tithonian, to the west of the basin through to the Berriasian–Valanginian, to the north and northeast of the basin. The final closure of the Mongol-Okhotsk Ocean to the north of the Mohe-Upper Amur Basin in the earliest Cretaceous significantly affected the sedimentological, structural and tectonic evolution of Northeast Asia.

Geochemical and Sr–Pb–Nd isotopic characteristics of the Shakhtama porphyry Mo–Cu system (Eastern Transbaikalia, Russia)

The Shakhtama Mo–Cu porphyry deposit is located within the eastern segment of the Central Asian Orogenic Belt, bordering the southern margin of the Mongol–Okhotsk suture zone. The deposit includes rocks of two magmatic complexes: the precursor plutonic (J2) and ore-bearing porphyry (J3) complexes. The plutonic complex was emplaced at the final stages of the collisional regime in the region; the formation of the porphyry complex may have overlapped with a transition to extension. The Shakhtama rocks are predominantly metaluminous, I-type high K calc-alkaline to shoshonitic in composition, with relatively high Mg#, Ni, Cr and V. They are characterized by crustal-like ISr (0.70741–0.70782), relatively radiogenic Pb isotopic compositions, εNd(T) values close to CHUR (−2.7 to +2.1) and Nd model ages from 0.8 to 1.2Ga. Both complexes are composed of rocks with K-adakitic features and rocks without adakite trace element signatures. The regional geological setting together with geochemical and isotopic data indicate that both juvenile and old continental crust contributed to their origin. High-Mg# K-adakitic Shakhtama magmas were most likely generated by partial melting of thickened lower crust during delamination and interaction with mantle material, while magmas lacking adakite-like signatures were probably generated at shallower levels of lower crust. The derivation of melts, related to the formation of plutonic and porphyry complexes involved variable amounts of old Precambrian lower crust and juvenile Phanerozoic crust. Isotopic data imply stronger contribution of juvenile mantle-derived material to the fertile magmas of the porphyry complex. Juvenile crust is proposed as an important source of fluids and metals for the Shakhtama ore-magmatic system.

Geochronological and geochemical constraints on the Erguna massif basement, NE China – subduction history of the Mongol–Okhotsk oceanic crust

We present new geochronological and geochemical data for granites and volcanic rocks of the Erguna massif, NE China. These data are integrated with previous findings to better constrain the nature of the massif basement and to provide new insights into the subduction history of Mongol–Okhotsk oceanic crust and its closure. U–Pb dating of zircons from 12 granites previously mapped as Palaeoproterozoic and from three granites reported as Neoproterozoic yield exclusively Phanerozoic ages. These new ages, together with recently reported isotopic dates for the metamorphic and igneous basement rocks, as well as Nd–Hf crustal-residence ages, suggest that it is unlikely that pre-Mesoproterozoic basement exists in the Erguna massif. The geochronological and geochemical results are consistent with a three-stage subduction history of Mongol–Okhotsk oceanic crust beneath the Erguna massif, as follows. (1) The Erguna massif records a transition from Late Devonian A-type magmatism to Carboniferous adakitic magmatism. This indicates that southward subduction of the Mongol–Okhotsk oceanic crust along the northern margin of the Erguna massif began in the Carboniferous. (2) Late Permian–Middle Triassic granitoids in the Erguna massif are distributed along the Mongol–Okhotsk suture zone and coeval magmatic rocks in the Xing’an terrane are scarce, suggesting that they are unlikely to have formed in association with the collision between the North China Craton and the Jiamusi–Mongolia block along the Solonker–Xra Moron–Changchun–Yanji suture zone. Instead, the apparent subduction-related signature of the granites and their proximity to the Mongol–Okhotsk suture zone suggest that they are related to southward subduction of Mongol–Okhotsk oceanic crust. (3) A conspicuous lack of magmatic activity during the Middle Jurassic marks an abrupt shift in magmatic style from Late Triassic–Early Jurassic normal and adakite-like calc-alkaline magmatism (pre-quiescent episode) to Late Jurassic–Early Cretaceous A-type felsic magmatism (post-quiescent episode). Evidently a significant change in geodynamic processes took place during the Middle Jurassic. Late Triassic–Early Jurassic subduction-related signatures and adakitic affinities confirm the existence of subduction during this time. Late Jurassic–Early Cretaceous post-collision magmatism constrains the timing of the final closure of the Mongol–Okhotsk Ocean involving collision between the Jiamusi–Mongolia block and the Siberian Craton to the Middle Jurassic.

Late Triassic 40Ar-39Ar ages of the Baga-Gazryn Chuluu granites (Central Mongolia)

New radiometric data have been obtained for two-mica granites from the Baga-Gazryn Chuluu Pluton in Central Mongolia, classified previously as A-type granites based on their geochemical characteristics and tectonic position. Three most common granite varieties were dated by 40Ar–39Ar method on separated biotite. The following ages were obtained: 201.0 ± 3.6 Ma (2σ) for the coarse-, 211.9 ± 4.0 Ma for the medium-, and 209.4 ± 3.2 Ma for the fine-grained granites. Such Late Triassic cooling ages agree well with the older assumptions on the time of emplacement of the Baga-Gazryn Chuluu Pluton, based on geological evidence and previous K–Ar dating. New geochronological data better constrain the age of granitic magmatism in post-collisional, extensional regime adjacent to the western part of the Mongol–Okhotsk suture zone in the Adaatsag area.

Re-deciphering the tectonic jigsaw puzzle of northern Eurasia

Northern Eurasia consists of the East European, Siberian, North China, and Alai-Tarim cratons, fragments of the supercontinent Rodinia, and the orogens of the Baikalides, Timanides, Uralides, Altaids, and Mongolides. These can be collectively classified as the Central Asian supercollage. The Baikalides and Timanides host Meso- and Neoproterozoic magmatic arc terranes that were sutured with the adjacent East European and Siberian cratons in the end of the Neoproterozoic. The Paleozoic part of the supercollage consists of three almost synchronous and subparallel Neoproterosoic to Paleozoic magmatic arc and turbidite superterranes, as well as overlap assemblages, bent into the world’s largest oroclines. Analysis of their structural pattern, supported by paleontological, lithological, and paleomagnetic data, indicates that these superterranes might have been produced via formation of arc–backarc systems at the margin of combined North China, East European and Siberian cratons and then deformed during Paleozoic westward-directed strike-slip translation between the clockwise rotating Siberian and eastward moving North China cratons. It is proposed that this development took place against the respective breakup of the above-mentioned cratons from the northern and southern margins of Eastern Europe in the Neoproterozoic, initially as a group of cratons called Nena, which reassembled in late Paleozoic to early Mesozoic times into Laurasia, part of the new supercontinent Pangea. In Mesozoic–Cenozoic times, the subduction-related continental growth of northern Eurasia continued in the Nipponide, Kamchatka and Kolyma–Alaska orogenic collages of the northern Circum-Pacific, which consist of Paleozoic to Cenozoic turbidite to island arc superterranes and overlap assemblages, generally younging towards the Pacific oceanic plate and also severely oroclinally bent. It is proposed that terranes of the Kolyma–Alaska and Kamchatka collages were translated westward, dextrally relative to Siberia, whereas Nipponides were translated northward, relative to North China, similarly to the better constrained Mesozoic–Cenozoic reconstructions of southeastern Asia. The two groups of collages started to collide along the Mongol-Okhotsk suture zone in the south of the Siberian craton in the end of the Mesozoic and then continued to collide along the presently active plate boundary at the island of Sakhalin. The proposed scenario suggests similarities in Paleozoic evolution of the Central Asian and Mesozoic–Cenozoic evolution of the northern Circum-Pacific supercollages, both possibly formed in response to westward subduction and related strike-slip translation of the (Paleo)-Pacific oceanic plates. The individual superterranes might have been consequently translated for as much as 4000–6000 km and oroclinally bent during such translation or/and rotation of the adjacent cratons.

Palaeomagnetism and K-Ar dating of Cretaceous basalts from Mongolia

We report a combined geochronology and palaeomagnetic study of Cretaceous igneous rocks from Shovon (44.4°N, 103.8°E) and Arts-Bogd (44.3°N, 102.2°E) localities in the Gobi Desert, south Mongolia. K—Ar dating based on seven rock samples, with two independent measurements for each sample, allows us to propose an age of 94.7 ± 1.3 Ma for Shovon locality and a 98.2 ± 1.4 to 118.3 ± 1.7 Ma age range for Arts-Bogd. Stepwise thermal and AF demagnetization generally isolated a high temperature component (HTC) of magnetization for both Shovon and Arts-Bogds basalts, eventually following a low temperature component (LTC) in some samples. The HTC directions display normal polarity, consistent with the Cretaceous Long Normal Superchron. Rock magnetic analysis identifies fine-grained pseudo-single domain (PSD) magnetite and titanomagnetite as primary carriers of the remanence. Mean HTC palaeomagnetic direction is Dm = 8.2°, Im = 63.7° (n = 18 flows, k = 41.1, α95 = 5.5°) for Shovon and Dm = 12.1°, Im = 66.4° (n = 27 flows, k = 53.0, α95 = 3.9°) for Arts-Bogd. Because of their similar ages, we combine data from Shovon and data previously obtained from Khurmen Uul (92.0 ± 4.0 Ma), recomputed in geographic coordinates, and not in tilt-corrected ones as in our previous interpretation, at the Shovon locality. The combined final average palaeomagnetic direction for Shovon-Khurmen Uul is Dm = 7.4°, Im = 62.7° (n = 23 flows, k = 41.4, α95 = 4.8°). The corresponding palaeopoles computed from these HTC lie at λ = 84.7°N, φ = 195.0°E, dp/dm = 5.8/7.5 for Shovon-Khurmen Uul (average age: 93.4 ± 2.6 Ma) and λ = 80.5°N, φ = 159.0°E, dp/dm = 5.2/6.3 for Arts-Bogd (average age: 104.6 ± 6.6 Ma). These poles are consistent with those from the European apparent polar wander path (APWP) at 90, 100 and 110 Ma, and other published pole from the Mongol-Okhotsk suture zone, Amuria and North China blocks. This confirms the lack of a discernable latitudinal motion between Amuria and Siberia since their final accretion by the Late Jurassic—Early Cretaceous, and reinforces the idea that Europe APWP can be used as a reference for Siberia by the mid-Cretaceous. We finally propose a mid-Cretaceous mean palaeomagnetic pole for the Siberia-Amuria-North China Block assemblage which lies at: λ = 86.4°N, φ = 191.1°E(n = 10, k = 74.9, A95 = 5.8°).

Late Jurassic-Early Cretaceous closure of the Mongol-Okhotsk Ocean demonstrated by new Mesozoic palaeomagnetic results from the Trans-Baïkal area (SE Siberia)

SUMMARY We present new palaeomagnetic results from the Transba¨ ikal area (SE Siberia), from the Mongol-Okhotsk suture zone, the boundary between the Amuria and Siberia blocks. In order to better constrain the time of closure of the Mongol-Okhotsk Ocean in the Mesozoic, we col- lected 532 rock samples at 68 sites in six localities of basalts, trachy-basalts and andesites, from both sides of the Mongol-Okhotsk suture: at Unda river (J 3; 51.7 ◦ N, 117.4 ◦ E), Kremljevka peak (K1; 51.8 ◦ N, 117.5 ◦ E) and Torey lakes (K 1; 50.1 ◦ N, 115.9 ◦ E) on the southern side of the suture, and at Monostoy river (J 1; 51.1 ◦ N, 106.8 ◦ E), Ingoda river (K 1; 51.2 ◦ N, 112.2 ◦ E) and Bichura town (K 1; 50.6 ◦ N, 107.6 ◦ E) on the northern side. Progressive thermal demagnetization enabled us to resolve low (LTC) and high (HTC) temperature components of magnetization at most sites. Jurassic palaeopoles computed from the HTCs show a large discrepancy with respect to the Apparent Polar Wander Path of Eurasia, which we interpret in terms of 1700-2700 km of post-Late Jurassic northward movement of Amuria with respect to Siberia. Although ge- ological data suggest a middle Jurassic closure of the Mongol-Okhotsk Ocean in the west Trans-Baikal region, our data give evidence of a large remaining palaeolatitude difference between the Amuria and Siberia blocks. In contrast, Early Cretaceous sites cluster remarkably well along a small-circle, which is centred on the average site location. This implies the absence of post-Early Cretaceous northward motion of Amuria relative to Siberia, and demonstrates the pre-Early Cretaceous closure of the Mongol-Okhotsk Ocean. Finally, we interpret the very large tectonic rotations about local vertical axes, evidenced by the small-circle distribution of poles, as arising both from collision processes and from left-lateral shear movement along the suture zone, due to the eastward extrusion of Amuria under the effect of the collision of India into Asia.

Zircon age and occurrence of the Adaatsag ophiolite and Muron shear zone, central Mongolia: constraints on the evolution of the Mongol–Okhotsk ocean, suture and orogen

The Adaatsag ophiolite in eastern Mongolia is situated in the Mongol–Okhotsk suture zone, which extends from central Mongolia through Transbaikalia to the Sea of Okhotsk and separates the Siberian and Amurian (Mongolian) plates. The ophiolite sequence passes upwards from serpentinite mélange and serpentinized dunite and harzburgite, through layered gabbro (with leucogabbro pegmatite dykes), wehrlite and clinopyroxenite, to isotropic gabbro and leucogabbro, sheeted mafic dykes, and olivine-rich basaltic lavas, overlain by red chert and meta-clastic sediments. A single-zircon mean 207 Pb/ 206 Pb evaporation age of 325.4 ± 1.1 Ma for a leucogabbro pegmatite dyke records the time of igneous crystallization of the plutonic suite, and thus the time of formation of oceanic crust in the Mongol–Okhotsk ocean that gave rise to the ophiolite. A U–Pb secondary ionization mass spectrometry (SIMS) and evaporation zircon age of 172 Ma for a mylonitized granite provides a maximum age for the left-lateral Muron shear zone, which occurs close to the left-lateral Mongol–Okhotsk suture, and indicates that the suture in eastern Mongolia formed at least by the mid-Jurassic. We review the evidence and models for subduction tectonics that gave rise to major calc-alkaline batholiths along active continental margins of the bordering plates and to extensive, post-collisional, alkaline to peralkaline magmatism. Whereas the magmatic history of the orogen is better known, the age of the ocean, and the time of formation and deformation of the suture zone are not. Our new data on the Adaatsag ophiolite and Muron shear zone provide key constraints on the early and late stages of development of the Mongol–Okhotsk ocean and orogen.

Architecture and mineral deposit settings of the Altaid orogenic collage: a revised model

The Altaids are an orogenic collage of Neoproterozoic–Paleozoic rocks located in the center of Eurasia. This collage consists of only three oroclinally bent Neoproterozoic–Early Paleozoic magmatic arcs (Kipchak, Tuva–Mongol, and Mugodzhar–Rudny Altai), separated by sutures of their former backarc basins, which were stitched by new generations of overlapping magmatic arcs. In addition, the Altaids host accreted fragments of the Neoproterozoic to Early Paleozoic oceanic island chains and Neoproterozoic to Cenozoic plume-related magmatic rocks superimposed on the accreted fragments. All these assemblages host important, many world-class, Late Proterozoic to Early Mesozoic gold, copper–molybdenum, lead–zinc, nickel and other deposits of various types. In the Late Proterozoic, during breakup of the supercontinent Rodinia, the Kipchak and Tuva–Mongol magmatic arcs were rifted off Eastern Europe–Siberia and Laurentia to produce oceanic backarc basins. In the Late Ordovician, the Siberian craton began its clockwise rotation with respect to Eastern Europe and this coincides with the beginning of formation of the Mugodzhar–Rudny Altai arc behind the Kipchak arc. These earlier arcs produced mostly Cu–Pb–Zn VMS deposits, although some important intrusion-related orogenic Au deposits formed during arc–arc collision events in the Middle Cambrian and Late Ordovician. The clockwise rotation of Siberia continued through the Paleozoic until the Early Permian producing several episodes of oroclinal bending, strike–slip duplication and reorganization of the magmatic arcs to produce the overlapping Kazakh–Mongol and Zharma-Saur–Valerianov–Beltau-Kurama arcs that welded the extinct Kipchak and Tuva–Mongol arcs. This resulted in amalgamation of the western portion of the Altaid orogenic collage in the Late Paleozoic. Its eastern portion amalgamated only in the early Mesozoic and was overlapped by the Transbaikal magmatic arc, which developed in response to subduction of the oceanic crust of the Paleo-Pacific Ocean. Several world-class Cu–(Mo)-porphyry, Cu–Pb–Zn VMS and intrusion-related Au mineral camps, which formed in the Altaids at this stage, coincided with the episodes of plate reorganization and oroclinal bending of magmatic arcs. Major Pb–Zn and Cu sedimentary rock-hosted deposits of Kazakhstan and Central Asia formed in backarc rifts, which developed on the earlier amalgamated fragments. Major orogenic gold deposits are intrusion-related deposits, often occurring within black shale-bearing sutured backarc basins with oceanic crust. After amalgamation of the western Altaids, this part of the collage and adjacent cratons were affected by the Siberian superplume, which ascended at the Permian–Triassic transition. This plume-related magmatism produced various deposits, such as famous Ni–Cu–PGE deposits of Norilsk in the northwest of the Siberian craton. In the early Mesozoic, the eastern Altaids were oroclinally bent together with the overlapping Transbaikal magmatic arc in response to the northward migration and anti-clockwise rotation of the North China craton. The following collision of the eastern portion of the Altaid collage with the Siberian craton formed the Mongol–Okhotsk suture zone, which still links the accretionary wedges of central Mongolia and Circum-Pacific belts. In the late Mesozoic, a system of continent-scale conjugate northwest-trending and northeast-trending strike–slip faults developed in response to the southward propagation of the Siberian craton with subsequent post-mineral offset of some metallogenic belts for as much as 70–400 km, possibly in response to spreading in the Canadian basin. India–Asia collision rejuvenated some of these faults and generated a system of impact rifts.

Evolution of the Mongol-Okhotsk Ocean as constrained by new palaeomagnetic data from the Mongol-Okhotsk suture zone, Siberia

SUMMARY This paper presents new data from palaeomagnetic investigations on the Upper Palaeozoic and Mesozoic geological units from the Siberian platform and the Mongol–Okhotsk suture zone. Within the southern portion of the Mongol–Okhotsk suture we collected palaeomagnetic samples from the Late Permian Belektuy formation (Borzya region; 50.7uN, 116.9uE) and the Middle–Late Jurassic Shadaron formation (Unda-Daya; 51.5uN, 117.5uE). We sampled the Late Permian Alentuy formation (Khilok region; 50.8uN, 107.2uE), the Early to Middle Jurassic Irkutsk sedimentary basin (ISB; 52.0uN, 104.0uE), the Late Jurassic Badin formation (Mogzon region; 51.8uN, 112.0uE), and the Early Cretaceous Gusinoozesk formation (Gusinoe Lake region; 51.2uN, 106.5uE) additionally in the northern region of the Mongol–Okhotsk suture. Apart from the results of the ISB and Gusinoozersk formations, which show very large ellipses of confidence and might be the present-day geomagnetic field overprint, our results allow us to constrain the evolution of the Mongol–Okhotsk Ocean palaeomagnetically from the Late Permian to the Middle–Late Jurassic. They confirm that this large Permian ocean closed during the Jurassic, ending up in the late Jurassic or the beginning of the Cretaceous in the eastern end of the suture zone, as suspected on geological grounds. However, although geological data suggest a Middle Jurassic closure of the Mongol–Okhotsk Ocean in the west Trans-Baikal region, our data show evidence of a still large palaeolatitude difference between the Amuria and Siberia blocks. This is interpreted as a result of the quite fast closure of the ocean after the Middle Jurassic. Finally, our new palaeomagnetic results exhibit very large tectonic rotations around local vertical axes, which we interpret as probably arising both from collision processes and from a left-lateral shear movement along the suture zone, due to the eastward extrusion of Mongolia under the effect of the collision of India into Asia.