Mongol-Okhotsk Belt Research Articles

From an accretionary margin to a sediment-rich collision: Spatiotemporal evolution of the magmatism during the closure of the Mongol-Okhotsk Ocean

The closure of the Mongol-Okhotsk Ocean (MOO) marks the final suturing of the Central Asian Orogenic Belt, one of the largest accretionary orogens on Earth and a region that is considered an archetype for crustal growth during the Phanerozoic. Abundant Permian to Triassic magmatism in Mongolia extended into the Jurassic on the eastern side of the Mongol-Okhotsk Belt (MOB), the orogenic belt produced by the closure of the MOO. Magmatic belts formed north and south of the suture along the MOB provide insight into the dynamics of the subduction system and the magmatic, crustal, and mantle processes pre-, syn- and post- collision within this accretionary margin. One of the main questions regarding the magmatism in the region is: Was the magmatism formed during active subduction or during the collision and closure of the basin? Here we compile geochemical data (major and trace elements, and isotopes) from the Permian to Jurassic magmatic rocks in the MOB and analyze their spatiotemporal characteristics. Our goal is to assess how magmatism changed in time and space during the closure of the Mongol-Okhotsk Ocean and how those changes relate to first-order tectono-magmatic processes right before or during the collisional event that closed the basin. Our results show a general enrichment in fluid mobile elements, LILE, and LREE and depletion in HFSE, and HREE in mafic and felsic rocks, which indicates a mantle metasomatized by subduction-related fluids regardless of crustal contamination. Our analysis supports higher enrichment in sediment melts, especially along its western and older extent, and the assimilation of juvenile crustal components without producing abundant S-type peraluminous magmatism which indicates mantle and crustal contributions. Thus, we conclude that magmatism formed above a sediment-rich retreating margin was able to recycle and stabilize young and compositionally evolved crustal material in the Central Asian Orogenic Belt.

The Early Mesozoic NE–SW Extensional Model and Exhumation Processes at the Southeastern Margin of the Central Asian Orogenic Belt: Insights from the Strain and Kinematic Vorticity Analysis of the Sonid Zuoqi Ductile Detachment Zone

AbstractThe Sonid Zuoqi ductile detachment zone is located at the southeastern margin of the Central Asian orogenic belt (CAOB), striking EW and dipping to the S. The major rock type of the Sonid Zuoqi ductile detachment zone is mylonite derived from granite. The sequence of mylonite features is: (1) S and C foliations of mylonite, and (2) extensional crenulation cleavage (ecc) or C and the kinematic vorticity (Wk) value changed from 0.70 to 0.95 and from 0.37 to 0.69, respectively; the strain type of the mylonites within the Sonid Zuoqi ductile detachment zone is compressional to planar strain. The strong deformation mylonite and Halatu plutons yielded a zircon U‐Pb age of 244 Ma and a zircon (U‐Th)/He age of 214 Ma, respectively. Based on the strain and kinematic vorticity analysis, together with the zircon U‐Pb and zircon (U‐Th)/He ages and the regional tectonic background, the study area experienced three stage evolution: tangential simple‐shear (244 Ma), simple‐shear‐dominated general shear represented by upper crustal extension (224 Ma) and pure–shear–dominated general shear represented by the Halatu pluton doming (214 Ma), which constrained the early Mesozoic NE–SW crustal extension at the southeastern margin of the CAOB. This NE–SW extension probably originated from the post‐orogenic extensional collapse of the CAOB, subsequent exhumation being controlled by the far afield effects of the closure of the Mongol–Okhotsk belt.

Composition and Sources of Clastic Material of Terrigenous Rocks of the Khabarovsk Accretionary Complex (Sikhote-Alin)

Abstract ––The results of comprehensive studies of terrigenous rocks of the Khabarovsk Sikhote-Alin accretionary complex are presented. It is established that the fragments of Jurassic and Permian–Triassic sandstones are dominated by poorly rounded and poorly separated material mainly from local provenance areas. The detrital part of the rocks is mainly represented by quartz, in a smaller amount by feldspar and rock fragments. Sandstones are characterized by high silica content, moderate alumina content, low concentrations of femic elements and calcium, moderate alkali content with significant variations in the K/Na ratio. Both Jurassic and Permian–Triassic rocks are typically characterized by reduced contents of LILE, REE, to a lesser extent HFSE and negative values of the ɛNd(T) parameter – compared to PAAS. The model Nd age of Jurassic sandstones varies from 1.36 to 1.71 Ga, Permian–Triassic – from 1.14 to 1.35 Ga. Most of the detrital zircon population is of late Paleozoic–early Mesozoic age, approximately 25% are older (pre-Paleoproterozoic). The studied sandstones are mainly rocks of the first cycle of weathering (petrogenic), formed during the erosion of igneous rocks of felsic composition. The synthesis of the obtained data suggests that the main source of the cluster material for the Mesozoic sedimentary rocks was the geological formations of the northern part of the Bureya–Khanka superterrane (Bureya and Malokhingan blocks), as well as, possibly, the eastern part of the Mongol–Okhotsk belt. The Khabarovsk terrane has not drifted significantly along the Tan Lu stike-slip system and is an “autochthonous” block in the present-day structure of Sikhote-Alin.

Spatial–temporal influence of the Mongol–Okhotsk and Palaeo–Pacific tectonic systems in NE China: Evidence from geochronological and geochemical data of felsic rocks in the east‐Central Jilin Province and southern Great Xing'an Range

The temporal and spatial extents of the Palaeo–Pacific and Mongol–Okhotsk tectonic systems, which together dominated the Mesozoic tectonics of NE China, still remain unclear. Here, we present new zircon U–Pb ages, isotopic analyses and whole‐rock major‐ and trace‐element data for felsic rocks from west and east Songliao Basin to explore the above question. The granites in Shancheng area formed between 176 and 164 Ma and have εHf(t) values of −22.09 to −7.59 with a TDM2 age range of 3622–2336 Ma. The granites are dominated by adakites and I‐type granite whose primary magma is derived from a thickened ancient crust. The rhyolites in Wushijiazi area formed between 151 and 141 Ma and have εHf(t) values of 3.34–12.38 with a TDM2 age range of 517–1328 Ma. They are A‐type granite and have a flat rare‐earth‐element (REE) pattern with obvious negative Eu anomalies, suggesting they were derived from partial melting of lower crustal rocks. The above findings indicate that the middle–late Jurassic granite in the Shancheng area formed in a compression setting related to the subduction of the Palaeo–Pacific Plate beneath the Eurasian continent and the Late Jurassic to early Cretaceous rhyolites in Wushijiazi area formed in an extensional setting related to the collapse of the Mongol–Okhotsk belt. In combination with regional geologic data, we conclude that the Palaeo–Pacific tectonic system mainly affected the east side of Songliao Basin, while the Mongol Okhotsk tectonic system mainly affected the Erguna Block, through the GXAR to the northern margin of the North China Craton (NCC).

Large-scale Permo-Triassic back-arc extensions of the Mongol- Okhotsk Ocean

The Late Paleozoic−Mesozoic Mongol-Okhotsk orogenic belt marks the final aggregation of East Asia. The geodynamics of the Mongol-Okhotsk oceanic plate subduction are still poorly understood due to its curved orogenic architecture, complex kinematics, and the protracted active continent margin that developed during oceanic subduction. Here, we report the discovery of an ophiolite within the southern paleo-active margin of the Mongol-Okhotsk orogenic belt. The ophiolitic slices are composed of serpentinites, metagabbros, and metabasalts, and interleaved with deformed volcaniclastic rocks. Using zircon U-Pb dating, we determined an age of 253 ± 2 Ma for the metagabbro, which suggests that the ophiolite formed during the Late Permian. Geochemical data and geologic relationships indicate that this ophiolite formed in a back-arc setting. The contemporaneous granitic dikes (ca. 255 Ma) intruding the basalts of the ophiolite were likely formed by crustal anatexis during the back-arc extension. Detrital zircon in sandstone associated with the ophiolite shows a prominent peak age of ca. 273 Ma that probably represents the sediments of the back-arc basin. Combining our discovery with the available data from the literature, we suggest that a >5000-km-long back-arc extension belt existed in the southwestern segment of the Mongol-Okhotsk belt, which indicates a probable Western Pacific-type active margin rather than the previously proposed Andean-type margin that formed during the Permo-Triassic period.

Geochronology and Petrogenesis of the Gol Mod Massif: Implications for the Geodynamic Evolution of the Orkhon-Selenge Belt, Northwestern Mongolia

The Orkhon-Selenge Belt is a Late Permian to Early Triassic volcanic plutonic belt located in northern Mongolia and is part of the Central Asian Orogenic Belt. The Selenge Complex, which is a part of the Orkhon-Selenge Belt, is a key area for studying the tectonic and magmatic evolution of the Central Asian Orogenic Belt. This study aims to contribute to understanding of the geodynamic evolution of the Orkhon-Selenge Belt by investigating the petrology, geochemistry and geochronology of the rocks in the region. Our results indicate that intrusive rocks were characterized as high-K, Calc-alkaline series and metaluminous to weakly peraluminous I-type granite affinities and their geochemical characteristics are indicating as arc-like geochemical signatures with depleted in elements such as Nb, Ta, Ti and Y and enriched in elements such as Rb, Cs, Th, K and light rare earth elements. Using zircon U-Pb dating, we determined an age of 257.3±0.73 Ma for the alkali granite, suggesting that south-western part of the Orkhon-Selenge Belt formed during the Late Permian time. The Selenge pluton, which is closely related to Erdenet-Ovoo porphyry type mineralization, is a composite intrusion. However, the zircon grains display magmatic and low oxygen fugacity conditions, which characteristics are likely the effect of weak mineralization of magma ascent with Late Permian tectonothermal event in the south-west part of the Orkhon-Selenge Belt. The results of this study will provide insights into the formation and evolution of the north-western segment of the Mongol-Okhotsk Belt, and will have implications for our understanding of the tectonic history of this region.

Taking a Fresh Look at the Stratigraphy of the Selemdzha and Tokur Terranes of the Mongol–Okhotsk Belt: The Results of U–Pb, Lu–Hf, and Sm–Nd Isotope Studies

Taking a Fresh Look at the Stratigraphy of the Selemdzha and Tokur Terranes of the Mongol–Okhotsk Belt: The Results of U–Pb, Lu–Hf, and Sm–Nd Isotope Studies

Mesozoic thermal events and related gold mineralization in the еastern Mongol-Okhotsk Orogenic Belt: constraints from regional geology and 40Ar/39Ar dating

ABSTRACT The Mongol–Okhotsk Orogenic Belt is a major structural element in East Asia. Several aspects of its evolution remain unknown or controversial. In this study, we report the results obtained using 40Ar/39Ar geochronological studies performed on the metasedimentary rocks from various terranes and formations as well as host rocks and hydrothermally altered rocks of gold deposits in the еastern Mongol–Okhotsk Belt. Our new 40Ar/39Ar data suggest that four thermal events occurred in the eastern Mongol–Okhotsk Belt at 172–166, 157–149, 141–135, and 133–128 Ma. The ages of these events do not correlate with the geological age of the formations and not depend on the location of the terranes. None of these events were accompanied by magmatic activity in the east of the belt. Therefore, we believe that these events are related to tectonic. The 172–166 Ma event corresponds to regional metamorphism associated with the collision of the Siberian Craton and Amurian Superterrane. The remaining three younger events of 157–149, 141–135, and 133–128 Ma occurred in the post-collisional setting. The Malomyr, Albyn, Unglichikan, and Elga gold deposits formed during two thermal stages at 141–135 and 133–128 Ma, after the collision. 40Ar/39Ar isotopic data suggest a gap (~30 Ma) between orogenic events caused by the collision of the Siberian Craton and Amurian Superterrane and gold mineralization. Our data are consistent with the idea that the formation of the Mongol–Okhotsk Orogenic Belt and gold ore deposits in this belt occurred in the relation to strike–slip (or transform displacement) along the regional E–W Mongol–Okhotsk fault/thrust system, but without relation of these deposits to granites.

Early Cretaceous rapid exhumation processes in the southern Great Xing’an Range, NE China: implications for extensional geodynamics

ABSTRACT The Mesozoic-Cenozoic geological evolution of the southern Great Xing’an Range (GXR) has long been controversial since it is located in the superposed region of different tectonic regimes and involves complex interactions. Here, we present low-temperature thermochronological data from granitoids emplaced in multiple magmatic stages to delineate the tectono-thermal history and further understand the geodynamic process. Most zircon and apatite (U-Th)/He ages, within error, indicate Early Cretaceous: the zircon average (U-Th)/He ages range from 146.7 ± 8.7 to 120.0 ± 7.1 Ma, except for the oldest data (162.5 ± 9.6 Ma) and the apatite (U-Th)/He ages vary from 138.7 ± 8.2 to 94.8 ± 7.5 Ma, except for the youngest age (70.8 ± 4.3 Ma). Systematic numerical modelling reveals that a rapid cooling and exhumation process with rates of 2.5-10 oC/Ma and 0.07-0.29 mm/yr occurred in the southern GXR during the early Cretaceous (145–110 Ma). We propose that the rapid uplift and exhumation of intrusive rocks during the early Cretaceous is also a manifestation of the extensional regime in the southern GXR, which is dominated by the rollback of the subducted Paleo-Pacific oceanic plate superposed by the post-orogenic extensional collapse of the Mongol-Okhotsk belt. In addition, the slow exhumation process since the late Cretaceous (~0.01 mm/yr) suggests that the Paleo-Pacific Ocean tectonic regime has a weak impact on the southern GXR area.

Age, petrogenesis, and tectonic implications of the late Permian magmatic rocks in the Middle Gobi volcanoplutonic Belt, Mongolia

AbstractThe Mongol–Okhotsk Belt, the youngest segment of Central Asian Orogenic Belt, was formed by the evolution and closure of the Mongol–Okhotsk Ocean. The oceanic closure formed two volcanoplutonic belts: Selenge Belt in the north and the Middle Gobi Belt in the south (in present day coordinates). However, the origin and tectonic evolution of the Mongol–Okhotsk Belt in general, and the origin and formation age of the Middle Gobi Belt in particular, remain enigmatic. To better understand the history of the magmatic activity in the Middle Gobi Belt, we conducted geochemical, U–Pb geochronological, zircon Hf, and whole‐rock Nd isotopic analyses of samples from the Mandalgovi volcanoplutonic suite, the major component of the Middle Gobi Belt. Our results show that the plutonic rock consists of ~285 Ma gabbro, ~265 Ma biotite‐granite and ~250 Ma hornblende‐granodiorite. The volcanic counterpart is represented by a Permian Sahalyn gol rhyolite and ~247 Ma Ikh khad andesite. The geochemical compositions of biotite‐granite and hornblende‐granodiorite indicate that their precursors were metagraywacke and amphibolite, respectively. They are characterized by positive whole‐rock εNd(t) and zircon εHf(t) values, indicating juvenile protoliths. The gabbro was derived by partial melting of a metasomatized lithospheric mantle source in a supra‐subduction setting. The biotite‐granite and Sahalyn gol rhyolite are formed by remelting of sediments in an inter‐arc extension setting. Later the hornblende‐granite and Ikh khad volcanic were emplaced at a volcanic arc formed by the subduction of the Mongol–Okhotsk Ocean. We conclude that the magmatic rocks of the Middle Gobi Belt formed in an active continental margin setting. Considering the consistent distribution of coeval arc‐derived magmatic formations along the southern margin of the Mongol–Okhotsk Belt, the oceanic basin was closed in a relatively simultaneous manner.

НОВЫЙ ВЗГЛЯД НА СТРАТИГРАФИЮ СЕЛЕМДЖИНСКОГО И ТОКУРСКОГО ТЕРРЕЙНОВ МОНГОЛО-ОХОТСКОГО ПОЯСА: РЕЗУЛЬТАТЫ U-Pb, Lu-Hf, Sm-Nd ИЗОТОПНЫХ ИССЛЕДОВАНИЙ

Results of U-Pb and Lu-Hf isotopic studies on detrital zircons from metasedimentary rocks in the eastern part of the Selemdzha and Tokur terranes of the Mongol-Okhotsk belt and Sm-Nd isotopic data for whole-rocks are in complete contradiction with existing ideas about the stratigraphy of the region under consideration and indicate the need to revise the traditional principles of mapping within the belt. Two types of deposits have been established, differing in Sm-Nd isotopic characteristics of whole-rocks and Lu-Hf isotopic composition of detrital zircons. Deposits of different types are involved in the formation of the two opposing accretion systems: in front of the Siberian Craton margin (type I) and the Amur superterrane (type II). Among type I deposits, discrete groups are distinguished that differ in the lower age limit: 553–498 Ma, Late Ediacaran-Cambrian; ~373 Ma, Upper Devonian; 333–327 Ma, Upper Mississippian; ~304 Ma, Pennsylvanian. The deposits of tectonic-stratigraphic units show a general pattern of younging from north to south. Thus, we observe a structure typical of accretionary wedge-shaped terranes with a rear part in the north and a frontal part in the south. Tectonic-stratigraphic units formed by type I deposits and previously considered part of the Selemdzha and Tokur terranes, apparently, are components of the Galam terrane of the accretionary wedge.

Late Paleozoic–Early Mesozoic granitoids in the Khangay-Khentey basin, Central Mongolia: Implication for the tectonic evolution of the Mongol-Okhotsk Ocean margin

The Mongol-Okhotsk Belt is the youngest segment of the Central Asian Orogenic Belt, which is the venue of the massive juvenile crust emplacement, and its formation and evolutions are still pending problems. This paper presents the first up-to-date U–Pb zircon ages, Hf-in-zircon isotope, geochemical and whole-rock Nd isotope data from igneous rocks of the Khangay-Khentey basin, Central Mongolia. The U–Pb zircon ages indicate three groups of magmatism at ~296 Ma, ~280 Ma, and ~230 Ma. The ~296 Ma magmatic rocks are characterized by negative εHf(t) and εNd(t) values and old Hf and Nd model ages suggesting their derivation by the melting of the crustal source. The ~280 Ma rocks are A2-type monzonites, granitoids, and rhyolites show positive εHf(t) and εNd(t) values and Neoproterozoic Hf and Nd model ages. The geochemical and isotope data suggest that ~280 Ma magmatism derived by the melting of a crustal source, induced by mantle upwelling. The ~230 Ma rock assemblage includes granitoids and volcanic rocks. The I-type calc-alkaline granitoids are enriched in K, Rb, U, and Th. The geochemical characteristics suggest that they have formed by the melting of a hornblende-bearing crustal source with the participation of fluids separated from the subducting slab. The positive εHf(t) and εNd(t) ~230 Ma rocks suggest partial melting of a depleted lower crustal material with the contribution of ancient crustal material. The ~296 Ma granitoids possess coherent/coupled Nd–Hf isotopic compositions supporting their origin from the ancient crust. Although the number of ~296 Ma samples are small, we suggest that they were probably emplaced at an active continental setting, ~280 Ma samples could have formed in a setting of local extension environment, ~230 Ma granitoids were also formed at an active continental margin. These magmatic rocks formed during the subduction of the Mongol-Okhotsk oceanic plate beneath the Central Mongolia-Erguna Block.

Late Mesozoic Intracontinental Deformation in the Northern Margin of the North China Craton: A Case Study From the Shangyi Basin, Northwestern Hebei Province, China

During the late Mesozoic, the East Asian continent underwent a complex tectonic history due to multiple episodes of plate convergence. How the crust responds to the multiple plate convergence in the North China Craton (NCC) remains unclear. Here we undertook field geological investigations and fault-slip vectors analysis of the Shangyi Basin in the western Yanshan fold and thrust belt, northern margin of the NCC. Combined with new geochronological data, we delineate three phases of intracontinental deformation in the area: 1) NW-SE compression during the Late Jurassic to earliest Cretaceous (ca. 151–141 Ma); 2) NW-SE extension during the middle–late Early Cretaceous (ca. 135–110 Ma); and 3) NE-SW compressional deformation later than 110 Ma. The early NW-SE compression controlled the present bulk architecture of the basin, and the subsequent two tectonic events only caused limited reworking of the previous structures. Through balanced cross-section restoration, we estimate the horizontal shortening ratio of the crust in the study area is over 27% due to the NW-SE compression. Moreover, the contribution of tectonic shortening from the north side of the basin is greater than that from the south side. NW-SE compressional deformation is consistent in time with the episode B of the Yanshanian movement (Yanshanian B), which may be influenced by the subduction of the Paleo-Pacific plate beneath East Asia and the closure of the Mongol-Okhosk Ocean. Subsequent NW–SE extension is likely to be associated with the destruction of the NCC during the Early Cretaceous. Extension may result from the roll-back of the Paleo-Pacific plate and post-orogenic collapse of the Mongol-Okhotsk belt. The last NE-SW compressional event may be linked to the remote effect of the final collision between the Qiangtang and Lhasa terranes.

Metamorphic rocks from the north-eastern part of the Ereendavaa terrane (Eastern Mongolia) :

In this paper, we have conducted geochronological and geochemical studies on the metamorphic rocks of the Khaychingol and Ereendavaa Formations in the Mogoitiin Gol, Khaychin Gol and Emgentiin Bulag areas from the Ereendavaa terrane and these rocks have been considered to be Precambrian in age. However, new LA–ICP–MS zircon U–Pb dating results indicate that the protolith of the studied metamorphic rocks was formed in two stages: 1) during ~ 296 - 285 Ma, the protolith of mafic, felsic and black schists formed; 2) during ~276 - 271 Ma, the protolith of gneiss and psammitic schists began to deposit. The Early Permian bimodal association composed of low-K basalt and comagmatic high-Na, low-K dacite with high-K calc-alkaline rhyolite, represent protolith of the mafic and felsic schists which were formed in back-arc basin environment. The Middle Permian gneiss, and psammitic schists with sedimentary protolith have geochemical signatures of island arc rocks, such as enrichment of LILE relative to HFSE, and markedly negative Nb, Ta and Ti anomalies, suggesting that they were formed in a continental arc environment. Considering a close spatial relationship of the Ereendavaa terrane with the Mongol-Okhotsk Belt in the north-west, we propose that accompanied with the emplacement of arc magmatic rocks, the arc rifting occurred and formed the Early Permian bimodal volcanic rocks. In the Late Permian, after the formation of the back-arc basin, deposition of the immature deposits as wacke, arkose and litharenite dominated sediments in a continental arc environment started.

Timing of formation and tectonic setting of Paleozoic granitoids in the eastern Mongol–Okhotsk Belt: Constraints from geochemical, U[sbnd]Pb, and Hf isotope data

The Mongol–Okhotsk Belt is one of the major structural elements in East Asia. However, many aspects of the evolution of this structure from the oceanic domain to the suture zone remain unresolved. The most controversial of these aspects are the timing and polarity of subduction processes as well as the timing of the final closure of the Mongol–Okhotsk Ocean. Here, we present whole-rock geochemical and UPb geochronological data for granitoids in the eastern Mongol–Okhotsk Belt to constrain their timing of formation and tectonic setting. Our new data suggest three stages of granitic magmatism in the eastern part of the Mongol–Okhotsk Belt: Cambrian (ca. 511 Ma), Middle Devonian (ca. 393 Ma), and Late Permian–Early Triassic (257–250 Ma). Cambrian (511 ± 4 Ma) plagiogranites in the Galam Terrane have adakitic geochemical signatures, and εHf(t) values of zircons from these granites vary mostly from +8.8 to +14.8 with Hf model ages (tHf(DM)) ranging from 0.77 to 0.53 Ga. These plagiogranites most likely represent the roots of an island arc that formed above a NW-dipping (present-day coordinates) subduction zone beneath the edge of the Siberian Craton. Middle Devonian (393 ± 7 Ma) granites from the Selemdzha Terrane, as well as Late Permian–Early Triassic (257 ± 4, 253 ± 3, 253 ± 2, 252 ± 3, and 250 ± 4 Ma) granodiorites and granites from the Selemdzha and Tokur terranes, have supra-subduction whole-rock geochemical signatures. Slight negative and positive zircon εHf(t) values of −6.1 to +8.4, tHf(DM) ages of 1.1–0.6 Ga, and tHf(C) ages of 1.4–0.6 Ga suggest that the studied rocks contain a continental crustal component. The Middle Devonian and late Permian–Early Triassic granodiorites and granites formed either in a continental island arc or on an active continental margin. During the Middle Devonian and at the late Permian–Early Triassic boundary, the Mongol–Okhotsk oceanic crust was subducting beneath the edge of the Siberian Craton. Thus, middle Cambrian, Middle Devonian, and Permian–Triassic granitoids in the eastern Mongol–Okhotsk Belt reflect three stages of subduction, meaning that the accretionary wedge terrane assemblages started to form during the Cambrian then continued during the Devonian and at the Permian–Triassic boundary, but was not started at Jurassic as previously proposed.

THE FIRST <sup>40</sup>Ar/<sup>39</sup>Ar GEOCHRONOLOGICAL DATA FOR ORE METASOMATITES OF THE UNGLICHIKAN GOLD DEPOSIT, EASTERN PART OF THE MONGOL-OKHOTSK FOLD BELT

In our isotope-geochronological study, the age of ore metasomatites of the Unglichikan gold deposit is determined at 140–136 Ma. Magmatism of this age is absent in the study area, and it is thus unreasonable to relate the ore mineralization in the Unglichikan deposit to any magmatic process. We conclude that in the mobilization and redistribution of the ore material and the formation of the Unglichikan deposit, a significant role was played by dislocation processes accompanied by hydrothermal activity during the final stages of orogenesis in the Mongol-Okhotsk belt.

Late Mesozoic intracontinental deformation at the northern margin of the North China Craton: A case study from the Kalaqin massif, southeastern Inner Mongolia, China

The East Asian continent underwent intense crustal shortening-thickening and following large-scale lithospheric extension in the late Mesozoic due to multiple plate convergence. How to respond to the multiple plate convergence in the North China Craton remains enigmatic. Here we undertaken structural measurements and fault kinematic analysis of the Kalaqin massif in the northern part of the North China Craton (NCC), together with new geochronological results, delineate two stages of intracontinental deformation in the area: (1) NE–SW-striking ductile sinistral shearing (Ds) at 150–135 Ma; and subsequent (2) NW–SE ductile–brittle extensional deformation (De) at 135–110 Ma. Based on electron-backscatter diffraction (EBSD) analysis of quartz c-axis fabric and the characteristics of dynamically recrystallized quartz and feldspar, we estimate that ductile deformation during Ds and De occurred at temperatures of 400–700 °C (i.e., greenschist–amphibolite facies conditions). Kinematic vorticity analysis shows that Ds deformation was characterized by vorticity values (Wk) of 0.72–0.87 (average = 0.793), corresponding to general shear with a dominant simple shear component. Somewhat lower vorticity values of 0.45–0.92 (average = 0.698) during De event indicate a dominant pure shear component. We suggest that Ds reflected the regional NE-striking sinistral shearing represented by the Tan-Lu Fault Zone. Combined with contemporaneous dextral transpression along the ~E–W-striking Chifeng–Kaiyuan Fault and Fengning–Longhua Fault, these structures constituted a regional conjugate strike-slip system in the northern edge of the NCC. Ds event was likely associated with the episode B of the Yanshanian movement, which might result from the subduction of the Paleo-Pacific plate beneath East Asia and the closure of the Mongol-Okhosk Ocean. The subsequent De represents the destruction of the NCC during the Early Cretaceous, which was likely influenced by the roll-back of the Paleo-Pacific plate and post-orogenic extensional collapse of the Mongol-Okhotsk belt.

Two types of accretionary complexes in the eastern Mongol–Okhotsk Belt: Constraints from U–Pb and Hf isotopic data of detrital zircons from metasedimentary rocks of the Selemdzha and Tokur terranes

The Mongol–Okhotsk Belt is a major structural element of East Asia. However, a number of important aspects of its evolution remain unresolved, in particular, the timing and scenarios of accretion and collision. This paper presents first detrital zircon U–Pb–Hf and whole-rock Nd isotope data from metasedimentary rocks of the Selemdzha and Tokur terranes in the eastern Mongol–Okhotsk Belt. Our new data suggest that these terranes are not part of a single sedimentary sequence, as has been previously assumed, but likely form a set of thrust sheets. The eastern Mongol–Okhotsk Belt hosts at least two types of accretionary complexes. The first type includes the Talyma, Zlatoust, Sagur-1, Tokur, and Ekimchan formations, which contain abundant Paleoproterozoic and Archean zircons, as well as Paleozoic zircons with two-stage model ages (tHf(C)) of >1.7 Ga. These formations are interpreted as fragments of a Paleozoic accretionary complex emplaced at the southern edge of the Siberian Craton. The second type includes the Afanas'ev and Sagur-2 formations, which lack Paleoproterozoic and Archean zircons but contain early Mesozoic zircons with tHf(C) ages of <1.4 Ga. The Afanas'ev and Sagur-2 formations are probably fragments of an early Mesozoic accretionary complex formed at the northern edge of the Amur Block. Our new model focuses on stratigraphic architecture of the Selemdzha and Tokur terranes and implies that two subduction zones of opposite polarities existed during the suturing of the Mongol–Okhotsk Ocean.

Tectonic Nature of the Ul’ban Terrane (Mongol–Okhotsk Fold Belt): Results of U–Pb and Lu–Hf Isotope Studies of Detrital Zircons

The U–Pb and Lu–Hf isotope studies of detrital zircons from Mesozoic metasedimentary rocks of the Sorukan and Naldinda formations of the Ul’ban terrane are performed for the first time. It is shown that Early Precambrian zircons are almost completely absent in these rocks, while Early Mesozoic zircons with Paleozoic, Neoproterozoic, and Mesoproterozoic Hf model ages dominate. Based on the data obtained, it is assumed that the Ul’ban terrane is a part of the accretionary complex of the northern framing of the Amur superterrane and is neither a depression superimposed on the Paleozoic formations of the Mongol–Okhotsk belt nor an accretionary complex in the southern framing of the North Asian Craton, as was believed earlier.

Re-Os Geochronology, Whole-Rock and Radiogenic Isotope Geochemistry of the Wulandele Porphyry Molybdenum Deposit in Inner Mongolia, China, and Their Geological Significance

The Wulandele molybdenum deposit is a porphyry-type Mo deposit in the Dalaimiao area of northern Inner Mongolia, China. Molybdenite Re-Os dating yields a model age of 134.8 ± 1.9 Ma, with the fine-grained monzogranite most closely related to the mineralization. The lithogeochemical data show that the monzogranite is weakly peraluminous, high-K calc-alkaline series, with reduced to slightly oxidized, highly fractionated I-type granite characteristics. The relatively low initial 87Sr/86Sr (range from 0.705347 to 0.705771), weakly negative εNd(t) (range from −2.0 to −1.3), and crust-mantle mixing of Pb isotopes suggest that the monzogranite originated from the partial melting of mafic juvenile lower continental crust derived from the depleted mantle, with a minor component of ancient continental crust. Combined with the regional tectonic evolution, we argue that the partial melting, then injection, of the monzogranite melt was probably triggered by collapse or delamination of the thickened lithosphere, which was mainly in response to the post-orogenic extensional setting of the Mongol–Okhotsk belt; this is possibly coupled with a back-arc extension related to Paleo-Pacific plate subduction. The extensively fractional crystallization of the monzogranite melt is the crucial enrichment process, resulting in magmatic hydrothermal Mo mineralization in the Wulandele deposit, and the Cretaceous granitoids are generally favorable to form Mo mineralization in the Dalaimiao area.