Since the closure of the Mongol-Okhotsk Ocean in the Mesozoic, Mongolia has been in an intraplate tectonic setting; the nearest plate boundary being similar to 3,000 km to the east, at the active Pacific subduction front. Throughout this time, Mongolia has experienced magmatism in the form of distinctive, small-volume volcanic fields dispersed along the central and eastern parts of the country. On the basis of geochemical, isotopic, palaeomagnetic and zircon data, the magmatism can be discriminated from preceding post-collisional magmatism. Gradual change from a lithospheric to an asthenospheric mantle source suggests lithospheric delamination occurred beneath Mongolia, starting at similar to 140 Ma and terminating at similar to 107 Ma. Accordingly, the onset of intraplate magmatism is set at 107 Ma. Regardless of the spatial and temporal occurrence and evolution of the intraplate magmatism in Mongolia, the geochemistry of the resultant volcanic rocks throughout time remains remarkably similar, although the cause of magmatism has been much debated. Through evaluation of available K-Ar and Ar-Ar data from the literature, along with newly-obtained data from three different volcanic fields, we have identified that the intraplate volcanism in Mongolia has been near-continuous since its onset, with hiatuses of only <6 Myr. Since 35 Ma, hiatuses have reduced to <1 My. In light of these findings, we re-evaluate the various models that have been proposed for the origins of this long-lived volcanism and suggest the cause of magmatism results from asthenospheric upwellings initiated by a delamination event in the Mesozoic, but have been prolonged by enhanced mantle flow relating to northward progression of India, the closure of Neo-Tethys, and ultimately the Indo-Asian collision. This example of intraplate magmatism is one of the longest-lived volcanic intraplate regimes on Earth that does not appear to relate to a geophysically-recognisable high heat-flux mantle plume. (c) 2024 The Author(s). Published by Elsevier B.V. on behalf of International Association for Gondwana Research. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/ 4.0/).

&lt;p&gt;The closure of the Mongol-Okhotsk Ocean in Jurassic &amp;#8211; Cretaceous times led to the final amalgamation of the interior of Eastern Asia, thus placing Mongolia in an intraplate tectonic setting. Small and widespread volcanic fields of Mesozoic and Cenozoic age are known through Eastern Asia, attributed to both post-collisional and intraplate mechanisms. In Mongolia, intraplate volcanic fields are scattered across the central and eastern parts of the country. Although several models have been proposed to explain the origin of this late Mesozoic &amp;#8211; Cenozoic intraplate magmatism in Mongolia, there is still on-going debate about the process(es) that lead to it. Moreover, there are no temporal reconstructions on the extent of post-collisional magmatism in the area preceding intraplate magmatic activity, nor any hypotheses on the timing of the onset of the latter. In this study, we differentiate between post-collisional and intraplate magmatism in Mongolia using a set of geochemical, isotopic, palaeomagnetic and zircon data, and define the onset of intraplate magmatic activity at 107 Ma. Through evaluation of nearly 700 published radiometric data from the various volcanic fields across Mongolia along with newly-obtained age constraints, we reveal a complex temporal and spatial evolution of the magmatism that runs parallel in different volcanic fields through time, and we identify the extent of hiatuses in the magmatic activity. Based on the assessed data we discuss the source of bias in our understanding of the magmatic history of Mongolia and evaluate the various proposed models for the origin of the Mongolian magmatism. Finally, we suggest that asthenospheric upwellings were induced through a delamination event beneath Mongolia in the late Mesozoic. This initiated the intraplate magmatism, the temporal evolution of which is prolonged due to enhanced mantle flow related to northward progression of Tethys and the Indian plate.&lt;/p&gt;

Much evidence points to a dramatic thinning of East Asian lithosphere during the Mesozoic, but with little precision on when, or over what time scale. Using geochemical constraints, we examine an extensive compilation of dated volcanic samples from Russia, Mongolia and North China to determine when the lithosphere thinned and how long that process took. Geochemical results suggest that magmatism before 107 Ma derived from metasomatised subcontinental lithospheric mantle (SCLM), whereas after 107 Ma, melt predominantly derived from an asthenospheric source. The switch to an asthenospheric magma source at ~107 Ma occurred in both Mongolia and North China (>1600 km apart), whereas in eastern Russia the switch occurred a little later (~85 Ma). Such a dramatic change to an asthenospheric contribution appears to have taken, from beginning to end, just ~30 Myrs, suggesting this is the duration for lithospheric mantle weakening and removal. Subsequent volcanism, through the Cenozoic in Mongolia and North China does not appear to include any contribution from the removed SCLM, despite melts predominantly deriving from the asthenosphere.

The eclogite-bearing Alag Khadny metamorphic complex in the Lake Zone, SW Mongolia occupies the central region of the Central Asian Orogenic Belt, the largest Phanerozoic orogenic belt in the world. The complex consists mainly of orthogneisses intercalated with eclogites and micaschists in a mélange zone. Most of eclogites are strongly amphibolitized. In this study, we examined petrography and mineral chemistry of eclogites and amphibolitized eclogites, respectively. The result of our research shows that Chandman eclogites experienced multiple events of metamorphism in throughout their subduction and subsequent collision history. We revealed that eclogites were subjected to blueschist facies metamorphism before the peak eclogite facies stage. In addition, we have studied amphibolitized eclogite, and revealed that another distinct progressive medium pressure (MP) epidote-amphibolite facies metamorphic event took place in the eclogite, consistent with collision process. The multiple events of metamorphism in eclogites have been revealed by zonation textures of HP amphiboles zoned with glaucophane→barroisite→Mg-hornblende and MP amphiboles zoned with actinolite/winchite→barroisite→Mg-hornblende/tschermakite/Fe-pargasite. These amphiboles with different zonation textures reflect their metamorphic history of subduction to collision events.

Although the closure of the Paleo-Asian Ocean in western China and western Mongolia occurred in the Late Carboniferous and Early Permian, widespread intra-continental magmatism continued to occur across this region from the Late Permian to the end of the Triassic. In this study we document field relationships and geochemical characterization of a Late Triassic felsic intrusive complex in the western Mongolian Altai. The plutonic complex occurs as sills, dikes, and small stocks and its composition varies from biotite granite, two-mica granite, to leucogranite. Structurally, the plutonic complex occurs in the hanging wall of a segment of the regionally extensively (>1500 km long) Irtysh-Ertix-Bulgan thrust zone. As the plutonic bodies both cut and are deformed by the shear fabrics in this regional thrust shear zone, the duration of felsic magmatism and regional thrusting was temporally overlapping. This suggests that magmatism was coeval with crustal thickening. Major- and trace-element data and isotopic analysis of granitoid samples from our study area indicate that the felsic intrusions were derived from partial melting of meta-sediments, with the biotite and two-mica granite generated through vapor-absent melting and the leucogranite from flux melting. Although the Mongolian Altai intrusions were clearly originated from anatexis, coeval granite in the Chinese Altai directly west of our study area in the hanging wall of the Irtysh-Ertix-Bulgan thrust was derived in part from mantle melting. To reconcile these observations, we propose a Himalayan-style intracontinental-subduction model that predicts two geologic settings for the occurrence of felsic magmatism: (1) along the intracontinental thrust zone where granite was entirely generated by anatexis and (2) in the hanging wall of the intracontinental thrust where convective removal and/or continental subduction induced mantle melting. (C) 2015 Elsevier Ltd. All rights reserved.

The eclogite-bearing Alag Khadny metamorphic complex in the Lake Zone, SW Mongolia located in the central part of the Central Asian Orogenic Belt, consist mainly of orthogneisses which interleaving with marbles including lenses of garnetchloritoid schists of Maykhan Tsakhir Formation. Eclogites have two modes of occurrence, i.e. lenses and boudins eclogite-1in orthogneisses and eclogite-2in marbles. Thermocalc calculations for the peak eclogite facies metamorphism for eclogite-1with the assemblage of Grt + Omp + Brs ± Ph± Ep yielded 570-630°C and 22-25 kbar (Javkhlan et al., 2013a). In contrast, pressure conditions of the garnet-chloritoid schists (10-11 kbar) are distinctly lower than those of the eclogite-1, whereas temperatures (560-590°C) are similar (Javkhlan et al., 2013b). Eclogite-2in marbles consists of small grains of garnet ( <0.1 mm) and omphacite with minor amounts of amphibole, epidote, paragonite, plagioclase, chlorite, calcite, biotite, quartz, titanite and rutile. The matrix of eclogite-2shows a pseudomorpous texture, where small grains of garnet crowd cemented by titanite forming isomorphic round shape. Some of cores of garnet grain contain relics of garnet (X Ca=0.32-0.42; XMg=0.06-0.08) indicating previous mineral were larger porphyroblastic garnet. In addition, small grains of omphacite forming rectangular prismatic nature surrounded by garnet grains. Garnet grains have compositionally zoning with core (X Ca=0.08-0.20; XMg=0.10-0.16), mantle (XCa=0.24-0.39; XMg=0.080.17), rim (XCa=0.22-0.26; XMg=0.18-0.23) and outer-rim (X Ca=0.20-0.22; XMg=0.12-0.18). Few omphacites preserved their core (XJd=0.27-0.31; Fe 2O3=1.34-2.22 wt%) whereas most of grains have compositional heterogeneity with X Jd from 0.34 to 0.48 (Fe2O3=0.04-2.31) and locally with rims of higher Fe 2O3 (3.79 wt%)-bearing omphacite (X Jd=0.32). Omphacites partially replaced by symplecites of Pl (An=15-17), amphibole (Ed, Act, Mg-hbl) and Ep. Three types of amphibole are distinguished by their texture, amphibole (Amp1) [zoned with Act (X Mg=0.79-0.81) core, Brs (X Mg=0.54-0.69) mantle and rims with Ts, Mg-Trm and Prg in compositions] coexisting with Grt and Omp, poikiloblastic barroisitic amphibole (Amp2) (X Mg=0.65-0.75) containing eclogitic minerals of Grt and Omp with their symplectitic assemblage and finally actinolitic amphiboles (Amp3) partially replacing Omp and Grt. Based on the textures we distinguished two metamorphic events, i.e. eclogite facies metamorphism and poikiloblastic barroisitic amphibibole metamorphism. The peak eclogite facies metamorphism characterized by assemblages of Grt (mantle) + Omp + Amp1 (Brs) + Ep + Pg + Rt. Thermocalc calculation yielded 487 ± 46°C and 19.7± 2.1 kbar (sigfit=1.80). Thermocalc calculation of Grt (rim) + Omp (rim) + Amp1 (Mg-Trm) + Ep + Pl yielded 666 ± 45°C and 13.7± 1.6 kbar (sigfit = 2.11) suggesting a decompression stage after the eclogitic metamorphism. The poikiloblastic barroisitic Amp2 shows decreasing Si (7.01-6.69 pfu) and increasing NaB (0.61-0.70 pfu) from core to rim, suggesting that the Amp2 grew after the peak eclogite facies metamorphism, and probably during the second prograde metamorphic event. Approximate P-T conditions of the poikiloblastic barroisitic Amp2 are estimated as 5-7 kbar at c. 450 °C. Ar/Ar muscovite ages for eclogites (543 ± 3.9 Ma) in marbles (probably eclogite-2) and the garnet-chloritoid schists (537 ± 2.7 Ma) were determined ( Stipska et al. 2010). K-Ar ages for eclogite-1 [603 ± 15 Ma, 602± 15 Ma (Amp) and 612± 15 Ma (Ph)] within orthogneisses have been obtained (Javkhlan et al., 2014). These ages are interpreted as the exhumation ages for the eclogites (-1 and -2) and the garnet-chloritoid schists. The peak temperature conditions of eclogite-2 considerably lower than eclogite-1 whereas the pressure conditions are similar. The peak P-T conditions garnet-chloritoid schists are correlated with the poikiloblastic Amp2 metamorphism of the eclogite-2.

Cretaceous volcanism occurred over a wide region of eastern Asia. Hypotheses for its development include intra-continental rifting, continental-arc magmatism, plume activities, and delamination of a previously thickened mantle lithosphere. One of the major obstacles in differentiating the above model is the lack of systematic studies of Cretaceous volcanic rocks in eastern Mongolia and far-east Russia. In this study, we address this issue by examining the timing and geochemistry of Cretaceous volcanic rocks in eastern Mongolia. Our 40Ar/39Ar dating of volcanic rocks together with the existing K–Ar ages of volcanic rocks indicate that Cretaceous volcanism in our study area occurred between 120Ma and 104Ma. Our field investigation and geochemical analysis reveal three types of volcanic rocks in the study area: (1) a bimodal series, (2) a shoshonite series, and (3) a basanite series. Geochemical analysis of the volcanic rocks indicates a highly heterogeneous source, which is characterized by a mixture of depleted, enriched and metasomatized mantle. At least three models may explain the timing, geochemistry, and regional tectonic setting of Cretaceous volcanism in eastern Mongolia: (1) partial delamination of a previously thickened mantle lithosphere generated by arc magmatism and collision tectonics, (2) back-arc extension and its related mantle upwelling due to rapid collapse of a flat subduction slab, and (3) mantle avalanche of a thick pile of cold subducted Paleo-Asian oceanic plates initially trapped at the 670-km endothermic phase transition triggered by westward Pacific subduction causing mixing of several mantle sources. Testing the above models require further studies on the spatial and temporal relationships between Cretaceous volcanism and structural history of NE Asia. In addition, a careful restoration of the position and history of past subducted slabs of the Paleo-Asian oceans may help differentiate the above competing models in the future.

Garnet-chloritoid schists occur in the Chandman district of the Lake Zone, SW Mongolia, which is located within the central part of the Central Asian Orogenic Belt. The Chandman district is composed of Neoproterozoic ophiolites of the Khantaishir Formation, eclogite-bearing orthogneisses and micaschists of the Alag Khadny metamorphic complex, marbles of the Maykhan Tsakhir Formation, and the basement block of the Zamtyn Nuruu Formation. Garnet-chloritoid schists occur as lenses or layers within marbles of the Maykhan Tsakhir Formation, which lie in contact with eclogite bodies. They consist mainly of garnet, chloritoid, phengitic muscovite, chlorite and quartz, with minor amounts of kyanite, rutile, ilmenite, zircon and carbonaceous matter. The texture and mineral chemistry of the garnet-chloritoid schists record prepeak and peak stages of high-pressure intermediate type metamorphism. The pre-peak stage of epidote-amphibolite facies conditions (T = 500-510 °C and P = 7-8 kbar) is deduced from cores of garnet containing mineral inclusions of chloritoid, Fe-rich chlorite, phengitic muscovite, and quartz. The peak mineral assemblage of the garnet-chloritoid schists defined by garnet rims containing chloritoid, phengitic muscovite, Mg-rich chlorite, kyanite and quartz as inclusions indicates T = 560-590 °C and P = 10-11 kbar, falling in transitional conditions between the epidote-amphibolite and the eclogites facies. The pressure conditions of the garnet-chloritoid schists are distinctly lower than those of the eclogites (T = 590-610 °C and P = 20-25 kbar), whereas temperature conditions are similar. The eclogites are considered to have formed by subduction of oceanic crust. In contrast, the metamorphism of the garnet-chloritoid schists was probably related to collisional tectonic event in the Central Asian Orogenic Belt.

We report first fluid inclusion data on amphibolite-facies pelitic schists from Bodonch area of western Mongolian Altai in the Central Asian Orogenic Belt. Three categories of fluid inclusions have been observed in quartz: dominant primary and secondary inclusions, and least dominant pseudosecondary inclusions. The melting temperatures of all the categories of inclusions lie in the narrow range of -57.5 °C to -56.6 °C, close to the triple point of pure CO2. Homogenization of fluids occurs into liquid phase at temperature between -33.3 °C to +19.4 °C, which convert into densities in the range of 0.78 g/cm3 to 1.09 g/cm3. The estimated CO2 isochores for primary and pseudosecondary high-density inclusions is broadly consistent with the peak metamorphic condition of the studied area (6.3-7.3 kbar at 655 °C). The results of this study, together with the primary and pseudosecondary nature of the inclusions, indicate CO2 was the dominant fluid component during the peak amphibolite-facies metamorphism of the study area. The examined quartz grains are texturally associated with biotite, kyanite and staurolite, which are regarded as high-grade minerals formed during prograde to peak metamorphism. Therefore quartz probably formed by high-grade metamorphism and the primary fluid inclusions trapped in the minerals probably preserve fluids at around peak metamorphism.

Intrusion of the Cambrian Burd Gol Granite Massif into the Proterozoic metamorphic rocks of the Zamtyn Nuruu and molasse-like sediments of the Boomyn Khudag Fm. culminated the collisional events of the Caledonian evolution in the SE part of the Lake Zone, Gobi Altay, SW Mongolia. The monotonous leucogranites of the Burd Gol Massif have a high-K calc-alkaline, subaluminous to slightly peraluminous geochemical signature with an intermediate initial Sr ratio (Sr-87/Sr-86(510) = 0.7064) and rather unradiogenic Nd (epsilon(510)(Nd) = -1.5 and -0.2). The laser ablation ICP-MS U-Th-Pb radiometric dating of monazites and zircons from these rocks yields intrusive ages between 506.4 +/- 5.4 and 513.4 +/- 4.3 Ma (2 sigma). The final stage of magmatic activity is documented by the Ar-Ar dating of muscovite from a pegmatite dyke at 485.1 +/- 3.2 Ma. The granites intruded and thermally influenced the terrestrial clastic sediments of the Boomyn Khudag Formation, whereby the thermal metamorphism of sediments was characterized by a muscovite growth. These new data argue against the previously assumed Permian age of the Burd Gol Massif and the Lower-Middle Devonian age of the Boomyn Khudag sedimentary formation and illustrates the terrestrial evolution in this part of the Lake Zone during the Cambrian.

Understanding the development of the Central Asian Orogenic System (CAOS), which is the largest Phanerozoic accretionary orogen in the world, is critical to the determination of continental growth mechanisms and geological history of central Asia. A key to unraveling its geological history is to ascertain the origin and tectonic setting of the large flysch complexes that dominate the CAOS. These complexes have been variably interpreted as deep-marine deposits that were accreted onto a long-evolving arc against large continents to form a mega-accretionary complex or sediments trapped in back-arc to fore-arc basins within oceanic island-arc systems far from continents. To differentiate the above models we conducted U-Pb geochronological analyses of detrital-zircon grains from turbidites in the composite Hangay-Hentey basin of central Mongolia. This basin was divided by a Cenozoic fault system into the western and eastern sub-basins: the Hangay Basin in the west and Hentey basin in the east. This study focuses on the Hentey basin and indicates two groups of samples within this basin: (1) a southern group that were deposited after the earliest Carboniferous (similar to 339 Ma to 354 Ma) and a northern group that were deposited after the Cambrian to Neoproterozoic (similar to 504 Ma to 605 Ma). The samples from the northern part of the basin consistently contain Paleoproterozoic and Archean zircon grains that may have been derived from the Tuva-Mongol massif and/or the Siberian craton. In contrast, samples from the southern part of the basin contain only a minor component of early Paleozoic to Neoproterozoic zircon grains, which were derived from the crystalline basement bounding the Hangay-Hentey basin. Integrating all the age results from this study, we suggest that the Hangay-Hentey basin was developed between an island-are system with a Neoproterozoic basement in the south and an Andean continental-margin arc in the north. The initiation of the southern are occurred at or after the early Carboniferous, allowing accumulation of a flysch complex in a long-evolving accretionary complex. (C) 2007 Elsevier B.V. All rights reserved.

Three geological domains (Gobi Tien Shan, Shargyn Gobi and Trans-Altay) have been distinguished in the Trans-Altay Gobi (SW Mongolia) consolidated during the Carboniferous and intruded by numerous plutons of the Devonian, Carboniferous and Permian ages. According to relationship to the Variscan orogenic cycle, pre-orogenic, syn-orogenic and post-orogenic intrusive bodies have been recognized. A pre-orogenic phase is represented by the Gurvan Khar Massif composed of geochemically primitive granodiorites that are geologically related to the relics of the Silurian-Devonian oceanic crust in the Zoolen Terrane (Shargyn Gobi Domain). Calc-alkaline intrusive rocks of I-type in the Gobi Tien Shan and Naran Sevest intrusive complexes exposed in the Gobi Tien Shan Domain are syn-orogenic to post-orogenic. Laser ablation U-Pb ICP-MS dating on zircon yielded an Early Carboniferous age for the Zamyn Belgekh Pluton, representing the largest plutonic body in this domain. Other plutons (Bayan Ayrag, Trans-Altay and Ikh Bayan) are of high-K chemistry, have Late Carboniferous to Permian age and are clearly post-orogenic. Minor gabbro massifs and small oval bodies of granites of the Ikh Bayan Massif in the Baytag Terrane (Shargyn Gobi Domain) are spatially related to the Trans-Altay shear zone of Late Carboniferous to Permian age. This zone accommodated dextral lateral movements between the Shargyn Gobi and Trans-Altay domains.