Mongolian Plate Tectonics Studies Groups - MPTG

Abstract

Metaharzburgite and metadunite in the ultramafic body of the Naran Massif in the Khantaishir Ophiolite, western Mongolia, record multi-stage processes of serpentinization (antigorite, lizardite + brucite, then chrysotile). Bulk-rock chemistry and the compositions of primary olivine (P-olivine) and Cr-spinel suggest that the alteration occurred in the forearc mantle. In the metaharzburgite, a novel occurrence of fine-grained (10–50 μm) secondary olivine (S-olivine) takes the form of aggregates (a few millimeters across) with bands of antigorite. The S-olivine has higher Mg# values (0.96–0.98) than the P-olivine (Mg# = 0.92–0.94) and contains inclusions of clinopyroxene and magnetite. The P-olivine has been replaced by antigorite and magnetite. Mesh textures of lizardite + brucite are developed in both P- and S-olivine. The microtextures and chemical compositions of minerals suggest that S-olivine aggregates were formed by pseudomorphic replacement of orthopyroxene related to multi-stage hydration processes. Assuming the mantle wedge conditions beneath a thin crust, orthopyroxene was first replaced by S-olivine + talc at high temperatures (500–650 °C at ~ 0.5 GPa). With cooling to ca. 400–500 °C and fluid supply, talc transformed to antigorite with the release of silica. During this stage, P-olivine was also transformed to antigorite by consumption of silica released from orthopyroxene decomposition. At temperatures below 300 °C, lizardite + brucite ± magnetite formed from the remaining P- and S-olivine grains. The formation of S-olivine presented in this study contrasts with the commonly ascribed process of deserpentinization. Taking into account the geochemical data for the studied ultramafic rocks and those previously reported for mafic rocks, our results suggest that mantle wedge beneath thin crust was hydrated in response to continuous cooling and fluid supply from a subducting slab after subduction initiation.

Abstract

The Mongolian Altai Zone of the Central Asian Orogenic Belt has been traditionally interpreted as a mosaic of Paleozoic magmatic arcs, back-arcs, and Precambrian continental terranes. In order to define its architecture and its tectonic evolution, three domains previously interpreted as terranes were investigated. The findings show that the Northern and Central domains are formed by a metamorphic sequence characterized by Barrovian S1 fabric transposed by recumbent folds and dominant sub-horizontal amphibolite facies S2 schistosity. The latter is associated with the intrusions of late Devonian syntectonic granite sheets and anatexis in the south. The Southern domain is formed by early Permian migmatites and anatectic granites separated from the metamorphic envelope by amphibolite to green-schist facies D3 shear zone cross-cutting S2 fabrics. All domains have been reworked by E-W upright folds associated with axial-planar greenschist facies cleavage, reflecting the final mid-Permian to Triassic D4 shortening. Lithological, geochemical, and U-Pb zircon analyses of metasediments of all domains indicate that they are formed by Ordovician mature quartzite derived from Precambrian basement intruded by Cambrian-Ordovician continental arc and Silurian immature graywacke which originated through erosion of an oceanic arc. Altogether, the whole sequence represents a fore-arc basin in front of a migrating arc affected by thickening and late Devonian extension. The Southern domain is interpreted as an early Permian core complex amplified by mid-Permian to Triassic compression. The apparent “terrane” architecture of the Mongol Altai Zone originated due to Devonian and Permian heterogeneous reworking of a giant Ordovician to Silurian fore-arc basin.

Abstract

In Mongolia, rare earth element (REE) mineralization of economic significance is related either to the Mesozoic carbonatites or to the Paleozoic peralkaline granitoid rocks. Carbonatites occur as part of alkaline silicate-carbonatite complexes, which are composed mainly of nepheline syenites and equivalent volcanic rocks. The complexes were emplaced in the Gobi-Tien Shan rift zone in southern Mongolia where carbonatites usually form dikes, plugs or intruded into brecciated rocks. In mineralized carbonatites, REE occur mainly as fluorocarbonates (bastnäsite, synchysite, parisite) and apatite. Apatite is also present in the carbonatite-hosted apatite-magnetite (mostly altered to hematite) bodies. Alkaline silicate rocks and carbonatites show common geochemical features such as enrichment of light REE but relative depletion of Ti, Zr, Nb, Ta and Hf and similar Sr and Nd isotopic characteristics suggesting the involvement of the heterogeneous lithospheric mantle in the formation of both carbonatites and associated silicate rocks. Hydrothermal fluids of magmatic origin played an important role in the genesis of the carbonatite-hosted REE deposits. The REE mineralization associated with peralkaline felsic rocks (peralkaline granites, syenites and pegmatites) mainly occurs in Mongolian Altai in northwestern Mongolia. The mineralization is largely hosted in accessory minerals (mainly elpidite, monazite, xenotime, fluorocarbonates), which can reach percentage levels in mineralized zones. These rocks are the results of protracted fractional crystallization of the magma that led to an enrichment of REE, especially in the late stages of magma evolution. The primary magmatic mineralization was overprinted (remobilized and enriched) by late magmatic to hydrothermal fluids. The mineralization associated with peralkaline granitic rocks also contains significant concentrations of Zr, Nb, Th and U. There are promising occurrences of both types of rare earth mineralization in Mongolia and at present, three of them have already established significant economic potential. They are mineralization related to Mesozoic Mushgai Khudag and Khotgor carbonatites in southern Mongolia and to the Devonian Khalzan Buregtei peralkaline granites in northwestern Mongolia.

Abstract

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.

Fluid inclusions in high- and ultrahigh-pressure metamorphic rocks provide direct information on the composition of the fluids that evolved during metamorphism and fluid-rock interactions in deep subduction zones. We investigate the fluid inclu- sions in the Khungui eclogite of the Zavkhan Terrane, Central Asian Orogenic Belt. Fluid inclusions are observed in garnet and quartz in the eclogite samples that underwent metamorphism during subduction. The primary fluid inclusions in quartz are composed of liquid and vapor with high salinities (15.7–16.4 wt.% NaCl eq.), whereas the secondary fluid inclusions in quartz are classified as: relatively high salinity (Type I:12.5–16.3 wt.% NaCl eq.) and low salinity (Type II:6.7–10.6 wt.% NaCl eq.). The garnet shows compositional zoning from Ca-poor cores to Ca-rich rims, and the rims that grew during the eclogite-stage metamorphism (2.1–2.2 GPa at 580–610 °C) preferentially contain numerous primary fluid inclusions. The primary fluid inclusions in garnet are commonly bi-phases (liquid and vapor); however, some are multiphase-solid fluid inclusions composed of fluids (liquid and vapor) and combinations of several minerals (halite, quartz, apatite, calcite, biotite, chlorite, and actinolite). Bi-phase fluid inclusions preferentially occur in the inner parts of the Ca-rich garnet rim, whereas multiphase-solid fluid inclusions occur along the margins of the Ca-rich rim. We hypothesize that the multiphase-solid fluid inclusions are formed via interactions between trapped fluids, trapped minerals, and the host garnet during exhumation. By combination of FIB–SEM and synchrotron X-ray CT analyses, the detailed occurrences, volumes, and compositions of the solid phases in the fluid inclusion was analyzed. We then conduct mass balance analysis to reconstruct accurate fluid compositions using data from the FIB–SEM and synchrotron X-ray CT images of the multiphase-solid fluid inclusion. The results of these analyses reveal that (1) fluid changed from an H2O-dominated saline fluid (13–16 wt. % NaCl eq.) at the prograde to the earlier eclogite stage to H2O–CO2-dominated hypersaline fluid at later eclogite stage (~ 32 wt. % NaCl eq., 7.3 wt. % CO2 and ~ 19 molal dissolved cations); (2) a variety of mineral assemblages in multiphase-solid fluid inclusions are produced by post-entrapment reactions between the trapped hypersaline fluid, trapped minerals and the fluid host mineral. The evolution of fluids from saline to hypersaline during the eclogite facies stage is probably caused by the formation of hydrous minerals (i.e., barroisite) under a near-closed system.

Abstract

The Bayankhongor Ophiolite Zone (BOZ) located at the northeastern margin of the Baidrag microcontinent in western-central Mongolia represents a key litho-tectonic unit of the Mongolian orogenic collage. The BOZ has been traditionally considered as one of the largest and major ophiolitic systems representing a vestige of a Neoproterozoic ocean-floor basin that developed between two ancient microcontinents named the Khangai in the northeast and Baidrag in the southwest. However, the age, petrology, and tectonic setting of many magmatic complexes of the BOZ are still poorly constrained. In order to fill the gap, we carried out geochemical and isotopic characteristics as well as zircon U-Pb ages and Hf isotopic data of magmatic rocks from the Khan-Uul area in the southeastern part of the BOZ. The rock assemblage of the Khan-Uul area is composed of volcanic rocks intercalated with carbonates and ultrabasic to felsic magmatic rocks commonly having both cumulate and mingling textures. The studied rocks were affected by the greenschist to lower amphibolite facies metamorphism. Nearly all samples including the serpentinite (Mg# = 81 – 90 mol.%), the gabbro (Mg# = 71 – 81 mol.%), and the TTG-type intermediate to felsic rocks (Mg# = 15 – 47 mol.%) reveal primitive geochemical characteristics and notable depletion in K2O (K2O/Na2O = 0.01 – 1 wt.%). Based on the geochemical characteristics, they indicate a transitional composition from mainly tholeiitic to calc-alkaline. The REE and trace-element patterns show obvious enrichment in large-ion lithophile elements (including Cs, Ba, K, Sr, and Pb) relative to highly depleted high-field strength elements such as Nb, Ta, Zr, and Ti. In general, such a geochemical characteristic indicates a magmatic arc source and oceanic subduction environment. A whole-rock Sr-Nd isotopic data of magmatic rocks from the Khan-Uul area reveal a broad range from negative to positive initial epsilon Nd values (ԑNd590 = -3.9 to +2.2) with relatively young Nd model ages (TDMNd.2stg= 1559 - 1079 Ma) pointing to limited crustal contamination. U-Pb ages of 10 dated samples revealed that the magmatic rocks were mainly emplaced during the Ediacaran (ca. 600 – 570 Ma). In-zircon Hf isotopic analyses exhibit significantly positive epsilon Hf values for zircons of Ediacaran age (εHf (t) = +3.9 to +13.8) with variable two-stage Hf model ages ranging from 1499 to 658 Ma. In contrast, samples of the trondhjemite show mostly negative (εHf (t) = -5.5 to -1.5) and a few positive (εHf (t) = +1.3 to +6.1) epsilon Hf values with relatively older Hf model ages ranging from 2389 to 1081 Ma. Our geochemistry data indicate that the studied magmatic rocks from the Khan-Uul area originated in the relatively primitive Ediacaran magmatic arc. The whole-rock Sr-Nd and zircon Hf isotopic data further suggest the dominant contribution of the juvenile material via partial melting of the depleted mantle with only minor crustal components. This study shows that a large part of the southeastern BOZ does not belong to the ophiolite suite as it was widely accepted. Contrary to broadly assumed knowledge, the current data point to an active margin evolution of the northeastern edge of the Baidrag Block during Ediacaran.

Abstract

The Mongolia Block (MOB), which is now sandwiched by the Siberia Craton (SIB) and the North China Craton (NCC), plays an essential role for understanding the late stage evolution of the Paleo-Asian Ocean and the early stage evolution of the Mongol-Okhotsk Ocean. Here, a paleomagnetic study is performed for the first time on the Early Permian volcanic strata in the Bayandun region of northeastern Mongolia and the data are used to uncover the late Paleozoic paleoposition of the MOB and better understand the evolution of both oceans. Zircon U-Pb dating results reveal an emplacement age of 283 ± 3 Ma for the studied volcanic strata. Rock magnetic analyses identify that titanium-poor magnetite is the main magnetic carrier. Characteristic remanent magnetization isolated from seven sites shows consistent reverse polarity, corresponding to the Permo-Carboniferous (Kiaman) Reverse Superchron. Site-mean directions pass fold tests, and an Early Permian paleomagnetic pole is calculated for the MOB at λ/φ = −14.9°N/76.8°E (A95 = 5.7°) with N = 7 sites. Comparison with published Permian paleomagnetic poles from surrounding blocks indicates that (1) the MOB should have welded with the NCC before the Early Permian or was at least very close to it. (2) The welded MOB-NCC was separated from the SIB by the Mongol-Okhotsk Ocean with ~30° latitudinal difference during the Early Permian. (3) Significant vertical-axis strike-slip related rotations occurred within and along the margins of the unified MOB-NCC due to the far-field stress effect produced by posterior orogenic events.

Reaction of carbon dioxide (CO2) with minerals to generate stable carbonates, also known as CO2 mineralization, has been regarded as one of the most promising methods for safe and permanent carbon storage. As a promising feedstock, basaltic rock has gained special interest, and elevating basalt carbonation efficiency with the reduction of negative environmental impact is the main challenge for CO2 mineralization system development. Considering multiple potential positive effects of the CO2 carrier, NaHCO3, we conducted this study to experimentally evaluate the CO2 storage efficiency during water-basalt-NaHCO3 interactions under hydrothermal conditions at 200–300°C. The inclusion of NaHCO3 was confirmed to drastically promote the alteration of basalt, especially at higher temperatures. As revealed by experiments conducted at the saturated vapor pressure of water, the carbon storage efficiency at 300°C reached 75 g/kg of basalt in 5 days, which was 12 times higher than that at 200°C. In such hydrothermal systems, basalt was carbonated to generate calcite (CaCO3), where the Ca was mainly from plagioclase; Mg and Fe were incorporated into smectite, and Na in the saline system participated in the formation of Na silicates (i.e., analcime in the case of basalt). Due to the presence of additional Na in solution, all the released elements were consumed quickly with generation of secondary minerals in turn promoted basalt dissolution to release more Ca for CO2 storage. This study illuminated the role of NaHCO3 in basalt carbonation and provided technical backup to the design of advanced CO2 mineralization systems.

We report here our investigations into the petrology and geochemistry of the Khungui eclogites and metamorphosed continental crust in the Central Asian Orogenic Belt (CAOB), Western Mongolia. The eclogites consist mainly of garnet, omphacite, amphibole, Ti-bearing minerals, phengite, quartz, epidote, and plagioclase. Two types of garnet occur: Grt1 in aggregates > 500 µm and discrete Grt2 crystals > 100 µm in size. The omphacite occurs in the matrix and as inclusions in the rims of the garnets and has a jadeite content (Xjd) ranging from 0.32 to 0.44. The phengite also occurs both in the matrix and as inclusions in the garnet rims, and its Si content is homogeneous. Amphibole in the matrix shows compositional zoning from barroisite to taramite. The mineral assemblage of the eclogite-metamorphism is found in the garnet rims and includes omphacite, rutile, phengite, epidote, and barroisite. Geothermobarometry indicates eclogite P–T conditions of 2.1–2.2 GPa and 580–610 °C. In contrast, the matrix mineral assemblage of taramite, epidote, and symplectite (hornblende + plagioclase) was formed during decompression at T = 575–635 °C and P = 0.1–0.5 GPa. Interestingly, the Khungui eclogite shows a geochemical signature of continental arc basalt (Nb/La vs La/Yb ratios), which contrasts with the MORB compositions of most other eclogite terranes in the CAOB. The geochemical and petrological features of the Khungui eclogite indicate that the collision of the continental and island arcs causes Precambrian continental crust thickening in the CAOB.

Serpentinization of ultramafic rocks in ophiolites is key to understanding the global cycle of elements and changes in the physical properties of lithospheric mantle. Mongolia, a central part of the Central Asian Orogenic Belt (CAOB), contains numerous ophiolite complexes, but the metamorphism of ultramafic rocks in these ophiolites has been little studied. Here we present the results of our study of the serpentinization of an ultramafic body in the Manlay Ophiolite, southern Mongolia. The ultramafic rocks were completely serpentinized, and no relics of olivine or orthopyroxene were found. The composition of Cr-spinels [Mg# = Mg/(Mg + Fe2+) = 0.54 and Cr# = Cr/(Cr + Al) = 0.56] and the bulk rock chemistry (Mg/Si = 1.21–1.24 and Al/Si < 0.018) of the serpentinites indicate their origin from a fore-arc setting. Lizardite occurs in the cores and rims of mesh texture (Mg# = 0.97) and chrysotile is found in various occurrences, including in bastite (Mg# = 0.95), mesh cores (Mg# = 0.92), mesh rims (Mg# = 0.96), and later-stage large veins (Mg# = 0.94). The presence of lizardite and chrysotile and the absence of antigorite suggests low-temperature serpentinization (<300 °C). The lack of brucite in the serpentinites implies infiltration of the ultramafic rocks of the Manlay Ophiolite by Si-rich fluids. Based on microtextures and mineral chemistry, the serpentinization of the ultramafic rocks in the Manlay Ophiolite took place in three stages: (1) replacement of olivine by lizardite, (2) chrysotile formation (bastite) after orthopyroxene and as a replacement of relics of olivine, and (3) the development of veins of chrysotile that cut across all previous textures. The complex texture of the serpentinites in the Manlay Ophiolite indicates multiple stages of fluid infiltration into the ultramafic parts of these ophiolites in southern Mongolia and the CAOB.

Magnetite veins are commonly observed in serpentinized peridotite, but the mobility of iron during serpentinization is poorly understood. The completely serpentinized ultramafic rocks (originally dunite) in the Taishir Massif in the Khantaishir ophiolite, western Mongolia, contain abundant antigorite + magnetite (Atg + Mag) veins, which show an unusual distribution of Mag. The serpentinite records multi–stage serpentinization in the order: (1) Atg + lizardite (Lz) with a hourglass texture (Atg–Lz); (2) thin vein networks and thick veins of Atg; (3) chrysotile (Ctl) that cuts all earlier textures. Mg# values of the Atg–Lz (0.94–0.96) are lower than those of the Atg (~ 0.99) and chrysotile (~ 0.98). In the Atg–Lz regions, magnetite occurs as arrays of fine grains (<50 µm) around the hourglass texture, and magnetite is absent in the thin Atg vein networks replacing Atg–Lz. Magnetite occurs as coarse grains (100–250 µm) in the center of some thick Atg veins. As the volume ratio of thin Atg veins to Atg–Lz increases, both the modal abundance of Mag and the bulk iron content decrease. These features indicate that hydrogen generation occurred mainly during Atg–Lz formation, and that the Mag distribution was largely modified by dissolution and precipitation in response to the infiltration of the higher temperature fluids associated with the Atg veins. The transport of iron during redistribution of Mag in the late–stage of serpentinization is potentially important for ore deposit formation and modifying the magnetic properties of ultramafic bodies.

Unmanned aerial vehicles (UAVs) or drones have revolutionized scientific research in multiple fields. Drones provide us multiple advantages over conventional geological mapping or high-altitude remote sensing methods, in which they allow us to acquire data more rapidly of inaccessible or risky outcrops, and can connect the spatial scale gap in mapping between manual field techniques and airborne, high-altitude remote sensing methods. Despite the decreased cost and technological developments of platforms, sensors and software, the use of drones for geological mapping in Mongolia has not yet been utilized. In this study, we present using of drone in two areas: the Chandman area in which eclogite is exposed and the Naran massif of the Khantaishir ophiolite in the Altai area. Drone yields images with high resolution that is reliable to use and reveals that it is possible to make better formulation of geological mapping. Our suggestion is that (1) Mongolian geoscientists are encouraged to add drones to their geologic toolboxes and (2) drone could open new advance of geological mapping in Mongolia in which geological map will be created in more effective and more detailed way combined with conventional geological survey on ground.

Fluids in subduction zones contain various aqueous species and cause metasomatic reactions as well as hydration (i.e., serpentinization) at the slab–mantle interface. However, the nature of elemental transfer during fluid infiltration into the dry mantle in the subduction zone is poorly understood. In this study, we describe novel textures related to orthopyroxene (Opx) pseudomorphs and antigorite veins in the Chandman meta-peridotite body in the Alag Khadny accretionary wedge, western Mongolia. This body consists mainly of meta-harzburgite and meta-dunite, and hydration proceeded pervasively within olivine domains associated with the development of antigorite vein networks. Pseudomorphs after orthopyroxene consist of secondary clinopyroxene (S-Cpx) + tremolite (Tr) ± [secondary olivine (S-Ol) + antigorite (Atg)] whereas the primary clinopyroxene (P-Cpx) is almost unaltered. The pseudomorphs after Opx are rarely cut by Atg veins. The mineral occurrences indicate two main stages of mantle alteration: (1) Opx replacement (Ca-metasomatism) and (2) Atg formation induced by Si-bearing fluid infiltration (i.e., the main hydration event), followed by low-temperature (T) alteration that formed lizardite, chrysotile, and brucite. The mineralogical sequence (S-Cpx + Tr ± S-Ol ± Atg) related to Opx replacement can be explained by replacement at 550–650 °C (assuming a pressure of 1.0–2.0 GPa), which is broadly consistent with temperatures experienced by eclogites in the Chandman area during exhumation. Atg in P-Ol domains could have formed at lower temperatures (<500 °C; assuming a pressure of 1.0–2.0 GPa). Mass balance calculations show that Opx replacement was characterized by gains of Ca and a small amount of H2O, whereas Ol hydration was characterized by gains of Si and H2O. A modeled slab-derived fluid, which was calculated to be in equilibrium with basaltic crustal rocks, indicates the time-integrated fluid flux required for Ca metasomatism of the Chandman meta-peridotite body, with an assumption of the alteration thickness of L = 1000 m, was 0.18–1.5 × 106 m3 fluid m−2 rock, whereas the time-integrated fluid flux required for Si metasomatism was 0.3–1.7 × 103 m3 fluid m−2 rock, which is 102–103 lower than that of the Ca metasomatism. The characteristics of the Chandman meta-peridotite body suggest that: (1) the alteration type and trapped elements in the mantle derived from slab-derived fluids vary with temperature and local Si activity; and (2) intense fracture networks facilitated extensive serpentinization related to the Si metasomatism.

Abstract

Geochemical Data Toolkit (GCDkit) is a program for handling and recalculation of geochemical data from igneous and metamorphic rocks. It is built using the Windows version of R, which provides a flexible and comprehensive language and environment for data analysis and graphics. GCDkit was designed to eliminate routine and tedious operations involving large collections of whole-rock data and, at the same time, provide access to the wealth of statistical functions built into R. Data management tools include import and export of data files in a number of formats, data editing, searching, grouping and generation of subsets. Included are a variety of calculation and normative schemes, for instance CIPW and Mesonorm, as are the common geochemical graphs (e.g. binary and ternary graphs, Harker plots, spider plots, and several dozens of classification and geotectonic discrimination diagrams). The graphical output is publication ready but can be further retouched if required. The system can be further expanded by means of plug-in modules that provide specialist applications. GCDkit is available as Free Software under the terms of the Free Software Foundation's GNU General Public License and can be downloaded from http://www.gla.ac.uk/gcdkit. The product is actively maintained and updated to provide additional functionality; Unix/Linux and Mac OS versions are being developed.