Solonker Suture Research Articles

Paleozoic tectonothermal history of the amalgamation of the Tarim–North China and Mongolian collages

In the Mongolian Collage, metamorphic pressure–temperature ( P – T ) and timing reveal a one-stage evolution defined by a duality of late Neoproterozoic–Ordovician subduction-related low T / P metamorphism and suprasubduction high T / P metamorphism recorded in the Mongolia–Manchuria and Baikal–Sayan belts. This was followed by gradual prevalence of suprasubduction high T / P metamorphism towards the late Paleozoic corresponding to the Altai and South Altai cycles. In the Tarim–North China Collage, metamorphic P – T and timing reveal a two-stage evolution, from dominant intermediate T / P metamorphism possibly resulting from Ordovician–Devonian amalgamation and Andean-type evolution of the collage, to dual low and high T / P metamorphism in the Carboniferous–Permian reflecting subduction–collision processes along the South Tianshan suture in the west and a suprasubduction evolution along the Solonker suture in the east. Altogether, the Paleozoic tectonometamorphic evolution of the two collages in the Central Asian Orogenic Belt shows remarkable differences, with the Mongolian Collage displaying features typical of peripheral accretionary style reflecting recurrent tectonic switches that can be regarded as a single orogenic system, and a two-stage evolution of the Tarim–North China Collage with features of both peripheral–accretionary and interior–collisional orogenic cycles, but mostly related to recurrent subductions of interior oceans.

Where and when the Paleo-Asian Ocean was closed: new evidence from the Tuquan ophiolite

The Solonker suture marks the location of the final collision in southeastern Central Asian Orogenic Belt (CAOB). However, it is observed that the suture is obscured at the western margin of the Songliao Basin. Fortunately, the recently discovered Tuquan ophiolite is one of the important fragments within the Solonker suture. Testings of samples from the Tuquan ophiolite yielded LA-ICP-MS zircon U–Pb chronological data with weighted mean of 206 Pb/ 238 U ages from 265 ± 5 Ma to 279 ± 6 Ma which were interpreted to represent the formation age of the Tuquan ophiolite from the late Cisuralian to Guadalupian. The geochemical characteristics of the Tuquan ophiolite is related to supra-subduction zone features. We suggest that it represents an important bidirectional subduction event involving the oceanic crust of the Paleo-Asian Ocean beneath the Mongolia Block and the North China Craton during the early to late Permian. Integrating these findings with previous research indicates that the final subduction of the Paleo-Asian Ocean persisted until at least the late Permian. Our study provides further insights, indicating that the discovery of the Tuquan ophiolite could be regarded as the eastern extension of the Solonker suture. This finding will provide new constraints on the evolution of the southeastern CAOB.

An Early Permian transpressional continental arc and arc-arc collision recorded in the Baidunzi Complex, southern Central Asian orogenic belt, NW China

Our multidisciplinary study of the southern Beishan orogen in NW China, situated between the Tienshan and Solonker suture zones, sheds light on the closure of the Paleo-Asian Ocean and termination of orogeneses in the Central Asian Orogenic Belt. We identify the Baidunzi Complex, an exotic Permian transpressional continental arc terrane, characterized by sheared and isoclinally folded metasedimentary and syn-kinematic plutonic rocks transformed into upper greenschist to amphibolite tectonites. The complex comprises basement rocks of thick-bedded orthoquartzite and orthogneiss, the Carboniferous to Permian meta-sedimentary strata including volcaniclastic sedimentary rocks, marble, and marl interlayered psammite, and the Baidunzi Intrusive Suite featuring 294–289 Ma hornblende diorite, tonalite, granodiorite, and granite. The Late intrusive rocks, including leuco-granite to trondhjemite sills and a stitching gabbro unit, suggest an amalgamation age of 281–280 Ma between the Baidunzi and Ganquan arcs. The spatial and temporal linkage between the Baidunzi continental arc and the Liuyuan backarc-Ganquan arc suggests they were either built on a coeval doubly vergent subduction system or from the same north-vergent subduction system and structurally juxtaposed by sinistral convergence. Geochronological and Hf isotope evidence, along with continental rock affinity, indicates a close association between the Baidunzi Complex and the northern margin of the Tarim Craton during the Gondwana breakup and Pangea assembly.

Late Paleozoic slab rollback events in the southeastern part of the Central Asian orogenic belt: Implications for Paleo-Asian Ocean subduction and continental crust growth

The Central Asian orogenic belt is considered to be the largest Phanerozoic accretive orogenic belt on Earth. The late Paleozoic magmatic rocks in central Inner Mongolia are crucial for understanding continental crust growth and the tectonic evolution of the southeastern part of the Central Asian orogenic belt. We present comprehensive geochemical, isotopic, and geochronological data from three late Paleozoic magmatic units in the Mandula area, west of the Solonker suture zone. Zircon U-Pb dating indicates that these rocks formed during the late Carboniferous (316−304 Ma). The Mandula high-Mg diorites exhibit high MgO (3.9−6.5 wt%), high Mg# (61−69), and depleted Nd-Hf isotopic compositions, generated through interaction between a metasomatized mantle and slab melts with the overlying sediments. The Mandula granodiorites display adakite geochemical characteristics with high Sr/Y mass ratios (29−52), high MgO (1.7−2.2 wt%), and high Mg# (52−54), formed by partial melting of the oceanic slab with the addition of overlying sediment. Mafic microgranular enclaves have consistent ages, Sr-Nd-Hf isotope compositions, and hornblende crystallization temperature-pressure conditions with their host granodiorite, formed from a cognate magma associated with the host granodiorites through cumulate. We propose that two phases of slab rollback took place during the late Paleozoic southward subduction-accretion of the Paleo-Asian Ocean. The first phase corresponded to the transformation of low- to medium-angle slab subduction, while the second phase led to subduction-related extension. Considering the tectonic-magmatic evolution, crustal maturity, and thickness variations in the late Paleozoic southeastern part of the Central Asian orogenic belt, we propose that prolonged subduction and slab rollback promoted continental crust growth. The Central Asian orogenic belt coincides temporally and spatially with the Phanerozoic Pangea cycle, suggesting that continuous subduction and supercontinent amalgamation significantly contributed to continental crust growth.

Sedimentary provenance supports a mid-paleozoic tectonic connection between the Junggar and Altai terranes in central Asia

The provenance of Precambrian detritus in the Junggar and Altai terranes provides crucial constraints on the peri-Siberian accretionary tectonic evolution in the middle Paleozoic. The Precambrian detrital zircons have no coeval magmatic equivalents in the Junggar terrane but show U–Pb age spectra and εHf(t) values comparable to those in the Altai terrane. The correlations suggest that the old detrital materials in the Junggar and Altai terranes were most likely derived from the Siberia craton and adjacent Tuva-Mongolian microcontinent. Paleozoic zircons in the Junggar terrane display a εHf(t) pattern from large spread to dominantly positive values at ca. 420–410 Ma. Such an abrupt change points to an accretionary tectonic transition from an advancing to retreating mode during mid-Paleozoic time, synchronous with similar tectonic switch occurring in the Altai terrane. Taking into account the temporal and spatial relations in sedimentation, tectonism and arc magmatism, we propose that the Junggar terrane had once collided onto the peri-Siberian Altai terrane to receive abundant old detritus from the Siberian continent in the Silurian–early Devonian. They were subsequently separated at ca. 420–410 Ma, possibly due to the slab rollback of the subducting Paleo-Asian Ocean (PAO) plate. These results constrain an Early Paleozoic tectono-paleogeographic boundary of the CAOB along the North Tianshan–Solonker suture zone, and also imply a long-lived PAO subduction was responsible for the Neoproterozoic to Paleozoic accretionary orogenesis at the margins of southern Siberia, eastern Kazakhstan, and northern Gondwana.

Эволюция кайнозойского вулканизма и его источники в зоне Дариганга-Чифэн (Дачи): вовлечение в тектоническую активизацию киля Северо-Китайского кратона и глубокой мантии под сопредельными геологическими структурами

In this paper, volcanism of the fields of the Dachi zone (Dariganga, Abaga, Dalinuoer, and Chifeng) that extends for more than 500 km in the border regions of southeastern Mongolia and northern China is considered. It is inferred that the zone has undergone a common evolution in the last 24 Ma, starting from the boundary of the North China Craton. Volcanism propagated northward, through the west-east Solonker suture zone into the Xing'an block. It is suggested that the Solonker suture zone was involved in left-lateral motions after 15–14 Ma eruptions in the Dariganga, Abaga, and Dalinuoer fields and after the final (ca. 5 Ma) eruptions in the northern margin of the Chifeng field. The nature of source changes during the evolution of volcanism is assessed from Pb isotopes of volcanic rocks with additional information on the Sm/Nd ratio based on Nd isotope variations. In sources of volcanic rocks from the southern part of the Dachi zone (Chifeng field), a component of the cratonic mantle keel is determined. The development of the process of cratonization of the mantle and crust is assumed to have occurred about 2.23 billion years ago due to degassing of the mantle originated from the Hadean magmatic ocean, which solidified 4.47 billion years ago. Volcanic rocks of the Abaga and Dalinuoer fields are characterized by the youngest protolith, corresponding to a geochron of 4.44 billion years.

Timing the Hegenshan Suture in the Central Asian Orogenic Belt: New Paleomagnetic and Geochronological Constraints From Southeastern Mongolia

AbstractThe timing of formation of the Hegenshan suture is crucial to understanding the evolution of the Central Asian Orogenic Belt (CAOB). Here we present a well‐dated paleomagnetic pole (103 samples from 15 sites, 36.7°N/29.2°E, A95 = 3.3°) that passed a positive fold test, a reversal test, and a conglomerate test from the ∼250 Ma upper Gunbayn Formation andesites in the southeastern Mongolia Block (MOB). The high‐quality paleomagnetic database may demonstrate that the MOB drifted rapidly southward and collided with the Xilinhot–Songliao Block between 256 and 250 Ma, resulting in the closure of the Hegenshan Ocean at ∼250 Ma. The Hegenshan suture and the Solonker suture were formed almost simultaneously and marked the final closure of the Paleo‐Asian Ocean.

Geochemistry and zircon U-Pb-Hf isotopes of Paleozoic granitoids along the Solonker suture zone in Inner Mongolia, China: Constraints on bidirectional subduction and closure timing of the Paleo-Asian Ocean

The closure of the Paleo-Asian Ocean resulted in formation of the Central Asian Orogenic Belt (CAOB) accompanied by considerable Phanerozoic juvenile crustal growth. As an international frontier topic, whether the Solonker suture zone is the site of final closure of the Paleo-Asian Ocean and records the terminal evolution of the CAOB in Inner Mongolia is still in controversial. This study presents the geochemistry and zircon U-Pb-Hf isotopes of granitoids along the Solonker suture zone to constrain the subduction and closure processes of the Paleo-Asian Ocean. Granitoids from the Xilinhot area in the northern accretionary zone comprise diorites and granodiorites with LA-ICPMS zircon U-Pb ages of 316 ± 2 Ma, 302 ± 2 Ma, 285 ± 2 Ma and 243 ± 4 Ma, positive εHf(t) values of +2.39 to +13.31 and TDM2 model ages of 481–1191 Ma. The ca. 316–285 Ma granitoids were formed by fractional crystallization of regional coeval calc-alkaline basaltic magma in an arc-related setting and the ca. 243 Ma granitoid was generated mainly from partial melting of dominant basaltic source rocks and minor metagraywackes in the northward subduction-related setting. The granitoids from the Linxi area in the southern accretionary zone include muscovite monzogranites and monzogranites with LA-ICPMS zircon U-Pb ages of 525 ± 6 Ma, 274 ± 2 Ma and 261 ± 4 Ma. The ca. 525 Ma muscovite monzogranites have negative zircon εHf(t) values of −19.36 to −13.61 with TDM2 model ages of 2348–2711 Ma. They were generated from partial melting of Neoarchean to Paleoproterozoic supracrustal materials. The ca. 274–261 Ma monzogranites show negative zircon εHf(t) values ranging from −10.48 to −0.66 with TDM2 model ages of 1335–1942 Ma. They were derived from remelting of Paleoproterozoic to Mesoproterozoic supracrustal materials in the southward subduction zone along the northern active margin of the North China Craton. Combining our results with existing data, we infer that the early Paleozoic southward subduction of the Paleo-Asian Ocean started before ca. 525 Ma, and the northward subduction zone lasted to ca. 243 Ma, which implies the bidirectional subduction of the ocean lasted at least until ca. 243 Ma. Our work suggests that along the Solonker suture zone, the regional amalgamation may have occurred after ca. 243 Ma, which closed the Paleo-Asian Ocean.

Crustal‐Scale Seismic Reflection Profiling Constrains How the Paleo‐Asian Ocean Was Closed

AbstractThe Central Asian Orogenic Belt (CAOB) is the most significant accretionary orogenic belt since the Phanerozoic and the most ideal site for studying continental growth evolution processes. A 460‐km‐long high‐resolution crustal‐scale seismic reflection study was conducted across the eastern CAOB in North‐Central China to constrain the closure mode and location of the Paleo‐Asian Ocean, that is, the previous ocean of the CAOB. The resultant seismic reflection profile revealed opposite‐dipping reflectors in the northern and southern parts of the profile, which converge at the profile center to form an inverted U‐shaped reflector pattern near the crust–mantle transition zone beneath the Solonker Suture. The dipping reflectors represent bidirectional fossil subduction zones sloping to the north and south, and the convergence reflector pattern represents the ocean closure location. Integration of these results with available geological data facilitated model construction whereby Paleo‐Asian Ocean closure was accomplished by divergent subduction of the Paleo‐Asian oceanic plate, with northward subduction beneath the southern margin of the Mongolian Block and southward subduction beneath the northern margin of the North China Craton. The oceanic lithosphere contracted and deformed, yielding the observed inverted U‐shaped reflector pattern, representing Paleo‐Asian Ocean closure. This subsurface location lies beneath the Solonker Suture surface exposure, suggesting that this suture marks the ocean closure location, rather than the previously proposed Hegenshan–Heihe Suture to the north or Xar Moron Suture to the south. Our study suggests that divergently dipping subduction and associated accretion and magmatism may constitute the primary continental growth mode for accretionary‐type orogens.

Eastward extension of the Solonker Suture beneath the central Songliao Basin, NE China: Evidence from a deep seismic reflection profile

The Solonker Suture is one of the most important structures in central Asia and marks the location of closure of the Paleo-Asian Ocean and the assemblage of the North China and Siberia cratons during the Late Permian - Early Triassic. The suture is traceable for ∼ 700 km within the Central Asian Orogenic Belt, but its eastward extension beneath the Songliao Basin has not been defined. In this paper, we present a 127.3-km-long N–S deep seismic reflection profile across the central Songliao Basin. The data acquired from five large-size and four medium-size dynamite shots were used to generate a single-fold reflection profile imaging the Moho and the structures of the crust and upper mantle. The southern end of the profile reveals a northward-dipping reflector that extends 80 km into the mantle. This dipping reflector is interpreted as a fossil subduction zone. The new data from this study suggest that the Songliao Terrane may not be a single micro-continent block or orogenic belt but a composite terrane that comprises the North Accretionary Zone, South Accretionary Zone, and northern margin of the North China Craton from north to south. The fossil subduction zone is regarded as part of the NE-trending extension of the Solonker Suture beneath the central Songliao Basin.

Electrical resistivity structure across the Late Cenozoic Abaga-Dalinor volcanic field, eastern China, from 3-D magnetotelluric imaging and its tectonic implications

The Abaga-Dalinor volcanic field (ADVF) in Inner Mongolia, China, located to the west of the North-South Gravity Lineament (NSGL), is the largest and less known area of Late Cenozoic intraplate volcanism in Northeast China. Knowledge of the subsurface structure beneath the ADVF will improve our understanding of its evolution process and formation mechanisms. New broadband magnetotelluric (MT) data were collected along a profile of about 300 km that crosses the major basaltic areas exposed at the surface in the ADVF and a series of east-north-east (ENE) trending faults. The electrical resistivity structure of the crust and uppermost mantle across the ADVF was generated by 3-D inversion of full impedance tensor and tipper data. This model reveals a thick high-resistivity layer (>1000 Ωm) in the upper crust that may represent the Precambrian basement and Late Paleozoic-Mesozoic granitic plutons. In contrast, high conductivity anomalies dominate the middle-lower crust. The northward-dipping high-conductivity feature at depths of 20–40 km below sea level under the Chagan Obo fault is suggested to be the involvement of the Early Paleozoic subducted oceanic crust. A remarkably high-conductivity zone (1–20 Ωm) exists in the middle-lower crust beneath the Abaga and Dalinor volcanic areas. It is explained to be a saline fluid zone of ∼0.45%–0.48% that exsolved from the partial melts as magma ascent rapidly. The impermeable upper crust traps these fluids in the middle-lower crust. A moderately high-conductivity anomaly in the uppermost mantle below near the Solonker Suture Zone (SSZ) is attributed to a minimum partial melt of ∼3–4% associated with the localized asthenospheric upwelling, which feeds the Abaga and Dalinor volcanos as a common magma source. Integrating our resistivity model with previous geophysical and geochemical studies, we suggest that intraplate volcanism in the ADVF is mainly controlled by the pre-existing tectonic weak zones. The lithosphere within the SSZ is susceptible to thermal perturbation from the upwelling mantle, providing the preconditions for melt generation. A set of ENE trending faults and an inferred NW-SE trending fault jointly dominated the vertical transport of magma and lateral extension along the faults, which may have influenced the spatial distribution of basalts and volcanic cones at the surface in the ADVF. In addition, we tend to suggest that Late Cenozoic intraplate volcanism in the ADVF could be the result of decompressing melting within the localized small-scale asthenospheric upwelling induced by the edge-driven convection near the NSGL, which is indirectly related to the westward subduction and retreat of the (Paleo-)Pacific plate during the Late Mesozoic to Cenozoic.

Современные сейсмотектонические деформации на вулканическом плато Асхатэ и их роль в структуре вулканического поля Дариганга, Юго-Восточная Монголия

From instrumental measurements in southeastern Mongolia and adjacent China, weak seismicity is determined. However, 7.5-kilometer long system of ditches, probably of seismic nature, was detected in the western part of the Dariganga volcanic field, on the edge of the Ashate Plateau that resulted from erosional dissection of the 4.8–4.3 Ma shield edifice. We propose that the modern seismotectonic reactivation of the Askhate volcano is related to tectonic motions in the Late Cenozoic Tamtsag foredeep of the Nukut-Daban range, spatially associated with neotectonic reactivation of the Solonker suture.

Palaeomagnetic constraints on a Late Permian to Early Triassic final closure of thePalaeo‐AsianOcean

AbstractFinal closure of the Palaeo‐Asian Ocean signifies the ultimate amalgamation of the North China, Tarim, Mongolia, Songliao, Xilinhot and other Central Asian blocks, and therefore, is critical in reconstructing the configuration of the Pangea supercontinent during the late Palaeozoic to early Mesozoic. However, the final closure time remains contentious. Here, we present a systematic palaeomagnetic study on the Upper Permian sedimentary sequences of the Xilinhot Block, located to the north of the Solonker Suture Zone. Rock magnetic results indicate stable pseudo‐domain magnetite is the dominant magnetic carrier. The characteristic components isolated between ~350 and 590°C display the existence of dual polarities. Combined with previous results, the merged site‐mean directions yield a negative reversal test and inconclusive fold tests. By comparing with coeval and posterior data, and correlating with stitching plutons that intruded the sampling sections, we argue for a remagnetization origin for the newly obtained data, which is most likely acquired during the thermal event when the stitching granitic plutons intruded at 242.0 ± 3.7 Ma. Then we synthesized available palaeomagnetic data from the Central Asian Orogenic Belt and adjacent blocks, and propose a Late Permian to Early Triassic final closure of the Palaeo‐Asian Ocean. Our reconstruction of Pangea in the Early Triassic negates the existence of a cut‐through seaway between the Mongol‐Okhotsk and Palaeo‐Asian Oceans. The Mongol‐Okhotsk Ocean was an extension of Panthalassa, in trumpet‐shaped and faced to the east, rather than an individual oceanic basin separating Siberia from the Cinamuria Block.

Crust and upper mantle electrical structure of the eastern Central Asian Orogenic Belt revealed by the MT line from Zhangwu County to East Ujimqin Banner

The Central Asian Orogenic Belt is bounded on the north by the Siberian Craton and on the south by the North China Craton and the Tarim Craton. It is one of the largest Phanerozoic accretionary orogenic belts on Earth. Since the early Paleozoic, its eastern part has experienced the compound orogenesis and mineralization of three major tectonic systems: the closure of the Paleo-Asian Ocean, the closure of the Mongolian–Okhotsk Ocean, and the subduction of the Paleo-Pacific Plate. From Zhangwu County in the south to East-Ujimqin Banner in the north, a 500 km magnetotelluric profile adjacent to Northeast China has been studied. With 100 sites of magnetotelluric data processing and analysis, we apply a two dimensional inversion in TE and TM modes and obtain a resistivity model up to a 100 km depth. We have discovered two high-resistivity anomalies with opposite dip directions in the upper mantle on both sides of the Solonker Suture Zone, which provide an evidence of the bi-directional subduction pattern of the oceanic crust and the position of the closure of the Paleo-Asian Ocean. In addition, the whole study area presents an approximate basin-range coupling relationship. In the northern part of the study area, the low-resistivity anomalies below it have an apparent north-dipping characteristic, which may be related to the asthenosphere upwelling from west to east. In addition, they may be related to the upwelling of mantle materials, and provide sources of ore-forming material for the Baiyinnuoer mining area through post-collision extension. In the central part of the study area, there are several large-scale high-resistivity anomalies below the Baolidao Belt. The different dip directions reveal the experiences of several subductions and collisions. In the southern part of the study area, the Bainaimiao Belt is located between the southern margin of the Songliao Basin and the northern margin of North China Craton. The main resistivity anomalies below are all south-dipping.Graphical

Lithospheric dripping in a soft collision zone: Insights from late Paleozoic magmatism suites of the eastern Central Asian Orogenic Belt

The closure of Paleo-Asian Ocean is considered to have occurred along the Solonker Suture in the southernmost segment of the Central Asian Orogenic Belt (CAOB), the largest Phanerozoic accretionary orogen on the globe. The suture branches to the east to form the northern Hegenshan–Heihe Suture and the southern Solonker–Changchun Suture. The Hegenshan–Heihe Suture is an ideal natural laboratory for studying the post-collisional geodynamic processes operating in a soft collision zone driven by divergent double-sided subduction. Here we report results from an integrated study of the petrology, geochronology, geochemistry, and Sr–Nd–Hf isotopic compositions of the Early Carboniferous–Early Permian magmatic suite in the Hailar Basin of the Xing’an–Erguna Block. The Early Carboniferous igneous rocks are represented by 356–349 Ma andesitic tuffs, exhibiting typical subduction-related features, such as enrichment in large-ion lithophile elements and depletion in high-field-strength elements. These features, together with the relatively depleted Sr–Nd–Hf isotopic compositions, constant Nb/Y values, but highly variable Rb/Y and Ba values indicate that these rocks were generated by partial melting of a depleted mantle wedge metasomatized by slab-derived fluids. The Late Carboniferous–Early Permian magmatic suite (317–295 Ma) is characterized by high Sr contents (313–1080 ppm) and low Y contents (5–13 ppm), and these can be subdivided into calc-alkaline adakitic rocks and high-K calc-alkaline adakitic rocks. The calc-alkaline adakitic rocks have higher values of Sr/Y, (Sm/Yb)source normalized, and Mg#, and lower values of Y, Ybsource normalized, and K2O/Na2O than the high-K calc-alkaline adakitic rocks, which suggests that the former was generated by partial melting of foundered lower continental crust and the latter by partial melting of normal lower continental crust. Based on our new data, in conjunction with those in previous studies, we conclude that the tectonic evolution of the Hegenshan–Heihe Suture involved Early Carboniferous double-sided subduction of the Nenjiang Ocean, latest Early Carboniferous soft collision between the Xing’an–Erguna and Songliao blocks, and Late Carboniferous–Early Permian post-collisional extension. We also propose a new geodynamic scenario in which removal of the lithospheric root might have occurred in a soft collision zone during the post-collision period via repeated and localized lithospheric dripping, which results from combinedeffects of hydration weakening of the lithosphere caused by pre-collision subduction and asthenospheric stirring triggered by slab break-off.

Constraints on the process and mode of the Paleo-Asian Ocean closure from the lithospheric conductivity structure of the south-eastern Central Asian Orogenic Belt

The Paleo-Asian Ocean eventually closed along the Solonker Suture Zone during the Late Permian and Early Triassic, before the collision between the Siberian Craton and the North China Craton (NCC) formed the eastern Central Asian Orogenic Belt (CAOB). In this paper, Magnetotelluric (MT) profile data across the south-eastern CAOB and the northern margin of the NCC are explored to image the two-dimensional resistivity structure of the region. According to the resistivity model, a north-dipping conductive layer to the north of the Xilinhot fault and a south-dipping conductive layer to the south of the Linxi fault are revealed. The two conductors are interpreted as evidence of the late-stage low-angle bidirectional subduction of the Paleo-Asian Ocean, which constrains the northern and southern deep boundaries of the Solonker Suture Zone. Another couple of mantle conductors with opposite dipping angles are revealed under the Solonker Suture Zone and Bainaimiao Belt, which are interpreted as evidence of the early-stage high-angle bidirectional subduction of the Paleo-Asian Ocean. Compared with the results from previously reported MT profiles in this area, the subduction angle becomes more gentle from west to east. The difference of convergence rate between the NCC and the Siberian Craton is considered to be the major cause of the closure difference along the strike. The northern margin of the NCC has a conductive lower crust, which may result from the magma underplating or crust-mantle decoupling, as it is imaged to be connected with the south-dipping conductive layer beneath the Solonker Suture Zone.

Deep Crustal Structure of the Eastern Central Asian Orogenic Belt Revealed by Integrated Magnetic-Gravity Imaging

The Central Asian Orogenic Belt (CAOB) is a globally magnificent accretionary orogenic belt that has been formed since the Phanerozoic as a result of the Paleozoic closing of the Paleo-Asian Ocean (PAO). The transition zone between the North China Craton (NCC) and the Siberia Plate is located in the eastern CAOB and has been thoroughly investigated by various seismic investigations. However, other types of geophysical approaches lag behind, especially integrated magnetic-gravity surveying, which could provide regional continent-scale constraints on the deep crustal structure. Here, the high-resolution ground gravity and airborne magnetic data covering the study region are newly processed by upward continuation, an improved potential field normalization differential algorithm, an analytical signal approach, and correlation analysis. The processed gravity and magnetic anomalies reveal dominant differences between the CAOB and the northern margin of the NCC; these regions are tectonically divided by the upper crustal Chifeng-Baiyan Obo fault, which is expressed by an important geological boundary. In the middle and lower crust, this tectonic boundary extends northward to the Xar Moron fault. Unexposed Mesozoic granites may be distributed extensively in the mid-lower crust along the Solonker suture zone. The local negative correlation characteristics of gravity and magnetic anomalies may be related to the structural fabrics derived from the convergence of the two terrains.

When did the final closure occur of the eastern Paleo-Asian Ocean: Constraints from the latest Early–Middle Triassic adakitic granites in the southeastern Central Asian Orogenic Belt

There is a broad consensus that the Solonker suture zone marked the final closure of the eastern Paleo-Asian Ocean, which led to the formation of the eastern segment of the Central Asian Orogenic Belt (CAOB). However, when and how the final closure occurred still remained controversial. Located in a key position in the southeastern CAOB, the Early Mesozoic magmatic rocks along the northern margin of the North China Craton provide a crucial window to evaluate the subduction and final closure processes of the eastern Paleo-Asian Ocean. To address these issues, data on geochronological, whole-rock geochemical, whole-rock Sr-Nd and in situ zircon Hf isotopic study of the Early Mesozoic granites, as well as major-trace elements compositions of the apatites are reported here. Zircon U-Pb dating results showed that our studied granites were emplaced in the Middle Triassic (243.1–242.6 Ma). These granites have high Sr (250–652 ppm) contents, low heavy rare earth elements (HREEs), Yb (0.46–0.77 ppm) and Y (4.48–8.15 ppm) contents, and show high Sr/Y and (La/Yb)N signatures (36–108 and 21–35, respectively), with negligibly negative to moderately positive Eu anomalies (δEu = 0.80–1.36), indicating adakite-like affinities. Most of these granite samples are characterized by high K2O contents (3.30–4.83 wt%), and belong to the high-K calc-alkaline and shoshonitic series, with peraluminous affinities (A/CNK = 1.02–1.28). Besides, their low MgO (0.10–0.34 wt%), Cr (0.99–3.50 ppm), and Ni (0.96–2.40 ppm) contents indicate a thickened continent crustal origin, rather than subducting slab-derived or delaminated lower crust-derived melts. Analyzed granite samples yielded in situ zircon εHf (t) values (–8.3 to + 5.8) and whole-rock εNd (t) values (–4.38 to –5.69), with predominately negative values, indicating mixture sources of major recycled old components and limited juvenile crustal materials. Their parental magma was originated by partial melting of thickened lower crust, under garnet amphibolite facies conditions. Thickened crust in the southeastern CAOB yielded crustal thickness of ~54–64 km estimated from (La/Yb)N ratios of these lower crust-derived granites. These constraints, in conjunction with the E-W trending collision-related adakitic granitoids (250–243 Ma) along the southeastern CAOB, and other geological evidence, recorded a tectonic transition from this prolonged subduction to final closure of the eastern Paleo-Asian Ocean.

Multi-phase metamorphism in the northern margin of the North China Craton: Records from metapelite in the Hongqiyingzi Complex

It is controversial about the tempo–spatial distribution and tectonic attributes of the Paleoproterozoic orogenic belts in the North China Craton (NCC). The Hongqiyingzi Complex in the northern margin of the NCC, comprising orthogneiss, supracrustal sequence and an ophiolitic mélange, was investigated to have experienced multiple tectonic evolution from Paleoproterozoic to Mesozoic. However, the depositional age and metamorphic P–T path of the supracrustal sequence have not been well addressed. Garnet mica schists are investigated on the basis of petrography, mineral chemistry, phase equilibria modelling and zircon and monazite dating, which record an extreme case with four phases of metamorphism. The first-phase is characterized by high pressure (HP) granulite facies with the peak conditions of 10–11 kbar/>780 °C, which occurred at ~1.95 Ga. It is considered to correspond to a collisional orogeny with crust thickening widespread in the Paleoproterozoic orogenic belts of the NCC, followed by an exhumation with heating at ~1.92 Ga. The second-phase is marked by the overprinting of staurolite-bearing assemblages with the peak conditions of 6–7 kbar/610–630 °C, which occurred at ~1.85 Ga. It may correlate a separate orogeny occurred along the northern margin of the NCC, following a ~1.90 Ga oceanic subduction. Phase modelling suggests this overprinting over the first-phase HP granulites is mainly controlled by water infiltration. The third-phase andalusite-type and the fourth-phase greenschist facies metamorphism are confined to local equilibrium domains and their ages have not been well constrained. With a comparison of regional geology, the third-phase metamorphism is inferred to be related with the 320–290 Ma extension in the northern part of the NCC during Late Paleozoic, and the fourth-phase metamorphism is correlated with the 250–230 Ma collision occurred along the Solonker Suture Zone in the Central Asia Orogenic Belt during Early Mesozoic. Moreover, detrital magmatic zircons yield ages of 2.0–2.1 Ga, providing a definite upper limit on the deposition of the Hongqiyingzi supracrustal sequence.

Geodynamic modeling on the formation mechanism of Linxi Basin: New constraints on the closure time of the Paleo-Asian Ocean

The Linxi Basin is situated at the eastern portion of the Tianshan-Solonker suture where the Paleo-Asian Ocean closed and represents the key region to understand the tectonic evolution of the Solonker suture. The Linxi Basin has the best preserved Permian sediments and its formation mechanism is much debated. Two controversial Permian tectonic backgrounds are proposed for this basin, i.e., oceanic-continental convergence (hence an arc basin related to subduction) versus post-orogenic extension (hence a crustal/lithospheric subsidence basin). Oceanic-continental convergence indicates subduction of oceanic lithosphere beneath the continental boundary of the North China Craton, whereas post-orogenic extension implies a thickened continental lithosphere experienced a drip-like gravitational instability. To address this controversy, we conduct subduction and drip (lithospheric instability) numerical experiments to calculate model predictions of surface topography for the Linxi Basin across the Solonker suture. Comparisons between the model predictions and the Permian sedimentary records in the Linxi Basin are made to determine which model can better explain the sedimentation. The persistent accumulation of Permian sediments indicates continuous crustal subsidence, which fits better the predicted subsidence from our drip model. By contrast, our modelled subduction-induced surface evolution in the Linxi Basin generally shows a switch from subsidence to uplift. The better fitting of the drip tectonics indicates that the post-orogenic extension rather than subduction tectonic setting would be more suitable to explain the Linxi Basin development during the Permian. In addition, the spatial and temporal distribution of magmatic activities is consistent with drip process. Our preferred Permian basin formation induced by drip model favors an early closure of the Paleo-Asian Ocean before the Permian.