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The features of ophiolites in the central sector of Inner Mongolia and its geological significance
... There are several ophiolite belts in this area, such as the Xar Moron, the Hegenshan, and the Solonker. These ophiolite belts likely represent the locations of ancient subduction zones between the Siberian and the Sino-Korean palaeoplates (Wang 1986;Tang 1989Tang , 1990; Wang et al. 1991;Liang 1994;Xiao et al. 2003;Li 2006;Zhang et al. 2015). Based on the latest marine sedimentary strata of Permian and the middle Permian radiolarian fossils in the siliceous rocks of the Xar Moron ophiolite, some authors infer that the Palaeo-Asian Ocean finally closed along the Xar Moron suture in middle-late Permian (Wang and Fan 1997;Li 2006). ...
... Central Inner Mongolia is situated at the southeastern part of the CAOB (Figure 1a). There are two major ophiolite belts in this area, the Erenhot-Hegenshan ophiolite belt to the north and the Solonker-Xar Moron ophiolite belt to the south ( Figure 1b) (Wang 1986;Tang 1989Tang , 1990Wang et al. 1991;Liang 1994;Wang and Fan 1997;Miao et al. 2007Miao et al. , 2008Jian et al. 2012;Zhang et al. 2015). Additionally, a late Carboniferous to early early Permian subduction-accretion complex near the Daqing pasture was identified between these two ophiolite belts, which represents another late Palaeozoic subduction zone along the southern margin of the Siberian craton (Liu et al. 2013). ...
The petrology, geochronology, and geochemistry of the early Permian volcanic rocks from Houtoumiao area, south Xiwuqi County in central Inner Mongolia of China, are studied to elucidate the early Permian tectonic setting of the region. The volcanic rocks, which are interbedded with sandstone, feature both mafic and felsic compositions and show a bimodal nature. Zircon U–Pb dating reveals that the volcanic rocks formed at 274–278 Ma, similar to the ages of bimodal magmatism in neighbouring areas. The mafic rocks are composed of tholeiitic basalt, basaltic andesite, basaltic trachyandesite, and trachyandesite. They are rich in Th, U, and LILEs, depleted in HFSEs Nb, Ta, and Ti, and have positive εNd(t) values (+3.6 to +7.9). Geochemical analyses indicate that the mafic rocks originated from metasomatized lithospheric mantle. The felsic volcanic rocks are mainly rhyolite, with minor trachyte and dacite. They have different evolutionary tendencies of major elements, chondrite-normalized REE patterns, and isotopic compositions from the mafic volcanic rocks, which preclude formation by fractional crystallization of mafic melts. The εNd(t) values of the felsic rocks are similar to those of the Carboniferous Baolidao arc rocks in the region. It is suggested that Permian felsic melts originated from the partial melting of Carboniferous juvenile arc-related rocks. By comparison with typical Cenozoic bimodal volcanism associated with several tectonic settings, including rift, post-collisional setting, back-arc basin, and the Basin and Range, USA, the bimodal volcanic rocks in central Inner Mongolia display similar petrological and geochemical characteristics to the rocks from back-arc basin and the Basin and Range, USA. Based on the analysis of regional geological data, it is inferred that the early Permian bimodal volcanic rocks in the study area formed on an extensional continental margin of the Siberian palaeoplate after late Carboniferous subduction–accretion.
... Inner Mongolia is located at the south-eastern margin of the CAOB where several discontinuous ophiolite belts are developed. The CAOB ophiolite belts are divided from south to north into the: Ondor Sum-Kedan Mountain (Xiramulun River) ophiolite belt, Solonker-Linxi ophiolite belt, Jiaoqier-Xilinhot ophiolite belt, and Erenhot-Hegen Mountain ophiolite belt (Huang et al., 2006;Li et al., 2015;Liang, 1994;Miao et al., 2008). The Erenhot-Hegenshan ophiolite belt was once considered to be the final suture location of the Siberian plate and the NCP; however, the most recent research results indicate that the two plates eventually closed along the Solonker-Linxi suture zone Huang et al., 2016;Tang, 1990;Xiao et al., 2003). ...
The Early Permian Yuejin Sumu intrusive rocks are located approximately 40 km northwest of Xilinhot City, Inner Mongolia. The magmatic rocks primarily composed of gabbro, diorite, and syenogranite. Through the study of zircon LA-ICP-MS U–Pb geochronology and rock geochemistry, age dates of the three lithologies show that the three formed simultaneously in the late Early Permian (gabbro formation age is 276.7 ± 1.3 Ma; diorite is 276.9 ± 0.8 Ma; syenogranite is 275.0± 1.4 Ma). The geochemical characteristics of the three lithologies are primarily calc-alkaline, and some samples, such as gabbro and diorite, show a transition to the alkaline series. The oxides in the gabbro and diorite are linearly related to SiO2. With magma MgO content, Al2O3 content increases first and then decreases, while CaO content decreases sharply, suggesting that the magma for the gabbro and diorite is a mantle-derived magma that undergoes separation and crystallization. The crystalline minerals include magnesia-rich pyroxene in the early stage and the plagioclase and pyroxene in the late stage. The syenogranite has high aluminum content (12.8–14.6%), and the ratio of FeOT/Mg# is high. Rare earth elements are characterized by a mid-range of the light and heavy rare earth elements. Eu shows significant negative anomalies, and the whole rare earth elements (REEs) are characterized by V-shaped distribution. The syenogranite are enriched in K, Rb, Th, U, Zr, and depleted in Eu, Ba, Sr, Ti, P, Nb, Ta, indicating that the magma may derived from crust in a typical post-orogenic environment. Petrology and geochemical characteristics of the Early Permian intrusive rocks indicate that in the Early Permian (~274–278 Ma) in the Xilinhot area was back-arc extensional environment at the active continental margin. Mantle-derived magma caused partial melting of the crustal material. The mantle-derived magma and the melt of the crustal material formed the Early Permian bimodal-like Yuejin Sumu intrusive rocks.
... Most researchers have considered the Kedanshan ultramafic-mafic intrusion as the basal peridotite of an ophiolite, related to the Paleo-Asian Ocean (e.g., Liang, 1994;Li et al., 2011). However, lines of evidence in this study confirm it is an Alaskan-type ultramafic-mafic intrusion, formed in a super-subduction environment. ...
The Xing’an-Inner Mongolia accretionary belt in the eastern Central Asian Orogenic Belt (CAOB) was produced by the subduction of three oceanic plates: the Paleo-Asian, Mongol-Okhotsk and Paleo-Pacific oceanic plates. The interactions between these plates remain unclear. Here we report an Alaskan-type ultramafic-mafic intrusion in the Kedanshan area, central Inner Mongolia, China. The main lithologies of this intrusion include cumulate dunite, pyroxene peridotite, olivine pyroxenite and cumulate gabbro, with late gabbroic/anorthositic dykes. Minerals and whole-rock compositional variations display characteristics of an arc cumulate trend (Alaskantype), through fractional crystallization of Mg-rich and hydrous basaltic magma associated with oceanic subduction. Zircons from two gabbro samples yield Early Jurassic ages of 193 ± 6 Ma and 179 ± 4 Ma, respectively. We conclude that this ultramafic-mafic complex is an accumulated intrusion from an arc-related, high-Mg magma chamber above a supra-subduction zone. Considering the ages, location and tectonic setting of the complex, we suggest that it was most likely generated by melting of a large and triangle-shaped mantle wedge during superimposed subduction between the Mongol-Okhotsk and the Paleo-Pacific oceanic plates in the Jurassic.
... The Eastern Erenhot ophiolite is located~60 km east of the Erenhot city, near the China-Mongolia border ( Figure 1). It has been recognized as one of the fragments of the Erenhot-Hegenshan ophiolite belt [Liang, 1994;Robinson et al., 1999] and recently studied by Z. . This ophiolite is composed of serpentinized ultramafic rocks with subordinate gabbros, mafic lavas, red-colored radiolarian chert, and minor plagiogranite dykes. ...
The Xing'an-Inner Mongolia accretionary belt (XIMAB) in the southeastern segment of the Central Asian Orogenic Belt (CAOB) was produced by the long-lived subduction and eventual closure of the Paleo-Asian Ocean, and by the convergence between the North China Craton and the Mongolian micro-continent. Two ophiolite belts have been recognized: the northern Erenhot-Hegenshan-Xi-Ujimqin ophiolite belt and the southern Solonker-Linxi ophiolite belt. Most basalts in the northern ophiolite belt exhibit characteristics of N-type to E-type MORB affinities with depleted Nd isotopic composition (εNd(t) > +5), comparable to modern Eastern Pacific mid-ocean ridge basalts. Most basaltic rocks in the southern belt show clear geochemical features of supra-subduction-zone type (SSZ-type) oceanic crust, probably formed in an arc/back-arc environment. The inferred back-arc extension along the Solonker-Linxi belt started at ca. 280 Ma. Statistics of all the available age data for the ophiolites indicates two cycles of seafloor spreading/subduction, which gave rise to two main epochs of magmatic activity at 500-410 Ma and 360-220 Ma, respectively, with a gap of ~50 million years (Myrs). The spatial and temporal distribution of the ophiolites and concurrent igneous rocks favor bilateral subduction towards the two continental margins in the convergence history, with final collision at ~230-220 Ma. In the whole belt, signals of continental collision and Himalayan-style mountain building are lacking. We thus conclude that the Xing'an-Inner Mongolia segment of the CAOB experienced two cycles of seafloor subduction, back-arc extension and final “Appalachian-type” soft collision.
... The Eastern Erenhot ophiolitic complex (EOC) was recognized as one of the numerous fragments of oceanic lithosphere in the EHOB (Fig. 1c) (Liang, 1994;Robinson et al., 1999). No detailed dating data and geochemical data of this complex are available partly because the area is remote, and in general displays poor exposure. ...
The Eastern Erenhot ophiolitic complex (EOC) is one of the numerous fragments of oceanic lithosphere in southeastern Central Asian Orogenic Belt. It is composed dominantly of serpentinized ultramafic rocks with subordinate gabbros, mafic lavas and minor plagiogranite dikes. Zircons from two gabbros and one plagiogranite yielded weighted mean 206Pb/238U ages of 354.2 ± 4.5 Ma, 353.3 ± 3.7 Ma and 344.8 ± 5.5 Ma. These ages suggest that the oceanic crust of the EOC formed in a maximum time period of 10 Ma, and that the plagiogranite may have formed later than the gabbroic section. An undeformed and unmetamorphosed dioritic porphyry dike intruded in the Carboniferous strata near the EOC has an intrusive age of 313.6 ± 2.9 Ma and provides a possible younger minimum time limit for the formation of the early Carboniferous ophiolitic complex. All the mafic rocks have similar chondrite normalized REE patterns characterized by moderate depletion in LREE with (La/Yb)N (0.20–0.75) similar to normal middle oceanic ridge basalt (N-MORB). The PM-normalized trace element patterns of the gabbros and massive basalts are also reasonably consistent, essentially similar to those of N-MORB except for some enrichment in LILE (e.g. Rb, Ba) and slightly negative Ti anomalies. The plagiogranite samples are characterized by lower K2O (0.45–0.73 wt%) comparable with oceanic plagiogranite. They have LREE-enriched, chondrite-normalized REE patterns with varying Eu anomalies and the trace elements (e.g. Rb, Y, Nb) show similarity to volcanic arc granite. These geochemical features of the EOC show a similar volcanic arc affinity, suggesting that they form in a back-arc-type environment. Their origin is attributed to asthenospheric upwelling and further lithospheric extension during early Carboniferous, formed as a consequence of slab breakoff on collision of the northern early to mid-Paleozoic orogenic terrane and the Hunshandake Block.
... Southeastern Inner Mongolia is situated in the southeastern part of the North Asian Orogenic Region (Fig. 1a). There are several ophiolite belts, such as the Xar Moron ophiolite, the Hegenshan ophiolite and the Solonker ophiolite, distributed discretely in this area (Fig. 1b), which are thought to be oceanic crust formed during the closure of the Paleo-Asian Ocean (Liang, 1994;Tang, 1989Tang, , 1990Wang, 1986;Wang et al., 1991). However, because of the incomplete preservation and lack of accurate ages for the ophiolites, there have significant debates on the late-Paleozoic tectonic division and evolution of this area (Jian et al., 2008(Jian et al., , 2010(Jian et al., , 2012Li, 2006;Wang, 1986;Xiao et al., 2003Xiao et al., , 2009bXu et al., 1996Xu et al., , 2013. ...
A subduction-accretion complex is identified from previously defined
late-Carboniferous and early-Permian strata in Daqing pasture, southern
Xiwuqi, Inner Mongolia. The subduction-accretion complex is composed of
a matrix of siltstone and exotic blocks of bioclastic limestone, pillow
basalt, foliated basalt, chert and asbestos. The pillow basalt possesses
the geochemical characteristics of mid-ocean ridge basalt (N-MORB),
whereas the foliated basalt displays the geochemical characteristics of
island arc basalt (IAB), indicating that these basalts are of different
origins. U-Pb (zircon) dating indicates that the foliated basalt formed
in the late-Carboniferous (314.5 to 318.4 Ma) and the bioclastic
limestone formed in the early early-Permian. Combined with regional
geological data, the subduction-accretion complex and coeval
calc-alkaline granitic belt to the north constitute the essential
elements of the late-Carboniferous to early early-Permian subduction
zone on the southern margin of the Siberian paleoplate. The zircon
ɛHf(t) values of the foliated basalt are positive (+
14.4 to + 23.9), suggesting that this basalt originated directly from
depleted mantle. The temporal-spatial distribution of the
subduction-accretion complex and ophiolite belts in southeastern Inner
Mongolia indicates that there was significant lateral crustal growth on
the southern margin of the Siberian paleoplate in the late Paleozoic.
Appinite commonly occurs in convergent plate tectonic settings and thus can constrain the tectonic evolution of ancient orogens. Geochemical and geochronological analyses were carried out on a newly identified Early Triassic appinitic complex in southeastern Inner Mongolia in the eastern segment of the Central Asian Orogenic Belt. Petrographically, the Luotuochang complex can be divided into two zones: an outer zone of intermediate rocks and inner zone dominated by mafic rocks. A monzonite sample from the outer zone yielded weighted mean zircon 206Pb/238U ages of 246 ± 1.6 Ma, whereas the gabbro from the inner zone yielded an age of 243 ± 1.2 Ma. The inner zone mafic rocks have SiO2 contents of 45.56 to 54.27 wt% with high MgO, Cr, Ni and Sr contents, elevated Ba/Nb, Ba/Zr, Rb/Y and Th/Zr, and low TiO2, Nb/Zr and Nb/Y. These features suggest that the metasomatized lithospheric mantle is the primitive magma source. SiO2 contents of the outer zone intermediate rocks range from 57.6 to 63.69 wt% and K2O, Ba and Sr contents are enriched; the εNd(t) (+2.2 to +4.0) and εHf(t) (+8.4 to +13.5) values are also high. These characteristics suggest that the magma of these rocks mainly derived from the mantle with possible juvenile lower crust involvement. Based on these geochemical data and results from regional geological investigations, we propose that the Luotuochang appinitic complex was formed in a post-orogenic extensional setting. Its formation was likely the result of lithospheric delamination, upwelling of new mantle material and partial melting of the overlying lower crust during crustal compression and thickening soon after the closure of the restricted Paleo-Asian Ocean basin.
In this study, plagiogranites in the Diyanmiao ophiolite of the southeastern Central Asian Orogenic Belt (Altaids) were investigated for the first time. The plagiogranites are composed predominantly of albite and quartz, and occur as irregular intrusive veins in pillow basalts. The plagiogranites have high SiO2 (74.37–76.68wt%) and low Al2O3 (11.99–13.30wt%), and intensively high Na2O (4.52–5.49wt%) and low K2O (0.03–0.40wt%) resulting in high Na2O/K2O ratios (11.3–183). These rocks are classified as part of the low‐K tholeiitic series. The plagiogranites have low total rare earth element contents (ΣREE)(23.62–39.77ppm), small negative Eu anomalies (δEu=0.44–0.62), and flat to slightly LREE‐depleted chondrite‐normalized REE patterns ((La/Yb)N=0.68–0.76), similar to N‐MORB. The plagiogranites are also characterized by Th, U, Zr, and Hf enrichment, and Nb, P, and Ti depletion, have overall flat primitivemantle‐normalized trace element patterns. Field and petrological observations and geochemical data suggest that the plagiogranites in the Diyanmiao ophiolite are similar to fractionation‐type plagiogranites. Furthermore, the REE patterns of the plagiogranites are similar to those of the gabbros and pillow basalts in the ophiolite. In plots of SREE–SiO2, La–SiO2, and Yb–SiO2, the plagiogranites, pillow basalts, and gabbros show trends typical of crystal fractionation. As such, the plagiogranites are oceanic in origin, formed by crystal fractionation from basaltic magmas derived from depleted mantle, and are part of the Diyanmiao ophiolite. LA–ICP–MS U–Pb dating of zircons from the plagiogranites yielded ages of 328.6±2.1 and 327.1±2.1Ma, indicating an early Carboniferous age for the Diyanmiao ophiolite. These results provide petrological and geochronological evidence for the identification of the Erenhot–Hegenshan oceanic basin and Hegenshan suture of the Paleo‐Asian Ocean.
In this paper we discuss the timing of final closure of the Paleo-Asian Ocean based on the field investigations of the Carboniferous–Permian stratigraphic sequences and sedimentary environments in southeastern Inner Mongolia combined with the geology of its neighboring areas. Studies show that during the Carboniferous–Permian in the eastern segment of the Tianshan-Hinggan Orogenic System, there was a giant ENE–NE-trending littoral-neritic to continental sedimentary basin, starting in the west from Ejinqi eastwards through southeastern Inner Mongolia into Jilin and Heilongjiang. The distribution of the Lower Carboniferous in the vast area is sparse. The Late Carboniferous or Permian volcanic-sedimentary rocks always unconformably overlie the Devonian or older units. The Upper Carboniferous–Middle Permian is dominated by littoral-neritic deposits and the Upper Permian, by continental deposits. The Late Carboniferous–Permian has no trace of subduction-collision orogeny, implying the basin gradually disappeared by shrinking and shallowing. In addition, it is of interest to note that the Ondor Sum and Hegenshan ophiolitic mélanges were formed in the pre-Late Silurian and pre-Late Devonian respectively, and the Solonker ophiolitic mélange formed in the pre-Late Carboniferous. All the evidence indicates that the eastern segment of the Paleo-Asian Ocean had closed before the Late Carboniferous, and most likely before the latest Devonian (Famennian).
The Central Asian Orogenic Belt (CAOB) is known for its massive Phanerozoic generation of juvenile crust. The tectonic evolution of the CAOB during the late Paleozoic era is still debated. The Eastern Erenhot ophiolite complex (EOC) has been recognized as one of the numerous late Paleozoic ophiolitic blocks in the southeastern part of the CAOB. Zircon U-Pb dating on rhyolite and plagiogranite from the EOC yielded a tight range of ages from 360 to 348 Ma, indicating that the complex formed in the early Carboniferous. The primitive mantle-normalized spider diagram of rhyolites (εNd(t) values of +6.8 and +7) and basalts almost overlaps. Such rhyolites may have been derived from partial melting of juvenile basaltic rocks during the initial opening of the Erenhot–Hegenshan oceanic basin. All of the plagiogranites exhibit similar trace element behaviours of High Field-Strength Elements, such as U, Zr and Hf, and Large Ion Lithophile Elements, such as Ba and Rb, to these of gabbros. These plagiogranites were considered products of episodes of partial melting of hydrous gabbros during ocean floor spreading. We conclude that the northern subduction of the Paleo-Asian Ocean stopped before 360 Ma and the southeastern CAOB experienced extension during the late Paleozoic era. The Erenhot–Hegenshan Ocean, which is comparable to the present Red Sea, originated from syn-collisional crustal thickening, subsequent lithosphere extension, and upwelling of the asthenosphere during orogenic quiescence with an age of 20 Ma.
The NE China area consists of several micro-continental blocks, such as Jiamusi Block in the southeast, the Xing’an - Songliao Block in the middle, and the Erguna blocks in the northwest. The studies of the suture zones between the blocks indicate that the amalgamation of these blocks have finished before Late Paleozoic, and formed a big continental block, i.e. Jia-Meng block. From Late Paleozoic the cover sequences started to develop and formed a continental margin cover sequences of Late Paleozoic.
The tectonic setting of southern margin of the Jia-Meng block was active continental margin during early Paleozoic. The Paleo-Asian ocean plate broke down during the north-ward subduction around 320 Ma, and formed a volcanic arc. Meanwhile the Hegenshan back-arc basin ocean was opened. The continuing north-ward subduction resulted in the arc-continent collision, and the Hegenshan Ocean was closed in 280 Ma. The tectonic setting changed from active margin to passive margin. Finally the Paleo-Asian ocean was closed in the end of Late Permian, and the whole area became to be intra-continental terrestrial setting.
The Hegenshan ophiolite is located in the northern part of Xing’an Mongolia orogenic belt. It consists of a complete rock assemblage including peridotite, cumulate rocks and mafic lava associated with radiolarian cherts. The formation age of the Hegenshan ophiolite has long been controversial, which makes a lot of troubles in dividing the tectonic evolution stages. Zircon U-Pb isotope chronology studies have shown that the crystallization age of the gabbro diorite (341±3Ma) and basalt (359±5Ma) in Hegenshan ophiolite are in the early Early Carboniferous and the peak age of the inherited zircons in basalt is early Late Devonian (375±2Ma). These inherited zircons in basalt are short prismatic and angularity with wide, patchy or planar growth zones which display the characteristics of basic magmatic zircons. All of these indicate that the oceanic crust already formed during early Late Devonian. The pyroclastic rocks of the Upper Carboniferous Gegenaobao Formation are unconformity with the Hegenshan ophiolite. The age of the tuff in this formation is 323±3Ma which support the minimum age for the emplacement of Hegenshan ophiolite. So we suggested that the age of the Hegenshan ophiolite is Late Devonian to Early Carboniferous and the emplacement time is Late Carboniferous. While the ages of some mafic dikes in Hegenshan ophiolite are Early Cretaceous (132±1Ma, 139±3Ma and 120±1Ma). They contain a lot of inherited zircons (144±1Ma~2698±25Ma). The peak ages of these inherited zircons closely respond the complex magma and tectonic events before Early Cretaceous in the northern Xing’an Mongolia orogenic belt. These mafic dikes might form in the extensional environment during the Mesozoic but not the Early Carboniferous ophiolite. Combined with previous achievements as well as temporal and spatial distribution characteristics of regional strata and magmatic rocks, we built the tectonic evolution of the northern Xing’an Mongolia orogenic belt during the Late Paleozoic. We suggested that the Erenhot and Hegenshan area was in the denudation stage in the Early Devonian and the continental crust expanded to form the oceanic basin in the Middle Devonian, the basin continued to expand and formed the oceanic crust in the early Late Devonian and then the crust began to subduct northward in the late Early Carboniferous and accreted to the Siberian continental margin which resulted in the gradually closing of the ocean basin during the Late Carboniferous, and most emplaced ophiolites were unconformity covered by the volcanic rocks of Late Carboniferous. The Hegenshan ophiolite is the residue of this oceanic lithosphere.
Diamond and other deep mantle minerals, previously found in the Mesozonic Tibet and the early Paleozoic Polar Ural, Russia, are now found in ophiolite mantle peridotite chromite. Hence the traditional opinion holding shallow origin of ophiolite and chromite should be reconsidered. In order to find out the distribution of diamond and other deep mantle minerals in different orogenic ophiolite chromites and to investigate the formation processes of the Hegenshan podiform chromitites, the authors carried out mineralogical study of chromitites in the Hegenshan ophiolite, Inner Mongolia. Approximately 2000 kg of mainly disseminated chromitite ore were collected from Hegenshan No. 3756 orebody. By mineral separation, preliminary studies identified more than 30 mineral species in addition to diamonds and moissanite. The other minerals include minerals of oxides, sulfides, silicates, and other compounds. This study demonstrates that the formation and tectonic setting of Hegenshan ophiolite and the chromitites deserve reevaluation.
There is a suit of shoshonite discovered recently in Siziwangqi area, Inner Mongolia. The shoshonitic basalts are characterized by low-Si (SiO2 48.11%∼52.77%), high-Ti (TiO2, 1.94% ∼ 2.77%, averaged by 2.48%), high-K (K2O + Na2O 5.32%∼7.02%, K2O/Na2O 0.48∼1.10); The rocks are also enriched in large ion lithophile element (LILE), light rare earth elements (LREE), and depleted in high field strength elements (HFSE), slightly negative Eu anomaly (δEu=0.81∼0.98). Their chemical characteristics are similar to those of oceanic island basalt (OIB). Trace element ratios such as Rb/Nb, Ba/Nb, La/Nb, K/Nb and Zr/Nb are consistent with the EM I OIB geochemical signatures. 87Sr/86Sr and 143Nd/144Nd ratios are in the ranges of 0.7063 to 0.7077 and of 0.51196 to 0.51243, respectively, showing the EM I geochemical signatures. The progressive enrichment of P2 O5, Rb, Sr, Ba and Zr with decreasingεNd values, exclude the possibility that the magmas were contaminated by crustal materials. Trace-element compositions and isotopic characteristics suggest that the shoshonitic parental magma was derived from the small degree decompression melting of phlogopite-bearing mantle peridotites. Based on the geochemistry, Siziwangqi shoshonites are the products of early Cretaceous lithospheric thinning of the North China craton.
Assemblages of the Upper Devonian ultrabasic rock, cumlative complex, diabase-dike swarm as well as basic pillow lava nipping radiolarian Si-sargillaceous rocks are discovered in Hujierte area, Inner Mongolia, China. They are typical ophiolite, which geochemical compositions accord with the characteristics of MORB. The age of 371.0±5.3 Ma using zircon U-Pb geologic dating method shows that there is the Upper Paleozoic era oceanic crust in this area, being a part of the Paleo-Asiatic Ocean, it is possibly a branch of ocean basin in archipelago paleogeographic pattern between the Siberian plate and the Sino-Korean plate, not inner the Sino-Korean plate, a northern part of the Sino-Korean plate should be on the south of Hujierte at the least.
Firstly recognized Diyanmiao ophiolite occurs in Xing'anling-Mongolian Orogenic Belt (XMOB) between the Sino-Korean paleoplate and the Siberian paleoplate in the central sector of Inner Mongolia. According to field geological investigation and the preliminary results of petrology and geochemistry, we found Diyanmiao ophiolite consists of Baiyinbulage ophiolite zone and Naolaiketu ophiolite zone, each of them is about 3km wide, 30km long, and Diyanmiao ophiolite outcrops completely, including augite peridotite, beded-massive gabbro, anorthosite, spilite, pillow basalt, keratophyre, baschtauite, and chert from bottom to top. The chondrite-normalized REE patterns of augite peridotite represent a typical mantle section of the SSZ-type ophiolite. Pillow basalt has the islandarc tholeiite (IAT) features. The values of A1 20 3/(A1 20 3 +Fe 20 3) of chert display the characteristics of continental margin.
Platinum-group elements (PGEs) abundances of mantle peridotites from the Kedanshan ophiolite, Inner Mongolia, were determined by using a Carius tube sample digestion and the MC-ICP-MS system. Compared with primitive mantle and typical mantle peridotites in the world, Kedanshan mantle peridotites are enriched in Pt and Pd, but depleted in Ir and Ru, with high Pd/Ir ratio. The PGEs distribution patterns have very steep positive slope, unlike the PGEs distribution patterns in the mantle peridotites of partial melting relict origin which usually are negative-slop patterns or flat patterns. Ir and Ru have a positive correlation with MgO, which might reflect the consumption of sulfide during partial melting, but have no relations with sulfide melt/silicate melt Nernst partition coefficients. Enriched Pt and Pd indicate that these samples cannot be the residuum of simple partial melting, but have a more complex origin (e. g. melt percolation). Similar with the behavior of Pt and Pd enriched in the abyssal peridotites(AP) and the subcontinental lithospheric mantle (SCLM) harzburgites, the kedanshan mantle peridotites can be explained by sulfide precipitation during melt percolation. The melt may come from 'filtered' sulfide during magma evolution.
Cumulates in ophiolites of the Har Tolgoi area, Damao Qi, Inner Mongolia, are represented by the peridotite+pyroxenite+gabbro association, i.e. the PPG type. Boninite is recognized for the first time from gabbro by petrochemical and geochemical studies, which provides petrological evidence for the tectonic environment of ophiolites in the area. Basalt has the characteristics of MORB and is enriched in components (e.g. Th) of the subduction zone. In the mélange zone there occur Devonian island arc-type intermediate and intermediate-acid volcanic rocks, which conformably overlie siliceous pelite. Based on the above-mentioned analysis, the authors suggest that ophiolites in the area formed in a SSZ-type environment, i.e. at the time of initial subduction. New oceanic crust formed above the subduction (fore-arc) zone and gradually evolved to an island arc.
The authors accumulate a large amount of new data of regional geology and geochronology in recent years and makes a generalization and summation of the tectonic nature and evolution in Northeastern China. A comprehensive study indicates that the NE China continent is composed mostly of relatively stablae massifs, rather than orogenic belt since Late Paleozoic. All of these massifs include the basements of Precambrian-Early Precambrian, even Archean, and were simultaneously subjected to metamorphism and granitic magmatism in early Early Paleozoic (490-510 Ma). During 350-450 Ma, at least two stable microcontinents of Erguna-Xing'an and Songnen-Jiamusi composed of massifs solidified in 490-510 Ma existed in NE China, marked by without magmatism, which are northwards linked together with Erguna and Buriya massifs in Russia, respectively. These two microcontinents collided together along a NNE-trending Kailu-Nenjiang-Heihe(NE China)-Nora-Soukhotin(Russia) in Early Carboniferous and formed a Northeast Asia continental plate(NACP). The well-known Xilamulunhe fault is commonly referred as a final collision zone between NACP and North China plate, but the west end of the fault is cut by NNE-trending fault, and east end linked together with Kailu-Nenjiang collision zone and not stretches eastwards into Songliao basin, indicating the Xilamulunhe fault is possibly related to the formation of Kailu-Nenjiang collision zone. From Late Carboniferous to Mid Permain, the NACP on the whole was at extension setting where at least in NE China, a series of faulted basins with predominant volcanics formed in early time and a large marine sedimentary basin formed in Mid Permian. All of the Late Paleozoic volcanics and sedimentary rocks do not suffer regional metamorphisam. The transformation of Mesozoic tectonic regime in NE China is related not only to superposition and transformation of Paleo-Asia ocean tectonic system by west Pacific tectonic system, but the evolution of Mongolia-Okhotsk tectonic system plays an important role in transforming tectonic regim, especially in Mid-Late Jurassic. The evidences of geology and geophysics show that the well-known three faults of Derbugan in Da-Xinganling Mountains, Hegenshan in central part of Inner-Mongolia and Nadanhada in east part of Heilongjiang Province are not deep faults controlling tectonic units probably. The parts under Mesozoic-Cenozoic basins in NE China for the most part are not metamorphosed crystalline basement, but a large marrine sedimentary basin.
Chaokeshan ophiolite , one of the best-outcropped ophiolitic blocks in Inner Mongolia region , likely formed in Late Carboniferous. The Chondrite-normalized REE patterns of the mafic rocks of the Chaokeshan ophiolite are of characteristics of LREE depletion , similar to N-MORB , while the spider diagram shows the enrichment of the Large Iron Lithosphile Elements and depletion of High Field Strength Elements-the unique feature of the arc volcanic rocks , so the tectonic setting of Chaokeshan ophiolite may be back-arc basin. Compared with modern intra-oceanic arc Mariana Trough basalts , continental marginal arc Okinawa Trough basalts and mafic rocks of the Kuerti ophiolite which formed in paleo-intra-oceanic back-arc basin , the Chaokeshan mafic rocks show geochemical characteristics similar to Mariana and Kuerti basalts , we suggest that the Chaokeshan ophiolite was likely produced in an intra-oceanic back-arc tectonic setting instead of continental margin back-arc basin.
Diamond, moissanite and a variety of other minerals, similar to those reported from ophiolites in Tibet and northern Russia, have recently been discovered in chromitites of the Hegenshan ophiolite of the Central Asian Orogenic Belt, north China. The chromitites are small, podiform and vein-like bodies hosted in dunite, clinopyroxene-bearing peridotite, troctolite and gabbro. All of the analysed chromite grains are relatively Al-rich, with Cr# [100Cr/(Cr+Al)] of about 47–53. Preliminary studies of mainly disseminated chromitite from ore body No. 3756 have identified more than 30 mineral species in addition to diamond and moissanite. These include oxides (mostly hematite, magnetite, rutile, anatase, cassiterite, and quartz), sulfides (pyrite, marcasite and others), silicates (magnesian olivine, enstatite, augite, diopside, uvarovite, pyrope, orthoclase, zircon, sphene, vesuvianite, chlorite and serpentine) and others (e.g., calcite, monazite, glauberite, iowaite and a range of metallic alloys). This study demonstrates that diamond, moissanite and other exotic minerals can occur in high-Al, as well as high-Cr chromites, and significantly extends the geographic and age range of known diamond-bearing ophiolites.
Recent research has identified an early to late Carboniferous magmatic arc that extends from Suzuo Qi to Xiwu Qi in Inner Mongolia, China, but the eastern extension of this arc is unknown. Understanding the relationship between this arc and the Hegenshan ophiolite belt and Xilamulun Solonker suture zone is important to our understanding of the tectonic evolution of the late Palaeozoic Palaeo-Asian Ocean. Here, we present new zircon laser ablation-inductively coupled plasma mass spectrometry U-Pb and geochemical data for the Maoliger quartz monzodiorites within the Jalaid Qi area. The Maoliger quartz monzodiorites formed at 329 +/- 2 Ma, are low-K and tholeiitic, and have geochemical signatures indicative of formation within a magmatic arc. These rocks are large-ion lithophile element (e.g. Rb, Ba, and Sr)-enriched and high-field-trength element (e.g. Nb and Ta)-depleted. Combined with previously published researches, it is suggested that the quartz monzodiorites within the Jalaid Qi area formed contemporaneously with and are geochemically similar to quartz diorites of the Xiwu Qi area and the Baolidao pluton in the Suzuo Qi area. This indicates that the early to late Carboniferous magmatic arc in this region extends eastward to the Jalaid Qi area. This arc is located in an area parallel to a southerly early Permian magmatic arc, suggesting that the Palaeo-Asian Ocean subduction zone migrated south between the early Carboniferous and early Permian. The new data show that the Palaeo-Asian Ocean closed after the late Carboniferous.
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