Xing'an–Mongolia Orogenic Belt Research Articles

Permian Granitic Plutons from the Northern Margin of the North China Craton: Implications for the Tectonic Evolution of the Central Asian Orogenic Belt

As the world’s largest accretionary orogen, the Central Asian Orogenic Belt (CAOB) underwent continuous juvenile crustal growth in the Phanerozoic. The northern margin of the North China Craton (NCC) and its adjacent area form the eastern segment of the CAOB, which is a key area for learning about the geological evolution of the Paleo-Asian Ocean (PAO). In the Permian, the west of the northern margin of the NCC was a post-collision extensional environment, while the east was in a subduction stage. As a connecting area, the Permian evolution of the PAO in the middle of the northern margin of the NCC has not been systematically studied. In order to fill the gap and understand the continuous temporal and spatial evolutionary process of the PAO, this paper focuses on the Permian granitic rocks in the Chifeng area. Zircon U-Pb dating and the geochemical analysis of whole-rock major and trace elements were conducted to build a granite chronological framework, and to discuss the genesis and tectonic background of the granitic rocks, along with tectono-magmatic evolutionary history in the Chifeng area. The respective LA-ICP-MS zircon U-Pb dating results from eight samples are 269 ± 1, 268 ± 3, 260 ± 4, 260 ± 1, 260 ± 1, 255 ± 2, 254 ± 2 and 256 ± 1 Ma, respectively. These results, combined with previous data, revealed that the Permian granitic rocks had undergone three events of magmatism: (1) monzogranitic-syenitic phase (294–284 Ma; Cisuralian); (2) monzogranitic phase (269–260 Ma; Guadalupian) and (3) late monzogranitic-syenitic phase (256–254 Ma; Lopingian). From the Early Permian (294–284 Ma) to the Middle Permian (269–260 Ma), granites with fine-medium-grained locally porphyritic texture and massive structure showed a high-potassium calc-alkaline series formed in a compressional setting, indicating a continuous collision between the Xing’an-Mongolian Orogenic Belt (XMOB) and the NCC. During the Late Permian-Early Triassic (256–248 Ma), granites with massive structure and medium-grained texture in the Chifeng area were magmatism dominated by A- and I-type granites of high-potassium calc-alkaline series, combined with the coeval basic rocks, which constituted a typical “bimodal” rock assemblage. This suggests that the Chifeng area was located in an extensional setting where the subducting slab broke off during the collision between the XMOB and NCC. These granitic plutons from the Permian are believed to have been generated by the subduction-collision of the Paleo-Asian oceanic crust beneath the NCC, according to emplacement time and occurrence location. Our findings provide strong evidence for Permian continuous temporal and spatial tectonic evolution and the characterization of the eventual closure of the PAO in Chifeng area at the northern margin of the NCC.

Geological characteristics and genesis of the Late Carboniferous extremely thick slate in the Shuangjianzishan Ag-Pb-Zn district, southern Great Xing’an Range, NE China: constraints on metallogenesis and tectonic setting

The Shuangjianzishan super-large Ag-Pb-Zn deposit is situated in the southern Great Xing’an Range (SGXR), which is part of the eastern Central Asian Orogenic Belt (CAOB) and the northeastern Xing’an-Mongolia Orogenic Belt (XMOB). The host rock of this deposit is the Dashizhai Formation, characterized by a widely distributed, extremely thick slate in the Shuangjianzishan basin. Petrographically, the slate contains secondary minerals such as secondary quartz, biotite, sphalerite, pyrite, galena, and chalcopyrite, in addition to primary quartz, feldspar, and rock fragment. The whole-rock geochemical analysis indicates that the slate was deposited in an oxygen-deficient shale basin, influenced by calcium-rich volcanic ash and magmatic-hydrothermal fluids. The mineralogical characteristics of sulfide minerals and the in-situ sulfur isotopic composition of pyrites suggest that the sulfide minerals were likely formed before the primary metallogenic epoch, indicating pre-enrichment of metals. The color variation from black to green observed in the drilling core of the slate is attributed to differences in feldspar content, with the denser “black” slate marking the metallogenic horizon. Based on previous research, it is inferred that the slate is a product of the post-orogenic extensional tectonic setting of the XMOB in the Late Carboniferous, and it underwent alteration by epithermal magmatic-hydrothermal fluids during the Mesozoic metallogenic episode.

Distinct Lithospheric Structure in the Xing'an‐Mongolian Orogenic Belt

AbstractDetailed knowledge of the lithospheric structure is essential for a better understanding of the tectonic evolution of the Xing'an‐Mongolian Orogenic Belt (XMOB). We constrained the lithospheric structure across the XMOB by receiver function imaging and shear wave splitting analysis from a dense seismic array. The lithosphere‐asthenosphere boundary (LAB) is coherently imaged in different tectonic blocks. The mid‐lithospheric discontinuity is also identified at ∼75–100 km depth over the deeper LAB (∼110–130 km) beneath the western side of the North‐South Gravity Lineament (NSGL), roughly at the same depth as the LAB beneath the eastern side of the NSGL. The distinct variations in both the lithospheric structure and fast shear‐wave polarization direction across the NSGL are likely caused by different tectonic processes in the two sides, indicating that the NSGL might represent the western boundary influenced by the subduction of the (Paleo‐)Pacific Plate. Lithosphere thinning in the XMOB is evidenced to have been limited mainly to the east of the NSGL.

The genesis and mineralization process of Xiaobaliang VHMS Cu-Au deposit, Central Asian Orogenic Belt: Constraints from the pyrite geochemistry and zircon geochronology

The Xiaobaliang Cu-Au deposit is located in the Hegenshan ophiolite complex belt of the western Xing’an–Mongolia Orogenic Belt (XMOB). Strata-bound sulfide ores are discovered within the Carboniferous volcanic tuff, with the oxidized Au-Cu ores at the top and Cu (-Au) sulfide ores at the bottom. Zircon U-Pb dating on volcanic tuff has yielded a concordia age of 326 ± 3 Ma (n = 21, MSWD = 5.6). Four types of pyrite have been identified, including lamellar pyrite (Py1) in the wall rocks, framboidal pyrite (Py2) in the Zn-rich massive ores, colloform pyrite (Py3) in the massive Cu ores, and euhedral pyrite (Py4) in the fractured Cu ores. Chalcopyrite comprises two types: early massive chalcopyrite aggregates (Cp1) and late chalcopyrite veinlets (Cp2). Here we present the in situ trace elements and sulfur isotopes analyses of different sulfide generations. Elements like Se (44.1 ppm), Co (227 ppm) and Cu (3211 ppm) are enriched in pyrites from Cu ores (Py3&Py4), whereas the pyrites from altered rocks (Py1) take high Mn (5027 ppm), Ti (134 ppm), V (13.0 ppm), Ni (41.9 ppm) and Au (5.07 ppm) concentrations. Similar elemental patterns are observed in chalcopyrite analyses where Co (312 ppm), Se (566 ppm) and Te (31.5 ppm) are enriched in massive Cu ores (Cp1), but Mn (147 ppm), V (174 ppm) and Ti (42.2 ppm) contents are mainly concentrated in chalcopyrite veinlets (Cp2). In situ S isotope analyses exhibit a large variation in Py1 (−30.70 to −1.50‰) and Cp2 (−7.81 ∼ −4.50‰) but are constant in Py2, Py3, Py4 and Cp1 (0.44 to 3.15 ‰). The high Co/Ni ratios and relatively lower δ34S values in Py1 suggest a diagenetic environment where the lower-temperature and extremely reduced conditions facilitate the enrichment of Mn, V and Au. The progressive enrichment of Sn, Te, Bi, Se and Cu in hydrothermal Py3 and Py4 then points out the rising of temperature in the weakly reduced environment. The negative sulfur isotopes in altered rocks (Py1) and fractured ores (Cp2) point out the contribution of the volcanic-sedimentary sequence. These compositional features, combined with sulfur isotopes, have demonstrated that Cu was mainly derived from magmatic-hydrothermal fluids whereas Au was released from Au-rich sediments on the seafloor. The Xiaobaliang Cu-Au deposit belongs to a Cyprus-type VHMS deposit formed in the Carboniferous extensional setting.

Geochronology and genesis of the newly discovered Mengdehe orogenic-type Au deposit in the Xing’an-Mongolia orogenic Belt, NE China

The Xing’an-Mongolia Orogenic Belt (XMOB) is a tectonic window to understand the subduction-orogenesis-magmatism and corresponding Au mineralization during the Mesozoic. Although epithermal gold mineralization associated with Paleo-oceanic plate subduction has been widely reported, information on orogenic Au metallogenesis during this period is insufficient. Geochronology and genesis study of the newly discovered orognenic Au deposit named as Mengdehe located at the Heihe-Nenjiang tectonic mélange in the eastern XMOB, can provide new information about regional metallogenetic mechanisms in this region. The Mengdehe Au deposit is the first discovered mylonitic rocks-hosted and brittle-ductile shear zone-dominated Au deposit in this area. Magmatic zircon U-Pb ages from the granitic mylonite and undeformed diorite in the Mengdehe deposit constrain that the strike-slip event probably has happened between 245.8 and 208.0 Ma. Rb-Sr dating of hydrothermal pyrites from the ore-bearing altered rocks indicates that Au mineralization occurred at 209.6 ± 3.1 Ma. We consider that the Mengdehe Au deposit was formed in the Late Triassic and associated with a strike-slip event. The trace elements, and He-Ar, S and Pb isotopic compositions of the ore-forming stage pyrites suggest that the ore-forming materials were derived from crustal sources with orogenic affinity. Combined with the early Mesozoic regional tectonic evolution in NE China and deformation characteristics of wall rocks, hydrothermal minerals and diorites, we conclude that the Au mineralization of the Mengdehe deposit was in response to the tectonic transformation from compression to extension associated with the extensional collapse of a previous thickened continental lithosphere related to the final closure of the Paleo-Asian Ocean.

Magmatism associated with lithospheric thinning, mantle upwelling, and extensional tectonics: Evidence from Carboniferous-Permian dyke swarms and granitoids from Inner Mongolia, Central Asian Orogenic Belt

The east-Ujimqin complex located north of the Erenhot-Hegenshan ophiolite belt in North China is composed of extensive Carboniferous and Permian magmatic rock suites, which preserve the records of an extensional tectonic regime following collision. These rocks provide important clues to the closure of the Hegenshan ocean and intraplate processes. Here we investigate the diorite dyke swarms in association with granitic rocks from the Xing'an-Mongolian Orogenic Belt (XMOB) through field investigations, petrology, geochemistry, zircon U-Pb and Lu-Hf studies, to gain insights into the timing, magma source and tectonic setting during Late Carboniferous and Early Permian. The diorite rocks display sub-alkaline, calc-alkaline, and high-K calc-alkaline features. Their extensional setting is suggested by plots in the fields of within plate basalt and active continental margin + alkaline oceanic arc. Zircon U-Pb data on magmatic zircons indicate that the granitoids crystallized at ca. 301 Ma, whereas the diorite dykes were emplaced at 286 Ma. Xenocryst zircon grains with 320 to 300 Ma ages in the dyke samples suggest incorporation from the surrounding granites. Zircon Lu-Hf isotopic data show consistently positive εHf(t) values (5.3 to 10.8) and plot between CHUR (Chondritic Uniform Reservoir) and depleted mantle lines suggesting that depleted mantle sources contributed significantly to the magma source. The computed model ages suggest that the magma source involved Neoproterozoic juvenile components.We propose a model where the magmatism occurred within an extensional tectonic setting caused by asthenosphere upwelling and lithospheric thinning. Our study also constrains the timing of Early Permian extension which suggests that the Hegenshan Ocean, as part of the Central Asian Orogenic Belt closed prior to 286 Ma.

Architecture and evolution of the lithospheric roots beneath circum-cratonic orogenic belts–The Xing'an Mongolia Orogenic Belt and its relationship with adjacent North China and Siberian cratonic roots

The accretionary mobile belts surrounding ancient cratonic cores are an important facet of the growth and preservation of continental landmasses. Peridotites from Nuominhe in the Xing'an Mongolia Orogenic Belt (XMOB) provide an additional opportunity to examine the age, structure and evolution of mantle lithosphere separating two of the largest existing ancient continental nuclei: the North China Craton and the Siberian Craton. This suite of mantle rocks comprises fertile to refractory garnet- and spinel-facies harzburgites and lherzolites. Their lithophile element systematics show that the peridotites were metasomatized to variable extent by silicate‑carbonate melts. Despite this, the highly siderophile element and Os isotope systematics appear to have been largely undisturbed. The Nuominhe peridotites have Re-depletion Os model ages (TRD) that range from 0.5 Ga to 2.4 Ga, with three peaks/major ranges at ~2.0–2.4 Ga, ~1.4–1.5 Ga and ~ 0.8 Ga, of which the latter two are closely similar to those data from other XMOB localities reported in the literature. The only section of the mantle that appears to have ages which correlate with crust formation is the suite with Neoproterozoic (~0.8 Ga) depletion ages, while the older mantle domains document older episodes of mantle depletion. Given the lack of correlation between equilibrium temperatures and bulk composition or TRD ages, the Nuominhe peridotites were inter-mixed in the mantle column, most likely as a result of incorporation of recycled older continental mantle fragments into juvenile Neoproterozoic mantle during the orogenic processes responsible for new lithosphere formation. Geothermobarometry of the Nuominhe peridotites indicates a conductive geotherm of ~60 mWm−2 and therefore a lithosphere thickness of ~125 km, which is thicker than most Phanerozoic continental terranes, and even thicker than Proterozoic regions that comprise the larger cratonic unit of the Siberian craton. This thick Proterozoic lithosphere sandwiched between the converging North China and Siberian cratons was evidently partly constructed from recycled refractory continental mantle fragments, perhaps extant in the convecting mantle, or in-part derived from the surrounding cratons, leading to a composite nature of the mantle in this re-healed continental suture. Re-accretion of recycled refractory old continental mantle fragments plays a significant role in affecting mantle composition and controlling the thickness of circum-cratonic landmasses between cratonic blocks.

Petrogenesis of the Neoproterozoic Xinlin ophiolite, northern Great Xing’an Range, northeastern China: Implications for the evolution of the northeastern branch of the Paleo-Asian Ocean

The Xinlin ophiolite in northeastern China marks the Xinlin–Xiguitu branch of the Paleo-Asian Ocean (PAO), and provides an opportunity to reconstruct the evolution of the Xing’an–Mongolian orogenic belt (XMOB). The main ophiolitic body is composed of mantle peridotites, mafic and ultramafic cumulates, isotropic gabbros, diabase dykes, and massive basaltic lava flows, from base to top. The uppermost pelagic sediment cover is missing and was presumably eroded. The cumulate gabbro yields a concordant zircon U–Pb age of 669 ± 8 Ma and has positive zircon εHf(t) values (+10.7 to +15.6). The mafic rocks of the ophiolite have rare earth element (REE) patterns that plot between the compositions of normal (N-) and enriched (E-) mid-ocean ridge basalt (MORB). The monzogranite that intruded into the Xinlin ophiolite yields a narrow age range (574–571 Ma), and is enriched in light REE and depleted in high field strength elements (HFSE). Zircon grains from the monzogranite have negative εHf(t) values (−5.6 to −10.0), suggesting the monzogranite was formed by partial melting of the Erguna continental crust, rather than oceanic crust. The geochemical characteristics of the ophiolite and spatially associated younger arc granites show that the Xinlin ophiolite developed at a mid-ocean ridge setting, and was subsequently accreted onto the Erguna continental margin before amalgamation with the Xing’an Block.

Age and composition of the subcontinental lithospheric mantle beneath the Xing'an–Mongolia Orogenic Belt: Implications for the construction of microcontinents during accretionary orogenesis

We present major- and trace-element concentrations, Sr–Nd–Hf isotopic compositions, ReOs isotopes, and highly siderophile element (HSE) abundances of 17 xenolithic mantle peridotites from the Nuomin volcanic field to constrain the assembly and evolution of the Xing'an–Mongolia Orogenic Belt (XMOB), Northeast China. The Nuomin peridotites are divided into five groups (harzburgite groups Haz-1, Haz-2, and Haz-3, and lherzolite groups Lhz-1 and Lhz-2). The Haz-1 peridotites are characterized by refractory chemical compositions (Al2O3 contents of 1.03–2.30 wt% and olivine forsterite (Fo) numbers of 90.7–92.5) that are best interpreted as residues after high degrees of mantle partial melting. The Haz-2 and Haz-3 peridotites exhibit slightly more fertile compositions (Al2O3 of 1.72–1.80 wt% and Fo of 89.0–89.9 for the Haz-2, and Al2O3 of 2.86 wt% and Fo of 91.2 for the Haz-3), resulting from refertilization by silicate melts. The Lhz-1 samples are more fertile (Al2O3 of 2.90–3.64 wt% and Fo of 89.1–90.3) than the Lhz-2 samples (Al2O3 of 1.64–2.16 wt% and Fo of 90.9–91.3). The combination of trace-element data (Ti/Eu ratio) and Sr–Nd–Hf isotopic compositions indicates the presence of at least three peridotite end-members (Haz-1, Haz-2, and Lhz-1–Lhz-2) beneath the Nuomin area, which can be explained by the infiltration of two distinct metasomatic agents (MA1: composition similar to potassic basaltic rocks in the Wudalianchi–Erkeshan–Keluo area, and MA2: Fe–Ti-enriched melt). HSE abundances and Os isotopic data show that the Os isotopic compositions of the Haz-1, Lhz-1, and Lhz-2 peridotites were not significantly affected by lithophile-element-dominated metasomatism and can be used to constrain the melt extraction ages of these peridotites. The lower 187Os/188Os ratios of Haz-1 peridotites define a prominent rhenium depletion age (TRD) of ~1.6 Ga and represent fragments of the Paleo–Mesoproterozoic lithospheric mantle rather than reflecting the heterogeneous characteristics of the asthenosphere or extraneous emplacement. In contrast, the Lhz-1 and Lhz-2 lherzolite groups together have a narrow range of 187Os/188Os ratios that give younger TRD ages (~0.5 to 1.0 Ga). Our data demonstrate that both the Paleo–Mesoproterozoic and Neoproterozoic subcontinental lithospheric mantle coexist beneath the XMOB and that they correspond respectively to the bimodal age peaks of crustal growth of ~1.6 Ga and ~0.8 Ga defined by the XMOB granitoids. The XMOB was assembled primarily during the Phanerozoic by multiple blocks that had formed from the Paleoproterozoic to Neoproterozoic. The similar chemical and Sr–Nd–Hf isotopic signatures of both young and old lithospheric mantle indicate that the large-scale metasomatism event involving potassic melts most likely occurred during the evolution of the Paleo–Asian Ocean. The crustal materials carried to the mantle by multiple subduction events could be an effective source of potassium for the origin of potassic basaltic rocks.

New insights on the early Mesozoic evolution of multiple tectonic regimes in the northeastern North China Craton from the detrital zircon provenance of sedimentary strata

Abstract. To investigate the timing of deposition and provenance of early Mesozoic strata in the northeastern North China Craton (NCC) and to understand the early Mesozoic paleotectonic evolution of the region, we combine stratigraphy, U–Pb zircon geochronology, and Hf isotopic analyses. Early Mesozoic strata include the Early Triassic Heisonggou, Late Triassic Changbai and Xiaoyingzi, and Early Jurassic Yihe formations. Detrital zircons in the Heisonggou Formation yield ∼ 58 % Neoarchean to Paleoproterozoic ages and ∼ 42 % Phanerozoic ages and were sourced from areas to the south and north of the basins within the NCC, respectively. This indicates that Early Triassic deposition was controlled primarily by the southward subduction of the Paleo-Asian oceanic plate beneath the NCC and collision between the NCC and the Yangtze Craton (YC). Approximately 88 % of the sediments within the Late Triassic Xiaoyingzi Formation were sourced from the NCC to the south, with the remaining ∼ 12 % from the Xing'an–Mongolia Orogenic Belt (XMOB) to the north. This implies that Late Triassic deposition was related to the final closure of the Paleo-Asian Ocean during the Middle Triassic and the rapid exhumation of the Su–Lu Orogenic Belt between the NCC and YC. In contrast, ∼ 88 % of sediments within the Early Jurassic Yihe Formation were sourced from the XMOB to the north, with the remaining ∼ 12 % from the NCC to the south. We therefore infer that rapid uplift of the XMOB and the onset of the subduction of the Paleo-Pacific Plate beneath Eurasia occurred in the Early Jurassic.

Geochronology and Geochemistry of the Early Permian A‐type Granite in the Hongol Area, Central Inner Mongolia: Petrogenesis and Tectonic Implications

We performed geochronological and geochemical analyses of the A‐type granite in the Hongol area, central Inner Mongolia, to determine its age, petrogenesis and tectonic setting, which are significant for clarifying the Late Paleozoic tectonic evolution of the Xing'an Mongolian Orogenic Belt (XMOB). The rock type of the A‐type granite in the Hongol area is alkali‐feldspar granite, and it constitutes a western part of the Baiyinwula–Dongujimqin A‐type granite belt. Zircon U–Pb geochronology yields 206Pb/238U ages ranging from 293 to 286 Ma for the alkali‐feldspar granite, indicating this granitic pluton formed in the Early Permian. The alkali‐feldspar granite is high in silica (SiO2=75.13wt%–80.17wt%), aluminum (Al2O3=10.59wt%–13.17wt%) and alkali (Na2O+K2O=7.33wt%–9.11wt%), and low in MgO (0.08wt%–0.39wt%) and CaO (0.19wt%–0.70wt%). It is obviously enriched in LILEs such as Rb, Th and K, depleted in HFSEs such as Nb, Ti, La and Ce, with pronounced negative anomalies of Nb, Ti, P, Eu, Sr and Ba. Its Sr–Nd–Pb isotopic compositions show positive εNd(t) (+0.72–+3.08), low TDM2 (805–997 Ma), and high radioactive Pb with (206Pb/204Pb)i of 18.710–19.304, (207Pb/204Pb)i of 15.557–15.604 and (208Pb/204Pb)i of 37.887–38.330. Petrological characteristics and geochemical data suggest that the alkali‐feldspar granite in the Hongol area belongs to aluminous A‐type granite. This A‐type granite formed in a post‐collisional extensional setting and was generated by the partial melting of felsic rocks in the middle‐lower crust resulting from post‐collisional slab breakoff. It is suggested that the Paleo‐Asian Ocean was closed before the Permian in central Inner Mongolia.

Zircon U–Pb and Wolframite Sm–Nd Dating of the Bayinsukhtu Tungsten Deposit in Southern Mongolia and its Geological Significance

AbstractThe Bayinsukhtu tungsten deposit is a newly discovered quartz‐vein tungsten deposit in the Xing'an–Mongolia Orogenic Belt (XMOB) in southern Mongolia, hosted by the Bayinsukhtu granite porphyry. The granite porphyry is located mainly south of the study area, over 3 km2. The rock consists of quartz and feldspar phenocrysts in a fine‐grained matrix, also mainly composed of feldspar and quartz. The granite porphyry samples demonstrate high SiO2 and high alkalinity. All samples also straddle the high‐potassium calc‐alkaline series. In a plot of the molar ratios of A/NK versus A/CNK, the granites are metaluminous. The chondrite‐normalized REE patterns are characterized by large negative Eu anomalies and fractionated LREEs. The U–Pb age of zircons from the granite porphyry is 298.8 ± 1.8 Ma, and the Sm–Nd age of the five wolframite samples from the tungsten deposit is 303 ± 19 Ma. The cooling age of the granite porphyry and tungsten mineralization is within the error of measurement and is of the Late Carboniferous age. Geological and geochronological evidence shows that the tungsten mineralization and the granite porphyry at Bayinsukhtu are genetically closely related and that they are results of Carboniferous magmatism. Their tectonic setting is related to the accretion of the Central Asian Orogenic Belt during the late Paleozoic era.

Hydrous parental magmas of Early to Middle Permian gabbroic intrusions in western Inner Mongolia, North China: New constraints on deep-Earth fluid cycling in the Central Asian Orogenic Belt

The role of fluids in the formation of the Permian-aged Xigedan and Mandula gabbroic intrusions in western Inner Mongolia was significant to the evolution of the Xing’an Mongolia Orogenic Belt (XMOB), and the active northern margin of the North China Craton (NCC). Secondary Ion Mass Spectroscopy (SIMS) U-Pb zircon geochronology establishes that the Xigedan gabbroic intrusion in the northern NCC was emplaced at 266Ma, and is therefore slightly younger than the ca 280Ma Mandula gabbroic intrusion in the XMOB. Along with their felsic counterparts, the mafic igneous intrusions record extensive bimodal magmatism along the northern NCC and in the XMOB during the Early to Middle Permian. The Mandula gabbroic rocks have low initial 87Sr/86Sr ratios (0.7040–0.7043) and positive εNd(t) (+6.2 to +7.3) and εHf(t) values (+13.4 to +14.5), resembling to those of contemporaneous Mandula basalts. These features, together with the presence of amphibole and the enrichment of large ion lithophile elements (LILE, e.g., Rb, Ba, U and Sr) and depletion of Nb-Ta suggest that the parental magmas of the Mandula mafic igneous rocks were derived from a depleted mantle source metasomatized by water-rich fluids. In contrast, the Xigedan gabbroic rocks are characterised by high 87Sr/86Sr ratios (0.7078–0.7080) and zircon δ18O values (5.84–6.61‰), but low εNd(t) (−9.3to−10.2) and εHf(t) values (−8.76to−8.54), indicative of a long-term enriched subcontinental lithosphere mantle source that was metasomatized by recycled, high δ18O crustal materials prior to partial melting. The high water contents (4.6–6.9wt%) and arc-like geochemical signature (enrichment of fluid-mobile elements and depletion of Nb-Ta) of the parental magmas of the Xigedan gabbroic rocks further establish the existence of a mantle hydration event caused by fluid/melts released from hydrated recycled oceanic crust. Incompatible element modelling shows that 5–10% partial melting of an enriched mantle source by adding respectively 0.5% and 2% sediment melts and fluids, could have produced the parental magmas of the Xigedan gabbroic rocks. A range of geological evidence establishes an intracontinental origin for Late Paleozoic mafic igneous rocks along the northern NCC and in the XMOB, rather than a subduction-related setting. We therefore propose a deep-Earth water cycling process to account for mantle hydration and subsequent Late Paleozoic magmatism, supporting a geodynamic link between deep-Earth water cycling, and intracontinental magmatism and lithospheric extension.

Late Carboniferous N-MORB-type basalts in central Inner Mongolia, China: Products of hydrous melting in an intraplate setting?

Petrogenesis of the ca. 310Ma Benbatu basalts in central Inner Mongolia is crucial for constraining the evolution of the Xing'an Mongolia Orogenic Belt (XMOB), eastern segment of the Central Asian Orogenic Belt. The Benbatu basalts have low initial 87Sr/86Sr ratios (0.7042–0.7048), positive εNd(t) (+8.99 to +9.24) and εHf(t) values (+15.38 to +15.65), and are characterized by relatively flat rare earth element patterns and enrichment of Rb, U, Pb, Zr and Hf, but depletion of Nb, Ta, Sr and Ti, resembling those of the normal Mid-Ocean-Ridge Basalt (N-MORB). Variations of trace element ratios (e.g., Sm/Yb and La/Sm) suggest that the basalts were derived from spinel peridotites, with a melting depth of <60–85km. The characteristics of enrichment of Zr and Hf and depletion of Sr distinguish the Benbatu basalts from typical arc basalts and back arc basin basalts. The arc-like geochemical signatures (i.e., enrichment of large ion lithophile elements and depletion of Nb and Ta) are attributed to hydrated mantle source that may be caused by fluids released from stagnant oceanic slabs in the mantle transition zone. Integrating geological evidences with geochemical and isotopic features of the Benbatu basalts, we proposed that these basalts were produced under an intraplate extensional setting during the Late Carboniferous. The genesis of the Benbatu basalts therefore argues for the pre-Carboniferous accretion of the XMOB and highlights the importance of the deep-Earth recycling water in the generation of the Late Carboniferous magmatism in this region.

Petrogenesis of Early–Middle Jurassic intrusive rocks in northern Liaoning and central Jilin provinces, northeast China: Implications for the extent of spatial–temporal overprinting of the Mongol–Okhotsk and Paleo-Pacific tectonic regimes

The Mesozoic tectonic evolution of NE China was controlled mainly by the Mongol–Okhotsk and Paleo-Pacific tectonic regimes. However, the extent of the spatial and temporal overprinting of these two regimes is poorly understood. Here, we report new zircon LA-ICP-MS U–Pb dating and geochemical analyses of Jurassic intrusive rocks in northern Liaoning and central Jilin provinces, northeast China, to discuss their petrogenesis and outline the extent of spatial and temporal overprinting of these two tectonic regimes. Dating results indicate that Jurassic magmatism occurred in two stages during the Early (ca. 175Ma) and Middle Jurassic (170–163Ma). These rocks represent two-stage typical bimodal igneous rock associations composed mainly of olivine gabbro, gabbro, and gneissic granitoids. The Early and Middle Jurassic gabbros have low rare earth element (REE) abundances, positive Eu anomalies, depletion in high field strength elements (HFSEs), and positive εHf(t) values (+4.0 to +10.3, except for one value of −17.8), suggesting that the primary magma was derived from partial melting of depleted lithospheric mantle metasomatized by subducted-slab-derived fluids. The Early Jurassic monzogranite exhibit high REE abundances (195–201ppm), weak negative Eu anomalies (δEu=0.63–0.64), and negative εHf(t) values (−11.9 to −8.2), suggesting a primary magma that was derived from partial melting of lower continental crust of the NCC. The Middle Jurassic granodiorites are enriched in light REEs (LREEs) and large ion lithophile elements (LILEs), and are depleted in heavy REEs (HREEs) and HFSEs, as well as high Sr/Y (29–132) and (La/Yb)N (15–44) ratios. In addition, the Middle Jurassic granitoids near or within the NCC exhibit negative εHf(t) values (−18.9 to +0.2), whereas those within the Xing'an–Mongolia Orogenic Belt (XMOB) have generally positive εHf(t) values (−0.6 to +6.4), suggesting their origin from partial melting of thickened ancient NCC and newly accreted continental crust of the XMOB, respectively.The above findings, combined with regional geologic data, suggest that the Early Jurassic bimodal igneous rocks formed in an extensional tectonic environment related to subduction of the Paleo-Pacific plate beneath the Eurasian continent. The Middle Jurassic bimodal igneous rocks possibly formed in an extensional environment related to the collapse of a thickened region of the lower crust after closure of the Mongol–Okhotsk Ocean.

Ordovician intrusive rocks from the eastern Central Asian Orogenic Belt in Northeast China: chronology and implications for bidirectional subduction of the early Palaeozoic Palaeo-Asian Ocean

ABSTRACTThe eastern segment of the Central Asian Orogenic Belt is traditionally called the Xing’an Mongolia Orogenic Belt (XMOB). Ordovician intrusive rocks exposed in the XMOB, from north to south, are the Abaga-East Ujimqin Qi-Duobaoshan belt, the Sonid Zuoqi-West Ujimqin Qi belt, and the Damaoqi-Baimaimiao-Tulinkai belt, respectively. Zircon U–Pb dating and geochemical data are presented for the intrusive rocks in East Ujimqin Qi and West Ujimqin Qi, Inner Mongolia. The intrusive rocks from East Ujimqin Qi consist of gabbro, diorite, and granodiorite. LA-MC-ICP-MS zircon U–Pb ages range 446 to 461 Ma. Geochemical data suggest that the gabbros and diorites from East Ujimqin are a tholeiitic series, both of arc-related and N-MORB (mid-ocean ridge basalt) signature, indicating a back-arc basin setting. The granodiorites have a shoshonitic series and arc-related signature. Rare earth element (REE) patterns and trace element characteristics suggest gabbros, diorites, and granodiorites are petrogenetically correlated. These intrusive rocks from East Ujimqin Qi have high light REE, Th, and U concentrations, suggesting the effect of middle–upper continental crustal contamination. Major oxides display positive or negative correlations, with increasing MgO or SiO2, indicating that fractional crystallization occurred during magma evolution. Geochemical data of diorite from West Ujimqin Qi indicate a tholeiitic series, arc-related signature. Zircon U–Pb dating yielded an age of 441.8 ± 1.5 Ma. Integrated with the regionally exposed Mid–Late Ordovician plutons and metasedimentary strata, we concluded that the northward subduction of the Palaeo-Asian Ocean (PAO) that occurred beneath the southern margin of the South Mongolian Micro-continent along the Sonid Zuoqi-Xilinhot gave rise to early Palaeozoic igneous rocks from the Abaga–East Ujimqin Qi–Duobaoshan and the Sonid Zuoqi–West Ujimqin Qi belts. Southward subduction beneath the North China Craton generated the Damaoqi–Baimaimiao–Tulinkai belt. The results support the bidirectional subduction model of the PAO in the early Palaeozoic.

Geochronology and geochemistry of late Carboniferous volcanic rocks from northern Inner Mongolia, North China: Petrogenesis and tectonic implications

Zircon U–Pb ages, geochemical and Sr–Nd isotopic data are presented for the late Carboniferous Baoligaomiao Formation (BG Fm.) and Delewula Formation (DW Fm.) volcanic rocks, widely distributed in northern Inner Mongolia, in the northern part of the Xing'an–Mongolia Orogenic Belt (XMOB). The BG Fm. rocks mainly consist of basaltic andesites and andesites while the DW Fm. rocks include dacites, trachytes, rhyolites, pyroclastic rocks and minor andesites. New LA-ICPMS zircon U–Pb analyses constrain their eruption to late Carboniferous (317–322Ma and 300–310Ma, respectively). The BG Fm. volcanic rocks are characterized by enriched large ion lithophile elements (LILE) and depleted high field strength elements (HFSE), with initial 87Sr/86Sr ratios of 0.70854–0.70869 and negative εNd(t) (−2.1 to −2.4) values. They have low La/Ba (0.03–0.05), high La/Nb (2.05–3.70) ratios and variable Ba/Th (59.5–211) ratios. Such features suggest that they are derived from melting of heterogeneous sources including a metasomatized mantle wedge and Precambrian crustal material. The DW Fm. volcanic rocks are more depleted in HFSE with significant Nb, Ta, P, Ti anomalies. They have high initial 87Sr/86Sr ratios (0.72037–0.72234) and strong negative εNd(t) (−11 to −11.6) values which indicate those igneous rocks were mainly derived from reworking of the Paleoproterozoic crust. The late Carboniferous volcanic rocks have geochemical characteristics similar to those of the continental arc rocks which indicate the northward subduction of the Paleo Asian Ocean may have continued to the late Carboniferous. The volcanic association of this study together with the early Permian post-collisional magmatic rocks suggests that a tectonic transition from subduction-related continental margin arc volcanism to post-collisional magmatism occurred in the northern XMOB between the late Carboniferous and the early Permian.

Pyrite Re–Os and zircon U–Pb dating of the Tugurige gold deposit in the western part of the Xing'an–Mongolia Orogenic Belt, China and its geological significance

The Tugurige gold deposit, in the northeastern margin of the North China Craton (NCC) and western part of the Xing'an–Mongolia Orogenic Belt (XMOB), is a large tonnage quartz-vein type deposit with a resource of ~24t at about 4g/t Au. Gold mineralization is hosted by porphyritic syenogranite and controlled by NW-trending faults. Our study provides the first precise age for the Tugurige gold mineralization and host porphyritic syenogranite. The U–Pb age of zircons from the syenogranite is 276±2Ma, and Re–Os dating of six pyrite separates from the deposit yields an isochron age of 268±15Ma. The results show that the ages of syenogranite and gold mineralization are the same within error. Furthermore, geological and geochronological evidence indicate that the syenogranite and gold mineralization have very close genetic relationships and that they are products of Permian magmatism related to post-collisional extension of the Central Asian Orogenic Belt (CAOB). The initial Os isotopic compositions (187Os/188Os=1.26±0.69) of pyrite indicate that the source of Os, and by inference other ore-related materials, are derived from the crust. The Late Paleozoic is an important period in which economic gold mineralization was deposited in the CAOB.

Permian magmatic sequences of the Bilihe gold deposit in central Inner Mongolia, China: Petrogenesis and tectonic significance

The Bilihe gold deposit is located in the eastern section of the Ondor Sum–Yanji Suture at the southern margin of the Xing'an–Mongolian Orogenic Belt (XMOB) and the northern margin of the North China Craton (NCC), central Inner Mongolia. The magmatic rocks in the ore district are generally high-K calc-alkaline, enriched in LREE, Zr, and Hf, and depleted in HREE, Nb, Ta, and P. The magmatic evolution sequences are norite gabbro→granodiorite porphyry→granite or norite gabbro→andesite→dacite porphyry→granodiorite, which show a trend of decreasing TiO2, FeO, MgO, CaO, and P2O5 with increasing SiO2. In the Bilihe ore district, hydrothermal processes were coeval with granitic magmatism for a period of ~17Myr (272–255Ma). The ages of the granite, granodiorite porphyry, granodiorite, and dacite porphyry are 271.5–264.1Ma, 269.8–255.8Ma, 268.3Ma, and 268.6–259.4Ma, respectively. The magmatic rocks contain magmatic, hydrothermal, and magmatic–hydrothermal zircons. The magmatic zircons have δCe>4, La<3ppm, and SmN/LaN>2.5; the hydrothermal zircons have δCe<4, La>3ppm, and SmN/LaN<2.5. The Nb/Ta and Zr/Hf ratios of granodiorite are 12.7–14.99 and 40.2–46.56, respectively. The Zr/Hf ratios successively increase in the sequence of granite (27.4–29.02)→granodiorite porphyry (29.19–32.18)→dacite porphyry (33.54–38.5)→norite gabbro (36.75–38.37), and their Nb/Ta ratios are 9.09–12.38. Zircons in granodiorite yield ε Hf (t) values of −0.29 to −56 (n=13) and 2.07–7.62 (n=5), and they give a Hf two-stage model age (tDM2) of 807–4765Ma. The ε Hf (t) values of the zircons in granite, granodiorite porphyry, and dacite porphyry are −0.46 to 8.03, 3.17 to 10.32, and −0.78 to 6.58, respectively, and their Hf tDM2 ages are 787–1324Ma, 638–1091Ma, and 868–1343Ma, respectively. Dehydration partial melting of subducted oceanic crust resulted in the formation of dacite porphyry; partial melting of depleted mantle resulted in the formation of norite gabbro; mixing of depleted mantle and lower crust resulted in the formation of granodiorite porphyry; partial melting of lower crust resulted in the formation of granite; and mixing of lower crust and old upper crust resulted in the formation of granodiorite. Magmatic rocks in the ore district with ages of 272–255Ma were formed during the late stages of closure of the Paleoasian Ocean; i.e., during the transformation from a collisional to extensional setting.

The age, geological setting, and types of gold deposits in the Yanbian and adjacent areas, NE China

The eastern part of the Xing'an–Mongolian Orogenic Belt (XMOB) in Jilin and Heilongjiang Provinces, NE China, is highly prospective for gold mineralization, and more than ten gold and gold–copper deposits have been found in the Yanbian and Dongning areas in the last decade. These deposits are classified, based on geological characteristics and mineralogical and geochemical information, into orogenic, intrusion-related, gold-rich porphyry, and epithermal deposits; the latter are further divided into high-sulfidation (HS) and low-sulfidation (LS) deposit types.We present the results of zircon laser ablation U–Pb dating of some representative gold deposits and associated magmatic rocks within the eastern XMOB; these are the orogenic Wudaogou deposit, the intrusion-related Naozhi deposit, and the epithermal Jiusangou gold deposit. The weighted mean age of the granodiorite that hosts mineralization at Wudaogou is 253.1±0.3Ma, whereas zircons from a porphyritic dacite dike that was intruded at the same time as the formation of the Naozhi deposit yielded two groups of weighted mean ages (198.9±3.6 and 126.3±1.7Ma). Zircons from a porphyritic quartz diorite within the breccia that hosts the Jiusangou deposit yielded a weighted mean age of 108.1±1.4Ma.These data, combined with data from previous studies, enable the identification of three periods of Phanerozoic gold mineralization within the Yanbian area and adjacent regions; these are ca. 270–240, ca. 130–120, and ca. 115–100Ma. The Late Permian to Early Triassic (270–240Ma) events formed the orogenic gold deposits at Yangjingou and Wudaogou; both are located near the Xar Moron–Changchun Suture Zone between the North China Craton and the XMOB. The Early Cretaceous (130–120Ma) events produced intrusion-related gold mineralization, such as the Naozhi and Miantian deposits, which are associated with Jurassic granodiorite plutons or volcanic rocks. Almost all of the gold-rich porphyry and epithermal gold deposits in this area formed during the late Early Cretaceous (115–110Ma). We propose that the orogenic gold mineralization in the Yanbian area was closely related to the collision between the North China Craton and the eastern XMOB, whereas the other gold deposit types formed at Early Cretaceous lithospheric extensions associated with post-subduction tectonics.