Bainaimiao Arc Research Articles

Geochronological data of igneous and metamorphic rocks from the Xing’an-Mongolia Orogenic Belt of the eastern Central Asian Orogenic Belt: implications for the final closure of the Paleo-Asian Ocean

In order to better constrain the evolution of the Xing’an-Mongolia Orogenic Belt and the resulting closure of the Paleo-Asian Ocean, we conducted an integrated investigation involving U-Pb dating of igneous and detrital zircon and a synthesis of existing work across the Erdaojing and Ondor Sum complexes in the eastern segment of the Central Asian Orogenic Belt. Geochronological analysis revealed a different tectonic setting between the Ondor Sum complex (Tulinkai area) and the Erdaojing accretion complex, as detrital zircon U-Pb analysis identified different major age provenances: (1) the Precambrian basement of the North China Craton and Paleozoic Bainaimiao Arc along the northern margin of North China to the south are the provenance rocks of the schists in the Tulinkai area, and (2) the Precambrian basement of the Xilinhot complex and Paleozoic Baolidao arc to the north are the provenance rocks of the schists in the Erdaojing area. We infer that the existence of the ocean between the above two regions in the late of Early Paleozoic in accordance with the result of this study. Furthermore, a comparison of the youngest age population in the intruded-diorite rock and metasedimentary rocks with U-Pb ages implies that the final closure of the Paleo- Asian Ocean along the Solonker Suture Zone most likely occurred in the period between 239 and 222 Ma (Late Paleozoic-Triassic).

Provenance of Late Carboniferous bauxite deposits in the North China Craton: New constraints on marginal arc construction and accretion processes

The North China Craton (NCC) is bounded by two Paleozoic accretionary arc terranes: the North Qinling terrane to the south and the Bainaimiao terrane to the north. The timing of arc accretion to the NCC and the architecture of the Bainaimiao arc remain unclear. During the building and accretion of the arcs along its margins, the NCC experienced a long sedimentary hiatus since the Ordovician, which ended with the deposition of bauxite-bearing sediments in the Late Carboniferous. In this paper we report the U–Pb and Hf isotopes of detrital zircons from the Late Carboniferous bauxite layer and use these data to constrain the tectonic evolution of the margin of the NCC. The detrital zircons yield a minimum U–Pb age of ca. 310Ma and a prominent age peak at ca. 450Ma. Zircon crystals with ages of ca. 330Ma and ca. 1900Ma are more common in the bauxite samples from the northern part of the NCC than in those from the central part. The εHf(t) values of the ca. 450Ma detrital zircon crystals of the bauxite samples from the NCC are similar to those of the contemporaneous detrital zircon crystals from the North Qinling arc terrane to the south, but different from those of the contemporaneous detrital zircon crystals from the Bainaimiao arc terrane to the north. The ca. 450Ma detrital zircon crystals in the ca. 310Ma bauxite deposits are therefore interpreted to have been derived from the North Qinling arc terrane. The source of the ca. 330Ma detrital zircon crystals of the bauxite deposits is interpreted to be the northern margin of the NCC, where intermediate-felsic plutons formed at ca. 330Ma are common. The results from this study support the interpretation that the Paleozoic continental arc terranes and their concomitant back-arc basins were developed along the margins of the NCC before ca. 450Ma, and the arc complexes were subsequently accreted to the craton in the Late Carboniferous. This was then followed by the formation of a walled continental basin within the NCC.

Neoarchaean quartz diorites in the Jiefangyingzi area, Central Asian Orogenic Belt: geological and tectonic significance

Quartz diorite intrusions in the Jiefangyingzi area associated with deformed Palaeozoic rocks of the Palaeozoic Bainaimiao arc magmatic belt on the northern margin of the North China Craton (NCC) were studied to determine their age, chemical composition, and isotopic characteristics. U–Pb dating of magmatic zircons indicates that the quartz diorites formed in Neoarchaean time between 2502.6 ± 9.1 Ma and 2551 ± 7.3 Ma. The quartz diorites have high Al2O3 and low K2O contents, A/CNK = 0.75–0.97, and belong to the low-K tholeiitic series. The quartz diorites are enriched in light rare earth elements (LREEs) with high (La/Yb)N ratios and exhibit weak positive or no Eu anomalies, characteristics of high-alumina tonalite–trondhjemite–granodiorite (TTG) igneous rocks. Zircon εHf(t) value for the quartz diorites ranges from +1.6 to +8.7, and the two-stage Hf-depleted mantle model age (TDM) ranges from 2705 to 2744 Ma, suggesting that the quartz diorite was derived from melting juvenile Neoarchaean crust formed from partial melting of the mantle at 2.7 Ga. Amphibolite xenoliths have low REE concentrations and are moderately depleted in LREE with (La/Yb)N ratios of 0.46–1.09. The trace element characteristics of the amphibolites are consistent with a mid-ocean-ridge basalt (MORB)-like protolith. This is the first time that Archaean rocks have been identified in the Bainaimiao arc magmatic belt and the age and nature of Jiefangyingzi quartz diorites suggest that they belonged to the NCC. The Early Palaeozoic Bainaimiao arc thus appears to represent an Andean-type continental arc on the northern margin of the NCC.

Devonian magmatism associated with arc-continent collision in the northern North China Craton: Evidence from the Longwangmiao ultramafic intrusion in the Damiao area

The northern margin of the North China Craton (NCC) witnessed extensive felsic and mafic magmatism during late Paleozoic to early Mesozoic. Here we investigate the ultramafic complex of Longwangmiao in Damiao comprising clinopyroxenite and hornblendite intrusion. The rocks are dominantly composed of clinopyroxene and hornblende similar to Alaskan-type intrusions. We present zircon U–Pb age data for zircon grains from the hornblendite and clinopyroxenite which show weighted mean 207Pb/206Pb ages of 382±10Ma and 399.1±4.4Ma respectively. The hornblendite and clinopyroxenite show similar rare earth element patterns indicating a co-magmatic nature and formation through differentiation and accumulation. The parental magma for this intrusion is inferred to be island-arc picrite in composition possibly derived from low degree partial melting of garnet-peridotite facies at the boundary of slightly enriched sub-continental lithospheric mantle and crust. The hydrous nature of the magma is correlated with subduction of the Mongolian oceanic slab beneath the NCC. The magma evolved into basaltic composition through input of silica-rich crustal components as suggested by the zircon εHf(t) compositions that plot between the evolution array of the 2.5–3.0Ga Neo- to Mesoarchean basement rocks and presumed sub-continental lithospheric mantle array. The enrichment in LILEs (e.g., Rb, K, Ba, Pb, and Sr) and LREEs, and conspicuous depletion of HFSEs (e.g., Nb, Zr, Ti) suggest an Andean-type continental arc signature. The tectonic setting for the final emplacement of the Devonian intrusion is correlated with the transformation from arc-related compression to back-arc extension as evidenced by the dual IAB and CRB signature of these rocks. The post-collisional extensional setting might be related to the arc-continent collision between the Bainaimiao arc and the northern NCC during the Late Silurian.

Age, igneous petrogenesis, and tectonic setting of the Bilihe gold deposit, China, and implications for regional metallogeny

The Bilihe gold deposit, along the northern margin of the North China Craton (NCC), is hosted in Permian intrusions that were emplaced into Bainaimiao Group greenschist- to amphibolite-facies Mesoproterozoic basement rocks, the early Paleozoic Bainaimiao Arc, and late Paleozoic sedimentary and volcanic cover rock sequences. The Middle Ordovician to Early Silurian calc-alkaline arc comprises volcanic sequences of basalt, andesite, and felsic lavas, and an intrusive complex ranging from gabbro through granite. The deposit is genetically related to ilmenite-series and highly fractionated quartz diorite porphyry host rocks. The most fractionated variety of the quartz diorite porphyry is granitic, which contains dendritic quartz phenocrysts and occurs at the top part of the intrusions. Gold mostly occurs as Au grains (>990 fineness) in trails in dendritic quartz, and the gold trails typically occur along certain crystallographic elements of dendritic quartz. The gold is believed to have crystallized directly from magma at temperatures of at least 750–800°C. The Fe2O3/(Fe2O3+FeO) ratio of the quartz diorite ranges from 0.21 to 0.29, and there is significantly more ilmenite than magnetite, suggesting that the magma was moderately reduced. There are also post-ore syenogranite porphyries in the deposit area. Zircon U-Pb LA-ICP-MS dating in this study shows that two samples of the volcanic wallrock have ages of 274±3Ma and 270±4Ma, two samples of gold-forming quartz diorite porphyries have ages of 261±2Ma and 259±3Ma, with 440–400Ma and 1344–1316Ma cores, and one syenogranite porphyry sample is dated at 253±3Ma.The mineralized quartz diorite porphyries exhibit a high-K calc-alkaline composition, high Mg# (46–51), and a high content of compatible elements (V: 114–164ppm; Cr: 80–93ppm; Ni: 37–42ppm) and Rb, Th, and U, as well as depletions in Ba, Sr, and HFSE (e.g., Ti, Nb, and Ta), suggesting melt from a slab fluid-metasomatized enriched mantle. The LREE enrichment, the intermediate oxidation state, and the presence of the Silurian and Mesoproterozoic zircon cores indicate that crustal materials also played a significant role in porphyry formation. In addition, the rocks have slightly negative εNd(t) (−2.5–0.1), but variable (87Sr/86Sr)i (0.70328–0.70573). Based on the above features, we propose that quartz diorite porphyries were produced by partial melting of the metasomatized mantle coupled with the assimilation of crustal materials of the Bainaimiao Group, Late Ordovician-Silurian subduction-related plutons, and Late Carboniferous-Early Permian marine sedimentary rocks. The assimilation of rocks of the Bainaimiao Group, which have an elevated gold content of 26.3ppb, is speculated to have played important roles in the formation of a rarely documented magmatic gold deposit. In contrast, syenogranite porphyries are characterized by high contents of K2O+Na2O (10.3–10.6wt%), Al2O3 (16.3–16.5wt%), K, Hf, and Zr; low CaO (0.6–1.2wt%), Mg# values (17–21), compatible elements (V: 16–21ppm; Cr: 4.9–7.9ppm; Ni: 2.8–4.5ppm), and Eu, Sr, and HFSE; and positive εNd(t) (1.4–1.5) with low (87Sr/86Sr)i (<0.70431), indicating that the porphyries were formed by partial melting of the subduction-modified lower crust with a minor input of asthenospheric materials.It is concluded that the quartz diorite porphyries formed during the southward subduction of the Paleo-Asian ocean underneath the northern margin of the NCC. This confirms that the northern margin was not a passive continental margin, but an active margin with subduction and development of a continental arc. In addition to Bilihe, there are at least four more gold deposits (Hadamiao, Changshanhao, Saiyinwusu, and Chaihulanzi) that formed at the northern margin of the NCC from Middle Permian to Early Triassic. Such a timing indicates that they may also be ultimately related to the tectonic regime imposed by southward subduction of the Paleo-Asian ocean. Based on the new understanding of the tectonic setting, with significant known mineral occurrences along the length of the margin, it is inferred that the >1000-km-long northern margin of NCC may have a large potential for subduction-related mineralization.

U–Pb, 39Ar/40Ar geochronology of the metamorphosed volcanic rocks of the Bainaimiao Group in central Inner Mongolia and its implications for ore genesis and geodynamic setting

The greenschist facies volcano-sedimentary sequence of the Bainaimiao Group in central Inner Mongolia hosts the Bainaimiao Cu–Mo–Au deposit and the Bainaimiao lode gold deposit. But the ore classification and genetic process of the Bainaimiao Cu–Mo–Au deposit is still disputed. In this study we have selected the interbedded metamorphosed volcanic rocks of the Bainaimiao Group for LA-ICP-MS zircon U–Pb analysis. 17 analyses form a tight cluster on the concordia and yield a weighted mean 206Pb/238U age of 465±1Ma (95% confidence level, MSWD=0.95, n=17), which represents the best estimate of the crystallization age of the volcanic rocks of the Bainaimiao Group. Biotite from the greenschist of the Bainaimiao Group yielded a well-defined plateau with an age of 429.2±4.1Ma. The inverse isochron age is 429.1±4.2Ma with an elevated (40Ar/36Ar)i ratio of 299.9±22.7, which represents the time of regional greenschist facies metamorphism. Previous studies showed that the Re–Os isochron age of the molybdenite from the Bainaimiao Cu–Mo–Au deposit is 445.0±3.4Ma and the mineralized granodiorite porphyry yielded a weighted mean 206Pb/238U age of 445±6Ma. These data indicate that the Bainaimiao volcanic rocks were formed at about 465Ma and the porphyry Cu–Mo–Au mineralization took place at about 445Ma.Volcanic rock emplacement is 20Ma older than the Cu–Mo–Au mineralization indicates it is a porphyry deposit rather than VMS deposit. The dynamic setting for the porphyry mineralization is the Paleo-Asian Oceanic plate subduction along the northern margin of the North China Craton in early Paleozoic. Subsequently, the collision between the Bainaimiao arc and the North China Craton led to regional metamorphism and deformation at about 430Ma and the old porphyry system may have been overprinted during this metamorphic process.

Origin and evolution of the Bainaimiao arc belt: Implications for crustal growth in the southern Central Asian orogenic belt

Recent results show that evolution of the huge Central Asian orogenic belt can be explained in terms of southwest Pacific–style accretion of arcs and microcontinents. A better understanding of the origin and evolution of arcs and microcontinents will enhance our knowledge on the evolution of the Central Asian orogenic belt. As one of the most important early Paleozoic arc systems south to the Solonker suture zone, the origin and evolution of the Bainaimiao arc belt are still not well constrained. New zircon U-Pb geochronological and geochemical results on magmatic rocks in the Bainaimiao arc belt indicate that the arc was active from 0.52 Ga to 0.42 Ga and can extend to east Siping in NE China. Zircon U-Pb geochronological results of metasedimentary rocks in the Bainaimiao arc belt indicate that they are early Paleozoic in age, not Precambrian as previously regarded. Detrital zircon analysis of metasedimentary rocks and Sr-Nd-Hf geochemical results of magmatic rocks indicate the existence of some Proterozoic basement rocks beneath the Bainaimiao arc, and it was built upon a Precambrian microcontinent that has a tectonic affinity to the Tarim or Yangtze cratons. The Bainaimiao arc is an ensialic island arc characterized by different evolution history and basement compositions from the northern North China craton. It was separated by a wide ocean from the northern North China craton during the Cambrian–Ordovician period. Successive northward subduction resulted in contraction of the ocean and final accretion of the Bainaimiao arc to the northern North China craton during the Late Silurian–earliest Devonian by arc-continent collision. Arc-continent collision could be an important mechanism for continental crustal growth and formation of the huge Central Asian orogenic belt.

Crustal structure of the northern margin of the North China Craton and adjacent region from SinoProbe02 North China seismic WAR/R experiment

Abstract The SinoProbe02 North China seismic experiment consisted of three seismic recording efforts along a 450 km long profile. The profile extended from northwest of Beijing, across the northern margin of the North China Craton (NCC), and across the southeastern portion of the Central Asian Orogenic Belt (CAOB) to the Solonker suture zone. We processed, analyzed, and modeled the data collected during a wide-angle reflection and refraction effort. The recording of seismic waves from 8 explosions was conducted in 4 deployments of 300 Texan recorders with station spacings of 1–1.5 km. The P-wave field provided good quality data for most of the profile. Arrivals of refracted and reflected waves from sediments and basement (Pg), intracrustal phases (PcP, PlP), and the Moho (PmP, Pn) were typically observed. Travel time modeling was done layer-by-layer using the top to bottom approach and was constrained by a coincident deep reflection seismic profile. The initial velocity model was altered by trial and error, and the forward model was updated by damped least-squares inversion. In our modeling, calculated travel times fit observed arrivals for all 1439 traces with an average RMS error of 0.113 s. The final 2-D P-wave velocity model contains many features of tectonic significance: (1) the velocity structure varies significantly from the NCC to the CAOB; (2) thicker crust appears beneath the Yinshan–Yanshan belt and was probably generated by compression in the early Jurassic and modified during the craton's destabilization and extension; the flat and relatively shallow Moho in the CAOB may be attributed to the extension; (3) strong velocity variations beneath the Bainaimiao arc and Ondor Sum subduction accretion complex indicate that multiple pulses of magmatism occurred during the complex tectonic evolution of this area.

Magmatic and metamorphic development of an early to mid-Paleozoic continental margin arc in the southernmost Central Asian Orogenic Belt, Inner Mongolia, China

The Bainaimiao arc in Inner Mongolia, China, comprises a weakly metamorphosed volcani-sedimentary sequence and a low-P/T metamorphic complex. In this study we present SHRIMP zircon ages and geochemical data to document the temporal and genetic relationships between these two tectonic units. Zircons from a rhyolite and two dacites (high-K calc-alkaline) of the volcani-sedimentary sequence yielded 206Pb/238U ages of 474±7Ma, 453±7Ma and 436±9Ma respectively, that we interpret as recording the timing of three volcanic arc episodes. Rocks from the low-P/T metamorphic complex yielded zircon ages of 462±11Ma for a sillimanite gneiss and 437±5Ma for a plagioclase–hornblende gneiss, reflecting two distinct anatectic events. Also from the low-P/T complex, the protolith age of a metadiorite is 438±2Ma. A diorite from a weakly deformed diorite–granodiorite pluton in the low-P/T complex has a zircon age of 419±10Ma that correlates in time with regional collisional magmatism. An undeformed pegmatite dike cutting the low-P/T metamorphic complex has an age of 411±8Ma, which postdates collision and provides an upper limit for the termination of orogeny. Accordingly, we conclude that the volcani-sedimentary sequence and the low-P/T metamorphic complex evolved coherently in the early to mid-Paleozoic and formed an integral continental margin arc in the southernmost Central Asian Orogenic Belt.

U–Pb and Re–Os geochronology of the Bainaimiao Cu–Mo–Au deposit, on the northern margin of the North China Craton, Central Asia Orogenic Belt: Implications for ore genesis and geodynamic setting

The Bainaimiao Cu–Mo–Au deposit, located at the northern margin of the North China Craton (NCC) in central Inner Mongolia, is a large Cu-dominant deposit. Orebodies are hosted in strongly deformed and metamorphosed granodiorite porphyry intrusions or in EW-trending shear zones in greenschist and sericite felsic schist of the Bainaimiao Group. Mineralization can be divided into early, middle and late stages, characterized by quartz–pyrite, polymetallic quartz–sulfides and carbonate–quartz, respectively. Hydrothermal activity led to pervasive silicification, K-feldspar, biotite and chlorite–epidote–carbonate alteration.Eight samples from Cu–Mo-bearing quartz veins and massive sulfide ores in greenschist were collected from the VIII orebody in the North ore belt and molybdenite separates were selected for Re–Os analyses. The concentrations of Re and 187Os range from 216.6 to 1051ppm and 2302 to 4913ppb, respectively. All samples give Re–Os model ages between 441.2 and 445.9Ma and yielded an isochron age of 445.0±3.4Ma (MSWD=0.26), with an initial 187Os of −4±17ppb. A granodiorite porphyry sample for SHRIMP U–Pb dating was collected from the underground of the XIII orebody of the North ore belt. Spots from 14 zircon grains have 206Pb/238U values between 426.2 and 462.9Ma and form a tight cluster on Concordia yielding a weighted mean 206Pb/238U age of 445±6Ma. This is considered to be the best estimate of crystallization age of the granodiorite porphyry. The Re–Os isochron age of the molybdenite is consistent with the SHRIMP U–Pb age of the granodiorite porphyry, which indicates that the deposit is a porphyry system of Paleozoic age.According to geodynamic setting and geological history of the northern margin of the NCC, the deposit was probably related to magmatic activity of the Middle Paleozoic Ondor Sum subduction–accretion complex-Bainaimiao arc. During the Devonian, the Bainaimiao arc was accreted to the northern margin of the NCC. Collision between the Bainaimiao arc and NCC led to regional metamorphism of the ore-hosting terrane and ore metals were remobilized in this process. Gold mineralization was probably overprinted onto the earlier Cu–Mo mineralization.

Accretion leading to collision and the Permian Solonker suture, Inner Mongolia, China: Termination of the central Asian orogenic belt

The Solonker suture records the termination of the central Asian Orogenic Belt (CAOB). However, tectonic development of the Solonker suture is poorly understood. We report new field data for the Ondor Sum melange in the Ulan valley, and present a new evaluation of the orogenic belt extending from the southern Mongolia cratonic boundary to the north China craton within the context of a new geological framework and tectonic model, which incorporates relevant data from the literature. The southern accretionary zone between the north China craton and the Solonker suture is characterized by the Mid‐Ordovician‐Early Silurian Ulan island arc‐Ondor Sum subduction‐accretion complex and the Bainaimiao arc. This zone was consolidated by the Carboniferous‐Permian when it evolved into an Andean‐type magmatic margin above a south dipping subduction zone. The northern accretionary zone north of the Solonker suture extends southward from a Devonian to Carboniferous active continental margin, through the Hegenshan ophiolite‐arc accretionary complex to the Late Carboniferous Baolidao arc associated with some accreted Precambrian blocks. This northern zone had consolidated by the Permian when it developed into an Andean‐type magmatic margin above a north dipping subduction zone. Final subduction of the central Asian ocean caused the two opposing active continental margins to collide, leading to formation of the Solonker suture in the end‐Permian. Predominant northward subduction during final formation of the suture gave rise in the upper northern plate to a large‐scale, postcollisional, south directed thrust and fold belt in the Triassic‐Jurassic. In summary, the CAOB underwent three final stages of tectonic development: early Japanese‐type accretion, Andean‐type magmatism, and Himalayan‐type collision.

The influence on EM-anomalies of the position of the generator in a Turam loop

The Bainaimiao Cu–Mo–Au deposit, located at the northern margin of the North China Craton (NCC) in central Inner Mongolia, is a large Cu-dominant deposit. Orebodies are hosted in strongly deformed and metamorphosed granodiorite porphyry intrusions or in EW-trending shear zones in greenschist and sericite felsic schist of the Bainaimiao Group. Mineralization can be divided into early, middle and late stages, characterized by quartz–pyrite, polymetallic quartz–sulfides and carbonate–quartz, respectively. Hydrothermal activity led to pervasive silicification, K-feldspar, biotite and chlorite–epidote–carbonate alteration.Eight samples from Cu–Mo-bearing quartz veins and massive sulfide ores in greenschist were collected from the VIII orebody in the North ore belt and molybdenite separates were selected for Re–Os analyses. The concentrations of Re and 187Os range from 216.6 to 1051 ppm and 2302 to 4913 ppb, respectively. All samples give Re–Os model ages between 441.2 and 445.9 Ma and yielded an isochron age of 445.0 ± 3.4 Ma (MSWD = 0.26), with an initial 187Os of − 4 ± 17 ppb. A granodiorite porphyry sample for SHRIMP U–Pb dating was collected from the underground of the XIII orebody of the North ore belt. Spots from 14 zircon grains have 206Pb/238U values between 426.2 and 462.9 Ma and form a tight cluster on Concordia yielding a weighted mean 206Pb/238U age of 445 ± 6 Ma. This is considered to be the best estimate of crystallization age of the granodiorite porphyry. The Re–Os isochron age of the molybdenite is consistent with the SHRIMP U–Pb age of the granodiorite porphyry, which indicates that the deposit is a porphyry system of Paleozoic age.According to geodynamic setting and geological history of the northern margin of the NCC, the deposit was probably related to magmatic activity of the Middle Paleozoic Ondor Sum subduction–accretion complex-Bainaimiao arc. During the Devonian, the Bainaimiao arc was accreted to the northern margin of the NCC. Collision between the Bainaimiao arc and NCC led to regional metamorphism of the ore-hosting terrane and ore metals were remobilized in this process. Gold mineralization was probably overprinted onto the earlier Cu–Mo mineralization.