Ondor Sum Research Articles

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.

Permian–Triassic amalgamation of Asia: Insights from Northeast China sutures and their place in the final collision of North China and Siberia

The Central Asia Orogenic Belt (CAOB) corresponds to the domain where Siberia and Mongolia were welded to North China. The eastern extension of the CAOB in Northeast China is disputed, since both suture location and timing are poorly documented. This paper reports for the first time the recognition of two suture zones in the southern part of Northeast China (Manchuria), between the Fushun Mishan and Yilan-Yitong faults. In the Jilin Province, west-directed thrust sheets involving successively, from west to east, passive continental margin rocks, metamorphic rocks and ophiolites, block-in-matrix formations and arc plutons indicate a Permian–Early Triassic collision. In the Liaoning Province, arc plutonism and top-to-the-north ductile shearing, coeval with the emplacement of an ophiolitic nappe, suggest a Palaeozoic collision. These two sutures are correlated with the Ondor Sum and Solonker sutures, described in Inner Mongolia. A new geodynamic model involving rifting and collision of the southern part of the Xilinhot Block with North China is proposed.

Phanerozoic evolution of the Inner Mongolia–Daxinganling orogenic belt in North China: constraints from geochronology of ophiolites and associated formations

Abstract The Inner Mongolia–Daxinganling Orogenic Belt (IMDOB), located between the North China and South Mongolia Blocks, consists of several ENE–WSW- to NE–SW-trending zones including dismembered ophiolite blocks, metamorphic rocks and granitoids. Although numerous studies have been carried out on this belt, its tectonic evolution has been a subject of controversy, chiefly because of the lack of reliable geochronological data. Based on a synthesis of newly published geochronological data and our unpublished data for the IMDOB, we define two oceanic basins: Ondor Sum and Hegenshan. The former, probably the main one, was initiated during the Ordovician (>467 Ma) period, whereas the latter, representing a back-arc basin, opened on a pre-Permian basement at, or earlier than, Early Permian times ( c . 295 Ma). These two oceanic basins were separated by a magmatic arc (Sunid–Baolidao), and were probably closed simultaneously when the final orogenesis of the IMDOB occurred during the Triassic period (240–220 Ma). Importantly, the Triassic timing of the final orogenesis of the IMDOB due north of the North China Craton is essentially coeval with that of the Qinling–Dabie–Su–Lu orogenic belt on the southern margin of the North China Craton. It is inferred that this two-sided subduction–collision scenario in the Triassic may have contributed to the Mesozoic lithospheric thinning event of the North China Craton, although the details are unclear.

Ordovician 40Ar/39Ar phengite ages from the blueschist-facies Ondor Sum subduction-accretion complex (Inner Mongolia) and implications for the early Paleozoic history of continental blocks in China and adjacent areas

We obtained 453.2 ± 1.8 Ma and 449.4 ± 1.8 Ma (2σ) laser step-heating ⁴⁰Ar/³⁹Ar plateau ages for phengite from quartzite mylonites from the blueschist-facies Ondor Sum subduction-accretion complex in Inner Mongolia (northern China). These ages are within error of the inverse isochron ages calculated using the plateau steps and the weighted mean ages of total fusion of single grains. The compositional change from glaucophane in the cores to crossite in the rims of blue amphiboles, as revealed by electron microprobe analysis, points to decompression, probably caused by progressive exhumation of the subducted material. The Late Ordovician ages were not affected by excess argon incorporation because in all likelihood the oceanic sediments were wet on arrival at the trench and free of older detrital mica. The ca. 450 Ma ages are, hence, interpreted as the time of crystallization during mylonitization under high fluid activity at fairly low temperatures. This means that accretion of the quartzite mylonite unit occured about 200 Ma before final closure of the Paleo-Asian Ocean, amalgamation of the Siberian, Tarim and North China cratons, and formation of the end-Permian Solonker suture zone. We argue that the Early Paleozoic evolution of the Ondor Sum complex occurred along the northeastern Cimmerian margin of Gondwana, which was composed of micro-continents fringed by subduction-accretion complexes and island arcs. The later evolution took place during the building of the Eurasian continent following middle Devonian and younger rifting along the East Gondwanan margin and northward drift of the detached North China craton. An extensive review shows that this type of two-stage scenario probably also applies to the geodynamic evolution of other micro-continents like, South China, Tarim, a number of Kazakh terranes, Alashan, Qaidam and Kunlun, as well as South Kitakami and correlatives in Japan, and probably Indochina. Like the North China craton, these were bordered by Early Paleozoic subduction-accretion complexes, island arcs or contained calc-alkaline volcanic margins, like for example, the central Tienshan, North Qinling, North Qaidam-Altun, North Qilian and Kunlun belts in China, as well as the Oeyama and Miyamori ophiolites and Matsugadaira-Motai blueschist belt of Japan and the dismembered Sergeevka ophiolite of the southern part of the Russian Far East. This implies that a vast orogenic system, comprising an archipelago of micro-continents, seems to have existed along the Cimmerian margin of East Gondwana in Early Paleozoic time in which the ultrahigh-pressure metamorphism that characterizes the early evolution of many of the Asian micro-continents occurred.

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.

Regional metamorphism and tectonic evolution of the Inner Mongolian suture zone

Abstract Regional metamorphism in central Inner Mongolia has occurred during four different periods: the middle Proterozoic, the early Palaeozoic, the middle Palaeozoic and the late Palaeozoic tectonic cycles. The middle Proterozoic and late Palaeozoic metamorphic events are associated with rifting and are characterized by low‐pressure facies series. The early Palaeozoic metamorphism occurred in two stages: (1) subduction zone metamorphism resulted in paired metamorphic belts in the Ondor Sum ophiolite and Bainaimiao island arc complex; and (2) orogenic metamorphism occurred during the collision of an island arc with the continent. Two types of middle Palaeozoic metamorphism are represented: (1) subduction zone metamorphism, which affected the melange; and (2) orogenic metamorphism that resulted from continent–continent collision.

A New Fauna—Qinghezhen Fauna—from the Northern Margin of the North China Platform

Abstract In the Inner Mongolia axis and Jiaoliao anteclise along the northern margin of the North China Platform, it has been found that the strata formerly considered as Archaean and Proterozoic are in fact an ophiolite suite similar to the Early Palaeozoic Ondor Sum Group in the Northern Geosyncline region of China. The strata have been named in northern Liaoning as the Qinghezhen Group. The emphasis of this paper is on the discussion of the small shelly fossils found in the siliceous rocks in the upper part of the Qinghezhen Group. This ophiolite suite stretches in an E‐W direction for about 1000 km along the northern margin of the North China Platform, in which 15 fossil localities with stable stratigraphic horizons have been discovered. In this paper. 4 types. 7 genera (including 5 new ones) and 10 species (including 8 new ones and 1 new subspecies) are described, which are collectively referred to as the Qinghezhen Fauna. The characteristics of these fossils are: shell form simple, the maximum length not exceeding 4 mm, with obvious shell wall and wall ornaments. The Qinghezhen Fauna is comparable in shell structure of some genera and species with the Meishucun Fauna in South China, but differs from the latter in having only monotonous fossil groups developed in a distinctly varied ecological environment. The two faunas may belong to the same evolutionary stage of the Early Cambrian but have developed parallelly in different regions. The existence of the Qinghezhen Fauna represents an important biological event in the Early Cambrian in the Northern Geosyncline region of China. The discovery of the Qinghezhen Fauna will bring new knowledge and profound influence to a series of problems on basic geology and mineral deposit prospecting such as the Cambrian faunas, biogeographical povincialism in China and the geotectonic features of the northern margin of the North China Platform.

High pressure metamorphic rocks and their tectonic environment in northeastern China

This paper deals with the high-pressure metamorphic rocks mainly of the Ondor Sum Group in Inner Mongolia and the Heilongjiang Group in the Jiamusi massif. A high pressure/low temperature metamorphic mineral assemblage of glaucophane schist facies and greenschist facies characterized by lawsonite, jadeitic pyroxene, glaucophane and pumpellyite has been recognized in the Ondor Sum Group. In northeastern China the high pressure metamorphic rocks are mainly distributed in the ancient intercontinental area, north of the Sino-Korean platform. A part of them is distributed on the outer margin of the Caledonian fold belt along the north edge of the Sino-Korean platform and it was dated at 445.6 ± 15 Ma by 39Ar/ 40Ar isochron method on glaucophane schist and at 509 ± 40 Ma by RbSr isochron method on whole rock of meta-basalt. These isotopic data indicate an early Paleozoic age of the subduction on the north edge of the Sino-Korean platform. The other part is the Heilongjiang Group of Jiamusi massif on the outer margin of the Xingkaiian fold belt. The Heilongjiang Group was intruded by a granite of 460 Ma (KAr). The lithological feature of the high pressure metamorphic rocks of the Heilongjiang Group is possibly correlated to that in the Altay-Sayan district (540–900 Ma). These facts suggest a probable Sinian-early Cambrian subduction.