Geological characteristics and tectonic signifcance of unconformities in Mesoproterozoic successions in the northern margin of the North China Block

Several stratigraphic breaks and unconformities exist in the Mesoproterozoic successions in the northern margin of the North China Block. Geologic characters and spatial distributions of five of these un- conformities, which have resulted from different geological processes, have been studied. The uncon- formity beneath the Dahongyu Formation is interpreted as a breakup unconformity, representing the time of transition from continental rift to passive continental margin. The unconformities beneath the Gaoyuzhuang and the Yangzhuang formations are considered to be the consequence of regional eustatic fluctuations, leading to the exposure of highlands in passive margins during low sea-level stands and transgressive deposition on coastal regions during high sea-level stands. The unconformity atop the Tieling Formation might be caused by uplift due to contractional deformation in a back-arc setting, whereas the uplift after the deposition of the Xiamaling Formation might be attributed to a continental collision event. It is assumed that the occurrences of these unconformities in the Mesoproterozoic successions in the northern margin of the North China Block had a close bearing on the assemblage and breakup of the Columbia and Rodinia supercontinents.

Archean (about 2500 Ma) anatexis in eastern North China Block

Two Neoarchean alkaline feldspar-rich granites sourced from partially melted granulite-facies granodioritic orthogneiss have been here recognised in the eastern part of the North China Block(NCB).These poorly foliated granites have previously been assumed to be Mesozoic in age and never dated,and so their significance has not been recognised until now.The first granite(AG1)is a porphyritic syenogranite with megacrystic K-feldspar,and the second(AG2)is a quartz syenite with perthitic megacryst.Zircons from the granites yield LA-ICP-MS U-Pb ages of 2499±10 Ma(AG1),and 2492±28 Ma(AG2),which are slightly younger than the granodioritic orthogneiss that they intrude with a crystallisation U-Pb age of 2537±34 Ma.The younger granites have higher assays for SiO_(2)(71.91%for AG1 and 73.22%for AG2)and K_(2)O(7.52%for AG1 and 8.37%for AG2),and much lower assays for their other major element than the granodioritic orthogneiss.All of the granodioritic orthogneiss and granite samples have similar trace element patterns,with depletion in Th,U,Nb,and Ti and enrichment in Rb,Ba,K,La,Ce,and P.This indicates that the granites are derived from the orthogneiss as partial melts.Although they exhibit a similar REE pattern,the granites have much lower total REE contents(30.97×10^(−6) for AG1,and 25.93×10^(−6) for AG2),but pronounced positive Eu anomalies(Eu/Eu^(*)=8.57 for AG1 and 27.04 for AG2).The granodioritic orthogneiss has an initial ^(87)Sr/^(86)Sr ratio of 0.70144,εNd(t)value of 3.5,and εHf(t)values ranging from−3.2 to+2.9.The orthogneiss is a product of fractional crystallisation from a dioritic magma,which was derived from a mantle source contaminated by melts derived from a felsic slab.By contrast,the AG1 sample has an initial ^(87)Sr/^(86)Sr ratio of 0.6926 that is considered too low in value,εNd(t)value of 0.3,andεHf(t)values between+0.57 and+3.82;whereas the AG2 sample has an initial ^(87)Sr/^(86)Sr ratio of 0.70152,εNd(t)value of 1.3,andεHf(t)values between+0.5 and+14.08.These assays indicate that a Sr-Nd-Hf isotopic disequilibrium exists between the granite and granodioritic orthogneiss.The elevatedεHf(t)values of the granites can be explained by the involvement of Hf-bearing minerals,such as orthopyroxene,amphibole,and biotite,in anatectic reactions in the granodioritic orthogneiss.Based on the transitional relationship between the granites and granodioritic orthogneiss and the geochemical characteristics mentioned above,it is concluded that the granites are the product of rapid partial-melting of the granodioritic orthogneiss after granulite-facies metamorphism,and their crystallisation age of about 2500 Ma provides the minimum age of the metamorphism.This about 2500 Ma tectonic-metamorphic event in NCB is similar to the other cratons in India,Antarctica,northern and southern Australia,indicating a possible connection between these cratons during the Neoarchean.

GEODYNAMICAL MODELING OF CRUSTAL DEFORMATION OF THE NORTH CHINA BLOCK

The North China block,the western portion of the Sino- Korean Craton,is rounded byYanshanian in the north and Qinling- Dabie orogenic belts in the south.The widespread de-velopment of extensional basins in thisblock indicates horizontal extension or continued thin-ning of a previousely thickened,unstable lithosphere throughout the Mesozoic.In this pa-per,we attempt to simulate numerically the geodynamical process of the basin formation byusing the mountain- basin evolution system.We assume thatthe formation of numeroussedi-mentary basins in the North China block is the resultofthe crustal extension,which destruc-ts rapidly the previously thickened crust.The gravitational collapse of the thickened crust ispossibly triggered by the re- orientation of the far- field stress regime,or the relaxation of theboundary resistantstress.

Tectonic Setting and Metallogenic System of North China Block Margins

The paleocontinental margins have frequent and intensive tectonic movement and various ore forming processes. According to their tectono dynamic characteristics, the paleocontinental margins can be classified into three types: the divergent, the convergent and the transformational. Each type has its specific geological geochemical processes and metallogenic system. The paper discusses the tectonic evolution and ore forming features of the North China block margins, puts forward conceptions such as complexity, variety and multi stage development of metallogenic evolution in the paleocontinental margins, and expounds five factors controlling the formation of large superlarge ore deposits in the paleocontinental margins: (1) channelway, (2) rendezvous of fluids, (3) abundance of ore source, (4) thermo dynamic anomaly, (5) long duration of structural activities.

The absolute paleoposition of the North China Block during the Middle Ordovician

Present-day hot spots and Phanerozoic large igneous provinces(LIPs) and kimberlites mainly occur at the edges of the projections of Large Low Shear Wave Velocity Provinces(LLSVPs) on the earth's surface. If a plate contains accurately dated LIPs or kimberlites, it is possible to obtain the absolute paleoposition of the plate from the LIP/kimberlite and paleomagnetic data. The presence of Middle Ordovician kimberlites in the North China Block provides an opportunity to determine the absolute paleoposition of the block during the Middle Ordovician. In addition to paleobiogeographical information and the results of previous work on global plate reconstruction for the Ordovician Period, we selected published paleomagnetic data for the North China Block during the Middle Ordovician and determined the most reasonable absolute paleoposition of the North China Block during the Middle Ordovician: paleolatitude of approximately 16.6°S to 19.1°S and paleolongitude of approximately 10°W. The block was located between the Siberian Plate and Gondwana, close to the Siberian Plate. During the Cambrian and Ordovician periods, the North China Block may have moved toward the Siberian Plate and away from the Australian Plate.

U-Pb geochronology of hydrothermal zircon from the Mesoproterozoic Gaoyuzhuang Formation on the northern margin of the North China Block and its geological implications

This paper reports LA-ICP-MS zircon U-Pb ages and REE compositions on detrital zircons （Type 1） and hydrothermal zircons （Type 2） from low-metasedimentary rocks in the Mesoproterozoic Gaoyuzhuang Formation, Pingquan area, Hebei Province that provide important constraints on the tectonic evolution along the northern margin of the North China Block. The detrital zircons are characterized by an oscillatory magmatic core, surrounded by a narrow structureless rim in CL images. They yield 2~7pb/2~tpb ages ranging from 1703 to 2543 Ma with two age peaks at 2473 and 1794 Ma, which is consistent with the Neoarchean and Paleoproterozoic tectono-thermal events recorded in the NCB basement, indicating that the source of the detritus was locally derived. By contrast, the hydrothermal ones are euhedral, sector zoning or internally structureless. They have relatively higher U and Th contents and Th/U ratios （U=139-2918 ppm, Th=35-1327 ppm, Th/U （average） =0.78 vs. U=15-1044 ppm, Th=8-341 ppm, Th/U （average） =0.57 for detrital zircons）. Moreover, they are also enriched in REEs relative to detrital zircons and show a lower positive Ce anomaly （ZREE = 659-2418 ppm vs. 231-611 ppm for detrital zircon; Ce/Ce＊=2-13 vs. 33-174 for detrital zircons）, similar to known hydrothermal zircons derived from many locations. These characteristics, combined with our field petrographic observations, indicate that the hydrothermal zircons possibly formed from a low temperature aqueous fluid. Twelve concordant or near-concordant analyses on hydrothermal zircons yield weighted mean 206pb/238U ages of 325-327 Ma, which provide unambiguous evidence that the northern NCB underwent late Paleozoic low temperature hydrothermal modification. This timing of this hydrothermal event is compatible with that of the late Paleozoic magmatic and metamorphic-deformational events occurred on the northern margin of the NCB, it appears likely that the hydrothermal activity resulted from southward subduction of the Paleo-Asian Oceanic plate underneath the NCB during late Paleozoic.

New Paleomagnetic Study on Early Triasic Liujiagou Formation of NorthChina Block

This paper introduces the result of studying on paleomagnetism of 91 specimens from 13 sites from Liujiagou Formation in Dashu Village, Wu ’an city, Hebei Province. The result reveals that North China block (NCB) was not united with Yangtze block (YB) in the Early Triassic. From the Early Triassic to now, NCB has rotated 30.0° counterclockwise, but YB has rotated 45.1° clockwise.

Carboniferous and Permian integrative stratigraphy and timescale of North China Block

The Carboniferous-Permian strata in the North China Block(NCB) contain abundant fossils, coals and natural gases.Establishing a high-resolution timescale for the Carboniferous and Permian in the NCB is essential to understand the geologic events and explore the spatial and temporal distributions of the natural resources. The upper Carboniferous and the basal part of Permian are relatively well correlated because they yield marine conodont and fusuline fossils. However, the Permian terrestrial strata mostly rely on poorly constrained palynostratigraphy and phytostratigraphy and are short of the precise geochronologic constraints on the correlation with the marine strata. This study provides a critical review on the state-of-the-art of the latest Carboniferous and Permian chronostratigraphic and biostratigraphic frameworks and stratigraphic correlation in the NCB. The Penchi Formation ranges from lower Bashkirian to lower Gzhelian;the Taiyuan Formation is assigned to the upper Gzhelian to lower Asselian;the Shansi and Lower Shihhotse formations are from middle Asselian to lower Sakmarian;the Upper Shihhotse Formation is assigned to upper Artinskian to lower Kungurian, and the Sunjiagou Formation was assigned to Lopingian, respectively. A long hiatus up to ~20 Myr between the Upper Shihhotse and Sunjiagou formations, mainly marked by a large-scale erosional surface at the base of a coarse conglomeratic sandstone unit and/or multiple paleosol layers as well as significant differences of floras between these two lithostratigraphic units, is present probably due to tectonic uplift in association with the closure of the Paleo Asian Ocean(PAO) during the Cisuralian and Guadalupian. The possible amplitude of sea-level changes from Carboniferous to Permian on the NCB is estimated from 0 to 40 m.The floral succession, depositional records and organic carbon isotope profiles suggest that the latest Carboniferous and earliest Permian was a favorable period for coal accumulation under an ever-wet and warm climate, followed by a prominent shift to dry climate from early-middle Cisuralian. This climatic shift during the Permian was mainly resulted from northward migration of the Pangea and the closure of the PAO, which is comparable with the Carboniferous and Permian trends in central Europe.

The Early Paleozoic paleogeography of the North China block and the other major blocks of China

With the summarization of the Early Paleozoic paleomagnetic data recently obtained from the three major blocks of China, the Early Paleozoic (i.e. Cambrian and Ordovician) paleogeographic positions of the North China, South China and Tarim blocks were discussed in detail. The North China, South China and Tarim blocks were inferred to be located adjacent to East Gondwana in low latitudes of the Southern Hemisphere during the Early Cambrian. During the Early-Middle Ordovician, the South China and Tarim blocks were also located in low latitudes of the Southern Hemisphere with some affinities of the Gondwanaland, whereas the North China block may have episodically separated from the Gondwanaland, and might be sited close to the North America and Siberia. The reestablished paleogeographic configurations are in agreement with the studies on the biogeography, paleoclimate and sedimental facies of the North China and South China blocks.

Apparent polar wander path and tectonic movement of the North China Block in Phanerozoic

The results on the Early Paleozoic from the North China Block (NCB) are reported, and a series of reliable poles are selected from the available Phanerozoic data, based on the conventional reliability criteria, e.g. the number of samples, the uncertainty limit, any suspected incomplete demagnetization or overprint and field test (including fold, reversal, conglomerate tests). Especially, paleopole data is excluded if the sampling area suffered from the tectonic (e.g. rotation) and thermal effects. A new Phanerozoic apparent polar wander (APW) path for the NCB is compiled, and its tectonic evolution is discussed.

New paleomagnetic results from the Neoproterozoic successions in southern North China Block and paleogeographic implications

A total of 312 oriented core samples collected from the Neoproterozoic to lower Cambrian strata in the Lushan region of western Henan Province, southern North China Block, were subjected to stepwise demagnetization. After removing recent and possibly Mesozoic overprint components, inferred primary magnetic remanence was isolated from the Precambrian Dongjia, Sanjiaotang and Cuizhuang Formations. Our new results, together with selected previous results, suggest that the North China Block was in low latitudinal positions for most of the studied time interval. Through comparison with paleogeographic positions of other continents, we suggest that the NCB could have been closed to Siberia during the ca. 800-600 Ma interval.

He-Ar isotopic system of fluid inclusions in pyrite from the molybdenum deposits in south margin of North China Block and its trace to metallogenetic and geodynamic background

The helium and argon isotopic compositions of the ore-forming fluids from the molybdenum deposits such as Jinduicheng, Donggou, Shijiawan, and Sandaozhuang, which are located in the East Qinling molybdenum belt in south margin of North China Block (SMCNB), are reported in this paper. The origin and the evolution of the ore-forming fluids and their coupled-relationships with the intra-continental collision and orogenic process of Qinling Orogen in Mesozoic-Cenozoic have been discussed. The 3He/4He and 40Ar/36Ar values (3He/4He=1.38―3.64 Ra, and 40Ar/36Ar=295.68―346.39, respectively) of the fluid inclusions in pyrite from the molybdenum deposits in East Qinling suggest that, the ore-forming fluid system is mixed by two end members. One is the high temperature deep-derived fluid congenetic with the porphyries generated by crust-mantle mixing, and the other is the low-temperature meteoric water which is rich in crustal radiogenic He with the component of atmospheric Ar. From the Pb isotopic composition, and ore-bearing potential of the porphyry and the regional stratum, we can conclude that the ore-forming materials of the deposits in the East Qinling molybdenum belt are derived from the deep source by the mixing of lower crust and upper mantle. Therefore, the formation of the molybdenum deposits in SMNCB can be related to the crust-mantle interaction, which is accompanied by the intra-continental orogenic and extension process in the post-collision period of Qinling Orogen. The granitic porphyries which are related to Mo mineralization are not simple crust-remelting type granites or S type granites, but belong to syntexis-type or mantle-derived granites, hence their formation has a profound and regional geodynamic background.

Discovery of Late Carboniferous strata and their tectonic significance in the southern margin of the North China Block(Huoqiu and Gushi areas)

Late Carboniferous fossils(such as Boultonia? sp., Tabulata, and spiriferoid specimens with smooth shells), bioclastic material(such as crinoid stems and sponge fragments), and Late Ordovician microfossils of the conodont Belodina have been discovered in the lower part of the strata typically referred to as the Neoproterozoic on the boundary of the provinces of Anhui and Henan in the southern margin of the North China Block. These findings prove that the strata contain macrofossils belonging to the Late Carboniferous, which belonged to a carbonate debris flow deposit that was formed under a carbonate slope environment. The conodont fossils might belong to a detrital deposit. Thus, it is possible to reset the stratigraphic sequences and tectonic attributes belonging to the North Huaiyang tectonic belt and limit the Shouxian fault to the boundary between the Dabie Orogen and North China Block.

Paleomagnetic Results of Later Paleozolic Rocks From Alashan Area of Western North China Block

In order to contribute to the Apparent Polar Wander Path (APWP) of the North China Block (NCB) , we collected paleomagnetic samples of Late Paleozoic rocks from the northern and southern margins of the Alashan (Alxa) terrane (the western part of Inner Mongolia and northern parts of Gansu and Ningxia Provinces) and adjacent regions. Laboratory work revealed different types of demagnetization behavior and isolated a higher temperature characteristic component in most of the samples. Results of IRM experiments show high thermal stability and high coercivities for the magnetic carriers in these rocks. The characteristic component passed the fold and/or reversal tests, suggesting that the characteristic components were probably primary. On the basis of comparing the paleomagnetic results with the geologic evidence, we discuss the tectonic implications. In the Late Paleozoic, the Alashan region had been part of the NCB and was not connected with the northern and southern blocks. During the Carboniferous Alashan wandered around the equator, and then moved northerly during the Permian. During the Jurassic the Alashan region rotated anticlockwise about 25° relative to the NCB when the Helanshan geosyncline closed.

Paleoproterozoic Potassic Granitoids in the Sushui Complex from the Zhongtiao Mountains,Northern China:Geochronology,Geochemistry and Petrogenesis

Paleoproterozoic potassic granitoids in the southern Sushui Complex from the Zhongtiao Mountains yielded SHRIMP zircon U-Pb ages of 1968-1944 Ma. Lithologically, the potassic granitoid series consists chiefly of monzodiorite, quartz monzonite and syenogranite. Their trace elements and Sm-Nd isotope characteristics indicate that they were derived from partial melting of Archean TTG rocks in an overthickened continental crust. Petrogenesis of this potassic granitoid series implies a collisional environment within the Trans-North China Orogen in the Paleoproterozoic, which supports a tectonic model of Eastern and Western Continental Blocks being amalgamated in the Paleoproterozoic.

Differential Tectonic Stress Pattern Between the Deep and Shallow Crust in North China and Its Possible Genesis

This paper approaches the neotectonic stress field based on the data of foeal mechanismsolution,ground stress measurement,tectonic mechanical analysis and geodetic surveying,and finds out that the orientations of the maximum principal comproessive sterss patterns arequite discordant with different methods and the stress patterns are widely differnt betweenthose in the shallow and deep part of the crust in North China.Based on the analysis ofabove-mentioned data,we established a duplex model by considering the diversities of theStress patterns in 3-dimentional spaces,the boundary conditions and the lithospheric media,and made an inverse calculation by using the finite element method.The calculated results fitwell with the reality in North China,i.e.the stress patterns in lower crust which is below thedetachment interface at the deptp of 10 km from ground surface are relatively consistent withnearly horizontal state and NE-ENE trending of the maximum principal compressive stressaxes,whereas the stress

Earthquake-induced Soft-sediment Deformation Structures in the Dengfeng Area,Henan Province,China:Constraints on Qinling Tectonic Evolution during the Early Cambrian

Soft-sediment deformation structures are abundant in the Cambrian Zhushadong and Mantou formations of the Dengfeng area, Henan Province, China. Soft-sediment deformation structures of the Zhushadong Formation consist of fluidized deformation, synsedimentary faults, seismo-folds and plastic deformation; the Mantou Formation is dominated by small-scale horst faults, intruded dikes, fluidized veins, and seismo-cracks. These structures are demonstrated to be earthquake-related by analysis of trigger mechanisms, and may indicate the activity of the Qinling tectonic belt during the early Cambrian. Furthermore, the assemblages of soft-sediment deformation structures altered with time： large-scale, intense deformation in the Zhushadong Formation alters to small-scale, weak deformation in the Mantou Formation. This striking feature may have been caused by changes in hypocentral depth from deep-focus to shallow-focus earthquakes, indicating that the Qinling tectonic belt developed from the subduction of the Shangdan Ocean to the extension of the Erlangping back-arc basin. This study suggests that soft-sediment deformation structures can be used to reveal the activity of a tectonic belt, and, more importantly, changes in deformation assemblages can track the evolution of a tectonic belt.

华北稳定参考框架NChina20的建立及应用

选用中国大陆地壳运动观测台网(CMONOC)在华北地区约8年(2011年10月~2019年9月)的连续观测数据,建立了华北稳定参考框架——NChina20。NChina20与全球参考框架(IGS14)保持坐标系缩放比例一致,两坐标系统在历元2020.0对齐。本文详细介绍了将相对于IGS14的位置时间序列转换到NChina20的方法,并列举了NChina20在城市地面升降长期观测领域的应用。NChina20的稳定性(精度)在水平方向约为0.5mm/a,在垂直方向约为0.6mm/a。参考框架的稳定性随时间的推移而退化,建议NChina20的使用范围在时间上限于2006~2025年的时间窗口内,在空间上限于华北活动地块区域内。选用华北地区5个基岩站20年(2000~2019年)的连续观测数据,建立了华北地区季节性地面升降预测模型。华北稳定参考框架将每隔几年更新一次,以缓解框架稳定性随时间的退化,并与IGS参考框架同步更新。

3D Velocity Structure and Its Tectonic Implications in the East China Sea and Yellow Sea

3D structure of the crust and upper mantle in the studied area has been analyzed from surface wave tomography. The velocity distribution in the uppermost crust is symmetrical on two sides of the central line of the sea, and coincides with the structure of crystalline basement. The essential difference in tectonics between the East China Sea and the Yellow Sea mainly lies in that the velocity structures of their lower crust and upper mantle are identical to those of South China and North China respectively. In the upper mantle there exists a high-velocity zone with a nearly EW strike from the Hangzhou Bay, China, to the Tokara Channel, Japan, along about the latitude of 30°N. It is found that between the East China Sea and the Yellow Sea there are systematical differences in geomorphology, geology, seismicity, heat flow, quality factor and gravity and aeromagnetic anomalies, which is related to both left-lateral shear dislocation and right-lateral tear of the Benioff zone from the Hangzhou Bay to the Tokara Channel.It is inferred that the East China Sea was formed by Cenozoic back-arc extension. The boundary between the North China and South China crustal blocks stretches along the southern piedmont of Mts. Daba-Dabie-Hangzhou Bay-Tokara Channel, and the subduction zone at the Okinawa trench is the eastern boundary of the South China crustal block. The movements of the Pacific plate, Indian plate and upper mantle rather than the Philippine plate subduction have played a dominant role for the modern tectonic movements in East Asia.

Hydrocarbon Potential of Pre-cenozoic Strata in the North Yellow Sea Basin

The North Yellow Sea Basin ( NYSB ), which was developed on the basement of North China (Huabei) continental block, is a typical continental Mesozoic-Cenozoic sedimentary basin in the sea area. Its Mesozoic basin is a residual basin, below which there is probably a larger Paleozoic sedimentary basin. The North Yellow Sea Basin comprises four sags and three uplifts. Of them, the eastern sag is a Mesozoic-Cenozoic sedimentary sag in NYSB and has the biggest sediment thickness; the current Korean drilling wells are concentrated in the eastern sag. This sag is comparatively rich in oil and gas resources and thus has a relatively good petroleum prospect in the sea. The central sag has also accommodated thick Mesozoic-Cenozoic sediments. The latest research results show that there are three series of hydrocarbon source rocks in the North Yellow Sea Basin, namely, black shales of the Paleogene, Jurassic and Cretaceous. The principal hydrocarbon source rocks in NYSB are the Mesozoic black shale. According to the drilling data of Korea, the black shales of the Paleogene, Jurassic and Cretaceous have all come up to the standards of good and mature source rocks. The NYSB owns an intact system of oil generation, reservoir and capping rocks that can help hydrocarbon to form in the basin and thus it has the great potential of oil and gas. The vertical distribution of the hydrocarbon resources is mainly considered to be in the Cretaceous and then in the Jurassic.