Chronology and Sources of Mesozoic Intrusive Complexes in the Xuzhou-Huainan Region, Central China: Constraints from SHRIMP Zircon U-Pb Dating

SHRIMP zircon U-Pb dating in the Liguo and Jiagou intrusives indicates that they were formed at -130 Ma in the Early Cretaceous. Most inherited zircons in the Liguo intrusive were formed at 2509±43 Ma. Most inherited and detrital zircons in the Jiagou intrusive were formed at -2500 Ma, -2000 Ma and -1800 Ma. The SHRIMP zircon U-Pb dating in two gneiss xenoliths from the Jiagou intrusive yields the ages of 2461±22 Ma and 2508±15 Ma, respectively. The dating results from inherited and detrital zircons in the intrusives and the gneiss xenoliths imply that the magmas could be derived from the partial melting of the basement of the North China Block (NCB). The magmatism is strong and extensive in the periods from 115 to 132 Ma, which is of typical bimodal characteristics. It is suggested that the lithospheric thinning in the eastern North China Block reached its peak in 115-132 Ma.

Thrusting of the North Lhasa Block in the Tibetan Plateau

A huge thrust system, the North Lhasa Thrust (NLT), was discovered in the northern Lhasa block of the Tibetan Plateau based on geological mapping of the Damxung region and its vicinity, the Deqen-Lunpola traverse and the Amdo-Bam Co profile. The NLT consists of the Dongqiao-Lunpola thrust (DLT), the west Namco thrust (WNT) and the south Damxung thrust (SDT) and ductile shear zones, ophiolite slices and folds extending in a WNW direction. Major thrust faults of the NLT seem to merge into a single deep-seated detachment of the upper-crust and totally displaced southward as far as 100-120 km. Chronological analyses with 39Ar-40Ar of plagioclase and hornblende, Rb-Sr isochron of minerals and fission-tracks of apatite from mylonite within the WNT yield ages of 174-173 Ma, 109 Ma and 44 Ma, showing 3 periods of thrusting in the north Lhasa block caused by subduction of the Tethys oceanic plate and the India-Eurasia continental collision respectively.

Evolution of an Ancient Large Lake in the Southeast of the Northern Tibetan Plateau

Nam Co is the largest (1920 km2 in area) and highest (4718 m above sea level) lake in Tibet. According to the discovery of lake terraces and highstand lacustrine deposits at several places in Nam Co and its adjacent areas, the authors confirm the existence of an ancient large lake in the southeastern part of the northern Tibetan Plateau. On the basis of the U-series, 14C and ESR dating, coupled with the levelling survey of lake deposits and geomorphology, the evolutionary process of the ancient large lake in the southeastern part of the northern Tibetan Plateau may fall into three stages: (1) the ancient large lake stage at 115-40 ka BP, when the ancient lake level was 140-26 m above the level of present Nam Co; (2) the outflow lake stage at 40-30 ka BP, when the ancient level was 26-19 m above the present lake level; and (3) the Nam Co stage since 30 ka BP, when the ancient lake level was < 19 m above the present lake level. During the ancient large lake stage, a large number of modern large, medium-sized and small lakes, including Nam Co, Siling Co and Zhari Namco, in the southeastern part of the northern Tibetan Plateau, were connected into a single large ancient lake, rather than several separate lakes connected by river channels. Its areal extent may have gone beyond the watersheds of the modern endorheic and exorheic drainage systems; so it may be called the 'ancient east lake', 'ancient south lake' and 'ancient west lake'. It might also be connected with other ancient lakes in the southern and western parts of the northern Tibetan Plateau to form a unified 'ancient large lake' on the northern Tibetan Plateau.

P-T-t Path of Mafic Granulite Metamorphism in Northern Tibet and Its Geodynamical Implications

Mafic granulites have been found as structural lenses within the huge thrust system outcropping about 10 km west of Nam Co of the northern Lhasa Terrane, Tibetan Plateau. Petrological evidence from these rocks indicates four distinct metamorphic assemblages. The early metamorphic assemblage (M1) is preserved only in the granulites and represented by plagioclase+hornblende inclusions within the cores of garnet porphyroblasts. The peak assemblage (M2) consists of garnet+clinopyroxene+hornblende+plagioclase in the mafic granulites. The peak metamorphism was followed by near-isothermal decompression (M3), which resulted in the development of hornblende+plagioclase symplectites surrounding embayed garnet porphyroblasts, and decompression-cooling (M4) is represented by minerals of hornblende+plagioclase recrystallized during mylonization. The peak (M2) P-T conditions of garnet+ clinopyroxene+plagioclase+hornblende were estimated at 769-905℃ and 0.86-1.02 GPa based on the geothermometers and geobarometers. The P-T conditions of plagioclase+hornblende symplectites (M3) were estimated at 720-800℃ and 0.55-0.68 GPa, and recrystallized hornblende+plagioclase (M4) at 594-708℃ and 0.26-0.47 GPa. It is impossible to estimate the P-T conditions of the early metamorphic assemblage (M1) because of the absence of modal minerals. The combination of petrographic textures, metamorphic reaction history, thermobarometric data and corresponding isotopic ages defines a clockwise near-isothermal decompression metamorphic path, suggesting that the mafic granulites had undergone initial crustal thickening, subsequent exhumation, and cooling and retrogression. This tectonothermal path is considered to record two major phases of collision which resulted in both the assemblage of Gondwanaland during the Pan-African orogeny at 531 Ma and the collision of the Qiangtang and Lhasa Terranes at 174 Ma, respectively.

Mangshan Loess in Central China and the Paleomonsoon Variations since the Last Interglaciation

The Mangshan Yuan is a loess platform on the southern bank of the Yellow River, which is located in northwestern Zhengzhou of Henan Province, China. The typical Zhaoxiayu section of the Mangshan Yuan preserves stratigraphical loess units above S10 with a total thickness of 172.1 m, which includes 15.7 m of the last interglacial paleosol S1, 77.3 m of the last glacial loess L1 that consist of 41.6 m of the late stade L1LL1, 13.2 m of the interstade L1SS1 and 22.5 m of the early stade L1LL2. Based on the age marking points by correlating magnetic susceptibility of the section with the SPECMAP curve, the timescale of the section was constructed, and the average accumulation rate and the resolution of each loess strata over the S2 were subsequently calculated using the susceptibility age model. The results indicate that strata units developed in the glacial, interglacial stages, stadial and interstadial show substantial differences in grain size, average accumulation rate and time resolution ub the Zhaoxiayu section. Specifically, the average accumulation rate of the loess L1LL1 is 3.45 mm/a, whereas that of paleosol S1 is only 0.28 mm/a. Based on the high-resolution records of magnetic susceptibility and >45μm fraction percentage of the loess-paleosol, the summer and winter monsoon variations as well as their interrelations since the last interglaciation have been discussed, which were correlated with the SPECMAP and the GRIP climate records.

Quaternary Geology and Faulting in the Damxung-Yangbajain Basin

The detailed geological mapping, conducted in the Damxung-Yangbajain basin, shows that there are many types of deposits formed since the Pliocene. The oldest sediments are formed during the Pliocene. The most prominent sediments are three sets of moraines and fluvioglacial deposits. The ESR, U-series and OSL dates indicate they are formed about 700-500 ka B.P., 250-125 ka B.P. and 75-12 ka B.P. respectively and indicate that there are three glacial periods since the mid-Pleistocene in the Nyainqentanglha Range. Along the southeast side of the Nyainqentanglha Range, the main southeast dipping fault zone which bounds the Damxung-Yangbajain Graben on its western edge was mapped. The fault zone consists of three secondary fault zones and their initiation ages that the fault zones became active gradually decrease southeastward. Prominent faulting occurred in about 700-500 ka B.P., 350-220 ka B.P., -140 ka B.P. and 70-50 ka B.P. since the mid-Pleistocene. The height of fault scarps which offset the sediments formed since the mid-Pleistocene suggest that the vertical slip rates change between 0.4 -2 mm/a and the cumulative average vertical movement at rates of 1.1±0.3 mm/a during the Quaternary period and the Holocene vertical throw rate is 1.4±0.6 mm/a along the fault zones on the western side of the Damxung-Yangbajain Graben.

Origin, Age and Significance of Pseudotachylites from the Eastern Dabieshan Orogenic Belt, China

Recent field survey in the eastern Dabieshan Mountains has revealed extensive occurrences of pseudotachylite. The pseudotachylite tends to occur as simple veins and injected networks along the NE-SW-trending fracture zones or shear zones, which are parallel to the Tanlu fault zone and cut all the pre-Cretaceous geological bodies. The characteristics of both the microstructures gained by the optical microscope and SEM imaging and the geochemistry between the pseudotachylites and their host rocks show that the pseudotachylites were formed mainly by ultracataclasis of their wall rocks in which they occur. The bulk K-Ar ages of the pseudotachylites yielded a narrow range of 81 -93 Ma, and moreover the laser-probe 40Ar/39Ar dating of phengite overprinting on the pseudotachylite gave a weighted mean age of 78.9 Ma. These results show that the pseudotachylites from the eastern Dabieshan Mountains formed along the NE-SW-trending fault zone during the uplifting of the orogenic belt at 80-90 Ma, which places important constrains on the cooling and exhumation history of the Dabieshan Mountains during and after the late Cretaceous.

Early Paleozoic Magmatism and Gold Mineralization in the Northern Altun, NW China

This paper discusses the relationships between granitic magmatism and gold mineralization and the exhumation history of the Dapinggou gold deposit in northern Altun, NW China based on the geochronological data, including zircon U-Pb ages, Rb-Sr isochron age and 40Ar-39Ar dating and MDD modeling data. The main granitic magmatism age in this area is attained from the ID TIMS U-Pb geochronology of zircons from the Kuoshibulak granite, the biggest granite in the northern Altun area, which gives a concordant age of 443±5 Ma in the Late Ordovician. Zircon ID TIMS U-Pb geochronology of the West Dapinggou biotite granite west of the Dapinggou gold deposit gives concordant ages around 485±10 Ma, representing the early stage of Ordovician magmatism. The Rb-Sr isochron age (487±21 Ma) of 6 quartz inclusion samples from quartz veins in this gold deposit is very close to that of the West Dapinggou granite. MDD modeling of step heating 40Ar-39Ar data of K-feldspar from the same West Dapinggou biotite granite gives a rapid cooling history from 300℃ to 150℃ during 200-185 Ma. According to the age data and the geological setting of this area, we conclude that the Dapinggou gold deposit was formed at the early stage of the Early Paleozoic granitic magmatism in northern Altun, and exhumed in the Early Jurassic due to the normal faulting of the Lapeiquan detachment. The Early Paleozoic magmatism may provide heat source and produce geological fluids, which are very important for gold mineralization. Exhumation in the Mesozoic caused the uplift of the deposit towards the ground surface.

Characteristics of Blueschist in Shuangjiang Tectonic Mélange Zone,West Yunnan Province

Glaucophane in Shuangjiang area, West Yunnan Province, supplies a chance for studying south segment of Lancangjiang tectonic zone. But people are at odds as to whether two stage glaucophane exists or not, glaucophane is the result of dynamic metamorphism later, or indicates a high P/T metamorphic belt when Paleozoic Tethys Sea closed. Authors discover in a recent research that there is only one stage glaucophane in Shuangjiang area, and three blueschist belts are distributed near N S tending, and glaucophane in Shuangjiang area is related to the eastward subduction of Changning Menglian basin.

Methods for Calculation of Geogenetic Depth

Some current methods for the calculation of the geogenetic depth are based on the hydrostatic model, it is induced that the depth in certain underground place is equal to the pressure divided by the specific weight of rock, on the assumption that the rock is hydrostatic and overlain by no other force but gravity. However, most of rock is in a deformation environment and non hydrostatic state, especially in an orogenic belt, so that the calculated depth may be exaggerated in comparison with the actual depth according to the hydrostatic formula. In the finite slight deformation and elastic model, the relative actual depth value from the 3 axis strain data was obtained with the measurement of strain including that of superimposed tectonic forces but excluding that of time factor for the strain. If some data on the strain speed are obtained, the depth would be more realistically calculated according to the rheological model because the geological body often experiences long term creep strains.

Mechanical Nature of Gravity and Tectonic Forces

There are two models of ultrahigh pressure metamorphism (UHPM) zone in Dabie: the model of under thrusting returning which even arrives at the mantle and the superimposed model of tectonics in the crust. There are two points of view in the argument about formation depth of ultrahigh pressure metamorphism: (1) the depth can be calculated by hydrostatic equation; (2) the high pressure was composed of gravity, tectonic and other forces instead of merely gravity force. Some misunderstandings of mechanical conceptions presented in the paper showing the hydrostatic viewpoints should be open to question. The main conceptions are: (1) the confining pressure was only formed by gravity, and the differential stress was only formed by tectonic force; (2) the differential stress is not big enough to lead to form ultrahigh pressure metamorphism; (3) once tectonic overpressure goes beyond the limited strength of rocks the tectonic force would disappear and the rocks would be broken or rheomorphied at the same time. A short discussion in basic mechanics is made in this paper for a perfect process for discussing the argument.

Effects of Tectonic Force on Hydrostatic Pressure in Crust

The research into the hydrostatic pressure in the crust has been previously conducted from the viewpoint that the hydrostatic pressure is equal to the gravity, based on the fact that the hydrostatic pressure is derived mainly from the gravity of its overlying rocks. In this paper, the stress state of any point in the crust is suggested to have been caused by both the gravity and the tectonic force. The author proposes that the hydrostatic pressure is a combination or superposition of two isotropic stresses in the tectonic force and gravity stress fields. The results obtained with a finite element simulation indicate that the additional hydrostatic pressure borne by rocks decreases gradually from the compression zone ( p s c), the shear zone ( p s sh ) to the tensile zone ( p s t), and that the difference in the additional tectonic hydrostatic pressure between these deformed zones tends to increase, following the increase in the absolute value and/or the difference in external forces between different directions. This paper presents the foundation for the research into the tectonic physicochemistry.

Pressure State in Deep Crust and Formation Depth of UHP Metamorphic Rocks

This paper presents some questions to the formula of pressure=depth×specific gravity from the viewpoint that the hydrostatic pressure is equal to the gravity of overlying rocks and the rocks in a static fluid state, which is drawn from the research and analysis of the research field and the corresponding problems of the pressure state in the deep crust and the formation depth of the UHP metamorphic rocks. In this research, the underground rocks are considered as the solid possessing some rheological behaviors to discuss the polysource stress state and to obtain a more reasonable method for the calculation of depths using the model of the unbalanced force solid. It is suggested from this paper that the P/SW method for the calculation of the ultrahigh pressure stemming only from the gravity has obviously overstated the formation depth of the UHP metamorphism. The formation model emphasizing the effect of the gravity, the tectonic force and the metamorphic force of the facies change concludes that such UHP minerals as coesite may have been produced in the inner crust.

Experimental Study of Confining Pressure Initiated by Tectonic Force

An experimental study of the confining pressure, i.e. additional hydrostatic pressure initiated by the tectonic force is presented. The experimental progress is that the σ 1 is gradually increasing from 0 in a limiting movement ( ε 1=0) in the σ 1 direction and the speed rate of the accelerating load is 0.4 MPa·s -1 in the lateral and level directions. When σ 2= σ 3<200 MPa, Δ σ l is nearly lacking, Δ σ l is increasing at a high speed only when the horizontal force reaches 250-380 MPa, and Δ σ l almost ceases to increase at the level force of 380 MPa. It is calculated that the tectonic force can produce the confining pressure which is gradually increasing with σ 2= σ 3 before it reaches 380 MPa in an experiment. It is supposed that the horizontal force is almost all transformed into the confining pressure with the increase of the creep deformation of rocks.

Preliminary Study on Loess Slumping Hazard between Northern Shaanxi and Western Shanxi along the Line of West-East Gas Transportation Project

The loess slumping hazard is a special type of slope failures in the northern area of loess plateau of China. The characteristics of paroxysm and high frequency of the hazard always lead to ruin of cave houses, as well as a high human casualties. The hazard is also seriously harmful to railways, highroads and long transporting pipelines. With the mechanisms of both landslide and falling, as well as the forming process of sliding followed by collapses, loess slumping is used to be treated as landslide or falling. Based on field investigations and lab analyses of the loess between Jingbian County of Shaanxi Province and Puxian County of Shanxi Province along the line of west east gas transportation project which is 300 km long, it is discovered that the hazards of loess slumping are controlled by the content of clay, which lead to their obvious regional properties: the area with a content of clay (<0.005 mm) less than 10 %, called sand loess area, is a seriously developing area of slumping; the area with a content of clay between 10 % and 20 %, called typical loess area, is a medium developing area of slumping; the area with a content of clay more than 20 %, called clay loess area, is a non developing area of slumping. Based on research of the formation mechanism and formation conditions of the loess slumping hazards, some corresponding engineering countermeasures are suggested in this paper.

Measurement of Metallogenic Depth and Deep Second Enrichment Belt Forecasting in Jiaodong Gold Deposit

The depth is important for ore finding in Jiaodong gold deposit. However, many geologists are still discussing how to confirm the depth for the tectonic and metallogenesis formation. The authors of this paper propose a new method-the correction of metallogenic depth via its structure to calculate the depth. This method, based on the crust rock in a solid stress state, emphasizes the elastic pattern rather than the static fluid pattern. In addition, this method is more appropriate to the actual situation in the crust than the method of weight/special weight. The authors of this paper illustrating, with the Jiaodong gold deposit as an example, the metallogenic depth correction via structure conclude that the depth of the most deposits, lower than 4-6 km, is often 2.5 km. Therefore, the authors suggest that there exists a second enrichment belt and that ore resources are more potential at the belt of Jiaodong area. These results have been demonstrated by years of exploration.

Primary Deformation Analyses of High Pressure Metamorphic Rock in Western Dabie Mountain Area

In the western Dabie Mountain area, the eclogites have similar compositions and tectonic environment, which could be contrastively researched. Except for the reservation of the early structural deformation inside and outside of the eclogite lens, there is no obvious difference between the characteristics of the foliation and lineation in the eclogite lens from the one in surrounding region. So this paper concludes that the eclogites or blueschists (high pressure metamorphic rocks, i. e. HPM) are basically situated in the original position. The eclogites are mostly superposed by the ductile shear zone and show the feature of structural displacement, but so far we have not discovered any large scale structural zone to uplift eclogite return. Based on the analyses of finite strain measure, petrofabric analysis and TEM image for some minerals such as quartz and garnet, we could efficiently know the deforming characteristics of the eclogite in the prophase and anaphase of the main deforming epoch, and finally determine the forming condition of eclogite according to the strain and the differential stress. This paper puts forward preliminary conclusion that some HPM rocks could be formed by the deep layer embedding and local stress concentration in the process of regional metamorphism.

Formation Depth of Coesite-Bearing Eclogite,Dabie UHPM Zone,China

The plastic deformation of garnet in coesite bearing eclogite, quartz eclogite and garnet amphibolite of the UHPM complex in Yingshan County in the Dabie Mountains has been studied. The stress generated by the strong tectonic movement was an important component of the total pressure that resulted in the formation of the eclogite in the Dabie UHPM zone. The three dimensional tectonic principal stresses and additional tectonic stress induced hydrostatic pressure [ p s=( σ 1+ σ 2+ σ 3)/3] are reconstructed according to the differential stress and the strain ratio ( α ) of the garnet in the minor coesite bearing eclogite of the Yingshan County. Then the gravity induced hydrostatic pressure ( p g) is calculated following the equation p minus p s, where p is estimated to be 2.8 GPa based on the quartz coesite geobarmeter. Therefore, the thickness of the rock column overlying the coesite bearing eclogite in the Ying shan County is determined ≥32 km. This estimation, significantly different from ≥100 km, the previous one obtained solely based on the weight/specific weight ratio (W/SW), offers a proper explanation for the puzzle that no tracer of the addition of mantle derived material has been found in the Dabie UHPM zone during the process of UHPM, although a number of researchers claim that this process took place at the depth of the mantle (≥100 km). It is concluded that attention should be paid to the additional tectonic stress induced hydrostatic pressure in the study of UHPM zones.

Geobarmal Gradient in Orogenic Belt and Metamorphism Caused by Ultrahigh Pressure

The components and structures of lithosphere, inhomogeneous, are changing incessantly in different periods. Therefore, the state of load, called the pressure, in lithosphere is also incessantly changing. When the lithosphere volume remains the same, namely on the premise of isovolumes, the geobarmal gradient is: (d p /d h) v =( p / h) v +( p / T) v (d T /d h) v . If β =(d p /d h) v /( p / h) v is supposed, then β =1+ rg -1 C v (d T /d h) v . When the geothermal degree in (d T /d h) v is zero, then the pressure grade of lithosphere is equal to the lithostatic grade, which is the minimum value in the pressure grade of lithosphere. Suppose that the lithosphere is made only up of quartz, C v ≌ C p =0.782+5.718×10 -4 T -1.883×10 4 T -2 (J· g -1 · K -1 ) is obtained, which is the minimum one in lithospheric rock, and then the geothermal grade value of 20 ℃/km is calculated according to the geophysical transection data of Qinling Mountain orogeny. The results show that the high pressure and ultrahigh pressure eclogites in Jiangsu Province and Huangzhen, Dabie may, on the condition of incompletely isovolumes, occur in the depths of 17-40 km with the increase in geothermal temperature, whose values of β do not correspond to the theoretic value of 3.08.

Structural Effects of Overpressure Fluid Activities in Yinggehai Basin

The characteristics and distribution of faults in Yinggehai basin discussed in this paper reveal the structural effects of the overpressure fluid expulsion. The rapid subsidence and mud rich intervals of the marine rocks dominate the formation of the overpressure systems and the enormous volumes of the overpressure fluids in the basin. Triggered by some faults, the overpressure fluids were expulsed rapidly from the overpressure compartments to form a series of diapirs in the basin, resulting in the dense fractures or faults and folds in the limbs of diapirs. These fractures and faults provided the migration pathway for the vertical flow of hydrocarbons, so that the gas fields arising from this process might migrate upwards to the sandstone reservoir. Therefore, the hydrocarbon accumulations are usually located in the upper parts of diapiric structures.