Structural Geochemical Study of the Yuxi Fold-Thrust Belt in the Southern North China Plate

The gigantic fold-thrust fracture belt that ex-tends for 900 kmto the NWin the southern NorthChina plate strides across three provinces : Shaanxi ,Henan and Anhui .It is one of the most characteristicstructural deformations in the North China plate .Along this structural belt a series of i mportantcoalfields occur ( e . g. Pingdingshan, Queshan,Huainan) inthe southernmost edge of the North Chi-na coal-concentrating basin. Consequently ,this beltis not only of key value for research onthe…     

Geochemical Features and Formation of the Auriferous Cherts in the Mojiang Gold Deposit,Yunnan

Auriferous cherts in the Middle Carboniferous Jinchang Formation are the dominant host rocks of auriferous quartz veins and mixed orebodies comprised of gold-bearing quartz veins and cherts in the Mojiang gold deposit.The rocks exhibit sedimentary texture and structure and are composed of hot-water deposited minerals.The FeO,Fe2O3,Au and Ag contents of the auriferous cherts are high;the Cr,Ni and Co contents are also high but significantly variable;MnO/TiO2 and TFe/TiO2 ratios are relatively higy.As viewed from a few diagrams that distinguish different chert formations,the auriferous cherts are in or near the range of hot-water deposited cherts.Because the correlation coefficients between Au contents and those of Cr, Ni of the rocks are negative,a great Au amount in the cherts might not be brought about by later hydrothermal alterations.The rare-earth elements,O and Si isotopic compositions of the auriferous cherts demonstrate that the cherts belong to hot-water deposited rocks.The later hydrothermal alterations made the petrochemical compositions of the cherts deviate from the characteristics of hot-water deposition.In general,the geological and geochemical features of the auriferous cherts demonstrate that the rocks were formed by hot water deposition.     

Geochemistry Characteristics of Sediment and Provenance Relations of Sediments in Core NT1 of the South Yellow Sea

The contribution of substance from Yellow River, Yangtze River, and Korean rivers to the sedimentation of Yellow Sea is studied through geochemical analysis and through characterization of the source of the substance about sediment from Core NT1 among the lutaceous area in Central South Yellow Sea. The research finds out that the sediment in Core NT1 mainly comes from Yangtze River and Yellow River, the sediment between 0-7.70 m in upper Core NT1 mainly belongs to Yangtze River source; the sediments between 7.70-16.60 m and 42.0-54.80 m in middle Core NT1 are mainly from Yellow River, the 26 m thick sediment interlayer in it mainly comes from Yangtze River; and the sediment between 54.80-69.76 m in the bottom of Core NT1 is mainly from Yangtze River. The results demonstrate that Yangtze River has been playing a main role in the lutaceous area in the Central South Yellow Sea since early Late Pleistocene, and Yellow River started to influence the continental sedimentation of Yellow Sea from early Warm Glaciation of late Late Pleistocene.     

Geochemical Characteristics of the Plagiogranites in the Vicinity of Bingdaban,Central Tianshan

The tectonic and geochemical characteristics suggest that the plagiogranites exposed in the vicinity of Bingdaban on the northen margin of the central Tianshan uplift zone show a distinct mantle-source character, and their enrichment in LREE and selected enrichment in LILE(large ion lithophile elements)reflect a setting related to an arc tectonic regime.These rocks represent the products formed at shallow levels from mantle-derived magmas modified with subduction components(or super crustal rocks).     

The Geochemical Features and Evolution of Laterite in the Karst Areas of Guizhou Province

Guizhou is one of the dominating karst regions,where laterite is widespread,in China.Seventy-two laterite samples were taken from twelve laterite sections in the karst areas of Guizhou Province,which are possessed of typical subtropical karst geomphological and ecological environmental features and have evolved completely from dolostones and limestones.In terms of the major,trace and REE deta for thd samples this paper discusses the geochemical characteristics of laterite in the karst areas with an attempt to disclose the geochemical process and evolutionary rule of laterite formation.There have been involved three important pedogenetic geochemical precesses in the formation of laterite in the karst areas:(1)enrichment of silicon and aluminum and depletion of calcium and magnesium;(2)enrichment of iron and manganese;and(3) enrichment of aluminum and depletion of silicon.During the formation and evolution of laterite,obvious enrichment and differentiation of trace elements such as Cu,Pb,Zn,Ni,Co,Cr,Pb,F,Cl,and As can be observed,but for the rare-earth elements,their enrichment is remarkable against a weak differentiation.The REE distribution patterns in the laterite are similar to those of its parental carbonate rocks.     

Geochemical Characteristics and Sedimentary Environment of the Salawusu Section

The chemical compositions of and the contents of CaCO3, free Fe2O3 and REE in the sediments of the Salawusu section have been analyzed to investigate the geochemical features of different sedimentary facies and explore their sedimentary enironmetnts during the Late Pleistocene.The con-tents of CaCO3, free Fe2O3 and ∑REE in lacustrine deposits of the middle part of the section are higher than those in the other parts ,except SiO2 which shows an opposite trend.According to the distribution characteristics of the components mentioned above, the section may be divided into four parts which are equivalent to those divided in terms of lithologic characters and sedimentary facies.More remarkable characteristics were observed with respect to the distribution of CaCO3, free Fe2O3,∑REE and SiO2/Al2O3 and FeO/Fe2O3 ratios, which can be adopted as the geochemical indices for paleoclimate division.From the above, four paleoclimate stages have been distinguished for the deposition process.The first stage ,dated at 0.20-0.15Ma, is characterized by a dry and cold climate,the second stage, ranging from 0.15 to 0.07 Ma ,by a warm and slightly dry climate,the third stage ,about 0.07-0.01Ma ,by a cold and humid climate;and prevailing in the last ten thorsand years is the fourth stage,with the climate changing graduall from warm-humid to warm-dry.     

Geochemical Features of the Two Early Paleozoic Ophiolitic Zones and Volcanic Rocks in the Central —Southern Tianshan Region ,Xinjiang

This paper deals with the geochemical features of the two Early Paleozoic ophiolite zones in the central-southem Tianshan region and the central Tianshan igneous rock belt between them.Study results suggest that the central Tianshan belt was an Ordovician volcanic arc with an affinity of continental crust, and the Kumux-Hongluhe ophiolitic zone that is located on the southern margin of central Tianshan has a crustal affinity to back-arc marginal sea.The Aqqikkudug-Weiya ophiolitic zone is an accretionary boundary between the Tuha continental block and the central Tianshan volcanic arc during Late Silurian to Devoniann;Ordovician ophi-olitic blocks,Silurian flysch sequence and HP metamorphic rock relics are distributed along the Aqqikkudug-Weiya zone.Geochemically,ophiolitic rocks in the Aqqikkudug-Weiya zone have an affinity to oceanic crust,reflecting a tectonic setting of paleo-trench or subduction zone .The Early Carboniferous red molasses were deposited unconformably on the pre-Carboniferous meta-mrophosed and ductile sheared volcanic and flysch rocks,providing an upper limit age of the central and southern Tianshan belts.     

A Comparison of the Geochemical Features of Some Loess Sections in New Zealand and China

A comparison of the geochemical features of loesses of New Zealand and China indicates that the distributions of the elements and their variations reflect the fluctuations of climate which can be compared with the oxygen isotope stages and glacial periods.New Zialand loess is different in source from Chinese loess.Therefore ,some differences are also noticed in their chemical compositions .Loess accumulation in New Zealand is later than that in China.Because of more rainfall in New Zealand and different distributions of loess the elements in loess have suffered stronger leaching than in China.     

Geological and Geochemical Studies of Heavy Oil Reservoirs in China

Thickened heavy oils in China are genetically characteristic of continenta .As to their physico-chemical properties,these oils are very high in viscosity and low in sulphur and trace element con-tents.In the group constituents,the concentrations of non-hydrocarbons and asphaltene are very high but those of saturated hydrocarbons and aromatics are very low.The gas chromatograms of alkanes show that these heavy oils have high abundances of iso-alkanes and cyclic hydrocarbons.In all the steroids and terpenoids ,bicyclic sesquiterpenoids,tricyclic diterpenoids,re-arranged steranes and gammacerane are strongly bildegradation-resistent.The formation of heavy oil reservoirs is controlled mainly by late basin ascendance,biodegradation,flushing by meteoric water and oxidation in the oil-bearing formations.Ac-cording to their formation mechanisms,heavy oil reservoirs can be classified as four categories:weathering and denudation,marginal oxidation,secondary migration and thickening of bottom water .Spacially,heavy thick oil reservoirs are distributed regularly:they usually show some paragenetic relationships with normal oil reservoirs.Heavy oil reservoirs often occur in structural highs or in overlying younger strata.Their burial depth is about 200m.Horizontally,most of them are distributed on the margins of basins or depressions.     

Geochemical Characteristics of Different Kinds of Crude Oils in the Tarim Basin,Northwest China

Based on the compositions and distributions of biomarkers in thirty-five representative oil samples, oils from the Tarim Basin of northwestern China are mainly divided into two oil families. One oil family contains relatively low amounts of C₁₅-C₂₀ isoprenoid hydrocarbons and shows pristane predominance with Pr/Ph ratios ranging from 1.50 to 3.00. The GC/MS analytical data of these oils show the occurrence of abundant hopanes, and low concentrations of steranes and tricyclic terpanes with hopanes/steranes ratios from 6.25 to 12.24 and tricyclic terpanes/hopanes ratios from 0.03 to 0.24. These oils contain low drimane relative to homodrimane (C₁₅/C₁₆ < 1.0) and abundant rearranged bicyclanes in bicyclic sesquiterpanes. They are dominated by low carbon number (C₁₉-C₂₁) compounds in the tricyclic terpanes, and are rich in rearranged hopanes, C₂₉Ts and an unknown C₃₀ compound in pentacyclic triterpanes. These geochemical characteristics suggest that the oils were generated mainly from terrigenous organic matter. The other oil family shows remarkably different biomarker compositions and distributions. The oils revealed Pr/Ph ratios of about 1.0, high drimane/homodrimane ratios (>1.0), low hopanes/steranes ratios (0.65–2.50), high tricyclic terpanes/hopanes ratios (0.30–2.00) and a dominant peak at C₂₃ in tricyclic tepanes, suggesting a marine organic origin. Oil-source rock correlation indicates that these two oil families seem to have been derived from Mesozoic Jurassic-Triassic terrestrial source rocks (shales and coal seams) and Lower Paleozoic Ordovician-Cambrian marine source rocks, respectively.     

Evidence from Fe- and Al-rich metapelites for thrust loading in the Transangarian region of the Yenisey Ridge, eastern Siberia

In the Transangarian region of the Yenisey Ridge in eastern Siberia (Russia), Fe‐ and Al‐rich metapelitic schists of the Korda plate show field and petrological evidence of two superimposed metamorphic events. An early middle Proterozoic event with age of c.1100 Ma produced low‐pressure, andalusite‐bearing assemblages at c. 3.5–4 kbar and 540–560 °C. During a subsequent late Proterozoic event at c. 850 Ma, a medium‐pressure, regional metamorphic overprint produced kyanite‐bearing mineral assemblages that replaced minerals formed in the low‐pressure event. Based on the results of geothermobarometry and P–T path calculations it can be shown that pressure increased from 4.5 to 6.7 kbar at a relatively constant temperature of 540–600 °C towards a major suture zone called the Panimba thrust. In order to produce such nearly isothermal loading of 1–7 °C km ^?1, we propose a model for the tectono‐metamorphic evolution of the study area based on crustal thickening caused by south‐westward thrusting of the 5–7 km‐thick upper‐plate metacarbonates over lower‐plate metapelites with velocity of c. 350 m Myr^?1. A small temperature increase (up to 20 ± 15 °C) of the upper part of the overlapped plate is explained by specific behaviour of steady‐state geotherms calculated using lower radioactive heat production of metacarbonates as compared with metapelites. The suggested thermal‐mechanical model corresponds well with P–T paths inferred from obtained thermobarometric data and correlates satisfactorily with P–T trajectories predicted by other two‐dimensional thermal models for different crustal thickening and exhumation histories.     

Neoproterozoic collisional metamorphism in overthrust terranes of the Trans-Angarian Yenisey Ridge,Siberia

Four polymetamorphic complexes in the vicinity of regional faults in the Trans-Angarian region of the Yenisey Ridge were studied to determine their metamorphic evolution and to elucidate distinctive features of the regional geodynamic processes. Based on our geological and petrological studies using geothermobarometry and P–T path calculations, we show that a Neoproterozoic medium-pressure metamorphism of the kyanite-sillimanite type at c. 850 Ma overprinted regionally metamorphosed low-pressure andalusite-bearing rocks. A positive correlation between rock ages and P–T estimates for the kyanite-sillimanite metamorphism provides evidence for regional structural and tectonic heterogeneity. The medium-pressure recrystallization was characterized by (1) localized distribution of metamorphic zones in the area directly underlying thrust faults with a measured thickness of 2.5–8 km; (2) syntectonic formation of kyanite-bearing mineral assemblages related to thrusting; (3) gradual increase in metamorphic pressure towards the thrust faults associated with a low metamorphic field gradient (from 1–7 to 12°C/km); and (4) equally steep burial P–T paths recorded for the highest grade rocks. These specific features are typical of collisional metamorphism during overthrusting of continental blocks and are evidence of near-isothermal loading in accordance with the transient emplacement of thrust sheets. The proposed model for tectono-metamorphic evolution of the study areas due to crustal thickening at high thrusting rates and subsequent rapid exhumation explains these tectonic features. Data analysis allowed us to consider the medium-pressure kyanite-bearing metapelites as a product of collisional metamorphism, reflecting unidirectional thrusting of Siberian cratonal blocks onto Yenisey Ridge along regional deep faults (Angara, Mayakon, and Chapa areas) and by opposite movements in the zone of secondary splay faults (Garevka area).     

Late Precambrian OIB magmatism in the Kuznetsk Alatau,Siberia: Geochemical features of the Kulbyurstyug Formation volcanics

Basaltoids of the Vendian–Cambrian Kulbyurstyug volcanic complex in the eastern part of Kuznetsk Alatau have a high content of titanium (TiO₂ 3–4 wt %). They are relatively enriched with LILE and HFSE (Ba 360–900, Zr 160–726, Nb 66–101, ΣREE up to 225–329 ppm), and demonstrate a fractionated spectrum of REEs (La/Yb ～ 13–17), high Nb/U (44–66), and low Th/Ta and Th/Nb. These features are comparable with derivatives of OIB (Ocean Island Basalts) magmatism. The rock geochemistry suggests the possible formation of an initial mafic melt in the garnet-bearing peridotite mantle with 2–4% degree of melting and the presence of a small amount of spinel. By the age and composition, the studied volcanics are correlated with the OIB magmatism products, occurring in the adjacent Gorny Altai and also considered to be derivatives of intraplate magmatism of Riphean–Early Cambrian age. This magmatism was caused by plume activity during formation of the Paleo-Asian Ocean.     

The age and petrologic and geochemical conditions of formation of the Kogtakh gabbro-monzonite complex in the Kuznetsk Alatau

We present new data on the U-Pb age of zircons, geochemistry of trace elements, and isotope (Nd, Sr, O) composition of rocks of the Kogtakh gabbro-monzonite complex on the eastern slope of the Kuznetsk Alatau Ridge. The established age of zircon in the rocks of the main intrusive phases (500-480 Ma) is taken as the time of formation of the Kogtakh complex in the Late Cambrian-Early Ordovician, during the accretion-collision stage of evolution of the Central Asian Orogenic Belt. The distribution and ratios of LILE and HFSE in the rocks suggest that the intrusions proceeded under interaction of the PREMA + EM/OIB mantle plume and the suprasubductional lithospheric IAB mantle. The mantle-crust interaction led to the heterogeneous isotopic composition of neodymium in the magma source (?_Nd(T) ~ 3.5-5.4). The mixing of different mantle and continental-crust materials resulted in an increase in isotope parameters: (⁸⁷Sr/⁸⁶Sr)_T ~ 0.7039-0.7052, ?_Sr(T) ~ 0-19, and δ¹⁸O ~ 6.5-8.8‰ (SMOW). The REE ratio in the least differentiated gabbroids indicates different depths of probable magma chambers and the formation of their primary (Tb/Yb_PM > 1.8) mafic melts at different degrees of melting of the model garnet peridotite.     

Cambrian paleomagnetism in the Bateni Ridge: evolution of the Kuznetsk Alatau island arc (southern Siberia)

We present paleomagnetic data on the Cambrian volcanosedimentary complexes of the Bateni Ridge, which correspond to the evolution of the Kuznetsk Alatau island arc. The paleomagnetic poles which served as the basis for the apparent polar-wander path (APWP) were determined. The paleomagnetic data were compared with those on the Cambrian poles for other fragments of the Kuznetsk Alatau island arc.For example, the APWP of the Bateni fragment corresponds to that of the Kiya (Martaiga) fragment of the eastern slope of the Kuznetsk Alatau. The distribution of the paleomagnetic poles suggests that the relative drift of the Kuznetsk Alatau arc fragments and the entire island arc system resulted from large-amplitude strike-slips. In the Cambrian, this paleoarc moved from ～10° N to ～10° S and rotated more than 50° clockwise. In the paleomagnetic record, its accretion is reflected in the coincidence of the Late Cambrian–Early Ordovician poles on the eastern slope of the Kuznetsk Alatau with one another and the coeval poles for the Siberian craton. This coincidence also suggests that the general structure of the present Kuznetsk Alatàu formed as early as the Early Caledonian. Nevertheless, the differences in the position of the poles on the western and eastern slopes of the Kuznetsk Alatau suggest that strike-slip activity along the Kuznetsk–Altai deep fault (the major one in the region) continued in the Late Paleozoic and, probably, in the Mesozoic.     

西南印度洋脊龙旂热液区富铝蚀变黏土矿物类型和地球化学特征研究

产出于不同地质背景下的热液成因黏土矿物组成、晶体结构及化学成分等信息,可指示与海底热液作用有关的水-岩反应过程和流体的物理化学条件变化。但目前对于以西南印度洋脊为代表的超慢速扩张脊热液区的黏土矿物研究程度较低,尚未了解其经历的热液蚀变作用及形成过程。本文综合应用SEM-EDS、XRD、FT-IR、EPMA和LA-ICP-MS等多种分析测试手段对采自龙旂热液区矿化蚀变角砾的形貌结构、矿物组成及其化学成分进行系统表征。研究表明:该蚀变角砾中的共生矿物相主要由具二八面体结构、富Al端元的蒙皂石族矿物贝得石与蛋白石组成,角砾中可见呈细粒浸染状的TiO_2。蚀变黏土矿物的化学成分较为单一,具有富Al、贫Mg和贫Fe的特征;其稀土元素总量普遍不高(2. 43～43. 45μg/g),配分模式呈负Eu异常(0. 31～0. 53)而未显示Ce异常(1. 09～1. 16)。推断产出于硫化物堆积丘体边部的矿化角砾长期受酸性、相对还原的、低温热液流体持续叠加和淋滤改造,除Al和Ti以外大部分元素被活化迁移,形成矿物组成简单的富铝黏土矿物相。本研究查明了龙旂热液区新的蚀变黏土矿物类型及其元素地球化学特征,反映该区广泛发育低温热液蚀变作用,为进一步探讨西南印度洋超慢速扩张脊热液成矿系统的水-岩反应过程提供了一定依据。     

Neoproterozoic accretionary and collisional events on the western margin of the Siberian craton: new geological and geochronological evidence from the Yenisey Ridge

The geological, structural and tectonic evolutions of the Yenisey Ridge fold-and-thrust belt are discussed in the context of the western margin of the Siberian craton during the Neoproterozoic. Previous work in the Yenisey Ridge had led to the interpretation that the fold belt is composed of high-grade metamorphic and igneous rocks comprising an Archean and Paleoproterozoic basement with an unconformably overlying Mesoproterozoic–Neoproterozoic cover, which was mainly metamorphosed under greenschist-facies conditions. Based on the existing data and new geological and zircon U–Pb data, we recognize several terranes of different age and composition that were assembled during Neoproterozoic collisional–accretional processes on the western margin of the Siberian craton. We suggest that there were three main Neoproterozoic tectonic events involved in the formation of the Yenisey Ridge fold-and-thrust belt at 880–860 Ma, 760–720 Ma and 700–630 Ma. On the basis of new geochronological and petrological data, we propose that the Yeruda and Teya granites (880–860 Ma) were formed as a result of the first event, which could have occurred in the Central Angara terrane before it collided with Siberia. We also propose that the Cherimba, Ayakhta, Garevka and Glushikha granites (760–720 Ma) were formed as a result of this collision. The third event (700–630 Ma) is fixed by the age of island-arc and ophiolite complexes and their obduction onto the Siberian craton margin. We conclude by discussing correlation of these complexes with those in other belts on the margin of the Siberian craton.     

Problems of phytostratigraphy and the correlation of the Lower Jurassic continental sediments in West Siberia and Kuznetsk and Kansk-Achinsk basins

Paleofloral and palynological records of Lower Jurassic sediments in West Siberia, Kuznetsk (Kuzbass), and Kansk-Achinsk basins and their correlation are discussed. In a number of recent papers dedicated to the Jurassic stratigraphy of Siberia this problem is ambiguously treated. The reference palynological scale has been developed for the Jurassic West Siberian sediments and an uninterrupted succession of floral assemblages associated with it and with regional stratigraphic units has been recognized. On this basis the scheme of the correlation between the Lower Jurassic sediments of the Kansk-Achinsk and Kuznetsk basins and West Siberia permitting a better age estimate of coal-bearing deposits, is proposed.     

Arsenic and other Trace Metals in Sediments from the Kara Sea and the Ob and Yenisey Estuaries, Russia

Major (Al and Fe), minor (Mn) andtrace (As, Cd, Co, Cr, Cu, Hg, Li, Mo, Ni, Pb, Sb, Vand Zn) metals along with material of grain size<63 m, TOC and TN have been determined insediment grab and core samples from the Kara Sea, andthe Ob and Yenisey estuaries, Russia. Surprisingly,the levels of trace metals, with the exception of As,were much lower than was anticipated from speculativereports of extensive contamination in the Arcticmarine areas adjacent to the Siberian coastline ofRussia. Lithium normalization indicates that theabundance and distribution of the metals, with theexception of As and Mo, are controlled by theaccumulation of their fine grained aluminosilicatehost minerals at sites determined by hydrodynamicconditions in the Kara Sea and in the estuaries. Metallevels in the Kara Sea and the Ob and Yeniseyestuaries, except for some anomalous As, Cu and Nivalues, are close to natural baseline levels of otherEurasian Arctic shelf sediments. High levels of As,however, occur in surface and subsurface sediments.The accumulation of As, as well as Mo, can beattributed to the post-depositional diagenetic effectsof Fe-Mn cycling both at and near the sediment waterinterface. Subsurface As and Fe maxima and minimasuggest alternating oxic and anoxic water conditionsduring post-glacial rises in sea level. In contrast tothe results from the adjacent Pechora Sea, in the KaraSea there is no correlation between the levels of Asand radionuclides in the sediments.     

Diffusion-controlled corona reaction and overstepping of equilibrium in a garnet granulite, Yenisey Ridge, Siberia

Diffusion modelling is applied to layered garnet–pyroxene–quartz coronas, formed by a pressure-induced reaction between plagioclase and primary pyroxene in a metabasic granulite. The reconstructed reaction involves some change in composition of reactant minerals. The distribution of minerals between layers is satisfactorily explained by diffusion-controlled reaction with local equilibrium, in which the diffusion coefficient for Al was smaller than those for Fe, Mg and Ca by a factor of approximately four. Diffusion of Mg towards plagioclase implies a chemical-potential gradient for MgO component in a direction opposite to the changing Mg content of garnet; this is explained by the influence of Al₂O₃ on the chemical potential of the pyrope end-member. Grain-boundary diffusion is suggested to have operated, possibly with composition gradients different from those in the bulk minerals. Chemical-potential differences across the corona are estimated from the variation in garnet composition, enabling affinity (the free energy change driving the reaction) to be estimated as 6.9±1.8  kJ per 24-oxygen mole of garnet produced. This implies that the pressure for equilibrium among the minerals was overstepped by 1.4±0.4  kbar. The probable P–T conditions of reaction were in the range 650–790  °C, 8–10  kbar. Assuming a timescale of reaction between 10⁶ and 10⁸ years, estimated diffusion coefficients for Fe, Mg and Ca are in the range 9×10⁻²³ to 5×10⁻²⁰ m² s⁻¹. These are consistent with experimental values in the literature for solid-state diffusion, including grain-boundary diffusion.