Distribution of clay minerals in marine sediments off Chennai,Bay of Bengal,India： Indicators of sediment sources and transport processes

Clay mineralogy, texture size and statistical analyses were carried out on surface sediments from the continental shelf of Chennai, Bay of Bengal, India. The purpose of this study is to characterize the clay mineral distribution and its relation to the hydrodynamics off Chennai to identify the sources and transport pathways of the marine sediments. Characterization of clay minerals in coastal sediments by Fourier Transform Infrared （FTIR） spectroscopy has provided the association of quartz, feldspar, kaolinite, chlorite, illite and iron oxides （magnetite and hematite） derived from river catchments and coastal erosion. Kaolinite, chlorite, illite, iron oxides, and organic matter are the dominant minerals in Cooum, and Adayar region. High quartz and feldspar zones were identified in Marina, which are being confined the sand zone and paralleling the coast. The strong relationships among the wave energy density, sand, quartz and carbonate revealed that wave induced littoral drift system play a dominant role in transportation and deposition of sediments in the Chennai coast. The sediment texture and minerals data are in agreement well with the previous results of hydrodynamics and littoral drift models in this region. Multivariate statistical analyses （correlation, cluster and factor analyses） were carried out and obtained results suggested that clay minerals and organic matter are trapped in silt and clay particles, whereas quartz, feldspar and carbonate are associated with sand particles. Results of sediment sources and transport processes from this study will be useful to predict the fate of the pollutants released from land or the potential change in sediment delivery to coastal areas.     

Variation characteristics of heavy metals and nutrients in the core sediments of Taihu Lake and their pollution history

The sedimentary environment change, trophic evolution and heavy metals pollution history of the northern Taihu Lake in the last 100 years are studied according to the sedimentary geo-chemical proxies of the core sediments, such as grain size, nutrients, heavy metals, diatom, etc. The nutrients in the sediments depended mainly on the lake internal circulation and the heavy metals were from natural geogenic sources before the 1920s, which were not influenced by human activities generally, and grain size was one of the key factors influencing heavy metals content in the sediments. The alternation of manner and strength of human activities in Taihu Lake catchment before and after the 1920s made the lake sediments coarser, and hence heavy metals and TP content decreased contrasted with that before the 1920s. TP content in sediments and water increased from the 1950s to late 1970s due to anthropogenic pollutants discharge, and the lake belonging to mesotrophic state. TN and TOC content and C/N ratio increased due to the increasing external pollutants into Taihu Lake by human activities, TP content also increased obviously in water and sediments, and the diatom association was dominated by eutrophic species since the late 1970s, indicating the eutrophication state of Taihu Lake in this period. Meanwhile the increasing in heavy metals content, such as Cu, Mn, Ni, Pb and Zn, and their proportion of valid fractions in the sediments indicates that they all result from human pollutants since the late 1970s. The heavy metals in the surface sediments have certain potential biological toxicity due to the higher SEM/AVS ratio.     

Rock-magnetic properties of Neogene sediments, northern flank of Tianshan Mountains

Analyses of rock-magnetic properties of Neogene sediments of the Taxihe section, northern Tianshan Mountains, show that the section can be classified into three categories including lacustrine facies, fluvial facies and alluvial facies, which correspond to the lower, middle and upper of the Taxihe section respectively. The magnetic minerals of the lacustrine facies may be affected by the process of weath- ering, lithogenesis and biolithogenesis besides the source of the sediments. The natural remanence intensities are between 10-3 A/m and 10-2 A/m. The minerals are dominated by magnetite and the high coercive magnetic mineral may be goethite. The magnetic grains are the mixture of PSD SD or SD SP. The natural remanence intensities of the strata of fluvial facies are between 10-2 A/m and 10-1 A/m, about ten times that of the lacustrine facies. The magnetic minerals are mainly magnetite and hematite, and the magnetic grains are mainly PSD. The characteristic remanence (ChRM) carriers are magnetites. In the alluvial facies, the natural remanence intensities are mostly less than 1×10-2 A/m. The magnetic minerals of the series are dominated by magnetite and hematite, almost the same as the fluvial facies. But the difference is that most of the stepwise demagnetization can reveal two components and the ChRM carriers are hematites. The magnetic grains are PSD in terms of the hysteresis parameters.     

Fractionation characteristics of rare earth elements(REEs) linked with secondary Fe,Mn, and Al minerals in soils

Soil secondary minerals are important scavengers of rare earth elements(REEs) in soils and thus affect geochemical behavior and occurrence of REEs. The fractionation of REEs is a common geochemical phenomenon in soils but has received little attention, especially fractionation induced by secondary minerals. In this study, REEs(La to Lu and Y) associated with soil-abundant secondary minerals Fe-, Al-, and Mn-oxides in 196 soil samples were investigated to explore the fractionation and anomalies of REEs related to the minerals. The results show right-inclined chondrite-normalized REE patterns for La–Lu in soils subjected to total soil digestion and partial soil extraction. Light REEs(LREEs) enrichment features were negatively correlated with a Eu anomaly and positively correlated with a Ce anomaly. The fractionation between LREEs and heavy REEs(HREEs) was attributed to the high adsorption affinity of LREEs to secondary minerals and the preferred activation/leaching of HREEs.The substantial fractions of REEs in soils extracted byoxalate and Dithionite-Citrate-Bicarbonate buffer solutions were labile(10 %–30 %), which were similar to the mass fraction of Fe(10 %–20 %). Furthermore, Eu was found to be more mobile than the other REEs in the soils, whereas Ce was less mobile. These results add to our understanding of the distribution and geochemical behavior of REEs in soils, and also help to deduce the conditions of soil formation from REE fractionation.     

Geochemical characteristics of Neogene sandstones of the East and West Siang Districts of Arunachal Pradesh,NE India:implications for source-area weathering,provenance,and tectonic setting

Sandstones from the Neogene Siwalik successions of the East and West Siang Districts of Arunachal Pradesh were analyzed to evaluate source-area weathering,provenance,and tectonic setting by using major,trace,and rare earth elements(REEs)as proxies.The sandstones are classified as lithic arenite and wacke arenite based on their mineralogical compositions.The values of different weathering indices such as Chemical Index of Alteration(CIA;60.93–89.86)and Chemical Index of Weathering(CIW;40–96.8)suggest moderate to intense weathering in the source area.The plot of Th/Sc versus Zr/Sc indicates enrichment of zircon by sediment sorting and/or recycling from a weathered source.The high positive correlation between Al2 O₃ and K2 O points towards a strong influence of the constituent clay minerals on the major oxide composition of the sandstones.Petrographic analysis together with enriched LREE,flat HREE,negative Eu anomalies(Eu/Eu^*=0.47 to 0.90)in the chondrite-normalized diagrams,and the ratios of La/Sc,La/Co,Th/Sc,Th/Co,Cr/Th collectively suggests that the Neogene sediments were derived from felsic igneous and/or reworked sedimentary/metasedimentary sources in an upper continental crustal setting.The geochemical characteristics of the studied Neogene Siwalik sandstones indicate that the sediments were sourced from pre-Himalayan gneisses and granitoids together with metabasic rocks,which had formed in a passive margin tectonic setup.     

Ra's Abdah of the north Eastern Desert of Egypt: the role of granitic dykes in the formation of radioactive mineralization,evidenced by zircon morphology and chemistry

Syenogranitic dykes in the north of Egypt's Eastern Desert are of geological and economic interest because of the presence of magmatic and supergene enrichment of radioactive mineralization. Zircon crystal morphology within the syenogranitic dykes allows precise definition of sub-alkaline series granites and crystallized at mean temperature of about 637 °C. The growth pattern of the zircons suggest magmatic and hydrothermal origins of radioactive mineralization. Hydrothermal processes are responsible for the formation of significant zircon overgrowth; high U-zircon margins might have occurred contemporaneously with the emplacement of syenogranitic dykes which show anomalous uranium(e U) and thorium(e Th) contents of up to 1386 and 7330 ppm, respectively.Zircon chemistry revealed a relative increase of Hf consistent with decreasing Zr content, suggesting the replacement of Zr by Hf during hydrothermal activity.Visible uranium mineralization is present and recognized by the presence of uranophane and autunite.     

Focal Depth of Earthquakes in the Qinghai-Xizang Plateau and Its Tectonic Implication

In the previous work of the present study,moment tensors of 11 major earthquakes in the Qinghai-Xizang Plateau and its surrounding region from 1966 to 1980 are estimated by generalized inversion technique.The seismic source time function and focal depth are immediately determined in the inversion.The results indicated that all earthquakes investigated here are shallow events within the upper crust.The purpose of this paper is to present a summary of the distribution of focal depths of earthquakes in and near the Qinghai-Xizang Plateau from the above result,combining the focal depths of 78 significant earthquakes from 1964 to 1986,which are relocated individually by other authors,as well as to discuss its tectonic implication.     

Geological characteristics and mineralization setting of the Zhuxi tungsten (copper) polymetallic deposit in the Eastern Jiangnan Orogen

The Zhuxi ore deposit is a super-large scheelite(copper) polymetallic deposit discovered in recent years. It grew above copper/tungsten-rich Neoproterozoic argilloarenaceous basement rocks and was formed in the contact zone between Yanshanian granites and Carboniferous-Permian limestone. Granites related to this mineralization mainly include equigranular, middle- to coarse-grained granites and granitic porphyries. There are two mineralization types: skarn scheelite(copper) and granite scheelite mineralization. The former is large scale and has a high content of scheelite, whereas the latter is small scale and has a low content of scheelite. In the Taqian-Fuchun Basin, its NW boundary is a thrust fault, and the SE boundary is an angular unconformity with Proterozoic basement. In Carboniferous-Permian rock assemblages, the tungsten and copper contents in the limestone are both very high. The contents of major elements in granitoids do not differ largely between the periphery and the inside of the Zhuxi ore deposit. In both areas, the values of the aluminum saturation index are A/CNK1.1, and the rocks are classified as potassium-rich strongly peraluminous granites. In terms of trace elements, compared to granites on the periphery of the Zhuxi ore deposit, the granites inside the Zhuxi ore deposit have smaller d Eu values, exhibit a significantly more negative Eu anomaly, are richer in Rb, U, Ta, Pb and Hf, and are more depleted in Ba, Ce, Sr, La and Ti, which indicates that they are highly differentiated S-type granites with a high degree of evolution. Under the influence of fluids, mineralization of sulfides is evident within massive rock formations inside the Zhuxi ore deposit, and the mean SO_3 content is 0.2%. Compared to peripheral rocks, the d Eu and total rare earth element(REE) content of granites inside the Zhuxi ore deposit are both lower, indicating a certain evolutionary inheritance relationship between the granites on the periphery and the granites inside the Zhuxi ore deposit. For peripheral and ore district plutons, U-Pb zircon dating shows an age range of 152–148 Ma. In situ Lu-Hf isotope analysis of zircon in the granites reveals that the calculated e_(Hf)(t) values are all negative, and the majority range from -6 to -9. The T_(DM2) values are concentrated in the range of 1.50–1.88 Ga(peak at 1.75 Ga), suggesting that the granitic magmas are derived from partial melting of ancient crust. This paper also discusses the metallogenic conditions and ore-controlling conditions of the ore district from the perspectives of mineral contents, hydrothermal alteration, and ore-controlling structures in the strata and the ore-bearing rocks. It is proposed that the Zhuxi ore deposit went through a multistage evolution, including oblique intrusion of granitic magmas, skarn mineralization, cooling and alteration, and precipitation of metal sulfides. The mineralization pattern can be summarized as "copper in the east and tungsten in the west, copper at shallow-middle depths and tungsten at deep depths, tungsten in the early stage and copper in the late stage".     

Earthquake-induced soft-sediment deformation structures in the Mesoproterozoic Wumishan Formation,North China,and their geologic implications

Soft-sediment structures are key to defining seismites. Two soft-sediment deformation horizons, bounded by undeformed carbonate strata, have been found in the Wumishan Formation in the Jumahe region, 175 km southwest of Beijing. One is in the lowest part of Wumishan Formation; and the other is in the uppermost part of Litho-member I. The soft-sediment structures in these two horizons fall into three categories: mould-and-sag structures, hydraulic shatterings and liquefaction dikes. The mould-and-sag structures are divided into two types: one developed in tidal-flat sediments, accompa-nied by many liquefaction-related structures and characterized by autochthonous post-earthquake sediments in sags, and the other type developed in deep-water environments, is not associated with liquefaction structures, and is overlain immediately by seismogenic tsunamites. The hydraulic shat-terings are composed of pockets of fluidization conglomerate, sand intrusions, and syndepositional faults. The liquefaction dikes fall into two categories: hydraulic-fracturing dikes and lateral-spreading dikes. The former are steep, planar, and pinch out upwards. The latter are snake-like and characterized by no diapir-related drag structures in surrounding rocks. Examination of the attitudes and strati-graphic positions of these structures suggests that these soft-sediment structures are seismogenic, and consequently, are seismites. Most seismites in the Wumishan Formation are developed near the former western, margin fault of Yanliao rift. This occurrence suggests that they could be related to movements on this fault. Other geological implications are discussed.     

Estimation of internal nutrient release in large shallow Lake Taihu, China

Based on field investigation of wave, sediment suspension and the changes in nutrient concentration of the water column in Lake Taihu, China, we proposed two release models to quantify nutrient release under static and dynamic conditions, respectively. Under static conditions, nutrient release from sediments to the overlying water mainly depends on chemical diffusion induced by concentration gradient, in which the nutrient release is controlled by the temperature, dissolved oxygen concentration in the sediment-water interface, oxidation-reduction potential and the concentration difference between porewater and overlying water. Under dynamic condition (or disturbed condition), both dissolved and particulate nutrients in sediments are released into the water column because of wind-induced sediment suspension. The amount of nutrient release under dynamic conditions is larger than that under the static condition. The release of dissolved nutrients, however, does not increase because the wind induced turbulence made oxidation of metallic elements such as Fe (ferric iron), Mn which are capable of precipitating soluble reactive phosphate (SRP). Under dynamic conditions, therefore, the release of total phosphorus (TP) increases dramatically but the release of SRP is close to those under static conditions. In sediments of Lake Taihu, high Fe content leads to a high ratio of Fe to P contents in sediments (Fe:P ratio). Under dynamic conditions, therefore, nutrient release is controlled by the intensity of disturbance, sediment consolidation and nutrient content in sediments. As for dissolved nutrients, especially SRP, the release is also controlled by the intensity of dynamic re-oxidation, Fe content in sediments and nutrient concentration gradient between porewater and overlying water. Based on these two release modes, the release flux in Lake Taihu has been estimated. In the static condition (i.e. laboratory experimental condition), total release of NH4 -N for whole lake is ca. 10,000 ton/a, and PO43--P is ca. 900 ton/a. In the dynamic condition, nutrient release following sediment suspension was estimated according to three different intensities of wind forcing which were defined as "calm" (wind speed is less than 2 m/s), "gentle" (wind speed is greater than 2 m/s and less than 6 m/s) and "gust" (wind speed is greater than 6 m/s). The release rate in the condition of "calm" was estimated in terms of the nutrient release in the laboratory experimental static condition; whereas the release rate in conditions of "gentle" and "gust" was estimated in terms of measurement during sediment resuspension conducted in flume experiments. With the observation of wind velocity and frequency in 2001, each type of wind forcing took the frequency of 12%, 82% and 6% for "calm", "gentle" and "gust", respectively. The yearly release of nitrogen was 81,000 ton and phos- phorus was 21,000 ton, which is about 2-6 folds of annual external loading, respectively.     

Rare earth element geochemistry of the Permo-Carboniferous clastic sedimentary rocks from the Spiti Region,Tethys Himalaya:significance of Eu and Ce anomalies

Siliciclastic sedimentary rocks,including sandstones and associated shales,from the Permo-Carboniferous Kanawar Group of NW Tethys Himalaya,Spiti Region,India were examined geochemically to monitor the evolutionary changes in the upper continental crust in the Himalaya.The rocks are characterized by consistent rare earth element(REE) patterns with light REE enrichment(La_N/Yb_N = 5.3-28.2) and flat heavy REE patterns.The∑REE values are high(up to 281 ppm) with large negative Eu anomalies(avg.Eu/Eu~* = 0.57).The REE characteristics of the sediments are similar to those of postArchean Australian shales and North American shale composite.La/Th values(avg.2.34) correspond to a relatively felsic composition of the terrestrial igneous rocks standard(La/Th of G-1 = 2.3).The evolved felsic composition of the sediments probably relates to widespread acidic activity in the source.The REE patterns and Th/U values seem to have been affected by the sedimentary environment as well as by the provenance.The presence of positive Ce anomalies in some sediments may be the result of post-depositional processes.Moreover,the Permo-Carboniferous sediments indicate that hydraulic sorting,even over short transport distances,is capable of concentrating enough accessory phases to influence REE composition and to develop negative Eu anomalies.High ∑REE,La/Yb,and Th/U contents and large negative Eu anomalies reveal that the sediments were deposited in an oxidizing environment,suggesting the surficial environment became oxidizing around the Carboniferous-Permian boundary in the Indian craton.     

Environmental magnetic comparisons between distal and proximal sediments of Huangqihai Lake,Inner Mongolia,China

Spatial variations in lake sediments are a product of their depositional heterogeneities.To better correlate synchronous strata in Huangqihai Lake,two sections were excavated from the lake-center and the lake-shore.The study involved detailed lithology,mineralogy,and rock magnetic analysis.The results show that the frequency-dependent susceptibility,which is related to the content of super-paramagnetic minerals,is relatively easy to contrast between the two sections.However,mass susceptibility and saturation isothermal remanence,which are related to the content of total magnetic minerals,are difficult to contrast.A possible reason for this difference is that small-particle super-paramagnetic minerals are formed mainly during pedogenic processes,which correlates well with climate change and avoids the effects of local factors when corrections are made at the whole-lake scale.Both sections can be divided into three stages,which appear to be well correlated with previous results obtained through other environmental proxies.     

Recognition of Gas Hydrate Using AVO-Attribute Crossplots Based on the Porous Medium Theory

Gas hydrate is gradually considered as a potential energy resource. The presence of gas hydrate is commonly inferred from the appearance of “bottom simulating reflector”(BSR) on seismic section. Understanding the properties of hydrate-bearing sediments and studying the AVO characteristics of BSR are of great significance. Although more and more domestic and international studies have been conducted on the subjects mentioned above, they are still in the primary stage and need a long way to go to be appled in practice, especially in the field of gas hydrate. Aiming at the identification of gas hydrate, we studied the characteristics of the AVO attributes based on the Biot‘s theory when the sediments were bearing gas hydrate or free gas. The AVO attribute crossplots obtained from seismic sections with the forward simulation by means of staggered-grid finite-difference were compared with that of theoretic models. The coincidence shows that utilization of AVO attribute crossplots is an effective way to recognize gas hydrate and free gas.     

Phosphate mineral accumulation in lake sediment to form a secondary phosphate source: A case study in lake sediment around Eppawala Phosphate Deposit (EPD) in Sri Lanka

Global phosphorus scarcity will result in significant consequences for future food security with the depletion of current phosphate reserves.Therefore,exploration of new phosphorus sources is essential to address future phosphorus scarcity.The current study investigated the geochemical potential of lake sediment around the Eppawala Phosphate Deposit(EPD)in Sri Lanka to be used as a low-grade phosphorus source for agricultural purposes.Jaya-Ganga is a man-made water canal that drains through the EPD feeding three lakes,namely,upstream Ihalahalmilla Lake and Koon Lake,and downstream Kiralogama Lake with respect to the EPD.Three cores(cores A,B,and C)were collected from the above three lakes and major oxides and minerals distributions along the cores were analyzed.Notable high enrichment of phosphorus pentoxide(P2O5)content and high P2O5 solubility values were measured in the top 60 cm sediment layer in Core B and throughout the Core C compared to the Core A.This high enrichment of P2O5 content in the same sediment columns were confirmed by the comparison with the Upper Continental Crust(UCC)values and literature survey.According to the X-ray Diffraction(XRD)results,phosphate minerals,such as fluorapatite,crandallite,and millisite were abundantly found in the same sediment columns.Therefore,these phosphate minerals can be considered as pathfinding minerals for soluble phosphates in sediment cores.Thus,sediment with high P2O5 content and high solubility in downstream Kiralogama Lake showed the potential for application of these sediments as a direct phosphate source in agricultural purposes.Furthermore,the current study has introduced a new area of interest,i.e.,soil and sediments around major phosphate deposits,for the exploration of new phosphate sources to meet future phosphorus demand.     

Aoyitake plagiogranite in western Tarim Block, NW China: Age, geochemistry, petrogenesis and its tectonic implications

SHRIMPP U-Pb zircon age and geochemical and Nd isotopic data are reported for the Aoyitake plagiogranite in western Tarim Block, NW China. The plagiogranite intruded the Middle Pro- terozoic and Lower Carboniferous with an exposure area of ca. 60 km2 and crystallized at 330.7±4.8 Ma. Rock types mainly include tonalite, trondhjemite and minor amounts of diorite and quartz-diorite. Feldspars in the rocks are dominated by oligoclase-andesine, and minor perthite observed locally. The granites are sodic with Na/K ratios (molar) between 4 and 87. Total REE (50-220 ppm) show a clear positive correlation with SiO2. There is no LRRE/HREE fractionation (LaN/YbN=0.5-1.5), me- dium negative Eu anomalies (δ Eu=0.3-0.6), high Y content and low Sr/Y ratio (~1.0). These granites exhibit relatively juvenile Nd T2DM model ages of 470 to 580 Ma and positive εNd(331 Ma) values of 6.23 to 7.65. The aforementioned characteristics are similar to those of ocean island or ocean ridge plagiogranites. However, the regional geology, especially its scale, precludes that the plagiogranite pluton was derived directly from fractionational crystallization of mantle-derived basaltic magma. We interpreted that the primary magma of the pluton might be tonalitic in composition generated by ca. 50% partial melting of the juvenile basaltic crust. The primary magma experienced intensive frac- tionational crystallization, and intruded into the middle to upper crusts to form the granite pluton. In combination with the previous regional geological data, it is concluded that the plagiogranite pluton was emplaced within the Tarim Block in respond to the Carboniferous continental rifting along the Tianshan orogenic belt.     

Study on Active Faults in Quaternary Unconsolidated Sediments by Microstructural Method

Active faults in Quaternary unconsolidated sediments are important indicators to determine paleoearthquake events,They can be studied by macroscopic geological survey,but some problems have been encountered,such as invisible active faults.The authors found an approach to solve these problems by microstructural observation.Firstly,oriented original-state samples of Quaternary Unconsolidated sediments in active fault zones are collected.Secondly,the samples are consolidated in laboratory.Thirdly,microstructureal slides are gound in three-dimension coordinate system.Lastly,microstructures are observed in the microscopic field.By this method,the movement properties of active faults can be determined in lack of the support of macroscopic data.The invisible faults in Quaternary unconsolidated sediments can be found and described.The mechanisms of die-out faults can be also studied.oreover,the boundaries between different unconsolidated sediments in engineering sites can be studied to judge whether they are active faults or not.     

Rare earth element chemistry indicates chemical alteration of zircons during the evolution of weathering profile

Although rare earth elements(REEs)in magmatic zircons have been widely used to identify the type and compositional evolution of host rocks,REE distribution patterns during the chemical alteration of zircons need clarification.We investigated REE characteristics in zircons with different degrees of chemical weathering through systematic observation of a granodiorite-weathering profile in southeast China.Despite the relatively stable provenance of the studied profile(zircon U–Pb ages are95.2±4.8 Ma),the zircon REEs exhibited systematic differences in abundance and fractionation patterns from the bedrock to upper layers,e.g.∑PREE,(LREE/HREE)_(CN),and Ce/Ce*.This evidence suggests chemical alteration of zircons during intensive chemical weathering and an expected influence on REE variability in the weathered products due to the presence of REE-bearing minerals.     

Petrogenesis and tectonic implications of Late-Triassic high εNd(t)-εHf(t) granites in the Ailaoshan tectonic zone(SW China)

High εNd(t)-εHf(t) granites are robust evidence for crustal growth. In this paper we report results of petrologic, geochronological and geochemical investigations on the Huashiban granites from the Ailaoshan tectonic zone in western Yunnan(SW China). Zircon grains separated from the two samples(10HH-119 A and 10HH-120A) yield the weighted mean 206Pb/238 U ages of 229.9 ± 2.0 Ma and 229.3 ± 2.3 Ma, respectively, interpreted as the crystallization ages of the granites. Based on our results, in combination with the existing U-Pb geochronological data for the Ailaoshan metamorphic rocks, we propose that the Ailaoshan Group might be a rock complex composed of the Mesoproterozoic, Neoproterozoic, Hercynian, Indosinian and Himalayan components, rather than a part of the crystalline basement of the Yangtze block. The zircon grains show highly depleted Lu-Hf isotope compositions, with positive εHf(t) values ranging from 8.4 to 13.1. The Huashiban granites have high SiO2(72.66 wt%–73.70 wt%), low Mg#(0.28–0.34) with A/CNK=1.01–1.05, and can be classified as peralumious high-K calc-alkaline I-type granites. A synthesis of these data indicates that the Ailaoshan tectonic zone had evolved into a post-collisional setting by the Late-Triassic(229 Ma). Genesis of the Huashiban high εNd(t)-εHf(t) granites involved into two processes:(1) underplating of the sub-arc mantle into the lower crust, and(2) remelting of the juvenile crustal materials in response to the upwelling of the asthenospheric mantle in the post-collisional setting.     

Heavy mineral assemblage characteristics and the Cenozoic paleogeographic evolution in southwestern Qaidam Basin

Based on the analysis of heavy mineral assemblages in Cenozoic southwestern Qaidam Basin, we found that different areas have variable heavy mineral assemblage characteristics, which suggested that there were two source areas—the Altyn Mountains and the Qimen Tagh-East Kunlun Mountains. In Ganchaigou-Shizigou-Huatugou(Area A), which was mainly source from the Altyn Mountains, its heavy minerals were mainly composed of zircon, Ti-oxides, and wollastonite in the Paleoceneearly Eocene and mainly of unstable minerals, especially amphibole, in the middle Eocene-Oligene. Since the late OligoceneMiocene, the heavy minerals were still mainly unstable minerals, but the content of epidote increased and the content of amphibole decreased. In Qigequan-Hongliuquan(Area B), which was the mixed source from the Altyn Mountains and the Qimen Tagh-East Kunlun Mountains, its heavy minerals were mainly garnet, epidote, and amphibole. The source of LücaotanDongchaishan-Kunbei(Area C) was mainly from the Qimen Tagh-East Kunlun Mountains, heavy minerals in the sediments in Area C were mainly zircon and Ti-oxides in Paleogene and garnet, epidote, and amphibole in Neogene. In Yuejin-Youshashan(Area D), where the stable minerals and unstable minerals were present simultaneously, the heavy mineral assemblages was controlled by multi-direction source. The variation of heavy mineral assemblages in southwestern Qaidam Basin shows that Altyn Mountains was of low-lying topographic relief in Paleocene-early Eocene, and the rapid uplift of Altyn Mountains started from the middle Eocene. In Paleogene, the Altyn Tagh Fault had a slow strike-slip velocity, but the strike-slip velocity increased greatly since the late Oligocene, leading to a strike-slip displacement above 300 km since Neogene. Meanwhile, the Qimen Tagh-East Kunlun fault zone was under a stable tectonic stage in Paleogene with the Qimen Tagh Mountain being lowlying hills; since the late Oligocene, the fault zone started to activate and the Qimen Tagh Mountain began to uplift rapidly.