The Paleo－Environmental Significance of Sr and Ca Contained in Acid－Soluble Fraction of Plateau Loess Deposits in Xining Basin

The Xining Basin,located on northeastern Qinghai-Xizang Plateau,is adjacent to the Qaidam Bsim and Gobi-desert.Therefore,thick,integrated deposits of loess or paleo-sol sequence were well developed.As is well known,there is much paleo-environmental information recorded in loess and plaeo-sol.So if we deal with paleo-environmental index from different an gles ,especially from loess geochemistry,some important and detailed paleo-environmental evolutional information will be acquired.Based on this,the authors determined the contents of Sr and Ca contained in acid-soluble fraction from the bottom to the top of the Panzishan section.This section was obtained through making a well about 17 m in depth,including S1 paleo-sol that was developed during the last interglacial period.According to the results,it is found that variations in contents of Sr and Ca are well correlated with conversion of loess and paleo-sol.In loess deposits,the contents of Sr and Ca are higher than those in paleo-sol deposits.It is known that loess was deposited under cold-dry climate conditions and paleo-sol was developed during warm-humid climate episodes.It is concluded that due to their mobile behaviors,under cold-dry climate conditions,Sr and Ca are stagnant during weak weathering,but under warm-humid climate conditions,Sr and Ca are easily leached and transported in aqueous solution.Therefore,they can be utilized as indicatiors of climate changes.Moreover,variations in contents of Sr and Ca are more sensitive than susceptibility.The mobile behaviors of Sr and Ca contained in acid-soluble fraction can serve as a new quantitative index to evaluate paleo-environmental changes on the plateau.     

陇西黄土剖面的古地磁研究

The Longxi loess section is quite different from that at eastern Liupanshan Meuntains in stratigraphic structure, but with better magnetic stability than that of Luochuan loess. 43 samples were collected from the 95 m-thick Longxi loess section. Results of palcomagnetic measurement indicates that those samples taken from 0 down to 48m in.the section are normally magnetized. So there is no doubt that this part of Longxi loess was accumulated during the Brunhes epoch corresponding to the Middle Pleistocene Lishi Loess (Q2) and the Late Pleistocene Maian Loess (Q3). On the other hand, samples between 48-82 m within the section are mainly of reversed polarity, formed during the Matuyama reversed polarity epoch, corresponding to the Early Pleistocene Wucheng Loess (Q1). Jaramillo normal event was recorded at between 64-69m within this section. The starting point of loess accumulation is estimated to be about 1.15 my according to the boundary ages of polarity epochs and event and an accumulation rate of 8 cm per thousand years was obtained.     

陕西洛川黄土剖面中的F／Cl比值——反映古气候的一种地球化学标志

The F/Cl ratio for loess and palcosol has been studied based on the difference in the geochemical behaviour of fluorine and chlorine under hypergene conditions. The variation of F/Cl ratio in the loess section indicates a paleoclimatic evolution from humid to arid since loess accumulation. The maximum F/Cl ratio is found .in the fifth paleosol layer, reflecting an optimum climatic period for pedogenesis. The variation of F/Cl ratio is synchronous with the climatic fluctuation from warm-humid to arid-cold.     

中国黄土物质来源的初步探讨——石英粉砂颗粒表面结构的电子显微镜研究

Loess of China, regardless of its ageexhibites similarity in the surface textures of silt quartz grains, all of them having conchoidal fractures, flat cleavage planes(or plates) and mechanical cracks, and having been rewrked by silica precipitation and chemical dissolution. Silica precipitation an different-degree chemical etching and dissolution are commonly found on thes silt quartz grains indicating that they would be separated from source rocks, such as granite, gneiss, etc. ; or they might have been subjected to relatively long weathering in situ before transported to loessic accumulation areas; or they probably underwent diagenesis when brought into the sedimentation system, followed by weathering and erosion, and then transported to loess accumulation areas. It is evident from the relationship between loess of China, deserts and Gobi, loessic cimposition and its regional distribution that MesozoicCenozoic land detrital sediments widely spreaded in the Ordos and Alxa regions are the principal material source of loessic materials in China.     

Geochemical Characteristics of Eolian Deposits on the Eastern Margin of the Tibetan Plateau and Implications for Provenance

Eolian dust deposition is intimately related to atmospheric circulation and environmental setting of the source region, and therefore is an invaluable tool for studying the evolutionary history of atmospheric circulation patterns and paleoclimatic change. Identifying the provenance of any eolian deposit is crucial not only for reconstructing the paleoenvironmental history of the dust source region, but also for understanding the paleoclimatic significance of various indices. Loess and paleosol samples from the Garze region on the eastern margin of the Tibetan Plateau(TP) were analyzed for their elemental(major and trace elements) and isotopic(Sm-Nd) geochemistry and compared with those of Northern Chinese(NC) loess formed at the same age. The results show that the geochemical compositions of the Garze loess and paleosol samples are similar to those eolian deposits on the Chinese Loess Plateau(CLP), and also resemble the average UCC. This indicates that the eolian deposits on the eastern margin of the TP were derived from well-mixed sedimentary protoliths that had undergone numerous upper crustal recycling processes, just as with the CLP loess deposits. However, compared with NC loess, the Garze samples have higher ∑REE, Li, Rb, Zr, Cs, Hf and Bi concentrations, higher TiO2/Al2O3, Hf/Nb, La/Nb, Th/Nb and lower K2O/TiO2, Zr/Hf, Ba/Rb ratios. From the Sm-Nd isotopic geochemistry, εNd(0) and 147Sm/144Nd values of Garze loess and paleosol samples are clearly lower than the NC loess. The higher Bi, Zr and Hf concentrations are relevant to the widely distributed acid-magmatic rocks in this region, whereas the higher contents of Li, Cs, Rb are attributed to the high background values of the TP. The geochemical characteristics of the Garze loess and paleosol samples further prove that the local glacial and other Quaternary detrital sediments are predominantly the contributors for the eolian deposits on the eastern margin of the TP. Stable element concentrations and their ratios in the Garze loess and paleosol samples formed at different times have relatively greater variation ranges in comparison with the NC loess, indicating that the source regions for eolian deposits have been unstable since the late Early Pleistocene. We attribute the instability of dust sources to variable earth surface conditions and the changeable TP winter monsoon in direction and intensity, which are in turn related to the uplift of the TP.     

黄土中稀土元素的初步探讨

REE oxides in loess are estimated to amount to about 200 ppm. The REE distribution patterns in loess and its clay fraction are characterized by the enrichment of rare earth elements of Ce family. The REE distribution patterns of loess in the middle Huanghe (Yellow River) Valley are consistent with those of sands from the Tengeli desert, probably indicating the consistency of their material sources. The REE distribution patterns are similar to each other in the elay fractions of Malan loess everywhere in the middle Huanghe Valley, indicating the homogeneity in their composition. Close to the average value of the earth‘s crust, the REE distribution patterns in loees and its clay fraction are similar to that of sedimentary rocks (e.g., North American shales), but different from that of chrondrites. It seems to show that large amounts of loessic material were transported from the provenance by moving water into sedimentarysy stems after it had been separated from its precursor, and then transported by wind to where it is now distributed.     

Hydrochemical and isotope characteristics of spring water discharging from Qiushe Loess Section in Lingtai,northwestern China and their implication to groundwater recharge

The loess plateau in northwestern China with an area of 640 000km~2, which has developed the loess deposits with a thickness up to 200m in typical areas, is regarded as a huge carbon stock like the karst area in southwestern China, and plays an important role in regional(even global) carbon cycle. But the spring discharging from loess is poorly known compared with karst spring so far. The objective of this study is to ascertain the characteristics and origin of spring at Qiushe Village, Lingtai County, Gansu Province by hydro-chemical and isotopic methods. The results show that the springs including LGQ, HMQ, YYQ and CZQ are the depression spring and belong to the same shallow aquifer with the well water JZJ. There are not distinct seasonal/diurnal-scale variations on the hydro-chemical characteristics of the spring water(LGQ, HMQ, YYQ, CZQ) and groundwater(JZJ). The hydro-chemical type of groundwater is Ca·Mg-HCO_3. The D and O isotope ratios indicate that the precipitation is the main recharge source of groundwater in study area. And the results of tritium(TU) and Cl concentration suggest that the recharge cycle of groundwater may be more than 60 yrs. Our study shows that the water cycle in loess plateau including rainfall, infiltration, recharge and discharge exerts a continuous impact on carbon stock in loess, which should be paid more attention to in future research on the quantitative reconstruction of paleoclimate.     

TL dating of loess in China

TL ages of 56 loess samples collected from Xinjiang and the middle reaches of the Yellow River were determined by using fine-grained quartz and man-made light source bleaching techniques.The results indicate that :1.The well-preserved loess sections in Xinjiang began depositing 400000 a ago.The loess/paleosol series may be comparable with that observed in the middle reaches of the Yellow River but the depositing time was probably delayed by 30000-50000a.2.The bottom boundary age of the Malan loess varies from one place to another.The TL dates can be divided into two group:70000-130000 a and 30000-50000 a.The TL data are con-sistent with the appearance of two warmer climate periods since the Pleistocene,indicating obvious regional differences in stratigraphic development of loess,i.e.,paleosol occurred in the warm climate period in the same location,but loess was deposited in other locations.The initial age of loess deposition tends to become older from west to east and from south to north .In contrast ,the cold cli-matic belt showed an opposite trend.The TL ages of loess provide strong evidence for the Quaternary stratigraphy throughout geological times and the shift tendency of climatic belt.3.The depositing rate of the Malan loess along the middle reaches of the Yellow River varies from place to place but the TL age shows a linear relationship with the depositing thickness.The de-positing rate of loess in the north is higher than that in the south,suggesting that loess materials are derived from desert areas of northern China.     

The Paleoclimatic Variation Records of Carbonate and Iron Oxides in the Weinan Loess Section

The contents and distributions of CaCO3,Fe2O3.FeO and free Fe2O3 in the Weinan loess section of Shaanxi Province of China were investigated through dense sampling.The results show that the contents of CaCO3 and the ration of Fe2O3/FeO may be chosen as proxy in dices for the precipitation and temperature changes in the formation time of the strata,respectively.According to these geochemical indices,six stages of plaeoclimate evolution are proposed in this region since 142 ka B.P.,and secondary climate changes are discussed as well based on the curves of geochemical indices.     

REE Geochemistry of Loess in Xinjiang ,China

The total rare-earth element values(ΣREE)of loess in the Xinjiang region vary over a range of 128-200 ppm ,with an average of 153ppm .The average REE content of loess lies between the earth‘s crust (155ppm) and sedimentary rocks(151ppm).The Xinjiang loess,with the REE distribu-tion patterns characterized by negative slopes ,is rich in the Ce-family elements, and has a distribu-tion pattern characteristic of sedimentary rocks.The North Xinjiang loess is relatively depleted in Tb,but rich in Yb and Lu.The South Xinjiang loess is relatively rich in light rare-earth elements.This is full proof that the Xinjiang loess comes partly from weathered materials(clay rock,sandstone)in the region studied.The REE distribution patterns in the Xinjiang loess are similar to those in the precipitated dust and Aeolian sand,indicating the same material source.The REE distribution pat-terns in the Xinjiang loess are also similar to those in loess from the middle Yellow River Valley,China and Taskent,the former USSR.This implies that loesses of the three locations(Xinjiang,the mid-dle Yellow River Valley and Taskent) come from a common material source.But the REE patterns in the Xinjiang loess are different from those in wall rocks (volcanic rock,K-bearing volcanic rock).Generally ,LREE/HREE,Eu/Eu* and Ce/Ce* ratios reflect the features of parent materials of loess,indicating that the parent rocks were probably in the early stage of alkaline weathering and the weathered materials existed in an oxidation environment with basic mediums under arid-climatic conditions before transport.As a result,the migration ability of the REE is weak.     

Aeolian dust dynamics in the Fergana Valley,Central Asia,since ～30 ka inferred from loess deposits

Knowledge of the interactions among atmospheric dynamics, dust emissions and climate system is essential to understand the physical mechanisms for the dust lifecycle, their role in loess formation as well as the predictions of future dust concentration. However, these issues still remain relatively poorly known in Central Asia(CA). The extensive loess deposits on the CA pediments provide a promising archive to explore atmospheric dust dynamics and climatic conditions in the past and their association with loess formation. This study investigates the granulometric and magnetic properties of a loess section(named Osh section) in the Fergana Valley, which provides a sensitive record of atmospheric dust dynamics since 30 ka based on radiometric(AMS~(14)C) dating. The frequency-dependent magnetic susceptibility(χfd) and the mean grain size are used to reconstruct the broadscale effective moisture and summer atmospheric dynamics pattern in CA, respectively. The results show that the precession forcing exerts a huge influence on the wind-regime variabilities in CA, but with different physical processes under the impact of the Northern Hemisphere ice sheet(NHIS) before and after 15 ka. The origin of the sedimentation rate variations in the Osh loess is also linked to the NHIS-modulated changes of the atmospheric circulation patterns. Either the strengthened westerlies or the increased surface roughness from higher vegetation cover in loess-deposition areas have significantly accelerated the loess accumulation. As a result, these complicated influence factors of sedimentation rate change in the Osh loess section, especially during the Holocene epoch, may hamper accessibility of the authentic dust emission flux and atmospheric dust concentration in CA.     

Geochronology, Petrology and Geochemistry of Xingdi No. 3 Mafic-Ultramafic Intrusions in the Northeastern Tarim Craton, NW China

The Xingdi mafic-ultramafic intrusions occur in the northeastern margin of the Tarim craton. The Xingdi No. 3 intrusion is the smallest of four intrusions, with an exposed area of 1.7 km~2, and the zircon U-Pb age of the intrusion is 752±4 Ma. The intrusion consists of gabbros, pyroxenites and peridotites, and exhibits a crystallization sequence of the main rock-forming minerals as olivine, orthopyroxene, clinopyroxene and plagioclase. Mineralization occurred at or near the boundaries of the intrusion between pyroxenites and peridotites, and appears as a layered or lenticular shape about 500 m long and 4–15 m wide. The primary sulfides have a relatively simple mineralogy dominated by pyrrhotite-pentlandite-chalcopyrite assemblages, which occur as droplet, star-like and graphic texture and locally sideronitic structures. Geochronological and geochemistry investigations suggest that the Xingdi mafic-ultramafic intrusions and coeval volcanic rock in the Kuluktag area of the Tarim craton formed in an intracontinental breakup environment. Based on the composition of the dominant rockforming minerals and covariant relationships of other oxides versus Mg O, the parental magma of the Xingdi No.3 intrusion belongs to high-Mg tholeiitic basaltic magmas with Mg O of 10.78 wt%. The Xingdi No.3 intrusive rocks are characterized by light REE enrichment relative to heavy REE, negative Nb-Ta anomalies, low ~(143)Nd/~(144)Nd ratios(from 0.511183 to 0.511793) and high initial ~(87)Sr/~(86)Sr ratios(from 0.7051 to 0.7113). The magma was derived from the enriched-lithospheric mantle and was contaminated during emplacement. According to rock assemblages, mineralization, olivine characteristics, geochemical characteristics and mass balance, there are better copper-nickel ore prospects in the Xingdi No.3 intrusion than in the other three intrusions in the area.     

The Medieval Warm Period and Little Ice Age in the Daihai Area, North China

Rb/Sr ratio, CaCO3 content, organic carbon (Corg) concentration, magnetic suscep-tibility and clay mineralogy of 4.0 m sediments samples recovered from Daihai Lake, northern China, are presented in the paper. Weathering and paleoclimatic change history during the last 2300 years is reconstructed in terms of the variations of Rb/Sr ratios in the sediment sequence, including the Little Ice Age and Medieval Warm Period. Our results suggest that the evolution processes of weathering and paleoclimate can be rebuilt in terms of the variations of Rb/Sr ratios in the lake sediment sequence, in combination with magnetic susceptibility, Corg, CaCO3 contents and clay mineralogy.     

Lead isotope and trace metal characteristics of particulates accumulated on vehicular tunnel ceilings in Hong Kong and Guangzhou, SE China

Ever-increasing accumulation of trace metals in the environment is a serious health concern. This is a valid apprehension particularly in urban settings where the population and the level of trace metal contamination are increasing rapidly. Hong Kong and Guangzhou are the most populated cities in the region of the Pearl River Delta, the fastest growing economic region in the world. In unconfined urban compartments, particulates deposited on the earth surface such as road dusts and gully sediments are good indicators to assess the level of trace metal contamination. In such locations, however, source partitioning of the pollution, which is essential to assess and then reduce the potential environmental risk, is a difficult task. This is mainly due to the complexity and variability of site-specific natural and anthropogenic conditions. This leads us to focus on the particulates accumulated in relatively confined settings, such as vehicular tunnels. Our recent studies demonstrated that particulates accumulated on the ceiling of tunnels are useful to characterize particulates mainly derived from traffic related activities. With this in mind, Pb isotope ratios and trace metal contents of the ceiling dusts from the Cross-Harbor Tunnel （CHT） in Hong Kong and the Pearl River Tunnel （PRT） in Guangzhou were characterized and compared with each other in order to evaluate the environmental impact of traffic derived inorganic pollutants in the region. The ceiling dusts from both tunnels were dominated by silt-sized particles （〈63 μm） which are significantly freer than those of road dusts, usually larger than （〉200 μm）. In general, Cr, Ni, Pb and especially Cu contents of the ceiling dusts from CHT were noticeably higher than those from PRT. This was partly attributed to longer residence time of particulates in CHT, which has been operating since 1976, while the PRT was open to public in 1996. On the other hand, Co, Fe, Mn and Zn contents of the ceiling dusts from CHT were similar to those from PRT, while Ca and Mg contents were noticeably lower.     

Molecular Records of Primary Producers and Sedimentary Environmental Conditions of Late Permian Rocks in Northeast Sichuan, China

A series of biomarkers were identified in the aliphatic and aromatic fractions of the extracts from Late Permian Dalong (大隆) and Wujiaping (吴家坪) formations in Shangsi (上寺) Section, Northeast Sichuan (四川), South China, on the basis of the analysis of gas chromatography-mass spectrometry (GC-MC). The dominance of lower-molecular-weight n-alkanes throughout the profile suggests the dominant contribution of algae and bacteria to the organics preserved in the marine section. Wujiaping Formation is characterized by the elevated contribution from algae as well as other photoantotrophs such as photosynthetic bacteria as shown by the molecular ratios of hopanes to steranes or tricyclic terpanes as well as the ratio of pristane (Pr) and phytane (Ph) to C17 and C18 n-alkanes. This is in accord with the data from the microscopic measurement on the calcareous algae. In contrast, Dalong Formation is featured by enhanced contribution from bacteria and probably terrestrial organics indicated by the enhanced C24 tetracyclic terpanes relative to tricyclic terpanes. The two formations also show a distinct discrimination in sedimentary environmental conditions including redox condition and salinity. The anoxic condition was only found in the middle of the Dalong Formation as shown by the ratios of Pr/Ph and dibenzothiophene to phenanthrene, consistent with the reported data of Mo and U. An enhancedsalinity indicated by the homohopane index is observed at the shallow Wujiaping Formation. On the basis of the composition of primary productivity and the redox condition, Dalong Formation is proposed, herein, to be potential hydrocarbon source rocks in the study site. It is notable that the topmost end-Permian is characterized by a large perturbance in both the redox condition and salinity, with oxic conditions being frequently interrupted by short-term anoxia, likely showing a causal relationship with the episodic biotic crisis across the Permian-Triassic boundary.     

A Mathematical Calculation Model Using Biomarkers to Quantitatively Determine the Relative Source Proportion of Mixed Oils

It is difficult to identify the source（s） of mixed oils from multiple source rocks, and in particular the relative contribution of each source rock. Artificial mixing experiments using typical crude oils and ratios of different biomarkers show that the relative contribution changes are non-linear when two oils with different concentrations of biomarkers mix with each other. This may result in an incorrect conclusion if ratios of biomarkers and a simple binary linear equation are used to calculate the contribution proportion of each end-member to the mixed oil. The changes of biomarker ratios with the mixing proportion of end-member oils in the trinal mixing model are more complex than in the binary mixing model. When four or more oils mix, the contribution proportion of each end-member oil to the mixed oil cannot be calculated using biomarker ratios and a simple formula. Artificial mixing experiments on typical oils reveal that the absolute concentrations of biomarkers in the mixed oil cause a linear change with mixing proportion of each end-member. Mathematical inferences verify such linear changes. Some of the mathematical calculation methods using the absolute concentrations or ratios of biomarkers to quantitatively determine the proportion of each end-member in the mixed oils are deduced from the results of artificial experiments and by theoretical inference. Ratio of two biomarker compounds changes as a hyperbola with the mixing proportion in the binary mixing model, as a hyperboloid in the trinal mixing model, and as a hypersurface when mixing more than three end- members. The mixing proportion of each end-member can be quantitatively determined with these mathematical models, using the absolute concentrations and the ratios of biomarkers. The mathematical calculation model is more economical, convenient, accurate and reliable than conventional artificial mixing methods.     

Jurassic-Cretaceous Boundary Strata of the Somanakamura Group in NE Japan and their Correlation with Coeval Terrestrial Deposits in China

Defining the Jurassic-Cretaceous boundary is a controversy in stratigraphic study of the world. It has been widely accepted that this boundary can be defined at the bottom of Berriasian in Tethys, with the appearance of the ammonite Berriasella jacobi dating to ca. 145 Ma. However, it is difficult for the widespread terrestrial deposits in China to correlate with the international standard of marine facies. The Somanakamura Group in Japan is represented by a succession of marine-continental transitional strata. It provides a bridge of marine and nonmarine stratigraphic correlation. The ammonite and radiolarian fossils preserved in this group suggest an age from Bajocian to early Valanginian. The J-K boundary was defined in or atop the Tomizawa Formation of the group according to the ammonite data. The present authors study the fossil spores and pollen newly found from the Tomizawa and Koyamada formations. Three assemblages have been recognized. They are Assemblage 1 (Cyathidites-Classopollis) from the upper part of the Tomizawa Formation, Assemblage 2 (Cyathidites-Jiaohepollis) from the lower part of the Koyamada Formation, and Assemblage 3 (Cyathidites-Spheripollenites-Ephedripites) from the middle to upper part of the Koyamada Formation. With the reference of ammonite evidence, the J-K boundary can be defined between Assemblage 1 and Assemblage 2. This palynological J-K boundary can be correlated with that of terrestrial sequence in China. However, local biostratigraphy imply that the continental J-K boundary in China is of 135 or 137 Ma age. It has a considerable discrepancy from the marine standard. Biogeographically, the distribution pattern of spores and pollen in southern China is in accordance with that in the Somanakamura Group, which parallels the Tuchengzi Formation in northeastern China. By the palynological correlation between the Somanakamura Group and the strata in southern China, and then with the sequence in northeastern China, it is suggested that the continental J-K boundary is located in the Tuchengzi Formation.     

Types and Characteristics of Volcanostratigraphic Boundaries and?Their Oil-Gas Reservoir?Significance

Just like in sedimentary stratigraphy, the volcanostratigraphic boundary is an important factor for constructing volcanostratigraphic framework. The fundamental factor of volcanostratigraphic boundaries is to classify the types and define their characteristics. Based on field investigation and cross-wells section analysis of Mesozoic volcanostratigraphy in NE China, 5 types of volcanostratigraphic boundaries have been recognized, namely eruptive conformity boundary(ECB), eruptive unconformity boundary(EUB), eruptive interval unconformity boundary(EIUB), tectonic unconformity boundary(TUB) and intrusive contacts boundary(ICB). Except ICB, the unconformity boundaries can be divided into angular unconformity and paraconformity. The time spans and signs of these boundaries are analyzed by using age data of some volcanic fields that have been published. The time spans of ECB and EUB are from several minutes to years. In lava flows, cooling crust is distributed above and below ECB and EUB; in pyroclastic flows, airfalls and lahars, a fine layer below these boundaries has no discernable erosion at every part of the boundary. EUB may be curved or cross curved and jagged. The scale of ECB/EUB is dependent on the scale of lava flow or pyroclastic flows. The time span of EIUB is from decades to thousands of years. There is also weathered crust under EIUB and sedimentary rock beds overlie EIUB. In most instances, weathered crust and thin sedimentary beds are associated with each other laterally. The boundary is a smooth curved plane. The scale of EIUB is dependent on the scale of the volcano or volcano groups. The characteristics of TUB are similar to EIUB’s. The time interval of TUB is from tens of thousands to millions of years. The scale of TUB depends on the scale of the basin or volcanic field. Both the lab data and logging data of wells in the Songliao Basin reveal that the porosity is greatly related to the boundaries in the lava flows. There is a high-porosity belt below ECB, EUB or EIUB, and the porosity decreases when it is apart from the boundary. The high-porosity belt below ECB and EUB is mainly contributed by primary porosity, such as vesicles. The high-porosity belt below EIUB is mainly contributed by primary and secondary porosity, such as association of vesicles and spongy pores, so the area near the boundary in lava flows is a very important target for reservoirs.     

Geochemistry, Geochronology and Lu-Hf Isotopes of Peraluminous Granitic Porphyry from the Walegen Au Deposit, West Qinling Terrane

Walegen Au deposit is closely correlated with granitic intrusions of Triassic age, which are composed of granite and quartz porphyries. Both granite porphyry and quartz porphyry consist of quartz, feldspar and muscovite as primary minerals. Weakly peraluminous granite porphyry(A/CNK=1.10–1.15) is enriched in LREE, depleted in HREE with Nb-Ta-Ti anomalies, and displays subduction-related geochemistry. Quartz porphyry is strongly peraluminous(A/CNK=1.64–2.81) with highly evolved components, characterized by lower TiO_2, REE contents, Mg~#, K/Rb, Nb/Ta, Zr/Hf ratios and higher Rb/Sr ratios than the granite porphyry. REE patterns of quartz porphyry exhibit lanthanide tetrad effect, resulting from mineral fractionation or participation of fluids with enriched F and Cl. LAICP-MS zircon U-Pb dating indicates quartz porphyry formed at 233±3 Ma. The ages of relict zircons from Triassic magmatic rocks match well with the detrital zircons from regional area. In addition, ε_(Hf)(t) values of Triassic magmatic zircons from the granite and quartz porphyries are -14.2 to -9.1(with an exception of +4.1) and -10.8 to -8.6 respectively, indicating a crustal-dominant source. Regionally, numerous Middle Triassic granitoids were previously reported to be formed under the consumption of Paleotethyan Ocean. These facts indicate that the granitic porphyries from Walegen Au deposit may have been formed in the processes of the closing of Paleotethyan Ocean, which could correlate with the arc-related magmatism in the Kunlun orogen to the west and the Qinling orogen to the east.     

陕西洛川黄土剖面中的稀土元素

The ∑REE in Ioesses of different ages in the Luochuan section shows a narrow range of variation, indicating the homogeneity of materials in chemical composition. The REE in loess are concentrated mainly in silt.sized detrital minerals. Loesses of different ages and paleosols all are relatively enriched in rare-earth elements of the Ce family, and show similar REE distribution patterns. The fractinnatlon among various REEs in the loess is different from that in morainic, marine and lacustrine sediments,but is similar to that in sand samples from deserts in Northwest China. The ratios of Ce/Ce^* and Eu/Eu^* reflect that the provenance of loessic materials and their accumulating area are all in an oxidation environment with weakly basic mediums under arid or sub-arid climate.