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2020 Vol. 31, No. 2

2020, 31(2): .
Petrology, Geochemistry and Ore Deposits
Geochemical Characteristics of Zoned Chromites in Peridotites from the Proterozoic Miaowan Ophiolitic Complex, Yangtze Craton: Implications for Element Mobility and Tectonic Setting
Huang Yang, Deng Hao
2020, 31(2): 223-236. doi: 10.1007/s12583-019-1278-x
The chrome spinel (chromite) in mantle peridotites from ophiolites can shed light on the formation and evolution process of ophiolites. Podiform chromites were found in the Late Proterozoic Miaowan ophiolitic complex (MOC), Yangtze Craton. Due to the metamorphism and intense deformation, most chromite grains in the MOC peridotites show typical chemical zoning (core-rim texture). The values of major and trace elements largely vary from core to rim within chromite grains, indicating that the chromites have undergone strong alteration and element mobility. Major and trace elements in the core parts of chromites are used to infer the tectonic origins and evolution of mantle peridotites in the MOC. The chromites from the MOC peridotites have higher Cr# values and lower Ni and Ga contents with respect to those from Phanerozoic mantle peridotites, indicating a higher degree of depletion. In-situ major and trace elements (e.g., Ga) characteristics of podiform chromites in the MOC show that chromites from both harzburgites and dunites have strong subduction-related signatures, indicating that the MOC has formed in a supra-subduction setting which is consistent with the geological and geochemical data presented in previous studies.
Geochemistry, Sr-Nd-Pb Isotopic Compositions and Zircon U-Pb Geochronology of Neoproterozoic Mafic Dyke in the Douling Complex, South Qinling Belt, China
Tong Li, Liang Liu, Xiao-Ying Liao, Yong-Sheng Gai, Tuo Ma, Chao Wang
2020, 31(2): 237-248. doi: 10.1007/s12583-020-1298-6
Mafic dyke swarm is widely developed in Proterozoic continental lithosphere. The Gangou diabase dyke from the Xixia area, intruded into the Douling complex of the South Qinling belt, yields zircon U-Pb age of 731 Ma. It has low SiO2 content (49.02 wt.%-49.37 wt.%) and Mg# (34.0-37.7) and shows characteristics of subalkaline tholeiite series. They show high ΣREE (155.5×10-6-184.7×10-6), weak negative Eu anomaly (δEu=0.88-0.93), slight depletion of Nb and Ta, and enrichment of LILE. Their major and trace element, Sr-Nd-Pb isotope and clinopyroxene compositions indicate that it originated from the partial melting of the asthenospheric mantle within-plate extension setting and was slightly contaminated by crust compositions. The similarities of formation age, petrogenesis, source characteristic and tectonic setting for Gangou diabase with mafic dykes in Wudang Block and mafic volcanic rocks in Yaolinghe Group indicate that the South Qinling belt underwent strong continental extension in Neoproterozoic during 796-685 Ma most likely correspondence to the breaking-up of the Rodinia supercontinent.
Magnesium Isotopic Homogeneity of GSR-1 and RGM-2: Two Potential Standards for Mg Isotope Analysis of Low MgO Felsic Rocks
Lu Chen, Zhian Bao, Honglin Yuan, Kaiyun Chen, Chunlei Zong
2020, 31(2): 249-253. doi: 10.1007/s12583-019-1261-6
In sample preparation and mass spectrometry analysis, sample dissolution, column chemistry, concentration mismatches, and matrix effects have significant potential for introducing analytical artifacts during Mg isotope analysis. Based on the low MgO content and undesirable matrix elements in felsic rocks, the development of well-characterized felsic standards is essential to reduce interlaboratory mass bias, enable the assessment of data accuracy, and facilitate the comparison of chemical separation procedures in different laboratories. In this work, the homogeneity and long-term stability of two felsic rock standards, GSR-1 and RGM-2, were evaluated due to their low MgO contents. Furthermore, synthetic solutions with doped matrix elements were used to evaluate potential Mg isotope analytical artifacts using multi-collector inductively coupled plasma mass spectrometry. The accuracy and precision of Mg isotopic compositions in GSR-1 and RGM-2 were assessed by repeated measurements over twelve months. The long-term tests show that the Mg isotopic compositions of the two low MgO felsic rocks (GSR-1 and RGM-2) are homogenous among batches and can be used as low MgO reference materials for accuracy assessments of Mg isotopic analyses. The Mg isotopic compositions (δ26Mg) of GSR-1 and RGM-2 were marked as -0.223‰±0.053‰ (2s, n=50) and -0.184‰±0.058‰ (2s, n=50) respectively.
Microstructures and Phase Transition in Omphacite: Constraints on the P-T Path of Shuanghe Eclogite (Dabie Orogen)
Zhanjun Xie, Xiangwen Liu, Zhenmin Jin, Zhuoyue Li
2020, 31(2): 254-261. doi: 10.1007/s12583-019-1279-9
Omphacite microstructures are important to decipher the metamorphic pressure-temperature (P-T) conditions of eclogite during subduction and exhumation processes. Here we present a systematic microstructural investigation of omphacite using transmission electron microscopy (TEM) in an ultrahigh-pressure (UHP) eclogite sample from Shuanghe, Dabie orogen, China. The omphacite can be divided into two phases:(1) Omp-1, exhibits as the main part of omphacite with P2/n space group containing small amount of dislocations and numerous antiphase domains (APDs). (2) Omp-2, shows as banded subgrains with C2/c space group containing large amount of dislocations. These two phases of omphacite have almost the same crystallography orientation and bounded by dislocation walls. Along these dislocation walls, we found some barroisite and albite microcrystals. The barroisite microcrystals contain large amount of dislocations and show a topological relation with host omphacite. While the albite microcrystals contain small amount of dislocations and does not show topological relation with host. These microstructures could give us the following metamorphic information to constrain the P-T path of eclogite:Firstly, this two omphacite space groups should be evolved from the same precursor C2/c omphacite, which had been underwent strongly plastic deformation during the syn-to late-peak metamorphic stage. Secondly, the precursor C2/c omphacite began to be retrograded and altered by small amount of barroisite microcrystals along its dislocation walls under the P-T condition of 2.2-2.6 GPa and 650-700℃ at the early amphibole eclogite stage. Thirdly, large amount of Na and other elements were exsolved from some precursor C2/c omphacite subgrains and crystallized numerous albite microcrystals at their boundaries, which is the necessary for the space group of C2/c surviving under the lower P-T conditions (1.7-1.9 GPa and 630-690℃) during the middle amphibole eclogite stage. Lastly, the rest of precursor C2/c omphacite subgrains were transformed into P2/n polymorph and formed the APDs structures under the P-T condition of ~1.5 GPa and 650-680℃ during the late amphibole eclogite stage.
Improved in-situ Determination of Sr Isotope Ratio in Silicate Samples Using LA-MC-ICP-MS and Its Wider Application for Fused Rock Powder
Lüyun Zhu, Ganglan Zhang, Yongsheng Liu, Jie Lin, Xirun Tong, Shao-Yong Jiang
2020, 31(2): 262-270. doi: 10.1007/s12583-019-1214-0
The in-situ Sr isotope determination of solid samples has the advantages of high spatial resolution and convenient and rapid sample preparation. However, due to the lack of column chemistry separation of matrix elements from Sr, the interference correction (especially the isobaric interference from 87Rb on 87Sr) becomes a big issue for high precise in-situ Sr isotope analytical results. In this study, we evaluated the influence of isobaric interference systematically and adopted an improved strategy for isobaric interference correction of Rb. Specifically, as an external standard for data calibration, an in-house silicate standard (Rb-std) was fused from pure oxide powders. The new standard (Rb-std) shows very low contents of Sr (< 1 ppm), Yb (< 0.09 ppm) and Er (< 0.05 ppm), which directly avoid the trouble of interference of doubly charged ions and isobar, when we determinate the mass fractionation factors of Rb (βRb). Our improved method can provide more accurate data than previous researches, in which the mass fractionation were calculated from an external reference material StHs6/80-G with high Rb (30.07 ppm), Sr (482 ppm), Yb (1.13 ppm) and Er (1.18 ppm). Then, these βRb from Rb-std were applied by linear interpolation to the unknown samples for correction of 87Rb on 87Sr. The analytical results of serial international reference materials (BIR-1G, BCR-2G, BHVO-2G, T1-G and ATHO-G) demonstrate that our improved method can raise the upper limit of Rb/Sr for in-situ Sr isotope determination and reduce the relative standard deviation of results. This improvement of the upper limit of Rb/Sr (< 0.33) will expand not only range of microanalysis for silicate minerals, but also bulk Sr isotope determination of volcanic rock combining with rapid fusion technique.
Factors Controlling Deposition of Metallic Minerals in the Meng'entaolegai Ag-Pb-Zn Deposit, Inner Mongolia, China: Evidence from Fluid Inclusions, Isotope Systematics, and Thermodynamic Model
He Yang, Wanli Ma, Rui Wang, Xueli Ma, Keyong Wang
2020, 31(2): 271-286. doi: 10.1007/s12583-019-1273-2
The Meng'entaolegai Ag-Pb-Zn vein-type deposit in Inner Mongolia,NE China is hosted in biotite/muscovite granite. This deposit includes the western (Zn-rich,deepest),middle (Zn-Pb rich) and eastern (Pb-Ag-rich,shallowest) ore-blocks. To better understand the metallogenic processes in ore district,we have undertaken a series of studies including fluid inclusion microthermometry,H-O-S-Pb isotope compositions and thermodynamic modeling. Based on fluid inclusion petrography,microthermometry results and H-O isotope compositions,the ore-forming H2O-NaCl fluid inclusions are characterized by medium temperature and medium salinity. And two kinds of fluid processes (boiling in western and middle ore-block and fluid mixing in the eastern ore-block) were identified to explain the ore fluid evolution. More importantly,log fO2-pH diagrams of δ34S contours with the stability fields of Fe-and Cu-,Zn-,Pb-,and Ag-bearing minerals were constructed to restore the physicochemical conditions of ore-forming fluid in the western (270℃ and 80 bars),middle (250℃ and 70 bars),and eastern (230℃ and 50 bars) ore-blocks. As a result,the ore-forming conditions in the western and middle ore-block were similar. In the eastern ore-block,the fluids may have changed from acidic,S-poor and δ34S(ΣS)≈2.8 to neutral,S-richer and δ34S(ΣS)≈0.5,which imply that neutral S-rich meteoric water was mixed with the magmatic fluid. Meanwhile,the activity of Ag+ was estimated to be about 10 ppm-9 ppm in the middle ore-block,but in the eastern ore-block it was about ~10 ppm-12 ppm. We proposed that the key for Ag ore deposition was likely to be neutralization led by fluid mixing.
Constraints on the Genesis of the Qixiashan Pb-Zn Deposit, Nanjing: Evidence from Sulfide Trace Element Geochemistry
Xuejuan Sun, Pei Ni, Yulong Yang, Zhe Chi, Shan Jing
2020, 31(2): 287-297. doi: 10.1007/s12583-019-1270-5
The large-scale Qixiashan Pb-Zn Deposit in the eastern Middle-Lower Yangtze metallogenic belt is hosted in carbonate rocks. Based on a detailed mineral paragenesis study, in-situ LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometer) trace element geochemistry data for pyrite and sphalerite from different stages in the Qixiashan Deposit are reported, the Pb-Zn mineralization processes are reconstructed, and a genetic model is constructed. Four paragenetic stages of Pb-Zn ore deposition are identified:the biogenic pyrite mineralization stage (Stage 1), the early stage of hydrothermal Pb-Zn mineralization (Stage 2), the late stage of hydrothermal Pb-Zn mineralization (Stage 3), and the carbonate stage (Stage 4). Stages 2 and 3 are the main ore stages. The trace element characteristics of the sulfide in stages 2 and 3, such as the higher Co/Ni and lower trace element contents of the pyrite and the Fe, Mn, and Ge contents of the sphalerite, indicate that they were generated by magmatic-hydrothermal processes. Furthermore, the lower Cu, Ag, Sb, and Pb contents of the pyrite and sphalerite of Stage 3 compared to Stage 2 suggest an increase in magmatic-hydrothermal activity from Stage 2 to Stage 3. The hydrothermal fluids leached trace elements (e.g., Cu, Ag, Sb, and Pb) from the previously deposited primary pyrite and sphalerite, which were precipitated in the later hydrothermal stage Cu, Au, Ag, Sb, and Pb bearing minerals and secondary pyrite and sphalerite with lower trace element contents (e.g., Cu, Au, Ag, Sb, and Pb). Compared with the pyrite from stages 2 and 3, the Stage 1 pyrite has relatively higher trace elements contents (Sb, Cu, Zn, Au, Ag, Pb, As, and Ni). However, their lower Co/Ni ratio suggests a syngenetic sedimentary origin. Based on the petrographic features and trace element data, a multi-stage mineralization model is proposed. The Stage 1 biogenic pyrite formed stratiform pyrite layers, which provided reducing conditions and a base for the subsequent Pb-Zn mineralization. During Stage 2, subsequent hydrothermal fluid interacted with the stratiform pyrite layers, which resulted in sulfide precipitation and the formation of stratiform Pb-Zn orebodies. In Stage 3, the hydrothermal fluid replaced the limestone along the fractures, which triggered the formation of Pb-Zn vein orebodies.
Ore Forming Fluids of Several Gold Deposits in the Irtysh Gold Belt, Xinjiang, China
Yingwei Wang, Jiuhua Xu, Rufu Ding, Hui Zhang, Xihui Cheng, Chunjing Bian
2020, 31(2): 298-312. doi: 10.1007/s12583-019-1274-1
The metallogenic environment of the Irtysh gold belt in Xinjiang is studied in detail. The metallogenic geological background, metallogenic conditions and ore-controlling factors of the gold deposits in eastern, central and western regions of the metallogenic belt are compared. The metallogenic structure of the Irtysh tectonic belt has the characteristics of diverging to the west and converging to the east. Composite ore controlling by ductile shearing and magmatic activity in Irtysh gold belt result in zoned and segmented distribution of gold mineralization. Through the fluid inclusion research and H-O-S isotope analysis, the evolution regularity of gold ore-forming fluids in the region was analyzed. Synchrotron radiation X-ray fluorescence was used to analysis the concentration of metal elements in a single fluid inclusion, explaining the occurrence and migration process of Au in hydrothermal fluid. The source of ore forming minerals in western gold deposit is more closely related to magmatic activity, and the structural metamorphism of eastern gold deposit has greater influence on mineralization. Metallogenic fluids of gold deposits are characterized by metamorphic water (and magmatic water) in the early stage and mixed with meteoric water in the late stage. And the metallogenic elements are enriched in CO2 rich fluid. The Au is mainly activated, migrated and enriched with the mixed fluid of magmatic hydrothermal, metamorphic hydrothermal and atmospheric precipitation in the medium-low temperature, shallow to medium-deep environment.
Geochronology, Geochemistry and Hf Isotope of the Late Mesozoic Granitoids from the Lushi Polymetal Mineralization Area: Implication for the Destruction of Southern North China Craton
Yuanyuan Zhai, Shuai Gao, Qingdong Zeng, Shaoxiong Chu
2020, 31(2): 313-329. doi: 10.1007/s12583-020-1277-y
The North China Craton (NCC) is the best example of an Archean craton that has lost its stability in the Late Mesozoic. Although the cratonic destruction is generally considered to have occurred in the Eastern Block and reached a peak in the Early Cretaceous, the exact areal extent of cratonic destruction is debated, especially the southern and northern margin of the NCC. Here we report geochronology, geochemical and Hf isotopic data of the Late Mesozoic granitoids from Lushi polymetal mineralization area (LPMA) in the southern margin of NCC. These results provide new insights into the destruction in the southern margin of the NCC during the Late Mesozoic. Zircon U-Pb dating indicates that eight granitoids intruded in the Late Jurassic to Early Cretaceous (136.8-154.1 Ma), respectively. Geochemical signatures define these granitoids being A-type or I-type granite that formed in an extension setting. In addition, Hf isotopic compositions of zircons from these granitoids vary in a relatively large range, with εHf(t) values and TDM2 ages ranginge from -26.1 to +15.2 and 215 to 2 849 Ma, respectively. The parental magmas were likely derived from diverse sources, including materials of the partial melting of ancient lower crust and mantle-derived mafic magmas in various proportions. Combining with previous studies on the contemporaneous magma-tectonic activities in circum of NCC, we suggest that the rim of NCC was already unstabilized from the Late Jurassic in the LPMA. The subduction of the Paleo-Pacific Plate was the main trigger to the destruction of the southern margin of NCC, which was responsible for the lithospheric extension and thinning, extensive magmatism and mineralization.
SEM-CL Study of Quartz Containing Fluid Inclusions in Wangjiazhuang Porphyry Copper (-Molybdenum) Deposit, Western Shandong, China
Lei Shu, Kun Shen, Renchao Yang, Yingxin Song, Yuqin Sun, Wei Shan, Yuxin Xiong
2020, 31(2): 330-341. doi: 10.1007/s12583-019-1025-3
The Wangjiazhuang porphyry copper (-molybdenum) deposit is located at Zouping volcanic basin in Shandong Province, East China and hosted in the Wangjiazhuang intrusive complex emplaced along a late volcanic conduit. There are two types of ores in this deposit:early disseminated and stockwork ores in the ore-bearing intrusion, and late massive sulfide-quartz veins above brecciated quartz monzonite. The ore minerals are mainly pyrite, chalcopyrite, and subordinately magnetite, tennantite, molybdenite with minor bornite, enargite, galena and sphalerite, etc., and gangue minerals including K-feldspar, biotite, quartz, muscovite-sericite, chlorite and calcite. Combined with fluid inclusion study, the scanning electron microscope-cathodoluminescence (SEM-CL) study of quartz in the deposit and wall rocks shows significant differences between the two types of quartz in the ores. In addition, four types of primary-pseudosecondary fluid inclusions in the quartz have been recognized. They are one-or two-phase aqueous inclusions with vapor/liquid ratios less than 30% to 40% (type I); gas-rich inclusions with vapor/liquid ratios more than 50% (type Ⅱ), some of which contain some small opaque minerals, probably chalcopyrite; multiphase fluid inclusions with daughter minerals of halite±anhydrite±opaque (chalcopyrite)±sylvite±hematite±unknown crystal (type Ⅲ); and mica-bearing fluid inclusions (type IV). Quartz containing abundant muscovite-bearing and halite-bearing fluid inclusions in the mineralized quartz monzonite with potassic-silicic alteration, have better oscillatory growth zoning with CL-colors from bright in the core to darker in the rim, indicating variations of element concentrations in the fluid media from which quartz grew during the later period of magmatic-hydrothermal process. In contrast, the quartz in the sulfide-quartz veins contains mainly fluid inclusions of low-to-medium salinities and does not show oscillatory zoning, indicating that there was less fluctuation in composition and element concentrations of the hydrothermal fluids. However, the quartz containing halite-bearing fluid inclusions and being associated with copper-molybdenum mineralization in the sulfide-quartz veins shows zoning in its rims, indicating variations in composition and element concentrations of the hydrothermal fluids.
Petroleum, Natural Gas Geology
Paired δ13Ccarb13Corg Evolution of the Dengying Formation from Northeastern Guizhou and Implications for Stratigraphic Correlation and the Late Ediacaran Carbon Cycle
Yi Ding, Daizhao Chen, Xiqiang Zhou, Taiyu Huang, Chuan Guo, Rumana Yeasmin
2020, 31(2): 342-353. doi: 10.1007/s12583-018-0886-1
This study provides δ13C profiles from a lower-slope (Well ZK102) to basin (Bahuang Section) environment to better understand the temporal and spatial variability in δ13Ccarb13Corg of the Yangtze Block during the Late Ediacaran. Our new δ13C profiles together with the reported data suggest that the Upper Ediacaran successions from different depositional environments are generally bounded by negative δ13Ccarb and/or δ13Corg excursions in the underlying and overlying strata. Moreover, the Upper Ediacaran δ13Ccarb profiles generally can be subdivided into two positive excursions and an interjacent negative excursion, whereas the paired δ13Corg profiles from different depositional environments have individual variation trends. On the other hand, these data show a large surface-to-deep water δ13C gradient (~5‰ variation in δ13Ccarb, >10‰ variation in δ13Corg) which can be reasonably explained by the heterogeneity of the biological activities in the redox-stratified water column. Furthermore, the decoupled δ13Ccarb13Corg pattern with large δ13Corg perturbations at the lower slope precluded the existence of a large dissolved organic carbon reservoir at the Yangtze Block during the Late Ediacaran. Thus, the high d13Ccarb values in the Upper Ediacaran succession could be balanced by large amounts of buried organic carbon likely associated with high productivity.
Formation Mechanism and Numerical Model of Quartz in Fine-Grained Organic-Rich Shales: A Case Study of Wufeng and Longmaxi Formations in Western Hubei Province, South China
Bao Zhang, Detian Yan, Hassan Jasmine Drawarh, Xiangrong Yang, Jin He, Liwei Zhang
2020, 31(2): 354-367. doi: 10.1007/s12583-019-1247-4
The difference in quartz types in shales not only affects the porosity and permeability of the rocks,but also reflects the difference in the sedimentary environments. We established the formation mechanism and numerical model of quartz in shales of Wufeng and Longmaxi formations in the Wangjiawan Section,South China,based on thin-section studies using SEM (scanning electron microscope),SEM-CL (cathodoluminescence),XRD (X-ray diffraction) and geochemical analyses. There are two types of quartz in the shales:detrital quartz and authigenic quartz. Detrital quartz is mostly silt-size,typically ranging from 10 to 60 μm in size and subangular to angular monocrystal in shape,and brighter than authigenic quartz by CL intensity; authigenic quartz is present in two phases in shape:grain overgrowths and crystallite grains. Overgrowth surfaces are subhedral. Crystallite grains are typically less than 10 μm in size,euhedral or subhedral monocrystal in shape. Authigenic quartz can be subdivided into biogenic quartz and clay mineral transformed quartz according to the source of silicon. In the numerical model,the content of detrital quartz is relatively consistent (20%); the content of biogenic quartz ranges from 40% to 70%,with a sharp fall (0-30%) in the Guanyinqiao mudstone. During the Katian,a lower anoxic and dense water column make the dissolution of biogenic silica well preserved. Biogenic quartz is the major contributor to the sediment. During the Early Hirnantian interval,due to the drop of sea level and the oxygenation of seafloor,the sediment is mainly composed of clay transformed quartz and detrital quartz. During the Latest Hirnatian and Rhuddanian,rapid sea level rise and anoxic ocean enhance the preservation of the biogenic silica,thereby biogenic quartz re-emerges as the major contributors to the sediment. Authigenic crystallite grains and grain overgrowths have filled in primary pore space and have decreased the interparticle porosity,however,as a rigid framework,they can suppress compaction and maintain the internal pore structure. The formation of authigenic quartz results in the increase of total quartz,which fortifies the brittleness of rocks and is beneficial to the development of shale gas.
Effect of Organic Matter Type and Maturity on Organic Matter Pore Formation of Transitional Facies Shales: A Case Study on Upper Permian Longtan and Dalong Shales in Middle Yangtze Region, China
Zhongrui Wu, Sheng He, Yuanjia Han, Gangyi Zhai, Xipeng He, Zhi Zhou
2020, 31(2): 368-384. doi: 10.1007/s12583-019-1237-6
Organic matter (OM) nanopores developed in transitional facies shales,i.e.,the Upper Permian Longtan and Dalong Formations in the Yangtze Platform,China,were investigated to determine the corresponding influence of thermal maturity and OM types within the geological conditions. A suite of 16 core samples were taken from Type-Ⅲ Longtan shales and Type-Ⅱ Dalong shales from two wells covering a maturity (Ro,vitrinite reflectance) ranging from 1.22% to 1.43% and 2.62% to 2.97%,respectively. Integrated analysis of the shale samples was carried out,including field-emission scanning electron microscopy (FE-SEM),low-pressure N2 and CO2 adsorption,high-pressure CH4 adsorption,and mercury intrusion capillary pressure (MICP) analysis. The fluid inclusions of liquid and gas hydrocarbons trapped in calcite vein samples in Dalong shales of two wells were studied using laser Raman and fluorescence spectroscopy. FE-SEM images indicated that OM pores in different formations varied substantially in terms of shape,size,and distribution density. OM pores in Type-Ⅱ Dalong shales of Well XY1 were mainly micropore,sparsely distributed in the gas-prone kerogen with a spot-like and irregular shape,while bitumen rarely developed observable pores. In contrast,the morphology of OM pores in Type-Ⅲ Longtan shales were significantly different,which was due to differences in the OM type. The primary OM pores in some terrestrial woody debris in Longtan shales had a relatively larger pore diameter,ranging from hundreds of nanometers to a few micrometers and were almost all rounded in shape,which might be one of the factors contributing to larger pore volume and gas adsorption capacity than Dalong shales of Well XY1. Comparing Dalong shales of Well XY1 with relatively lower thermal maturity,there were abundant spongy-like pores,densely developed in the pyrobitumen in Type-Ⅱ Dalong shales of Well EY1,with an irregular shape and diameter ranging from several to hundreds of nanometers. Many blue fluorescent oil inclusions and a small number of CH4 inclusions mixed with C2H6 could be observed within calcite veins in Dalong shales of Well XY1,whereas only CH4 inclusions could be identified within calcite veins in Dalong shales of Well EJ1. Therefore,thermal maturity not only controlled the type of hydrocarbons generated,but also makes a significant contribution to the formation of OM pores,resulting in larger pore volumes and adsorption capacity of Type-Ⅱ shale samples in the dry gas window.
Structural Geology and Engineering Geology
Non-Impact Origin of the Baisha Structure in Hainan Province, China
Jiang Pu, Zhiyong Xiao, Long Xiao, Cheng Huang
2020, 31(2): 385-392. doi: 10.1007/s12583-018-0887-0
The Baisha Structure,with a rim-to-rim diameter of ~3.7 km,in the center of the Hainan Province,southern China has been considered to be an impact crater. Field investigation and petrological study are presented in this paper to investigate the impact hypothesis for this structure. The ~600-m-thick Lower Cretaceous feldspathic quartz sandstones from the Lumuwan Formation are the major outcrops both within and outside of the structure. The amphitheater-shaped rim of the structure is composed of granite porphyries that are intruded in the Lumuwan Formation. Previously interpreted impact breccia and impact melt rocks are actually granite porphyries different cooling rates and weathering status. Rocks from locations that most likely have recorded shock metamorphic signatures are sampled,but petrographic analyses reveal no indications of shock metamorphism. While subtle structural deformation occurs at the contact boundary between the granite porphyries and the feldspathic quartz sandstones,the feldspathic quartz sandstones exhibit uniformed dipping strata across the crater floor and walls. All the evidence suggests that the Baisha Structure was not formed by impact cratering. It most likely has been shaped by a combination of magmatic intrusion and long-term differential erosion.
Influence of Tectonics and Morphometric Features on the Landslide Distribution: A Case Study from the Mesima Basin (Calabria, South Italy)
Massimo Conforti, Fabio Ietto
2020, 31(2): 393-409. doi: 10.1007/s12583-019-1231-z
Identifying the instability areas provides useful information about the recognition of the vulnerable zones and,thus,helps to potentially decrease the harm to the infrastructure and the hazard conditions in inhabited areas. In this regard,morphometric parameters and tectonic setting are widely employed in mapping,modelling and understanding of instability processes. The present study is aimed to analyse the combined effects of slope morphometry (slope gradient,slope curvature,local relief),of lithology,of drainage density and of tectonic setting on the distribution of landslides. The study area is located in the Mesima Basin that lies in the central part of the Calabria region (southern Italy). The morphometric analysis was carried out by geographic information system (GIS) techniques,whereas the geomorphic attributes were computed by a digital elevation model (DEM). The recognized landslides and faults were digitized and stored in a GIS geo-database that allowed the construction of relative density maps. A total of 1 991 landslides and 231 tectonic lineaments were recognized and mapped. The distribution of the collected data suggests that the occurrence of landslides is not random. Indeed,the research highlights as the lithology,the fault density,the slope gradient and the local relief play an important role in determining the landslide occurrence. The used approach is aimed to reinforce the knowledge about the validity of the application of morphometric analysis to the landslide susceptibility. The Mesima Basin is a poorly studied area of the Calabria from a geomorphological viewpoint. Indeed,previous researches focused mainly on geological and on tectonic aspects,as well as on instability phenomena in the Mesima surroundings. Thus,this paper takes a step forward with respect to previous studies because it augments the knowledge about the relationship between faults,morphometric features and the occurrence of landslide.
Environmental Geology
Potential Effect of Black Carbon on Glacier Mass Balance during the Past 55 Years of Laohugou Glacier No. 12, Western Qilian Mountains
Jizu Chen, Xiang Qin, Shichang Kang, Wentao Du, Weijun Sun, Yushuo Liu
2020, 31(2): 410-418. doi: 10.1007/s12583-019-1238-5
This study reconstructed the annual mass balance (MB) of Laohugou Glacier No. 12 in the western Qilian Mountains during 1961-2015. The annual MB was calculated based on a temperature-index and an accumulation model with inputs of daily air temperature and precipitation recorded by surrounding meteorological stations. The model was calibrated by in-situ MB measurements conducted on the glacier during 2010-2015. Change in constructed annual MB had three phases. During Phase Ⅰ (1961-1984),glacier-wide MB values were slightly positive with an average MB of 24±276 mm w.e. (water equivalent). During Phase Ⅱ (1984-1995),the MB values became slightly negative with an average MB of -50±276 mm w.e.. The most negative MB values were found during Phase Ⅲ (1996-2015),with an average MB of -377±276 mm w.e. Climatic analysis showed that the warming led to accelerated glacier mass loss despite a persistent increase of precipitation during the analysis period. However,an increase of black carbon deposited on the glacier surface since the 1980s could have contributed to intensified glacier melt. From simulations and measurements of MB on the Urumqi Glacier No. 1,26% of glacier melt caused by black carbon could be identified.
Stable Isotope Evidence for Recent Global Warming Hiatus
Rui Wang, Zhongfang Liu
2020, 31(2): 419-424. doi: 10.1007/s12583-019-1239-4
Global mean surface air temperature (SAT) has remained relative stagnant since the late 1990s,a phenomenon known as global warming hiatus. Despite widespread concern and discussion,there is still an open question about whether this hiatus exists,partly due to the biases in observations. The stable isotopic composition of precipitation in mid-and high-latitude continents closely tracks change of the air temperature,providing an alternative to evaluate global warming hiatus. Here we use the available long-term precipitation δ18O records to investigate changes in SAT over the period 1970-2016. The results reveal slight decline in δ18O anomaly from 1998 to 2012, with a slope of -0.000 4‰ decade-1 which is significantly different from that of pre-1998 interval. This downward δ18O anomaly trend suggests a slight cooling for about -0.001℃ decade-1,corroborating the recent hiatus in global warming. Our work provides new evidence for recent global warming hiatus and highlights the potential of utilizing precipitation isotope for tracking climate changes.