2018 Vol. 29, No. 2
Two anatomically preserved calamitean axes are reported for the first time from the Late Permian Wutonggou Formation in the southern Bogda Mountains, Xinjiang Uygur Autonomous Region, Northwest China. Based on the anatomical features, these axes are assigned to Arthropitys. A new species Arthropitys taoshuyuanensis sp. nov. is established. A. taoshuyuanensis sp. nov. possesses a large pith and comprises large pith cavity and a narrow perimedullary zone at the nodes and diaphragms at the internodes. Carinal canals are circular and surrounded by a single layer of metaxylem tracheids. Secondary xylem is divided into interfascicular rays and fascicular wedges. Interfascicular rays are initially four to five cells wide and taper abruptly centrifugally. Fascicular wedge consists of thick-walled tracheids and thin-walled fascicular ray cells. Radial tracheid walls have uniseriate or biseriate circular pits, or scalariform pits. The absence of growth rings in the Arthropitys specimens indicates that they probably lived in the wetland area under stable annual temperature and water sufficient conditions.
Partial melting experiments were carried on KLB-1 peridotite, a xenolith sample from the Earth's upper mantle, at 1.5 GPa and temperatures from 1 300 to 1 600 ℃, with heating time varies from 1 to 30 min. We quantify the axial temperature gradient in the deformation-DIA apparatus (D-DIA) and constrain the time scale of partial melting by comparing experimental observations with calculated result from pMELTS program. The compositions of the liquid phase and the coexisting solid phases (clinopyroxene, orthopyroxene, and olivine) agree well with those calculated from pMELTS program, suggesting that local chemical equilibrium achieves during partial melting, although longer heating time is required to homogenize the bulk sample. The Mg# (=Mg/(Mg+Fe) mol.%) of olivines from the 1-minute heating experiment changed continuously along the axial of the graphite capsule. A thermal gradient of 50 ℃/mm was calculated by comparing the Mg# of olivine grains with the output of pMELTS program. Olivine grains at the hot end of the graphite capsule from the three experiments heated at 1 400 ℃ but with different annealing time show consistence on Mg#, indicating that partitioning of Fe2+ between the olivine grains and the silicate melt happened fast, and partial melting occurs in seconds.
This paper presents a synthesis and analysis of geochronological, geochemical, and zircon Hf isotopic data of Phanerozoic granitoids within the Khanka massif, with the aim of revealing the accretion and reworking processes of continental crust within the massif. Zircon U-Pb dating indicates that Phanerozoic granitic magmatism within the Khanka massif can be subdivided into eight stages: Late Cambrian, Middle–Late Ordovician, Middle Silurian, Late Carboniferous, Early Permian, Middle–Late Permian to Early Triassic, Late Triassic–Early Jurassic, and Early Cretaceous. The zircon Hf isotopic compositions reveal that crustal accretionary events took place mainly in the Mesoproterozoic and Neoproterozoic. Through time, the zircon εHf(t) values gradually increase, indicating that the Phanerozoic granitic magmas were derived from the melting of progressively less ancient and more juvenile crust. The zircon εHf(t) values exhibit a gradual decrease with the increases in latitude, which implies that the amounts of ancient crustal components within the lower continental crust of the Khanka massif increased from south to north. At the same latitude range, the zircon Hf isotopic compositions also display some variations. We conclude, therefore, that significant horizontal and vertical heterogeneities existed in the lower continental crust of the Khanka massif during the Phanerozoic.
Volcanic rocks in the Bangxi-Chenxing tectonic zone provide important carries for better understanding the Late Paleozoic tectonic evolution in Hainan and its temporal-spatial pattern of the eastern Paleotethyan evolution. This paper presents a set of new geochronological and geochemical data on the andesitic and dacitic rocks along the Bangxi-Chenxing tectonic zone in central Hainan. The representative andesitic and dacitic samples yield similar zircon U-Pb ages of 353±3 and 351±7 Ma, respectively, being of Early Carboniferous origin. These volcanic rocks are characterized by low TiO2 and high Al2O3 contents and are enriched in LILEs and LREEs but depletion in HFSEs, along with negative εNd(t) values of -1.4– -4.7 and high 87Sr/86Sr(i) ratios of 0.707 2–0.710 1. Geochemical signatures suggest that the andesitic and dacitic samples might originate from a metasomatized wedge modified by the slab-derived component in a continental arc setting. In combination with the available data, it is proposed for the development of a Carboniferous continental arc in response to the eastern Paleotethyan evolution. The Bangxi-Chenxing tectonic zone might westerly link with the Jinshajiang-Ailaoshan-Song Ma suture zone, constituting an assemblage boundary between the South China and Indochina blocks.
Geochronological and geochemical analyses were performed on K-feldspar granites and monzonitic granites from the Xilinhot area, Inner Mongolia, China. Zircon U/Pb ages indicate that the two types granites were emplaced during the Lower Carboniferous. The K-feldspar granites (332 Ma) have the typical A-type granite characteristics of a post-collision setting. The monzonitic granites have an emplacement age of 323 Ma. Zircon εHf values of the former range from +12.8 to +14.2, with an average TDM2 of 453 Ma. The latter have lower zircon εHf values, ranging from +5.4 to +10.7, with an average TDM2 of 798 Ma. The strong, positive εHf values of the zircon indicate that both sets of samples are from a juvenile crust formed in an oceanic crust subduction stage, although the monzonitic granite may have undergone a hybridization of crustal materials. These results indicate a younger post orogenic event. The Paleo-Asian Ocean had closed before the Early Carboniferous and the Xilinhot area started its post-orogenic evolution with an extensional tectonic environment during the Early Carboniferous.
Major elements, trace elements and Sr isotopic compositions of apatite from the granite in the Chengchao skarn-type Fe deposit of Edong ore district of Middle–Lower Yangtze River metallogenic belt were measured using EMPA (electron microprobe), LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometer) and LA-MC (multicollector)-ICP-MS methods in order to reveal the petrogenetic and metallogenic significance of the skarn-type iron deposits. The results show that the apatite in Chengchao granite is fluorapatite, which displays slight variation in major elements. The REE distribution pattern of the apatite is similar to that of the whole rocks, with strong negative Eu anomaly and low Sr/Y ratio. The concentration of Mn in apatite is low (140 ppm–591 ppm) and the Sr isotopic composition shows a limited variation from 0.706 9 to 0.708 2. The high oxygen fugacity of the Chengchao granite, implied by the low Mn content in apatite, is possibly attributed to contamination of the gypsum from sedimentary rock strata, which has long been thought to be an important factor that controls the Fe mineralization in the Middle–Lower Yangtze River metallogenic belt. This study also proves that the Eu/Eu* value and Sr/Y ratio in apatite can be effectively used to identify the adakitic affinity. The in situ Sr isotope analysis of apatite is in consistent with the bulk rock analysis, which indicates that the apatite Sr isotope can represent the initial Sr isotopic compositions of the magma. The Sr isotope and negative Eu anomaly in apatite imply that the Chengchao granite is likely sourced from crust-mantle mixed materials.
Sichuan Basin is one of the uppermost petroliferous basins in China. It experienced three evolutionary phases which were marine carbonate platform (Ediacaran to Late Triassic), Indosinian-Yanshanian orogeny foreland basin (Late Triassic to Late Cretaceous) and uplift and tectonic modification (Late Cretaceous to Quaternary). The present-day tectonics of the Sichuan Basin and its periphery are characterized by three basic elements which are topography, basement type and surface structure, and two settings (plate margin and interior). Therefore, be subdivided into five units which have different structure and tectonic history. The basin contains five different sets of source rocks with thickness up to 2 500 m. These source rocks were well preserved due to the presence of Middel–Lower Triassic evaporites (> ~200 m) and thick terrestrial sediments filling in the Indosinian-Yanshanian foreland basin (> 3 000 m). The uplift and erosion since Late Cretaceous has significant influence on cross-strata migration and accumulation of oil and gas. The multi-phase evolution of the basin and its superimposed tectonic elements, good petroleum geologic conditions and diverse petroleum systems reveal its bright exploration prospects.
Rare earth element (REE) and Y concentrations, and 87Sr/86Sr ratios were analyzed in 33 carbonate samples from the Paleogene Shahejie Formation in the Qikou depression, eastern China, with the goal of determining depositional environments and post-depositional conditions of carbonates in the region. The REE and Y concentrations were normalized to the post-Archean Australian shale (PAAS) standard. The La *PAAS/Yb*PAAS ratios of 0.35–1.52, where *PAAS indicates values for the PAAS standard, show light REE enrichment and heavy REE depletion in most samples. Values of La *PAAS (0.775–1.284) and Ce *PAAS (0.822–1.224), coupled with a relatively flat REE distribution, indicate that the Shahejie carbonates were deposited in lacustrine environments. Values of Y anomalies (1.009–1.527) and Y/Ho ratios (28.43–45.00) in the Shahejie Formation are greater than those of lacustrine carbonates and closer to those of marine carbonates, indicating that diagenetic fluids were probably influenced by seawater. In the carbonates from Well Kou-42, Eu anomalies (1.171–1.604), 87Sr/86Sr ratios (0.708 001–0.710 893), and high homogenization temperatures (104–151 C) suggest that the carbonates were affected by hydrothermal fluids. The REEs and Sr isotope ratios show that the carbonates from the Shahejie Formation in the Qikou depression were deposited in lacustrine environments, and were influenced by seawater and hydrothermal fluids.
In the present study, we carried out trace element analyses of black shales of the Ediacaran Doushantou Formation from two sections (Jiulongwan, Baiguoyuan) in Hubei Province, South China. Mo-U characteristics of black shales from the two sections and compiled Mo-U data of Doushantuo black shales from sections of a variety of sedimentary facies described the temporal/spatial variability in the redox conditions of paleo-seawater during deposition of the Doushantuo Formation. Changes in Mo-U patterns of the Doushantuo Member Ⅱ (DST2) shales of open marine environments are consistent with a shift from a predominately oxic to a predominately anoxic ocean during their deposition. Mo-U patterns of the DST2 black shales from intra-shelf sections reflect basin restriction may have happened in the intra-shelf basin and are compatible with the redox-stratified model of the intra-shelf basin. Mo-U patterns of black shales of the Doushantuo Member Ⅳ (DST4) reveal that the shales from intra-shelf sections have more pronounced Mo enrichment and more significant enrichment of Mo over U than the slope shales, indicating the operation of a Mn particulate shuttle in the intra-shelf basin. High Mo/TOC ratios of the DST4 at the intra-shelf sections, in combination with similar Mo-TOC patterns of the DST4 from both intra-shelf and slope sections, indicate the intra-shelf basin was well connected to the open ocean during deposition of the DST4.
Seismic stratigraphy, core logging and the data from cores are used to identify the development and evolution of the east provenance-sedimentary system and to analyze its influence on the quality of reservoirs in the eastern depression of the North Yellow Sea Basin. The eastern provenance-sedimentary system is a fan delta (delta) and subaqueous fan sedimentary system with volcanic provenance. Its evolution is closely related to internal uplift and coeval volcanic activity. During faulting and volcanic activity, an internal uplift gradually formed and the eastern provenance-sedimentary system began to develop. Clastic materials from the weathered uplift and volcanic activity were the main sources of sediment and were deposited in the Middle Jurassic to the Early Cretaceous. A subaqueous fan sedimentary system developed in the Middle Jurassic, a deltaic to subaqueous fan sedimentary system developed in the Late Jurassic, and a fan delta to subaqueous fan sedimentary system developed in the Early Cretaceous. The volcaniclastic detritus was mainly composed of acidic materials with a small amount of intermediate materials. The reservoir in the eastern sedimentary system is a type of tight sandstone with ultra-low porosity and very low permeability due to the influence of volcanism and diagenesis. The reservoir quality is worse than those of the northern and the western sedimentary systems. However, the high volume of volcanic materials and deep-marine sedimentary conditions resulted in good-quality hydrocarbon source rocks that ensures the exploration potential in the eastern sedimentary system reservoir.
The Xujiaweizi (XJWZ) fault depression is a primary area for deep-seated gas exploration in Daqing. Through seismic profile interpretation, the main sag-controlling fault and structural characteristics in the XJWZ fault depression were analyzed based on folds and faults analysis. A three-dimensional geological model of the XJWZ fault depression was also set up to enable further discussion by means of structural physical modeling. According to our research, the basic structural framework of the XJWZ fault depression was the result of regional extension. Its formation and evolution were mainly controlled by the Xuxi fault. The structural framework of S-N blocking was due to changes in the fracture attitude. The Xuzhong uplift and the depressions on both sides belong to a normal-fault-related fold and were formed by controlling of the ramp/flat normal fault in the same tectonic movement period.
The Jingdezhen ductile shear zone is evolved from the Neoproterozoic Zhangyuan ophiolite mélange belt in the eastern Jiangnan Orogen, South China. Comprehensive study of geometry, kinematics, quartz c-axis fabric, temperature-pressure conditions and geochronology were conducted in this study. The Jingdezhen shear zone extends ~180 km along the NE orientation with two groups of subvertical foliation and subhorizontal lineation. One group of foliation strikes NEN orientation whereas another one NEE orientation. Field investigation, microscopic observation and quartz c-axis fabric show that sinistral shearing along NEN-striking foliation occurred earlier than dextral shearing along NEE-striking foliation. Syn-tectonic staurolite porphyroblasts and deformation manner of feldspar imply that sinistral shearing occurred at 530–420 C and 6–2 kbar. Deformation manner and c-axis fabric of quartz and pre-tectonic staurolite porphyroblasts indicate that dextral shearing took place at 420–300 C. LA-ICP-MS zircon U-Pb and mica 40Ar/39Ar dating indicate that the sinistral shearing occurred during Neoproterozoic orogeny (830–800 Ma) whereas the dextral shearing at 447±12 Ma. The sinistral shearing resulted from the Neoproterozoic final assembly between the Yangtze and Cathaysia blocks. The dextral shearing was caused by Early Paleozoic orogen parallel extension and clockwise rotation.
A combined study of magnetic fabrics, zircon U-Pb geochronology and structural deformation was carried out for Late Paleozoic sedimentary and Mesozoic magmatic rocks in the southwestern Fujian rift basin, South China, aiming at deciphering the tectonic evolution during Late Mesozoic. Field observations showed that the Late Mesozoic structure deformations in southwestern Fujian were categorized into four phases: NW-SE compression, ENE-WSW extension, NNE-SSW compression and NNW-SSE extension, sequentially. Zircons picked out from Juzhou granite and WNW-trending diabase dykes showed complete crystal shapes and clear oscillatory zonings on their edges, and the U-Pb dating yielded ages of 132 and 141 Ma, respectively. The susceptibility ellipsoid magnitude parameters of the Juzhou granite are characterized by flaser type strain ellipsoid, with pole density center of K3 falling into the first and the third quadrants, these features revealed that the Juzhou granite formed in ENE-WSW compressional stress field, indicating the early stage of Early Cretaceous extrusion in southwestern Fujian. The late stage of Early Cretaceous NNE-SSW extension was limited by the widespread WNW-trending diabase dykes, which were usually regarded as important indications for a regional extensional setting. On the basic of the previous researches, structural deformation studies, and the deductions above, it can be concluded that southwestern Fujian experienced five main tectonic stages during Late Mesozoic: Early Jurassic extension, Middle–Late Jurassic thrusting, early stage of Early Cretaceous extension, late stage of Early Cretaceous compression and Late Cretaceous extension.
A new fully automatic 40Ar/39Ar laboratory with a Thermo Scientific© ARGUS Ⅵ mass spectrometer has been established in China University of Geosciences (Wuhan). We designed and developed a mini efficient preparation system (80 mL), a CO2 laser for heating samples, a crusher for extracting fluid inclusions within K-poor minerals and an air reservoir (31 L) and pipette (0.1 mL) system. The ARGUS Ⅵ mass spectrometer is operated by the Qtegra Noble Gas software, which can control the peripheral accessories, such as pneumatic valves, CO2 laser and crusher through a PeriCon (peripheral controller). The experimental procedures of atmospheric argon analyses, 40Ar/39Ar dating by laser stepwise heating and by progressive crushing in vacuo, can be fully automatically performed. The weighted mean of atmospheric 40Ar/36Ar ratios is 302.22±0.03 (1σ, MSWD=0.74, n=200), indicating that air reservoir and pipette system and the whole instrument system are very stable. This laboratory is a successful pioneer example in China to establish a new noble gas laboratory with self-made peripheral accessories expect for the mass spectrometer.
The joint methods of 40Ar/39Ar laser stepwise heating and in vacuo crushing have been applied to date amphiboles from the North Qaidam ultra-high pressure metamorphic amphibolites. Two amphibole samples analyzed by laser heating yielded saddle-shaped age spectra with total gas ages of 574.5±2.5 and 562.5±2.5 Ma. These ages are much older than the reported zircon U-Pb ages (~495 Ma) from Yuka eclogite, indicating the presence of excess 40Ar. In order to decipher the occurrence of excess 40Ar and constrain the age of amphibolite-facies retrogression, two duplicate amphibole samples were further employed for 40Ar/39Ar in vacuo crushing analyses. Both samples exhibit similar monotonically declining release spectra, which are characterized by rapid decline of anomalously old apparent ages in the early steps. The data of the late steps yielded concordant apparent ages with plateau ages of 460.9±1.2 and 459.6±1.8 Ma. We interpret that gases released in the early steps derive from the significant excess 40Ar containing secondary fluid inclusions (SFIs) due to their distribution characteristics along cracks leading to be easily extracted, whereas those released in the later steps represent the contribution of the small primary fluid inclusions (PFIs).
Coulomb stress accumulation and releasing history and its relationship with the occurrence of strong historical earthquakes could deepen our understanding of the occurrence pattern of strong earthquakes and hence its seismic potential in future. The sinistral strike-slip Xianshuihe-Xiaojiang fault zone (XXFS) is one of the most dangerous fault zones in China, extending 1 500-km-long from the central Tibetan Plateau to the Red River fault zone. There are 35 M≥6.5 historical earthquakes occurred since 1327, hence it is an ideal site for studying the Coulomb stress evolution history and its relationship with the occurrences of strong earthquakes. In this study, we evaluated the Coulomb stress change history along the XXFS by synthesizing fault geometry, GPS data and historical earthquakes. Coulomb stress change history also revealed different patterns of historical earthquakes on different segments of the XXFS, such as characteristic recurrence intervals along the Salaha-Moxi fault and super-cycles along the Xianshuihe fault. Based on the occurrence pattern of past historical earthquakes and current Coulomb stress field obtained in this study, we suggest positive ΔCFS and hence high seismic potential along the Salaha-Moxi fault and the Anninghe fault.
The two eastern segments of the Sertengshan piedmont fault have moved considerably since the Holocene. Several paleoseismic events have occurred along the fault since 30 ka BP. Paleoearthquake studies have been advanced by digging new trenches and combining the results with the findings of previous studies. Comprehensive analyses of the trenches revealed that 6 paleoseismic events have occurred on the Kuoluebulong segment since approximately 30 ka BP within the following successive time periods: 19.01–37.56, 18.73, 15.03–15.86, 10.96, 5.77–6.48, and 2.32 ka BP. The analyses also revealed that 6 paleoseismic events have occurred on the Dashetai segment since approximately 30 ka BP, and the successive occurrence times are 29.07, 19.12–28.23, 13.92–15.22, 9.38–9.83, 6.08–8.36, and 3.59 ka BP. The results indicate that quasi-periodic recurrences occurred along the two segments with an approximate 4 000 a mean recurrence interval. The consistent timing of the 6 events between the two segments indicates that the segments might conform to the cascade rupturing model between the two segments. As recorded by a large number of Chinese historical texts, the latest event on the Kuoluebulong segment is the historical M 8.0 earthquake occurred on November 11, 7 BC.
Geothermal resources in China are distributed throughout the country, with hydrothermal systems of high temperature in the Tibet Autonomous Region, Yunnan Province and Taiwan Island and hydrothermal systems of low-medium temperature mainly in various sedimentary basins. Development and exploration of geothermal energy in China are below expectations. The purpose of this study is to comparatively review the characteristics (geology, hydrogeology, hydrochemistry and geophysical data) of typical hydrothermal fields/areas and suggest development and utilization approaches in the future. Hydrothermal systems formed by mountain lifting contain a considerable amount of energy for geothermal power generation, especially in the Tibet Autonomous Region, Yunnan Province and Taiwan Island. However, geothermal water in the Tatun geothermal field has high TDS (total dissolved solids), an issue that requires more research to resolve this problem for power generation. The large storage of geothermal resources has been investigated in Meso–Cenozoic sedimentary basins; it is basically used for heating, bathing or greenhouse plantation. Moreover, hydrothermal resources of low-medium temperature can also be used in binary power plants. Although the enhanced geothermal systems (EGS) in China are promising, the resources have not yet been commercially exploited, because the emerging technologies (hydraulic fracturing) and concerns over environmental impacts (induced micro-seismicity) lead to slow development. On the contrary, shallow geothermal energy has been directly utilized mainly for heating and cooling buildings. Cities like Beijing, Tianjin and Shenyang have established a series of ground-source heat-pump systems, which has led to a massive reduction of CO2 emission of 19.87×106 t.
Xi'an ground fractures are the most typical ground fractures in China. Fourteen fractures have nearly divided the historical city into several distinct sections. These fractures are parallel and distributed in NEE direction at the same interval, with all features exhibiting a down dropping southerly block which extends to connect with the underlying fault. The activities of fractures are primarily expressed as normal faults. The faulted strata are well defined and dislocation displacement increases with depth. Thus, fractures have the characteristics of syn-sedimentary faults, which constitute the hanging wall of the Lintong-Chang'an fault branch system. Crustal thinning caused by the uplifting of upper mantle provides a power source for extension and stretching along the fracture surface of the upper crust, which results in a series of extensional faults and the suitable conditions for forming massive ground fractures. The movement of tectonic blocks influences the normal dip-slipping tension of Lintong-Chang'an fault branches, and produces a series of secondary tectonic fractures adjacent to surface, which constitute the prototype of ground fractures. The recent regional tensile stress produced by modern mainland deformation, also profoundly influences the current activity of Xi'an ground fractures.