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摘要:The progressive indentation of India into Eurasia generated an E–W-trending orthogonal collision belt and a N– S-trending oblique collision belt. Compiling available data reveals that ~70% of the Cenozoic igneous rocks in eastern and southeastern Tibet are concentrated within an ENE-trending, ~550-km long and ~250-km wide magmatic zone (CMZ) that once separated the orthogonal and oblique collision belts. The Latitude 26°N Line is now its southern boundary. The onset timing of magmatism of the CMZ varies gradually from ~55 Ma in the westernmost part to ~27 Ma in the easternmost. Then the magmatism successively occurred and suddenly stopped at ~25 Ma. The segmented and coherent chemical variation trends found suggest that the CMZ magmatic rocks were formed due to partial melting of the heterogeneous upper mantle and crusts of Eurasia. Subduction of Paleo- and Neotethyan oceanic plates generated this compositional and mineralogical heterogeneity. Combined with available geophysical data, the CMZ was diachronously formed in response to asthenosphere upwelling induced by NNW?SSE-direction lithosphere stretching. The difference in responses of the orthogonal and oblique collision belts to the indentation of the Indian continent has led to this lithosphere stretching.

摘要:Phosphorus (P) is a key biological nutrient and probably the ultimate limiter of marine productivity during Earth history. In recent years, a wealth of new knowledge has revolutionized our understanding of the global P cycle, yet its long-term evolution remains incompletely documented. In this paper, we review the effects of three major controlling factors on the long-term evolution of the global P cycle, i.e., tectonics, marine redox conditions, and bio-evolution, on the basis of which a five-stage model is proposed: Stage I (>~2.4 Ga), tectonic-lithogenic-controlled P cycling; Stage II (~2.4 Ga to 635 Ma), low-efficiency biotic P cycling; Stage III (~635 Ma to 380 Ma), transitional biotic P cycling; Stage IV (~380 Ma to near-modern), high-efficiency biotic P cycling; and Stage V (Anthropocene), human-influenced P cycling. This model implies that the earlier-proposed Ediacaran reorganization of the marine P cycle may represent only the start of a ~250-Myr-long transition of the Earth’s P cycle (Stage III) between the low-efficiency biotic mode of the Proterozoic (Stage II) and the high-efficiency biotic mode of the Phanerozoic (Stage IV). The development of biologically-driven, high-efficiency P cycling may have been a key factor for the increasing frequency and volume of phosphorite deposits since the late Neoproterozoic.

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摘要:Spatangoid echinoids belonging to Heteraster found in the Lower Cretaceous limestones and calcareous deposits of the Tirgan and Sarcheshmeh formations (Barremian–Aptian) in the Bahman jan-Bala stratigraphic section on the northern flank of the Borouj syncline, situated in the eastern Kopet-Dagh Basin, northeastern Iran are assessed as a sexually dimorphic species. Sexual dimorphism is a common feature in echinoids and, in this study of Heteraster renngarteni Poretzkaja, 1961, sexual dimorphism has been detected for the first time in the family Toxasteridae.

摘要:The Susong complex zone is a relatively low-grade metamorphic unit located in the southern part of the Dabie orogen and preserves a variety of metasedimentary rocks, mostly with epidote–amphibolite facies. However, their depositional age, provenance and tectonic setting of sedimentary protolith remain controversial due to the lack of fossils, precise dating and integrated geochemical investigation. This study has conducted whole-rock elemental, and zircon U-Pb SHRIMP dating and Lu-Hf isotope analyses on three types of representative metasedimentary rocks including garnet-bearing mica-quartz schist, graphite-muscovite-quartz schist and dolomitic marble from the Susong complex zone. The U-Pb SHRIMP dating data indicate that the sedimentary protolith of these rocks have the maximum depositional ages of less than 840–750 Ma and have various sedimentary provenances. In combination with zircon Lu-Hf isotope compositions, the provenance of the metasedimentary rocks in the region is for the first time documented to be mainly derived from four groups of magmatic rocks formed at ~2.5 Ga, ~2.0 Ga, ~1.4 Ga and ~0.8 Ga in response to four episodes of igneous activity in the northeastern margin of the Yangtze Block. The conventional geothermobarometry combined with rare metamorphic zircon ages indicate that the studied rocks underwent the Late Triassic continental subduction-related metamorphism with peak epidote–amphibolite facies conditions at P = 0.34–0.91 GPa and T = 427–532°C. In addition, results of the elemental contents (i.e., La, Ce, Th and ΣREE) and ratios (i.e., Eu/Eu* and LaN/YbN) suggest that the protoliths of the metasedimentary rocks were mainly shales, wackes and limestones, most probably related to the Rodinia supercontinent rifting along the northern margin of the Yangtze Block during the Neoproterozoic.

摘要:Tonstein layers are found worldwide in the Permo–Carboniferous coal-bearing strata. This study investigates the geochronology, mineralogy, and geochemistry of four tonstein samples from the Permo–Carboniferous Benxi Formation, Ordos Basin, North China Craton (NCC). The typical features of the studied tonsteins include thin beds, lateral continuity, angular quartz grains, and euhedral zircons with similar U-Pb ages, indicating a significant pyroclastic origin. In addition, the tonstein samples have low TiO2/Al2O3 ratios (<0.02) and rare earth elements and yttrium (REY) concentrations with obvious negative Eu anomalies, indicating that the tonsteins have a felsic magma origin. Moreover, compared with the mean composition of clay shale, the studied tonsteins are characterized by high concentrations of the elements Nb and Ta, which may affect the concentration of the corresponding elements in surrounding coal seams. The zircon U-Pb ages of the tonsteins (293.9–298.8 Ma) provide a precise chronological framework on the Benxi Formation in the Ordos Basin, constraining the Gzhelian–Aselian stages. The tonsteins were probably sourced from arc volcanism along the western margin of the NCC during the early Permian, implying that the Alxa Terrane had not amalgamated with the NCC at that time.

摘要:The East Tianshan contains many late Paleozoic magmatic and polymetallic deposits. Recent studies demonstrate that the early Paleozoic volcanic rocks discovered in the northern region of East Tianshan can be subdivided into the Daliugou, Hongliuxia and Kalatag formations. Here, we report zircon U-Pb ages of volcanic rocks and quartz diorite porphyry, together with whole-rock geochemical and Sr-Nd isotope data of early Paleozoic volcanic rocks, in order to investigate their petrogenesis and geodynamic setting. Zircon U-Pb analyses of the rocks suggest that the Kalatag Formation formed at ca. 438–413 Ma. Geochemical characteristics of the early Paleozoic volcanic rocks exhibit typical subduction-related features. They have high Mg# (44–75), positive εNd(t) values (0.25–7.88), low 87Sr/86Sr ratios (0.70457 to 0.70588) and young two-stage Hf isotope model ages (551–446 Ma), consistent with a depleted mantle origin with limited crustal contamination. The basaltic andesite and andesite have relatively high MgO contents (6.1–9.4 wt%), suggesting that they belong to high-magnesian andesites. They were most likely derived from the partial melting of mantle peridotite caused by the addition of fluids released by subducted oceanic slab. Based on regional geology, geochemical characteristics and previous studies, we infer that the southward subduction of the Junggar Ocean slab resulted in the generation of early Paleozoic volcanic rocks and arc setting-related metallogenic systems in the Kalatag area.

摘要:Orogenic peridotite is an important component of orogenic belts and retains crucial information on mantle magmatic activity, slab subduction, and melt or fluid metasomatism. To determine the source of the mantle-derived parental magma of the peridotite and to investigate the metasomatism that it experienced, we undertook an integrated study of the petrography, whole-rock major- and trace-element compositions, in situ zircon U-Pb geochronology, and mineral major- and trace-element compositions of an early Paleozoic ultramafic complex in the North Wulan area of North Qaidam. The Halihatu ultramafic–mafic complex is composed of dunite, pyroxene peridotite, and gabbro, which are characteristic of Alaskan-type complexes. The dunite yields a weighted mean 206Pb/238U age of 479 ± 5 Ma (MSWD = 0.7), which reflects the age of the metasomatism rather than the crystallization age of the ultramafic magma. The peridotites have high Mg# (89.8–91.8) and Cr contents (2419–5190 ppm), low Al2O3 (0.20–1.68 wt%) and Ni (289–1012 ppm) contents, and high olivine Fo contents (87–91), suggesting a large degree (~15%–22%) of partial melting of lithospheric ultramafic rocks followed by variable degrees of fractional crystallization of olivine and pyroxene. This is consistent with estimates of 15%–22.3% partial melting calculated using the Cr# of spinel crystals and with the low Yb (0.04–0.33 ppm) and Y (0.72–1.29 ppm) contents of clinopyroxene crystals. Whole-rock trace-element patterns show enrichment in large ion lithophile elements and depletion in high field strength elements, along with high Al2O3 (2.10–6.47 wt%) and low TiO2 (0.01–0.21 wt%) contents of clinopyroxene crystals, suggesting an arc magma cumulate trend. These features, along with the high olivine Fo contents (87–91 ppm), imply that the Halihatu peridotite is an Alaskan-type crustal cumulates derived from Mg-rich hydrous basaltic melts. The high estimated fO2 (FMQ +1.97 to FMQ +3.81) further supports the idea that they formed in an arc setting. The Ni/Co and Ni/Mn ratios and cumulate textures of the olivine, quenched boundaries between mafic and felsic melts, and the occurrence of tremolite and phlogopite reflect interactions between the Halihatu peridotite and injected silicate and carbonatitic melts in the lower crust. Therefore, we propose a new cumulate-infiltration model for the petrogenesis of Alaskan-type ultramafic complexes, which improves our understanding of the nature of Alaskan-type continental arc root.

摘要:Jiama is a giant, high-grade porphyry copper system in the Gangdese metallogenic belt, Tibet. Multistage intermediate–felsic porphyries intruded in this deposit, some of which are strongly associated with copper-polymetallic mineralization. These ore-bearing porphyries include monzogranite, granodiorite, and quartz diorite porphyries. A new granite aplite dyke was found in the south of Jiama. Its age, genesis, and relationship with ore-related magmatism are obscure. Here, its emplacement age and petrogenesis were determined using mineralogy, zircon U-Pb dating, geochemistry, and Sr-Nd-Pb isotope studies. The zircon LA-ICP-MS U-Pb age of the aplite dyke is 16.66 ± 0.21 Ma (n = 14, MSWD = 0.66), earlier than that of the ore-bearing porphyries (~15 Ma) in Jiama. Furthermore, the aplite exhibits high amounts of silicon (SiO2 = 73.39%–74.74%), potassium (K2O = 5.12%–6.61%), aluminum (Al2O3 = 14.25%–14.69%), and light/heavy rare earth elements (LREE/HREE = 12.12–16.19) as well as negative europium (δEu = 0.47–0.72) and weak negative cerium anomalies (δCe = 0.84–0.93). The aplite dyke is characteristic of metaluminous–peraluminous I-type granite, which is rich in large-ion lithophile elements (Rb, Ba, Th and U) and depleted in high-field-strength elements (Nb, P and Ti). The aplite dyke and ore-bearing porphyries in the Jiama deposit are the results of a partial melting of the juvenile lower crust, according to whole-rock geochemistry and Sr-Nd-Pb isotope data, but the dyke and ore-bearing porphyries were emplaced from the same magma chamber at different times. Thus, the aplite dyke shows the composition of the early evolution stage of shallow magma in the Jiama deposit and is the product of rapid condensation and crystallization.

摘要:Deep-water gravity depositional processes and evolution in arc systems have become topics of intense research focus in recent years. This study discusses the co-evolution of volcanism and deep-water gravity flow deposits at the southern margin of the Junggar Basin, based on petrology, geochronology and geochemical analyses. The results show that a massive collapse of unstable sediments from the slope was triggered by volcanism, resulting in the formation of slumping gravity flows. The occurrence of volcanic beds in the slump deposits confirm that synchronous volcanism likely affected sediment instability, triggering gravity flows. The Th/Yb, Ta/Yb and Th/Ta elemental ratios, U-Pb ages of detrital zircons and paleocurrent directions indicate that the North Tianshan (NTS) island arc represents the provenance of the Qianxia Formation. Moreover, statistical data on the pyroclastic components in the gravity flow deposits reveal an intensity index of volcanism, indicating that volcanism is strongly related to gravity flow deposits, especially in terms of the type and distribution of the deposits. A model for volcanically-triggered deep-water gravity flow deposits is established, in order to provide a more in-depth understanding of the co-evolution of volcanism and gravity flow deposits within the depositional setting of the late Paleozoic NTS oceanic subduction margin in the Junggar Basin.

摘要:The Panzhihua mafic intrusion, which hosts a world-class Fe-Ti-V ore deposit, is in the western Emeishan region, SW China. The formation age (~260 Ma), and Sr and Nd isotopes indicate that the Panzhihua intrusion is part of the Emeishan large igneous province and has little crustal contamination. To assess ore genesis of the Panzhihua Fe-Ti-V ore deposit, two different models have been provided to explain the formation, namely silicate immiscibility and normal fractional crystallization. Silicate immiscibility occurring around 1,000°C at the late stage of basaltic magma evolution argues against the silicate immiscibility model. Apatite-hosted melt inclusion research indicates that silicate immiscibility occurred at the late stage of Panzhihua magma evolution, which may not have offered potential to form such large ore deposits as Panzhihua. Alternatively, continuous compositional variations of the Panzhihua intrusion and calculations using thermodynamic modelling software support the hypothesis that the Panzhihua deposit was formed by normal fractional crystallization. Reciprocal trace element patterns of the Panzhihua intrusion and nearby felsic rocks also coincide with the fractional crystallization model. Normal fractional crystallization of high-Ti basaltic magma played a key role in the formation of the Panzhihua Fe-Ti-V ore deposit.

摘要:Oxygen fugacity (?O2) is a key intensity variable during the entire magmatic-hydrothermal mineralization courses. The redox state and its variations between different stages of the ore-forming fluids of intermediate sulfidation epithermal deposits are rarely deciphered due to the lack of appropriate approaches to determine ?O2 of the fluids. Here, we reported the δ34S of the sulfides from three different stages (stage I, II, III) of Zhengguang, an Early Ordovician Au-rich intermediate sulfidation (IS) epithermal deposit, to decipher the redox evolution of the ore-forming fluids. The increasing δ34S values from stage I pyrite (py1, average ?2.6‰) through py2 (average ?1.9‰) to py3 (average ?0.2‰) indicates a decrease of the oxygen fugacity of the ore-forming fluids. A compilation of δ34S values of sulfides from two subtypes of IS deposits (Au-rich and Ag-rich) from NE China shows that the δ34S values of sulfides from Au-rich IS deposits are systematically lighter than those of Ag-rich IS Ag-Pb-Zn deposit, indicating the ore-forming fluids of the former are more oxidized than the latter. We highlight that sulfur isotopic composition of hypogene sulfides is an efficacious proxy to fingerprint the oxygen fugacity fluctuations of epithermal deposits and could potentially be used to distinguish the subtypes of IS deposits.

摘要:Located along the southern part of the West Qinling orogenic belt, the Yangshan gold deposit is one of the largest in China. The major gold ores of Yangshan are disseminated in metasedimentary host rocks with minor native gold amounts in stibnite-gold quartz veins. Pyrite and arsenopyrite are the major Au-bearing minerals. Hydrothermal muscovite from gold-bearing quartz veins was dated using the in situ Rb-Sr method to determine the formation age of the Yangshan gold deposit. The Rb-Sr isochron date of the muscovite yielded 210.1 ± 5.6 Ma (MSWD = 1.2). This date is near the lower end of the period of the mineralized granitic dykes (210.49–213.10 Ma). Two stages of gold enriching process are recognized in the gold-bearing pyrite: the first is incorporated with the Co, Cu, As, Ni enrichment; and the second is accompanied by Bi, Co, Ni, Pb, Cu, Sb concentration. The in-situ sulfur isotopic values of pyrites show a restricted Δ34s range of ?1.43 ‰ to 2.86 ‰ with a mean value of 0.43 ‰. Trace-element mapping and in-situ sulfur isotopic analysis of pyrite suggest that the sulfur deposits are likely derived from a magmatic source and likely assimilated by sulfur from the sedimentary bedrock. Thus, magmatism plays a critical role in the formation of the Yangshan gold deposit.

摘要:To explore the genesis of the laminated calcite veins developed in the black shale of Chang 73 submember of the Ordos Basin, the petrology, microstructure and geochemistry of calcite veins are studied using thin sections, fluid inclusions, trace elements and isotopic geochemistry. The source, the time of formation, the mechanism of formation, and the dynamic background of the veins are discussed. The veins are mostly made up of calcite, mixed with lenticular or spindle-shape solid wall rock inclusions. Three structures are identified in the calcite minerals: fibrous, rhombic cleavage, and wedge-like structure. Trace elements and isotopes of carbon and oxygen confirm that the calcite veins were formed from a high density hydrothermal fluid. It is assumed that calcite veins formed prior to wall rock consolidation during the formation of the Qinling Orogenic Belt in the Middle and Late Triassic. The results show that the sedimentary sources of Chang 73 submember were influenced by hydrothermal materials besides terrigenous detritus. The fibrous calcite is different from the fibrous calcite reported in the morphology and mechanism of formation. In this way, the research complements and improves the morphology and the mechanism of formation of fibrous calcite.

摘要:The lacustrine shale of deep Shahezi Formation in the Songliao basin has great gas potential, but its pore evolution, heterogeneity, and connectivity characteristics remain unclear. In this work, total organic carbon analysis, rock pyrolysis, X-ray diffraction, field emission scanning electron microscopy, the particle and crack analysis system software, low-temperature nitrogen adsorption experiment, fractal theory, high-pressure mercury injection experiment and nuclear magnetic resonance experiment were used to study the Shahezi shale from Well SK-2. The result indicated that the organic pores in Shahezi shale are not developed, and the intergranular and intragranular pores are mainly formed by illite-dominated clay. As the burial depth increases, the pore size and slit-shaped pores formed by clay decrease, and dissolved pores in the feldspar and carbonate minerals and dissolved fractures in the quartz increase. The pore evolution is affected by clay, compaction, and high-temperature corrosion. Based on the pore structure characteristics reflected by the pore size distribution and pore structure parameters obtained by multiple experimental methods, the pore development and evolution are divided into three stages. During stage I and II, the pore heterogeneity of the shale reservoirs increases with the depth, the physical properties and pore connectivity deteriorate, but the gas-bearing property is good. In stage III, the pore heterogeneity is the highest, its gas generation and storage capacity are low, but the increase of micro-fractures makes pore connectivity and gas-bearing better.

摘要:Early Cambrian shale is an important petroleum source rock around the world. Because of little drilling data and poor seismic data, until recently, organic matter enrichment of the Lower Cambrian Yuertusi and Xishanbulake formations shale is still an enigma in the Tarim Basin, northwestern China. Total organic carbon (TOC), major and trace element data of Cambrian shale samples from five boreholes have been analyzed to decipher the mechanism of the organic matter enrichment. The results show that the shales deposited in the western restricted intraplatform have much higher TOC contents (3.2%–19.8%, on average 11.0%) than those from the eastern basin (2.2%–10.2%, on average 4.5%). The paleoproductivity proxies (Ba, Ba/Al, P/Al) in the western restricted platform are much higher than those in the eastern basin. The trace element indicators such as V/Cr, Ni/Co, Mo–TOC and MoEF–UEF suggest an anoxic environment across the basin, but a more restricted environment in the western intraplatform. The paleoproductivity rather than anoxic condition and hydrothermal activity are concluded to have resulted in the differentiation of the organic matter enrichment from the western intraplatform to eastern basin in the early Cambrian shales; the restricted environment was favorable for paleoproductivity and preservation of organic matter.

摘要:We investigated diagenesis of the sandstones from the DN2 Gas Field of the Kuqa Foreland Basin (KFB), in order to infer the timing of fluid migration and discuss the linkage between fluids and tectonics. The textures and chemical composition of authigenic minerals, fluid evidence from fluid inclusions and formation water measurements were all used to fulfill this aim. Eodiagenesis occurred with the participation of meteoric water and connate water. Mesodiagenesis is related to high salinity fluids, which were attributed as originating from the overlying Neogene Jidike Formation evaporite (principal minerals including halite, anhydrite, glauberite, carnallite and thenardite). The onset of high salinity fluid migration is inferred to occur during the late Miocene (12.4–9.2 Ma) through the use of homogenization temperatures measured in the present study and K-Ar dating of authigenetic illites from previous work. This period is consistent with the crucial phase (13–10 Ma) that witnessed the rapid uplift of the southern Tianshan Mts and the stage when calcite and anhydrite veins formed in the studied strata. We thus argue that diagenesis related to high salinity fluids occurred as a response to the Tianshan Mts’ rapid uplift and related tectonic processes. The flow of high salinity fluids was probably driven by a density gradient and channeled and focused by fractures formed contemporaneously.

摘要:The bedrock weathered crust in front of the Altun Mountains in the Qaidam Basin, western China, is different from others because this is a salt-lake basin, where saline water fluid infiltrates and is deposited in the overlying strata. A large amount of gypsum infills the bedrock weathered crust, and this has changed the pore structure. Using core observation, polarized light microscopy, electron probe, physical property analysis and field emission scanning electron microscopy experiments, the characteristics of the weathered bedrock have been studied. There are cracks and a small number of dissolved pores in the interior of the weathered crust. Matrix micropores are widely developed, especially the various matrix cracks formed by tectonics and weathering, as well as the stress characteristics of small dissolved pores, and physical properties such as porosity and permeability. This ‘dual structure’ developed in the bedrock is important for guiding the exploration of the lake basin bedrock for natural gas.

摘要:In this study, organic geochemical and petrological analyses were conducted on 111 shale samples from a well to understand the retention, intraformational migration and segmented accumulation (shale oil enrichment in different intervals is unconnected) features of shale oil within the organic-rich shale in the Qingshankou Formation of the Gulong Sag. Our study shows that retained petroleum characteristics in the investigated succession are mainly influenced by three factors: organic richness, intraformational migration and segmented accumulation. Organic matter richness primarily controls the amount of retained petroleum, especially the ‘live’ component indicated by the S2 value rather than the total organic carbon (TOC) figure alone. The negative expulsion efficiencies determined by mass-balance calculations of hydrocarbons reveal that petroleum from adjacent organic-rich intervals migrates into the interval of about 2386–2408 m, which is characterized by high free hydrocarbon (S1), OSI and saturated hydrocarbons content, along with a greater difference in δ13C values between polar compounds (including resins and asphaltenes) and saturated hydrocarbons. The depth-dependent heterogeneity of carbon isotope ratios (δ13C) of mud methane gas, δ13C of extracts gross composition (SARA), δ13C of kerogen and SARA content of extracts suggest that the studied succession can be subdivided into four intervals. The shale oil sealing enrichment character in each interval is further corroborated by the distinct δ13C values of mud methane gas in different intervals. Due to the migration of petroleum into the 2386–2408 m interval, the S1, OSI and saturated hydrocarbons content of the interval show higher relative values. The maturity of organic matter in the 2471–2500 m interval is at the highest with the smaller size molecular components of the retained petroleum. Thus, favorable ‘sweet spots’ may be found in the 2386–2408 m interval and the 2471–2500 m interval, according to the experiment results in this study.