2021 Vol. 46, No. 6
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2021, 46(6): 1909-1940.
doi: 10.3799/dqkx.2020.392
Abstract:
The Gangdese copper-molybdenum polymetallic metallogenic belt in Tibet was not included in the national important metallogenic belts before 2001. However, the subsequent theoretical understanding and method innovation of mineralization and prospecting have led to a historic breakthrough in ore prospecting in this belt, newly finding of a series of large and super-large deposits such as Qulong, Jiama, Zhunuo, Xiongcun, Nuri, Chongjiang, Bangpu, Mengya'a, Dongzhongsongduo and Chagele. The proven copper resources alone exceed 56 million tons, forming the largest world-class copper polymetallic exploration and development base in China. The newly discovered deposits are mainly distributed in the southern Lhasa terrane and Gangdese back-arc fault uplift belt, and show the spatial distribution of east-west banding, parallel north-east and interjunction mineralization. The isotopic data show five-stage porphyry mineralization (213 Ma, 173-165 Ma, ~45 Ma, ~30 Ma, 17-13 Ma), five-stage skarn mineralization (~112 Ma, ~77 Ma, 67-55 Ma, ~41-37 Ma, ~23-16 Ma) and two-stage epithermal mineralization (~126 Ma, ~65-55 Ma). With the formation and subsequent subduction of the Neo-Tethys ocean, and India-Asia collision, the Gangdese belt has experienced four orogenic processes, including accretionary orogenesis, collisional orogenesis, intracontinental orogenesis and balanced orogenic process. The ore-bearing magma was derived from partial melting of basaltic oceanic crust dominated by mantle source material, or ancient crust, or juvenile lower crust during different periods, in which a series of Cu-Mo-W-Ag-Sn-Au polymetallic deposits have been formed including single types of porphyry, skarn, epithermal-hydrothermal vein, thermal spring deposits, or complex types of porphyry-skarn-epithermal deposits and so on. These deposits show the metallogenic characteristics of same source with multiple locations, same location with multiple genesis, deep source with shallow mineralization, as well as multiple genesis and complex formation, due to the superposition of different orogenies during superimposed orogenesis process. Therefore, nine minerogenetic series are classified: Late Triassic porphyry Cu-Au series related to arc-magmatism; Middle Jurassic porphyry Cu-Au series related to island arc magmatism; Early Cretaceous skarn-epithermal Fe-Ag-Pb-Zn(-Sn) series related to intermediate-acidic magma; Late Cretaceous Fe-Cu polymetallic series related to intermediate-acidic magma; Paleocene-Eocene Fe-Cu polymetallic series related to intermediate-acidic magma; Paleocene Ag-Sn-Au polymetallic series related to continental (sub) volcanic rocks; Oligocene porphyry-skarn Cu-W-Mo(-Au) series; Miocene porphyry-skarn-epithermal Cu-Mo-Au-Pb-Zn-Ag series; Cenozoic hydrothermal spring Au-S-Cs deposits and salt mineralization series. Finally it proposes some scientific questions that remains to be further study and exploration in Gangdese belt, and predictes that Zhunuo deposit cluster still has the potential of finding large- superlarge porphyry copper deposits, where a major breakthrough can be achieved in the future ore-prospecting in the Gangdese metallogenic belt, indicating directions for the further exploration deployment and evaluation.
The Gangdese copper-molybdenum polymetallic metallogenic belt in Tibet was not included in the national important metallogenic belts before 2001. However, the subsequent theoretical understanding and method innovation of mineralization and prospecting have led to a historic breakthrough in ore prospecting in this belt, newly finding of a series of large and super-large deposits such as Qulong, Jiama, Zhunuo, Xiongcun, Nuri, Chongjiang, Bangpu, Mengya'a, Dongzhongsongduo and Chagele. The proven copper resources alone exceed 56 million tons, forming the largest world-class copper polymetallic exploration and development base in China. The newly discovered deposits are mainly distributed in the southern Lhasa terrane and Gangdese back-arc fault uplift belt, and show the spatial distribution of east-west banding, parallel north-east and interjunction mineralization. The isotopic data show five-stage porphyry mineralization (213 Ma, 173-165 Ma, ~45 Ma, ~30 Ma, 17-13 Ma), five-stage skarn mineralization (~112 Ma, ~77 Ma, 67-55 Ma, ~41-37 Ma, ~23-16 Ma) and two-stage epithermal mineralization (~126 Ma, ~65-55 Ma). With the formation and subsequent subduction of the Neo-Tethys ocean, and India-Asia collision, the Gangdese belt has experienced four orogenic processes, including accretionary orogenesis, collisional orogenesis, intracontinental orogenesis and balanced orogenic process. The ore-bearing magma was derived from partial melting of basaltic oceanic crust dominated by mantle source material, or ancient crust, or juvenile lower crust during different periods, in which a series of Cu-Mo-W-Ag-Sn-Au polymetallic deposits have been formed including single types of porphyry, skarn, epithermal-hydrothermal vein, thermal spring deposits, or complex types of porphyry-skarn-epithermal deposits and so on. These deposits show the metallogenic characteristics of same source with multiple locations, same location with multiple genesis, deep source with shallow mineralization, as well as multiple genesis and complex formation, due to the superposition of different orogenies during superimposed orogenesis process. Therefore, nine minerogenetic series are classified: Late Triassic porphyry Cu-Au series related to arc-magmatism; Middle Jurassic porphyry Cu-Au series related to island arc magmatism; Early Cretaceous skarn-epithermal Fe-Ag-Pb-Zn(-Sn) series related to intermediate-acidic magma; Late Cretaceous Fe-Cu polymetallic series related to intermediate-acidic magma; Paleocene-Eocene Fe-Cu polymetallic series related to intermediate-acidic magma; Paleocene Ag-Sn-Au polymetallic series related to continental (sub) volcanic rocks; Oligocene porphyry-skarn Cu-W-Mo(-Au) series; Miocene porphyry-skarn-epithermal Cu-Mo-Au-Pb-Zn-Ag series; Cenozoic hydrothermal spring Au-S-Cs deposits and salt mineralization series. Finally it proposes some scientific questions that remains to be further study and exploration in Gangdese belt, and predictes that Zhunuo deposit cluster still has the potential of finding large- superlarge porphyry copper deposits, where a major breakthrough can be achieved in the future ore-prospecting in the Gangdese metallogenic belt, indicating directions for the further exploration deployment and evaluation.
2021, 46(6): 1941-1959.
doi: 10.3799/dqkx.2020.216
Abstract:
To find out the metallogenic age and environment of Lunggar iron-rich ore deposit and its indication of regional iron mineralization in Gangdese Belt, the petrogeochemistry, zircon U-Pb chronology, zircon Lu-Hf isotopes, and Ar-Ar dating of phlogopite have been carried out. The results show that: (1) there are two stages of magmatism: Early Cretaceous granodiorite (116.3 Ma) and Late Cretaceous diorite (94.3-93.8 Ma). The 40Ar/39Ar isochron age of gold mica in the main orebody is 93.71±2.96 Ma, indicating that the deposit was formed in the Late Cretaceous. (2) The diorite related to the mineralization is characterized by low SiO2 content (52.17%-55.32%), enrichment of LREE and large ion lithophile elements such as Rb, Ba, and Pb, and depletion of HREE and high field strength elements such as Nb, Ta, and Ti, which belongs to the High-K calc-alkaline and metaluminous rock series. The zircon εHf(t) is between 1.4 and 3.6, and the Mg# index is very high (0.59-0.64), suggesting that more mantle-derived materials are added to the ore-forming pluton, which has the characteristics of crust-mantle mixing. (3) According to the ages of other iron deposits in Gangdese Belt, the iron mineralization can be divided into three stages, including ~115 Ma, ~94 Ma and 50-65 Ma. The Lunggar iron deposit suggests that it belongs to a new stage (Late Cretaceous) of iron-rich mineralization in Gangdese Belt. (4) Compared with the multi-stage skarn iron mineralization in the Gangdese metallogenic belt, it is found that there are obvious geochemical differences between the Lunggar iron ore-forming body and other periods of iron ore-forming body. The intermediate-basic magma with more mantle-derived materials may be easier to form iron-rich deposits than acid magma.
To find out the metallogenic age and environment of Lunggar iron-rich ore deposit and its indication of regional iron mineralization in Gangdese Belt, the petrogeochemistry, zircon U-Pb chronology, zircon Lu-Hf isotopes, and Ar-Ar dating of phlogopite have been carried out. The results show that: (1) there are two stages of magmatism: Early Cretaceous granodiorite (116.3 Ma) and Late Cretaceous diorite (94.3-93.8 Ma). The 40Ar/39Ar isochron age of gold mica in the main orebody is 93.71±2.96 Ma, indicating that the deposit was formed in the Late Cretaceous. (2) The diorite related to the mineralization is characterized by low SiO2 content (52.17%-55.32%), enrichment of LREE and large ion lithophile elements such as Rb, Ba, and Pb, and depletion of HREE and high field strength elements such as Nb, Ta, and Ti, which belongs to the High-K calc-alkaline and metaluminous rock series. The zircon εHf(t) is between 1.4 and 3.6, and the Mg# index is very high (0.59-0.64), suggesting that more mantle-derived materials are added to the ore-forming pluton, which has the characteristics of crust-mantle mixing. (3) According to the ages of other iron deposits in Gangdese Belt, the iron mineralization can be divided into three stages, including ~115 Ma, ~94 Ma and 50-65 Ma. The Lunggar iron deposit suggests that it belongs to a new stage (Late Cretaceous) of iron-rich mineralization in Gangdese Belt. (4) Compared with the multi-stage skarn iron mineralization in the Gangdese metallogenic belt, it is found that there are obvious geochemical differences between the Lunggar iron ore-forming body and other periods of iron ore-forming body. The intermediate-basic magma with more mantle-derived materials may be easier to form iron-rich deposits than acid magma.
2021, 46(6): 1960-1972.
doi: 10.3799/dqkx.2020.196
Abstract:
Study on high T/P metamorphic rocks in the southern Yili plate is essential for our understanding of the evolution of the Tianshan orogenic belt. Previous researchers suggested these rocks to belong to the Early Precambrian basement of the Nalati Group. However, recent studies show that they were generated in Paleozoic. Here in this study, it discusses the origin and formation environment for rocks in different litho-tectonic units in the southern Yili plate. Combined with other studies, it proposes that the high T/P metamorphic rocks were formed due to the emplacement and transportation of large volume magma in the arc region when the South Tianshan oceanic slab subducted under the Yili plate. The magma intrusion significantly elevated the thermal gradient, resulted in the regional high T/P metamorphism along the southern Yili plate. Therefore, high T/P rocks in the southern Yili plate were not a constituent of the Early Precambrian basement, but represented an active continental margin formed by the subduction of the South Tianshan ocean.
Study on high T/P metamorphic rocks in the southern Yili plate is essential for our understanding of the evolution of the Tianshan orogenic belt. Previous researchers suggested these rocks to belong to the Early Precambrian basement of the Nalati Group. However, recent studies show that they were generated in Paleozoic. Here in this study, it discusses the origin and formation environment for rocks in different litho-tectonic units in the southern Yili plate. Combined with other studies, it proposes that the high T/P metamorphic rocks were formed due to the emplacement and transportation of large volume magma in the arc region when the South Tianshan oceanic slab subducted under the Yili plate. The magma intrusion significantly elevated the thermal gradient, resulted in the regional high T/P metamorphism along the southern Yili plate. Therefore, high T/P rocks in the southern Yili plate were not a constituent of the Early Precambrian basement, but represented an active continental margin formed by the subduction of the South Tianshan ocean.
2021, 46(6): 1973-1992.
doi: 10.3799/dqkx.2020.007
Abstract:
In order to explore the closure time and evolution process of the Paleo Asian ocean in Wenduermiao-Jining area, it analyzes the petrology, geochemistry, geochronology and zircon Hf isotope composition. Zircon U-Pb dating shows that, Kuaziliang granite porphyry formed in the Middle Permian (270±3 Ma), while Wulannaoer granite porphyry formed in the Middle Triassic (241±2 Ma) that is the first Triassic granite discovered in the region. These rocks characterize with high contents of SiO2, Al2O3, very low Mg# values. They also display enrichment in LREEs and LILEs, depletion in HREEs and HFSEs, with obviously negative Eu anomalies. Kuaziliang granite porphyry exhibits negative εHf(t) values (0.25 to -12.33), old Hf isotopic tDM2 from 1 278 Ma to 2 071 Ma, indicating that it was derived from partial melting of ancient crustal material, while Wulannaoer granite porphyry exhibits positive εHf(t) values (2.03-5.94), relatively new Hf isotopic tDM2 from 892 Ma to 1 144 Ma, indicating that it was derived from partial melting of juvenile crustal material. Analyzed by synthesis, it speculates that before the Middle Permian, the Paleo-Asian ocean was closed in the Wenduertemple-Jining area; during the Middle Permian, the region is in the post-collision stage, forming the Kuaziliang granite porphyry. During Late Permian through Early Triassic, continuous collision orogeny in the region; the Middle Triassic, the region is in the post-orogenic extension stage, forming the Wulannaoer granite porphyry.
In order to explore the closure time and evolution process of the Paleo Asian ocean in Wenduermiao-Jining area, it analyzes the petrology, geochemistry, geochronology and zircon Hf isotope composition. Zircon U-Pb dating shows that, Kuaziliang granite porphyry formed in the Middle Permian (270±3 Ma), while Wulannaoer granite porphyry formed in the Middle Triassic (241±2 Ma) that is the first Triassic granite discovered in the region. These rocks characterize with high contents of SiO2, Al2O3, very low Mg# values. They also display enrichment in LREEs and LILEs, depletion in HREEs and HFSEs, with obviously negative Eu anomalies. Kuaziliang granite porphyry exhibits negative εHf(t) values (0.25 to -12.33), old Hf isotopic tDM2 from 1 278 Ma to 2 071 Ma, indicating that it was derived from partial melting of ancient crustal material, while Wulannaoer granite porphyry exhibits positive εHf(t) values (2.03-5.94), relatively new Hf isotopic tDM2 from 892 Ma to 1 144 Ma, indicating that it was derived from partial melting of juvenile crustal material. Analyzed by synthesis, it speculates that before the Middle Permian, the Paleo-Asian ocean was closed in the Wenduertemple-Jining area; during the Middle Permian, the region is in the post-collision stage, forming the Kuaziliang granite porphyry. During Late Permian through Early Triassic, continuous collision orogeny in the region; the Middle Triassic, the region is in the post-orogenic extension stage, forming the Wulannaoer granite porphyry.
2021, 46(6): 1993-2015.
doi: 10.3799/dqkx.2020.394
Abstract:
In this paper it presents a combined study of zircon U-Pb ages, trace elements and Hf isotopes, and whole-rock major-trace elements and Sr-Nd isotopes for the Jingshan granites from the southeast margin of the North China block (NCB). LA-ICP-MS zircon U-Pb dating yielded Late Jurassic ages of 160.9±0.8 to 161.6±1.5 Ma. Relict zircons have U-Pb ages of mainly Triassic and Neoproterozoic, in agreement with metamorphic and protolith ages for the ultrahigh-pressure (UHP) meta-igneous rocks in the Dabie-Sulu orogenic belt, respectively. These granites are calc-alkaline to high-K calc-alkaline, and exhibit arc-type trace element features and enriched Sr-Nd-Hf isotope compositions, i.e. high whole-rock (87Sr/86Sr)i ratios of 0.708 0 to 0.709 1, low εNd(t) values of -15.6 to -13.5 and zircon εHf(t) values of -23.1 to -9.5, with two-stage Nd-Hf model ages mainly of Paleoproterozoic. These zircon U-Pb geochronological and geochemical characteristics are consistent with those of the UHP meta-igneous rocks in the Dabie-Sulu orogenic belt, indicating a genetic link between them. In particular, the Neoproterozoic and Triassic U-Pb ages of relict zircons are characteristic features of the subducted continental crust of the South China block (SCB). Therefore, the Jingshan granites are the product of partial melting of the subducted SCB continental crust, which would be incorporated into the crust of the NCB during the Triassic continental collision. These granites have low Rb contents, high Sr, Ba contents, and thus low Rb/Sr ratios, and low zirconium saturation and Ti-in-zircon temperatures (~700℃), suggesting their derivation from low-temperature partial melting of the subducted SCB continental crust with water addition, which might be related to the subduction of the paleo-Pacific plate beneath eastern China in the Jurassic.
In this paper it presents a combined study of zircon U-Pb ages, trace elements and Hf isotopes, and whole-rock major-trace elements and Sr-Nd isotopes for the Jingshan granites from the southeast margin of the North China block (NCB). LA-ICP-MS zircon U-Pb dating yielded Late Jurassic ages of 160.9±0.8 to 161.6±1.5 Ma. Relict zircons have U-Pb ages of mainly Triassic and Neoproterozoic, in agreement with metamorphic and protolith ages for the ultrahigh-pressure (UHP) meta-igneous rocks in the Dabie-Sulu orogenic belt, respectively. These granites are calc-alkaline to high-K calc-alkaline, and exhibit arc-type trace element features and enriched Sr-Nd-Hf isotope compositions, i.e. high whole-rock (87Sr/86Sr)i ratios of 0.708 0 to 0.709 1, low εNd(t) values of -15.6 to -13.5 and zircon εHf(t) values of -23.1 to -9.5, with two-stage Nd-Hf model ages mainly of Paleoproterozoic. These zircon U-Pb geochronological and geochemical characteristics are consistent with those of the UHP meta-igneous rocks in the Dabie-Sulu orogenic belt, indicating a genetic link between them. In particular, the Neoproterozoic and Triassic U-Pb ages of relict zircons are characteristic features of the subducted continental crust of the South China block (SCB). Therefore, the Jingshan granites are the product of partial melting of the subducted SCB continental crust, which would be incorporated into the crust of the NCB during the Triassic continental collision. These granites have low Rb contents, high Sr, Ba contents, and thus low Rb/Sr ratios, and low zirconium saturation and Ti-in-zircon temperatures (~700℃), suggesting their derivation from low-temperature partial melting of the subducted SCB continental crust with water addition, which might be related to the subduction of the paleo-Pacific plate beneath eastern China in the Jurassic.
2021, 46(6): 2016-2036.
doi: 10.3799/dqkx.2020.233
Abstract:
In order to study the tectonic environment and dynamic background of the magmatic activity in the West Qinling orogenic belt in Early Paleozoic, it chooses the granite and diorite in Longwanggou rock mass and granodiorite in Tangzang rock mass, which near the Shang-Dan fault zone, for study. Geochemistry and LA-ICP-MS zircon U-Pb chronology were tested for all type of rocks and Hf isotope analysis for granite and diorite in Longwanggou rock mass. The dating results show that the age of Longwanggou granite is 433±4 Ma and 439±5 Ma, the diorite emplacement time is later than granite with the U-Pb age of 453±3 Ma. ΣREEs of Longwanggou diorite and Tangzang granodiorite are (107-181)×10-6 and (209-321)×10-6, respectively. Rare earth element distribution curve characteristics are similar, both showing clearly to the right-inclined pattern, strong light and heavy rare earth element fractionation strongly ((La/Yb) N at 7-13 and 31-57, respectively), and no obvious negative Eu anomaly (δEu are 0.88-1.00 and 0.79-0.87, respectively). Relative to the Longwanggou diorite, Tangzang granodiorites have high ΣREE, light and heavy rare earth element fractionation is more intense, less negative Eu anomaly. The emplacement times of Longwanggou diorite and Tangzang rock mass are consistent, both of originate and granodiorites are from the lower crust, mixed with mantle-based magma to a certain extent during the rising process. The Tangzang granodiorite experienced relatively sufficient magma evolution. Hf isotopic compositions have distinct differences between the two Longwanggou granite samples. Although the εHf(t) values are all positive, they concentrated on the values of 0-3 and 7-10 respectively, with corresponding two-stage model ages (tDM2) 1.1-1.4 Ga and 0.8-1.0 Ga. Combined with geochemical characteristics, the granite formed in the active continental margin tectonic environment (island arc) and derived from the partial melting of Qinling Group. At the same time, much of basic rocks from oceanic crust were brought into the melt during the plate subduction process, the magma have experienced a relatively strong crystallization differentiation and no obvious mingling process during the emplacement occurred. Longwanggou rock mass is a polyphase complex, experienced the tectonic regime transformation from subduction to collision and compression, and recorded the tectonic evolution information of Qinling ocean from subduction to closure. The subduction and orogenesis of Qinling area in the Paleozoic lasted for a long time and the accompanying magmatic activity also continued from Early Paleozoic to Middle Paleozoic.
In order to study the tectonic environment and dynamic background of the magmatic activity in the West Qinling orogenic belt in Early Paleozoic, it chooses the granite and diorite in Longwanggou rock mass and granodiorite in Tangzang rock mass, which near the Shang-Dan fault zone, for study. Geochemistry and LA-ICP-MS zircon U-Pb chronology were tested for all type of rocks and Hf isotope analysis for granite and diorite in Longwanggou rock mass. The dating results show that the age of Longwanggou granite is 433±4 Ma and 439±5 Ma, the diorite emplacement time is later than granite with the U-Pb age of 453±3 Ma. ΣREEs of Longwanggou diorite and Tangzang granodiorite are (107-181)×10-6 and (209-321)×10-6, respectively. Rare earth element distribution curve characteristics are similar, both showing clearly to the right-inclined pattern, strong light and heavy rare earth element fractionation strongly ((La/Yb) N at 7-13 and 31-57, respectively), and no obvious negative Eu anomaly (δEu are 0.88-1.00 and 0.79-0.87, respectively). Relative to the Longwanggou diorite, Tangzang granodiorites have high ΣREE, light and heavy rare earth element fractionation is more intense, less negative Eu anomaly. The emplacement times of Longwanggou diorite and Tangzang rock mass are consistent, both of originate and granodiorites are from the lower crust, mixed with mantle-based magma to a certain extent during the rising process. The Tangzang granodiorite experienced relatively sufficient magma evolution. Hf isotopic compositions have distinct differences between the two Longwanggou granite samples. Although the εHf(t) values are all positive, they concentrated on the values of 0-3 and 7-10 respectively, with corresponding two-stage model ages (tDM2) 1.1-1.4 Ga and 0.8-1.0 Ga. Combined with geochemical characteristics, the granite formed in the active continental margin tectonic environment (island arc) and derived from the partial melting of Qinling Group. At the same time, much of basic rocks from oceanic crust were brought into the melt during the plate subduction process, the magma have experienced a relatively strong crystallization differentiation and no obvious mingling process during the emplacement occurred. Longwanggou rock mass is a polyphase complex, experienced the tectonic regime transformation from subduction to collision and compression, and recorded the tectonic evolution information of Qinling ocean from subduction to closure. The subduction and orogenesis of Qinling area in the Paleozoic lasted for a long time and the accompanying magmatic activity also continued from Early Paleozoic to Middle Paleozoic.
2021, 46(6): 2037-2056.
doi: 10.3799/dqkx.2020.286
Abstract:
The study of granite is of great significance to the inversion of the material composition of the lower crust and the deep dynamic process of orogeny. The quartz diorite in Balong region is located in the east of the East Kunlun orogen. In this paper, it presents LA-ICP-MS zircon U-Pb age for the Balong quartz diorite to determine precisely the time of the magmatism, and also presents geochemical, Sr-Nd-Hf isotope data for the Balong quartz diorite to constrain the petrogenesis and tectonic setting. The LA-ICP-MS U-Pb analyses of zircon yielded a weighted mean age of 229.5±1.4 Ma, indicating that it was emplaced in the Late Triassic. The quartz diorites have contents of SiO2 (59.86%-61.83%), Na2O (3.38%-3.55%), Al2O3 (16.38%-17.03%) with Na2O/K2O ratios ranging from 1.25 to 1.39 and Mg# values ranging from 50.1 to 51.2. They are characterized by high silicon and belonging to the high-potassium-calcium-alkaline rock. Meanwhile, they are enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements(HFSEs). In addition, the quartz diorite shows characteristics of high Sr/Y (32.31-40.86) and (La/Yb)N (13.34-15.32) and low contents of Yb (1.34×10-6-1.75×10-6) and Y (13.40×10-6-15.60×10-6). These features indicate that the quartz diorite is similar to adakite. All rock samples are enriched in large ion lithophile elements and light rare earth elements, but depleted in high field strength elements. The (87Sr/86Sr)i ratios range from 0.708 186 to 0.708 428, εNd(t) values range from -5.75 to -5.27 with corresponding two-stage Nd model ages ranging from 1 432 to 1 471 Ma. The εHf(t) values are from -5.2 to -3.2 and two-stage Hf model ages rang from 1 305 to 1 420 Ma. Integrated geological, geochemical and isotopic data suggest that the quartz diorite from Balong region is most likely generated via partial melting of thickened mafic lower continental crust and with subordinate mantle-derived basic magma. In combination with the tectonic evolution of the East Kunlun orogenic belt and the geochronological and geochemical characteristics of contemporary intrusive rocks, it is concluded that the quartz diorite from Balong region was formed in post-collisional extensional tectonic environment. Slab break-off, triggered by continuous collision between the Bayanhar block and EKOB, led to underplating of basic magma formed by partial melting of enriched mantle. The quartz diorite from Balong region was formed by partial melting of lower crust.
The study of granite is of great significance to the inversion of the material composition of the lower crust and the deep dynamic process of orogeny. The quartz diorite in Balong region is located in the east of the East Kunlun orogen. In this paper, it presents LA-ICP-MS zircon U-Pb age for the Balong quartz diorite to determine precisely the time of the magmatism, and also presents geochemical, Sr-Nd-Hf isotope data for the Balong quartz diorite to constrain the petrogenesis and tectonic setting. The LA-ICP-MS U-Pb analyses of zircon yielded a weighted mean age of 229.5±1.4 Ma, indicating that it was emplaced in the Late Triassic. The quartz diorites have contents of SiO2 (59.86%-61.83%), Na2O (3.38%-3.55%), Al2O3 (16.38%-17.03%) with Na2O/K2O ratios ranging from 1.25 to 1.39 and Mg# values ranging from 50.1 to 51.2. They are characterized by high silicon and belonging to the high-potassium-calcium-alkaline rock. Meanwhile, they are enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements(HFSEs). In addition, the quartz diorite shows characteristics of high Sr/Y (32.31-40.86) and (La/Yb)N (13.34-15.32) and low contents of Yb (1.34×10-6-1.75×10-6) and Y (13.40×10-6-15.60×10-6). These features indicate that the quartz diorite is similar to adakite. All rock samples are enriched in large ion lithophile elements and light rare earth elements, but depleted in high field strength elements. The (87Sr/86Sr)i ratios range from 0.708 186 to 0.708 428, εNd(t) values range from -5.75 to -5.27 with corresponding two-stage Nd model ages ranging from 1 432 to 1 471 Ma. The εHf(t) values are from -5.2 to -3.2 and two-stage Hf model ages rang from 1 305 to 1 420 Ma. Integrated geological, geochemical and isotopic data suggest that the quartz diorite from Balong region is most likely generated via partial melting of thickened mafic lower continental crust and with subordinate mantle-derived basic magma. In combination with the tectonic evolution of the East Kunlun orogenic belt and the geochronological and geochemical characteristics of contemporary intrusive rocks, it is concluded that the quartz diorite from Balong region was formed in post-collisional extensional tectonic environment. Slab break-off, triggered by continuous collision between the Bayanhar block and EKOB, led to underplating of basic magma formed by partial melting of enriched mantle. The quartz diorite from Balong region was formed by partial melting of lower crust.
2021, 46(6): 2057-2072.
doi: 10.3799/dqkx.2020.241
Abstract:
How crust-mantle magma interaction controls the petrological diversity of the felsic igneous rocks is one of the key scientific issues in petrology research. In this study it takes the Bairiqili felsic pluton which is characterized by its various rock types, as well as its mafic microgranular enclaves (MMEs) in East Kunlun as the research object, and presents its zircon U-Pb chronology, mineralogy, whole-rock geochemistry and Sr-Nd-Hf isotopic data to discuss the key scientific issue. LA-ICPMS zircon U-Pb geochronology indicates that the MMEs (247.8±2.0 Ma), monzogranite (247.5±1.4 Ma), granodiorite (248.8±2.1 Ma) and quartz diorite (248.8±1.5 Ma) all emplaced and crystallized in Early Triassic. Petrographic and mineralogical studies show that the petrogenesis of the felsic rocks and MMEs is closely related to the crust-mantle magma mixing or mingling. Elemental and Sr-Nd-Hf isotopical geochemistry reveals that the mafic magma was originated from partial melting of the enriched mantle which was metasomatized by subduction-related fluid, while the felsic end-member magma was derived from partial melting of the ancient metagreywackes. This study proposes that the mantle-derived mafic magma firstly intruded into the felsic crystal mushy magma chamber, promoting the rejuvenation of this felsic crystal mush. Subsequently, the mixing and mingling interaction occurred between the crust and mantle end-member magmas in different proportions and in different ways, thus formed a variety of igneous rocks including mafic dyke, MMEs, quartz diorite and granodiorite. Crust-mantle magma interaction in crystal mushy state is an important way to control the petrological diversity of felsic rocks and the growth and evolution of continental crust in East Kunlun.
How crust-mantle magma interaction controls the petrological diversity of the felsic igneous rocks is one of the key scientific issues in petrology research. In this study it takes the Bairiqili felsic pluton which is characterized by its various rock types, as well as its mafic microgranular enclaves (MMEs) in East Kunlun as the research object, and presents its zircon U-Pb chronology, mineralogy, whole-rock geochemistry and Sr-Nd-Hf isotopic data to discuss the key scientific issue. LA-ICPMS zircon U-Pb geochronology indicates that the MMEs (247.8±2.0 Ma), monzogranite (247.5±1.4 Ma), granodiorite (248.8±2.1 Ma) and quartz diorite (248.8±1.5 Ma) all emplaced and crystallized in Early Triassic. Petrographic and mineralogical studies show that the petrogenesis of the felsic rocks and MMEs is closely related to the crust-mantle magma mixing or mingling. Elemental and Sr-Nd-Hf isotopical geochemistry reveals that the mafic magma was originated from partial melting of the enriched mantle which was metasomatized by subduction-related fluid, while the felsic end-member magma was derived from partial melting of the ancient metagreywackes. This study proposes that the mantle-derived mafic magma firstly intruded into the felsic crystal mushy magma chamber, promoting the rejuvenation of this felsic crystal mush. Subsequently, the mixing and mingling interaction occurred between the crust and mantle end-member magmas in different proportions and in different ways, thus formed a variety of igneous rocks including mafic dyke, MMEs, quartz diorite and granodiorite. Crust-mantle magma interaction in crystal mushy state is an important way to control the petrological diversity of felsic rocks and the growth and evolution of continental crust in East Kunlun.
2021, 46(6): 2073-2082.
doi: 10.3799/dqkx.2020.108
Abstract:
In this study it attempts to use the Li isotope geochemistry to preliminarily limit the surficial environmental processes associated with Archean seawater. It performed Li isotope analysis on marine carbonate samples from the Kaapvaal craton in South Africa and finds that the carbonate shows light Li compositions of~+1‰ during the period of 3.0-2.9 Ga, and increase of +7‰ to +10‰ during the period of 2.6-2.5 Ga. Through inversion calculation, the Li isotope compositions of seawater in the two periods are~+12‰ and~+ 20‰, respectively, which are significantly lower than modern seawater (~+31‰). However, the δ7Li value of seawater during 2.6-2.5 Ga is more than 8‰ higher than that at 3.0-2.9 Ga. As an effective tracer for continental weathering of silicate rocks, Archean seawater shows relatively low δ7Li values, which indicates that the surficial weathering at that age was dominated by the dissolution of source rocks, and secondary minerals were rarely formed. During 3.0-2.5 Ga, the decrease in the overall temperature of the seawater and the increase in the formation of secondary minerals may jointly lead to an increase in the seawater δ7Li value during Late Archean. The study of Li isotopes of Archean carbonate can effectively invert the Li isotopic composition of paleoseawater, and provide new information for understanding the surficial environmental processes related to the Archean seawater.
In this study it attempts to use the Li isotope geochemistry to preliminarily limit the surficial environmental processes associated with Archean seawater. It performed Li isotope analysis on marine carbonate samples from the Kaapvaal craton in South Africa and finds that the carbonate shows light Li compositions of~+1‰ during the period of 3.0-2.9 Ga, and increase of +7‰ to +10‰ during the period of 2.6-2.5 Ga. Through inversion calculation, the Li isotope compositions of seawater in the two periods are~+12‰ and~+ 20‰, respectively, which are significantly lower than modern seawater (~+31‰). However, the δ7Li value of seawater during 2.6-2.5 Ga is more than 8‰ higher than that at 3.0-2.9 Ga. As an effective tracer for continental weathering of silicate rocks, Archean seawater shows relatively low δ7Li values, which indicates that the surficial weathering at that age was dominated by the dissolution of source rocks, and secondary minerals were rarely formed. During 3.0-2.5 Ga, the decrease in the overall temperature of the seawater and the increase in the formation of secondary minerals may jointly lead to an increase in the seawater δ7Li value during Late Archean. The study of Li isotopes of Archean carbonate can effectively invert the Li isotopic composition of paleoseawater, and provide new information for understanding the surficial environmental processes related to the Archean seawater.
2021, 46(6): 2083-2116.
doi: 10.3799/dqkx.2020.306
Abstract:
In order to determine the formation age of lamprophyre accurately in Du'an-Mashan belt, Central Guangxi, and to discuss its source property, tectonic environment and dynamic setting, LA-ICP-MS zircon U-Pb dating, 40Ar-39Ar phlogopite dating, εHf isotope and whole-rock geochemistry were obtained. It shows that zircon U-Pb dating fails to determine the fomation age of lamprophyres, however, a mass of captured zircons ranging from 2 578 Ma to 1 650 Ma indicates the existence of the Archean-Paleoproterozoic metamorphic basement. The phlogopite 40Ar/39Ar age of 100.4±0.99 Ma displayes the formation age of the lamprophyres. Zircons yield both positive and negative εHf(t) values (with mostly positive), suggesting that the protolith was derived from newborn crust that incorporated a few evolved ancient continental crustal components. Geochemical data exhibit that lamprophyres have low SiO2 (47.66%-50.93%) and high K2O (4.98%-6.77%) contents, with relatively high LILE and LREE and low HFSE (such as Nb, Ta and Ti) contents, indicating the characteristics of enriched mantle and subducted fluid metasomatism. During the formation of lamprophyres in Central Guangxi, crustal contamination is very limited, and the main formation process is as follows: under the background of lithospheric extension in the Late Yanshanian, the enriched mantle which was metasomatic was partially melted to form lamprophyre magma, and the Nandan-Kunlunguan fault extending in the NNW direction provided a channel for the magma fluid. The lamprophyres formed under the dynamic setting of subduction and collision of Pacific and Indian plate to Eurasian plate.
In order to determine the formation age of lamprophyre accurately in Du'an-Mashan belt, Central Guangxi, and to discuss its source property, tectonic environment and dynamic setting, LA-ICP-MS zircon U-Pb dating, 40Ar-39Ar phlogopite dating, εHf isotope and whole-rock geochemistry were obtained. It shows that zircon U-Pb dating fails to determine the fomation age of lamprophyres, however, a mass of captured zircons ranging from 2 578 Ma to 1 650 Ma indicates the existence of the Archean-Paleoproterozoic metamorphic basement. The phlogopite 40Ar/39Ar age of 100.4±0.99 Ma displayes the formation age of the lamprophyres. Zircons yield both positive and negative εHf(t) values (with mostly positive), suggesting that the protolith was derived from newborn crust that incorporated a few evolved ancient continental crustal components. Geochemical data exhibit that lamprophyres have low SiO2 (47.66%-50.93%) and high K2O (4.98%-6.77%) contents, with relatively high LILE and LREE and low HFSE (such as Nb, Ta and Ti) contents, indicating the characteristics of enriched mantle and subducted fluid metasomatism. During the formation of lamprophyres in Central Guangxi, crustal contamination is very limited, and the main formation process is as follows: under the background of lithospheric extension in the Late Yanshanian, the enriched mantle which was metasomatic was partially melted to form lamprophyre magma, and the Nandan-Kunlunguan fault extending in the NNW direction provided a channel for the magma fluid. The lamprophyres formed under the dynamic setting of subduction and collision of Pacific and Indian plate to Eurasian plate.
2021, 46(6): 2117-2132.
doi: 10.3799/dqkx.2020.210
Abstract:
It reports the first occurrence of Early Neoproterozoic dike swarm located in the southern Huangling anticline, Yangtze craton, which is composed of diabase, granodiorite and syengranite. Zircon U-Pb dating of diabasic and granodioritic dikes yields weighted mean 206Pb/238U ages of 856±6.4 Ma and 860±6.0 Ma, respectively, which are interpreted as the formation ages of the dike swarm. Whole-rock geochemical results show that diabasic dikes show high contents of Pb, variable Ba/Nb and La/Nb ratios, however, the granodioritic and syengranitic dikes have relative low contents of Y and Nb. In combination with age spectrums of inherited zircons from analytical dikes, it is proposed that the dike swarm experienced a certain crustal contamination of juvenile island arc during formed process. Document of ca. 860 Ma dike swarm in the southern Huangling anticline indicates that the Yangtze craton entered into the post-collisional extensional environment before 860 Ma. Therefore, it is inferred that Precambrian basement breakup of the Yangtze craton shows difference in spatial-temporal distribution.
It reports the first occurrence of Early Neoproterozoic dike swarm located in the southern Huangling anticline, Yangtze craton, which is composed of diabase, granodiorite and syengranite. Zircon U-Pb dating of diabasic and granodioritic dikes yields weighted mean 206Pb/238U ages of 856±6.4 Ma and 860±6.0 Ma, respectively, which are interpreted as the formation ages of the dike swarm. Whole-rock geochemical results show that diabasic dikes show high contents of Pb, variable Ba/Nb and La/Nb ratios, however, the granodioritic and syengranitic dikes have relative low contents of Y and Nb. In combination with age spectrums of inherited zircons from analytical dikes, it is proposed that the dike swarm experienced a certain crustal contamination of juvenile island arc during formed process. Document of ca. 860 Ma dike swarm in the southern Huangling anticline indicates that the Yangtze craton entered into the post-collisional extensional environment before 860 Ma. Therefore, it is inferred that Precambrian basement breakup of the Yangtze craton shows difference in spatial-temporal distribution.
2021, 46(6): 2133-2144.
doi: 10.3799/dqkx.2020.048
Abstract:
Kalimantan Island is the largest island in the south of South China Sea that has undergone complex tectonic evolution, resulting from convergence of the Indian-Australian, Pacific and Philippine Sea plates. The Kuching area of Kalimantan Island has a series of Late Cretaceous magmatic rocks, but the distribution of basic rocks is less. The petrogenesis of basic rocks is significant to reveal the tectonic framework and evolution history in that period. In this study, it presents new petrographic, geochronological and geochemical data for the Lundu gabbroic pluton in the Kuching area. The basic rocks are predominantly made of dolerite and gabbro. Zircon U-Pb dating result shows that the crystallization age of the gabbro is 83.4±0.9 Ma, suggesting that the basic rocks intruded in the Late Cretaceous. These samples have low SiO2 ranging from 52.01% to 52.38%, K2O from 0.37% to 0.53% and TiO2 ranging from 0.81% to 0.92% with high Al2O3 of 14.00%-14.54% and MgO of 7.40%-7.86%. These samples are geochemically characterized by enrichment of light rare earth elements (LREE) and large-ion lithophile elements (LILE) and flat distribution of heavy rare earth elements (HREE) with light low REE abundance (∑REE=43.96×10-6-48.19×10-6). The representative samples show low initial 87Sr/86Sr ratios (0.705 1 to 0.705 3) and positive εNd(t) values (2.1 to 3.3). Combination of trace elemental and isotopic results suggest that the parental magmas were likely derived from the mantle source modified by subduction-related fluid and sediments in the back-arc basin tectonic setting in the response of the subduction of the Paleopacific plate, and may link with Southeast China, Hainan island and Vietnam.
Kalimantan Island is the largest island in the south of South China Sea that has undergone complex tectonic evolution, resulting from convergence of the Indian-Australian, Pacific and Philippine Sea plates. The Kuching area of Kalimantan Island has a series of Late Cretaceous magmatic rocks, but the distribution of basic rocks is less. The petrogenesis of basic rocks is significant to reveal the tectonic framework and evolution history in that period. In this study, it presents new petrographic, geochronological and geochemical data for the Lundu gabbroic pluton in the Kuching area. The basic rocks are predominantly made of dolerite and gabbro. Zircon U-Pb dating result shows that the crystallization age of the gabbro is 83.4±0.9 Ma, suggesting that the basic rocks intruded in the Late Cretaceous. These samples have low SiO2 ranging from 52.01% to 52.38%, K2O from 0.37% to 0.53% and TiO2 ranging from 0.81% to 0.92% with high Al2O3 of 14.00%-14.54% and MgO of 7.40%-7.86%. These samples are geochemically characterized by enrichment of light rare earth elements (LREE) and large-ion lithophile elements (LILE) and flat distribution of heavy rare earth elements (HREE) with light low REE abundance (∑REE=43.96×10-6-48.19×10-6). The representative samples show low initial 87Sr/86Sr ratios (0.705 1 to 0.705 3) and positive εNd(t) values (2.1 to 3.3). Combination of trace elemental and isotopic results suggest that the parental magmas were likely derived from the mantle source modified by subduction-related fluid and sediments in the back-arc basin tectonic setting in the response of the subduction of the Paleopacific plate, and may link with Southeast China, Hainan island and Vietnam.
2021, 46(6): 2145-2156.
doi: 10.3799/dqkx.2020.205
Abstract:
In order to identify the expression and spatial distribution of Eocene tectonic transition event in petroliferous basins offshore China, and reveal the geodynamic origin ultimately, based on the new released geological data, investigation on interface was carried out from a case study of Weixi'nan sag. The results show that the basin experienced the rifting change to N-S-directed, transformation of fault system, flexural deformation of strata and the migration of the depocenters. The Eocene tectonic transition is closely related to the stress change due to the reorganization of neighboring plates, especially to the orientation change of Pacific plate subduction and collision of Indian plate and Eurasian plate. The occurrence of this tectonic event reveals the coexistence of consistency and diversity, inheritance and reworked structures, extension and strike-slip deformation. It deeply influences the coupling between different accumulation elements. The configuration of the high-quality source rocks and the shallower strike-slip structures determines the advantageous section of hydrocarbon accumulation.
In order to identify the expression and spatial distribution of Eocene tectonic transition event in petroliferous basins offshore China, and reveal the geodynamic origin ultimately, based on the new released geological data, investigation on interface was carried out from a case study of Weixi'nan sag. The results show that the basin experienced the rifting change to N-S-directed, transformation of fault system, flexural deformation of strata and the migration of the depocenters. The Eocene tectonic transition is closely related to the stress change due to the reorganization of neighboring plates, especially to the orientation change of Pacific plate subduction and collision of Indian plate and Eurasian plate. The occurrence of this tectonic event reveals the coexistence of consistency and diversity, inheritance and reworked structures, extension and strike-slip deformation. It deeply influences the coupling between different accumulation elements. The configuration of the high-quality source rocks and the shallower strike-slip structures determines the advantageous section of hydrocarbon accumulation.
2021, 46(6): 2157-2171.
doi: 10.3799/dqkx.2020.284
Abstract:
In recent years, glutenite oil and gas reservoir exploration has continuously made major discoveries, making the conglomerate body a new field of oil and gas exploration. At present, the conglomerate body is focused on sedimentary response analysis, and its characteristics of lithofacies combination division and differences are relatively weak, and there is a lack of a unified division standard, especially in the Paleogene in the southern slope area of the Bonan low uplift in the Bohai Bay basin. There are multiple gravel components and composite diagenesis in the sandy conglomerate, which greatly restricts the distribution prediction and evaluation of high-quality sandy conglomerate reservoir. Due to the multiple sources of supply in the study area, the gravel composition, support type and matrix type are diverse. Based on the typical drilling core in the area, comprehensive observation under the microscope and scanning electron microscope analysis, it proposes the "three parent rocks of sedimentary rock, metamorphic rock and magmatic rock. The type is the first-level classification standard, and the support type and matrix type are the second-level classification standards." The sandstone and conglomerate lithofacies are divided into three major categories and seven subcategories, followed by (1) matrix support mainly based on volcanic matrix support-sand-level matrix-structural fracture-conglomerate facies, particle support-sand-level matrix-intra-gravel fracture-conglomerate facies, matrix support-mud-level matrix-conglomerate facies, (2) matrix support mainly composed of carbonate conglomerate-sand-level matrix-conglomerate facies (carbonate rock and gravel), matrix support-sand grade matrix-breccia facies, (3) cement support mainly composed of metamorphic rock and gravel-sand grade matrix-concrete sandstone facies, and particle support-sand grade matrix-gravel-edge fracture-concrete sandstone facies. In reservoirs of different gravel types, the matrix type and fracture development degree are obviously different. The reservoir with high mud content has poor permeability. The fracture development will improve the connectivity of its pores and induce the formation of intra-gravel dissolution and matrix dissolution to improve storage properties.
In recent years, glutenite oil and gas reservoir exploration has continuously made major discoveries, making the conglomerate body a new field of oil and gas exploration. At present, the conglomerate body is focused on sedimentary response analysis, and its characteristics of lithofacies combination division and differences are relatively weak, and there is a lack of a unified division standard, especially in the Paleogene in the southern slope area of the Bonan low uplift in the Bohai Bay basin. There are multiple gravel components and composite diagenesis in the sandy conglomerate, which greatly restricts the distribution prediction and evaluation of high-quality sandy conglomerate reservoir. Due to the multiple sources of supply in the study area, the gravel composition, support type and matrix type are diverse. Based on the typical drilling core in the area, comprehensive observation under the microscope and scanning electron microscope analysis, it proposes the "three parent rocks of sedimentary rock, metamorphic rock and magmatic rock. The type is the first-level classification standard, and the support type and matrix type are the second-level classification standards." The sandstone and conglomerate lithofacies are divided into three major categories and seven subcategories, followed by (1) matrix support mainly based on volcanic matrix support-sand-level matrix-structural fracture-conglomerate facies, particle support-sand-level matrix-intra-gravel fracture-conglomerate facies, matrix support-mud-level matrix-conglomerate facies, (2) matrix support mainly composed of carbonate conglomerate-sand-level matrix-conglomerate facies (carbonate rock and gravel), matrix support-sand grade matrix-breccia facies, (3) cement support mainly composed of metamorphic rock and gravel-sand grade matrix-concrete sandstone facies, and particle support-sand grade matrix-gravel-edge fracture-concrete sandstone facies. In reservoirs of different gravel types, the matrix type and fracture development degree are obviously different. The reservoir with high mud content has poor permeability. The fracture development will improve the connectivity of its pores and induce the formation of intra-gravel dissolution and matrix dissolution to improve storage properties.
2021, 46(6): 2172-2187.
doi: 10.3799/dqkx.2020.225
Abstract:
In order to penetrate into discussion the "intersection type" uranium mineralization in the Changjiang uranium ore field, South China. Here, it collected representative samples from geological structure that formed during the process of mafic dick penetrating silicified surrounding rock and then carried out a study applying in-situ U-Pb dating and elementary analysis via LA-ICP-MS system. Results show that the weighting average age of pitchblende in this region is 71.3±1.1 Ma. Besides, trace elements are characterized by the enrichment of sulfophilic elements (such as W, Bi, and Mo) and by the depletion of high field strength elements (like Nb, Ta, Zr, Hf, and Th). The signature of rare earth element (REE) curve shows low total amount with negative Eu anomalies, slight enrichment in light REE, and indistinct fractionation between light and heavy REEs. The uranium source of pitchblende probably relates to the Changjiang granite, as indicated by a REE pattern comparison between them. In addition, the pitchblende was formed under low-temperature and low-middle salinity condition. This is validated through a combination of ΣREE-(U/Th) plot, ΣREE-(LREE/HREE)N plot, and comparison against REE distribution pattern in vein-type uranium deposits. Moreover, δCe value indicates reducing environment. In this region, the pitchblende was controlled by the interplay among Nanxiong fault (during 80-60 Ma), uranium-rich Changjiang granite, and mafic dyke, therefore forming the "intersection type" uranium mineralization in the active region.
In order to penetrate into discussion the "intersection type" uranium mineralization in the Changjiang uranium ore field, South China. Here, it collected representative samples from geological structure that formed during the process of mafic dick penetrating silicified surrounding rock and then carried out a study applying in-situ U-Pb dating and elementary analysis via LA-ICP-MS system. Results show that the weighting average age of pitchblende in this region is 71.3±1.1 Ma. Besides, trace elements are characterized by the enrichment of sulfophilic elements (such as W, Bi, and Mo) and by the depletion of high field strength elements (like Nb, Ta, Zr, Hf, and Th). The signature of rare earth element (REE) curve shows low total amount with negative Eu anomalies, slight enrichment in light REE, and indistinct fractionation between light and heavy REEs. The uranium source of pitchblende probably relates to the Changjiang granite, as indicated by a REE pattern comparison between them. In addition, the pitchblende was formed under low-temperature and low-middle salinity condition. This is validated through a combination of ΣREE-(U/Th) plot, ΣREE-(LREE/HREE)N plot, and comparison against REE distribution pattern in vein-type uranium deposits. Moreover, δCe value indicates reducing environment. In this region, the pitchblende was controlled by the interplay among Nanxiong fault (during 80-60 Ma), uranium-rich Changjiang granite, and mafic dyke, therefore forming the "intersection type" uranium mineralization in the active region.
2021, 46(6): 2188-2203.
doi: 10.3799/dqkx.2020.177
Abstract:
Existing studies have shown that magmatic intrusion has effects on shale mineralogical characteristics, but the lack of a detailed anatomy of its scope and extent has restricted the accurate understanding of the influence mechanism of magmatic activities on shale mineralogical characteristics.Based on contact with diabase in southern Songliao basin shale as an object of the Nenjiang Formation, by means of core observation, XRD, polarizing microscope, and scanning electron microscopy, the contact zone shale is divided from the diabase intrusion from far and near into five zones: gray shale zone, 19 m thick; dark gray shale zone, thick 11 m; gray-black shale zone, 12.9 m thick; blue-gray shale zone, 1.5 m thick; gray-white shale zone, 2.2 m thick. Quartz content increases, with average contents of 27.0%, 33.6%, 51.7%, 56.7%, and 52.7%, respectively. The transition from terrestrial clastic quartz to high-temperature in-situ quartz (microcrystalline self-shaped quartz and microcrystalline spherical quartz), the recrystallization phenomenon intensified. The content of feldspar increases, with average contents of 8.8%, 12.0%, 14.0%, 15.1%, and 18.2%, respectively, which changes from semi-morphic to semi-self-shaped, and the recrystallization phenomenon intensified. The content of carbonate minerals decreases first and then increased, with the average contents of 9.0%, 10.0%, 7.5%, 4.5%, and 6.0%, respectively. The shape of siderite and calcite changes from self-shaped to colloidal and veined. The pyrite content gradually decreases, with average contents of 5.4%, 3.5%, 1.0%, 0%, and 0%, respectively, and transformed from strawberry-like and self-shaped to other-shaped granular. The clay mineral content gradually decreases, with average contents of 46.0%, 36.5%, 22.5%, 20.1%, and 19.0%, respectively, transformed from platy montmorillonite to floc illite.The results show that the intrusion of basic rock into shale not only accelerates the diagenetic evolution of shale, but also increases the brittle minerals and decreases the plastic minerals, which can improve the brittleness and compressibility of shale reservoirs but weaken the ability of shale gas to exist.
Existing studies have shown that magmatic intrusion has effects on shale mineralogical characteristics, but the lack of a detailed anatomy of its scope and extent has restricted the accurate understanding of the influence mechanism of magmatic activities on shale mineralogical characteristics.Based on contact with diabase in southern Songliao basin shale as an object of the Nenjiang Formation, by means of core observation, XRD, polarizing microscope, and scanning electron microscopy, the contact zone shale is divided from the diabase intrusion from far and near into five zones: gray shale zone, 19 m thick; dark gray shale zone, thick 11 m; gray-black shale zone, 12.9 m thick; blue-gray shale zone, 1.5 m thick; gray-white shale zone, 2.2 m thick. Quartz content increases, with average contents of 27.0%, 33.6%, 51.7%, 56.7%, and 52.7%, respectively. The transition from terrestrial clastic quartz to high-temperature in-situ quartz (microcrystalline self-shaped quartz and microcrystalline spherical quartz), the recrystallization phenomenon intensified. The content of feldspar increases, with average contents of 8.8%, 12.0%, 14.0%, 15.1%, and 18.2%, respectively, which changes from semi-morphic to semi-self-shaped, and the recrystallization phenomenon intensified. The content of carbonate minerals decreases first and then increased, with the average contents of 9.0%, 10.0%, 7.5%, 4.5%, and 6.0%, respectively. The shape of siderite and calcite changes from self-shaped to colloidal and veined. The pyrite content gradually decreases, with average contents of 5.4%, 3.5%, 1.0%, 0%, and 0%, respectively, and transformed from strawberry-like and self-shaped to other-shaped granular. The clay mineral content gradually decreases, with average contents of 46.0%, 36.5%, 22.5%, 20.1%, and 19.0%, respectively, transformed from platy montmorillonite to floc illite.The results show that the intrusion of basic rock into shale not only accelerates the diagenetic evolution of shale, but also increases the brittle minerals and decreases the plastic minerals, which can improve the brittleness and compressibility of shale reservoirs but weaken the ability of shale gas to exist.
2021, 46(6): 2204-2217.
doi: 10.3799/dqkx.2020.372
Abstract:
In order to evaluate the shale gas exploration potential of Permian marine shale in the Lower Yangtze region, the shale characteristics and sedimentary environment were analyzed by core photos, thin sections, X-ray diffraction and total organic carbon tests from three drilling cores in Xuanjing area of south Anhui Province. It shows that in Xuanjing area the middle-upper parts of Gufeng Formation are rich in organic matter, and Yinping Formation is a poor organic matter interval. The middle and lower parts of Dalong Formation in the southeast of Xuanjing are high-rich organic matter intervals, and the middle and upper parts of northwest are rich organic matter intervals. In the early stage of Gufeng Formation, with the tectonic subsidence and the seawater deepening, Xuanjing area formed a high biological productivity and anoxic environment formed in Xuanjing area, and a thick siliceous rock layer deposited. In the middle and late periods, the sea level decreased slowly, the living space of organisms decreased, and the sedimentation of high organic matter made the water sulfide, and the TS content increased, and the TOC content reached 10%. During the sedimentary period of Yinping Formation, the sea level dropped rapidly and changed into shallow sea oxidation environment with TOC content less than 1%. In the early stage of Dalong Formation, the position of southeast shallow water slope was affected by upwelling, forming a high biological productivity and anoxic environment, and deposited siliceous rock layer with TOC content greater than 4%. In the middle and late period, the sea level dropped, and the TOC content increased from southeast shallow water to northwest deep water.
In order to evaluate the shale gas exploration potential of Permian marine shale in the Lower Yangtze region, the shale characteristics and sedimentary environment were analyzed by core photos, thin sections, X-ray diffraction and total organic carbon tests from three drilling cores in Xuanjing area of south Anhui Province. It shows that in Xuanjing area the middle-upper parts of Gufeng Formation are rich in organic matter, and Yinping Formation is a poor organic matter interval. The middle and lower parts of Dalong Formation in the southeast of Xuanjing are high-rich organic matter intervals, and the middle and upper parts of northwest are rich organic matter intervals. In the early stage of Gufeng Formation, with the tectonic subsidence and the seawater deepening, Xuanjing area formed a high biological productivity and anoxic environment formed in Xuanjing area, and a thick siliceous rock layer deposited. In the middle and late periods, the sea level decreased slowly, the living space of organisms decreased, and the sedimentation of high organic matter made the water sulfide, and the TS content increased, and the TOC content reached 10%. During the sedimentary period of Yinping Formation, the sea level dropped rapidly and changed into shallow sea oxidation environment with TOC content less than 1%. In the early stage of Dalong Formation, the position of southeast shallow water slope was affected by upwelling, forming a high biological productivity and anoxic environment, and deposited siliceous rock layer with TOC content greater than 4%. In the middle and late period, the sea level dropped, and the TOC content increased from southeast shallow water to northwest deep water.
2021, 46(6): 2218-2229.
doi: 10.3799/dqkx.2020.033
Abstract:
The passive-margin salt basins in the central segment of the South Atlantic are areas with rapid growth of petroleum reserves in the world, where the distribution and structural deformation of the Aptian salt rocks have significant impacts on hydrocarbon accumulation. According to the Hotspur transform fault zone, the typical conjugated salt basins of Espirito Santo and Kwanza are selected in this study. Based on the previous seismic profiles and gravity and magnetism data, the Bouguer gravity anomaly data are superimposed on the plane in the Gplates software for reconstruction of the plates, and the Move software is used to restore balanced profile on the section. Then forward models designed according to the inversion results, and the contrast analogue experments are carried out. According to the studies, under the superposition of gravity sliding and gravity spreading, salt tectonic zones of extension-transition-contraction developed in the passive-margin basins, while factors resulted in the of salt structure deformation process among the conjugated basins, including slope angle, basement extension, the outer high barrier, salt deposition load and syn-depositional rate.(1) From Aptian to early Albian, the Espirito basin and the Kwanza basin were initially a unified salt basin. The east side of the unified basin was strongly stretched, forming faults and salt rafts in the extensional domain. At the same time, the unified basin was gradually divided, accompanied by the formation of the outer high, and the gravity deformation of salt rocks gradually slowed down. (2) From late Albian to Early Paleogene, the inherited upflit of the outer high in the Espirito Santo basin resulted in the vertical upward migrations of salt rocks; the uplift of the thick salt plateau in the Kwanza basin caused salt rocks to cross the outer high and form the thrust nappe. (3) From Late Paleogene to the present, the basement of the Espirito basin had steady thermal subsidence, and the salt structures were successively developed, then the salt rocks in the whole basin exhibit vertical migration characteristics. In the Kwanza basin, the basement was uplifted on a large scale, and the transport of salt rocks to the sea started again. Salt rafts in the extensional domain were further developed into salt rollovers; salt diapirs in the transitional zone and the contractional domain continued to rise; and the thrust nappe continued to develop, forming a large number of faults and folds in the supra salt layers. Then the salt rocks of the entire basin are continuously accumulating downslope with obvious structural zonation.
The passive-margin salt basins in the central segment of the South Atlantic are areas with rapid growth of petroleum reserves in the world, where the distribution and structural deformation of the Aptian salt rocks have significant impacts on hydrocarbon accumulation. According to the Hotspur transform fault zone, the typical conjugated salt basins of Espirito Santo and Kwanza are selected in this study. Based on the previous seismic profiles and gravity and magnetism data, the Bouguer gravity anomaly data are superimposed on the plane in the Gplates software for reconstruction of the plates, and the Move software is used to restore balanced profile on the section. Then forward models designed according to the inversion results, and the contrast analogue experments are carried out. According to the studies, under the superposition of gravity sliding and gravity spreading, salt tectonic zones of extension-transition-contraction developed in the passive-margin basins, while factors resulted in the of salt structure deformation process among the conjugated basins, including slope angle, basement extension, the outer high barrier, salt deposition load and syn-depositional rate.(1) From Aptian to early Albian, the Espirito basin and the Kwanza basin were initially a unified salt basin. The east side of the unified basin was strongly stretched, forming faults and salt rafts in the extensional domain. At the same time, the unified basin was gradually divided, accompanied by the formation of the outer high, and the gravity deformation of salt rocks gradually slowed down. (2) From late Albian to Early Paleogene, the inherited upflit of the outer high in the Espirito Santo basin resulted in the vertical upward migrations of salt rocks; the uplift of the thick salt plateau in the Kwanza basin caused salt rocks to cross the outer high and form the thrust nappe. (3) From Late Paleogene to the present, the basement of the Espirito basin had steady thermal subsidence, and the salt structures were successively developed, then the salt rocks in the whole basin exhibit vertical migration characteristics. In the Kwanza basin, the basement was uplifted on a large scale, and the transport of salt rocks to the sea started again. Salt rafts in the extensional domain were further developed into salt rollovers; salt diapirs in the transitional zone and the contractional domain continued to rise; and the thrust nappe continued to develop, forming a large number of faults and folds in the supra salt layers. Then the salt rocks of the entire basin are continuously accumulating downslope with obvious structural zonation.
2021, 46(6): 2230-2245.
doi: 10.3799/dqkx.2020.186
Abstract:
The coupling relationship between hydrocarbon accumulation and the involvement of organic matter in metal mineralization has been a frontier scientific question in recent years. On the basis of the petrography and Raman spectral analysis of fluid inclusions, high-density methane inclusions were discovered in the samples of quartz and calcite veins from Sinian Doushantuo shales, Lower Cambrian Niutitang shales, and MVT (Mississippi Valley type) lead-zinc deposit of Sinian Dengying Formation in Yichang area. The Raman scatter peak v1 of methane inclusions was applied to calculate the density of pure methane inclusions, and the ore-forming ages of MVT lead-zinc deposit were determined by using Rb-Sr and Sm-Nd isochron dating. The density of methane inclusions trapped in the quartz veins within the Doushantuo shales of well EYY 1 mainly ranges from 0.237 to 0.278 g/cm3, and the density of methane inclusions trapped in the calcite veins within the Hejiaping MVT lead-zinc deposit mainly ranges from 0.213 to 0.271 g/cm3, which signifies methane inclusions of high density. Paragenetic mineral association sphalerite and galena within the Hejiaping lead-zinc deposit have a Rb-Sr isochron age of 189.1±1.8 Ma, and calcite within the Hejiaping lead-zinc deposit yields a Sm-Nd isochron age of 189.9±2.0 Ma, which indicates is the mineralization of Hejiaping lead-zinc deposit was closely related to the Yanshanian tectonic compressions. The 87Sr/86Sr values of the paragenetic mineral association (0.711 92) and calcite (0.712 03-0.712 27) indicate that the ore-forming fluids of the Hejiaping lead-zinc deposit were derived largely from shale sources. The high-density methane trapped in fluid inclusions within the Hejiaping lead-zinc deposit was most likely derived from the high-density overpressure methane generated by the Doushantuo shales and/or Niutitang shales. The discovery of high-density methane inclusion in the shale gas layer and MVT lead-zinc deposit, and the determination of ore-forming ages of MVT lead-zinc deposit provide new evidence for the study of organic matter participation in the mineralization of MVT lead-zinc deposit.
The coupling relationship between hydrocarbon accumulation and the involvement of organic matter in metal mineralization has been a frontier scientific question in recent years. On the basis of the petrography and Raman spectral analysis of fluid inclusions, high-density methane inclusions were discovered in the samples of quartz and calcite veins from Sinian Doushantuo shales, Lower Cambrian Niutitang shales, and MVT (Mississippi Valley type) lead-zinc deposit of Sinian Dengying Formation in Yichang area. The Raman scatter peak v1 of methane inclusions was applied to calculate the density of pure methane inclusions, and the ore-forming ages of MVT lead-zinc deposit were determined by using Rb-Sr and Sm-Nd isochron dating. The density of methane inclusions trapped in the quartz veins within the Doushantuo shales of well EYY 1 mainly ranges from 0.237 to 0.278 g/cm3, and the density of methane inclusions trapped in the calcite veins within the Hejiaping MVT lead-zinc deposit mainly ranges from 0.213 to 0.271 g/cm3, which signifies methane inclusions of high density. Paragenetic mineral association sphalerite and galena within the Hejiaping lead-zinc deposit have a Rb-Sr isochron age of 189.1±1.8 Ma, and calcite within the Hejiaping lead-zinc deposit yields a Sm-Nd isochron age of 189.9±2.0 Ma, which indicates is the mineralization of Hejiaping lead-zinc deposit was closely related to the Yanshanian tectonic compressions. The 87Sr/86Sr values of the paragenetic mineral association (0.711 92) and calcite (0.712 03-0.712 27) indicate that the ore-forming fluids of the Hejiaping lead-zinc deposit were derived largely from shale sources. The high-density methane trapped in fluid inclusions within the Hejiaping lead-zinc deposit was most likely derived from the high-density overpressure methane generated by the Doushantuo shales and/or Niutitang shales. The discovery of high-density methane inclusion in the shale gas layer and MVT lead-zinc deposit, and the determination of ore-forming ages of MVT lead-zinc deposit provide new evidence for the study of organic matter participation in the mineralization of MVT lead-zinc deposit.
2021, 46(6): 2246-2257.
doi: 10.3799/dqkx.2020.200
Abstract:
Multiphase calcite veins were developed in the carbonate reservoirs of Middle-Lower Ordovician, which have been an important indicator for both the history of fluid activity and the hydrocarbon accumulation process in the northern Shuntuoguole low-uplift, Tarim basin. Based on the thin section observation, cathodoluminescence, in-situ element analysis, fluid inclusion and strontium isotope analysis of calcite vein, the research divides the phases of calcite vein and determines the origin and formation time of calcite vein in different stages. The results show that there are four phases of calcite vein in the northern Shuntuoguole low-uplift. The Fe/Mn and U/Th values of calcite veins in different periods are different. The cathodoluminescence colors of C1, C2, C3 and C4 calcite are dark red, non-luminous or dark blue, orange yellow and bright yellow, respectively. C1 calcite vein-forming fluid came from deep fluid with high 87Sr/86Sr. C2 and C3 calcite vein-forming fluids are similar and both derived from diagenetic fluids with seawater properties in the same formation. C1, C2, and C4 calcite veins all formed in the partial reduction environment, and C3 calcite veins formed in the partial oxidation environment. Based on the homogenization temperature of the primary aqueous fluid inclusions developed in the veins in different periods, combined with the burial history and thermal history of the single well, it is determined that the C1 calcite vein was formed at about 445 Ma, corresponding to the Middle Caledonian act Ⅲ, C2 and C3 calcite veins formed at about 430-428 Ma and 418 Ma respectively, corresponding to the Late Caledonian period.
Multiphase calcite veins were developed in the carbonate reservoirs of Middle-Lower Ordovician, which have been an important indicator for both the history of fluid activity and the hydrocarbon accumulation process in the northern Shuntuoguole low-uplift, Tarim basin. Based on the thin section observation, cathodoluminescence, in-situ element analysis, fluid inclusion and strontium isotope analysis of calcite vein, the research divides the phases of calcite vein and determines the origin and formation time of calcite vein in different stages. The results show that there are four phases of calcite vein in the northern Shuntuoguole low-uplift. The Fe/Mn and U/Th values of calcite veins in different periods are different. The cathodoluminescence colors of C1, C2, C3 and C4 calcite are dark red, non-luminous or dark blue, orange yellow and bright yellow, respectively. C1 calcite vein-forming fluid came from deep fluid with high 87Sr/86Sr. C2 and C3 calcite vein-forming fluids are similar and both derived from diagenetic fluids with seawater properties in the same formation. C1, C2, and C4 calcite veins all formed in the partial reduction environment, and C3 calcite veins formed in the partial oxidation environment. Based on the homogenization temperature of the primary aqueous fluid inclusions developed in the veins in different periods, combined with the burial history and thermal history of the single well, it is determined that the C1 calcite vein was formed at about 445 Ma, corresponding to the Middle Caledonian act Ⅲ, C2 and C3 calcite veins formed at about 430-428 Ma and 418 Ma respectively, corresponding to the Late Caledonian period.
2021, 46(6): 2258-2266.
doi: 10.3799/dqkx.2021.020
Abstract:
It is vital to prevent the pollution of petroleum hydrocarbons in the groundwater environment globally. In the paper it uses numerical simulation method and hydrogeochemical techniques to simulate the natural attenuation processes of petroleum hydrocarbons in the fractured aquifer and calculate the natural attenuation mechanisms quantitatively. BIOSCREEN model was used to simulate the natural attenuation processes of petroleum hydrocarbons, the contribution rates of physical processes and biodegradation processes to the total natural attenuation are 31.53% and 68.47%, respectively, and biodegradation was the main mechanism for the natural remediation ability of fractured karst aquifer. Inter-relationships between water chemistries (HCO3-, NO3-) and isotopes (δ15NNO3, δ18ONO3 and δ13CDIC) were analyzed by the principle of quality conservation in the research. The average contribution rate of biodegradation to the concentration of HCO3- in the groundwater system was 33.93%. Ion of NO3- was the main electron acceptor in the anaerobic biodegradation processes of petroleum hydrocarbons without methanogenic activity. The process of petroleum hydrocarbon biodegradation consuming NO3- contributes 30.77% to the δ13CDIC in the groundwater system, which accounts for 90.69% of total biodegradation of petroleum hydrocarbons in the fractured karst aquifer.
It is vital to prevent the pollution of petroleum hydrocarbons in the groundwater environment globally. In the paper it uses numerical simulation method and hydrogeochemical techniques to simulate the natural attenuation processes of petroleum hydrocarbons in the fractured aquifer and calculate the natural attenuation mechanisms quantitatively. BIOSCREEN model was used to simulate the natural attenuation processes of petroleum hydrocarbons, the contribution rates of physical processes and biodegradation processes to the total natural attenuation are 31.53% and 68.47%, respectively, and biodegradation was the main mechanism for the natural remediation ability of fractured karst aquifer. Inter-relationships between water chemistries (HCO3-, NO3-) and isotopes (δ15NNO3, δ18ONO3 and δ13CDIC) were analyzed by the principle of quality conservation in the research. The average contribution rate of biodegradation to the concentration of HCO3- in the groundwater system was 33.93%. Ion of NO3- was the main electron acceptor in the anaerobic biodegradation processes of petroleum hydrocarbons without methanogenic activity. The process of petroleum hydrocarbon biodegradation consuming NO3- contributes 30.77% to the δ13CDIC in the groundwater system, which accounts for 90.69% of total biodegradation of petroleum hydrocarbons in the fractured karst aquifer.
2021, 46(6): 2267-2277.
doi: 10.3799/dqkx.2020.259
Abstract:
High salinity and shallow level of shallow groundwater are the important causes of soil secondary salinization in Hetao irrigation area. In this paper, it takes the western part of Hetao irrigation area as the research area. The hydrochemical analysis, isotopic characteristics of hydrogen and oxygen and hydrogeochemical modeling were investigated to identify the recharge sources of shallow groundwater and the main water-rock interaction processes, and estimate quantitatively the effect of evaporation on shallow groundwater. The results show that shallow groundwater was weak alkaline salt water, the pH and total dissolved solids (total dissolved solids, TDS) are 7.23-8.45 and 371-7 599 mg/L respectively in the research area. With the accumulation of salt, the hydrochemical type of shallow groundwater changed from HCO3-Na·Mg·Ca to Cl-Na. The main recharge resources of shallow groundwater are the irrigation by using Yellow River water and precipitation. The shallow groundwater was affected by evaporation and transpiration during the runoff process. The hydrochemical compositions of shallow groundwater mainly came from the dissolution of halite and silicate weathering, and experienced intense evaporation and the cation exchange. The results of hydrogeochemical modeling and principal component analysis show that evaporation and halite dissolution contributed the most part to the salinity of shallow groundwater; Na-Ca exchange, topography variations and dissolution of gypsum, dolomite and other minerals also contributed to the salinity of groundwater.
High salinity and shallow level of shallow groundwater are the important causes of soil secondary salinization in Hetao irrigation area. In this paper, it takes the western part of Hetao irrigation area as the research area. The hydrochemical analysis, isotopic characteristics of hydrogen and oxygen and hydrogeochemical modeling were investigated to identify the recharge sources of shallow groundwater and the main water-rock interaction processes, and estimate quantitatively the effect of evaporation on shallow groundwater. The results show that shallow groundwater was weak alkaline salt water, the pH and total dissolved solids (total dissolved solids, TDS) are 7.23-8.45 and 371-7 599 mg/L respectively in the research area. With the accumulation of salt, the hydrochemical type of shallow groundwater changed from HCO3-Na·Mg·Ca to Cl-Na. The main recharge resources of shallow groundwater are the irrigation by using Yellow River water and precipitation. The shallow groundwater was affected by evaporation and transpiration during the runoff process. The hydrochemical compositions of shallow groundwater mainly came from the dissolution of halite and silicate weathering, and experienced intense evaporation and the cation exchange. The results of hydrogeochemical modeling and principal component analysis show that evaporation and halite dissolution contributed the most part to the salinity of shallow groundwater; Na-Ca exchange, topography variations and dissolution of gypsum, dolomite and other minerals also contributed to the salinity of groundwater.
2021, 46(6): 2278-2286.
doi: 10.3799/dqkx.2021.041
Abstract:
A shield tunnel of Zhengzhou Metro passes through the existing pile foundations of railway bridge at ultra close distance.Due to the limitation of ground space and clear distance between tunnel and bridge pile, conventional protection measures such as isolation pile cannot be adopted.In this paper, three protection measures are put forward, which are "soil grouting reinforcement within the scope of shield passing", "bridge cap reinforcement" and "grouting + bridge cap reinforcement". By means of numerical simulation, the deformation law of bridge piles under different protection measures was analyzed.The results show that the maximum settlement of the bridge deck can be reduced by about 45% by adopting the measures of "grouting + bridge cap reinforcement", besides, the uneven settlement of bridge deck in transverse direction and horizontal displacement of bridge pile can be significantly reduced.
A shield tunnel of Zhengzhou Metro passes through the existing pile foundations of railway bridge at ultra close distance.Due to the limitation of ground space and clear distance between tunnel and bridge pile, conventional protection measures such as isolation pile cannot be adopted.In this paper, three protection measures are put forward, which are "soil grouting reinforcement within the scope of shield passing", "bridge cap reinforcement" and "grouting + bridge cap reinforcement". By means of numerical simulation, the deformation law of bridge piles under different protection measures was analyzed.The results show that the maximum settlement of the bridge deck can be reduced by about 45% by adopting the measures of "grouting + bridge cap reinforcement", besides, the uneven settlement of bridge deck in transverse direction and horizontal displacement of bridge pile can be significantly reduced.
