2020 Vol. 45, No. 8
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2020, 45(8): 2735-2763.
doi: 10.3799/dqkx.2020.152
Abstract:
The study of suture zones in Tibet is always one of the hottest scientific problems related to the fundamental geology of Tibetan Plateau. Based on the composition, tectonic and formation ages for the four suture zones in the Tibetan Plateau, and through the analysis of spatial-temporal structure, it is proposed that the evolution of the Paleo-Tethys was related to the transform faults. The expansion of the Yarlung Zangbo Ocean Basin shows space-time transformation coupled with the Indosinian orogeny in the Qiangtang-Sanjiang area. Focusing on the Paleo-Tethys and Neo-Tethys transformation and expansion properties of the Bangong-Nujiang belt, it is found in this study that its tectonic mechanism is left-lateral strike-slip and pull-apart, which have resulted in a series of extensional complexes parallel to the strike-slip and pull-apart, and so it is as important as that of the thrust back sliding detachment system on the basin-ridge metamorphic core complex in the western United States. In this paper, it also discusses the difference of geological structures in eastern and western Tibet, and proposes that the expansion and widening of the Gangdese magmatic arc were constrained by the double subduction of the Yarlung Zangbo Tethys Ocean.
The study of suture zones in Tibet is always one of the hottest scientific problems related to the fundamental geology of Tibetan Plateau. Based on the composition, tectonic and formation ages for the four suture zones in the Tibetan Plateau, and through the analysis of spatial-temporal structure, it is proposed that the evolution of the Paleo-Tethys was related to the transform faults. The expansion of the Yarlung Zangbo Ocean Basin shows space-time transformation coupled with the Indosinian orogeny in the Qiangtang-Sanjiang area. Focusing on the Paleo-Tethys and Neo-Tethys transformation and expansion properties of the Bangong-Nujiang belt, it is found in this study that its tectonic mechanism is left-lateral strike-slip and pull-apart, which have resulted in a series of extensional complexes parallel to the strike-slip and pull-apart, and so it is as important as that of the thrust back sliding detachment system on the basin-ridge metamorphic core complex in the western United States. In this paper, it also discusses the difference of geological structures in eastern and western Tibet, and proposes that the expansion and widening of the Gangdese magmatic arc were constrained by the double subduction of the Yarlung Zangbo Tethys Ocean.
2020, 45(8): 2764-2784.
doi: 10.3799/dqkx.2020.083
Abstract:
The Shiquanhe-Laguoco-Yongzhu-Jiali ophiolite mélange zone (SYMZ) is located between the Bangongco-Nujiang suture zone and the Yarlung-Zangbo suture zone in the central Qinghai-Tibet Plateau. The tectonic property of the SYMZ remains controversial, which restricts the application of archipelagic arc-basin system theory to the evolution of Qinghai-Tibet Plateau. Based on recent geological survey information, research results and comprehensive data analyses, in this paper, it summarizes the geological feature and discusses the tectonic process of the SYMZ. The SYMZ represents an independent ophiolitic mélange zone dividing the north Lhasa Terrane and central Lhasa Terrane, which is part of the Tethyan archipelagic arc-basin system. The typical ophiolite mélange suites are distributed in such places as Shiquanhe, Laguoco, Aso, Yongzhu and Kaimeng, and high precision geochronology data show that the main ocean basin was formed during 178-160 Ma, which was about 10 Ma later than the formation of the Bangongco-Nujiang ocean basin (188-162 Ma). The Aso ophiolitic mélange reflects that the SYMZ ocean basin had existed until 113 Ma. The Jurassic mafic rocks of the SYMZ show the geochemistry characteristics of MORB and IAT, belonging to the Oceanic Arc and MORB type ophiolites. The Early Cretaceous mafic rocks show both MORB and arc basalt characters, suggesting they probably formed in the tectonic setting of SSZ, different from the contemporaneous Bangongco-Nujiang ocean which was influenced by the mantle plume. The Jurassic boninites and bonibasalts series places like Laguoco, Yongzhu and Kaimeng were found in this study, indicating the intra-oceanic subduction of the SYMZ oceanic slab. Combined all these new discoveries with regional geological data, the spatial-temporal framework of the SYMZ Tethyan Ocean and its tectonic evolution history are established. The SYMZ Tethyan Ocean opened and spread in Late Triassic-Early Jurassic, the intra-oceanic subduction lasted during Middle-Late Jurassic, oceanic slab subduction begun in the Early Cretaceous, and finally ocean basin was extinct in the late Early Cretaceous. This study provides important geological evidences for understanding the lithosphere evolution and tectonic process of the Tethyan.
The Shiquanhe-Laguoco-Yongzhu-Jiali ophiolite mélange zone (SYMZ) is located between the Bangongco-Nujiang suture zone and the Yarlung-Zangbo suture zone in the central Qinghai-Tibet Plateau. The tectonic property of the SYMZ remains controversial, which restricts the application of archipelagic arc-basin system theory to the evolution of Qinghai-Tibet Plateau. Based on recent geological survey information, research results and comprehensive data analyses, in this paper, it summarizes the geological feature and discusses the tectonic process of the SYMZ. The SYMZ represents an independent ophiolitic mélange zone dividing the north Lhasa Terrane and central Lhasa Terrane, which is part of the Tethyan archipelagic arc-basin system. The typical ophiolite mélange suites are distributed in such places as Shiquanhe, Laguoco, Aso, Yongzhu and Kaimeng, and high precision geochronology data show that the main ocean basin was formed during 178-160 Ma, which was about 10 Ma later than the formation of the Bangongco-Nujiang ocean basin (188-162 Ma). The Aso ophiolitic mélange reflects that the SYMZ ocean basin had existed until 113 Ma. The Jurassic mafic rocks of the SYMZ show the geochemistry characteristics of MORB and IAT, belonging to the Oceanic Arc and MORB type ophiolites. The Early Cretaceous mafic rocks show both MORB and arc basalt characters, suggesting they probably formed in the tectonic setting of SSZ, different from the contemporaneous Bangongco-Nujiang ocean which was influenced by the mantle plume. The Jurassic boninites and bonibasalts series places like Laguoco, Yongzhu and Kaimeng were found in this study, indicating the intra-oceanic subduction of the SYMZ oceanic slab. Combined all these new discoveries with regional geological data, the spatial-temporal framework of the SYMZ Tethyan Ocean and its tectonic evolution history are established. The SYMZ Tethyan Ocean opened and spread in Late Triassic-Early Jurassic, the intra-oceanic subduction lasted during Middle-Late Jurassic, oceanic slab subduction begun in the Early Cretaceous, and finally ocean basin was extinct in the late Early Cretaceous. This study provides important geological evidences for understanding the lithosphere evolution and tectonic process of the Tethyan.
2020, 45(8): 2785-2804.
doi: 10.3799/dqkx.2020.180
Abstract:
The breakoff of the Neo-Tethyan slab and related magmatism after India-Asia continental collision has been controversial topics in the geological study of the southern Tibetan Plateau. In this study, it reviews the unsolved problems and petrological methods for exploring the process on the Neo-Tethyan slab breakoff. Based on the systematic geochronology, major and trace elements and Sr-Nd-Hf isotope geochemical analyses of the Early Cenozoic mafic rocks in the typical areas of the Lhasa Terrane, it is found that two suites of mafic rocks with ages of ~57 Ma and ~50 Ma have close relationship with the Neo-Tethyan slab breakoff. The ~57 Ma mafic rocks are characterized by high Zr/Y and Ti/Y ratios, and their geochemistry indicates an intraplate affinity rather than arc magmas, indicating that they likely correspond to the magmatic expression of the initial stage of Neo-Tethyan slab breakoff. The ~50 Ma mafic rocks are appinites, reflecting the water-rich environment in the lithospheric mantle of the southern Lhasa Terrane after the India-Asia collision, suggesting the flux of slab fluids through the lithospheric mantle during breakoff of the Neo-Tethyan slab. Integrating the geochronological and geochemical data of the Early Cenozoic mafic magmatism in the Lhasa Terrane, we have reconstructed the processes from initial tear to completely breakoff of the Neo-Tethyan lithosphere after India-Asia continent collision. The initial tear of the Neo-Tethyan lithosphere occurred at ~57 Ma, then the slab detached from the India lithosphere with a high angle subduction, which resulted in the simultaneous occurrence of extensive mafic magmatism in the central and southern Lhasa Terrane. The complete slab breakoff happened at ~50 Ma. The isotopic compositions of Early Cenozoic magmatic rocks reveal that there was great geochemical heterogeneity of lithospheric mantle beneath the Lhasa Terrane. There were ancient lithospheric materials in the central Lhasa Terrane and eastern part of the southern Lhasa Terrane, while there was mainly depleted juvenile lithosphere in the central part of the southern Lhasa Terrane. The local ancient enriched lithosphere in the Lhasa Terrane is likely to transform into a juvenile depleted lithosphere by the upwelling of the deep mantle material, which may promote the eruption of the felsic magmatism and the growth of continental crust in the Lhasa Terrane.
The breakoff of the Neo-Tethyan slab and related magmatism after India-Asia continental collision has been controversial topics in the geological study of the southern Tibetan Plateau. In this study, it reviews the unsolved problems and petrological methods for exploring the process on the Neo-Tethyan slab breakoff. Based on the systematic geochronology, major and trace elements and Sr-Nd-Hf isotope geochemical analyses of the Early Cenozoic mafic rocks in the typical areas of the Lhasa Terrane, it is found that two suites of mafic rocks with ages of ~57 Ma and ~50 Ma have close relationship with the Neo-Tethyan slab breakoff. The ~57 Ma mafic rocks are characterized by high Zr/Y and Ti/Y ratios, and their geochemistry indicates an intraplate affinity rather than arc magmas, indicating that they likely correspond to the magmatic expression of the initial stage of Neo-Tethyan slab breakoff. The ~50 Ma mafic rocks are appinites, reflecting the water-rich environment in the lithospheric mantle of the southern Lhasa Terrane after the India-Asia collision, suggesting the flux of slab fluids through the lithospheric mantle during breakoff of the Neo-Tethyan slab. Integrating the geochronological and geochemical data of the Early Cenozoic mafic magmatism in the Lhasa Terrane, we have reconstructed the processes from initial tear to completely breakoff of the Neo-Tethyan lithosphere after India-Asia continent collision. The initial tear of the Neo-Tethyan lithosphere occurred at ~57 Ma, then the slab detached from the India lithosphere with a high angle subduction, which resulted in the simultaneous occurrence of extensive mafic magmatism in the central and southern Lhasa Terrane. The complete slab breakoff happened at ~50 Ma. The isotopic compositions of Early Cenozoic magmatic rocks reveal that there was great geochemical heterogeneity of lithospheric mantle beneath the Lhasa Terrane. There were ancient lithospheric materials in the central Lhasa Terrane and eastern part of the southern Lhasa Terrane, while there was mainly depleted juvenile lithosphere in the central part of the southern Lhasa Terrane. The local ancient enriched lithosphere in the Lhasa Terrane is likely to transform into a juvenile depleted lithosphere by the upwelling of the deep mantle material, which may promote the eruption of the felsic magmatism and the growth of continental crust in the Lhasa Terrane.
2020, 45(8): 2805-2825.
doi: 10.3799/dqkx.2020.204
Abstract:
The continuous Xiongmei-Baingoin-Qinglong-Sangxiong granite zone, located in central Tibet, belongs to the middle part of the Bangong-Nujiang metallogenic belt. Small- and medium-sized deposits were discovered in recent years, such as Xiongmei, Kuga, Ri'a copper deposits and Qinglong lead-zinc deposit, but no bigger deposits were found in this zone. Based on comprehensive data collection and field investigation, research on geochemical properties, magma sources and ore-forming factors of the granitic zone was carried out. The granitic intrusive stages can be identified at 140-125 Ma, 120-110 Ma, 94-72 Ma, and some Cenozoic intrusions, with a magmatic flare-up at 120-110 Ma. In western part of this area, the Early Cretaceous Shesuo, Xiongmei, Kuga granitic intrusives and the Late Cretaceous Xueru, and Sangxinri granitic intrusives are significant ore-forming granites with copper, gold and iron deposits. However, they are different from granitic intrusives forming super-large porphyry copper deposits regarding to their magma sources and surrounding rock. They have prospecting potential to form medium- or large-sized porphyry copper, gold deposits or skarn-type deposits similar to the Qingcaoshan copper deposit. The Early Cretaceous granites distributed continuously along Baingoin, Qinglong and Sangxiong are closely related to hydrothermal and skarn-type iron, lead and zinc polymetallic mineralization.
The continuous Xiongmei-Baingoin-Qinglong-Sangxiong granite zone, located in central Tibet, belongs to the middle part of the Bangong-Nujiang metallogenic belt. Small- and medium-sized deposits were discovered in recent years, such as Xiongmei, Kuga, Ri'a copper deposits and Qinglong lead-zinc deposit, but no bigger deposits were found in this zone. Based on comprehensive data collection and field investigation, research on geochemical properties, magma sources and ore-forming factors of the granitic zone was carried out. The granitic intrusive stages can be identified at 140-125 Ma, 120-110 Ma, 94-72 Ma, and some Cenozoic intrusions, with a magmatic flare-up at 120-110 Ma. In western part of this area, the Early Cretaceous Shesuo, Xiongmei, Kuga granitic intrusives and the Late Cretaceous Xueru, and Sangxinri granitic intrusives are significant ore-forming granites with copper, gold and iron deposits. However, they are different from granitic intrusives forming super-large porphyry copper deposits regarding to their magma sources and surrounding rock. They have prospecting potential to form medium- or large-sized porphyry copper, gold deposits or skarn-type deposits similar to the Qingcaoshan copper deposit. The Early Cretaceous granites distributed continuously along Baingoin, Qinglong and Sangxiong are closely related to hydrothermal and skarn-type iron, lead and zinc polymetallic mineralization.
2020, 45(8): 2826-2845.
doi: 10.3799/dqkx.2020.124
Abstract:
The identification of oceanic island-seamount/oceanic plateau has not been attended sufficiently in the regional tectonic analysis of China. In order to study the ocean plate geology of China in depth, the basic concepts, characteristics and accretionary procession of the oceanic island-seamount and ocean floor plateau are reviewed systematically in this paper. Ocean island-seamount/oceanic plateau is an area with abnormal thickness ocean crust formed by hot spot/mantle plume in the process of ocean floor spreading and ocean crust evolution. It is an important part of the oceanic lithosphere. The ocean island-seamount/oceanic plateau has a typical binary structure, with the lower part dominated by mafic and ultramafic rocks, and the upper part composed of carbonate rock formation. According to the principles of comparative tectonics, the existence of the ancient ocean island-seamount indicates the relics of the fossil ocean basin. It is an important carrier to study the orogenic belt. It is considered that there are quite a number of oceanic island-seamounts in the oceanic basin during the geological history. During the subduction-accretion to collision orogeny, the remaining fossil oceanic island-seamounts are sandwiched in the subduction-accretion complex in the form of tectonic slices (blocks), which is one of the important criteria to identify the suture zone, when the ocean closed.
The identification of oceanic island-seamount/oceanic plateau has not been attended sufficiently in the regional tectonic analysis of China. In order to study the ocean plate geology of China in depth, the basic concepts, characteristics and accretionary procession of the oceanic island-seamount and ocean floor plateau are reviewed systematically in this paper. Ocean island-seamount/oceanic plateau is an area with abnormal thickness ocean crust formed by hot spot/mantle plume in the process of ocean floor spreading and ocean crust evolution. It is an important part of the oceanic lithosphere. The ocean island-seamount/oceanic plateau has a typical binary structure, with the lower part dominated by mafic and ultramafic rocks, and the upper part composed of carbonate rock formation. According to the principles of comparative tectonics, the existence of the ancient ocean island-seamount indicates the relics of the fossil ocean basin. It is an important carrier to study the orogenic belt. It is considered that there are quite a number of oceanic island-seamounts in the oceanic basin during the geological history. During the subduction-accretion to collision orogeny, the remaining fossil oceanic island-seamounts are sandwiched in the subduction-accretion complex in the form of tectonic slices (blocks), which is one of the important criteria to identify the suture zone, when the ocean closed.
2020, 45(8): 2846-2856.
doi: 10.3799/dqkx.2020.102
Abstract:
There are relatively few reports on the Mesozoic magmatic rocks in the Shiquanhe area of western Lhasa Block, which limits the understanding of the Mesozoic magmatism in this area. In this paper, it presents the major element, trace element and zircon U-Pb age data of bulk-rock for the host quartz diorite and dioritic enclaves from the Shiquanhe area. The zircon dating yields U-Pb age of 161.1±1.7 Ma for the host quartz diorite, of 159.8±1.6 Ma and 157.0±1.3 Ma respectively for dioritic enclaves, indicating that the host quartz diorite and dioritic enclaves have similar crystallization ages and they both formed at Late Jurassic. The host quartz diorite is metaluminous and calc-alkaline to high K calc-alkaline I-type granitoids. They are enriched in large ion lithopile elements, and depleted in high field strength elements. The dioritic enclaves are metaluminous and calc-alkaline to high K calc-alkaline. Based on the data in this study and previous ones, it is proposed that Late Jurassic quartz diorite was formed in tectonic setting of the southward subduction of Bangongco-Nujiang Neo-Tethyan oceanic seafloor, and generated by the magma mixing between mantle-derived basic magma and acidic magma formed by the melting of the ancient crustal material of Lhasa Block.
There are relatively few reports on the Mesozoic magmatic rocks in the Shiquanhe area of western Lhasa Block, which limits the understanding of the Mesozoic magmatism in this area. In this paper, it presents the major element, trace element and zircon U-Pb age data of bulk-rock for the host quartz diorite and dioritic enclaves from the Shiquanhe area. The zircon dating yields U-Pb age of 161.1±1.7 Ma for the host quartz diorite, of 159.8±1.6 Ma and 157.0±1.3 Ma respectively for dioritic enclaves, indicating that the host quartz diorite and dioritic enclaves have similar crystallization ages and they both formed at Late Jurassic. The host quartz diorite is metaluminous and calc-alkaline to high K calc-alkaline I-type granitoids. They are enriched in large ion lithopile elements, and depleted in high field strength elements. The dioritic enclaves are metaluminous and calc-alkaline to high K calc-alkaline. Based on the data in this study and previous ones, it is proposed that Late Jurassic quartz diorite was formed in tectonic setting of the southward subduction of Bangongco-Nujiang Neo-Tethyan oceanic seafloor, and generated by the magma mixing between mantle-derived basic magma and acidic magma formed by the melting of the ancient crustal material of Lhasa Block.
2020, 45(8): 2857-2867.
doi: 10.3799/dqkx.2020.231
Abstract:
The formation age of the Zenong Group along the Shiquanhe-Yongzhu-Namucuo-Jiani mélange in the middle Lhasa terrane remains controversial between the Late Jurassic and the Early Cretaceous, and there is a lack of comparative study along the east-west direction. In this study, it focuses on the Zenong Group tuff from the Shiquanhe area in the western segment, and systematic analyses of zircon U-Pb age, trace element and Hf isotope for zircon were done for the tuff samples. Zircons from the tuff samples are elongated grains, showing obvious oscillatory zones, and the trace elements also verify the magmatic origin of zircons. Two tuff samples show LA-ICP-MS zircon U-Pb ages as 157.5±0.9 Ma and 159.2±1.3 Ma. Combined with previous research results, the Zenong Group was formed in the Late Jurassic to Early Cretaceous. Zircon trace elements and Hf isotopes of the tuff samples indicate that the Zenong Group were formed in the continental arc setting, and the hybrid of ancient continent crustal basement material and minor mantel material might be the magma source. Combined with the previous studies, the initial time of the subduction might vary from the earlier one in the western part to the later one in the eastern part.
The formation age of the Zenong Group along the Shiquanhe-Yongzhu-Namucuo-Jiani mélange in the middle Lhasa terrane remains controversial between the Late Jurassic and the Early Cretaceous, and there is a lack of comparative study along the east-west direction. In this study, it focuses on the Zenong Group tuff from the Shiquanhe area in the western segment, and systematic analyses of zircon U-Pb age, trace element and Hf isotope for zircon were done for the tuff samples. Zircons from the tuff samples are elongated grains, showing obvious oscillatory zones, and the trace elements also verify the magmatic origin of zircons. Two tuff samples show LA-ICP-MS zircon U-Pb ages as 157.5±0.9 Ma and 159.2±1.3 Ma. Combined with previous research results, the Zenong Group was formed in the Late Jurassic to Early Cretaceous. Zircon trace elements and Hf isotopes of the tuff samples indicate that the Zenong Group were formed in the continental arc setting, and the hybrid of ancient continent crustal basement material and minor mantel material might be the magma source. Combined with the previous studies, the initial time of the subduction might vary from the earlier one in the western part to the later one in the eastern part.
2020, 45(8): 2868-2881.
doi: 10.3799/dqkx.2020.092
Abstract:
There are still many controversies about the tectonic evolution of Bangongco-Nujiang suture zone in the late period. In this paper, it presents a study of chronology and geochemistry, taking the newly discovered high magnesium rhyolite in East Ritu of Tibet as the object. The concordance age of 113.69±0.82 Ma (MSWD=2.7) measured by LA-ICP-MS zircon U-Pb indicates that it is the product of magmatic activity in the late Early Cretaceous. The geochemical analyses show that the samples are rich in SiO2 (70.27%-75.72%), MgO (3.50%-4.12%) and Mg# (82.68-84.79). It is relatively enriched in light rare earth elements, and depleted in heavy rare earth elements (LaN/YbN=7.00-9.34), with obvious negative Eu anomalies (δEu=0.50-0.65). The results show enrichment of Th, U, depletion of Ba, Sr, P and Ti, and weak depletion of Nb and Ta. In the diagram of tectonic environment discrimination, the samples are projected in the continental margin arc area, indicating that they were formed in the tectonic background of oceanic lithosphere subduction. This study provides a precise isotopic age to determine the marine strata of Shamulo, and new evidence for further studies on the structural evolution of the Bangongco-Nujiang suture zone in Cretaceous period.
There are still many controversies about the tectonic evolution of Bangongco-Nujiang suture zone in the late period. In this paper, it presents a study of chronology and geochemistry, taking the newly discovered high magnesium rhyolite in East Ritu of Tibet as the object. The concordance age of 113.69±0.82 Ma (MSWD=2.7) measured by LA-ICP-MS zircon U-Pb indicates that it is the product of magmatic activity in the late Early Cretaceous. The geochemical analyses show that the samples are rich in SiO2 (70.27%-75.72%), MgO (3.50%-4.12%) and Mg# (82.68-84.79). It is relatively enriched in light rare earth elements, and depleted in heavy rare earth elements (LaN/YbN=7.00-9.34), with obvious negative Eu anomalies (δEu=0.50-0.65). The results show enrichment of Th, U, depletion of Ba, Sr, P and Ti, and weak depletion of Nb and Ta. In the diagram of tectonic environment discrimination, the samples are projected in the continental margin arc area, indicating that they were formed in the tectonic background of oceanic lithosphere subduction. This study provides a precise isotopic age to determine the marine strata of Shamulo, and new evidence for further studies on the structural evolution of the Bangongco-Nujiang suture zone in Cretaceous period.
2020, 45(8): 2882-2893.
doi: 10.3799/dqkx.2020.163
Abstract:
The Chazi syenite mass is a newly discovered Miocene potassic-ultrapotassic intrusive rock in the middle Lhasa block, and its lithology is mainly composed of medium-coarse grained quartz hornblende syenite and porphyraceous quartz syenite. Mafic magmatic Enclaves were found in the syenite body. Petrography, zircon U-Pb dating and geochemical analysis were done, in order to study the petrogenesis of the Chazi syenite. Zircon U-Pb ages of 10.37±0.24 Ma and 11.06±0.39 Ma were obtained for these two rock types respectively, which represent that they were formed in the Miocene Epoch and product of post-collision magmatism in the Lhasa block. The Chazi syenite rocks have relatively high K2O (6.75%-7.39%) and low MgO (1.44%-2.97%), with K2O/Na2O > 1, so they are potassic rocks. They have similar characteristics of trace elements to the ultrapotassic rocks:strongly enriched in large ion lithophile elements (LILE) such as Rb, Th, U and K, and light rare earth elements (LREE), relatively depleted in high field strength elements (HFSE) such as Nb, Ta, Zr, Hf, Ti and P, and heavy rare earth elements (HREE), but the contents of Cr (22.7×10-6-64.6×10-6) and Ni (18.9×10-6-46.6×10-6) of the syenite are obviously lower. Compared with porphyraceous quartz syenite rocks, the quartz hornblende syenite rocks have relatively higher SiO2, and lower elements sucn as TiO2, FeOT, MgO, K2O, Cr, Ni, REE and Y. By comprehensive analyses, it is proposed that the Chazi syenite was mainly formed by magma mixing, and produced by the mixing between the ultrapotassic magma that originated from the lithosphere mantle partial melting, and acid magma that originated from the continental crust partial melting. The chemical differcence of two rock types is mainly due to the different degrees of magma mixing and ratio of mixed magma.The Chazi syenite possibly resulted from convective thinning or delamination of over thickened mantle lithosphere.
The Chazi syenite mass is a newly discovered Miocene potassic-ultrapotassic intrusive rock in the middle Lhasa block, and its lithology is mainly composed of medium-coarse grained quartz hornblende syenite and porphyraceous quartz syenite. Mafic magmatic Enclaves were found in the syenite body. Petrography, zircon U-Pb dating and geochemical analysis were done, in order to study the petrogenesis of the Chazi syenite. Zircon U-Pb ages of 10.37±0.24 Ma and 11.06±0.39 Ma were obtained for these two rock types respectively, which represent that they were formed in the Miocene Epoch and product of post-collision magmatism in the Lhasa block. The Chazi syenite rocks have relatively high K2O (6.75%-7.39%) and low MgO (1.44%-2.97%), with K2O/Na2O > 1, so they are potassic rocks. They have similar characteristics of trace elements to the ultrapotassic rocks:strongly enriched in large ion lithophile elements (LILE) such as Rb, Th, U and K, and light rare earth elements (LREE), relatively depleted in high field strength elements (HFSE) such as Nb, Ta, Zr, Hf, Ti and P, and heavy rare earth elements (HREE), but the contents of Cr (22.7×10-6-64.6×10-6) and Ni (18.9×10-6-46.6×10-6) of the syenite are obviously lower. Compared with porphyraceous quartz syenite rocks, the quartz hornblende syenite rocks have relatively higher SiO2, and lower elements sucn as TiO2, FeOT, MgO, K2O, Cr, Ni, REE and Y. By comprehensive analyses, it is proposed that the Chazi syenite was mainly formed by magma mixing, and produced by the mixing between the ultrapotassic magma that originated from the lithosphere mantle partial melting, and acid magma that originated from the continental crust partial melting. The chemical differcence of two rock types is mainly due to the different degrees of magma mixing and ratio of mixed magma.The Chazi syenite possibly resulted from convective thinning or delamination of over thickened mantle lithosphere.
2020, 45(8): 2894-2904.
doi: 10.3799/dqkx.2020.110
Abstract:
The Higher Himalayan Crystalline Sequence is composed mainly of middle-high grade metamorphic rocks and leucogranites, forming a natural laboratory for studying the formation and evolution of the Himalayan orogen. However, the geological significance of the U-Pb ages remains controversial because zircons and monazites of the migmatites and leucogranites from the Higher Himalayan Crystalline Sequence commonly have yielded variable U-Pb ages, which significantly restricts our understanding of timing and duration of metamorphism, partial melting and melt crystallization of the orogen.Here it presents zircon U-Pb geochronological study of the Naiduila migmatites in Yadong region from the upper structural level of the Higher Himalayan Crystalline Sequence, middle Himalaya. The results show that zircons from the melanosomes of the migmatites in Naiduila area have variable U-Pb ages ranging from 29.1 to 24.7 Ma, and zircons from the leucosomes of the migmatites yield various U-Pb ages of 25.0-13.7 Ma. The former may be interpreted as the timing and duration of prograde metamorphism and partial melting, and the latter represents timescales of retrogressive metamorphism and melt crystallization.Therefore, it is proposed that the partial melting of the Higher Himalayan Crystalline Sequence in the Yadong region initiated at ca.30 Ma and lasted to ca.13 Ma, indicating that it is a long-term and sustained process. The study also indicates that the timing of partial melting of the Higher Himalayan Crystalline Sequence is earlier than the starting activation of the South Tibetan detachment and the Main Central Thrust. This may further indicates that the partial melting occurred during subduction. The study provides new information on the structural evolution model of the orogen.
The Higher Himalayan Crystalline Sequence is composed mainly of middle-high grade metamorphic rocks and leucogranites, forming a natural laboratory for studying the formation and evolution of the Himalayan orogen. However, the geological significance of the U-Pb ages remains controversial because zircons and monazites of the migmatites and leucogranites from the Higher Himalayan Crystalline Sequence commonly have yielded variable U-Pb ages, which significantly restricts our understanding of timing and duration of metamorphism, partial melting and melt crystallization of the orogen.Here it presents zircon U-Pb geochronological study of the Naiduila migmatites in Yadong region from the upper structural level of the Higher Himalayan Crystalline Sequence, middle Himalaya. The results show that zircons from the melanosomes of the migmatites in Naiduila area have variable U-Pb ages ranging from 29.1 to 24.7 Ma, and zircons from the leucosomes of the migmatites yield various U-Pb ages of 25.0-13.7 Ma. The former may be interpreted as the timing and duration of prograde metamorphism and partial melting, and the latter represents timescales of retrogressive metamorphism and melt crystallization.Therefore, it is proposed that the partial melting of the Higher Himalayan Crystalline Sequence in the Yadong region initiated at ca.30 Ma and lasted to ca.13 Ma, indicating that it is a long-term and sustained process. The study also indicates that the timing of partial melting of the Higher Himalayan Crystalline Sequence is earlier than the starting activation of the South Tibetan detachment and the Main Central Thrust. This may further indicates that the partial melting occurred during subduction. The study provides new information on the structural evolution model of the orogen.
2020, 45(8): 2905-2919.
doi: 10.3799/dqkx.2020.221
Abstract:
As one of the important suture belts in the Sanjiang Tethyan tectonic domain, the Jinshajiang suture belt has long been the focus of study on the process of Paleo-Tethys ocean-continent subduction and collision. However, controversies remain about the exact time for the collision and closure of the Jinshajiang Paleo-Tethys. In this paper, the Gonjo granite body was selected as the research object, which is located on the west side of the Jinshajiang suture belt in the eastern Tibet. Based on the chronological, geochemical and Hf isotope analyses of the large-scale exposed quartz monzonite, the geochemical characteristics and tectonic setting of the Gonjo granite body are revealed. LA-ICP-MS zircon U-Pb dating results yield a weighted mean 206Pb/238U age of 231±1 Ma for the sample, indicating that quartz monzonite was emplaced in Late Triassic. Zircon from the quartz monzonite exhibits a narrow range of initial Hf isotope ratios (εHf(t)=-8.3 to -5.5), with corresponding TDMC of 1 611-1 788 Ma.Geochemical studies show that the quartz monzonite is typical I-type granite, belonging to shoshonite and high K calc-alkali series, enriched in Rb, Th, K, and LILEs, but depleted in Nb, Ta, Zr, Hf and HFSEs. In addition, geochemical features show that the quartz monzonite was formed in the geological background of continent collision, suggesting that the Gonjo area was in a transition phase of syn-collision compressional to post-collisional extension stage in the Early Triassic (~231 Ma) and the quartz monzonite was a product of partial melting of lower crustal intermediate-basic igneous rocks. Combined with previous studies, it can be proposed that the Paleo-Tethys ocean of Jinshajiang suture was gradually closed from south to north, and the differences of regional geological background and the complexity of the Paleo-Tethys ocean oblique collision are the main reasons for the different collision-closure time of different regions of the Jinshajiang suture belt.
As one of the important suture belts in the Sanjiang Tethyan tectonic domain, the Jinshajiang suture belt has long been the focus of study on the process of Paleo-Tethys ocean-continent subduction and collision. However, controversies remain about the exact time for the collision and closure of the Jinshajiang Paleo-Tethys. In this paper, the Gonjo granite body was selected as the research object, which is located on the west side of the Jinshajiang suture belt in the eastern Tibet. Based on the chronological, geochemical and Hf isotope analyses of the large-scale exposed quartz monzonite, the geochemical characteristics and tectonic setting of the Gonjo granite body are revealed. LA-ICP-MS zircon U-Pb dating results yield a weighted mean 206Pb/238U age of 231±1 Ma for the sample, indicating that quartz monzonite was emplaced in Late Triassic. Zircon from the quartz monzonite exhibits a narrow range of initial Hf isotope ratios (εHf(t)=-8.3 to -5.5), with corresponding TDMC of 1 611-1 788 Ma.Geochemical studies show that the quartz monzonite is typical I-type granite, belonging to shoshonite and high K calc-alkali series, enriched in Rb, Th, K, and LILEs, but depleted in Nb, Ta, Zr, Hf and HFSEs. In addition, geochemical features show that the quartz monzonite was formed in the geological background of continent collision, suggesting that the Gonjo area was in a transition phase of syn-collision compressional to post-collisional extension stage in the Early Triassic (~231 Ma) and the quartz monzonite was a product of partial melting of lower crustal intermediate-basic igneous rocks. Combined with previous studies, it can be proposed that the Paleo-Tethys ocean of Jinshajiang suture was gradually closed from south to north, and the differences of regional geological background and the complexity of the Paleo-Tethys ocean oblique collision are the main reasons for the different collision-closure time of different regions of the Jinshajiang suture belt.
2020, 45(8): 2920-2931.
doi: 10.3799/dqkx.2020.211
Abstract:
The Gonjo Triassic volcanic rocks in the northern area of the Nujiang-Lancangjiang-Jinshajiang are the products of subduction of the Jinshajiang Paleo-Tethyan Ocean to the west and subsequent arc-continent collision-orogeny effect. It is mainly composed of basalt, andesite fused tuff and andesite, etc.The study of Gonjo volcanic rocks is of great significance for the exploration of the Jinshajiang Paleo-Tethyan tectonic-magmatic evolution process. In this paper, it presents experiments on Gonjo volcanic rocks of LA-ICP-MS zircon U-Pb dating and master-trace element research. The LA-ICP-MS zircon U-Pb dating of representative samples of andesite fused tuff shows that the age of 206Pb/238U of 23 analysis points of zircon is 243-233 Ma. The LA-ICP-MS zircon U-Pb age of the representative sample of andesite fused tuff is 238.3±1.3 Ma, which indicates that the magmatic crystallization age is Middle-Late Triassic. Geochemical analysis shows that volcanic rocks contains low titanium (0.52%-0.91%), medium magnesium (3.38%-7.22%), rich sodium (3.63%-4.66%), depleted potassium (0.09%-4.05%), enriched light rare earth elements (LREE/HREE=4.09-13.27) and large ionic lithophilic elements (Th, U, La, Sm), deficient high field strength elements Nb and Ta (Nb=2.51×10-6-24.5×10-6, Ta=0.22×10-6-1.89×10-6), relatively high La/Nb (2.13-2.89) ratio and relatively low Nb/Th (1.12-1.62), which possesses the composition characteristics of main-trace elements similar to island arc or late collision orogenic volcanic rocks. It is concluded that the Middle-Late Triassic Gonjo volcanic rocks represent the magmatic response of the Jinshajiang during the evolution of Paleo-Tethys, which is probably the result of the interaction between the Jinshajiang oceanic crust material and the lithospheric mantle material remaining during the late collision.
The Gonjo Triassic volcanic rocks in the northern area of the Nujiang-Lancangjiang-Jinshajiang are the products of subduction of the Jinshajiang Paleo-Tethyan Ocean to the west and subsequent arc-continent collision-orogeny effect. It is mainly composed of basalt, andesite fused tuff and andesite, etc.The study of Gonjo volcanic rocks is of great significance for the exploration of the Jinshajiang Paleo-Tethyan tectonic-magmatic evolution process. In this paper, it presents experiments on Gonjo volcanic rocks of LA-ICP-MS zircon U-Pb dating and master-trace element research. The LA-ICP-MS zircon U-Pb dating of representative samples of andesite fused tuff shows that the age of 206Pb/238U of 23 analysis points of zircon is 243-233 Ma. The LA-ICP-MS zircon U-Pb age of the representative sample of andesite fused tuff is 238.3±1.3 Ma, which indicates that the magmatic crystallization age is Middle-Late Triassic. Geochemical analysis shows that volcanic rocks contains low titanium (0.52%-0.91%), medium magnesium (3.38%-7.22%), rich sodium (3.63%-4.66%), depleted potassium (0.09%-4.05%), enriched light rare earth elements (LREE/HREE=4.09-13.27) and large ionic lithophilic elements (Th, U, La, Sm), deficient high field strength elements Nb and Ta (Nb=2.51×10-6-24.5×10-6, Ta=0.22×10-6-1.89×10-6), relatively high La/Nb (2.13-2.89) ratio and relatively low Nb/Th (1.12-1.62), which possesses the composition characteristics of main-trace elements similar to island arc or late collision orogenic volcanic rocks. It is concluded that the Middle-Late Triassic Gonjo volcanic rocks represent the magmatic response of the Jinshajiang during the evolution of Paleo-Tethys, which is probably the result of the interaction between the Jinshajiang oceanic crust material and the lithospheric mantle material remaining during the late collision.
2020, 45(8): 2932-2944.
doi: 10.3799/dqkx.2020.103
Abstract:
In recent years,great progress has been made in the exploration of graphite deposits from Leiwuqi-Zuogong metallogenic belt,Tibet. The Diguo graphite deposit is one of the three large-scale graphite deposits which have been discovered recently. However,the geological background,genetic type and mineralization mechanism are still not clear,which has restricted the resources potential assessment of the metallogenic belt. Field geological survey,zircon U-Pb,carbon isotope analysis,and scanning electron microscopy (SEM) have been done for the Diguo deposit. Zircon U-Pb dating results show that the age of the ore-bearing monzogranite is 239.1 ±1.8 Ma,and the ages of the inherit zircon are concentrated in the Early Carboniferous (347-326 Ma). The carbon δ13C isotopic values reveal the organic origin of the carbon in the ore. This study shows that Diguo graphite deposit was constrained by both carbonous strata and the Middle Triassic intrusions,and belongs to the complex type which is the mix of magmatic type and contact metamorphic type. The emplacement time of the ore-bearing monzogranite represents the mineralization age of the graphite deposit approximately,which is the Middle Triassic. The regional metallogenic conditions show that the Leiwuqi-Zuogong metallogenic belt has a good exploration potential for the graphite deposits,the connection zone between the Middle Triassic intrusion and the carbonous Kagong stratum might be the key areas of the prospecting breakthrough.
In recent years,great progress has been made in the exploration of graphite deposits from Leiwuqi-Zuogong metallogenic belt,Tibet. The Diguo graphite deposit is one of the three large-scale graphite deposits which have been discovered recently. However,the geological background,genetic type and mineralization mechanism are still not clear,which has restricted the resources potential assessment of the metallogenic belt. Field geological survey,zircon U-Pb,carbon isotope analysis,and scanning electron microscopy (SEM) have been done for the Diguo deposit. Zircon U-Pb dating results show that the age of the ore-bearing monzogranite is 239.1 ±1.8 Ma,and the ages of the inherit zircon are concentrated in the Early Carboniferous (347-326 Ma). The carbon δ13C isotopic values reveal the organic origin of the carbon in the ore. This study shows that Diguo graphite deposit was constrained by both carbonous strata and the Middle Triassic intrusions,and belongs to the complex type which is the mix of magmatic type and contact metamorphic type. The emplacement time of the ore-bearing monzogranite represents the mineralization age of the graphite deposit approximately,which is the Middle Triassic. The regional metallogenic conditions show that the Leiwuqi-Zuogong metallogenic belt has a good exploration potential for the graphite deposits,the connection zone between the Middle Triassic intrusion and the carbonous Kagong stratum might be the key areas of the prospecting breakthrough.
2020, 45(8): 2945-2956.
doi: 10.3799/dqkx.2020.117
Abstract:
The lacustrine paleo-seismic study is an important supplement to the research of a complete paleo-seismic sequence. A large number of earthquake-induced soft-sediment deformation structures (seismites) were newly discovered in the Holocene lacustrine deposits in Xuru Co area by detailed field geological survey. Soft-sediment deformation markers mainly include liquefied vein,liquefied convolute,liquefied breccia,hydraulic structure,liqufied droplets and cusps,conglomerate mound,load structure and flance structure. The syn-seismic structure deformation markers were also developed including syn-seismic fault,seismic fissure,and syn-fold. Based on the empirical statistical relation between the earthquake magnitude and soft-sediment deformation markers,combining historical earthquake statistics with the range of liquefied particles,it is suggested that the maximum magnitude may exceed 7.5. The C14 and OSL dating results for samples suggest that the age of the paleoearthquake is around 7.5 ka. The work fills the gap of historical earthquakes in the area,and provides material for restoring the history and migration law of earthquakes in the NS-trending grabens of the Tibetan Plateau. A large amount of gravel liquefaction was found in seismite,which poses a new challenge to the investigation of sand liquefaction,which is dominated by sand and silt.
The lacustrine paleo-seismic study is an important supplement to the research of a complete paleo-seismic sequence. A large number of earthquake-induced soft-sediment deformation structures (seismites) were newly discovered in the Holocene lacustrine deposits in Xuru Co area by detailed field geological survey. Soft-sediment deformation markers mainly include liquefied vein,liquefied convolute,liquefied breccia,hydraulic structure,liqufied droplets and cusps,conglomerate mound,load structure and flance structure. The syn-seismic structure deformation markers were also developed including syn-seismic fault,seismic fissure,and syn-fold. Based on the empirical statistical relation between the earthquake magnitude and soft-sediment deformation markers,combining historical earthquake statistics with the range of liquefied particles,it is suggested that the maximum magnitude may exceed 7.5. The C14 and OSL dating results for samples suggest that the age of the paleoearthquake is around 7.5 ka. The work fills the gap of historical earthquakes in the area,and provides material for restoring the history and migration law of earthquakes in the NS-trending grabens of the Tibetan Plateau. A large amount of gravel liquefaction was found in seismite,which poses a new challenge to the investigation of sand liquefaction,which is dominated by sand and silt.
2020, 45(8): 2957-2963.
doi: 10.3799/dqkx.2020.077
Abstract:
The study of the Late Permian-Triassic strata of the Lhasa Block is significant to understanding pre-Cenozoic evolution of Tethys in the Qinghai-Tibet Plateau. However,there are still disputes on the sedimentary sequence and paleogeographic environment in this period. On the basis of the detailed analyses of the stratigraphy and conodonts from the Konglong area,Ngamring County,Tibet,we found the Late Triassic Norian conodonts Epigondolella sp. from the upper part of the Mujiucuo Formation,and the Late Permian Changhsingian to Early Triassic Induan conodont Hindeodus typicalis bearing in the top of the Xiala Formation. The age of the Mujiucuo Formation in the study area was redefined to the Late Permian Changhsingian to Late Triassic Norian,indicating that the paleogeographic environment should still be a continuous neritic facies from Late Permian to Late Triassic in western Lhasa Block.
The study of the Late Permian-Triassic strata of the Lhasa Block is significant to understanding pre-Cenozoic evolution of Tethys in the Qinghai-Tibet Plateau. However,there are still disputes on the sedimentary sequence and paleogeographic environment in this period. On the basis of the detailed analyses of the stratigraphy and conodonts from the Konglong area,Ngamring County,Tibet,we found the Late Triassic Norian conodonts Epigondolella sp. from the upper part of the Mujiucuo Formation,and the Late Permian Changhsingian to Early Triassic Induan conodont Hindeodus typicalis bearing in the top of the Xiala Formation. The age of the Mujiucuo Formation in the study area was redefined to the Late Permian Changhsingian to Late Triassic Norian,indicating that the paleogeographic environment should still be a continuous neritic facies from Late Permian to Late Triassic in western Lhasa Block.
2020, 45(8): 2964-2977.
doi: 10.3799/dqkx.2020.085
Abstract:
In the aftermath of the Permian-Triassic mass extinction during the Paleozoic-Mesozoic transition,Triassic oceans were characterized by the long-term and complex biological and environmental changes. Here it reports a stratigraphic study of the Tulong section using ammonites,conodonts and stable carbon isotope in South Tibet. The carbon isotope curves show six negative shifts and five positive peaks from earliest Triassic to middle Late Triassic. Within these carbon isotopic perturbations,the four negative and four positive shifts of the Early Triassic are well correlative with the profiles of South China and Japan,suggesting that the studied area records global signals for multi-phase environmental and biotic changes. Meanwhile,a distinct negative carbon isotope shift,with a decrease of 3.3‰,occurred in the Late Triassic of the Tulong section,is indicative to the Carnian climatic event,which has been extensively found in Italy,Japan and South China. This study presents a successive carbon isotope curve with few biostratigraphic records throughout the Triassic,firstly reporting the Carnian climatic event in Tibet. The rapid temperature rise in the humid climate,intensive weathering and strong terrestrial input might contribute to the distinct lithological changes of the Middle Carnian in the study area.
In the aftermath of the Permian-Triassic mass extinction during the Paleozoic-Mesozoic transition,Triassic oceans were characterized by the long-term and complex biological and environmental changes. Here it reports a stratigraphic study of the Tulong section using ammonites,conodonts and stable carbon isotope in South Tibet. The carbon isotope curves show six negative shifts and five positive peaks from earliest Triassic to middle Late Triassic. Within these carbon isotopic perturbations,the four negative and four positive shifts of the Early Triassic are well correlative with the profiles of South China and Japan,suggesting that the studied area records global signals for multi-phase environmental and biotic changes. Meanwhile,a distinct negative carbon isotope shift,with a decrease of 3.3‰,occurred in the Late Triassic of the Tulong section,is indicative to the Carnian climatic event,which has been extensively found in Italy,Japan and South China. This study presents a successive carbon isotope curve with few biostratigraphic records throughout the Triassic,firstly reporting the Carnian climatic event in Tibet. The rapid temperature rise in the humid climate,intensive weathering and strong terrestrial input might contribute to the distinct lithological changes of the Middle Carnian in the study area.
2020, 45(8): 2978-2988.
doi: 10.3799/dqkx.2020.166
Abstract:
Retrograde eclogite was newly discovered in Jinshajiang suture zone in Ludian area of West Yunnan Province,which occurs as lenses in garnet muscovite quartz schist. The EPMA and LRS analyses reveal that there were residual omphacite inclusions in garnet and zircon. The muscovites developed in garnet and matrix are phengite(Si(p.f.u)=3.27-3.53),indicating that the rock has undergone a high pressure process of metamorphism. Garnets in this rock have preserved prograde metamorphic zonation. Petrographic and mineral chemical characteristics indicate that the retrograde eclogite has roughly undergone 4 generations of mineral assemblages including the progressive amphibolite facies mineral assemblage,the peak phase eclogite facies mineral assemblage,the early retrograde metamorphic mineral assemblage and the late strongly retrograde mineral assemblage. The typical paragenetic association of minerals of each stage is Grt+AmpI+Qtz,Grt+Omp+Rt+Qtz+Phe,Pl+Di+AmpⅡ+Ilm+Spn+Qtz,AmpⅢ+Pl+Czo+Ilm+Qtz,respectively. These new findings will provide important evidences and more restraints to the key scientific problems such as the complex P-T-t path style of metamorphic evolution and chronostratigraphic framework in Jinshajiang suture zone,the complex tectonic evolution history of Paleo-Tethys ocean of subduction,collision,orogenesis,and the collage mechanism of microcontinent and so on. It has important scientific significance.
Retrograde eclogite was newly discovered in Jinshajiang suture zone in Ludian area of West Yunnan Province,which occurs as lenses in garnet muscovite quartz schist. The EPMA and LRS analyses reveal that there were residual omphacite inclusions in garnet and zircon. The muscovites developed in garnet and matrix are phengite(Si(p.f.u)=3.27-3.53),indicating that the rock has undergone a high pressure process of metamorphism. Garnets in this rock have preserved prograde metamorphic zonation. Petrographic and mineral chemical characteristics indicate that the retrograde eclogite has roughly undergone 4 generations of mineral assemblages including the progressive amphibolite facies mineral assemblage,the peak phase eclogite facies mineral assemblage,the early retrograde metamorphic mineral assemblage and the late strongly retrograde mineral assemblage. The typical paragenetic association of minerals of each stage is Grt+AmpI+Qtz,Grt+Omp+Rt+Qtz+Phe,Pl+Di+AmpⅡ+Ilm+Spn+Qtz,AmpⅢ+Pl+Czo+Ilm+Qtz,respectively. These new findings will provide important evidences and more restraints to the key scientific problems such as the complex P-T-t path style of metamorphic evolution and chronostratigraphic framework in Jinshajiang suture zone,the complex tectonic evolution history of Paleo-Tethys ocean of subduction,collision,orogenesis,and the collage mechanism of microcontinent and so on. It has important scientific significance.
2020, 45(8): 2989-3002.
doi: 10.3799/dqkx.2020.076
Abstract:
The pelagic abyssal sediments in orogenic belt are important basis for the restoration of the ancient ocean. The Changning-Menglian suture zone preserves a large number of marine sedimentary strata fragments,but it is not clear whether there are ocean basin facies pelagic sediments. Petrology,radiolarian age and detrital zircon U-Pb-Hf isotopes of the Carboniferous and Devonian fragments,which were thought to be the ocean basin facies sediments in the Nongba area,are studied in this paper. Early Carboniferous radiolarian fossils have been identified in the Carboniferous fragments. The detrital zircon ages of the Devonian fragment range from 3 266 Ma to 387 Ma,with the youngest group of 387-413 Ma. The detrital zircon ages of the basic tuff associated with radiolarian chert in the Carboniferous fragments range from 3 403 Ma to 341 Ma,and the youngest group is 341-354 Ma. Based on the detrital zircon ages and fossil data,it can be concluded that the primary depositional age of the Devonian fragments is limited to the Early-Middle Devonian,and that of the Carboniferous fragment is early-middle Early Carboniferous. The U-Pb age spectrum and Hf isotopic composition of detrital zircons indicate that Devonian and Carboniferous fragments have similar provenance,mainly from the Gondwana-related continental crust,and a small amount from the Paleo-Tethyan island arc. The Early-Middle Devonian fragments were originally deposited in the continental slope environment of Gondwana,while the early-middle Early Carboniferous fragments were originally deposited in the Gondwana-related continental slope to the edge of Paleo-Tethys ocean basin environment. It is one of the important research directions of Paleo-Tethys to find and study the ocean basin facies sedimentary rocks in the Changning-Menglian belt.
The pelagic abyssal sediments in orogenic belt are important basis for the restoration of the ancient ocean. The Changning-Menglian suture zone preserves a large number of marine sedimentary strata fragments,but it is not clear whether there are ocean basin facies pelagic sediments. Petrology,radiolarian age and detrital zircon U-Pb-Hf isotopes of the Carboniferous and Devonian fragments,which were thought to be the ocean basin facies sediments in the Nongba area,are studied in this paper. Early Carboniferous radiolarian fossils have been identified in the Carboniferous fragments. The detrital zircon ages of the Devonian fragment range from 3 266 Ma to 387 Ma,with the youngest group of 387-413 Ma. The detrital zircon ages of the basic tuff associated with radiolarian chert in the Carboniferous fragments range from 3 403 Ma to 341 Ma,and the youngest group is 341-354 Ma. Based on the detrital zircon ages and fossil data,it can be concluded that the primary depositional age of the Devonian fragments is limited to the Early-Middle Devonian,and that of the Carboniferous fragment is early-middle Early Carboniferous. The U-Pb age spectrum and Hf isotopic composition of detrital zircons indicate that Devonian and Carboniferous fragments have similar provenance,mainly from the Gondwana-related continental crust,and a small amount from the Paleo-Tethyan island arc. The Early-Middle Devonian fragments were originally deposited in the continental slope environment of Gondwana,while the early-middle Early Carboniferous fragments were originally deposited in the Gondwana-related continental slope to the edge of Paleo-Tethys ocean basin environment. It is one of the important research directions of Paleo-Tethys to find and study the ocean basin facies sedimentary rocks in the Changning-Menglian belt.
2020, 45(8): 3003-3013.
doi: 10.3799/dqkx.2020.141
Abstract:
A large number of records of magmatic activities and sedimentary strata in the Late Paleozoic have been obtained in the Changning-Menglian suture zone,but the records of Early Paleozoic are very rare,which greatly limits understanding of the Early Paleozoic evolution process in this area. In this paper,the zircon U-Pb age and geochemical characteristics of the Early Paleozoic tonalite are reported in the Nantinghe area,and its petrogenesis is discussed to reveal the evolution process of the Early Paleozoic Tethys. LA-ICPMS zircon U-Pb dating shows that the weighted mean 206Pb/238U ages are 454.1±2.7 Ma and 443.2±2.4 Ma for tonalite,representing the crystallization age of magmatic rocks. Geochemical analyses of the tonalite show that its SiO2 content is 63.41%-68.57% and the K2O content is 1.31%-2.17%,and it is rich in Na (Na2O=4.11%-5.07%) and poor in Mg (MgO=0.98%-1.39%). The REE distribution curve shows the enrichment of LREE and the depletion of HREE,large ion lithophile elements Sr,Ba enrichment,high field strength elements Nb,Ta,Ti depletion,which is consistent with the characteristics of island arc magma,indicating that they were derived from the partial melting of subduction oceanic slab and was contaminated by subduction sediments in the magma rising process,and formed in the arc volcanic environment related to the subduction of the original Tethys ocean,further indicating that the subduction of the proto-Tethys Ocean occurred in 454-443 Ma. The chronology and geochemsitry characteristics of the Nantinghe ophiolite in Changning-Menglian suture zone can be well correlated with the Taoxing Lake ophiolite in the Longmuco-Shuanghu suture zone of Qiangtang area. Thus,it is suggested that both the Changning-Menglian and Longmuco-Shuanghu sutures probably represent the original of a unified paleo-Tethys oceanic crust.
A large number of records of magmatic activities and sedimentary strata in the Late Paleozoic have been obtained in the Changning-Menglian suture zone,but the records of Early Paleozoic are very rare,which greatly limits understanding of the Early Paleozoic evolution process in this area. In this paper,the zircon U-Pb age and geochemical characteristics of the Early Paleozoic tonalite are reported in the Nantinghe area,and its petrogenesis is discussed to reveal the evolution process of the Early Paleozoic Tethys. LA-ICPMS zircon U-Pb dating shows that the weighted mean 206Pb/238U ages are 454.1±2.7 Ma and 443.2±2.4 Ma for tonalite,representing the crystallization age of magmatic rocks. Geochemical analyses of the tonalite show that its SiO2 content is 63.41%-68.57% and the K2O content is 1.31%-2.17%,and it is rich in Na (Na2O=4.11%-5.07%) and poor in Mg (MgO=0.98%-1.39%). The REE distribution curve shows the enrichment of LREE and the depletion of HREE,large ion lithophile elements Sr,Ba enrichment,high field strength elements Nb,Ta,Ti depletion,which is consistent with the characteristics of island arc magma,indicating that they were derived from the partial melting of subduction oceanic slab and was contaminated by subduction sediments in the magma rising process,and formed in the arc volcanic environment related to the subduction of the original Tethys ocean,further indicating that the subduction of the proto-Tethys Ocean occurred in 454-443 Ma. The chronology and geochemsitry characteristics of the Nantinghe ophiolite in Changning-Menglian suture zone can be well correlated with the Taoxing Lake ophiolite in the Longmuco-Shuanghu suture zone of Qiangtang area. Thus,it is suggested that both the Changning-Menglian and Longmuco-Shuanghu sutures probably represent the original of a unified paleo-Tethys oceanic crust.
2020, 45(8): 3014-3027.
doi: 10.3799/dqkx.2020.093
Abstract:
The Middle Triassic strata in the Changning-Menglian suture zone record the evolution of the Paleo-Tethys Ocean in the late stage, providing an important window for the better understanding of the structure and geography of the Paleo-Tethys. The original Devonian Manxin Formation composed of siliceous, mudstone and sandstone interlayer (lens bodies) in Tuanjie area, Cangyuan County, Yunnan Province. The zircon U-Pb dating of arkose and geochemical analysis of siliceous rocks were carried out, respectively, and the significance of the Paleo-Tethys evolution was explored on the basis of limiting the formation age. Zircon U-Pb dating of arkose lens bodies and interlayers shows that their ages are not earlier than 237.8±1.3 Ma and 237.7±1.4 Ma, respectively, the stratum can be compared with the Muyinhe Formation. The discovery of the Middle Triassic stratum fills the blank of the sedimentary records in this area. Geochemical analysis of the siliceous rock in this stratum indicates that it was formed in the continental marginal environment. Combining with composition of granule in sedimentary rock and para-mineral characteristics, it shows that the scale of the residual oceanic basin of Paleo-Tethys is limited. The angular inconsistency of the molassic-type deposition of the Sanchahe Formation above the early stratum indicates the final closure of the residual ocean basin.
The Middle Triassic strata in the Changning-Menglian suture zone record the evolution of the Paleo-Tethys Ocean in the late stage, providing an important window for the better understanding of the structure and geography of the Paleo-Tethys. The original Devonian Manxin Formation composed of siliceous, mudstone and sandstone interlayer (lens bodies) in Tuanjie area, Cangyuan County, Yunnan Province. The zircon U-Pb dating of arkose and geochemical analysis of siliceous rocks were carried out, respectively, and the significance of the Paleo-Tethys evolution was explored on the basis of limiting the formation age. Zircon U-Pb dating of arkose lens bodies and interlayers shows that their ages are not earlier than 237.8±1.3 Ma and 237.7±1.4 Ma, respectively, the stratum can be compared with the Muyinhe Formation. The discovery of the Middle Triassic stratum fills the blank of the sedimentary records in this area. Geochemical analysis of the siliceous rock in this stratum indicates that it was formed in the continental marginal environment. Combining with composition of granule in sedimentary rock and para-mineral characteristics, it shows that the scale of the residual oceanic basin of Paleo-Tethys is limited. The angular inconsistency of the molassic-type deposition of the Sanchahe Formation above the early stratum indicates the final closure of the residual ocean basin.
2020, 45(8): 3028-3039.
doi: 10.3799/dqkx.2020.143
Abstract:
The combination of subsurface geology and deep geophysics data is a new tendency and method for the mineralizing prognosis of deep zone in the three-dimensional space, of which the results can deepen the cognitions of subsurface feature and reduce risks from multiplicities of geophysical interpretations. Focused on the Beiya gold deposit, 3D visualization of Wandongshan key ore sections was realized by using the GOCAD modeling platform based on the geological, geochemical, and deep geophysical data in this study. Furthermore, metallogenetic regularity and deep metallogenic potential of geological bodies were analyzed, and three favorable factors were screened out for mineralization including hidden faults, beschtauite, and Qingtianbao sandstone.In addition, corresponding sgrid was established. Comprehensive multi-source information synthesis method was adopted to locate deep buried ore body of Wandongshan ore section with an altitude between 1 100-900 m via obtaining results from intersections of three favorable areas for mineralization. This study provides an important reference for the prediction of deep target area in this belt as well as in similar type of ore sections in the surrounding areas.
The combination of subsurface geology and deep geophysics data is a new tendency and method for the mineralizing prognosis of deep zone in the three-dimensional space, of which the results can deepen the cognitions of subsurface feature and reduce risks from multiplicities of geophysical interpretations. Focused on the Beiya gold deposit, 3D visualization of Wandongshan key ore sections was realized by using the GOCAD modeling platform based on the geological, geochemical, and deep geophysical data in this study. Furthermore, metallogenetic regularity and deep metallogenic potential of geological bodies were analyzed, and three favorable factors were screened out for mineralization including hidden faults, beschtauite, and Qingtianbao sandstone.In addition, corresponding sgrid was established. Comprehensive multi-source information synthesis method was adopted to locate deep buried ore body of Wandongshan ore section with an altitude between 1 100-900 m via obtaining results from intersections of three favorable areas for mineralization. This study provides an important reference for the prediction of deep target area in this belt as well as in similar type of ore sections in the surrounding areas.
2020, 45(8): 3040-3053.
doi: 10.3799/dqkx.2020.095
Abstract:
Discovery of ancient zircons and rocks is the key to exploring the early geological evolution of the earth. In order to further decipher the basement material composition and early crustal formation and evolution of the Yangtze Block, it conducted the chronological study on the metamorphic sandstone of the Dongchuan Group in the Lufeng area of southwestern Yangtze Block with LA-ICP-MS zircon U-Pb dating, and the detrital zircon of 3 822±21 Ma was found. This is the second zircon with the age older than 3.8 Ga in the Yangtze Block, and it's also the oldest age record that has been found in the southwestern Yangtze Block. The clastic material of the metasandstone mainly includes four age peaks (~2 320 Ma, ~2 162 Ma, ~2 036 Ma and~1 915 Ma). Two youngest zircons basically limit the maximum deposition age of the Dongchuan Group, which coincides with the volcanic rock age of Yinmin Formation in the region. In addition, it also contains a small amount of clastic materials with the ages of 2.6-2.9 Ga and 3.7-3.8 Ga. The Hf isotopic composition shows that these detrital zircons have different origins. The clastic zircons with the age of 2 674-3 822 Ma have positive εHf (t) values and two-stage Hf model age of 2.9-3.9 Ga, suggesting a certain scale of juvenile crust distribution within the Yangtze Block during the Hadean-Archean period. The Paleoproterozoic (1.9-2.4 Ga) magmatic activities were characterized by partial melting of the Archean (2.5-3.7 Ga) ancient crust, except for a small amount of melting of the Paleoproterozoic (2.3-2.4 Ga) juvenile crust. The Mesoproterozoic is more manifested in the melting and recycling of ancient crust. This study provides new information for deepening the understanding of the early geological evolution of the Yangtze Block.
Discovery of ancient zircons and rocks is the key to exploring the early geological evolution of the earth. In order to further decipher the basement material composition and early crustal formation and evolution of the Yangtze Block, it conducted the chronological study on the metamorphic sandstone of the Dongchuan Group in the Lufeng area of southwestern Yangtze Block with LA-ICP-MS zircon U-Pb dating, and the detrital zircon of 3 822±21 Ma was found. This is the second zircon with the age older than 3.8 Ga in the Yangtze Block, and it's also the oldest age record that has been found in the southwestern Yangtze Block. The clastic material of the metasandstone mainly includes four age peaks (~2 320 Ma, ~2 162 Ma, ~2 036 Ma and~1 915 Ma). Two youngest zircons basically limit the maximum deposition age of the Dongchuan Group, which coincides with the volcanic rock age of Yinmin Formation in the region. In addition, it also contains a small amount of clastic materials with the ages of 2.6-2.9 Ga and 3.7-3.8 Ga. The Hf isotopic composition shows that these detrital zircons have different origins. The clastic zircons with the age of 2 674-3 822 Ma have positive εHf (t) values and two-stage Hf model age of 2.9-3.9 Ga, suggesting a certain scale of juvenile crust distribution within the Yangtze Block during the Hadean-Archean period. The Paleoproterozoic (1.9-2.4 Ga) magmatic activities were characterized by partial melting of the Archean (2.5-3.7 Ga) ancient crust, except for a small amount of melting of the Paleoproterozoic (2.3-2.4 Ga) juvenile crust. The Mesoproterozoic is more manifested in the melting and recycling of ancient crust. This study provides new information for deepening the understanding of the early geological evolution of the Yangtze Block.
2020, 45(8): 3054-3069.
doi: 10.3799/dqkx.2020.199
Abstract:
The Cuoke Complex, a newly identified Early Precambrian basement complex in central Yunnan, bears key information for the early evolution of the Yangtze Block. Here, we present zircon U-Pb geochronological and Hf isotopic data of four representative samples collected from this complex. Magmatic zircons of one trondjemitic gneiss sample yield a crystallization age of 2 845±33 Ma, with positive εHf(t) values of 1.7-4.6 and relatively young depleted mantle two stage (TDM2) model ages of 2.97-3.12 Ga, suggestive of the derivation from remelting of juvenile crust. Meta-monzogranite and gneissic granite yield ages of 2 401±15 Ma and 2 320±16 Ma respectively. They show negative zircon εHf(t) values of -6.2 to -0.8 and older TDM2 model ages of 2.90-3.11 Ga, indicating that they were formed by remelting of ancient crustal materials. Metamorphic-origin zircons of one plagioclase-biotite cataclasite sample give an age of 1 948±16 Ma. Together with available metamorphic age data, it is revealed that one episode of regional metamorphism occurred at 1.96-1.95 Ga. Our new dataset confirms the existence of Archean basement rocks in the Cuoke Complex. Moreover, multiple magmatic and metamorphic events related to the assembly of the Nuna supercontinent extensively took place in the southwestern Yangtze Block, South China.
The Cuoke Complex, a newly identified Early Precambrian basement complex in central Yunnan, bears key information for the early evolution of the Yangtze Block. Here, we present zircon U-Pb geochronological and Hf isotopic data of four representative samples collected from this complex. Magmatic zircons of one trondjemitic gneiss sample yield a crystallization age of 2 845±33 Ma, with positive εHf(t) values of 1.7-4.6 and relatively young depleted mantle two stage (TDM2) model ages of 2.97-3.12 Ga, suggestive of the derivation from remelting of juvenile crust. Meta-monzogranite and gneissic granite yield ages of 2 401±15 Ma and 2 320±16 Ma respectively. They show negative zircon εHf(t) values of -6.2 to -0.8 and older TDM2 model ages of 2.90-3.11 Ga, indicating that they were formed by remelting of ancient crustal materials. Metamorphic-origin zircons of one plagioclase-biotite cataclasite sample give an age of 1 948±16 Ma. Together with available metamorphic age data, it is revealed that one episode of regional metamorphism occurred at 1.96-1.95 Ga. Our new dataset confirms the existence of Archean basement rocks in the Cuoke Complex. Moreover, multiple magmatic and metamorphic events related to the assembly of the Nuna supercontinent extensively took place in the southwestern Yangtze Block, South China.
2020, 45(8): 3070-3081.
doi: 10.3799/dqkx.2020.153
Abstract:
The Liangjiehe Formation is the interglacial stratum between Chang'an and Gucheng glaciations in the southeastern Yangtze Block, and the lithology is dominated by detrital sandstone and quartz sandstone. For a better understanding of the temporal and spatial distribution of the Sturtian glacigenic strata in South China, it is important to determine the depositional age of the Liangjiehe Formation. In this study, systematical morphology and zircon U-Pb dating were carried out on detrital zircons from the Liangjiehe Formation in the eastern Guizhou. Most of the detrital zircons are of magmatic origin, and have a main 206Pb/238U age range of 740-900 Ma and minor populations of Paleoproterozoic-Archean ages, with peaks at~760 Ma, ~780 Ma, ~800 Ma, ~820 Ma and 880-900 Ma. The youngest single-grain zircon obtained at the bottom of the Liangjiehe Formation gives the age of 708±15 Ma, and the youngest single-grain zircon from the upper part gives the age of 703±22 Ma. Taking into account the topmost age~690 Ma of the contemporary stratum (Xieshuihe Formation), it is suggested that the Liangjiehe Formation was most likely deposited at 708-690 Ma. The detrital zircon U-Pb age spectrum of the Liangjiehe Formation records the Neoproterozoic episodic tectonic-magmatic events and the information for the early crustal evolution of Yangtze Block. The zircon morphology indicates that the provenances of the Liangjiehe Formation may include the underlying Neoproterozoic magmatic and sedimentary strata, and the basement rocks from the northwestern and southwestern Yangtze Block. The detrital zircon U-Pb dating at the bottom of the Liangjiehe Formation constrained the interglacial deposition later than~708 Ma. Considering that the stratigraphic distribution in the early Nanhua period is partly controlled by basin tectonic activities, the Chang'an glacial deposit may exist locally in eastern Guizhou.
The Liangjiehe Formation is the interglacial stratum between Chang'an and Gucheng glaciations in the southeastern Yangtze Block, and the lithology is dominated by detrital sandstone and quartz sandstone. For a better understanding of the temporal and spatial distribution of the Sturtian glacigenic strata in South China, it is important to determine the depositional age of the Liangjiehe Formation. In this study, systematical morphology and zircon U-Pb dating were carried out on detrital zircons from the Liangjiehe Formation in the eastern Guizhou. Most of the detrital zircons are of magmatic origin, and have a main 206Pb/238U age range of 740-900 Ma and minor populations of Paleoproterozoic-Archean ages, with peaks at~760 Ma, ~780 Ma, ~800 Ma, ~820 Ma and 880-900 Ma. The youngest single-grain zircon obtained at the bottom of the Liangjiehe Formation gives the age of 708±15 Ma, and the youngest single-grain zircon from the upper part gives the age of 703±22 Ma. Taking into account the topmost age~690 Ma of the contemporary stratum (Xieshuihe Formation), it is suggested that the Liangjiehe Formation was most likely deposited at 708-690 Ma. The detrital zircon U-Pb age spectrum of the Liangjiehe Formation records the Neoproterozoic episodic tectonic-magmatic events and the information for the early crustal evolution of Yangtze Block. The zircon morphology indicates that the provenances of the Liangjiehe Formation may include the underlying Neoproterozoic magmatic and sedimentary strata, and the basement rocks from the northwestern and southwestern Yangtze Block. The detrital zircon U-Pb dating at the bottom of the Liangjiehe Formation constrained the interglacial deposition later than~708 Ma. Considering that the stratigraphic distribution in the early Nanhua period is partly controlled by basin tectonic activities, the Chang'an glacial deposit may exist locally in eastern Guizhou.
2020, 45(8): 3082-3093.
doi: 10.3799/dqkx.2020.145
Abstract:
Although the evolution of the mid-Neoproterozoic basins in South China is closely related to the Rodinia supercontinent breakup, it still lacks fine characterization. In this study, detrital zircon LAICP-MS U-Pb dating and systematic sedimentary analyses were performed on the Chengjiang Formation in the western Yangtze Block. Most of the clastic zircon U-Pb ages in Chengjiang Formation are Neoproterozoic (870-780 Ma), and a few are pre-Neoproterozoic (2 850-1 010 Ma).The most significant peak of U-Pb ages in Chengjiang Formation is 820 Ma, and the youngest age population yields a weighted mean of 804.5±5.4 Ma. Taking into account the available age data, it is suggested that the deposition time of Chengjiang Formation is further limited to 800-720 Ma. The Neoproterozoic detrital zircons of Chengjiang Formation were mainly denuded from neighboring Neoproterozoic magmatic rocks. The pre-Neoproterozoic zircons may come from the denudation of pre-Neoproterozoic pluton or the sedimentary recycling of strata. Mid-Neoproterozoic sedimentary rocks on the western margin of the Yangtze have a sedimentary evolution sequence that is characterized by gradual transitions from alluvial fan facies to anterior fan delta facies, and finally forms a "wedge formation" with rift filling characteristics. The sedimentary overlap process is common throughout the South China rift system, indicating that South China entered the maturity stage of the rift basin at 800 Ma.
Although the evolution of the mid-Neoproterozoic basins in South China is closely related to the Rodinia supercontinent breakup, it still lacks fine characterization. In this study, detrital zircon LAICP-MS U-Pb dating and systematic sedimentary analyses were performed on the Chengjiang Formation in the western Yangtze Block. Most of the clastic zircon U-Pb ages in Chengjiang Formation are Neoproterozoic (870-780 Ma), and a few are pre-Neoproterozoic (2 850-1 010 Ma).The most significant peak of U-Pb ages in Chengjiang Formation is 820 Ma, and the youngest age population yields a weighted mean of 804.5±5.4 Ma. Taking into account the available age data, it is suggested that the deposition time of Chengjiang Formation is further limited to 800-720 Ma. The Neoproterozoic detrital zircons of Chengjiang Formation were mainly denuded from neighboring Neoproterozoic magmatic rocks. The pre-Neoproterozoic zircons may come from the denudation of pre-Neoproterozoic pluton or the sedimentary recycling of strata. Mid-Neoproterozoic sedimentary rocks on the western margin of the Yangtze have a sedimentary evolution sequence that is characterized by gradual transitions from alluvial fan facies to anterior fan delta facies, and finally forms a "wedge formation" with rift filling characteristics. The sedimentary overlap process is common throughout the South China rift system, indicating that South China entered the maturity stage of the rift basin at 800 Ma.
2020, 45(8): 3094-3103.
doi: 10.3799/dqkx.2020.098
Abstract:
The Luoping Biota is a famous Triassic marine fossil Lagerstätten in the world, and its age is assigned to Pelsonian, Anisian, Middle Triassic. Previously the fossils of the Luoping Biota were reported only from the area around the Dawazi, Jiuguang villages in the Luoping County. Here it reports the discovery of abundant fossils, including marine reptiles, fishes, athropods, echinoderms, mollusks and plants from Member Ⅱ of the Guanling Formation in Luxi County, Yunnan Province. The fossil assemblage in Luxi County is similar to that of the Luoping Biota. However, the placoid scales found in Luxi represent the first report of Chondrichthyes from the Luoping Biota. Conodont Nicoraella germanicus, Nicoraella cf. kockeli and Neogondolella? bifurcate, were recognized from the Baishuitang section, indicating an age of Plesonian, Anisian, Middle Triassic, which is equal to that of Luoping Biota. The fossil locality found in Luxi is of great significance in understanding the spatial distribution of the Luoping Biota. This work also provides new materials for studying the Middle Triassic Chondrichthyes in South China.
The Luoping Biota is a famous Triassic marine fossil Lagerstätten in the world, and its age is assigned to Pelsonian, Anisian, Middle Triassic. Previously the fossils of the Luoping Biota were reported only from the area around the Dawazi, Jiuguang villages in the Luoping County. Here it reports the discovery of abundant fossils, including marine reptiles, fishes, athropods, echinoderms, mollusks and plants from Member Ⅱ of the Guanling Formation in Luxi County, Yunnan Province. The fossil assemblage in Luxi County is similar to that of the Luoping Biota. However, the placoid scales found in Luxi represent the first report of Chondrichthyes from the Luoping Biota. Conodont Nicoraella germanicus, Nicoraella cf. kockeli and Neogondolella? bifurcate, were recognized from the Baishuitang section, indicating an age of Plesonian, Anisian, Middle Triassic, which is equal to that of Luoping Biota. The fossil locality found in Luxi is of great significance in understanding the spatial distribution of the Luoping Biota. This work also provides new materials for studying the Middle Triassic Chondrichthyes in South China.
2020, 45(8): 3104-3118.
doi: 10.3799/dqkx.2020.114
Abstract:
The Luoping Biota is typical of the full recovery of the marine ecosystem after the end-Permian mass extinction. In order to more fully understand the sedimentary environment background of the Luoping Biota, the outcrop observation and microfacies analysis were used to study the sedimentary environment of Member Ⅱ of the Middle Triassic Guanling Formation in the Yishida section of Luoping County, Yunnan Province. Five sedimentary facies types are recognized:open platform, restricted platform (lagoon), tidal flat, intra-platform basin, and intra-platform shoal. Member Ⅱ of the Guanling Formation presents multiple stages of intra-platform shoal. The types of particles in the shoal are mainly composed of peloids and coated particles, followed by a small amount of ooids and biological debris. Intergranular pores are mainly filled with sparry calcite cement and minor carbonate mud. The size of the shoal is small. Based on the results of this study and neighboring areas, it is concluded that there was a large-scaled sea level rise during the Pelsonian substage of the early Anisian, which resulted in a rapid increase in the water depth of the sedimentary environment and the development of intra-platform basins. Its relatively low-energy and anoxic environment facilitates the exceptionally well preservation of the Luoping Biota.
The Luoping Biota is typical of the full recovery of the marine ecosystem after the end-Permian mass extinction. In order to more fully understand the sedimentary environment background of the Luoping Biota, the outcrop observation and microfacies analysis were used to study the sedimentary environment of Member Ⅱ of the Middle Triassic Guanling Formation in the Yishida section of Luoping County, Yunnan Province. Five sedimentary facies types are recognized:open platform, restricted platform (lagoon), tidal flat, intra-platform basin, and intra-platform shoal. Member Ⅱ of the Guanling Formation presents multiple stages of intra-platform shoal. The types of particles in the shoal are mainly composed of peloids and coated particles, followed by a small amount of ooids and biological debris. Intergranular pores are mainly filled with sparry calcite cement and minor carbonate mud. The size of the shoal is small. Based on the results of this study and neighboring areas, it is concluded that there was a large-scaled sea level rise during the Pelsonian substage of the early Anisian, which resulted in a rapid increase in the water depth of the sedimentary environment and the development of intra-platform basins. Its relatively low-energy and anoxic environment facilitates the exceptionally well preservation of the Luoping Biota.
