2018 Vol. 43, No. 8
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2018, 43(8): 2527-2550.
doi: 10.3799/dqkx.2018.160
Abstract:
The tectonic evolution of the Changning-Menglian Tethys, as well as the transformation from Proto-Tethys to Paleo-Tethys, is one of hottest research topics concerning the geology of Qinghai-Tibetan Plateau and adjacent regions. Based on the recent geological findings and our new data, we summarized the geological features of the tectonic units from the Sanjiang orogenic system. Then we investigated the evolution history of the Changning-Menglian Tethys during the Early Paleozoic to Late Paleozoic. In view of the spatio-temporal structure of the different tectonic units, and the associated magmatism, sedimentary and metamorphic records. It is suggested in this study that (1) both crustal remnants of Proto-and Paleo-Tethys retain in the Changning-Menglian suture zone, (2) a magmatic arc belt developed from Lincang to Menghai in the Early Paleozoic, (3) the 'Lancang rock Group' was not the basement rock series, but Early Paleozoic tectonic-accretionary complexes formed by the eastward subduction of the Changning-Menglian Tethys Ocean, (4) the eclogite belt in western Yunnan might be the production of rapid exhumation after the deep subduction process of the accretionary complex during the arc-continent collision process. All new lines of evidence suggest that the Changning-Menglian Tethys probably was a continuous ocean from Early Paleozoic to Late Paleozoic. Combined the new findings with the regional geological data, the preliminary temporal-spatial framework and evolution process of the Sanjiang Tethys orogenic system are established, which mainly includes the following four stages:(1) Longmucuo-Shuanghu-Changning-Menglian proto-Tethys seafloor spreading in the Early Paleozoic, (2) the Tethys subduction and formation of the island arc belt from middle-late Early Paleozoic to Late Paleozoic, (3) the convergence and main collision processes from the end of Late Permian to Early Triassic, and (4) the late-collision orogeny and basin-mountain transition in the Late Triassic.
The tectonic evolution of the Changning-Menglian Tethys, as well as the transformation from Proto-Tethys to Paleo-Tethys, is one of hottest research topics concerning the geology of Qinghai-Tibetan Plateau and adjacent regions. Based on the recent geological findings and our new data, we summarized the geological features of the tectonic units from the Sanjiang orogenic system. Then we investigated the evolution history of the Changning-Menglian Tethys during the Early Paleozoic to Late Paleozoic. In view of the spatio-temporal structure of the different tectonic units, and the associated magmatism, sedimentary and metamorphic records. It is suggested in this study that (1) both crustal remnants of Proto-and Paleo-Tethys retain in the Changning-Menglian suture zone, (2) a magmatic arc belt developed from Lincang to Menghai in the Early Paleozoic, (3) the 'Lancang rock Group' was not the basement rock series, but Early Paleozoic tectonic-accretionary complexes formed by the eastward subduction of the Changning-Menglian Tethys Ocean, (4) the eclogite belt in western Yunnan might be the production of rapid exhumation after the deep subduction process of the accretionary complex during the arc-continent collision process. All new lines of evidence suggest that the Changning-Menglian Tethys probably was a continuous ocean from Early Paleozoic to Late Paleozoic. Combined the new findings with the regional geological data, the preliminary temporal-spatial framework and evolution process of the Sanjiang Tethys orogenic system are established, which mainly includes the following four stages:(1) Longmucuo-Shuanghu-Changning-Menglian proto-Tethys seafloor spreading in the Early Paleozoic, (2) the Tethys subduction and formation of the island arc belt from middle-late Early Paleozoic to Late Paleozoic, (3) the convergence and main collision processes from the end of Late Permian to Early Triassic, and (4) the late-collision orogeny and basin-mountain transition in the Late Triassic.
2018, 43(8): 2551-2570.
doi: 10.3799/dqkx.2018.274
Abstract:
There are always different understandings about the age and tectonic affinity of the Tuanliangzi Formation in the Lancangjiang tectonic belt.In this paper, petrological investigation, zircon U-Pb geochronology and geochemical compositions of the Tuanliangzi Formation in the Jinggu area are studied in detail in order to constrain its timing and tectonic affinity.The Tuanliangzi Formation in the Jinggu area is a set of tectonic strata overprinted by strong deformation and weak metamorphism, which is mainly composed of marine argillaceous rocks in the deep ocean and clastic rocks near island arc and is interlayered by some mid-ocean ridge basalt and chert.The minimum detrital zircon U-Pb ages obtained from two metasedimentay rocks reveal that the protolith deposition ages are younger than 272±2 Ma and 259±1 Ma, respectively.Furthermore, an andesitic porphyrite occurred as intrusion in the Tuanliangzi Formation yields a zircon concordia U-Pb age of 244±1 Ma.Metabasalts in the Tuanliangzi Formation are classified into low-K tholeiitic magma series and are characterized by the typical trace element compositions of the normal mid-ocean ridge basalt (N-MORB).Thus, we suggest that the Tuanliangzi Formation could be represented by a set of the Late Paleozoic oceanic subduction-related accretionary complex, which is similar to other relict oceanic basins in the southern Lanchangjiang tectonic belt.The southern Lanchangjiang tectonic belt can be considered as a Paleo-Tethys suture zone resulted from the closure of the small Late Paleozoic back-arc oceanic basin instead of the wide ocean.
There are always different understandings about the age and tectonic affinity of the Tuanliangzi Formation in the Lancangjiang tectonic belt.In this paper, petrological investigation, zircon U-Pb geochronology and geochemical compositions of the Tuanliangzi Formation in the Jinggu area are studied in detail in order to constrain its timing and tectonic affinity.The Tuanliangzi Formation in the Jinggu area is a set of tectonic strata overprinted by strong deformation and weak metamorphism, which is mainly composed of marine argillaceous rocks in the deep ocean and clastic rocks near island arc and is interlayered by some mid-ocean ridge basalt and chert.The minimum detrital zircon U-Pb ages obtained from two metasedimentay rocks reveal that the protolith deposition ages are younger than 272±2 Ma and 259±1 Ma, respectively.Furthermore, an andesitic porphyrite occurred as intrusion in the Tuanliangzi Formation yields a zircon concordia U-Pb age of 244±1 Ma.Metabasalts in the Tuanliangzi Formation are classified into low-K tholeiitic magma series and are characterized by the typical trace element compositions of the normal mid-ocean ridge basalt (N-MORB).Thus, we suggest that the Tuanliangzi Formation could be represented by a set of the Late Paleozoic oceanic subduction-related accretionary complex, which is similar to other relict oceanic basins in the southern Lanchangjiang tectonic belt.The southern Lanchangjiang tectonic belt can be considered as a Paleo-Tethys suture zone resulted from the closure of the small Late Paleozoic back-arc oceanic basin instead of the wide ocean.
2018, 43(8): 2571-2585.
doi: 10.3799/dqkx.2018.102
Abstract:
There are little research on the Early Paleozoic magma events of the giant Lingcang batholith in Sangjiang area.Early Paleozoic granitic gneiss was recognized for the first time from the Lincang batholith in Shuangjiang area of western Yunnan.LA-ICP-MS U-Pb dating and Hf isotopic analysis of zircons, combined with geochemical characteristics of two granitic gneiss samples are applied in this paper.The zircon U-Pb ages of 476.9±1.9 Ma and 465.7±1.9 Ma are obtained from two samples, which indicates that two granitic gneisses were formed in Middle-Late Ordovician.SiO2 content of the granitic gneisses ranges from 70.67% to 74.03%.The K2O/Na2O ratio is greater than 1, ranging from 1.04 to 1.55, and belonging to peraluminous S-type granitoids.They also enriched in LILEs (Rb, Th and U), but strongly depleted in HFSEs (Nb, Ta, Ti and Zr), showing strongly fractionated REE pattern, with apparently negative Eu anomalies.Their zircons have negative εHf(t) values (-7.2 to -0.7, average -3.0), and Hf isotope crust model ages (tDMC) are basically the same (average 1 639 Ma, 1 630 Ma).It is suggested that granitic gneiss may have originated from partial melting of ancient crustal materials.Based on the above analyses, granitic gneiss is formed by partial melting of ancient crustal materials that was caused by the eastward subduction of the Paleo-Tethys, and is the response of the magmatic events during subduction of the Paleo-Tethys.It shows that there was subduction in the Changning-Menglian ocean in the Early Ordovician, and the evolution history of the Tethys in the Changning-Menglian combination can be traced back to the Early Ordovician.
There are little research on the Early Paleozoic magma events of the giant Lingcang batholith in Sangjiang area.Early Paleozoic granitic gneiss was recognized for the first time from the Lincang batholith in Shuangjiang area of western Yunnan.LA-ICP-MS U-Pb dating and Hf isotopic analysis of zircons, combined with geochemical characteristics of two granitic gneiss samples are applied in this paper.The zircon U-Pb ages of 476.9±1.9 Ma and 465.7±1.9 Ma are obtained from two samples, which indicates that two granitic gneisses were formed in Middle-Late Ordovician.SiO2 content of the granitic gneisses ranges from 70.67% to 74.03%.The K2O/Na2O ratio is greater than 1, ranging from 1.04 to 1.55, and belonging to peraluminous S-type granitoids.They also enriched in LILEs (Rb, Th and U), but strongly depleted in HFSEs (Nb, Ta, Ti and Zr), showing strongly fractionated REE pattern, with apparently negative Eu anomalies.Their zircons have negative εHf(t) values (-7.2 to -0.7, average -3.0), and Hf isotope crust model ages (tDMC) are basically the same (average 1 639 Ma, 1 630 Ma).It is suggested that granitic gneiss may have originated from partial melting of ancient crustal materials.Based on the above analyses, granitic gneiss is formed by partial melting of ancient crustal materials that was caused by the eastward subduction of the Paleo-Tethys, and is the response of the magmatic events during subduction of the Paleo-Tethys.It shows that there was subduction in the Changning-Menglian ocean in the Early Ordovician, and the evolution history of the Tethys in the Changning-Menglian combination can be traced back to the Early Ordovician.
2018, 43(8): 2586-2599.
doi: 10.3799/dqkx.2018.105
Abstract:
Researches on the lamprophyres from the Jinshajiang-Red River tectonic belt are mainly concentrated in the southern Ailaoshan area.The zircon U-Pb geochronology and the whole rock geochemistry of the newly discovered Ludian lamprophyres located in the middle segment of the belt are applied in this paper.Zircon U-Pb dating proves Eocene emplacement age of Ludian lamprophyres, which is consistent with the peak age of the Cenozoic magmatism in the Jingshajiang-Red River alkali-rich intrusion belt.Geochemical data indicate that Ludian lamprophyres have the characteristics of high K2O, high (K2O + Na2O) and Mg#, low TiO2, being enriched in LILE and LREE, depleted in HFSE (especially Ta-Nb-Ti).The source of the Ludian lamprophyres should be the lithosphere mantle which had been enriched by subduction-related fluids and melts, and the source should be phlogopite bearing spinel harzburgite.Combined with the study of coeval mafic lava and felsic intrusion, we put forward that the mantle in western Yunnan might have been enriched by the Protozoic subduction related to the assembly of the Rodinia supercontinent.In the Eocene, following the collision between Indian and Asia, the continental lithosphere in the Jingshajiang-Red River tectonic belt underwent convective thinning or delamination, which induced rising of the asthenosphere, and then partial melting of the enriched lithosphere mantle which generated the lamprophyre melt.
Researches on the lamprophyres from the Jinshajiang-Red River tectonic belt are mainly concentrated in the southern Ailaoshan area.The zircon U-Pb geochronology and the whole rock geochemistry of the newly discovered Ludian lamprophyres located in the middle segment of the belt are applied in this paper.Zircon U-Pb dating proves Eocene emplacement age of Ludian lamprophyres, which is consistent with the peak age of the Cenozoic magmatism in the Jingshajiang-Red River alkali-rich intrusion belt.Geochemical data indicate that Ludian lamprophyres have the characteristics of high K2O, high (K2O + Na2O) and Mg#, low TiO2, being enriched in LILE and LREE, depleted in HFSE (especially Ta-Nb-Ti).The source of the Ludian lamprophyres should be the lithosphere mantle which had been enriched by subduction-related fluids and melts, and the source should be phlogopite bearing spinel harzburgite.Combined with the study of coeval mafic lava and felsic intrusion, we put forward that the mantle in western Yunnan might have been enriched by the Protozoic subduction related to the assembly of the Rodinia supercontinent.In the Eocene, following the collision between Indian and Asia, the continental lithosphere in the Jingshajiang-Red River tectonic belt underwent convective thinning or delamination, which induced rising of the asthenosphere, and then partial melting of the enriched lithosphere mantle which generated the lamprophyre melt.
2018, 43(8): 2600-2613.
doi: 10.3799/dqkx.2018.308
Abstract:
The Pulang copper deposit is a super-large porphyry deposit located in Geza island arc in Yunnan Province.Previous studies were mostly concentrated on the ore-bearing quartz-monzonite porphyry, while the research on the diorite and granodiorite porphyry was relatively weak.In this paper, we present geochemistry and zircon chronology analysis on the diorite and granodiorite porphyry in Pulang mine, northwest of Yunnan Province.The diorite is characterized by low SiO2 (62.46%-62.65%), high K2O (5.53%-6.27%) and total alkali(K2O+Na2O=7.93%-8.72%), K2O (5.53%-6.27%), MgO (3.58%-3.69%), and K2O/Na2O is bigger than 2.Likewise, the granodiorite-porphyry is characterized by medium SiO2 (66.00%-66.98%) and high total alkali (7.60%-7.81%), with MgO (2.04%-2.17%), and the K2O/Na2O is bigger than 1.They are enriched in large-ion lithophile and light earth elements, but depleted in high field-strength and heavy rare earth elements, and with weak to negative Eu anomalies.Furthermore, diorite yields lower (87Sr/86Sr)i ratios (0.705 24-0.705 31) than granodiorite-porphyry ((87Sr/86Sr)i 0.705 58-0.705 62), and the granodiorite-porphyry yields negative εNd(t) values from -3.34 to -1.01, while the diorite yields variable εNd(t) values from -1.65 to 7.78.New U-Pb dating of zircons of diorite and granodiorite-porphyry are 227.0±2.9 Ma and 211.5±3.7 Ma respectively.Comprehensively, we propose that a transformation from compression to extension process may have occurred in the Pulang area during the long period of westward subduction of Garze-litang ocean (about 25 Ma), which has triggered the mantle source magmatic activity, and has resulted in the emplacement of deep source (ultra)potassium alkaline.And the transformation process is crucial to the formation of super-large porphyry deposit.
The Pulang copper deposit is a super-large porphyry deposit located in Geza island arc in Yunnan Province.Previous studies were mostly concentrated on the ore-bearing quartz-monzonite porphyry, while the research on the diorite and granodiorite porphyry was relatively weak.In this paper, we present geochemistry and zircon chronology analysis on the diorite and granodiorite porphyry in Pulang mine, northwest of Yunnan Province.The diorite is characterized by low SiO2 (62.46%-62.65%), high K2O (5.53%-6.27%) and total alkali(K2O+Na2O=7.93%-8.72%), K2O (5.53%-6.27%), MgO (3.58%-3.69%), and K2O/Na2O is bigger than 2.Likewise, the granodiorite-porphyry is characterized by medium SiO2 (66.00%-66.98%) and high total alkali (7.60%-7.81%), with MgO (2.04%-2.17%), and the K2O/Na2O is bigger than 1.They are enriched in large-ion lithophile and light earth elements, but depleted in high field-strength and heavy rare earth elements, and with weak to negative Eu anomalies.Furthermore, diorite yields lower (87Sr/86Sr)i ratios (0.705 24-0.705 31) than granodiorite-porphyry ((87Sr/86Sr)i 0.705 58-0.705 62), and the granodiorite-porphyry yields negative εNd(t) values from -3.34 to -1.01, while the diorite yields variable εNd(t) values from -1.65 to 7.78.New U-Pb dating of zircons of diorite and granodiorite-porphyry are 227.0±2.9 Ma and 211.5±3.7 Ma respectively.Comprehensively, we propose that a transformation from compression to extension process may have occurred in the Pulang area during the long period of westward subduction of Garze-litang ocean (about 25 Ma), which has triggered the mantle source magmatic activity, and has resulted in the emplacement of deep source (ultra)potassium alkaline.And the transformation process is crucial to the formation of super-large porphyry deposit.
2018, 43(8): 2614-2627.
doi: 10.3799/dqkx.2018.126
Abstract:
Zhongdian arc, located in Sanjiang Tethyan Orogenic belt in Southwest China southern part of Tethys belt, is one of the most important districts for polymetallic ore deposits.Previous studies mainly focus on the ore-bearing intrusions in the central and northern part of Zhongdian arc, however, there are still limited knowledge about the barren intrusions in the southern part.In this paper, we present detailed petrology, major and trace elemental geochemistry and geochronology of A're granitoids, southern part of Zhongdian arc.Zircon U-Pb dating results show the timing of emplacement of A're intrusion (216.3±1.8 Ma) is comparable to those of the other intrusive rocks in Zhongdian arc.A're quartz monzonites have high Sr (mean 1810.24×10-6), low Y (mean 19.05×10-6) concentrations and thus high Sr/Y(86.0-120.5) ratios.In addition, they also display enrichment in LILE (Rb, Ba, Th, U), depletion in HFSE (Nb, Ta, Ti) and slightly Eu negative anomaly, which suggests that they have geochemical characteristics of both arc-related and adakitic affinities.The low MgO (1.97%-3.30%), Ni (10.7×10-6-26.8×10-6), Cr (18.0×10-6-75.8×10-6) and Co (12.4×10-6-17.4×10-6) contents of the A're granitoids support that they were, generated from partial melting of thicken lower crust.Therefore, the A're quartz monzonites belong to a part of the continental marginal arc during westward subduction of Ganzi-Litang ocean, which represents the products of transformation from widespread Tethys ocean to residual sea in Zhongdian arc.
Zhongdian arc, located in Sanjiang Tethyan Orogenic belt in Southwest China southern part of Tethys belt, is one of the most important districts for polymetallic ore deposits.Previous studies mainly focus on the ore-bearing intrusions in the central and northern part of Zhongdian arc, however, there are still limited knowledge about the barren intrusions in the southern part.In this paper, we present detailed petrology, major and trace elemental geochemistry and geochronology of A're granitoids, southern part of Zhongdian arc.Zircon U-Pb dating results show the timing of emplacement of A're intrusion (216.3±1.8 Ma) is comparable to those of the other intrusive rocks in Zhongdian arc.A're quartz monzonites have high Sr (mean 1810.24×10-6), low Y (mean 19.05×10-6) concentrations and thus high Sr/Y(86.0-120.5) ratios.In addition, they also display enrichment in LILE (Rb, Ba, Th, U), depletion in HFSE (Nb, Ta, Ti) and slightly Eu negative anomaly, which suggests that they have geochemical characteristics of both arc-related and adakitic affinities.The low MgO (1.97%-3.30%), Ni (10.7×10-6-26.8×10-6), Cr (18.0×10-6-75.8×10-6) and Co (12.4×10-6-17.4×10-6) contents of the A're granitoids support that they were, generated from partial melting of thicken lower crust.Therefore, the A're quartz monzonites belong to a part of the continental marginal arc during westward subduction of Ganzi-Litang ocean, which represents the products of transformation from widespread Tethys ocean to residual sea in Zhongdian arc.
2018, 43(8): 2628-2637.
doi: 10.3799/dqkx.2018.162
Abstract:
Tengchong-Lianghe area in western Yunnan is becoming a hot area for ion-adsorption type REE deposit prospecting.However, there is few study on the ore-controlling factors and formation conditions of this type of deposit in high altitude areas in western Yunnan.In this paper, we have analyzed the composition of weathering crust and the variation of orebody in vertical-strike of Tuguanzhai deposit.We consider that the ion-adsorption type REE deposit is controlled by REE of parent rock and topography.The mountainous and hilly regions and low mountains and hills are favorable mineralization regions.8 favorable areas which have satisfactory ore-prospecting results have been found in Jiangdong area, indicating the effectiveness of the prospecting method.
Tengchong-Lianghe area in western Yunnan is becoming a hot area for ion-adsorption type REE deposit prospecting.However, there is few study on the ore-controlling factors and formation conditions of this type of deposit in high altitude areas in western Yunnan.In this paper, we have analyzed the composition of weathering crust and the variation of orebody in vertical-strike of Tuguanzhai deposit.We consider that the ion-adsorption type REE deposit is controlled by REE of parent rock and topography.The mountainous and hilly regions and low mountains and hills are favorable mineralization regions.8 favorable areas which have satisfactory ore-prospecting results have been found in Jiangdong area, indicating the effectiveness of the prospecting method.
2018, 43(8): 2638-2650.
doi: 10.3799/dqkx.2018.530
Abstract:
The Tethys Himalaya is characterized by the S-N trending and E-W trending structures, and the North Himalaya gneiss domes (NHGD).The Cuonadong dome, located at the eastern part of the North Himalaya, is a recently newly identified dome.The Cuonadong dome is divided into three units from outer to inner:the upper unit (the cover rocks), middle unit (the detachment layer) and lower unit (the core).The middle unit mainly consists of a series of strong deformation schist, pegmatite, granite, marble, and skarn.The main schist types include garnet mica schist, garnet staurolite schist, kyanite garnet staurolite schist, sillimanite kyanite garnet schist, and mica quartz schist.This unit is characterized by a ductile shear zone including the sheath fold, augen structure, rotating porphyroclast, S-C structure, and pressure shadow structure.The Cuonadong dome preserves evidences for four major deformational events:N-S thrust (D1), early approximately S-N extensional deformation (D2), approximately E-W extensional deformation (D3), and collapse structural deformation (D4).Ar-Ar dating of muscovite from the mylonitic schist in the Cuonadong dome yielded Ar-Ar plateau age of 14.0±0.2 Ma and inverse isochron age of 13.7±0.5 Ma, meanwhile the presence of subgrain rotation recrystallization (SGR) in quartz shows that the schist was deformed under high deformation temperature (450~550℃), which is clearly higher than the closure temperature of muscovite.Therefore, we suggest that the Ar-Ar plateau age of 14.0±0.2 Ma represents the age of the E-W extensional deformation in the Cuonadong dome, also the age of the S-N trending Sangri-Cuona rift.Combined with the structural deformation and thermochronology, we suggest that the formation of the Cuonadong dome resulted from both the earlier S-N and later E-W extensional deformations, especially the S-N extensional deformation, i.e.the STDS.
The Tethys Himalaya is characterized by the S-N trending and E-W trending structures, and the North Himalaya gneiss domes (NHGD).The Cuonadong dome, located at the eastern part of the North Himalaya, is a recently newly identified dome.The Cuonadong dome is divided into three units from outer to inner:the upper unit (the cover rocks), middle unit (the detachment layer) and lower unit (the core).The middle unit mainly consists of a series of strong deformation schist, pegmatite, granite, marble, and skarn.The main schist types include garnet mica schist, garnet staurolite schist, kyanite garnet staurolite schist, sillimanite kyanite garnet schist, and mica quartz schist.This unit is characterized by a ductile shear zone including the sheath fold, augen structure, rotating porphyroclast, S-C structure, and pressure shadow structure.The Cuonadong dome preserves evidences for four major deformational events:N-S thrust (D1), early approximately S-N extensional deformation (D2), approximately E-W extensional deformation (D3), and collapse structural deformation (D4).Ar-Ar dating of muscovite from the mylonitic schist in the Cuonadong dome yielded Ar-Ar plateau age of 14.0±0.2 Ma and inverse isochron age of 13.7±0.5 Ma, meanwhile the presence of subgrain rotation recrystallization (SGR) in quartz shows that the schist was deformed under high deformation temperature (450~550℃), which is clearly higher than the closure temperature of muscovite.Therefore, we suggest that the Ar-Ar plateau age of 14.0±0.2 Ma represents the age of the E-W extensional deformation in the Cuonadong dome, also the age of the S-N trending Sangri-Cuona rift.Combined with the structural deformation and thermochronology, we suggest that the formation of the Cuonadong dome resulted from both the earlier S-N and later E-W extensional deformations, especially the S-N extensional deformation, i.e.the STDS.
2018, 43(8): 2651-2663.
doi: 10.3799/dqkx.2017.573
Abstract:
The initial break-up time between Tethyan-Himalaya and Indian terrane has been controversial for a long time.In this study, we report newly discovered pillow basalts which provide new insights to the solution of the problem.The pillow basalts trending northwestward unconformable overlap on the Middle Jurassic Zhela Formation.The zircon SHRIMP U-Pb isotopic dating shows the formation age of 92.1±1.2 Ma, representing the Late Cretaceous volcanic activity of the Tethys Himalaya.The major and trace elements analyses show the basalt is alkaline basalt with (La/Yb)N=5.7~7.1 and without Nb-Ta, Eu, Zr-Hf negative anomalies, and it has high Fe, P, Ti contents, low (La/Nb)PM and (Th/Ta)PM ratios.Zircon εHf(t) values range from 9.02 to 12.97, with an average of 10.50 and tDM1 is 241-399 Ma.Geochemical and isotopic data show that the Kada basalt is OIB-type basalt without any contamination from the crust and the magma originates from partial melting of garnet-spinel bearing lherzolite.Kada basalt is present within Tethyan Himalaya passive continental margin.Mafic volcanics spreading within passive continental margins is usually related with the processes of continental break-up and formation of a new ocean basin.Combined with the latest research result of paleomagnetic and evolution of Neo-Tethys oceanic basin, we suggest that the Kada basalt is related to the initial break-up between Tethyan-Himalaya and Indian terrane.The Kada basalt provides new clues for the study of tectonic evolution of the Tethyan Himalaya.
The initial break-up time between Tethyan-Himalaya and Indian terrane has been controversial for a long time.In this study, we report newly discovered pillow basalts which provide new insights to the solution of the problem.The pillow basalts trending northwestward unconformable overlap on the Middle Jurassic Zhela Formation.The zircon SHRIMP U-Pb isotopic dating shows the formation age of 92.1±1.2 Ma, representing the Late Cretaceous volcanic activity of the Tethys Himalaya.The major and trace elements analyses show the basalt is alkaline basalt with (La/Yb)N=5.7~7.1 and without Nb-Ta, Eu, Zr-Hf negative anomalies, and it has high Fe, P, Ti contents, low (La/Nb)PM and (Th/Ta)PM ratios.Zircon εHf(t) values range from 9.02 to 12.97, with an average of 10.50 and tDM1 is 241-399 Ma.Geochemical and isotopic data show that the Kada basalt is OIB-type basalt without any contamination from the crust and the magma originates from partial melting of garnet-spinel bearing lherzolite.Kada basalt is present within Tethyan Himalaya passive continental margin.Mafic volcanics spreading within passive continental margins is usually related with the processes of continental break-up and formation of a new ocean basin.Combined with the latest research result of paleomagnetic and evolution of Neo-Tethys oceanic basin, we suggest that the Kada basalt is related to the initial break-up between Tethyan-Himalaya and Indian terrane.The Kada basalt provides new clues for the study of tectonic evolution of the Tethyan Himalaya.
2018, 43(8): 2664-2683.
doi: 10.3799/dqkx.2018.141
Abstract:
The structure assemblage, rock composition, and kinematics characteristics of metamorphic deformation of the newly discovered Cuonadong dome are not clear currently, which seriously hinders the restoration of its evolutionary process and the anatomy of the coupled relationship between the formation of plutonium and mineralization. Based on the detailed field geological survey, the newly discovered magmatic rocks in dome were additionally collected for chronological study in this paper. The results show that the Cuonadong dome is divided into three structural layers of upper (margin)-middle (mantle)-lower (core) by upper and lower detachment faults. The core rock assemblages are primarily composed of granite gneiss, leucogranite, and a little migmatite, with a large amount of intruded pegmatite veins. The mantle rock assemblage is a set of strongly metamorphic and deformable schist with interlayer carbonate. This rock assemblage has Barrovian metamorphism confirmed by mineral assemblages from the core to the margin of kyanite+staurolite+garnet+biotite→taurolite+garnet+biotite→garnet+cordierite+biotite→chlorite+biotite; The margin is primarily comprised of slightly metamorphic Triassic-Jurassic sedimentary rocks, which is constituted of sericite chlorite sand slate and a small amount of phyllite. The dome from early to late has experienced N-S trending thrusting, N-S extension and E-W extension, and the formation of the dome is primarily associated with the N-S extension. There are five episodes of magmatism in the dome at ~500 Ma (gneiss), 140 Ma (diabase), 26 Ma (deformed two-mica granite/pegmatite), 18 Ma (weakly oriented two-mica granite), 16.8-15.9 Ma (garnet-tourmaline-bearing granite). The study shows that the formation of the Cuonadong dome is the result of the combined effect of the early extension and detachment and the late magmatic diapir, and the movement of the South Tibetan detachment system during the Eocene-Oligocene is the primal factor.
The structure assemblage, rock composition, and kinematics characteristics of metamorphic deformation of the newly discovered Cuonadong dome are not clear currently, which seriously hinders the restoration of its evolutionary process and the anatomy of the coupled relationship between the formation of plutonium and mineralization. Based on the detailed field geological survey, the newly discovered magmatic rocks in dome were additionally collected for chronological study in this paper. The results show that the Cuonadong dome is divided into three structural layers of upper (margin)-middle (mantle)-lower (core) by upper and lower detachment faults. The core rock assemblages are primarily composed of granite gneiss, leucogranite, and a little migmatite, with a large amount of intruded pegmatite veins. The mantle rock assemblage is a set of strongly metamorphic and deformable schist with interlayer carbonate. This rock assemblage has Barrovian metamorphism confirmed by mineral assemblages from the core to the margin of kyanite+staurolite+garnet+biotite→taurolite+garnet+biotite→garnet+cordierite+biotite→chlorite+biotite; The margin is primarily comprised of slightly metamorphic Triassic-Jurassic sedimentary rocks, which is constituted of sericite chlorite sand slate and a small amount of phyllite. The dome from early to late has experienced N-S trending thrusting, N-S extension and E-W extension, and the formation of the dome is primarily associated with the N-S extension. There are five episodes of magmatism in the dome at ~500 Ma (gneiss), 140 Ma (diabase), 26 Ma (deformed two-mica granite/pegmatite), 18 Ma (weakly oriented two-mica granite), 16.8-15.9 Ma (garnet-tourmaline-bearing granite). The study shows that the formation of the Cuonadong dome is the result of the combined effect of the early extension and detachment and the late magmatic diapir, and the movement of the South Tibetan detachment system during the Eocene-Oligocene is the primal factor.
2018, 43(8): 2684-2700.
doi: 10.3799/dqkx.2018.170
Abstract:
The current study of the deposits on the Middle Gangdese metallogenic belt is mainly related to geological features, metallogenic epochs and the origin of ore-forming rock masses.However, there are less detailed reports on the formation process, especially the evolution of ore-forming fluids.Qiagong polymetallic deposit located in the belt is genetically related to monzogranite porphyry formed at early India-Asia continental collision.The mineralization styles include skarn type Fe(-Cu), distal vein type Pb-Zn-Ag(-Cu) and carbonate replacement type Pb-Zn-Ag.Based on field observation and petrographic results the ore-forming process in this deposit can be divided into at least six stages including complexed garnet-magnetite stage (Ⅰ), epidote-magnetite stage (Ⅱ), quartz-hematite stage (Ⅲ), fluorite-chalcopyrite stage (Ⅳ), calcite-galena-sphalerite stage (Ⅴ) and calcite-quartz stage (Ⅵ).Petrographic, microthermometric and laser Raman microprobe results of fluid inclusions in quartz, garnet, epidote, calcite and fluorite from these six stages demonstrate that the pressure, temperature, density and salinity of the ore forming fluid decrease from early to later stage, and the fluid system change from early H2O-NaCl (Ⅰ-Ⅱ) via H2O-NaCl-FeCl2-3±MgCl2(Ⅲ), H2O-CO2-NaCl (Ⅳ) to late H2O-NaCl-CaCl2(Ⅴ & Ⅵ).The deposition of Cu in Qiagong was triggered by the boiling of the ore-forming fluid of H2O-CO2-NaCl system from stage Ⅳ.The deposition of Pb and Zn might be ascribed to the temperature decrease, or it might also be the result of mineralization superposition.
The current study of the deposits on the Middle Gangdese metallogenic belt is mainly related to geological features, metallogenic epochs and the origin of ore-forming rock masses.However, there are less detailed reports on the formation process, especially the evolution of ore-forming fluids.Qiagong polymetallic deposit located in the belt is genetically related to monzogranite porphyry formed at early India-Asia continental collision.The mineralization styles include skarn type Fe(-Cu), distal vein type Pb-Zn-Ag(-Cu) and carbonate replacement type Pb-Zn-Ag.Based on field observation and petrographic results the ore-forming process in this deposit can be divided into at least six stages including complexed garnet-magnetite stage (Ⅰ), epidote-magnetite stage (Ⅱ), quartz-hematite stage (Ⅲ), fluorite-chalcopyrite stage (Ⅳ), calcite-galena-sphalerite stage (Ⅴ) and calcite-quartz stage (Ⅵ).Petrographic, microthermometric and laser Raman microprobe results of fluid inclusions in quartz, garnet, epidote, calcite and fluorite from these six stages demonstrate that the pressure, temperature, density and salinity of the ore forming fluid decrease from early to later stage, and the fluid system change from early H2O-NaCl (Ⅰ-Ⅱ) via H2O-NaCl-FeCl2-3±MgCl2(Ⅲ), H2O-CO2-NaCl (Ⅳ) to late H2O-NaCl-CaCl2(Ⅴ & Ⅵ).The deposition of Cu in Qiagong was triggered by the boiling of the ore-forming fluid of H2O-CO2-NaCl system from stage Ⅳ.The deposition of Pb and Zn might be ascribed to the temperature decrease, or it might also be the result of mineralization superposition.
2018, 43(8): 2701-2714.
doi: 10.3799/dqkx.2018.165
Abstract:
There are few related studies on the Cretaceous acid rocks currently, and this paper focuses on a wide range of acid (dyanite)-basic (diabase-basalt) volcanic magmatism recently discovered in the Quzhuomu area where bimodal volcanic magmatic activity is obvious.Geochronology, geochemistry and Hf-Sr-Nd isotope analyses are carried out on these dacites.The results show that the diagenetic age of dacite is 142.0±2.5 Ma.The dacites show high contents of LREE and high field-strength elements (etc. Nb, Zr, Hf, Y) and high ratios of Ga/Al, indicating the characteristics of A-type granite.εHf(t) values of zircons range from -13.1 to -2.4, with an average of -7.1.The dacites also show high initial Sr isotope contents (87Sr/86Sr ratios of 0.722 024 to 0.722 698) and low initial Nd isotope contents with εNd(t) values from -12.2 to -11.8).The comprehensive research indicates that the acid volcanic rocks of Quzhuomu region are the products of the continental crust anataxis under extension setting.These dacites show the similar geochemical characteristics and diagenetic age with the Sangxiu formation acid volcanic rocks, Laguila acid intrusive rocks and rhyolites of Sangrize region.They are probably all the products of the Comei-Bunbury large igneous province, which are the products of the early magmatic activity of Kerguelen mantle plume.The magma activities of Kerguelen mantle plume may be advanced to 140 Ma.
There are few related studies on the Cretaceous acid rocks currently, and this paper focuses on a wide range of acid (dyanite)-basic (diabase-basalt) volcanic magmatism recently discovered in the Quzhuomu area where bimodal volcanic magmatic activity is obvious.Geochronology, geochemistry and Hf-Sr-Nd isotope analyses are carried out on these dacites.The results show that the diagenetic age of dacite is 142.0±2.5 Ma.The dacites show high contents of LREE and high field-strength elements (etc. Nb, Zr, Hf, Y) and high ratios of Ga/Al, indicating the characteristics of A-type granite.εHf(t) values of zircons range from -13.1 to -2.4, with an average of -7.1.The dacites also show high initial Sr isotope contents (87Sr/86Sr ratios of 0.722 024 to 0.722 698) and low initial Nd isotope contents with εNd(t) values from -12.2 to -11.8).The comprehensive research indicates that the acid volcanic rocks of Quzhuomu region are the products of the continental crust anataxis under extension setting.These dacites show the similar geochemical characteristics and diagenetic age with the Sangxiu formation acid volcanic rocks, Laguila acid intrusive rocks and rhyolites of Sangrize region.They are probably all the products of the Comei-Bunbury large igneous province, which are the products of the early magmatic activity of Kerguelen mantle plume.The magma activities of Kerguelen mantle plume may be advanced to 140 Ma.
2018, 43(8): 2715-2726.
doi: 10.3799/dqkx.2018.546
Abstract:
The Jinshajiang Tethys is one of the most important ocean basins in the Sanjiang region.However, it is not clear when the oceanic subduction started.The research of arc volcanic rock is significant to constrain the subduction process of the oceanic plate.This paper reports the Late Middle Carboniferous volcanic rocks in Jomda area, eastern Tibet, which are dominated by basalts and andesites.Zircon U-Pb age results show they were formed at 332±3.3 Ma.The arc volcanic rocks have SiO2 contents of 45.65%-55.73%, Al2O3 contents of 15.48%-16.79%, TiO2 contents of 0.82%-1.12%, and have high Na/K ratios.They show obvious differentiation between LREE and HREE, and without Eu anomaly.They are enriched in strong incompatible trace elements, such as Th and U, and depleted in high field strength element, such as Nb and Ta.The zircons from basalts have positive εHf(t) values(+11.0 to +15.6), which indicates that the magma source might be the enriched lithosphere mantle.It is believed that Late Middle Carboniferous volcanic rocks were formed at the island arc tectonic enviorment, which is related to subduction of Jinshajiang Tethys ocean.Combining with the regional geological data, we propose that the subduction of the Jinshajiang Tethys ocean have started before 332 Ma.This study provides a lithology evidence for the evolution of Jinshajiang Tethys ocean.
The Jinshajiang Tethys is one of the most important ocean basins in the Sanjiang region.However, it is not clear when the oceanic subduction started.The research of arc volcanic rock is significant to constrain the subduction process of the oceanic plate.This paper reports the Late Middle Carboniferous volcanic rocks in Jomda area, eastern Tibet, which are dominated by basalts and andesites.Zircon U-Pb age results show they were formed at 332±3.3 Ma.The arc volcanic rocks have SiO2 contents of 45.65%-55.73%, Al2O3 contents of 15.48%-16.79%, TiO2 contents of 0.82%-1.12%, and have high Na/K ratios.They show obvious differentiation between LREE and HREE, and without Eu anomaly.They are enriched in strong incompatible trace elements, such as Th and U, and depleted in high field strength element, such as Nb and Ta.The zircons from basalts have positive εHf(t) values(+11.0 to +15.6), which indicates that the magma source might be the enriched lithosphere mantle.It is believed that Late Middle Carboniferous volcanic rocks were formed at the island arc tectonic enviorment, which is related to subduction of Jinshajiang Tethys ocean.Combining with the regional geological data, we propose that the subduction of the Jinshajiang Tethys ocean have started before 332 Ma.This study provides a lithology evidence for the evolution of Jinshajiang Tethys ocean.
2018, 43(8): 2727-2741.
doi: 10.3799/dqkx.2018.230
Abstract:
There are different understandings of the genesis and geodynamic mechanism for the Late Cretaceous adakite of the Gangdese belt.In this paper, we present zircon U-Pb data, geochemical and Hf isotopic data for the quartz diorite porphyry from Nuri area.The results show that the quartz diorite porphyry was emplaced at 96.5±1.3 Ma.These rocks are characterized by high SiO2(63.96%-65.75%), Al2O3(14.37%-15.99%), MgO (2.12%-2.39%), Sr (362×10-6-575×10-6, 467×10-6 on average), low Y (8.94×10-6-11.50×10-6), Yb (0.81×10-6-1.06×10-6) and high Sr/Y ratio (33.52-60.65), implying adakitic geochemical features.These rocks are low to medium-K, calc-alkaline and metalumious.They are enriched in LREE and depleted in HREE, together with enrichment in large-ion lithophile elements (LILE) and depletion in high field strength elements (HFSE), as well as small negative Eu anomalies.εHf(t) values of zircons range from -0.3 to +15.2 (mainly between +10.0 and +15.2), with tDM2 ranging from 187 Ma to 1 173 Ma (mainly from 187 Ma to 516 Ma), which indicates that the sources of these rocks were derived from subducted oceanic crust probably with a minor proportion of subducted sediments.High Mg# and compatible element Cr and Ni indicate that the melts have interacted with the mantle during ascent.By comparing with adakitic rocks from South Gangdese, we propose that high heat flow through a slab window induced partial melting of the oceanic crust at slab window edges to form the quartz diorite porphyry from Nuri, under the geodynamic setting of ridge subduction.
There are different understandings of the genesis and geodynamic mechanism for the Late Cretaceous adakite of the Gangdese belt.In this paper, we present zircon U-Pb data, geochemical and Hf isotopic data for the quartz diorite porphyry from Nuri area.The results show that the quartz diorite porphyry was emplaced at 96.5±1.3 Ma.These rocks are characterized by high SiO2(63.96%-65.75%), Al2O3(14.37%-15.99%), MgO (2.12%-2.39%), Sr (362×10-6-575×10-6, 467×10-6 on average), low Y (8.94×10-6-11.50×10-6), Yb (0.81×10-6-1.06×10-6) and high Sr/Y ratio (33.52-60.65), implying adakitic geochemical features.These rocks are low to medium-K, calc-alkaline and metalumious.They are enriched in LREE and depleted in HREE, together with enrichment in large-ion lithophile elements (LILE) and depletion in high field strength elements (HFSE), as well as small negative Eu anomalies.εHf(t) values of zircons range from -0.3 to +15.2 (mainly between +10.0 and +15.2), with tDM2 ranging from 187 Ma to 1 173 Ma (mainly from 187 Ma to 516 Ma), which indicates that the sources of these rocks were derived from subducted oceanic crust probably with a minor proportion of subducted sediments.High Mg# and compatible element Cr and Ni indicate that the melts have interacted with the mantle during ascent.By comparing with adakitic rocks from South Gangdese, we propose that high heat flow through a slab window induced partial melting of the oceanic crust at slab window edges to form the quartz diorite porphyry from Nuri, under the geodynamic setting of ridge subduction.
2018, 43(8): 2742-2754.
doi: 10.3799/dqkx.2018.154
Abstract:
Researchers have not paid attention to the tungsten-tin mineralization of the Tethys Himalayan lead-zinc-gold-metallogenic belt.The Cuonadong W-Sn polymetallic deposit is located in a newly recognized giant gneiss dome, eastern Tethys Himalaya.The deposit is rich in W, Sn and Be, accompanied by Cu, Pb, Zn, Bi and Mo, of which mineralization type is skarn-type. Besides, the mineralization of beryllium contains pegmatite type.Main ore minerals are scheelite, cassiterite and bertrandite, with minor chalcopyrite, galena, sphalerite, bismuthinite and molybdenited.There are also some zeunerite, uraninite and thorite, indicating the uranium mineralization potential.Geological survey shows that the Cuonadong W-Sn polymetallic deposit could be a large-or super large-scale ore deposit.Low concentration of Zr and relatively low ratios of Zr/Hf and Nb/Tb demonstrate that leucogranite within the Cuonadong dome is highly fractional crystallization.Meanwhile, there is absolutely little magnetite but relatively rich in ilmenite with Fe2O3/FeO < 0.5 and low Ce/Ce* ratio (average 23) in leucogranite, indicating characteristics of reductive granite.The reductive and highly fractional crystallization granite is a necessary condition for the formation of the tungsten-tin polymetallic deposit, additionally, the gneiss dome controls its spatial distribution.Therefore, the Cuonadong W-Sn polymetallic deposit is both controlled by dome structure and leucogranite.
Researchers have not paid attention to the tungsten-tin mineralization of the Tethys Himalayan lead-zinc-gold-metallogenic belt.The Cuonadong W-Sn polymetallic deposit is located in a newly recognized giant gneiss dome, eastern Tethys Himalaya.The deposit is rich in W, Sn and Be, accompanied by Cu, Pb, Zn, Bi and Mo, of which mineralization type is skarn-type. Besides, the mineralization of beryllium contains pegmatite type.Main ore minerals are scheelite, cassiterite and bertrandite, with minor chalcopyrite, galena, sphalerite, bismuthinite and molybdenited.There are also some zeunerite, uraninite and thorite, indicating the uranium mineralization potential.Geological survey shows that the Cuonadong W-Sn polymetallic deposit could be a large-or super large-scale ore deposit.Low concentration of Zr and relatively low ratios of Zr/Hf and Nb/Tb demonstrate that leucogranite within the Cuonadong dome is highly fractional crystallization.Meanwhile, there is absolutely little magnetite but relatively rich in ilmenite with Fe2O3/FeO < 0.5 and low Ce/Ce* ratio (average 23) in leucogranite, indicating characteristics of reductive granite.The reductive and highly fractional crystallization granite is a necessary condition for the formation of the tungsten-tin polymetallic deposit, additionally, the gneiss dome controls its spatial distribution.Therefore, the Cuonadong W-Sn polymetallic deposit is both controlled by dome structure and leucogranite.
2018, 43(8): 2755-2766.
doi: 10.3799/dqkx.2018.593
Abstract:
The Cenozoic volcanic rocks in Linzizong Group are widely exposed in southern Tibet Plateau and it is considered to have been formed in the transitional process from the end of subduction of Neo-Tethyan oceanic plate to India-Asia continent collision.The age of Dianzhong Formation is significant to constrain the time of collision between India and Asia.However, research on the Linzizong Group volcanic rocks is mainly concentrated on the Linzhou basin and its vicinity in the eastern part of the Lhasa block.There are few studies on volcanic rocks in the central and western parts of the Lhasa block, and very few systematic chronology studies.After the stratigraphic and petrological study, we conducted systematic U-Pb zircon dating on 3 selected samples taken from the geological section of Dianzhong Formation in Chazi area.The results show the ages of Dianzhong Formation are 70.7±1.4 Ma, 69.9±1.5 Ma and 68.3±1.2 Ma, respectively.Combining with previously reported chronological data, it is revealed that there were certain differences of the ages from bottom layer of the Linzizong volcanic rocks in Lhasa block.The initial collision between India and Asia continent took place earlier in middle Lhasa block but later in eastern and western parts.
The Cenozoic volcanic rocks in Linzizong Group are widely exposed in southern Tibet Plateau and it is considered to have been formed in the transitional process from the end of subduction of Neo-Tethyan oceanic plate to India-Asia continent collision.The age of Dianzhong Formation is significant to constrain the time of collision between India and Asia.However, research on the Linzizong Group volcanic rocks is mainly concentrated on the Linzhou basin and its vicinity in the eastern part of the Lhasa block.There are few studies on volcanic rocks in the central and western parts of the Lhasa block, and very few systematic chronology studies.After the stratigraphic and petrological study, we conducted systematic U-Pb zircon dating on 3 selected samples taken from the geological section of Dianzhong Formation in Chazi area.The results show the ages of Dianzhong Formation are 70.7±1.4 Ma, 69.9±1.5 Ma and 68.3±1.2 Ma, respectively.Combining with previously reported chronological data, it is revealed that there were certain differences of the ages from bottom layer of the Linzizong volcanic rocks in Lhasa block.The initial collision between India and Asia continent took place earlier in middle Lhasa block but later in eastern and western parts.
2018, 43(8): 2767-2779.
doi: 10.3799/dqkx.2018.167
Abstract:
The tectonic background of Lhasa terrane located at the northern margin of Gondwana in the Late Paleozoic is still in dispute. Seismites have been newly discovered in the Late Paleozoic strata (Laga Formation and Xiala Formation) in Nixiong area in the western section of the Lhasa terrane. The seismite unit of the first member of Laga Formation is found in pebbled fine sandstone, which is ice-rafting deposit formed by the melting of sediment-laden ice blocks. In this unit, we have found massive seismites related sedimentary and structure features. Furthermore, typical seismite rock types, including seismic corrugated rock, shattering rock, and plastic breccia are recognized. The authors have also found soft sediment deformation structures related to earthquake in the tidal-flat sedimentary in the second member of Laga Formation, such as convolute bedding, sand ball-pillow structure. Chert limestone with micrite veins, located at the bottom of Xiala Formation, is another seismite horizon. Based on the paragenetic relationship between chert limestones and seismite horizons of the Angjie Formation and the Xiala Formation, it can be inferred that partial chert limestones in the Late Paleozoic in Lhasa terrane may be closely related to the earthquake. Combined with the regional data, it is concluded that a series of seismic events in the Late Paleozoic recorded the rifting process at the northern margin of Gondwana, and proved a new seismic activity for Late Paleozoic basin evolution of the Lhasa terrane.
The tectonic background of Lhasa terrane located at the northern margin of Gondwana in the Late Paleozoic is still in dispute. Seismites have been newly discovered in the Late Paleozoic strata (Laga Formation and Xiala Formation) in Nixiong area in the western section of the Lhasa terrane. The seismite unit of the first member of Laga Formation is found in pebbled fine sandstone, which is ice-rafting deposit formed by the melting of sediment-laden ice blocks. In this unit, we have found massive seismites related sedimentary and structure features. Furthermore, typical seismite rock types, including seismic corrugated rock, shattering rock, and plastic breccia are recognized. The authors have also found soft sediment deformation structures related to earthquake in the tidal-flat sedimentary in the second member of Laga Formation, such as convolute bedding, sand ball-pillow structure. Chert limestone with micrite veins, located at the bottom of Xiala Formation, is another seismite horizon. Based on the paragenetic relationship between chert limestones and seismite horizons of the Angjie Formation and the Xiala Formation, it can be inferred that partial chert limestones in the Late Paleozoic in Lhasa terrane may be closely related to the earthquake. Combined with the regional data, it is concluded that a series of seismic events in the Late Paleozoic recorded the rifting process at the northern margin of Gondwana, and proved a new seismic activity for Late Paleozoic basin evolution of the Lhasa terrane.
2018, 43(8): 2780-2794.
doi: 10.3799/dqkx.2018.153
Abstract:
Many studies have been focused on the central and southern Lhasa terrane, but it remains controversial as to the genetic mechanism of the Mesozoic volcanic rocks in the central and northern Lhasa terrane due to less attention paid to the distribution characteristics of volcanic magmatism, the origin of volcanic rocks, and the tectonic significance of the volcanic rocks. Early Cretaceous volcanic rocks are widely exposed in Nixiong area, which record abundant pre-Cenozoic evolutionary geohistory of the Tibetan Plateau. Petrological and zircon U-Pb dating analyses show that the volcanic rocks are mainly composed of basalitic andesite, trachyandesite and rhyolites. They have variable SiO2 contents ranging from 55.76% to 77.78%, and alumina saturation index (A/CNK) of 0.89-3.04, indicative of high-K calc-alkaline to alkaline-calc and metaluminous to peraluminous. They are characterized by the enrichment of Th and U, and the depletion of HFSEs (such as Nb and Ta), typical of A-type granitoids. In addition, rhyolites show distinct high SiO2, but low MgO, TiO2, P2O5 and δEu, and display fiercely negative Ba, Nb, Ta, Sr and Eu anomalies, suggesting that they are highly fractionated A-type rocks. LA-ICP-MS U-Pb dating of magmatic zircons from one basalitic andesite, one trachyandesite and two rhyolites samples indicate that they were formed at 117 Ma, 127 Ma and 126-127 Ma, respectively. It is proved that results of previous studies are wrong in that the volcanic rocks in Nixiong area are all Eocene Nianbo Formation of Lingzizhong group or Oligocene Rigongla Formation. In addition, it is found that the basalitic andesite, trachyandesite and rhyolites are likely derived from partial melting of a crust-mantle mixed source, and have experienced significant fractional crystallization. We speculate that the studied rocks have been affected by double subduction of southward subduction of Bangong Co-Nujiang Tethys oceanic crust, and northward subduction of Yalung-Zangbo oceanic crust.
Many studies have been focused on the central and southern Lhasa terrane, but it remains controversial as to the genetic mechanism of the Mesozoic volcanic rocks in the central and northern Lhasa terrane due to less attention paid to the distribution characteristics of volcanic magmatism, the origin of volcanic rocks, and the tectonic significance of the volcanic rocks. Early Cretaceous volcanic rocks are widely exposed in Nixiong area, which record abundant pre-Cenozoic evolutionary geohistory of the Tibetan Plateau. Petrological and zircon U-Pb dating analyses show that the volcanic rocks are mainly composed of basalitic andesite, trachyandesite and rhyolites. They have variable SiO2 contents ranging from 55.76% to 77.78%, and alumina saturation index (A/CNK) of 0.89-3.04, indicative of high-K calc-alkaline to alkaline-calc and metaluminous to peraluminous. They are characterized by the enrichment of Th and U, and the depletion of HFSEs (such as Nb and Ta), typical of A-type granitoids. In addition, rhyolites show distinct high SiO2, but low MgO, TiO2, P2O5 and δEu, and display fiercely negative Ba, Nb, Ta, Sr and Eu anomalies, suggesting that they are highly fractionated A-type rocks. LA-ICP-MS U-Pb dating of magmatic zircons from one basalitic andesite, one trachyandesite and two rhyolites samples indicate that they were formed at 117 Ma, 127 Ma and 126-127 Ma, respectively. It is proved that results of previous studies are wrong in that the volcanic rocks in Nixiong area are all Eocene Nianbo Formation of Lingzizhong group or Oligocene Rigongla Formation. In addition, it is found that the basalitic andesite, trachyandesite and rhyolites are likely derived from partial melting of a crust-mantle mixed source, and have experienced significant fractional crystallization. We speculate that the studied rocks have been affected by double subduction of southward subduction of Bangong Co-Nujiang Tethys oceanic crust, and northward subduction of Yalung-Zangbo oceanic crust.
2018, 43(8): 2795-2810.
doi: 10.3799/dqkx.2018.589
Abstract:
The opening and subduction time of the Neo-Tethys ocean in the southern part of the Lhasa block on the Qinghai-Tibet Plateau ramains controversial.The widely developed Mesozoic granitoids in the southern Lhasa subterrane of Tibetan Plateau, are essential samples to explore the prolonged evolution of subduction process of the Neo-Tethys before collision between the Asia and India continents.Here, we present detailed zircon U-Pb dating, trace element and Hf isotope, whole-rock major and trace elements, and Sr-Nd isotope data for the granodiorites from the Renqinze area, central part of southern Lhasa subterrane.The granodiorites were crystallized at ca.180 Ma, which belongs to Early Jurassic.They are characterized by relatively high SiO2 (62.77%-64.18%) contents and low K2O/Na2O (0.29-0.60) and A/CNK values (0.90-0.98).These geochemical characteristics are similar to Ⅰ-type calc-alkaline rocks.Renqinze granodiorites are enriched in LILEs (e.g. Ba and U) and depleted in HFSEs (e.g. Nb and Ta), showing the geochemical affinity of arc-related magmatism.The granodiorites fall into the range of partial melts of meta-basaltic rocks due to their high CaO, low total alkaline and Al2O3 contents.The accordant results of Ti-in-zircon and whole-rock zircon saturation temperature suggest that the Renqinze granitoid rocks were derived from the lower continental crust.Moreover, the granodiorites show low (87Sr/86Sr)i(0.703 671-0.703 794), high εNd(t)(5.41-5.66) and zircon εHf(t)(12.6-14.8) values, indicating they were likely generated from partial melting of a juvenile mafic lower crust.The Renqinze granitodiorites represent the products of subduction of the Neo-Tethys ocean.The timing for the opening of the Neo-Tethys is at least before Late Triassic.
The opening and subduction time of the Neo-Tethys ocean in the southern part of the Lhasa block on the Qinghai-Tibet Plateau ramains controversial.The widely developed Mesozoic granitoids in the southern Lhasa subterrane of Tibetan Plateau, are essential samples to explore the prolonged evolution of subduction process of the Neo-Tethys before collision between the Asia and India continents.Here, we present detailed zircon U-Pb dating, trace element and Hf isotope, whole-rock major and trace elements, and Sr-Nd isotope data for the granodiorites from the Renqinze area, central part of southern Lhasa subterrane.The granodiorites were crystallized at ca.180 Ma, which belongs to Early Jurassic.They are characterized by relatively high SiO2 (62.77%-64.18%) contents and low K2O/Na2O (0.29-0.60) and A/CNK values (0.90-0.98).These geochemical characteristics are similar to Ⅰ-type calc-alkaline rocks.Renqinze granodiorites are enriched in LILEs (e.g. Ba and U) and depleted in HFSEs (e.g. Nb and Ta), showing the geochemical affinity of arc-related magmatism.The granodiorites fall into the range of partial melts of meta-basaltic rocks due to their high CaO, low total alkaline and Al2O3 contents.The accordant results of Ti-in-zircon and whole-rock zircon saturation temperature suggest that the Renqinze granitoid rocks were derived from the lower continental crust.Moreover, the granodiorites show low (87Sr/86Sr)i(0.703 671-0.703 794), high εNd(t)(5.41-5.66) and zircon εHf(t)(12.6-14.8) values, indicating they were likely generated from partial melting of a juvenile mafic lower crust.The Renqinze granitodiorites represent the products of subduction of the Neo-Tethys ocean.The timing for the opening of the Neo-Tethys is at least before Late Triassic.
2018, 43(8): 2811-2832.
doi: 10.3799/dqkx.2018.307
Abstract:
There was little research on the formation and evolution of the Triassic clastic rocks in the Jinshajiang tectonic belt, undermining the overall understanding of the evolution history of the Jinshajiang ocean basin.It is found in this study the protolithes of Shanglan Formation are mainly sandstone and shale, which is in agreement with the classification discriminate diagram of clastic rocks.The U-Pb dating of detrital zircon show 4 age groups, which are 426-603 Ma, 713-980 Ma, 1 000-1 600 Ma and 1 600-2 500 Ma respectively, indicative of the provenance from basement of the Yangtze block and its vincinity, magmatism of homochronous Pan-African and Caledonian.The content of ∑REE are slightly higher compared with that of upper crust.The LREE and HREE show obvious fractionation, (La/Yb)N=5.23-12.33.Meanwhile, δEu is slightly abnormal.The characteristics of detrital component and discrimination diagram show the provenance are mainly from collisional orogenic belt and a little from volcanic arcs.The quality of the matters belong to old sediment and quartz-feldspathic volcanic.The tectonic setting of provenance in Shanglan Formation is mainly continental arc and active continental margin.The information above common indicates that the tectonic setting of Shanglan Formation is depression which was formed by extruding of subduction collision.Combined with regional tectonic evolution, we consider its tectonic environment was retroarc foreland basin.
There was little research on the formation and evolution of the Triassic clastic rocks in the Jinshajiang tectonic belt, undermining the overall understanding of the evolution history of the Jinshajiang ocean basin.It is found in this study the protolithes of Shanglan Formation are mainly sandstone and shale, which is in agreement with the classification discriminate diagram of clastic rocks.The U-Pb dating of detrital zircon show 4 age groups, which are 426-603 Ma, 713-980 Ma, 1 000-1 600 Ma and 1 600-2 500 Ma respectively, indicative of the provenance from basement of the Yangtze block and its vincinity, magmatism of homochronous Pan-African and Caledonian.The content of ∑REE are slightly higher compared with that of upper crust.The LREE and HREE show obvious fractionation, (La/Yb)N=5.23-12.33.Meanwhile, δEu is slightly abnormal.The characteristics of detrital component and discrimination diagram show the provenance are mainly from collisional orogenic belt and a little from volcanic arcs.The quality of the matters belong to old sediment and quartz-feldspathic volcanic.The tectonic setting of provenance in Shanglan Formation is mainly continental arc and active continental margin.The information above common indicates that the tectonic setting of Shanglan Formation is depression which was formed by extruding of subduction collision.Combined with regional tectonic evolution, we consider its tectonic environment was retroarc foreland basin.
2018, 43(8): 2833-2847.
doi: 10.3799/dqkx.2018.158
Abstract:
The Triassic volcanic and sedimentary rocks are the important components in the geological records of the southern Lancangjiang zone.However, little is known about the biostratigraphic correlation in this area.In this study, three species and one indeterminate species in one genera of conchostracans were recognized for the first time from the Manghuai Formation of the study area, including Euestheria minuta, Euestheria yipinglangensis, Euestheria dazuensis and Euestheria sp..For the purpose of the comparison, quantitative and semi-quantitative descriptions of the specimens, we took eight standard linear measurements from each well-preserved specimen.A reduced major axis (RMA) regression line has been fitted to the scatter of carapace length versus carapace height, showing a significant linear correlation between them.In addition, the principal component analysis of the linear measurements and fourier coefficients for the outlines of the conchostracan specimens indicates a great deal of intraspecific variations among these four species.Finally, it is suggested that the upper part of the Manghuai Formation in the southern Lancangjiang zone is the Middle Triassic (Ladinian) depositions according to the fossils assemblage and isotopic dating evidences.
The Triassic volcanic and sedimentary rocks are the important components in the geological records of the southern Lancangjiang zone.However, little is known about the biostratigraphic correlation in this area.In this study, three species and one indeterminate species in one genera of conchostracans were recognized for the first time from the Manghuai Formation of the study area, including Euestheria minuta, Euestheria yipinglangensis, Euestheria dazuensis and Euestheria sp..For the purpose of the comparison, quantitative and semi-quantitative descriptions of the specimens, we took eight standard linear measurements from each well-preserved specimen.A reduced major axis (RMA) regression line has been fitted to the scatter of carapace length versus carapace height, showing a significant linear correlation between them.In addition, the principal component analysis of the linear measurements and fourier coefficients for the outlines of the conchostracan specimens indicates a great deal of intraspecific variations among these four species.Finally, it is suggested that the upper part of the Manghuai Formation in the southern Lancangjiang zone is the Middle Triassic (Ladinian) depositions according to the fossils assemblage and isotopic dating evidences.
2018, 43(8): 2848-2857.
doi: 10.3799/dqkx.2018.103
Abstract:
The process and cause about the mass extinction event across the Paleozoic-Mesozoic transition have always been one of the key paleontological issues in the past few decades. The conodont zoning of the Selong Xishan section has made important progress, providing new evidence for understanding the stratigraphic sequence and sedimentary history of the area. Carbon isotope analyses of bulk samples in this study indicate that there are negative shifts across the Changshingian-Induan Boundary, which are comparable to South China. Newly identified carbon isotope negative shift at earliest Olenekian, base of Neospathodus waageni zone and the following positive shift coincide with the pattern recognized in South China and other places, implying the multi-phase extinction-recovery processes in Early Triassic.
The process and cause about the mass extinction event across the Paleozoic-Mesozoic transition have always been one of the key paleontological issues in the past few decades. The conodont zoning of the Selong Xishan section has made important progress, providing new evidence for understanding the stratigraphic sequence and sedimentary history of the area. Carbon isotope analyses of bulk samples in this study indicate that there are negative shifts across the Changshingian-Induan Boundary, which are comparable to South China. Newly identified carbon isotope negative shift at earliest Olenekian, base of Neospathodus waageni zone and the following positive shift coincide with the pattern recognized in South China and other places, implying the multi-phase extinction-recovery processes in Early Triassic.
2018, 43(8): 2858-2874.
doi: 10.3799/dqkx.2018.111
Abstract:
The Qingcaoshan porphyry Cu-Au deposit, located in the northern Bangong Co-Nujiang belt, is a newly discovered porphyry deposit with huge potential.However, the age, origin and source area of the ore-bearing porphyry have not been effectively constrained.In this paper, we present the zircon geochronology, Hf isotope, and geochemistry of Qingcaoshan granodiorite and granodiorite porphyry.The analysis results show that the ages of granodiorite porphyry and granodiorite are 117.9±0.8 Ma and 131.2±0.3 Ma, respectively, which represent porphyry mineralization of Bangong Co-Nujiang metallogenic belt.They also exhibit relatively uniform in-situ zircon Hf isotopic compositions.In conclusion, Qingcaoshan ore-bearing porphyries were derived from the partial melting of mafic lower crust, which were induced by the subduction of the Bangong Co-Nujiang ocean crust, and they were also mixed by a small amount of enriched lithospheric mantle.
The Qingcaoshan porphyry Cu-Au deposit, located in the northern Bangong Co-Nujiang belt, is a newly discovered porphyry deposit with huge potential.However, the age, origin and source area of the ore-bearing porphyry have not been effectively constrained.In this paper, we present the zircon geochronology, Hf isotope, and geochemistry of Qingcaoshan granodiorite and granodiorite porphyry.The analysis results show that the ages of granodiorite porphyry and granodiorite are 117.9±0.8 Ma and 131.2±0.3 Ma, respectively, which represent porphyry mineralization of Bangong Co-Nujiang metallogenic belt.They also exhibit relatively uniform in-situ zircon Hf isotopic compositions.In conclusion, Qingcaoshan ore-bearing porphyries were derived from the partial melting of mafic lower crust, which were induced by the subduction of the Bangong Co-Nujiang ocean crust, and they were also mixed by a small amount of enriched lithospheric mantle.
2018, 43(8): 2875-2892.
doi: 10.3799/dqkx.2018.112
Abstract:
Orogenic peridotites(M-type) are uncommon in high-pressure(HP) and ultrahigh-pressure(UHP) metamorphic belts. However, they are not only a key to study the metamorphic and metasomatic information from subducting slabs and mantle wedges in subduction channel, but also of great significance to the evolution of orogenic belt. There is a set of peridotite in Kaipinggou near the edge of the Iqe eclogite-gneiss region, of which rock types, genesis, and age are short of research. Whole-rock major and trace and platinum-group elements, olive mineral elements, and zircon U-Pb dating and Hf isotopes for peridotite at Kaipinggou are studied in this paper. Kaipinggou peridotites are characterized by high contents of Mg#, Mg/Si and Ni, showing similar distribution patterns of REE and trace elements, depletion in HFSE and HREE, and slightly enrichment in LILE and LREE. Olivines have high Fo(90.11-92.77)and NiO(0.32%-0.45%) contents, but low contents of CaO(0.02%). Whole-rock platinum group elements(PGEs) are similar to those of metamorphic and residual peridotites. There are two groups of metamorphic zircons in the Kaipinggou peridotites, with ages of 459.5±3.6 Ma and 417.5±2.7 Ma, corresponding to εHf(t) of -0.71 to 9.45 and -11.96 to -1.20, respectively, indicating the nature and timing of the fluid derived from oceanic crust(or early continental crust subduction) and continental crust. In conclusion, the Kaipinggou peridotites are derived from the mantle wedge of the lithosphere and record the crustal features due to the metasomatism of the oceanic and continental crust fluids. The novel findings provide new evidence for the evolution of oceanic lithosphere subduction to continental collision/subduction in the North Qaidam.
Orogenic peridotites(M-type) are uncommon in high-pressure(HP) and ultrahigh-pressure(UHP) metamorphic belts. However, they are not only a key to study the metamorphic and metasomatic information from subducting slabs and mantle wedges in subduction channel, but also of great significance to the evolution of orogenic belt. There is a set of peridotite in Kaipinggou near the edge of the Iqe eclogite-gneiss region, of which rock types, genesis, and age are short of research. Whole-rock major and trace and platinum-group elements, olive mineral elements, and zircon U-Pb dating and Hf isotopes for peridotite at Kaipinggou are studied in this paper. Kaipinggou peridotites are characterized by high contents of Mg#, Mg/Si and Ni, showing similar distribution patterns of REE and trace elements, depletion in HFSE and HREE, and slightly enrichment in LILE and LREE. Olivines have high Fo(90.11-92.77)and NiO(0.32%-0.45%) contents, but low contents of CaO(0.02%). Whole-rock platinum group elements(PGEs) are similar to those of metamorphic and residual peridotites. There are two groups of metamorphic zircons in the Kaipinggou peridotites, with ages of 459.5±3.6 Ma and 417.5±2.7 Ma, corresponding to εHf(t) of -0.71 to 9.45 and -11.96 to -1.20, respectively, indicating the nature and timing of the fluid derived from oceanic crust(or early continental crust subduction) and continental crust. In conclusion, the Kaipinggou peridotites are derived from the mantle wedge of the lithosphere and record the crustal features due to the metasomatism of the oceanic and continental crust fluids. The novel findings provide new evidence for the evolution of oceanic lithosphere subduction to continental collision/subduction in the North Qaidam.
2018, 43(8): 2893-2910.
doi: 10.3799/dqkx.2018.200
Abstract:
The previous work is mainly concentrated in the Gangdise metallogenic belt in Cenozoic magmatite and mineral resources, but the study of Late Paleozoic sedimentary strata of the Gangdise belt is rarely reported.The recent 1:5 000 regional geological survey on western part of Gangdise metallogenic belt shows that there are a large number of exposed Late Paleozoic sedimentary strata of Yunzhug Formation.Taking the exposed Yunzhug Formation in Tibet area as the research object, the Yunzhug Formation sedimentary environment is analyzed through the geological survey, profile measurement, thin section identification, microfossils, particle size analysis and whole rock analysis firstly, and then the relationship with Gangdis is discussed in this study.It is found that Yunzhug Formation formed in Late Carboniferous (C2) Bashikirian-Moscovian (Middle and Late Bashki-Moscow period), sporopollen fossils by gymnosperm pollen is predominant (80%) and fern spore in subordinate status (20%).Typical sedimentary structures such as horizontal bedding, parallel bedding, graded bedding, turbidite bedding, slump deformation structure and the Bauma sequence are developed in the Yunzhug Formation.Yunzhug Formation sedimentary rocks show high Si(SiO2=60.94%-76.24%), Al(Al2O3=9.24%-13.20%), K(K2O=2.81%-5.47%)and low Mg(MgO=1.32%-2.82%), Ca(CaO=1.19%-2.78%)and Na(Na2O=0.2%-1.87%)compositions, with obvious high FeO content (3.31%-6.67%) and low Fe2O3 content (0.14%-0.65%).The ∑REE(174.24×10-6-278.39×10-6) is higher, and there is no Ce anomaly in the North American shale normalized REE pattern, and its Ce/Ce* is 0.94-1.03.Trace elements showed enriched elements such as Rb, Th, K, U, Nb, Ta, Sr, P, Ti and other elements.Yunzhug Formation formed in active continental margin environment, deposited on the continental slope and deep basin, as the product of warm climate, regression deposition in salinity environment, is in response to the Paleo-Tethys deposits in the Gangdise area of activity.
The previous work is mainly concentrated in the Gangdise metallogenic belt in Cenozoic magmatite and mineral resources, but the study of Late Paleozoic sedimentary strata of the Gangdise belt is rarely reported.The recent 1:5 000 regional geological survey on western part of Gangdise metallogenic belt shows that there are a large number of exposed Late Paleozoic sedimentary strata of Yunzhug Formation.Taking the exposed Yunzhug Formation in Tibet area as the research object, the Yunzhug Formation sedimentary environment is analyzed through the geological survey, profile measurement, thin section identification, microfossils, particle size analysis and whole rock analysis firstly, and then the relationship with Gangdis is discussed in this study.It is found that Yunzhug Formation formed in Late Carboniferous (C2) Bashikirian-Moscovian (Middle and Late Bashki-Moscow period), sporopollen fossils by gymnosperm pollen is predominant (80%) and fern spore in subordinate status (20%).Typical sedimentary structures such as horizontal bedding, parallel bedding, graded bedding, turbidite bedding, slump deformation structure and the Bauma sequence are developed in the Yunzhug Formation.Yunzhug Formation sedimentary rocks show high Si(SiO2=60.94%-76.24%), Al(Al2O3=9.24%-13.20%), K(K2O=2.81%-5.47%)and low Mg(MgO=1.32%-2.82%), Ca(CaO=1.19%-2.78%)and Na(Na2O=0.2%-1.87%)compositions, with obvious high FeO content (3.31%-6.67%) and low Fe2O3 content (0.14%-0.65%).The ∑REE(174.24×10-6-278.39×10-6) is higher, and there is no Ce anomaly in the North American shale normalized REE pattern, and its Ce/Ce* is 0.94-1.03.Trace elements showed enriched elements such as Rb, Th, K, U, Nb, Ta, Sr, P, Ti and other elements.Yunzhug Formation formed in active continental margin environment, deposited on the continental slope and deep basin, as the product of warm climate, regression deposition in salinity environment, is in response to the Paleo-Tethys deposits in the Gangdise area of activity.
