2019 Vol. 44, No. 7
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2019, 44(7): 2197-2206.
doi: 10.3799/dqkx.2019.114
Abstract:
The Lalong dome is located in the southeast margin of the Tethys-Himalayan tectonic belt, which constitutes another important dome belt parallel to the North Himalayan dome belt along with Cuonadong dome and Kuju dome. The dome is separated by fault into a core unit, a transitional zone and a caprock. The core unit is mainly composed of granitic gneiss, leucogranites and pegmatite. The zircon U-Pb age of granitic gneisses is 474.3±5.5 Ma. Leucogranties mainly consist of two-mica granite and muscovite granite. The transitional zone is composed of garnet schist, cross schist, andalusite schist, kyanite schist, marble, skarn and so on. The caprock is composed of Mesozoic epimetamorphic clastic sedimentary rocks. Three types of rare metal mineralization are recognized. The first type is pegmatite beryllium mineralization. The main beryllium-bearing mineral is beryl. The monazite U-Pb age of pegmatite is 23.19 ±0.12 Ma. The second type is skarn beryllium, tungsten, niobium and tantalum with rare polymetallic mineralization. The monazite U-Pb age of muscovite granite in the contact zone with skarn is 23.23±0.27 Ma. The third type is tectonically controlled hydrothermal Cu-Pb-Zn-Ag mineralization. The rare polymetallic mineralization in Lalong dome shows the zonation characteristics of Be, Be-W-Nb-Ta, Cu-Pb-Zn-Ag mineralization with leucogranite as the core. The determination of the Lalong dome indicates the existence of a "double dome" structural belt in the Tethys-Himalayan metallogenic belt. The discovery of rare polymetallic mineralization in the Lalong dome further confirms that the Tethys-Himalayan metallogenic belt has a huge potential for rare metal prospecting and is expected to become the third rare metal metallogenic belt after South China belt and Altai Xinjiang belt.
The Lalong dome is located in the southeast margin of the Tethys-Himalayan tectonic belt, which constitutes another important dome belt parallel to the North Himalayan dome belt along with Cuonadong dome and Kuju dome. The dome is separated by fault into a core unit, a transitional zone and a caprock. The core unit is mainly composed of granitic gneiss, leucogranites and pegmatite. The zircon U-Pb age of granitic gneisses is 474.3±5.5 Ma. Leucogranties mainly consist of two-mica granite and muscovite granite. The transitional zone is composed of garnet schist, cross schist, andalusite schist, kyanite schist, marble, skarn and so on. The caprock is composed of Mesozoic epimetamorphic clastic sedimentary rocks. Three types of rare metal mineralization are recognized. The first type is pegmatite beryllium mineralization. The main beryllium-bearing mineral is beryl. The monazite U-Pb age of pegmatite is 23.19 ±0.12 Ma. The second type is skarn beryllium, tungsten, niobium and tantalum with rare polymetallic mineralization. The monazite U-Pb age of muscovite granite in the contact zone with skarn is 23.23±0.27 Ma. The third type is tectonically controlled hydrothermal Cu-Pb-Zn-Ag mineralization. The rare polymetallic mineralization in Lalong dome shows the zonation characteristics of Be, Be-W-Nb-Ta, Cu-Pb-Zn-Ag mineralization with leucogranite as the core. The determination of the Lalong dome indicates the existence of a "double dome" structural belt in the Tethys-Himalayan metallogenic belt. The discovery of rare polymetallic mineralization in the Lalong dome further confirms that the Tethys-Himalayan metallogenic belt has a huge potential for rare metal prospecting and is expected to become the third rare metal metallogenic belt after South China belt and Altai Xinjiang belt.
2019, 44(7): 2207-2223.
doi: 10.3799/dqkx.2019.038
Abstract:
The Cuonadong gneiss dome is a newly discovered dome in the North Himalaya gneiss domes (NHGD) belt. Mineralization of the Cuonadong super-large Be-W-Sn polymetallic deposit is developed in the Cuonadong dome. The Cuonadong Be-W-Sn polymetallic ore bodies exist in skarn, fractures and (giant) granite. The main genetic type of deposit is skarn type, and the mother rock of skarn type ore body is weakly oriented two-mica granite. In this paper, chronology and geochemical characteristics of metallogenic mother rock (weakly oriented two-mica granite) of skarn type ore body in the Cuonadong super-large Be-W-Sn polymetallic deposit are studied. Zircon U-Pb dating results show that the weakly oriented two-mica granite's formation time is 16.5±0.3 Ma, which belongs to magmatic activity of Miocene leucogranite. It shows that the Cuonadong super-large Be-W-Sn polymetallic deposit, which is the product of the extension stage of Himalayan collision orogenic process, was formed in Miocene. Geochemical data show that metallogenic mother rock (weakly oriented two-mica granite) is the calcium alkaline and strong peraluminous granite with high silica (73.36%-73.89%), low iron(0.96%-1.58%). Its total rare earth is relatively low, with enrichment of LREE, depletion of HREE and obvious negative Eu anomaly. The two-mica granite is characterized by an enrichment of some large iron lithophile elements (Rb, Th) and loss of high field strength elements (Zr, Ti).The geochemical characteristics of two-mica granite show that it is a set of highly fractionated leucogranite. The study shows that the weakly oriented two-mica granite may be the product of remelting of metaargillaceous rocks, which is closely related to the activities of the South Tibetan detachment system.
The Cuonadong gneiss dome is a newly discovered dome in the North Himalaya gneiss domes (NHGD) belt. Mineralization of the Cuonadong super-large Be-W-Sn polymetallic deposit is developed in the Cuonadong dome. The Cuonadong Be-W-Sn polymetallic ore bodies exist in skarn, fractures and (giant) granite. The main genetic type of deposit is skarn type, and the mother rock of skarn type ore body is weakly oriented two-mica granite. In this paper, chronology and geochemical characteristics of metallogenic mother rock (weakly oriented two-mica granite) of skarn type ore body in the Cuonadong super-large Be-W-Sn polymetallic deposit are studied. Zircon U-Pb dating results show that the weakly oriented two-mica granite's formation time is 16.5±0.3 Ma, which belongs to magmatic activity of Miocene leucogranite. It shows that the Cuonadong super-large Be-W-Sn polymetallic deposit, which is the product of the extension stage of Himalayan collision orogenic process, was formed in Miocene. Geochemical data show that metallogenic mother rock (weakly oriented two-mica granite) is the calcium alkaline and strong peraluminous granite with high silica (73.36%-73.89%), low iron(0.96%-1.58%). Its total rare earth is relatively low, with enrichment of LREE, depletion of HREE and obvious negative Eu anomaly. The two-mica granite is characterized by an enrichment of some large iron lithophile elements (Rb, Th) and loss of high field strength elements (Zr, Ti).The geochemical characteristics of two-mica granite show that it is a set of highly fractionated leucogranite. The study shows that the weakly oriented two-mica granite may be the product of remelting of metaargillaceous rocks, which is closely related to the activities of the South Tibetan detachment system.
2019, 44(7): 2224-2233.
doi: 10.3799/dqkx.2019.024
Abstract:
The Sumdo Formation, which is widely developed in the central Gangdise, has been controversial in its stratigraphic age due to lack of fossil evidence, seriously restricting our understanding of structural evolution in the Sumdo area. In this paper, on the basis of detailed field geological investigation, LA-ICP-MS U-Pb dating was carried out on detrital zircons from metamorphic sandstones of Sumdo Formation. The minimum zircon age of detrital zircon in metamorphic sandstones is 316 Ma, the Late Carboniferous, representing the lower limit of the main depositional age of the Sumdo Formation. Provenance analysis indicates that the sedimentary detritus was sourced from the Lhasa terrane itself (South and North Lhasa terranes), and may be the sedimentary record of the initial ocean basin of Sumdo Paleo-Tethys Ocean. The determination of the age of the Sumdo Formation has further improved the Late Paleozoic stratigraphic system in this area and is of great significance to the study of lithofacies and paleogeography of the Late Carboniferous-Early Permian in the northern margin of the Gondwana.
The Sumdo Formation, which is widely developed in the central Gangdise, has been controversial in its stratigraphic age due to lack of fossil evidence, seriously restricting our understanding of structural evolution in the Sumdo area. In this paper, on the basis of detailed field geological investigation, LA-ICP-MS U-Pb dating was carried out on detrital zircons from metamorphic sandstones of Sumdo Formation. The minimum zircon age of detrital zircon in metamorphic sandstones is 316 Ma, the Late Carboniferous, representing the lower limit of the main depositional age of the Sumdo Formation. Provenance analysis indicates that the sedimentary detritus was sourced from the Lhasa terrane itself (South and North Lhasa terranes), and may be the sedimentary record of the initial ocean basin of Sumdo Paleo-Tethys Ocean. The determination of the age of the Sumdo Formation has further improved the Late Paleozoic stratigraphic system in this area and is of great significance to the study of lithofacies and paleogeography of the Late Carboniferous-Early Permian in the northern margin of the Gondwana.
2019, 44(7): 2234-2248.
doi: 10.3799/dqkx.2019.148
Abstract:
The Sumdo eclogite is an important material for constraining the evolution of the Paleo-Tethys in the central part of the Lhasa plate. Detailed studies have been carried out on its geochronological data and metamorphic P-T conditions. The study of eclogite protoliths is relatively weak. The geochemical analysis of eclogites is an important means to restore the composition of the original rocks, and it is also an important evidence for retrieving the evolution process of the Paleo-Tethys in the Sumdo area. Based on the systematic collection and classification of the previous geochemical data of Sumdo eclogite, 17 geochemical samples were collected from Sumdo eclogite for systematic analysis, which is intended to provide new constraints on regional tectonic evolution. Two types of eclogites were exposed in the Sumdo area, which are bimineralic eclogite and retrograded eclogite. Both types of eclogites are characterized by sub-alkaline basalts. In the trace elements and various tectonic environment discrimination diagrams, two kinds of eclogites are all in the N-MORB and E-MORB areas. Based on the above analysis results, combined with regional geological data, the interaction of the mantle plum mass and the normal mid-ocean ridge during the process of Paleo-Tethys Ocean has existed for a long time.
The Sumdo eclogite is an important material for constraining the evolution of the Paleo-Tethys in the central part of the Lhasa plate. Detailed studies have been carried out on its geochronological data and metamorphic P-T conditions. The study of eclogite protoliths is relatively weak. The geochemical analysis of eclogites is an important means to restore the composition of the original rocks, and it is also an important evidence for retrieving the evolution process of the Paleo-Tethys in the Sumdo area. Based on the systematic collection and classification of the previous geochemical data of Sumdo eclogite, 17 geochemical samples were collected from Sumdo eclogite for systematic analysis, which is intended to provide new constraints on regional tectonic evolution. Two types of eclogites were exposed in the Sumdo area, which are bimineralic eclogite and retrograded eclogite. Both types of eclogites are characterized by sub-alkaline basalts. In the trace elements and various tectonic environment discrimination diagrams, two kinds of eclogites are all in the N-MORB and E-MORB areas. Based on the above analysis results, combined with regional geological data, the interaction of the mantle plum mass and the normal mid-ocean ridge during the process of Paleo-Tethys Ocean has existed for a long time.
2019, 44(7): 2249-2264.
doi: 10.3799/dqkx.2019.100
Abstract:
The Sumdo green schist is distributed in the Sumdo area in the segment of the Lhasa block. Because of the low level of research on its age and tectonic characteristics, it has seriously constrained our understanding of the evolution of the Sumdo Paleo-Tethys Ocean. Based on the detailed field geological survey, petrology, whole-rock geochemistry, LA-ICP-MS zircon U-Pb isotope and zircon Hf isotope analysis were carried out on the green schist. The zircon U-Pb age is approximately 232 Ma in the Sumdo green schist, which is the Middle Triassic, representing the protolith's crystallization of the green schist. The εHf(t) of zircons is 3.6 on average, showing the characteristics of a weakly depleted mantle. Petrology and geochemistry studies reveal that the protolith of green schist has characteristics of tholeiite, which appears as a flat curve in the chondrite-normalized REE patterns and primitive mantle-normalized trace element spidergrams, similar to N-MORB attributes. In all kinds of discriminative diagrams, they all fall into the N-MORB field, indicating that the protolith of Sumdo green schist may be an oceanic crust fragment from the Sumdo Paleo-Tethys Ocean. The identification of the Middle Triassic green schist in Sumdo indicates the Sumdo Paleo-Tethys Ocean may hold oceanic basin during the Middle Triassic.
The Sumdo green schist is distributed in the Sumdo area in the segment of the Lhasa block. Because of the low level of research on its age and tectonic characteristics, it has seriously constrained our understanding of the evolution of the Sumdo Paleo-Tethys Ocean. Based on the detailed field geological survey, petrology, whole-rock geochemistry, LA-ICP-MS zircon U-Pb isotope and zircon Hf isotope analysis were carried out on the green schist. The zircon U-Pb age is approximately 232 Ma in the Sumdo green schist, which is the Middle Triassic, representing the protolith's crystallization of the green schist. The εHf(t) of zircons is 3.6 on average, showing the characteristics of a weakly depleted mantle. Petrology and geochemistry studies reveal that the protolith of green schist has characteristics of tholeiite, which appears as a flat curve in the chondrite-normalized REE patterns and primitive mantle-normalized trace element spidergrams, similar to N-MORB attributes. In all kinds of discriminative diagrams, they all fall into the N-MORB field, indicating that the protolith of Sumdo green schist may be an oceanic crust fragment from the Sumdo Paleo-Tethys Ocean. The identification of the Middle Triassic green schist in Sumdo indicates the Sumdo Paleo-Tethys Ocean may hold oceanic basin during the Middle Triassic.
2019, 44(7): 2265-2274.
doi: 10.3799/dqkx.2019.099
Abstract:
The Gangqiongla Mo-Cu deposit is a newly discovered quartz vein type Mo-Cu deposit in the eastern Gangdise metallogenic belt. This paper first presents the Re-Os geochronology of molybdenite in the Gangqiongla quartz vein Mo-Cu deposit. Eight molybdenite samples from the quartz vein molybdenite yielded a weighted average age of 22.6±0.2 Ma(MSWD=1.3)and an isochron age of 24.1±1.3 Ma (MSWD=1.1), which is interpreted to represent the age of main stage ore, and is earlier than the age ranging from 17 Ma to 12 Ma of other porphyry Cu-Mo mineralization in the southern Gangdise metallogenic belt. The Re contents on molybdenite samples range from 239.1×10-6 to 314.7×10-6, suggesting that mantle materials more or less contribute to the main stage mineralization. Based on the Oligocene-Early Miocene regional mineralization data, the formation of Gangqiongla Mo-Cu deposit was associated with the Early Miocene transition stage from collision to extension between the India and Eurasia continents.
The Gangqiongla Mo-Cu deposit is a newly discovered quartz vein type Mo-Cu deposit in the eastern Gangdise metallogenic belt. This paper first presents the Re-Os geochronology of molybdenite in the Gangqiongla quartz vein Mo-Cu deposit. Eight molybdenite samples from the quartz vein molybdenite yielded a weighted average age of 22.6±0.2 Ma(MSWD=1.3)and an isochron age of 24.1±1.3 Ma (MSWD=1.1), which is interpreted to represent the age of main stage ore, and is earlier than the age ranging from 17 Ma to 12 Ma of other porphyry Cu-Mo mineralization in the southern Gangdise metallogenic belt. The Re contents on molybdenite samples range from 239.1×10-6 to 314.7×10-6, suggesting that mantle materials more or less contribute to the main stage mineralization. Based on the Oligocene-Early Miocene regional mineralization data, the formation of Gangqiongla Mo-Cu deposit was associated with the Early Miocene transition stage from collision to extension between the India and Eurasia continents.
2019, 44(7): 2275-2294.
doi: 10.3799/dqkx.2019.034
Abstract:
The genesis of volcanic rocks in Dianzhong Formation of Linzizong Group has been deeply studied, but a large number of granitic porphyry emplaced has been neglected. Based on the field geological survey, the chronology, petrogeochemistry and Sr-Nd-Pb-Hf isotopes of granitic porphyry of Daruo area in the western Lhasa block were studied. The results show that the diagenetic ages of the two granitic porphyry samples are 61.9±0.3 Ma (MSWD=0.17) and 61.1±0.6 Ma (MSWD=0.69), respectively, indicating that they are products of Paleocene magmatic activities. Hornblende and aluminum-rich minerals are not found in the rocks, indicating they belong to the series of high potassium calc-potassium basaltic rocks, which are characterized by high SiO2 (76.16%-82.78%, average 78.28%), high alkali (K2O+Na2O=4.16%-6.93%, average 6.09%), low CaO(0.11%-0.16%, average 0.14%) and P2O5(0.02%-0.04%, average 0.03%). It is enriched in Rb, Th, K and LREE, and depleted in Ba, Nb, Sr, P, Ti and HREE. LREE and HREE have strong fractionation, and the negative Eu is very significant. These characteristics indicate that it belongs to weakly peraluminous and highly fractionated I-type granites. Daruo granite porphyry is rich in radiogenetic Pb, and the values of (208Pb/204Pb)t, (207Pb/204Pb)t and (206Pb/204Pb)t are 38.737-38.944, 15.661-15.682 and 18.079-18.624, respectively. Meanwhile, the granite porphyry has high (87Sr/86Sr)i value (0.722 739-0.744 497) and εNd(t) values (-6.82—-6.67), the εHf(t) values of zircon are only weakly and diffusely negative, and TDM2 is between 1 083 and 1 273 Ma, which means that there is a certain degree of decoupling between Nd-Hf isotopes. According to the comprehensive study, Daruo granite porphyry was formed in the early stage of the main collision convergence of the India-Eurasia plate (65-41 Ma), and the parent magma, formed by the interaction of the hysteretic subduction Neo-Tethys oceanic crust and mantle rocks, underplated beneath the ancient crust of Lhasa block, causing it to remelt and mix with a small part of mantle magma, then the granite porphyry was generated after a high degree of crystallization differentiation.
The genesis of volcanic rocks in Dianzhong Formation of Linzizong Group has been deeply studied, but a large number of granitic porphyry emplaced has been neglected. Based on the field geological survey, the chronology, petrogeochemistry and Sr-Nd-Pb-Hf isotopes of granitic porphyry of Daruo area in the western Lhasa block were studied. The results show that the diagenetic ages of the two granitic porphyry samples are 61.9±0.3 Ma (MSWD=0.17) and 61.1±0.6 Ma (MSWD=0.69), respectively, indicating that they are products of Paleocene magmatic activities. Hornblende and aluminum-rich minerals are not found in the rocks, indicating they belong to the series of high potassium calc-potassium basaltic rocks, which are characterized by high SiO2 (76.16%-82.78%, average 78.28%), high alkali (K2O+Na2O=4.16%-6.93%, average 6.09%), low CaO(0.11%-0.16%, average 0.14%) and P2O5(0.02%-0.04%, average 0.03%). It is enriched in Rb, Th, K and LREE, and depleted in Ba, Nb, Sr, P, Ti and HREE. LREE and HREE have strong fractionation, and the negative Eu is very significant. These characteristics indicate that it belongs to weakly peraluminous and highly fractionated I-type granites. Daruo granite porphyry is rich in radiogenetic Pb, and the values of (208Pb/204Pb)t, (207Pb/204Pb)t and (206Pb/204Pb)t are 38.737-38.944, 15.661-15.682 and 18.079-18.624, respectively. Meanwhile, the granite porphyry has high (87Sr/86Sr)i value (0.722 739-0.744 497) and εNd(t) values (-6.82—-6.67), the εHf(t) values of zircon are only weakly and diffusely negative, and TDM2 is between 1 083 and 1 273 Ma, which means that there is a certain degree of decoupling between Nd-Hf isotopes. According to the comprehensive study, Daruo granite porphyry was formed in the early stage of the main collision convergence of the India-Eurasia plate (65-41 Ma), and the parent magma, formed by the interaction of the hysteretic subduction Neo-Tethys oceanic crust and mantle rocks, underplated beneath the ancient crust of Lhasa block, causing it to remelt and mix with a small part of mantle magma, then the granite porphyry was generated after a high degree of crystallization differentiation.
2019, 44(7): 2295-2307.
doi: 10.3799/dqkx.2019.023
Abstract:
The petrogenetic differences of the Early Jurassic magmatic rocks within the central and southern Lhasa subterranes are indicative of the early-stage evolution of the Neo-Tethyan Ocean. In this study, we carry out petrography, geochronology and geochemistry analysis on Zongwo granitic pluton and Zhongda granitic pluton on both sides of the Luobadui-Milashan Fault. The zircon U-Pb dating results from two plutons are 193.8±2.2 Ma and 197.5±1.8 Ma, indicating the Early Jurassic magmatism in the study area. Granites samples from the Early Jurassic plutons are characterized by high SiO2 (69.80%-74.64%) contents, low A/CNK (0.98-1.07), enrichments in lighter rare earth elements (LREEs) and the large-ion-lithophile elements (LILEs) such as Rb, Th, K, and depletions in high-field strength elements (HFSEs) such as Nb, Ta, Ti. Geochemical characteristics show that both samples are typical Ⅰ-type volcanic arc magmatic rocks. Combined with previous research results, it is shown that the Early Jurassic magmatic rocks in the central and southern Lhasa subterranes were formed by the northward subduction of the Neo-Tethyan oceanic crust. Our data, in combination with geochemical and isotopic data indicate that the Early Jurassic magmatic rocks in the southern Lhasa subterrane were mainly derived from the juvenile lower crust, while the magmatic source of the magmatic rocks in the central Lhasa subterrane had more involvement of ancient lower crustal components.
The petrogenetic differences of the Early Jurassic magmatic rocks within the central and southern Lhasa subterranes are indicative of the early-stage evolution of the Neo-Tethyan Ocean. In this study, we carry out petrography, geochronology and geochemistry analysis on Zongwo granitic pluton and Zhongda granitic pluton on both sides of the Luobadui-Milashan Fault. The zircon U-Pb dating results from two plutons are 193.8±2.2 Ma and 197.5±1.8 Ma, indicating the Early Jurassic magmatism in the study area. Granites samples from the Early Jurassic plutons are characterized by high SiO2 (69.80%-74.64%) contents, low A/CNK (0.98-1.07), enrichments in lighter rare earth elements (LREEs) and the large-ion-lithophile elements (LILEs) such as Rb, Th, K, and depletions in high-field strength elements (HFSEs) such as Nb, Ta, Ti. Geochemical characteristics show that both samples are typical Ⅰ-type volcanic arc magmatic rocks. Combined with previous research results, it is shown that the Early Jurassic magmatic rocks in the central and southern Lhasa subterranes were formed by the northward subduction of the Neo-Tethyan oceanic crust. Our data, in combination with geochemical and isotopic data indicate that the Early Jurassic magmatic rocks in the southern Lhasa subterrane were mainly derived from the juvenile lower crust, while the magmatic source of the magmatic rocks in the central Lhasa subterrane had more involvement of ancient lower crustal components.
2019, 44(7): 2308-2318.
doi: 10.3799/dqkx.2019.172
Abstract:
The Tethys Himalayan metallogenic belt contains dozens of gold, gold-antimony and lead-zinc polymetallic ore deposits with variable sizes. Some Be-Rb-Sn-W polymetallic deposits in the gneiss dome have been found in recent mineral explorations. A 2-stage gold-antimony-lead-zinc mineralization in the orogenic belt has been recognized, one of which is the orogenic gold deposit represented by the Bangbu and Mayum gold deposit. These deposits were formed at 59-45 Ma, belonging to the main collision stage of the India-Asia continental collision. The others are hydrothermal-type gold-antimony-lead-zinc deposits represented by the Jienagepu gold deposit, Cheqiongzhuobu vein-type antimony deposit, Zhaxikang lead-zinc polymetallic vein-type deposit and Jisong lead-zinc deposit. The formation of the deposits is concentrated in the post-collision orogenic stage of 21-12 Ma. A large number of fluid inclusion researches indicate that the ore-forming fluids of Himalayan gold-antimony-lead-zinc metallogenic belt are mainly medium-low temperature (less than 300℃) and medium-low salinity fluid (< 10% NaCleqv). This paper presented a total of 169 H-O isotopic data of quartz, sericite and rhodochrosite that have been published and newly obtained from experiments. It is found that these isotopic compositions constitute three endmembers in the δ18OH2O-δDV-SMOW phase diagram. The endmember A (the Cheqiongzhuobu-type) has characteristics of very low δ18OH2O value (< -13‰) and low δDV-SMOW value (< -111‰, which is close to the Modern Rainwater Line(MRL), and completely falls into the H-O isotope range of Tibet geothermal water. The endmember B (the Shalagang-type) shows the lowest δDV-SMOW value (lowest to -172‰) and higher δ18OH2O (up to 12‰) value than A, that falls within range of the Formation Water. The endmember C constitutes very high δDV-SMOW value (up to -43‰) and a medium δ18OH2O value, which has two typical representative types of the Bangbu-type and the Langkazi-type. The Bangbu-type has the same oxygen isotope range(6‰-13‰) as that of orogenic gold deposit. However, the Langkazi-type shows same oxygen isotope range with the Original Magma Water(OMW). The ore-forming fluids hydrogen and oxygen isotopes of most deposits in Himalayan collisional belt fall among endmembers A, B and C, indicating that most of ore-forming fluids are not of a single fluid source, but have the characteristics of fluid mixing with multiple sources.
The Tethys Himalayan metallogenic belt contains dozens of gold, gold-antimony and lead-zinc polymetallic ore deposits with variable sizes. Some Be-Rb-Sn-W polymetallic deposits in the gneiss dome have been found in recent mineral explorations. A 2-stage gold-antimony-lead-zinc mineralization in the orogenic belt has been recognized, one of which is the orogenic gold deposit represented by the Bangbu and Mayum gold deposit. These deposits were formed at 59-45 Ma, belonging to the main collision stage of the India-Asia continental collision. The others are hydrothermal-type gold-antimony-lead-zinc deposits represented by the Jienagepu gold deposit, Cheqiongzhuobu vein-type antimony deposit, Zhaxikang lead-zinc polymetallic vein-type deposit and Jisong lead-zinc deposit. The formation of the deposits is concentrated in the post-collision orogenic stage of 21-12 Ma. A large number of fluid inclusion researches indicate that the ore-forming fluids of Himalayan gold-antimony-lead-zinc metallogenic belt are mainly medium-low temperature (less than 300℃) and medium-low salinity fluid (< 10% NaCleqv). This paper presented a total of 169 H-O isotopic data of quartz, sericite and rhodochrosite that have been published and newly obtained from experiments. It is found that these isotopic compositions constitute three endmembers in the δ18OH2O-δDV-SMOW phase diagram. The endmember A (the Cheqiongzhuobu-type) has characteristics of very low δ18OH2O value (< -13‰) and low δDV-SMOW value (< -111‰, which is close to the Modern Rainwater Line(MRL), and completely falls into the H-O isotope range of Tibet geothermal water. The endmember B (the Shalagang-type) shows the lowest δDV-SMOW value (lowest to -172‰) and higher δ18OH2O (up to 12‰) value than A, that falls within range of the Formation Water. The endmember C constitutes very high δDV-SMOW value (up to -43‰) and a medium δ18OH2O value, which has two typical representative types of the Bangbu-type and the Langkazi-type. The Bangbu-type has the same oxygen isotope range(6‰-13‰) as that of orogenic gold deposit. However, the Langkazi-type shows same oxygen isotope range with the Original Magma Water(OMW). The ore-forming fluids hydrogen and oxygen isotopes of most deposits in Himalayan collisional belt fall among endmembers A, B and C, indicating that most of ore-forming fluids are not of a single fluid source, but have the characteristics of fluid mixing with multiple sources.
2019, 44(7): 2319-2338.
doi: 10.3799/dqkx.2019.147
Abstract:
The volcanic rocks of the Yeba Formation on the Gangdese magmatic arc, Tibet are of importance for limiting the initiation of the Neo-Tethyan oceanic slab subduction. This paper reports LA-ICP-MS zircon U-Pb ages and whole-rock geochemistry compositions of the tuffs from the Yeba Formation in Jialulang area. LA-ICP-MS U-Pb isotopic dating of the tuffs obtained 206Pb/238U age weighted average values are 207.8±1.6 Ma, 204.8±1.7 Ma and 209.3±3.4 Ma, based on this analysis above and the evidence of fossil organism as well, the tuffs of the Yeba Formation were formed in the Late Triassic. The tuffs exhibit LREE and LILE (Rb, K, Th, U, Pb) enrichment and HREE and HFSE (Nb, Ta, Ti, P) depletion, depletion of Sr as well, with negative Eu anomaly. Combining with geochemical characteristics, it has been suggested a mixing source from crust and depleted mantle for the intermediate-acid rocks from the Yeba Formation. Based on the previous studies, it is indicated that the volcanic rocks of the Yeba Formation were formed in the active continental margin arc setting with the subduction of the Neo-Tethyan oceanic slab, the initiation of the subduction occurred at Late Triassic. This study provides new constraints for the ages and dynamic settings of the Yeba Formation.
The volcanic rocks of the Yeba Formation on the Gangdese magmatic arc, Tibet are of importance for limiting the initiation of the Neo-Tethyan oceanic slab subduction. This paper reports LA-ICP-MS zircon U-Pb ages and whole-rock geochemistry compositions of the tuffs from the Yeba Formation in Jialulang area. LA-ICP-MS U-Pb isotopic dating of the tuffs obtained 206Pb/238U age weighted average values are 207.8±1.6 Ma, 204.8±1.7 Ma and 209.3±3.4 Ma, based on this analysis above and the evidence of fossil organism as well, the tuffs of the Yeba Formation were formed in the Late Triassic. The tuffs exhibit LREE and LILE (Rb, K, Th, U, Pb) enrichment and HREE and HFSE (Nb, Ta, Ti, P) depletion, depletion of Sr as well, with negative Eu anomaly. Combining with geochemical characteristics, it has been suggested a mixing source from crust and depleted mantle for the intermediate-acid rocks from the Yeba Formation. Based on the previous studies, it is indicated that the volcanic rocks of the Yeba Formation were formed in the active continental margin arc setting with the subduction of the Neo-Tethyan oceanic slab, the initiation of the subduction occurred at Late Triassic. This study provides new constraints for the ages and dynamic settings of the Yeba Formation.
2019, 44(7): 2339-2352.
doi: 10.3799/dqkx.2019.051
Abstract:
The study on the evolution model of the New Tethys Ocean in Yarlung Zangbo has been seriously restricted due to the lack of in-depth research on magmatism during and even before the Late Triassic In order to discuss the geodynamic background of the Late Triassic magmatism in Dajia Co area of western Gangdise, this paper presents a study on petrology, geochemistry and chronology of the Ruerma monzodiorite. LA-ICP-MS zircon U-Pb analyses suggest that monzodiorite has two weighted mean 206Pb/238U ages of 212.1±0.6 Ma (mean square weighted deviation=0.97), and 212.8±0.2 Ma (mean square weighted deviation=0.74), which suggests that the Luerma magmatism event took place during the late Triassic epoch. The rocks contain plagioclase, potassium feldspar and amphibole, and contain a small amount of quartz, pyroxene, biotite, magnetite and apatite. They have moderate contents of SiO2 (50.75%-54.69%), and A12O3 (13.77%-19.17%), high contents of K2O (2.71%-3.99%), total K2O+Na2O (5.84%-8.65%), and CaO (4.90%-10.14%), with a Reitman index (σ43) of 3.40 to 7.65, and A/CNK values of 0.56 to 1.00. These characteristics suggest that the Luerma monzodiorite is a quasi-aluminous and alkaline-shoshonite series. The rocks are enriched in light rare earth elements (LREE) and large ion lithofile elements (LILE), and depleted in heavy rare earth elements (HREE) and high field strength elements(HFSE), with LREE/HREE ratios of 7.45 to 11.10. They have weakly negative Eu anomalies of 0.83 to 0.95 without obvious Ce anomalies (δCe=0.93-1.04). The relatively low initial 87Sr/86Sr ratios of 0.705 532 to 0.706 135, positive εHf(t) values of 4.97 to 14.10, and two-stage Hf model ages (TDM2) ranging from 348 Ma to 930 Ma, indicate that the rocks derived from a juvenile crust. The monzodiorite's (143Nd/144Nd)t values range from 0.512 639 to 0.512 669, their εHf(t) values range from 0.08 to 0.67, their (206Pb/204Pb)t values range from 18.235 to 18.521, their (207Pb/204Pb)t values range from 15.593 to 15.651, and their (208Pb/204Pb)t values range from 39.948 to 38.579. These data indicate that the Luerma monzodiorite was formed by partial melting of the juvenile crust. We propose that the Luerma monzodiorite was formed during the northward subduction of Yarlu Zangbo oceanic crust of the Neo Tethys Ocean.
The study on the evolution model of the New Tethys Ocean in Yarlung Zangbo has been seriously restricted due to the lack of in-depth research on magmatism during and even before the Late Triassic In order to discuss the geodynamic background of the Late Triassic magmatism in Dajia Co area of western Gangdise, this paper presents a study on petrology, geochemistry and chronology of the Ruerma monzodiorite. LA-ICP-MS zircon U-Pb analyses suggest that monzodiorite has two weighted mean 206Pb/238U ages of 212.1±0.6 Ma (mean square weighted deviation=0.97), and 212.8±0.2 Ma (mean square weighted deviation=0.74), which suggests that the Luerma magmatism event took place during the late Triassic epoch. The rocks contain plagioclase, potassium feldspar and amphibole, and contain a small amount of quartz, pyroxene, biotite, magnetite and apatite. They have moderate contents of SiO2 (50.75%-54.69%), and A12O3 (13.77%-19.17%), high contents of K2O (2.71%-3.99%), total K2O+Na2O (5.84%-8.65%), and CaO (4.90%-10.14%), with a Reitman index (σ43) of 3.40 to 7.65, and A/CNK values of 0.56 to 1.00. These characteristics suggest that the Luerma monzodiorite is a quasi-aluminous and alkaline-shoshonite series. The rocks are enriched in light rare earth elements (LREE) and large ion lithofile elements (LILE), and depleted in heavy rare earth elements (HREE) and high field strength elements(HFSE), with LREE/HREE ratios of 7.45 to 11.10. They have weakly negative Eu anomalies of 0.83 to 0.95 without obvious Ce anomalies (δCe=0.93-1.04). The relatively low initial 87Sr/86Sr ratios of 0.705 532 to 0.706 135, positive εHf(t) values of 4.97 to 14.10, and two-stage Hf model ages (TDM2) ranging from 348 Ma to 930 Ma, indicate that the rocks derived from a juvenile crust. The monzodiorite's (143Nd/144Nd)t values range from 0.512 639 to 0.512 669, their εHf(t) values range from 0.08 to 0.67, their (206Pb/204Pb)t values range from 18.235 to 18.521, their (207Pb/204Pb)t values range from 15.593 to 15.651, and their (208Pb/204Pb)t values range from 39.948 to 38.579. These data indicate that the Luerma monzodiorite was formed by partial melting of the juvenile crust. We propose that the Luerma monzodiorite was formed during the northward subduction of Yarlu Zangbo oceanic crust of the Neo Tethys Ocean.
2019, 44(7): 2441-2460.
doi: 10.3799/dqkx.2019.092
Abstract:
Precambrian meta-sedimentary rock is an important part of the Alxa Block, and accurate determination of the depositional and metamorphic timing of the meta-sedimentary rock is of great scientific significance for further understanding the origin, formation, evolution and affinity of the Alxa Block. This paper presents a detailed study on the field geological investigation, petrological observation, and zircon and monazite U-Pb dating of kyanite-bearing garnet mica quartz schists of the Deerhetongte Formation in the Alatanaobao area. The results of detrital zircon U-Pb dating and field geological investigation show that the crystallization age of detrital zircons of the Deerhetongte and Zuzongmaodao formations in Alatanaobao area ranges from 3 306 Ma to 1 146 Ma, and is characterized by the occurrence of high-frequency regions of Mesoproterozoic zircon age (1 800-1 100 Ma). The crystallization age of the youngest detrital zircons is circa 1 123 Ma, and the meta-sedimentary rocks are intruded by circa 900 Ma granitic gneisses. Thus, the depositional timing of the meta-sedimentary rocks of the the Deerhetongte and Zuzongmaodao formations in the Alatanaobao area is limited to 1 123~900 Ma, and it is a set of Late Mesoproterozoic-Early Neoproterozoic continental margin sedimentary rocks. In addition, the results of U-Pb dating and trace element analysis of metamorphic monazites and zircons indicate that a large number of metamorphic monazites are identified in the kyanite-bearing garnet mica quartz schists of the Deerhetongte Formation. They have typical REE distribution patterns of metamorphic monazites (strong enrichment of light REEs and strong depletion of heavy REEs). These monazites record a weighted average age of 419± 3 Ma (MSWD=7.1, n=40). Similarly, a zircon metamorphic rim also records 206Pb/238U age of 406±7 Ma. The metamorphic ages are interpreted as the timing of the late Early Paleozoic amphibolite-facies metamorphism-deformation of the kyanite-bearing garnet-mica quartz schist of the Deerhetongte Formation, probably in response to the late Early Paleozoic collision orogeny between the Alxa Block and the surrounding micro-continental blocks.
Precambrian meta-sedimentary rock is an important part of the Alxa Block, and accurate determination of the depositional and metamorphic timing of the meta-sedimentary rock is of great scientific significance for further understanding the origin, formation, evolution and affinity of the Alxa Block. This paper presents a detailed study on the field geological investigation, petrological observation, and zircon and monazite U-Pb dating of kyanite-bearing garnet mica quartz schists of the Deerhetongte Formation in the Alatanaobao area. The results of detrital zircon U-Pb dating and field geological investigation show that the crystallization age of detrital zircons of the Deerhetongte and Zuzongmaodao formations in Alatanaobao area ranges from 3 306 Ma to 1 146 Ma, and is characterized by the occurrence of high-frequency regions of Mesoproterozoic zircon age (1 800-1 100 Ma). The crystallization age of the youngest detrital zircons is circa 1 123 Ma, and the meta-sedimentary rocks are intruded by circa 900 Ma granitic gneisses. Thus, the depositional timing of the meta-sedimentary rocks of the the Deerhetongte and Zuzongmaodao formations in the Alatanaobao area is limited to 1 123~900 Ma, and it is a set of Late Mesoproterozoic-Early Neoproterozoic continental margin sedimentary rocks. In addition, the results of U-Pb dating and trace element analysis of metamorphic monazites and zircons indicate that a large number of metamorphic monazites are identified in the kyanite-bearing garnet mica quartz schists of the Deerhetongte Formation. They have typical REE distribution patterns of metamorphic monazites (strong enrichment of light REEs and strong depletion of heavy REEs). These monazites record a weighted average age of 419± 3 Ma (MSWD=7.1, n=40). Similarly, a zircon metamorphic rim also records 206Pb/238U age of 406±7 Ma. The metamorphic ages are interpreted as the timing of the late Early Paleozoic amphibolite-facies metamorphism-deformation of the kyanite-bearing garnet-mica quartz schist of the Deerhetongte Formation, probably in response to the late Early Paleozoic collision orogeny between the Alxa Block and the surrounding micro-continental blocks.
2019, 44(7): 2461-2477.
doi: 10.3799/dqkx.2018.537
Abstract:
The Qinghai Nanshan tectonic belt, situated in the northern margin of Gonghe basin, is the conjunction area among the West Qinling orogenic belt, South Qilian tectonic belt and Zongwulong tectonic belt. A large quantity of intrusive rocks outcropped in the Qinghai Nanshan tectonic belt, but the distribution of basic rocks is less and dispersed. The petrogenesis of basic rocks is significant to reveal the tectonic framework and evolution history in that period.The Lari basic complex is composed of gabbro and pyroxenite, which situates in the middle of the Qinghai Nanshan tectonic belt. In this paper, a detailed study on petrology, mineralogy, geochemistry and LA-ICP-MS zircon U-Pb dating was carried out for the Lari basic complex. The results show that the crystallization ages of the gabbro and pyroxenite are 247.7±2.8 Ma and 247.9±2.5 Ma, respectively, suggesting that the complex intruded in the late Early Triassic. The whole rock geochemical data show that the rocks of Lari basic complex are relatively rich in Na, Mg and Fe, but poor in Si and K, suggesting that the gabbros belong to the tholeiitic series. What's more, they have high ratios of Al2O3/TiO2, low contents of TiO2, which are typical features of boninite and boni-basalts. The rocks are enriched in LREE, LILEs (Cs, Rb, K, Sr) and depleted in HREE (Nb, Ta, Zr, Hf, Ti, P), which are similar to the arc magma, but different from ridge and intraplate basalt. The mineral electron microprobe analysis shows that plagioclase in the rocks of Lari basic complax is similar to the high calcium plagioclase in gabbro which originates in the island arc or active continental margin. The gabbro and pyroxenite of the Lari basic complex were formed by partial melting of the spinel lherzolite. In combination with analyses of regional geological setting, we suggest that there existed a Late Paleozoic-Early Mesozoic limited oceanic basin in the Qinghai Nanshan area. The Lari basic complex formed during the early stage of the oceanic basin subduction.
The Qinghai Nanshan tectonic belt, situated in the northern margin of Gonghe basin, is the conjunction area among the West Qinling orogenic belt, South Qilian tectonic belt and Zongwulong tectonic belt. A large quantity of intrusive rocks outcropped in the Qinghai Nanshan tectonic belt, but the distribution of basic rocks is less and dispersed. The petrogenesis of basic rocks is significant to reveal the tectonic framework and evolution history in that period.The Lari basic complex is composed of gabbro and pyroxenite, which situates in the middle of the Qinghai Nanshan tectonic belt. In this paper, a detailed study on petrology, mineralogy, geochemistry and LA-ICP-MS zircon U-Pb dating was carried out for the Lari basic complex. The results show that the crystallization ages of the gabbro and pyroxenite are 247.7±2.8 Ma and 247.9±2.5 Ma, respectively, suggesting that the complex intruded in the late Early Triassic. The whole rock geochemical data show that the rocks of Lari basic complex are relatively rich in Na, Mg and Fe, but poor in Si and K, suggesting that the gabbros belong to the tholeiitic series. What's more, they have high ratios of Al2O3/TiO2, low contents of TiO2, which are typical features of boninite and boni-basalts. The rocks are enriched in LREE, LILEs (Cs, Rb, K, Sr) and depleted in HREE (Nb, Ta, Zr, Hf, Ti, P), which are similar to the arc magma, but different from ridge and intraplate basalt. The mineral electron microprobe analysis shows that plagioclase in the rocks of Lari basic complax is similar to the high calcium plagioclase in gabbro which originates in the island arc or active continental margin. The gabbro and pyroxenite of the Lari basic complex were formed by partial melting of the spinel lherzolite. In combination with analyses of regional geological setting, we suggest that there existed a Late Paleozoic-Early Mesozoic limited oceanic basin in the Qinghai Nanshan area. The Lari basic complex formed during the early stage of the oceanic basin subduction.
2019, 44(7): 2478-2489.
doi: 10.3799/dqkx.2018.514
Abstract:
The Tianshan orogeny, which is located in the southwestern part of the Central Asian orogenic belt (CAOB), is a window to explore the accretionary orogenic process of the CAOB. The Akeshake Formation clastic rocks, widespread in the Wenquan area, is an important geological record for the study of the Early Carboniferous basin and orogenic evolution of Chinese West Tianshan. Zircon LA-MC-ICP-MS U-Pb dating of mylonitic sandstone from the Akehsake Formation shows that the detrital zircons have a wide range of zircon ages from 428±5 Ma to 331±4 Ma, which can be divided into 3 groups:428 Ma, 381-364 Ma and 343-331 Ma. Based on the dating results, combined with the mineralogical characteristics and CL images of the detrital zircons as well as the regional geological data, we propose that the deposition age of the Akeshake Formation is the later Early Carboniferous, which is consistent with the fossil record. It is also inferred that the detrital materials of the studied sandstone were mainly derived from intermediate to acidic volcanic rocks and sedimentary rock of the Dahalajunshan Formation. Therefore, the formation of sedimentary rocks of Akeshake Formation was in the active continental margin.
The Tianshan orogeny, which is located in the southwestern part of the Central Asian orogenic belt (CAOB), is a window to explore the accretionary orogenic process of the CAOB. The Akeshake Formation clastic rocks, widespread in the Wenquan area, is an important geological record for the study of the Early Carboniferous basin and orogenic evolution of Chinese West Tianshan. Zircon LA-MC-ICP-MS U-Pb dating of mylonitic sandstone from the Akehsake Formation shows that the detrital zircons have a wide range of zircon ages from 428±5 Ma to 331±4 Ma, which can be divided into 3 groups:428 Ma, 381-364 Ma and 343-331 Ma. Based on the dating results, combined with the mineralogical characteristics and CL images of the detrital zircons as well as the regional geological data, we propose that the deposition age of the Akeshake Formation is the later Early Carboniferous, which is consistent with the fossil record. It is also inferred that the detrital materials of the studied sandstone were mainly derived from intermediate to acidic volcanic rocks and sedimentary rock of the Dahalajunshan Formation. Therefore, the formation of sedimentary rocks of Akeshake Formation was in the active continental margin.
2019, 44(7): 2490-2504.
doi: 10.3799/dqkx.2018.531
Abstract:
The Heimahe pluton is located in the Qinghai Nanshan tectonic belt, northern margin of the West Qinling orogeny. Mafic microgranular enclaves (MMEs) are widely distributed in the Heimahe pluton. However, the petrogenesis of the MMEs is an issue of debate. We present the petrology, mineral chemistry, rock geochemistry and geochronology of the granodiorite and MMEs from the pluton in this paper. The outcrop conditions, shapes, structural and mineral characters of the MMEs reflect that they were products of rapid crystallization when mafic magma injected into the intermediate-acid magma. In the binary diagram of major elements, the granodiorites and MMEs show a mixing trend between mantle- and crust-derived magmas. Besides, their total REE contents and similar REE patterns and Eu anomalies also support the magma mixing model. LA-ICPMS zircon U-Pb dating results show that the granodiorite formed at 246 Ma, and the MMEs formed at 245 Ma. The same crystallization age suggests that the MMEs were not restites or host-rock xenoliths. Combined with regional geology, we suggest that the Heimahe pluton formed by the southward subduction of the limited oceanic basin in the northern margin of the West Qinling, and the Qinghai Nanshan tectonic belt experienced the same evolutionary process with the Tianjun Nanshan belt.
The Heimahe pluton is located in the Qinghai Nanshan tectonic belt, northern margin of the West Qinling orogeny. Mafic microgranular enclaves (MMEs) are widely distributed in the Heimahe pluton. However, the petrogenesis of the MMEs is an issue of debate. We present the petrology, mineral chemistry, rock geochemistry and geochronology of the granodiorite and MMEs from the pluton in this paper. The outcrop conditions, shapes, structural and mineral characters of the MMEs reflect that they were products of rapid crystallization when mafic magma injected into the intermediate-acid magma. In the binary diagram of major elements, the granodiorites and MMEs show a mixing trend between mantle- and crust-derived magmas. Besides, their total REE contents and similar REE patterns and Eu anomalies also support the magma mixing model. LA-ICPMS zircon U-Pb dating results show that the granodiorite formed at 246 Ma, and the MMEs formed at 245 Ma. The same crystallization age suggests that the MMEs were not restites or host-rock xenoliths. Combined with regional geology, we suggest that the Heimahe pluton formed by the southward subduction of the limited oceanic basin in the northern margin of the West Qinling, and the Qinghai Nanshan tectonic belt experienced the same evolutionary process with the Tianjun Nanshan belt.
2019, 44(7): 2505-2518.
doi: 10.3799/dqkx.2018.101
Abstract:
The Nagengkangqieer silver polymetallic deposit located in the East Kunlun orogenic belt is a newly discovered large-scale silver deposit in Qinghai Province. Orebodies of the deposit are mainly hosted in rhyolite porphyry. This paper presents zircon U-Pb dating and Lu-Hf isotopes, whole-rock major and trace elements for rhyolite porphyry in the Nagengkangqieer deposit. The LA-ICP-MS zircon U-Pb analyses for rhyolite porphyry show that rocks formed during the Late Triassic with ages of 217.4±3.1 Ma. The rhyolite porphyry has high contents of SiO2 (73.08%-75.78%), Al2O3 (14.05%-16.04%), with K2O+Na2O ranging from 4.31% to 4.77%, and K2O=4.20%-4.61%, belonging to the high-potassium calc-alkaline strong peralkaline volcanic rocks. These rocks are characterized by high silicon, aluminum and high-potassium, the Mg# values range from 45 to 54; and rhyolite porphyry is depleted in high field strength elements (HFSEs; e.g., Nb, Ta, P, and Ti), and enriched in large-ion lithophile elements (LILEs; e.g., Rb, Th and K); εHf(t) values vary from -4.4 to -9.7, and the two-stage model age of TDM2 ranges from 1 533 Ma to 1 864 Ma. According to the petrology, geochemistry and regional tectonic background, we conclude that the rhyolite porphyry was predominantly derived from partial melting of the Paleoproterozoic-Mesoproterozoic continental crust and some mantle derived material, it formed in extension tectonic settings. The Nagengkangqieer silver polymetallic deposit reflects the Late Triassic mineralization, which is essential for future exploration of polymetallic deposit in the area where intrusive rocks or volcanic rocks in East Kunlun orogenic belt would be very obvious.
The Nagengkangqieer silver polymetallic deposit located in the East Kunlun orogenic belt is a newly discovered large-scale silver deposit in Qinghai Province. Orebodies of the deposit are mainly hosted in rhyolite porphyry. This paper presents zircon U-Pb dating and Lu-Hf isotopes, whole-rock major and trace elements for rhyolite porphyry in the Nagengkangqieer deposit. The LA-ICP-MS zircon U-Pb analyses for rhyolite porphyry show that rocks formed during the Late Triassic with ages of 217.4±3.1 Ma. The rhyolite porphyry has high contents of SiO2 (73.08%-75.78%), Al2O3 (14.05%-16.04%), with K2O+Na2O ranging from 4.31% to 4.77%, and K2O=4.20%-4.61%, belonging to the high-potassium calc-alkaline strong peralkaline volcanic rocks. These rocks are characterized by high silicon, aluminum and high-potassium, the Mg# values range from 45 to 54; and rhyolite porphyry is depleted in high field strength elements (HFSEs; e.g., Nb, Ta, P, and Ti), and enriched in large-ion lithophile elements (LILEs; e.g., Rb, Th and K); εHf(t) values vary from -4.4 to -9.7, and the two-stage model age of TDM2 ranges from 1 533 Ma to 1 864 Ma. According to the petrology, geochemistry and regional tectonic background, we conclude that the rhyolite porphyry was predominantly derived from partial melting of the Paleoproterozoic-Mesoproterozoic continental crust and some mantle derived material, it formed in extension tectonic settings. The Nagengkangqieer silver polymetallic deposit reflects the Late Triassic mineralization, which is essential for future exploration of polymetallic deposit in the area where intrusive rocks or volcanic rocks in East Kunlun orogenic belt would be very obvious.
2019, 44(7): 2519-2531.
doi: 10.3799/dqkx.2018.164
Abstract:
The conventional Liuling Group is widespread in the northern part of South Qinling structure zone, which has been regarded as a key factor for revealing the convergence process between South Qinling structure zone and North Qinling structure zone in Early Paleozoic for its significant tectonic position. Compared to the Liuling Group exposed in Shanyang-Zhashui area, Liuling Group in Danfeng-Shangnan-Xiping area is more complicated in the degree of deformation and metamorphism, which is named as low-metamorphosed Liuling sedimentary assemblage and Liuling sedimentary assemblage in this paper, respectively.Liuling sedimentary assemblage in Shanyang-Zhashui area was defined as Middle-Upper Devonian based on reliable fossils, while there are none fossil evidences for low-metamorphosed Liuling sedimentary assemblage, defined as Lower Paleozoic.In this paper, LA-ICP-MS detrital zircon U-Pb ages of two garnet-bearing two-mica quartz schist samples in low-metamorphosed Liuling sedimentary assemblage were tested, results ranged from 2 598-390 Ma, yield two prominent age clusters at~1.0-0.6 Ga and~0.5-0.4 Ga, as well as three sub-prominent age clusters at~2.6-2.3 Ga, ~1.71-1.60 Ga and~1.3-1.2 Ga.Combined with regional geology, we suggest that:the Liuling sedimentary assemblage and low-metamorphosed Liuling sedimentary assemblage were mainly fed by South Qinling structure zone and North Qinling structure zone, own double-direction sources, they may have deposited at the same time in a forland basin, and experienced different degree of deformation and metamorphism lately.The maximum depositional age of low-metamorphosed Liuling sedimentary assemblage is~390 Ma, which most likely deposited in Middle-Late Devonian.
The conventional Liuling Group is widespread in the northern part of South Qinling structure zone, which has been regarded as a key factor for revealing the convergence process between South Qinling structure zone and North Qinling structure zone in Early Paleozoic for its significant tectonic position. Compared to the Liuling Group exposed in Shanyang-Zhashui area, Liuling Group in Danfeng-Shangnan-Xiping area is more complicated in the degree of deformation and metamorphism, which is named as low-metamorphosed Liuling sedimentary assemblage and Liuling sedimentary assemblage in this paper, respectively.Liuling sedimentary assemblage in Shanyang-Zhashui area was defined as Middle-Upper Devonian based on reliable fossils, while there are none fossil evidences for low-metamorphosed Liuling sedimentary assemblage, defined as Lower Paleozoic.In this paper, LA-ICP-MS detrital zircon U-Pb ages of two garnet-bearing two-mica quartz schist samples in low-metamorphosed Liuling sedimentary assemblage were tested, results ranged from 2 598-390 Ma, yield two prominent age clusters at~1.0-0.6 Ga and~0.5-0.4 Ga, as well as three sub-prominent age clusters at~2.6-2.3 Ga, ~1.71-1.60 Ga and~1.3-1.2 Ga.Combined with regional geology, we suggest that:the Liuling sedimentary assemblage and low-metamorphosed Liuling sedimentary assemblage were mainly fed by South Qinling structure zone and North Qinling structure zone, own double-direction sources, they may have deposited at the same time in a forland basin, and experienced different degree of deformation and metamorphism lately.The maximum depositional age of low-metamorphosed Liuling sedimentary assemblage is~390 Ma, which most likely deposited in Middle-Late Devonian.
2019, 44(7): 2532-2550.
doi: 10.3799/dqkx.2018.378
Abstract:
Granitic pegmatites are widely distributed in the inner and adjacent region of the Late Mesozoic Mufushan granitic complex in South China. Some of them are rich in Li-Nb-Ta and related elements, forming large-ultra-large rare metal deposits. This study focuses on the lithium-poor pegmatites from Duanfengshan area in the northern Mufushan and the newly discovered lithium-rich pegmatite from Renli area in the south. Hereby we provided detailed petrographic and in situ EPMA and LA-ICP-MS geochemical analysis of major and characteristic minerals (feldspar, mica, tourmaline, garnet, beryl, columbite-tantalite) from the Mufushan pegmatites, aiming to constrain the classification, evolution and mineralization of the investigated pegmatites. Based on the combination of the characteristics minerals, the Mufushan pegmatites have been divided into five groups:Tur-pegmatite, Tur-Brl-pegmatite, Brl-pegmatite, Col-Brl-pegmatite in Duanfengshan area and Elb-Lpd-pegmatite in Renli area. Compositional variation of feldspar, mica, tourmaline and garnet from the studied pegmatites display continuous magmatic evolution sequence. From low to high evolution degrees, the Mufushan pegmatites are Tur-pegmatite→Tur-Brl-pegmatite→Brl-pegmatite→Col-Brl-pegmatite→Elb-Lpd-pegmatite, corresponding to the mineralization stages of metal barren→(Be-bearing)→Be-rich→Be, Nb, Ta-rich→Li, Be, Nb, Ta-rich, respectively. This result indicates that the Renli pegmatites have evolved to a relatively late-stage with various rare metal enrichment, which thus has higher potential of Li-Nb-Ta polymetallic mineralization. In contrast, the evolutionary degrees of Duanfengshan pegmatites are relatively low. A strongly zoned tourmaline crystal has been observed in the Tur-Brl-pegmatite from Duanfengshan area, in which five compositional zones from inside to outside can be divided. The content of incompatible elements such as Li, Zn, Ga, Ge, Nb, Ta, Sn, Pb gradually increased from inner to outter, which clearly recording the normal magmatic evolution sequences and the rare metal enrichment processes. Combined with previous studies, our work suggests that the Mufushan pegmatites are generated by extremely prolonged fractional crystallization of the Mufushan massive granitic magma, representing the final-stage product of granitic magma.
Granitic pegmatites are widely distributed in the inner and adjacent region of the Late Mesozoic Mufushan granitic complex in South China. Some of them are rich in Li-Nb-Ta and related elements, forming large-ultra-large rare metal deposits. This study focuses on the lithium-poor pegmatites from Duanfengshan area in the northern Mufushan and the newly discovered lithium-rich pegmatite from Renli area in the south. Hereby we provided detailed petrographic and in situ EPMA and LA-ICP-MS geochemical analysis of major and characteristic minerals (feldspar, mica, tourmaline, garnet, beryl, columbite-tantalite) from the Mufushan pegmatites, aiming to constrain the classification, evolution and mineralization of the investigated pegmatites. Based on the combination of the characteristics minerals, the Mufushan pegmatites have been divided into five groups:Tur-pegmatite, Tur-Brl-pegmatite, Brl-pegmatite, Col-Brl-pegmatite in Duanfengshan area and Elb-Lpd-pegmatite in Renli area. Compositional variation of feldspar, mica, tourmaline and garnet from the studied pegmatites display continuous magmatic evolution sequence. From low to high evolution degrees, the Mufushan pegmatites are Tur-pegmatite→Tur-Brl-pegmatite→Brl-pegmatite→Col-Brl-pegmatite→Elb-Lpd-pegmatite, corresponding to the mineralization stages of metal barren→(Be-bearing)→Be-rich→Be, Nb, Ta-rich→Li, Be, Nb, Ta-rich, respectively. This result indicates that the Renli pegmatites have evolved to a relatively late-stage with various rare metal enrichment, which thus has higher potential of Li-Nb-Ta polymetallic mineralization. In contrast, the evolutionary degrees of Duanfengshan pegmatites are relatively low. A strongly zoned tourmaline crystal has been observed in the Tur-Brl-pegmatite from Duanfengshan area, in which five compositional zones from inside to outside can be divided. The content of incompatible elements such as Li, Zn, Ga, Ge, Nb, Ta, Sn, Pb gradually increased from inner to outter, which clearly recording the normal magmatic evolution sequences and the rare metal enrichment processes. Combined with previous studies, our work suggests that the Mufushan pegmatites are generated by extremely prolonged fractional crystallization of the Mufushan massive granitic magma, representing the final-stage product of granitic magma.
2019, 44(7): 2551-2566.
doi: 10.3799/dqkx.2018.570
Abstract:
Xinkailing-Keluo complex, located in Nenjiang-Heihe tectonic belt, NE China, has fully undergone regional tectonic movement and therefore recorded plentiful evolution information. Its residual deformation signatures and genetic mechanisms could provide key clues about regional tectonic evolution history of not only the adjacent region but also the whole north-east area of China. Based on detailed field survey, this paper conducted systematic studies on different types of structure units, foliation of mylonite rock, stretching lineation of minerals, A-type fold, asymmetrical fold, augen-shaped rotational structure and microscopic book skew structure.The results revealed a significant fact two wings of the complex have the extension of NW and SE respectively, The mineral of zircon have been identified from internal biotite-plagioclase gneiss and intrusive dioritic-porphyrite body in mylonite country rock.Its U-Pb dating of zircon and regional study both indicated these deformation mainly occurred during Middle-Jurasic (167 Ma), and reached metamorphic peak during 170-163 Ma.Then it was uplifted by tectonic movement and exposed to the ground in late Middle-Jurasic. Xinkailing-Keluo complex was composed of three layers architecture, the upper cap rock, middle zone and lower complex.The internal part is mainly composed by granite, which are much younger than its outer part.The metamorphic temperature ranged from 400 to 500 degree, reaching extent of greenschist facies and was followed by retrograde metamorphism. Later, the regional uplift happened due to the granite magma underplating and the Xinkailing-Keluo magmatic core complex formed under regional stretch tectonic background.
Xinkailing-Keluo complex, located in Nenjiang-Heihe tectonic belt, NE China, has fully undergone regional tectonic movement and therefore recorded plentiful evolution information. Its residual deformation signatures and genetic mechanisms could provide key clues about regional tectonic evolution history of not only the adjacent region but also the whole north-east area of China. Based on detailed field survey, this paper conducted systematic studies on different types of structure units, foliation of mylonite rock, stretching lineation of minerals, A-type fold, asymmetrical fold, augen-shaped rotational structure and microscopic book skew structure.The results revealed a significant fact two wings of the complex have the extension of NW and SE respectively, The mineral of zircon have been identified from internal biotite-plagioclase gneiss and intrusive dioritic-porphyrite body in mylonite country rock.Its U-Pb dating of zircon and regional study both indicated these deformation mainly occurred during Middle-Jurasic (167 Ma), and reached metamorphic peak during 170-163 Ma.Then it was uplifted by tectonic movement and exposed to the ground in late Middle-Jurasic. Xinkailing-Keluo complex was composed of three layers architecture, the upper cap rock, middle zone and lower complex.The internal part is mainly composed by granite, which are much younger than its outer part.The metamorphic temperature ranged from 400 to 500 degree, reaching extent of greenschist facies and was followed by retrograde metamorphism. Later, the regional uplift happened due to the granite magma underplating and the Xinkailing-Keluo magmatic core complex formed under regional stretch tectonic background.
2019, 44(7): 2567-2578.
doi: 10.3799/dqkx.2019.955
Abstract:
The Earth is a giant system composed of inorganic materials, organic materials and biological materials and a huge factory for material synthesis and processing that has been running for 4.6 billion years. It contains abundant integrated information of material compositions, structural properties, preparation processes and services. It has become a new research field worthy of attention in material science to learn from nature and obtain the key codes of natural materials to improve the prediction ability and manufacturing ability of new materials. In this study, the types and distribution of elements, natural mineral and natural biological material of Earth system were systematically summarized, and the core-shell structure in different scales and superwettability surface structure in different types in nature were discussed. In addition, the preparation and processing technologies of natural inorganic materials, such as separation and compaction, casting and sintering, condensation and baking, were analyzed. The research framework of Natural Materialogy was preliminarily established. The results of this study will provide a reference for the construction of natural materials database on Earth.
The Earth is a giant system composed of inorganic materials, organic materials and biological materials and a huge factory for material synthesis and processing that has been running for 4.6 billion years. It contains abundant integrated information of material compositions, structural properties, preparation processes and services. It has become a new research field worthy of attention in material science to learn from nature and obtain the key codes of natural materials to improve the prediction ability and manufacturing ability of new materials. In this study, the types and distribution of elements, natural mineral and natural biological material of Earth system were systematically summarized, and the core-shell structure in different scales and superwettability surface structure in different types in nature were discussed. In addition, the preparation and processing technologies of natural inorganic materials, such as separation and compaction, casting and sintering, condensation and baking, were analyzed. The research framework of Natural Materialogy was preliminarily established. The results of this study will provide a reference for the construction of natural materials database on Earth.
2019, 44(7): 2353-2367.
doi: 10.3799/dqkx.2018.945
Abstract:
The Azhalang pluton is located in the Sumdo area in the east-central part of the Gangdise magmatic arc. Zircon U-Pb dating and geochemical analysis of the whole rock indicate that the Azhalang intrusion is quartz monzonite porphyry and the formation period is Miocene (17.9±0.2 Ma). Geochemical characteristics show high Sr (1 052×10-6-1 150×10-6), low Y (8.51×10-6-9.04×10-6) and Yb (0.85×10-6-0.94×10-6), high Sr/Y (118-128) and La/Yb (30.9-40.8) ratios, no obvious Eu anomalies, high K2O (3.17%-3.84%) content, low Cr (6.46×10-6-7.78×10-6) and Ni (5.41×10-6-7.45×10-6) content, high Mg# value (43.8-49.8), high Rb/Sr ratio, large ion lithophile element content, such as Rb, Rb, Rb. Ba, Th and U were significantly higher than LREE.These geochemical characteristics indicate that the petrogenesis of the rocks may be formed by partial melting of the thicker lower crust and formed in the post-collisional tectonic setting of the India-Eurasia collision orogeny, and some mantle-derived materials may be involved.This study provides important constraints for revealing the genesis and metallogenic geological setting of the Miocene adakite in the Gangdise metallogenic belt.
The Azhalang pluton is located in the Sumdo area in the east-central part of the Gangdise magmatic arc. Zircon U-Pb dating and geochemical analysis of the whole rock indicate that the Azhalang intrusion is quartz monzonite porphyry and the formation period is Miocene (17.9±0.2 Ma). Geochemical characteristics show high Sr (1 052×10-6-1 150×10-6), low Y (8.51×10-6-9.04×10-6) and Yb (0.85×10-6-0.94×10-6), high Sr/Y (118-128) and La/Yb (30.9-40.8) ratios, no obvious Eu anomalies, high K2O (3.17%-3.84%) content, low Cr (6.46×10-6-7.78×10-6) and Ni (5.41×10-6-7.45×10-6) content, high Mg# value (43.8-49.8), high Rb/Sr ratio, large ion lithophile element content, such as Rb, Rb, Rb. Ba, Th and U were significantly higher than LREE.These geochemical characteristics indicate that the petrogenesis of the rocks may be formed by partial melting of the thicker lower crust and formed in the post-collisional tectonic setting of the India-Eurasia collision orogeny, and some mantle-derived materials may be involved.This study provides important constraints for revealing the genesis and metallogenic geological setting of the Miocene adakite in the Gangdise metallogenic belt.
2019, 44(7): 2368-2378.
doi: 10.3799/dqkx.2018.382
Abstract:
A new Late Jurassic-Early Cretaceous magmatic belt has been identified in the central Tibetan Plateau. This paper reports two zircon U-Pb ages and also new whole-rock major and trace element data of the newly discovered volcanic rocks in Mami area. These volcanic rocks were dated as Late Jurassic (152-150 Ma). The studied samples are characterized by high SiO2 content, high alkali, low MgO, and low P2O5 abundances, with significant depletion in Eu, Sr, and Ba, suggesting a geochemical affinity with highly fractionated I-type granites. It is concluded that the volcanic rocks were derived by partial melting of ancient mafic lower crust, followed by fractional crystallization. Our research propose that Late Jurassic-Early Cretaceous magmatic magmatism occurred in an oceanic subduction setting, likely related to the northward subduction of the Neo-Tethys oceanic lithosphere.
A new Late Jurassic-Early Cretaceous magmatic belt has been identified in the central Tibetan Plateau. This paper reports two zircon U-Pb ages and also new whole-rock major and trace element data of the newly discovered volcanic rocks in Mami area. These volcanic rocks were dated as Late Jurassic (152-150 Ma). The studied samples are characterized by high SiO2 content, high alkali, low MgO, and low P2O5 abundances, with significant depletion in Eu, Sr, and Ba, suggesting a geochemical affinity with highly fractionated I-type granites. It is concluded that the volcanic rocks were derived by partial melting of ancient mafic lower crust, followed by fractional crystallization. Our research propose that Late Jurassic-Early Cretaceous magmatic magmatism occurred in an oceanic subduction setting, likely related to the northward subduction of the Neo-Tethys oceanic lithosphere.
2019, 44(7): 2379-2391.
doi: 10.3799/dqkx.2019.010
Abstract:
Beryllium and rubidium mineralization is newly found in northern Namling, Tibet, China. However, the U-Pb ages are not reliable representation of magmatic emplacement age since U concentration in zircons from the mineralized pegmatitic muscovite monzogranite is too high. LA-ICP-MS is taken on monazites from this pyrogenic rock using 24 μm spot 193 nm laser, yielding reliable age. The weighted average age of 206Pb/238U is 207.0±0.7 Ma (n=22, MSWD=0.101), and the lower intercepted age is 207.0±0.8 Ma (MSWD=0.107). These ages obtained by LA-ICP-MS analysis show the mineralized pegmatitic muscovite monzogranite emplaced in Late Triassic and later than granodiorite in this region. It may represent an important diagenetic event that has not yet been fully identified in the late stage of Indosinian orogeny in Paleo-Gangdise. This new discovery may expand minerals and deposit types for further regional prospecting in the central Gangdise metallogenic belt. Furthermore, this discovery also has great importance for the re-understanding of the Mesozoic tectonic evolution, magmatic evolution and mineralization in this belt. And further enhances the orogenic and metallogenic theories related to the evolution of multi-island-arc-basin in the Qinghai-Tibet Plateau.
Beryllium and rubidium mineralization is newly found in northern Namling, Tibet, China. However, the U-Pb ages are not reliable representation of magmatic emplacement age since U concentration in zircons from the mineralized pegmatitic muscovite monzogranite is too high. LA-ICP-MS is taken on monazites from this pyrogenic rock using 24 μm spot 193 nm laser, yielding reliable age. The weighted average age of 206Pb/238U is 207.0±0.7 Ma (n=22, MSWD=0.101), and the lower intercepted age is 207.0±0.8 Ma (MSWD=0.107). These ages obtained by LA-ICP-MS analysis show the mineralized pegmatitic muscovite monzogranite emplaced in Late Triassic and later than granodiorite in this region. It may represent an important diagenetic event that has not yet been fully identified in the late stage of Indosinian orogeny in Paleo-Gangdise. This new discovery may expand minerals and deposit types for further regional prospecting in the central Gangdise metallogenic belt. Furthermore, this discovery also has great importance for the re-understanding of the Mesozoic tectonic evolution, magmatic evolution and mineralization in this belt. And further enhances the orogenic and metallogenic theories related to the evolution of multi-island-arc-basin in the Qinghai-Tibet Plateau.
2019, 44(7): 2392-2407.
doi: 10.3799/dqkx.2019.149
Abstract:
Miocene is an important period for the thickening and uplifting of the Lhasa terrane. A set of Miocene strata were recently identified in the eastern Lhasa terrane. It can be divided into three eruption cycles, and consists of rhyolite, dacite, pyroclastic rock, obsidian and lithic sandstone. Zircon U-Pb dating shows that the strata formed during 17.2-18.2 Ma. Whole-rock major and trace element analysis and zircon Hf isotope analysis show that the rhyolite has the geochemical characteristics of potassic volcanic rocks, and is derived from the partial melting of a middle-lower crust. The dacite has the geochemical characteristics of adakite related with A-type granite, and is derived from the partial melting of a new thickened lower crust. The coexistence of the potassic and adakitic rocks enriches the research of the Miocene volcanic rocks, and provides new evidence for the Miocene lithospheric detachment model in the Tibetan Plateau.
Miocene is an important period for the thickening and uplifting of the Lhasa terrane. A set of Miocene strata were recently identified in the eastern Lhasa terrane. It can be divided into three eruption cycles, and consists of rhyolite, dacite, pyroclastic rock, obsidian and lithic sandstone. Zircon U-Pb dating shows that the strata formed during 17.2-18.2 Ma. Whole-rock major and trace element analysis and zircon Hf isotope analysis show that the rhyolite has the geochemical characteristics of potassic volcanic rocks, and is derived from the partial melting of a middle-lower crust. The dacite has the geochemical characteristics of adakite related with A-type granite, and is derived from the partial melting of a new thickened lower crust. The coexistence of the potassic and adakitic rocks enriches the research of the Miocene volcanic rocks, and provides new evidence for the Miocene lithospheric detachment model in the Tibetan Plateau.
2019, 44(7): 2408-2425.
doi: 10.3799/qkx.2018.394
Abstract:
The closure time of the Bangong-Nujiang Ocean is one of the focal points of the Meso-Tethys Ocean evolution debate, and the mafic dikes play an important role in the identification of tectonic setting transformation. Here we report zircon LA-ICP-MS U-Pb ages and whole-rock geochemical data from the mafic dikes invaded in the tectonic mélange. Zircon U-Pb dating using LA-ICP-MS techniques yields the concordant age with a weighted mean 206Pb/238U age of 102.90±0.86 Ma (MSWD=1.4) for the gabbro dike. The whole-rock geochemical characteristics from the mafic dikes suggest that these rocks are calc-alkaline series. The dikes are enriched in large ion lithophile element (LILE) and show negative Nb and Ta anomalies with slightly enrichment of light rare earth element (LREE). The geochemical compositions suggest that the dikes are originated from a depleted mantle source metasomatized by slab-derived components and formed by the high degree of partial melting of spinel peridotite with the lack of mixing of crustal materials. Combined with the tectonic environment discriminant diagram and previous research results, we believe that the dikes formed in the late Early Cretaceous extension environment. We found that the Bangong-Nujiang suture zone and the two sides are in an extension environment between 110-100 Ma by collecting data on magmatic rocks (120-95 Ma) near the study area. The times of regional extension differ from area to area, which may be related to the diachronism of closure time of Bangong-Nujiang Ocean. The final closure time of Bangong-Nujiang Ocean may be around 103 Ma.
The closure time of the Bangong-Nujiang Ocean is one of the focal points of the Meso-Tethys Ocean evolution debate, and the mafic dikes play an important role in the identification of tectonic setting transformation. Here we report zircon LA-ICP-MS U-Pb ages and whole-rock geochemical data from the mafic dikes invaded in the tectonic mélange. Zircon U-Pb dating using LA-ICP-MS techniques yields the concordant age with a weighted mean 206Pb/238U age of 102.90±0.86 Ma (MSWD=1.4) for the gabbro dike. The whole-rock geochemical characteristics from the mafic dikes suggest that these rocks are calc-alkaline series. The dikes are enriched in large ion lithophile element (LILE) and show negative Nb and Ta anomalies with slightly enrichment of light rare earth element (LREE). The geochemical compositions suggest that the dikes are originated from a depleted mantle source metasomatized by slab-derived components and formed by the high degree of partial melting of spinel peridotite with the lack of mixing of crustal materials. Combined with the tectonic environment discriminant diagram and previous research results, we believe that the dikes formed in the late Early Cretaceous extension environment. We found that the Bangong-Nujiang suture zone and the two sides are in an extension environment between 110-100 Ma by collecting data on magmatic rocks (120-95 Ma) near the study area. The times of regional extension differ from area to area, which may be related to the diachronism of closure time of Bangong-Nujiang Ocean. The final closure time of Bangong-Nujiang Ocean may be around 103 Ma.
2019, 44(7): 2426-2440.
doi: 10.3799/dqkx.2018.944
Abstract:
The Mugagangri Group is an oceanic basin sedimentary record of Bangong-Nujiang Ocean. However, the age of Mugagangri Group has been controversial since it was named. In this paper, the Mugagangri Group in Yaduo Village of Gerze County, central part of Bangong-Nujiang suture zone, is selected as the research object. We have carried out detailed petrological and clastic zircons chronology studies on it. Our research shows the Mugagangri Group in this area is mainly composed of thin sandstone and shale interbeds, which have characteristic of flysch. The chronological analysis of sandstone clastic zircons shows that the minimum clastic zircon is 149 Ma and the minimum age peak is 156 Ma, indicating that the age of the Mugagangri Group in this area is from Late Jurassic to Early Cretaceous. In addition, the age information of Mugagangri Group in different areas of the central and western parts of the Bangong-Nujiang suture zone are collected and analyzed in detail as well. The results show that the age span of the Mugagangri Group is at least Late Triassic-Early Cretaceous, suggesting that the middle and western parts of the Bangong-Nujiang Ocean may have evolved continuously during the Late Triassic-Early Cretaceous.
The Mugagangri Group is an oceanic basin sedimentary record of Bangong-Nujiang Ocean. However, the age of Mugagangri Group has been controversial since it was named. In this paper, the Mugagangri Group in Yaduo Village of Gerze County, central part of Bangong-Nujiang suture zone, is selected as the research object. We have carried out detailed petrological and clastic zircons chronology studies on it. Our research shows the Mugagangri Group in this area is mainly composed of thin sandstone and shale interbeds, which have characteristic of flysch. The chronological analysis of sandstone clastic zircons shows that the minimum clastic zircon is 149 Ma and the minimum age peak is 156 Ma, indicating that the age of the Mugagangri Group in this area is from Late Jurassic to Early Cretaceous. In addition, the age information of Mugagangri Group in different areas of the central and western parts of the Bangong-Nujiang suture zone are collected and analyzed in detail as well. The results show that the age span of the Mugagangri Group is at least Late Triassic-Early Cretaceous, suggesting that the middle and western parts of the Bangong-Nujiang Ocean may have evolved continuously during the Late Triassic-Early Cretaceous.
