2019 Vol. 44, No. 4
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2019, 44(4): 1406-1423.
doi: 10.3799/dqkx.2018.386
Abstract:
The Boluositanmiao complex is a typical intermediate to high-grade metamorphic complex in the Alxa block, which is significant for better understanding of the origin, formation and evolution of Alxa block. In this paper, it presents a detailed study on the field geological observation, petrology and zircon U-Pb dating of the quartz dioritic gneisses, amphibolites and granitic pegmatites of the Boluositanmiao complex, and the results indicate that the magmatic zircons from the quartz dioritic gneisses, amphibolites and granitic pegmatites record the mean ages of 284±2 Ma and 278±3 Ma, 276±2 Ma and 271±3 Ma, and 242±7 Ma, respectively, which is interpreted as the protolith timing of the complex in our study, rather than Neoachean-Paleoproterozoic as suggested in previous studies.Moreover, the metamorphic zircons from the quartz dioritic gneisses and amphibolites also record the mean ages of 274±6 Ma and 272±5 Ma, and 269±3 Ma and 268±2 Ma, respectively, which is interpreted as the Late Paleozoic metamorphic timing of the complex.In conclusion, it is proposed that the Boluositanmiao complex mainly consists of the Paleoproterozoic-Late Paleozoic geological bodies, rather than Neoarchean-Paleoproterozoic rocks. Besides, they have also undergone 280-260 Ma amphibolite-facies metamorphism, indicating that the complex could be related to the Late Paleozoic orogenesis of the central Asian orogenic belt.
The Boluositanmiao complex is a typical intermediate to high-grade metamorphic complex in the Alxa block, which is significant for better understanding of the origin, formation and evolution of Alxa block. In this paper, it presents a detailed study on the field geological observation, petrology and zircon U-Pb dating of the quartz dioritic gneisses, amphibolites and granitic pegmatites of the Boluositanmiao complex, and the results indicate that the magmatic zircons from the quartz dioritic gneisses, amphibolites and granitic pegmatites record the mean ages of 284±2 Ma and 278±3 Ma, 276±2 Ma and 271±3 Ma, and 242±7 Ma, respectively, which is interpreted as the protolith timing of the complex in our study, rather than Neoachean-Paleoproterozoic as suggested in previous studies.Moreover, the metamorphic zircons from the quartz dioritic gneisses and amphibolites also record the mean ages of 274±6 Ma and 272±5 Ma, and 269±3 Ma and 268±2 Ma, respectively, which is interpreted as the Late Paleozoic metamorphic timing of the complex.In conclusion, it is proposed that the Boluositanmiao complex mainly consists of the Paleoproterozoic-Late Paleozoic geological bodies, rather than Neoarchean-Paleoproterozoic rocks. Besides, they have also undergone 280-260 Ma amphibolite-facies metamorphism, indicating that the complex could be related to the Late Paleozoic orogenesis of the central Asian orogenic belt.
2019, 44(4): 1057-1066.
doi: 10.3799/dqkx.2019.952
Abstract:
Orogenic peridotite is one of major research targets not only for mantle geochemistry, but also for the formation and evolution of the orogenic belts. There are mainly three types of orogenic peridotites: (1) Alpine peridotite, i. e. lithospheric mantle tectonic-thermal diaper into the shallow orogenic crust; (2) mafic-ultramafic cumulates of layered intrusions underwent subduction and metamorphism; (3) ophiolitic peridotite.Detailed petrological and geochemical investigation on Songshugou mylonitized peridotites and their related high-grade metamorphic rocks demonstrates that the Songshugou peridotites recorded the whole cycle from the formation of an oceanic lithosphere to amphibolite-facies retrograde metamorphism. i. e. dunites produced during the formation of an oceanic lithosphere at 1 000-800 Ma; followed by the period of oceanic-continental transition during < 800-500 Ma, lithospheric dunites metasomatized to produce an amounts of metasomatic harzburgites; rapid deep subdution and eclogite-facies metamorphism at 500-480 Ma; and amphibolite-facies retrograde metasomatism during slow exhumation at 460-335 Ma, magnesium-rich anthophyllites in Songshugou peridotites, including tremolite, actinolite, and magnesium amphibole, were emerged at this stage.Thus it can be seen that ophiolitic orogenic peridotites can record the whole cycle from the orogen formation and its evolution.They often underwent four stages:(1) formation of an oceanic lithosphere (ophiolite) to produce dunites; (2) the period of oceanic-continental transition and the formation of metasomatic harzburgite due to mantle metasomatism; (3) deep lithosphere subduction and eclogite-facies metamorphism; (4) retrograde metamorphism when exhumed to the depth of amphibolite-facies, lots of anthophyllites in peridotites emerged in this stage. Different orogens may have a diverse rock types just in the depth of the deep subduction and in degrees of retrograde metamorphism. Finally, the age formation of ophiolites must be emphasized.Meantime, the high grade metamorphosed rocks can be completely destroyed due to the later retrograde metamorphism, this deserves a great attention.
Orogenic peridotite is one of major research targets not only for mantle geochemistry, but also for the formation and evolution of the orogenic belts. There are mainly three types of orogenic peridotites: (1) Alpine peridotite, i. e. lithospheric mantle tectonic-thermal diaper into the shallow orogenic crust; (2) mafic-ultramafic cumulates of layered intrusions underwent subduction and metamorphism; (3) ophiolitic peridotite.Detailed petrological and geochemical investigation on Songshugou mylonitized peridotites and their related high-grade metamorphic rocks demonstrates that the Songshugou peridotites recorded the whole cycle from the formation of an oceanic lithosphere to amphibolite-facies retrograde metamorphism. i. e. dunites produced during the formation of an oceanic lithosphere at 1 000-800 Ma; followed by the period of oceanic-continental transition during < 800-500 Ma, lithospheric dunites metasomatized to produce an amounts of metasomatic harzburgites; rapid deep subdution and eclogite-facies metamorphism at 500-480 Ma; and amphibolite-facies retrograde metasomatism during slow exhumation at 460-335 Ma, magnesium-rich anthophyllites in Songshugou peridotites, including tremolite, actinolite, and magnesium amphibole, were emerged at this stage.Thus it can be seen that ophiolitic orogenic peridotites can record the whole cycle from the orogen formation and its evolution.They often underwent four stages:(1) formation of an oceanic lithosphere (ophiolite) to produce dunites; (2) the period of oceanic-continental transition and the formation of metasomatic harzburgite due to mantle metasomatism; (3) deep lithosphere subduction and eclogite-facies metamorphism; (4) retrograde metamorphism when exhumed to the depth of amphibolite-facies, lots of anthophyllites in peridotites emerged in this stage. Different orogens may have a diverse rock types just in the depth of the deep subduction and in degrees of retrograde metamorphism. Finally, the age formation of ophiolites must be emphasized.Meantime, the high grade metamorphosed rocks can be completely destroyed due to the later retrograde metamorphism, this deserves a great attention.
2019, 44(4): 1067-1082.
doi: 10.3799/dqkx.2018.375
Abstract:
Orogenic peridotites record the complex geological processes of continental subduction, collision and exhumation, and they can be divided into two types: crustal origin and mantle origin. Zircons are rarely found in orogenic peridotites. The in-situ zircons and zircon inclusions with mantle-rock mineral assemblages indicate that the zircons can grow within the peridotites. During the complex processes of plate aggregation (such as UHP metamorphism), the orogenic peridotites experienced the melt/fluid interaction in different periods, which have an important impact on the mineral and elemental compositions of peridotites. Zircon is one of the typical metasomatic minerals of orogenic peridotites. The zircon formation is controlled by the composition of the melt/fluid, source properties, and the formation of a physical and chemical environment. The mantle-derived zircons from orogenic peridotites have three origins:(1) Zircon has strong crystallization ability, and Zr prefers to combine with Si from silicate minerals in mantle rocks to form zircons. (2) Metamorphic destruction of Zr-bearing mineral phases and precipitation from intergranular melts generated can nucleate zircons under sub-solidus conditions. (3) The melt/fluid from the recycled crust can metasomatize the mantle wedge and form zircons. Thus, zircons can be used to unravel the history of specific lithospheric domains and thus contribute to our understanding of the evolution of continental cratons and their margins.
Orogenic peridotites record the complex geological processes of continental subduction, collision and exhumation, and they can be divided into two types: crustal origin and mantle origin. Zircons are rarely found in orogenic peridotites. The in-situ zircons and zircon inclusions with mantle-rock mineral assemblages indicate that the zircons can grow within the peridotites. During the complex processes of plate aggregation (such as UHP metamorphism), the orogenic peridotites experienced the melt/fluid interaction in different periods, which have an important impact on the mineral and elemental compositions of peridotites. Zircon is one of the typical metasomatic minerals of orogenic peridotites. The zircon formation is controlled by the composition of the melt/fluid, source properties, and the formation of a physical and chemical environment. The mantle-derived zircons from orogenic peridotites have three origins:(1) Zircon has strong crystallization ability, and Zr prefers to combine with Si from silicate minerals in mantle rocks to form zircons. (2) Metamorphic destruction of Zr-bearing mineral phases and precipitation from intergranular melts generated can nucleate zircons under sub-solidus conditions. (3) The melt/fluid from the recycled crust can metasomatize the mantle wedge and form zircons. Thus, zircons can be used to unravel the history of specific lithospheric domains and thus contribute to our understanding of the evolution of continental cratons and their margins.
2019, 44(4): 1083-1095.
doi: 10.3799/dqkx.2018.318
Abstract:
Subduction is one of the key factors that change the evolution of the interior of the Earth, and there have been controversies over the starting time of modern-style plate tectonics. The discovery of majorite garnet inclusions and eclogite xenoliths hosted by the Paleoproterozoic carbonatites in the Fengzhen area, Inner Mongolia, provides a rare window into the origin depth of magma and plate tectonics. Mineralogy and high temperature and pressure experiments constrain the ferric iron-rich (Fe3+/∑Fe~0.8) majoritic garnets (Si ~3.18 pfu) originated from about ~400 km in depth, indicating that the carbonatitic magma originated from the mantle transition zone. The mineral-pair thermobarometer and P-T phase diagram determine the peak metamorphic pressure and temperature at ~660 ℃ and ~2.65 GPa, which gives a peak thermal gradient of ~250 ℃·GPa-1, similar to the product of modern plate deep subduction, indicating that the modern plate tectonics has started since the Paleoproterozoic. Statistics show that the global Paleoproterozoic carbonatites are closely associated with high-pressure metamorphic rocks in Paleoproterozoic orogens. Global Paleoproterozoic slab subduction might be linked to the Columbia supercontinent amalgamation. Large-scale slab subduction inputted crustal sediments into the deep mantle, forming carbonatitic magmas and oxidized ferric iron-rich majoritic garnets. Crustal materials recycled for about 2 billion years in the mantle source region, leading to the compositional heterogeneities and carbon cycle in the deep mantle.
Subduction is one of the key factors that change the evolution of the interior of the Earth, and there have been controversies over the starting time of modern-style plate tectonics. The discovery of majorite garnet inclusions and eclogite xenoliths hosted by the Paleoproterozoic carbonatites in the Fengzhen area, Inner Mongolia, provides a rare window into the origin depth of magma and plate tectonics. Mineralogy and high temperature and pressure experiments constrain the ferric iron-rich (Fe3+/∑Fe~0.8) majoritic garnets (Si ~3.18 pfu) originated from about ~400 km in depth, indicating that the carbonatitic magma originated from the mantle transition zone. The mineral-pair thermobarometer and P-T phase diagram determine the peak metamorphic pressure and temperature at ~660 ℃ and ~2.65 GPa, which gives a peak thermal gradient of ~250 ℃·GPa-1, similar to the product of modern plate deep subduction, indicating that the modern plate tectonics has started since the Paleoproterozoic. Statistics show that the global Paleoproterozoic carbonatites are closely associated with high-pressure metamorphic rocks in Paleoproterozoic orogens. Global Paleoproterozoic slab subduction might be linked to the Columbia supercontinent amalgamation. Large-scale slab subduction inputted crustal sediments into the deep mantle, forming carbonatitic magmas and oxidized ferric iron-rich majoritic garnets. Crustal materials recycled for about 2 billion years in the mantle source region, leading to the compositional heterogeneities and carbon cycle in the deep mantle.
2019, 44(4): 1096-1112.
doi: 10.3799/dqkx.2019.055
Abstract:
Recently, few researches have been made on the thermal evolution of the mantle in Northeast China. An effective means to solve this issue is to study the variation characteristics in composition of basalts in Northeast China. In this paper, it summarizes and discusses the composition variations of the Mesozoic and Cenozoic basalts in the northern Great Xing'an Range, with the aim of revealing the thermal evolution of the mantle within the study area. The Early Cretaceous basalts in the northern Great Xing'an Range geochemically belong to tholeiitic series, which are characterized by depletion in high field strength elements (e.g., Nb, Ta and Ti).Their La/Nb and La/Ta ratios range from 1.8 to 5.6 and from 30 to 87, respectively, implying the basaltic magmas originated from the partial melting of the lithospheric mantle. Their initial 87Sr/86Sr ratios of 0.704 5-0.706 9, εNd(t) values of -1.52-+3.60 and εHf(t) values of +1.74-+7.77 further indicate that the magma source is weakly depleted-weakly enriched lithospheric mantle. Additionally, the Sr-Nd-Pb isotope compositions of the Early Cretaceous basalts suggest that their magmatic sources are characterized by mixing between DM and EMⅡ modified by subduction-derived fluids.Taking the above-mentioned into consideration, it is suggested that the Early Cretaceous basaltic magma was derived from partial melting of a lithospheric mantle metasomated by subduction-related fluids.The Cenozoic ultrapotassic and potassic basalts have weakly negative Nb-Ta anomalies, 87Sr/86Sr ratios of 0.704 7-0.705 7 and εNd(t) values of -6.3 to -0.8 whereas the mantle xenoliths entrained by Cenozoic ultrapotassic and potassic basalts show the depleted Sr-Nd isotopic characteristics. The Cenozoic sodium basalts have positive Nb-Ta anomalies, and lower 87Sr/86Sr ratios of 0.703 5-0.704 2 and higher εNd(t) values of +3.4-+6.6 than those of the ultrapotassic and potassic basalts, similar to those of MORB. These geochemical features suggest that the Cenozoic basaltic magmas in the northern Great Xing'an Range were primarily produced by melting of asthenospheric mantle. The composition variations of the Early Cretaceous and Cenozoic basalts in the northern Great Xing'an Range not only indicate that the basaltic magma sources had changed from lithospheric to asthenospheric mantle, but also reveal the thermal evolution of the mantle in the study area, i.e., high geothermal gradient in the late Early Cretaceous changed to low geothermal gradient in the Cenozoic. Combined with the regional tectonic evolution, it is concluded that the Early Cretaceous basaltic magmatism in the northern Great Xing'an Range is related to the taphrogeny caused by the lithospheric extension and thinning, while the Cenozoic basaltic magmatism is related to the intracontinental rifting.
Recently, few researches have been made on the thermal evolution of the mantle in Northeast China. An effective means to solve this issue is to study the variation characteristics in composition of basalts in Northeast China. In this paper, it summarizes and discusses the composition variations of the Mesozoic and Cenozoic basalts in the northern Great Xing'an Range, with the aim of revealing the thermal evolution of the mantle within the study area. The Early Cretaceous basalts in the northern Great Xing'an Range geochemically belong to tholeiitic series, which are characterized by depletion in high field strength elements (e.g., Nb, Ta and Ti).Their La/Nb and La/Ta ratios range from 1.8 to 5.6 and from 30 to 87, respectively, implying the basaltic magmas originated from the partial melting of the lithospheric mantle. Their initial 87Sr/86Sr ratios of 0.704 5-0.706 9, εNd(t) values of -1.52-+3.60 and εHf(t) values of +1.74-+7.77 further indicate that the magma source is weakly depleted-weakly enriched lithospheric mantle. Additionally, the Sr-Nd-Pb isotope compositions of the Early Cretaceous basalts suggest that their magmatic sources are characterized by mixing between DM and EMⅡ modified by subduction-derived fluids.Taking the above-mentioned into consideration, it is suggested that the Early Cretaceous basaltic magma was derived from partial melting of a lithospheric mantle metasomated by subduction-related fluids.The Cenozoic ultrapotassic and potassic basalts have weakly negative Nb-Ta anomalies, 87Sr/86Sr ratios of 0.704 7-0.705 7 and εNd(t) values of -6.3 to -0.8 whereas the mantle xenoliths entrained by Cenozoic ultrapotassic and potassic basalts show the depleted Sr-Nd isotopic characteristics. The Cenozoic sodium basalts have positive Nb-Ta anomalies, and lower 87Sr/86Sr ratios of 0.703 5-0.704 2 and higher εNd(t) values of +3.4-+6.6 than those of the ultrapotassic and potassic basalts, similar to those of MORB. These geochemical features suggest that the Cenozoic basaltic magmas in the northern Great Xing'an Range were primarily produced by melting of asthenospheric mantle. The composition variations of the Early Cretaceous and Cenozoic basalts in the northern Great Xing'an Range not only indicate that the basaltic magma sources had changed from lithospheric to asthenospheric mantle, but also reveal the thermal evolution of the mantle in the study area, i.e., high geothermal gradient in the late Early Cretaceous changed to low geothermal gradient in the Cenozoic. Combined with the regional tectonic evolution, it is concluded that the Early Cretaceous basaltic magmatism in the northern Great Xing'an Range is related to the taphrogeny caused by the lithospheric extension and thinning, while the Cenozoic basaltic magmatism is related to the intracontinental rifting.
2019, 44(4): 1113-1127.
doi: 10.3799/dqkx.2018.357
Abstract:
Carbonate metasomatism, one of the important ways to modify the mantle, is the interaction between carbonate melt and peridotite in the mantle. It can significantly change the petrology and geochemistry of the mantle peridotite. Firstly, composition and proportion of minerals in peridotite can be modified by carbonate metasomatism. Although results of carbonate metasomatism depend on the initial reactant composition and temperature and pressure conditions, most reactions result in the pyroxene enrichment in peridotite, and occurrence of accessory minerals such as apatite and monazite. In addition, minerals of peridotite having experienced significant metasomatism by carbonate melts are generally featured with abundant CO2-fluid and -melt inclusions, and distinctive spongy texture and melt pockets. Secondly, the carbonate metasomatism can be well identified by some geochemical fingerprints as well. As to major elements, the clinopyroxenes in peridotite having experienced carbonate metasomatism are characterized by high Mg# value and Ca/Al ratio (>5). In terms of trace elements, clinopyroxenes in peridotite having experienced carbonate metasomatism generally have higher (La/Yb)N, Eu/Ti, Zr/Hf and Y/Ho ratios, and show depletions in HFSE. It is worth noting that the geochemical features may vary with the degree of carbonate metasomatism. In order to trace the source of carbonate melt caused metasomatism, Mg-Zn-Ca-Sr isotopic systems can be well used. Especially, in-situ Sr isotopic analysis method established in recent years provides us an important way to unravel the overlap of multiple carbonate metasomatism.
Carbonate metasomatism, one of the important ways to modify the mantle, is the interaction between carbonate melt and peridotite in the mantle. It can significantly change the petrology and geochemistry of the mantle peridotite. Firstly, composition and proportion of minerals in peridotite can be modified by carbonate metasomatism. Although results of carbonate metasomatism depend on the initial reactant composition and temperature and pressure conditions, most reactions result in the pyroxene enrichment in peridotite, and occurrence of accessory minerals such as apatite and monazite. In addition, minerals of peridotite having experienced significant metasomatism by carbonate melts are generally featured with abundant CO2-fluid and -melt inclusions, and distinctive spongy texture and melt pockets. Secondly, the carbonate metasomatism can be well identified by some geochemical fingerprints as well. As to major elements, the clinopyroxenes in peridotite having experienced carbonate metasomatism are characterized by high Mg# value and Ca/Al ratio (>5). In terms of trace elements, clinopyroxenes in peridotite having experienced carbonate metasomatism generally have higher (La/Yb)N, Eu/Ti, Zr/Hf and Y/Ho ratios, and show depletions in HFSE. It is worth noting that the geochemical features may vary with the degree of carbonate metasomatism. In order to trace the source of carbonate melt caused metasomatism, Mg-Zn-Ca-Sr isotopic systems can be well used. Especially, in-situ Sr isotopic analysis method established in recent years provides us an important way to unravel the overlap of multiple carbonate metasomatism.
2019, 44(4): 1357-1370.
doi: 10.3799/dqkx.2018.161
Abstract:
Hainan Island, which is located between the Indochina and South China blocks, has undergone multiphased structural overprinting and complex tectonic evolution.However, the Late Paleozoic tectonic regime and amalgamation process of the South China with Indochina blocks are still controversial. The newly-identified felsic volcanic rocks in the Lizhigou area are important for understanding the Early Mesozoic tectonic evolution in Hainan. LA-ICP-MS zircon U-Pb dating results show that the representative rhyolite samples yield a weighted mean 206Pb/238U age of 241±6 Ma (MSWD=0.9). These samples contain SiO2 ranging from 72.34% to 77.83%, Al2O3 from 10.51% to 13.53%, K2O from 2.85% to 4.85% and Na2O from 1.75% to 3.79%.The A/CNK values range from 0.99 to 2.07. These samples have REE concentrations of 87×10-6 to 177×10-6, and Eu/Eu* of 0.49-0.65.They are characterized by subparallel right-sloping REE pattern and enrichment in LREEs and LILEs, and depletion in HFSEs, similar to those of arc volcanics. Their εNd(t) values range from -12.1 to -11.3. These felsic volcanic rocks are the products of partial melting of metasediments with significant fractionation crystallization. In combination with other available data, it is inferred that the Lizhigou felsic volcanic rocks can be compared with the Middle-Late Triassic igneous rocks along the Jinshajiang-Ailaoshan-Songma zone, and formed during the assemblage of the South China with Indochina blocks related to East Paleotethyan consumption.
Hainan Island, which is located between the Indochina and South China blocks, has undergone multiphased structural overprinting and complex tectonic evolution.However, the Late Paleozoic tectonic regime and amalgamation process of the South China with Indochina blocks are still controversial. The newly-identified felsic volcanic rocks in the Lizhigou area are important for understanding the Early Mesozoic tectonic evolution in Hainan. LA-ICP-MS zircon U-Pb dating results show that the representative rhyolite samples yield a weighted mean 206Pb/238U age of 241±6 Ma (MSWD=0.9). These samples contain SiO2 ranging from 72.34% to 77.83%, Al2O3 from 10.51% to 13.53%, K2O from 2.85% to 4.85% and Na2O from 1.75% to 3.79%.The A/CNK values range from 0.99 to 2.07. These samples have REE concentrations of 87×10-6 to 177×10-6, and Eu/Eu* of 0.49-0.65.They are characterized by subparallel right-sloping REE pattern and enrichment in LREEs and LILEs, and depletion in HFSEs, similar to those of arc volcanics. Their εNd(t) values range from -12.1 to -11.3. These felsic volcanic rocks are the products of partial melting of metasediments with significant fractionation crystallization. In combination with other available data, it is inferred that the Lizhigou felsic volcanic rocks can be compared with the Middle-Late Triassic igneous rocks along the Jinshajiang-Ailaoshan-Songma zone, and formed during the assemblage of the South China with Indochina blocks related to East Paleotethyan consumption.
2019, 44(4): 1371-1388.
doi: 10.3799/dqkx.2018.201
Abstract:
An Early Mesozoic volcanic belt with nearly EW distribution develops in South Hunan, Southeast Hunan, South Jiangxi and Southwest Fujian of the South China continental margin, which is represented by the volcanic rocks of Fankeng Formation from Tangpu and Wuhu villages of Yongding County in the Southwest Fujian. The zircon U-Pb dating results indicate that the volcanic rocks of Fankeng Formation formed in 189±6 Ma-195±2 Ma and 184±2 Ma, i.e., the Early Jurassic. In order to reveal the magma source and tectonic setting of the Early Jurassic volcanic rocks in the Southwest Fujian, the lithology identification, major and trace elements, and Sr-Nd isotope data are presented in this paper. The Daly gap of the basic end-member and the acid end-member of volcanic rocks of Fankeng Formation reaches to about 20, and they therefore constitute a typical bimodal association of volcanic rocks. The basic end-member basalt (tephrite) has high TiO2 and TFe2O3 (all Fe) values, and high ratios of (Rb/Sr)N, (La/Nb)N, (Ba/Nb)N and (Rb/Yb)N ratios, which are all higher than the original mantle values. They are also characterized by the higher ∑REE, enrichment in light rare earth elements, and negative anomalies of Eu. Their Isr values are between 0.704 4 and 0.707 5 and the εNd(t) are between -1.19 and 4.30, respectively, indicating that the magma source was derived from the deeper asthenosphere mantle and some components were derived from the lithosphere mantle. The (Rb/Sr)N and (Rb/Yb)N ratios of acid end-member rhyolite (dacite) are all higher than the initial mantle values. However, the (Ba/Nb)N ratios are all less than the initial mantle values and the (La/Nb)N ratios are both high and low. They are characterized by the high ∑REE, enrichment in light rare earth elements, and obvious anomalies of Eu. Their Isr values are between 0.701 2 and 0.705 3 and the εNd(t) are between -5.40 and -1.95, respectively, indicating that the magma source is the crust-mantle misciblemagma, which is mainly composed of the crust-derived materials with involvement of the mantle-derived components. The Early Jurassic bimodal volcanic rocks in Southwest Fujian were formed in the intraplate with a special tectonic position near the continental margin. They were influenced by the effect of low angle subduction of paleo-Pacific plate-back arc extension-basaltic magma underplating.
An Early Mesozoic volcanic belt with nearly EW distribution develops in South Hunan, Southeast Hunan, South Jiangxi and Southwest Fujian of the South China continental margin, which is represented by the volcanic rocks of Fankeng Formation from Tangpu and Wuhu villages of Yongding County in the Southwest Fujian. The zircon U-Pb dating results indicate that the volcanic rocks of Fankeng Formation formed in 189±6 Ma-195±2 Ma and 184±2 Ma, i.e., the Early Jurassic. In order to reveal the magma source and tectonic setting of the Early Jurassic volcanic rocks in the Southwest Fujian, the lithology identification, major and trace elements, and Sr-Nd isotope data are presented in this paper. The Daly gap of the basic end-member and the acid end-member of volcanic rocks of Fankeng Formation reaches to about 20, and they therefore constitute a typical bimodal association of volcanic rocks. The basic end-member basalt (tephrite) has high TiO2 and TFe2O3 (all Fe) values, and high ratios of (Rb/Sr)N, (La/Nb)N, (Ba/Nb)N and (Rb/Yb)N ratios, which are all higher than the original mantle values. They are also characterized by the higher ∑REE, enrichment in light rare earth elements, and negative anomalies of Eu. Their Isr values are between 0.704 4 and 0.707 5 and the εNd(t) are between -1.19 and 4.30, respectively, indicating that the magma source was derived from the deeper asthenosphere mantle and some components were derived from the lithosphere mantle. The (Rb/Sr)N and (Rb/Yb)N ratios of acid end-member rhyolite (dacite) are all higher than the initial mantle values. However, the (Ba/Nb)N ratios are all less than the initial mantle values and the (La/Nb)N ratios are both high and low. They are characterized by the high ∑REE, enrichment in light rare earth elements, and obvious anomalies of Eu. Their Isr values are between 0.701 2 and 0.705 3 and the εNd(t) are between -5.40 and -1.95, respectively, indicating that the magma source is the crust-mantle misciblemagma, which is mainly composed of the crust-derived materials with involvement of the mantle-derived components. The Early Jurassic bimodal volcanic rocks in Southwest Fujian were formed in the intraplate with a special tectonic position near the continental margin. They were influenced by the effect of low angle subduction of paleo-Pacific plate-back arc extension-basaltic magma underplating.
2019, 44(4): 1389-1405.
doi: 10.3799/dqkx.2018.122
Abstract:
According to previous studies, there are short of standard fossils to define the formed age of Shilidun Formation, which leads to many disputes about the depositionl time, and correlation studies on the provenance of Shilidun Formation are rare. LA-ICP-MS detrital zircon U-Pb dating was conducted for clastic rocks in the upper Shilidun Formation, Lintan area, to investigate the age and provenance in this study. The results show that the weighted mean age of the youngest zircon age group in the two samples is 265.7±6.5 Ma, representing the maximum depositional age of the upper Shilidun Formation. Taking the paleontological data into consideration, it is suggested that the upper Shilidun Formation was deposited in the Middle-Late Permian. The detrital zircon ages are divided into 4 groups:① Phanerozoic age group (265-467 Ma); ② Neoproterozoic age group (564-996 Ma), with a small peak at 955 Ma; ③ Mesoproterozoic age group (1 099-1 539 Ma); ④ Paleoproterozoic-Archaean age group (1 622-3 153 Ma), which can be subdivided into two groups:the Middle and Late Paleoproterozoic age group (1 622-2 194 Ma), with an obvious peak at 1 902 Ma; the Early Paleoproterozoic age group (2 343-3 153 Ma), with an obvious peak at 2 516 Ma.According to the characteristics of the detrital zircon spectra, it is thought that the upper Shilidun Formation has multi-sources, including the southern margin of the North China block, the eastern section of the Qilian orogenic belt, the northern margin of the West Qinling tectonic zone, and ancient basement of the southern margin of the North China block is main source region. Based on the sedimentary facies, lithofacies paleogeography and regional tectonic evolution, it is proposed that the upper Shilidun Formation should be formed in the northern slope of the back arc basin formed by the subduction of the Yangtze plate and the convergence of the Mianlue Ocean.
According to previous studies, there are short of standard fossils to define the formed age of Shilidun Formation, which leads to many disputes about the depositionl time, and correlation studies on the provenance of Shilidun Formation are rare. LA-ICP-MS detrital zircon U-Pb dating was conducted for clastic rocks in the upper Shilidun Formation, Lintan area, to investigate the age and provenance in this study. The results show that the weighted mean age of the youngest zircon age group in the two samples is 265.7±6.5 Ma, representing the maximum depositional age of the upper Shilidun Formation. Taking the paleontological data into consideration, it is suggested that the upper Shilidun Formation was deposited in the Middle-Late Permian. The detrital zircon ages are divided into 4 groups:① Phanerozoic age group (265-467 Ma); ② Neoproterozoic age group (564-996 Ma), with a small peak at 955 Ma; ③ Mesoproterozoic age group (1 099-1 539 Ma); ④ Paleoproterozoic-Archaean age group (1 622-3 153 Ma), which can be subdivided into two groups:the Middle and Late Paleoproterozoic age group (1 622-2 194 Ma), with an obvious peak at 1 902 Ma; the Early Paleoproterozoic age group (2 343-3 153 Ma), with an obvious peak at 2 516 Ma.According to the characteristics of the detrital zircon spectra, it is thought that the upper Shilidun Formation has multi-sources, including the southern margin of the North China block, the eastern section of the Qilian orogenic belt, the northern margin of the West Qinling tectonic zone, and ancient basement of the southern margin of the North China block is main source region. Based on the sedimentary facies, lithofacies paleogeography and regional tectonic evolution, it is proposed that the upper Shilidun Formation should be formed in the northern slope of the back arc basin formed by the subduction of the Yangtze plate and the convergence of the Mianlue Ocean.
2019, 44(4): 1424-1438.
doi: 10.3799/dqkx.2018.127
Abstract:
A suit of gray-green sericite-chlorite-quartz phyllite, gray-white sericite-albite-quartz phyllite and gray biotite-quartz-phyllite is exposed in the Huoshenmiao area, Lueyang County. The sequence is previously regarded as the Guanjiagou Formation, but the depositional age and tectonic setting of the protolith are still in dispute. In this paper, detrital zircons from the metasedimentary rocks of the Guanjiagou Formation in the Huoshenmiao area were tested by LA-ICP-MS zircon U-Pb dating. The zircon U-Pb ages range from 932 to 723 Ma, and the main age groups are 727 to 723 Ma, 760 to 758 Ma, 897 to 809 Ma and 932 Ma, respectively; a peak at 848 Ma and the secondary peak ages at 725 Ma, 758 Ma and 932 Ma. The youngest age group is 723 Ma to 727 Ma (725 Ma on average), which indicates that the depositional age of this formation was not earlier than the Nanhua period. Combined with the regional geological data, it is proposed that the detrital material is mainly sourced from magmatic rocks within the Mianlue tectonic belt and Bikou microblock, and less from magmatic rocks exposed in the Hannan area and the northern margin of the Yangtze plate. It should be noticed that the ages of the detrital zircons are fairly concentrated, from Qingbaikou to Nanhua period. Combined with data of the sedimentary strata in this area, it is suggested that this sedimentary sequences might be deposited in a rift environment.
A suit of gray-green sericite-chlorite-quartz phyllite, gray-white sericite-albite-quartz phyllite and gray biotite-quartz-phyllite is exposed in the Huoshenmiao area, Lueyang County. The sequence is previously regarded as the Guanjiagou Formation, but the depositional age and tectonic setting of the protolith are still in dispute. In this paper, detrital zircons from the metasedimentary rocks of the Guanjiagou Formation in the Huoshenmiao area were tested by LA-ICP-MS zircon U-Pb dating. The zircon U-Pb ages range from 932 to 723 Ma, and the main age groups are 727 to 723 Ma, 760 to 758 Ma, 897 to 809 Ma and 932 Ma, respectively; a peak at 848 Ma and the secondary peak ages at 725 Ma, 758 Ma and 932 Ma. The youngest age group is 723 Ma to 727 Ma (725 Ma on average), which indicates that the depositional age of this formation was not earlier than the Nanhua period. Combined with the regional geological data, it is proposed that the detrital material is mainly sourced from magmatic rocks within the Mianlue tectonic belt and Bikou microblock, and less from magmatic rocks exposed in the Hannan area and the northern margin of the Yangtze plate. It should be noticed that the ages of the detrital zircons are fairly concentrated, from Qingbaikou to Nanhua period. Combined with data of the sedimentary strata in this area, it is suggested that this sedimentary sequences might be deposited in a rift environment.
2019, 44(4): 1128-1142.
doi: 10.3799/dqkx.2019.026
Abstract:
The origin, growth and reworking of continental crust have always been a hot topics in the international geosciences, and there are still some disputes, particularly about the mode and rate of continental crust growth.In order to discuss continental crustal growth style, it briefly reviews the research achievements of subduction-related magmatism and continental crustal growth.Subduction zones can be divided into oceanic-oceanic subduction zones, oceanic-continental subduction zones and continental-continental subduction zones, characterized by arc magmatic rocks.Subduction-related magmatism has been widely accepted as the main continental crustal growth style.At present, there are two hypotheses of the growth of continental crust:basalt model and andesite model.The basalt model mainly highlights the evolution of the new crust to the continental crust through the processes of delamination and relamination, while the andesite model emphasizes that the continental crust is directly formed in the magmatic arc environments of the subduction zones where andesitic magmas occur.Plate convergent boundaries such as subduction and collision zones are considered to be the main locations for the growth and reworking of the Phanerozoic continental crust, and subduction-related magmatism is of great significance to continental crustal growth.
The origin, growth and reworking of continental crust have always been a hot topics in the international geosciences, and there are still some disputes, particularly about the mode and rate of continental crust growth.In order to discuss continental crustal growth style, it briefly reviews the research achievements of subduction-related magmatism and continental crustal growth.Subduction zones can be divided into oceanic-oceanic subduction zones, oceanic-continental subduction zones and continental-continental subduction zones, characterized by arc magmatic rocks.Subduction-related magmatism has been widely accepted as the main continental crustal growth style.At present, there are two hypotheses of the growth of continental crust:basalt model and andesite model.The basalt model mainly highlights the evolution of the new crust to the continental crust through the processes of delamination and relamination, while the andesite model emphasizes that the continental crust is directly formed in the magmatic arc environments of the subduction zones where andesitic magmas occur.Plate convergent boundaries such as subduction and collision zones are considered to be the main locations for the growth and reworking of the Phanerozoic continental crust, and subduction-related magmatism is of great significance to continental crustal growth.
2019, 44(4): 1143-1158.
doi: 10.3799/dqkx.2019.951
Abstract:
In order to further explore the nature of mantle source beneath the Northeast China, it presents major, trace element, and Sr-Nd-Pb-Hf isotopic compositions for the Cenozoic intra-plate volcanic rocks from the Halaha-Chaihe field in the Greater Khingan Range. These volcanic rocks are mainly alkaline (sodic) basalts, and generally exhibit OIB-like incompatible trace element characteristics, e.g.enrichment in large lithophile elements (LILEs) and positive Nb-Ta anomalies. They show moderate depleted Sr-Nd-Hf isotopic compositions (87Sr/86Sr=0.703 5-0.703 9, εNd=5.21-6.55, εHf=10.00-11.25) and almost represent the most depleted mantle end-member among the Cenozoic basalts in eastern China. Their Pb isotopic compositions (206Pb/204Pb=18.37-18.57, 207Pb/204Pb=15.52-15.54, 208Pb/204Pb=38.24 -38.43) range between 4.42 Ga and 4.45 Ga geochrons on the 207Pb/204Pb versus 206Pb/204Pb diagram. They also show similar Sr-Nd-Pb isotopic compositions with those mantle plume-derived ocean island basalts (3He/4He>30 Ra), which implies a deep mantle source. The high MgO (8.49%-11.58%), Ni(174×10-6-362×10-6) contents and high Mg# values (59.1-66.9) of these basalts imply that their compositions are close to those of the primary magmas. The calculated primitive compositions of Halaha-Chaihe basalts show moderate SiO2, low Al2O3 contents and high CaO/Al2O3 ratios, which are accordant with the compositions of experimental melts of garnet peridotite under high pressure (>2.5 GPa) conditions, suggesting a garnet peridotitic mantle source. Moreover, trace-element modeling suggests low-degree melts from a primitive mantle (rather than a depleted mantle) are consistent with these basalts. In summary, it is suggested that the mantle source of the Halaha-Chaihe basalts from the Greater Khingan Range contains ancient, primordial, peridotitic component from the deep mantle.
In order to further explore the nature of mantle source beneath the Northeast China, it presents major, trace element, and Sr-Nd-Pb-Hf isotopic compositions for the Cenozoic intra-plate volcanic rocks from the Halaha-Chaihe field in the Greater Khingan Range. These volcanic rocks are mainly alkaline (sodic) basalts, and generally exhibit OIB-like incompatible trace element characteristics, e.g.enrichment in large lithophile elements (LILEs) and positive Nb-Ta anomalies. They show moderate depleted Sr-Nd-Hf isotopic compositions (87Sr/86Sr=0.703 5-0.703 9, εNd=5.21-6.55, εHf=10.00-11.25) and almost represent the most depleted mantle end-member among the Cenozoic basalts in eastern China. Their Pb isotopic compositions (206Pb/204Pb=18.37-18.57, 207Pb/204Pb=15.52-15.54, 208Pb/204Pb=38.24 -38.43) range between 4.42 Ga and 4.45 Ga geochrons on the 207Pb/204Pb versus 206Pb/204Pb diagram. They also show similar Sr-Nd-Pb isotopic compositions with those mantle plume-derived ocean island basalts (3He/4He>30 Ra), which implies a deep mantle source. The high MgO (8.49%-11.58%), Ni(174×10-6-362×10-6) contents and high Mg# values (59.1-66.9) of these basalts imply that their compositions are close to those of the primary magmas. The calculated primitive compositions of Halaha-Chaihe basalts show moderate SiO2, low Al2O3 contents and high CaO/Al2O3 ratios, which are accordant with the compositions of experimental melts of garnet peridotite under high pressure (>2.5 GPa) conditions, suggesting a garnet peridotitic mantle source. Moreover, trace-element modeling suggests low-degree melts from a primitive mantle (rather than a depleted mantle) are consistent with these basalts. In summary, it is suggested that the mantle source of the Halaha-Chaihe basalts from the Greater Khingan Range contains ancient, primordial, peridotitic component from the deep mantle.
2019, 44(4): 1159-1168.
doi: 10.3799/dqkx.2019.021
Abstract:
Late Mesozoic magmatism in southeastern China was controlled by the subduction of paleo-Pacific plate, and the mantle beneath the area should also have been affected by such process.Late Mesozoic mafic dikes are widely distributed in southeastern China, especially in the coastal areas, which provide an ideal setting for studying the mantle evolution and subduction process of paleo-Pacific plate. Here it summarizes the chronological and geochemical data of mafic dikes in Hunan, Jiangxi, Zhejiang, Fujian and Guangdong provinces. Source lithologies of these rocks are identified by the compositions of fractionation-corrected primary magmas. The study indicates that the mantle lithologies of these mafic dikes in southeastern China do not show obvious differences in space, but significant changes with time. The source lithologies of mafic dikes during the period of 150-110 Ma are composed of SiO2-rich pyroxenite and SiO2-poor pyroxenite, whereas the source lithologies during 110-64 Ma are primarily composed of SiO2-poor pyroxenite, with minor peridotite. Therefore, it is suggested that the lithological variation of Late Mesozoic mantle beneath southeastern China is controlled by the subduction of paleo-Pacific plate via the variation of the subduction angle.
Late Mesozoic magmatism in southeastern China was controlled by the subduction of paleo-Pacific plate, and the mantle beneath the area should also have been affected by such process.Late Mesozoic mafic dikes are widely distributed in southeastern China, especially in the coastal areas, which provide an ideal setting for studying the mantle evolution and subduction process of paleo-Pacific plate. Here it summarizes the chronological and geochemical data of mafic dikes in Hunan, Jiangxi, Zhejiang, Fujian and Guangdong provinces. Source lithologies of these rocks are identified by the compositions of fractionation-corrected primary magmas. The study indicates that the mantle lithologies of these mafic dikes in southeastern China do not show obvious differences in space, but significant changes with time. The source lithologies of mafic dikes during the period of 150-110 Ma are composed of SiO2-rich pyroxenite and SiO2-poor pyroxenite, whereas the source lithologies during 110-64 Ma are primarily composed of SiO2-poor pyroxenite, with minor peridotite. Therefore, it is suggested that the lithological variation of Late Mesozoic mantle beneath southeastern China is controlled by the subduction of paleo-Pacific plate via the variation of the subduction angle.
2019, 44(4): 1169-1185.
doi: 10.3799/dqkx.2018.360
Abstract:
Changle is located both in the eastern section of the North China craton (NCC) and the middle part of Tancheng-Lujiang (Tan-Lu) fault zone that is characterized with widespread Cenozoic alkaline basalts wrapping a number of mantle xenoliths inside. For the better understanding of the evolution mechanism of the North China craton, three major types of clinopyroxenes are recognized on the basis of the petrography, mineralogy and mineral geochemical characteristics in this study. The first type is the protosomatic clinopyroxene occurred in mantle-derived xenoliths.The content of major and trace elements in this kind of clinopyroxenes show decoupling phenomenon with inconsistent tendencies. Moreover, the LREE and HREE characteristics of the protosomatic clinopyroxenes show that they had experienced multi-stage mantle metasomatism and different degrees of partial melting (no more than 10% partial melting in the spinel phase) and the metasomatic melts include mantle-derived alkali (K) and Al-enriched silicate melts/fluids, possibly contain the carbonate melts. The second kind is cribriform clinopyroxene in mantle-derived xenoliths known as the co-product of partial melting and melt metasomatism. The effect of partial melting exhibits a gradually strengthening from the non-meshing part to the areas far away from sieve mesh, then the areas around the sieve mesh. The areas far away from the sieve mesh record characteristics of a prior metasomatism of alkali (K) and Al-enriched melt before the basaltic magmatism while the areas around the sieve mesh show the strongest effect of late stage basaltic melts. The third type is the the clinopyroxenes in the corona of the protosomatic clinopyroxene or sieve-textured clinopyroxene in mantle-derived xenoliths and the clinopyroxene phenocrystsin alkaline basalts that share the similar geochemical compositions and P-T conditions that reveal a HFSE positive anomaly and a LILE negative anomaly that are strongly influenced by the OIB characteristic host basalt. The characteristics of multiple-stage "melt-effect" and different degrees of partial melting of the clinopyroxenes in the Changle Cenozoic alkaline basalt are the performance of the embodiment of lithospheric mantle heterogeneity in the study area.
Changle is located both in the eastern section of the North China craton (NCC) and the middle part of Tancheng-Lujiang (Tan-Lu) fault zone that is characterized with widespread Cenozoic alkaline basalts wrapping a number of mantle xenoliths inside. For the better understanding of the evolution mechanism of the North China craton, three major types of clinopyroxenes are recognized on the basis of the petrography, mineralogy and mineral geochemical characteristics in this study. The first type is the protosomatic clinopyroxene occurred in mantle-derived xenoliths.The content of major and trace elements in this kind of clinopyroxenes show decoupling phenomenon with inconsistent tendencies. Moreover, the LREE and HREE characteristics of the protosomatic clinopyroxenes show that they had experienced multi-stage mantle metasomatism and different degrees of partial melting (no more than 10% partial melting in the spinel phase) and the metasomatic melts include mantle-derived alkali (K) and Al-enriched silicate melts/fluids, possibly contain the carbonate melts. The second kind is cribriform clinopyroxene in mantle-derived xenoliths known as the co-product of partial melting and melt metasomatism. The effect of partial melting exhibits a gradually strengthening from the non-meshing part to the areas far away from sieve mesh, then the areas around the sieve mesh. The areas far away from the sieve mesh record characteristics of a prior metasomatism of alkali (K) and Al-enriched melt before the basaltic magmatism while the areas around the sieve mesh show the strongest effect of late stage basaltic melts. The third type is the the clinopyroxenes in the corona of the protosomatic clinopyroxene or sieve-textured clinopyroxene in mantle-derived xenoliths and the clinopyroxene phenocrystsin alkaline basalts that share the similar geochemical compositions and P-T conditions that reveal a HFSE positive anomaly and a LILE negative anomaly that are strongly influenced by the OIB characteristic host basalt. The characteristics of multiple-stage "melt-effect" and different degrees of partial melting of the clinopyroxenes in the Changle Cenozoic alkaline basalt are the performance of the embodiment of lithospheric mantle heterogeneity in the study area.
2019, 44(4): 1186-1201.
doi: 10.3799/dqkx.2018.586
Abstract:
This study presents zircon U-Pb dating, geochemistry and zircon Lu-Hf isotopic data for the Fengzishan granitic intrusion located in South Qinling to constrain its formation age, petrogenesis and tectonic setting. LA-ICP-MS zircon U-Pb dating results suggest that the Fengzishan granitic intrusion was formed in Late Neoproterozoic (716-710 Ma). Zircon Hf isotopic data show the εHf(t) values of -3.16-1.17 and TDM2 values of 1.84-1.52 Ga. Geochemical characteristics show that they are high in silica (62.92%-73.57%) and Na (3.33%-4.22%) contents and relatively enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE) (e.g. K, Rb, Ba, Th, U), poor in high field strength elements (HFSE) (Nb, Ta, Ti, P) and slightly negative Eu anomalies (δEu =0.79-0.95), displaying metaluminous to weak peraluminous and medium potassium calc-alkaline Ⅰ-type granites. These characteristics suggest that the magma was primarily derived from partial melting of gneiss of the Douling Group. The Fengzishan granitic intrusion was produced by subduction of oceanic crust into continental crust and was formed in the tectonic setting of rift expansion coexisting with small ocean basin.The formation of the Fengzishan granitoid was probably the response to the amalgamation and separation of the global Rodinia supercontinent.
This study presents zircon U-Pb dating, geochemistry and zircon Lu-Hf isotopic data for the Fengzishan granitic intrusion located in South Qinling to constrain its formation age, petrogenesis and tectonic setting. LA-ICP-MS zircon U-Pb dating results suggest that the Fengzishan granitic intrusion was formed in Late Neoproterozoic (716-710 Ma). Zircon Hf isotopic data show the εHf(t) values of -3.16-1.17 and TDM2 values of 1.84-1.52 Ga. Geochemical characteristics show that they are high in silica (62.92%-73.57%) and Na (3.33%-4.22%) contents and relatively enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE) (e.g. K, Rb, Ba, Th, U), poor in high field strength elements (HFSE) (Nb, Ta, Ti, P) and slightly negative Eu anomalies (δEu =0.79-0.95), displaying metaluminous to weak peraluminous and medium potassium calc-alkaline Ⅰ-type granites. These characteristics suggest that the magma was primarily derived from partial melting of gneiss of the Douling Group. The Fengzishan granitic intrusion was produced by subduction of oceanic crust into continental crust and was formed in the tectonic setting of rift expansion coexisting with small ocean basin.The formation of the Fengzishan granitoid was probably the response to the amalgamation and separation of the global Rodinia supercontinent.
2019, 44(4): 1202-1215.
doi: 10.3799/dqkx.2018.592
Abstract:
The geodynamic mechanisms of the Early Paleozoic orogeny in the South China block (SCB) remain highly debated, and two major models involving subduction-collision orogeny and intra-continental orogeny have been proposed. The Early Paleozoic granitic rocks are widely developed in the eastern SCB and the synchronous mafic rocks have recently been reported.However, the petrogenesis of these Early Paleozoic mafic rocks are still controversial, thus hindering our understanding for the Early Paleozoic tectonic setting. In this study, it presents new petrographic, geochronological and geochemical data for the Chidong gabbroic pluton in the Yunkai domain.The representative sample for the gabbric pluton yields a SIMS zircon U-Pb age of 433.9±1.5 Ma (MSWD=0.18). They have SiO2 of 47.81%-48.83%, MgO of 13.02%-14.65%, Cr of 278×10-6-356×10-6 and Ni of 120×10-6-184×10-6.They are enriched in large ion lithophile elements with significantly negative Nb-Ta and Ti anomalies.These samples show EMⅡ-like geochemical affinities with high initial 87Sr/86Sr ratios (0.708 5 to 0.710 7) and low εNd(t) values (-6.0 to -8.2), suggestive of derivation from a paleosubduction-modified wedge beneath the Yunkai Domain. The wedge was undisturbed until Silurian and the Kwangsian igneous rocks in the SCB were associated with the intra-continental orogenism rather than the subduction and subsequent collision.
The geodynamic mechanisms of the Early Paleozoic orogeny in the South China block (SCB) remain highly debated, and two major models involving subduction-collision orogeny and intra-continental orogeny have been proposed. The Early Paleozoic granitic rocks are widely developed in the eastern SCB and the synchronous mafic rocks have recently been reported.However, the petrogenesis of these Early Paleozoic mafic rocks are still controversial, thus hindering our understanding for the Early Paleozoic tectonic setting. In this study, it presents new petrographic, geochronological and geochemical data for the Chidong gabbroic pluton in the Yunkai domain.The representative sample for the gabbric pluton yields a SIMS zircon U-Pb age of 433.9±1.5 Ma (MSWD=0.18). They have SiO2 of 47.81%-48.83%, MgO of 13.02%-14.65%, Cr of 278×10-6-356×10-6 and Ni of 120×10-6-184×10-6.They are enriched in large ion lithophile elements with significantly negative Nb-Ta and Ti anomalies.These samples show EMⅡ-like geochemical affinities with high initial 87Sr/86Sr ratios (0.708 5 to 0.710 7) and low εNd(t) values (-6.0 to -8.2), suggestive of derivation from a paleosubduction-modified wedge beneath the Yunkai Domain. The wedge was undisturbed until Silurian and the Kwangsian igneous rocks in the SCB were associated with the intra-continental orogenism rather than the subduction and subsequent collision.
2019, 44(4): 1216-1235.
doi: 10.3799/dqkx.2018.526
Abstract:
Currently, predecessors hold different views on metamorphic ages and formation background of the Chencai Group. As the representative rock unit of the Chencai Group, the amphibolite is an important window for understanding its protolith property and tectonic evolution. Therefore, in this paper in detailed it analyzes the amphibolite from the Chencai Group, including geochronology and whole rock geochemistry.SHRIMP U-Pb dating data from the amphibolite, the weighted mean age of 424.0±3.6 Ma records the timing of the main metamorphic event and simultaneously preserves the record of the metamorphic age from Late Neoproterozoic to Ordovician. Based on the geochemical characteristics and discrimination diagrams, it is proposed that the protolith of the amphibolite from the Chencai Group is ocean island alkali basalt (OIB). Field observation presents the paragenetic association of the amphibolite with marble, further implying that the protolith of the amphibolite-marble assemblage could be OIB-carbonate rock assemblage from the oceanic island-seamount system. The protolith of the Chencai Group is probably the rock assemblage of the accretionary wedge which formed in the subduction of the South China Proterozoic ocean beneath the Yangtze block. The amphibolite-facies metamorphism occurred in the late period of Early Paleozoic.
Currently, predecessors hold different views on metamorphic ages and formation background of the Chencai Group. As the representative rock unit of the Chencai Group, the amphibolite is an important window for understanding its protolith property and tectonic evolution. Therefore, in this paper in detailed it analyzes the amphibolite from the Chencai Group, including geochronology and whole rock geochemistry.SHRIMP U-Pb dating data from the amphibolite, the weighted mean age of 424.0±3.6 Ma records the timing of the main metamorphic event and simultaneously preserves the record of the metamorphic age from Late Neoproterozoic to Ordovician. Based on the geochemical characteristics and discrimination diagrams, it is proposed that the protolith of the amphibolite from the Chencai Group is ocean island alkali basalt (OIB). Field observation presents the paragenetic association of the amphibolite with marble, further implying that the protolith of the amphibolite-marble assemblage could be OIB-carbonate rock assemblage from the oceanic island-seamount system. The protolith of the Chencai Group is probably the rock assemblage of the accretionary wedge which formed in the subduction of the South China Proterozoic ocean beneath the Yangtze block. The amphibolite-facies metamorphism occurred in the late period of Early Paleozoic.
2019, 44(4): 1236-1247.
doi: 10.3799/dqkx.2018.172
Abstract:
The Early Paleozoic magmatites are sporadically distributed along the western side of the Langshan Mountains, Inner Mongolia.The Early Silurian rocks are mainly exposed in the zone of Hariyinaorui-Maihantaolegai.LA-ICP-MS zircon U-Pb dating shows that the 206Pb/238U weighted average age of quartz diorite is 440±1 Ma.The rocks show similar chondrite-normalized REE patterns, and display relatively high concentration of light rare earth elements (LREEs) but low contents of heavy rare earth elements (HREEs) with minor negative Eu anomalies. The overall chemical similarities of these rocks on the primitive mantle-normalized variation diagrams display affinity to arc signature; considering both tectonic discriminated diagrams and the Hf isotopic signature, it is proposed that the Langshan area was under the tectonic setting of continental margin arc during Early Silurian. It is concluded that the subduction zone of Early Paleozoic had been extended from the area of Damaoqi to Langshan, since these rocks show similar geochemical characteristics with the recently published data of Early Silurian quartz diorite in the area of Damaoqi and Bainaimiao.
The Early Paleozoic magmatites are sporadically distributed along the western side of the Langshan Mountains, Inner Mongolia.The Early Silurian rocks are mainly exposed in the zone of Hariyinaorui-Maihantaolegai.LA-ICP-MS zircon U-Pb dating shows that the 206Pb/238U weighted average age of quartz diorite is 440±1 Ma.The rocks show similar chondrite-normalized REE patterns, and display relatively high concentration of light rare earth elements (LREEs) but low contents of heavy rare earth elements (HREEs) with minor negative Eu anomalies. The overall chemical similarities of these rocks on the primitive mantle-normalized variation diagrams display affinity to arc signature; considering both tectonic discriminated diagrams and the Hf isotopic signature, it is proposed that the Langshan area was under the tectonic setting of continental margin arc during Early Silurian. It is concluded that the subduction zone of Early Paleozoic had been extended from the area of Damaoqi to Langshan, since these rocks show similar geochemical characteristics with the recently published data of Early Silurian quartz diorite in the area of Damaoqi and Bainaimiao.
2019, 44(4): 1248-1265.
doi: 10.3799/dqkx.2018.270
Abstract:
Research on the quartz diorite porphyry in Dunbasitao gold deposit, Xinjiang, will throw light on the evolution of the paleo-Asian ocean in Late Devonian. The quartz diorite porphyry in Dunbasitao gold deposit is located in the northern margin of the East Junggar basin, Xinjiang Uygur Autonomous Region. LA-ICP-MS zircon U-Pb dating yields a weighted mean age of 378±2 Ma for the intrusion, indicating Late Devonian magmatism.The quartz diorite porphyry shows variable SiO2 contents (54.94%-68.64%), with high Na2O (average 4.34%) and Na2O/K2O (1.19%-2.72%), and it has medium Fe2O3T(2.23%-5.65%), MgO (0.81%-2.77%), Al2O3 (14.31%-16.99%), low TiO2 (0.32%-1.02%) with Rittman index (σ) < 3.3.Mg# values of the rocks range from 38.1 to 51.2 with CaO content of 2.58%-4.97 %, A/CNK of 0.81 to 1.08.The quartz diorite porphyry is metaluminous calc-alkaline-high-K calc-alkaline. It is enriched in LILE (Rb, Ba, Tu, K, La, Sr) and LREE, and depleted in HFSE (Nb, Ta, P, Ti) relative to the primitive mantle; the REE exhibits right-dipping patterns having significant LREE and HREE differentiation. All the samples have low (87Sr/86Sr)i (0.703 783-0.703 938), high εNd(t) (+6.2 to +6.7) and zircon εHf(t)=+12.1 to +15.1, as well as young Nd isotopic model ages (TDM ranging in 546-608 Ma) and zircon Hf isotopic model age (TDM1 ranging in 403-516 Ma), indicating a depleted mantle source. The rock was sourced from subducting slab during the subduction of the paleo-Asian ocean in Middle-Late Devonian. Trace element discrimination diagrams indicate that rock formed in volcanic arc environment, which implies bidirectional subduction (north and south) existing in early Late Paleozoic, combined with spatial distribution of ophiolite, granodiorites and volcanic rocks in the northern Xinjiang and northern margin of East Junggar.
Research on the quartz diorite porphyry in Dunbasitao gold deposit, Xinjiang, will throw light on the evolution of the paleo-Asian ocean in Late Devonian. The quartz diorite porphyry in Dunbasitao gold deposit is located in the northern margin of the East Junggar basin, Xinjiang Uygur Autonomous Region. LA-ICP-MS zircon U-Pb dating yields a weighted mean age of 378±2 Ma for the intrusion, indicating Late Devonian magmatism.The quartz diorite porphyry shows variable SiO2 contents (54.94%-68.64%), with high Na2O (average 4.34%) and Na2O/K2O (1.19%-2.72%), and it has medium Fe2O3T(2.23%-5.65%), MgO (0.81%-2.77%), Al2O3 (14.31%-16.99%), low TiO2 (0.32%-1.02%) with Rittman index (σ) < 3.3.Mg# values of the rocks range from 38.1 to 51.2 with CaO content of 2.58%-4.97 %, A/CNK of 0.81 to 1.08.The quartz diorite porphyry is metaluminous calc-alkaline-high-K calc-alkaline. It is enriched in LILE (Rb, Ba, Tu, K, La, Sr) and LREE, and depleted in HFSE (Nb, Ta, P, Ti) relative to the primitive mantle; the REE exhibits right-dipping patterns having significant LREE and HREE differentiation. All the samples have low (87Sr/86Sr)i (0.703 783-0.703 938), high εNd(t) (+6.2 to +6.7) and zircon εHf(t)=+12.1 to +15.1, as well as young Nd isotopic model ages (TDM ranging in 546-608 Ma) and zircon Hf isotopic model age (TDM1 ranging in 403-516 Ma), indicating a depleted mantle source. The rock was sourced from subducting slab during the subduction of the paleo-Asian ocean in Middle-Late Devonian. Trace element discrimination diagrams indicate that rock formed in volcanic arc environment, which implies bidirectional subduction (north and south) existing in early Late Paleozoic, combined with spatial distribution of ophiolite, granodiorites and volcanic rocks in the northern Xinjiang and northern margin of East Junggar.
Geochemistry and Significance of Paleoproterozoic Granitoids from Huangqikou, Central Helanshan Area
2019, 44(4): 1266-1277.
doi: 10.3799/dqkx.2018.295
Abstract:
Widespread rocks in the khondalite belt record magmatism, metamorphism and tectonic evolution of the North China craton (NCC). Whereas the further research on Huangqikou granitic plutons exposed in the central Helanshan area is needed to offer information about the genetic mechanisms and the dynamic backgroud. Based on the field geological and petrographic studies, major, trace elements and Sr-Nd isotopic compositions were carried out on the Huangqikou granitic plutons, so as to discuss their petrogenesis, source characteristics and geological significance. The early intruded Huangqikou granites consist of alkali granite and monzogranite, while the later intrusions are mainly tonalite. The Huangqikou granites display typical geochemical characteristics of S-type granite, such as high contents of K2O (2.97%-6.71%), A/CNK>1.1. However, the later intrusions generally show lower SiO2 and higher Al2O3 relative to those early ones. All the studied granitic rocks show enriched LREE, and negative Eu anomalies. The samples generally show similar variation trend in the trace-element patterns, such as systematic enrichment of LILE (K, Rb, Th) and depletion of HFSE, e.g. Nb, Ta, P, Ti. All these geochemical features, combined with the majority of positive εNd(t)(+1.81-+4.90) and Paleoproterozoic Nd model ages (TDM1=2.10-2.37 Ga, TDM2=2.10-2.35 Ga), suggest that the Huangqikou granites were mainly derived from the partial melting of the sedimentary rocks in the Zhaochigou Formation, but probably with minor mafic magma contributed from the mantle. Combined with previous regional metamorphic and magmatic studies, it is suggested that the early and later intruded granitic rocks in the Huangqikou region belong to orogenic granitoids, and were probably formed during the collision between the Ordos and Yinshan blocks and post-collisional extension setting, respectively.
Widespread rocks in the khondalite belt record magmatism, metamorphism and tectonic evolution of the North China craton (NCC). Whereas the further research on Huangqikou granitic plutons exposed in the central Helanshan area is needed to offer information about the genetic mechanisms and the dynamic backgroud. Based on the field geological and petrographic studies, major, trace elements and Sr-Nd isotopic compositions were carried out on the Huangqikou granitic plutons, so as to discuss their petrogenesis, source characteristics and geological significance. The early intruded Huangqikou granites consist of alkali granite and monzogranite, while the later intrusions are mainly tonalite. The Huangqikou granites display typical geochemical characteristics of S-type granite, such as high contents of K2O (2.97%-6.71%), A/CNK>1.1. However, the later intrusions generally show lower SiO2 and higher Al2O3 relative to those early ones. All the studied granitic rocks show enriched LREE, and negative Eu anomalies. The samples generally show similar variation trend in the trace-element patterns, such as systematic enrichment of LILE (K, Rb, Th) and depletion of HFSE, e.g. Nb, Ta, P, Ti. All these geochemical features, combined with the majority of positive εNd(t)(+1.81-+4.90) and Paleoproterozoic Nd model ages (TDM1=2.10-2.37 Ga, TDM2=2.10-2.35 Ga), suggest that the Huangqikou granites were mainly derived from the partial melting of the sedimentary rocks in the Zhaochigou Formation, but probably with minor mafic magma contributed from the mantle. Combined with previous regional metamorphic and magmatic studies, it is suggested that the early and later intruded granitic rocks in the Huangqikou region belong to orogenic granitoids, and were probably formed during the collision between the Ordos and Yinshan blocks and post-collisional extension setting, respectively.
2019, 44(4): 1278-1294.
doi: 10.3799/dqkx.2018.208
Abstract:
The strong Late Mesozoic magmatic intruding events happened in Northwest Zhejiang area, which is located in the active western Pacific margin. The granites are classified as Ⅰ- and aluminous A-types, and the former is further divided into Ⅰ-type and highly fractionated Ⅰ-type granites. The Ⅰ-type granites of Mid-Late Jurassic to early Early Cretaceous(172-135 Ma) have the initial 87Sr/86Sr ratios ranging from 0.707 004 to 0.711 896, εNd(t) values from -6.70 to -2.00, εHf(t) values from -5.08 to -1.67, indicating that these granites are originated from mixing between remelted Meso-Neoproterozoic Jiangnan island arc and little mantle materials rose along slab crack. The formation of these granites may be closely related to the subduction of Paleo-Pacific plate. The highly fractionated Ⅰ-type granites in the early Early Cretaceous(147-135 Ma) have the initial 87Sr/86Sr ratio ranging from 0.706 890 to 0.709 880, εNd(t) values from -6.80 to -4.50, εHf(t) values from -6.59 to -5.23, suggesting that they are the products of magma mixing by remelting crustal materials of the Meso-Neoproterozoic and rising mantle mafic magma due to asthenosphere uplifting. It also indicates that the tectonic environment transformed from compression to extension in this period.The A-type granites of middle Early Cretaceous(135-123 Ma) have the initial 87Sr/86Sr ratios ranging from 0.703 503 to 0.710 171, εNd(t) values from -8.90 to -0.30, εHf(t) values from -9.70 to 2.48, indicating that the magma during Middle-Early Cretaceous was mainly originated from remelting of the Meso-Neoproterozoic crustal materials, mixed by more and more mantle materials, in an extension setting.
The strong Late Mesozoic magmatic intruding events happened in Northwest Zhejiang area, which is located in the active western Pacific margin. The granites are classified as Ⅰ- and aluminous A-types, and the former is further divided into Ⅰ-type and highly fractionated Ⅰ-type granites. The Ⅰ-type granites of Mid-Late Jurassic to early Early Cretaceous(172-135 Ma) have the initial 87Sr/86Sr ratios ranging from 0.707 004 to 0.711 896, εNd(t) values from -6.70 to -2.00, εHf(t) values from -5.08 to -1.67, indicating that these granites are originated from mixing between remelted Meso-Neoproterozoic Jiangnan island arc and little mantle materials rose along slab crack. The formation of these granites may be closely related to the subduction of Paleo-Pacific plate. The highly fractionated Ⅰ-type granites in the early Early Cretaceous(147-135 Ma) have the initial 87Sr/86Sr ratio ranging from 0.706 890 to 0.709 880, εNd(t) values from -6.80 to -4.50, εHf(t) values from -6.59 to -5.23, suggesting that they are the products of magma mixing by remelting crustal materials of the Meso-Neoproterozoic and rising mantle mafic magma due to asthenosphere uplifting. It also indicates that the tectonic environment transformed from compression to extension in this period.The A-type granites of middle Early Cretaceous(135-123 Ma) have the initial 87Sr/86Sr ratios ranging from 0.703 503 to 0.710 171, εNd(t) values from -8.90 to -0.30, εHf(t) values from -9.70 to 2.48, indicating that the magma during Middle-Early Cretaceous was mainly originated from remelting of the Meso-Neoproterozoic crustal materials, mixed by more and more mantle materials, in an extension setting.
2019, 44(4): 1295-1310.
doi: 10.3799/dqkx.2018.585
Abstract:
Genetic types and tectonic backgrounds of miarolitic granite are still controversial. Previous studies have suggested that miarolitic alkali feldspar granites and miarolitic syenite granites belong to A-type granite formed in extensional environment. However, recent studies have found the diversity of the genetic types. In this paper, the petrology, geochemistry and geochronology of Boketu miarolite pluton are present. The zircon SHRIMP U-Pb dating results yield an age of 140.8±2.2 Ma, belonging to the Early Cretaceous. The pluton is characterized by high SiO2 content, and full high alkali content (7.99%-8.43%), with high K2O/Na2O (1.0-1.1); low total Fe content (1.15%-1.30%). On the primitive mantle-normalized spider diagrams, the granite is characterized by positive Th, U, Cs, Rb anomalies and negative Ba, Sr, P, Ti anomalies. 10 000*Ga/Al ratio, the total rare earth content and Zr+Nb+Y+Ce values are lower than the minimum value of the A-type granites. It is indicated that it should belong to Ⅰ-type granite. Hf isotope compositions show the source of juvenile crust. It is mainly restricted by the closure of the Mongolian-Okhotsk ocean, during which the lithosphere transformation from thickening to thinning took place. The granites are formed by the low-degree partial melting of juvenile crust in the underplating of the mantle.
Genetic types and tectonic backgrounds of miarolitic granite are still controversial. Previous studies have suggested that miarolitic alkali feldspar granites and miarolitic syenite granites belong to A-type granite formed in extensional environment. However, recent studies have found the diversity of the genetic types. In this paper, the petrology, geochemistry and geochronology of Boketu miarolite pluton are present. The zircon SHRIMP U-Pb dating results yield an age of 140.8±2.2 Ma, belonging to the Early Cretaceous. The pluton is characterized by high SiO2 content, and full high alkali content (7.99%-8.43%), with high K2O/Na2O (1.0-1.1); low total Fe content (1.15%-1.30%). On the primitive mantle-normalized spider diagrams, the granite is characterized by positive Th, U, Cs, Rb anomalies and negative Ba, Sr, P, Ti anomalies. 10 000*Ga/Al ratio, the total rare earth content and Zr+Nb+Y+Ce values are lower than the minimum value of the A-type granites. It is indicated that it should belong to Ⅰ-type granite. Hf isotope compositions show the source of juvenile crust. It is mainly restricted by the closure of the Mongolian-Okhotsk ocean, during which the lithosphere transformation from thickening to thinning took place. The granites are formed by the low-degree partial melting of juvenile crust in the underplating of the mantle.
2019, 44(4): 1311-1326.
doi: 10.3799/dqkx.2018.256
Abstract:
Zijinshan ore field is located in the eastern part of South China fold system, belonging to the Southwest Fujian depression, where develops many large and super-large epithermal Au-Cu-Ag deposits which are closely related with magmatic activities. The study in this paper is based on a detailed research of the fine-grained biotite granite in the southeastern ore section including field geological survey and petrography, zircon rare earth elements, in-situ zircon U-Pb chronology and Hf isotope et al.. The LA-ICP-MS zircon U-Pb dating shows that the 206Pb/238U-207Pb/235U correlated age of the biotite granite is 109.5±1.9 Ma (MSWD=0.74, N=16), and 206Pb/238U weighed age is 107.44±0.94 Ma(MSWD=1.06, N=16), and the two ages are within the error range. According to zircon REE and magmatic oscillatory zoning characteristics and the Th/U ratios, the age can represent the rock crystallization age, which suggests fine-grained biotite granite emplaced in the Early Cretaceous of the Yanshanian epoch. The Hf isotope initial ratio εHf(t) values are negative, ranging from -4.99 to -1.06 (mean value is -2.99). Two stage Hf model ages (tDM2) range from 1 233.7 Ma to 1 485.4 Ma (mean value is 1 362.4 Ma). The εHf(t) and Hf values show small variations in distribution, which implies that magmatic rocks having a more uniform zircon Hf isotope composition. The in-situ zircon U-Pb chronology and Hf isotope show time and source characteristics of Early Cretaceous magmatic activity in south-western part of Fujian Province, which results from the evolution of the extension and thinning, and crust-derived magma substance involved from lithosphere are closely related. In this paper, it provides new evidence on the mineralization and tectonic evolution of the Zijinshan ore field.
Zijinshan ore field is located in the eastern part of South China fold system, belonging to the Southwest Fujian depression, where develops many large and super-large epithermal Au-Cu-Ag deposits which are closely related with magmatic activities. The study in this paper is based on a detailed research of the fine-grained biotite granite in the southeastern ore section including field geological survey and petrography, zircon rare earth elements, in-situ zircon U-Pb chronology and Hf isotope et al.. The LA-ICP-MS zircon U-Pb dating shows that the 206Pb/238U-207Pb/235U correlated age of the biotite granite is 109.5±1.9 Ma (MSWD=0.74, N=16), and 206Pb/238U weighed age is 107.44±0.94 Ma(MSWD=1.06, N=16), and the two ages are within the error range. According to zircon REE and magmatic oscillatory zoning characteristics and the Th/U ratios, the age can represent the rock crystallization age, which suggests fine-grained biotite granite emplaced in the Early Cretaceous of the Yanshanian epoch. The Hf isotope initial ratio εHf(t) values are negative, ranging from -4.99 to -1.06 (mean value is -2.99). Two stage Hf model ages (tDM2) range from 1 233.7 Ma to 1 485.4 Ma (mean value is 1 362.4 Ma). The εHf(t) and Hf values show small variations in distribution, which implies that magmatic rocks having a more uniform zircon Hf isotope composition. The in-situ zircon U-Pb chronology and Hf isotope show time and source characteristics of Early Cretaceous magmatic activity in south-western part of Fujian Province, which results from the evolution of the extension and thinning, and crust-derived magma substance involved from lithosphere are closely related. In this paper, it provides new evidence on the mineralization and tectonic evolution of the Zijinshan ore field.
2019, 44(4): 1327-1337.
doi: 10.3799/dqkx.2017.591
Abstract:
The recently discovered Wenguangling deposit is a Pb-Zn-Cu polymetallic deposit, located in the Yunkai area. It was firstly investigated of the granodiorites from the Wenguangling deposit by integrating in situ zircon U-Pb dating and geochemical analysis. Eighteen zircons from sample WGL-02 yield excellently concordant results with a weighted mean 206Pb/238U age of 165.7±1.9 Ma (n=18, MSWD=3.1). The Wenguangling granodiorites belong to metaluminous Ⅰ-type granite. Their SiO2 and Al2O3 contents are 64.11%-66.64% and 14.9%-15.57%, respectively. The MgO contents are between 1.39% and 1.61%. The alkali (K2O+Na2O) contents range from 6.5% to 7.21%, with K2O/Na2O ratios from 1.25 to 2.74. Their aluminum indexes (A/CNK ratios) are 0.93-1.11. The granodiorites are also characterized by noteworthy fractionation between LREE and HREE ((La/Yb)N=13.03-17.83), with weak negative Eu anomalies (δEu=0.70-0.88). The trace element analyses show clear fractionation between HFSE and LILE, and the granodiorites are strongly enriched in Rb, Th, U and K but depleted in Nb, Ta, P and Ti. Based on the geochemistry, zircon U-Pb dating of the Wenguangling granodiorite and regional tectonic evolution, it is infered that the formation of the Wenguangling Pb-Zn-Cu polymetallic deposit is closely associated with the subduction of the Paleo-Pacific plate.
The recently discovered Wenguangling deposit is a Pb-Zn-Cu polymetallic deposit, located in the Yunkai area. It was firstly investigated of the granodiorites from the Wenguangling deposit by integrating in situ zircon U-Pb dating and geochemical analysis. Eighteen zircons from sample WGL-02 yield excellently concordant results with a weighted mean 206Pb/238U age of 165.7±1.9 Ma (n=18, MSWD=3.1). The Wenguangling granodiorites belong to metaluminous Ⅰ-type granite. Their SiO2 and Al2O3 contents are 64.11%-66.64% and 14.9%-15.57%, respectively. The MgO contents are between 1.39% and 1.61%. The alkali (K2O+Na2O) contents range from 6.5% to 7.21%, with K2O/Na2O ratios from 1.25 to 2.74. Their aluminum indexes (A/CNK ratios) are 0.93-1.11. The granodiorites are also characterized by noteworthy fractionation between LREE and HREE ((La/Yb)N=13.03-17.83), with weak negative Eu anomalies (δEu=0.70-0.88). The trace element analyses show clear fractionation between HFSE and LILE, and the granodiorites are strongly enriched in Rb, Th, U and K but depleted in Nb, Ta, P and Ti. Based on the geochemistry, zircon U-Pb dating of the Wenguangling granodiorite and regional tectonic evolution, it is infered that the formation of the Wenguangling Pb-Zn-Cu polymetallic deposit is closely associated with the subduction of the Paleo-Pacific plate.
2019, 44(4): 1338-1356.
doi: 10.3799/dqkx.2018.163
Abstract:
Located in the Ningqiang area of the northwest margin of the Yangtze block, the Daan granite consists mainly of biotite granodiorite. However, the petrogenesis and geological significances of the Daan pluton remain unclear in addition to the controversial formation age. In this paper, a detailed study of LA-ICP-MS zircon U-Pb geochronology and geochemistry is presented for the Daan granite.The results show that the age of granodiorite is 212.3±1.6 Ma and 212.48±0.43 Ma, belonging to the Late Triassic. Geochemically, the Daan granites have relatively high SiO2 (67.61%-69.02%) and Al2O3 (16.14%-16.80%) contents, and are enriched in LILE (Cs, Ba) and LREE. The samples display an insignificant negative Eu anomaly, low Y (3.10×10-6-3.90×10-6) contents, and high Sr contents (538×10-6-907×10-6) and Sr/Y ratios (138-291), exhibiting marked geochemical features of adakite. Combined with regional geological data, it is proposed that the Daan granite was formed in a post-collisional tectonic environment. The extensional regime after collision between the North China block and the Yangtze block induced the upwelling of mantle material, resulting in the dehydration and melting of thickened basic lower crust and formed the adakitic magma.
Located in the Ningqiang area of the northwest margin of the Yangtze block, the Daan granite consists mainly of biotite granodiorite. However, the petrogenesis and geological significances of the Daan pluton remain unclear in addition to the controversial formation age. In this paper, a detailed study of LA-ICP-MS zircon U-Pb geochronology and geochemistry is presented for the Daan granite.The results show that the age of granodiorite is 212.3±1.6 Ma and 212.48±0.43 Ma, belonging to the Late Triassic. Geochemically, the Daan granites have relatively high SiO2 (67.61%-69.02%) and Al2O3 (16.14%-16.80%) contents, and are enriched in LILE (Cs, Ba) and LREE. The samples display an insignificant negative Eu anomaly, low Y (3.10×10-6-3.90×10-6) contents, and high Sr contents (538×10-6-907×10-6) and Sr/Y ratios (138-291), exhibiting marked geochemical features of adakite. Combined with regional geological data, it is proposed that the Daan granite was formed in a post-collisional tectonic environment. The extensional regime after collision between the North China block and the Yangtze block induced the upwelling of mantle material, resulting in the dehydration and melting of thickened basic lower crust and formed the adakitic magma.
