Unlike the magma intrusion model,the in- situ melting hypothesis advanced in the lastdecade regards the upper crustas a closed system,and granite as the resultof the materialswithin system changing from order (protoli...Unlike the magma intrusion model,the in- situ melting hypothesis advanced in the lastdecade regards the upper crustas a closed system,and granite as the resultof the materialswithin system changing from order (protolith) to disorder (melts) and to new order(granite) with the variations of entropy of the system.The various geological and geochemi-cal data from the Mesozoic granitesof southeast China are explained logically and systemical-ly by the hypothesis,concluding that they should be originated from the melting of pro-toliths.According to the hypothesis,melts generated from in- situ melting are of layer- likewithin the crustand batholithsare the protruding parts of the uppersurface of the layer (de-fined as the Melting Interface,MI for short) .On the basis the author tries to discuss thesource of heatfor the Mesozoic crustal melting in southeast China.展开更多
The early Cretaceous trachytes of Donglingtai Formation in Xishan, Beijing are characterized by slight Eu negative anomaly and significant enrichment in LREE, LILE (Ba, K and Sr) and depletion of Nb-Ta-Ti, Th-U. These...The early Cretaceous trachytes of Donglingtai Formation in Xishan, Beijing are characterized by slight Eu negative anomaly and significant enrichment in LREE, LILE (Ba, K and Sr) and depletion of Nb-Ta-Ti, Th-U. These trachytes have been highly enriched by Sr and Nd isotopic signatures (87Sr/86Sr(t)= 0.70638～ 0.70672, εNd(t) = -16.3～ -15.7), overlapping Sr-Nd isotopic range of late Mesozoic mafic igneous rocks in the region. Taking into account Nb-Ta fractionation and high Zr/Sm ratio for these trachytes, we consider that the trachytes of Donglingtai Formation are derived from the garnet-bearing amphibolite in the lower crust composed of garnet + plagioclase ± amphibole ± pyroxene residual phases. On the basis of the melting experimental results of crustal materials and regional extensional tectonics during late Mesozoic, it is concluded that the thermal input from underplating basaltic melt results in the partial melting of lower continental crust to generate the trachytes of Donglingtai Formation. And the characteristics of high Sr and Ba for these trachytes suggest that part of underplating basalts should take a share in partial melting at least.展开更多
Several types of felsic granitoid rocks have been recognized, intrusive in both the mantle and the crustal sequence of the Semail ophiolite. Several models have been proposed for the source of this suite of tonalites,...Several types of felsic granitoid rocks have been recognized, intrusive in both the mantle and the crustal sequence of the Semail ophiolite. Several models have been proposed for the source of this suite of tonalites, granodiorites, trondhjemites intrusions, however their genesis is still not clearly understood.The sampled Dadnah tonalites that intruded in the mantle section of the Semail ophiolite display arctype geochemical characteristics, are high siliceous, low-potassic, metaluminous to weakly peraluminous, enriched in LILE, show positive peaks for Ba, Pb, Eu, negative troughs for U, Ti and occur with low δ^(18)O_(H_2 O), moderate ε_(Sr) and negative eNd values. They have crystallized at temperatures that range from～550 ℃ to ～720 ℃ and pressure ranging from 4.4 kbar to 6.5 kbar. The isotopic ages from our tonalite samples range between 98.6 Ma and 94.9 Ma, slightly older and overlapping with the age of the metamorphic sole. Our field observations, mineralogical, petrological, geochemical, isotopic and melt inclusion data suggest that the Dadnah tonalites formed by partial melting(～10%-15% continuous or ～12% batch partial melting), accumulation of plagioclase, fractional crystallization(～55%-57%), and interaction with their host harzburgites. These tonalites were the end result of partial melting and subsequent contamination and mixing of ～4% oceanic sediments with ～96% oceanic lithosphere from the subducted slab. This MORB-type slab melt composed from ～97% recycled oceanic crust and ～3% of the overlying mantle.We suggest that a possible protolith for these tonalites was the basaltic lavas from the subducted oceanic slab that melted during the initial stages of the supra-subduction zone(SSZ), which was forming synchronously to the spreading ridge axis. The tonalite melts mildly modified due to low degree of mixing and interaction with the overlying lithospheric mantle. Subsequently, the Dadnah tonalites emplaced at the upper part of the mantle sequence of the Semail ophiolite and are geochemically distinct from the other mantle intrusive felsic granitoids to the south.展开更多
Recent studies have shown that Cu-rich sulfide accumulates in the lower continental crust and serves as a critical reservoir to balance Cu depletion in the upper crust.Recycling of Cu in the lower crust is also assume...Recent studies have shown that Cu-rich sulfide accumulates in the lower continental crust and serves as a critical reservoir to balance Cu depletion in the upper crust.Recycling of Cu in the lower crust is also assumed to be a major metal source for non-arc setting porphyry Cu deposits.To test this hypothesis and further explore the behavior of Cu in the lower crust,we analyzed the elemental and Cu isotopic compositions of lower crustal rocks from different geological domains.The collected samples include hornblendites from the Kohistan arc,granulite xenoliths and hornblendites from the Gangdese arc,hornblendites and gabbros from the Laiyuan complex in the North China Craton,and hornblendite xenoliths from the western margin of the Yangtze Craton.These lower crustal rocks have experienced varying degrees of primary or secondary sulfide accumulation,with significantly varied Cu contents(11.2 to 145 ppm)andδδ^(65)(1.05‰to 1.40‰).Petrography and geochemistry reveal varying degrees of metasomatism and fluid interaction in these rocks,and based on this,they can be further divided into three groups:Group I includes the Gangdese granulites and Yunnan hornblendites,which perhaps experienced significant metasomatism.This suite of rocks shows enrichment ofδ^(65)(dδ^(65)=0.01‰to 1.40‰),positively correlated with metasomatism(dδ^(65)vs.Ce/Pb).We suggest the secondary sulfides which transformed from sulfates during the interaction between lower crust and arc magma are dominant in these rocks,so the feature of heavy isotope enrichment is inherited.Group II includes Laiyuan hornblendites and gabbros,derived from the same parental magma and emplaced at different depths(hornblendites,23.3–28.1 km;gabbros 8.4–11.1 km).The Cu isotopic compositions are strongly fractionated between these two kinds of rocks,with low dδ^(65)in the hornblendites(0.00‰to 0.28‰)and highly polarized dδ^(65)in the gabbros(1.05‰to 0.81‰).Geochemical indicators and mineral assemblages reveal that fluid interaction is most likely responsible for this feature.Primary sulfides were decomposed by fluids and reprecipitated at shallower depths.Since this process involves multiple redox reactions,the Cu isotopic composition in the shallowed emplaced gabbros was large fractionated.Group III includes the Gangdese hornblendites and Kohistan hornblendites which show negligible impacts of subduction-like metasomatism and fluid interaction.The Gangdese hornblendites show a homogeneous and unfractionated Cu isotopic composition(-0.09‰to 0.18‰)and Cu content(83.4 to 128 ppm),suggesting insignificant Cu migration and isotope fractionation.In contrast,the Cu isotopic composition of the Kohistan hornblendites is strongly fractionated(-0.36‰to 1.27‰).Geochemistry and modeling results suggest partial melting plays a role in the Cu isotope fractionation.The light Cu isotopes are preferentially distributed into sulfide melts and removed from the source region during partial melting of the lower crust,resulting in a decrease in Cu content and enrichment of heavy Cu isotopes in residues.Results suggest that partial melting and fluid interaction are two efficient mechanisms that encourage Cu migration in the lower crust.展开更多
It is proposed in the subduction channel model that the plate interface interaction is a basic mechanism for the mass and energy exchange between Earth’s surface and interior.The significant difference in composition...It is proposed in the subduction channel model that the plate interface interaction is a basic mechanism for the mass and energy exchange between Earth’s surface and interior.The significant difference in composition and nature between continental lithosphere and oceanic lithosphere inevitably leads to variations in deep physical and chemical processes as well as crust-mantle interaction products in these two settings.Many studies of experimental petrology have provided constraints on the potential partial melting and crust-mantle interaction in oceanic subduction channels for silicate and carbonate rocks.The partial melts of mafic and felsic compositions are adakitic or non-adakitic granitic melts depending on melting pressure or depth.A trivial amount of CO2 can lower significantly the melting temperature of peridotites and lead to pronounced enrichment of incompatible elements in carbonate melt.The silica saturated or unsaturated melts can react with mantle-wedge peridotites in subduction channels to generate complex products.However,the existing experiments are mostly dedicated to island arc settings above oceanic subduction zones rather than dehydration melting above continental subduction zones.It is crucial to conduct high pressure and high temperature experiments to investigate all possible reactions between peridotites and crustal materials and their derivatives under the conditions responsible for the slab-mantle interface in continental subduction channels.Experimental results,combined with natural observations,are possible to elucidate the processes of metamorphic dehydration,partial melting and mantle metasomatism in continental subduction channels.展开更多
It is commonly agreed that seismic anisotropy, most likely caused by lattice preferred orientation （LPO） of major minerals, is a very important indicator of intracrustal deformation. Ultra- sonic velocity measuremen...It is commonly agreed that seismic anisotropy, most likely caused by lattice preferred orientation （LPO） of major minerals, is a very important indicator of intracrustal deformation. Ultra- sonic velocity measurements on the rocks from higher Himalayan crystallines （HHC） and Honghe （红河） strike-slip fault zone in Southwest China showed an average anisotropic magnitude of about 5%. However, a series of seismic measurements conducted in Tibet indicated marked anisotropy with a magnitude ranging from 8% to 18% within middle to lower crust. What causes the anomalously strong anisotropy within Tibetan crust？ Parts of HHC rocks, to some extent, had undergone granulitic-grade metamorphism, the temperature and pressure of which were in excess of their solidus. Additionally, oriented leucocratic portions, which are accepted to be products crystallized from localized melt bands and aligned melt pocket （AMP）, are present in HHC pervasively. If melt is oriented, it is expected to be an extremely important factor to influence anisotropy behavior. Experiments performed on analogue materials composed of plexiglass matrix and chocolate demonstrated that aligned melt could result in an extra anisotropy whose magnitude might increase two to three times. The contribution of AMP on anisotropy is likely comparable to or larger than that induced by LPO of major minerals, possibly amphiboles and micas, in middle to lower crust. It is implied that aligned melt may be a potential factor to induce anomalously strong anisotropy within Tibetan middle to lower crust.展开更多
文摘Unlike the magma intrusion model,the in- situ melting hypothesis advanced in the lastdecade regards the upper crustas a closed system,and granite as the resultof the materialswithin system changing from order (protolith) to disorder (melts) and to new order(granite) with the variations of entropy of the system.The various geological and geochemi-cal data from the Mesozoic granitesof southeast China are explained logically and systemical-ly by the hypothesis,concluding that they should be originated from the melting of pro-toliths.According to the hypothesis,melts generated from in- situ melting are of layer- likewithin the crustand batholithsare the protruding parts of the uppersurface of the layer (de-fined as the Melting Interface,MI for short) .On the basis the author tries to discuss thesource of heatfor the Mesozoic crustal melting in southeast China.
基金This study is being financially supported by Chinese Academy of Science(KZCX1-107 and KZCX2-104)National Nature Science Foundation(No.40073011)
文摘The early Cretaceous trachytes of Donglingtai Formation in Xishan, Beijing are characterized by slight Eu negative anomaly and significant enrichment in LREE, LILE (Ba, K and Sr) and depletion of Nb-Ta-Ti, Th-U. These trachytes have been highly enriched by Sr and Nd isotopic signatures (87Sr/86Sr(t)= 0.70638～ 0.70672, εNd(t) = -16.3～ -15.7), overlapping Sr-Nd isotopic range of late Mesozoic mafic igneous rocks in the region. Taking into account Nb-Ta fractionation and high Zr/Sm ratio for these trachytes, we consider that the trachytes of Donglingtai Formation are derived from the garnet-bearing amphibolite in the lower crust composed of garnet + plagioclase ± amphibole ± pyroxene residual phases. On the basis of the melting experimental results of crustal materials and regional extensional tectonics during late Mesozoic, it is concluded that the thermal input from underplating basaltic melt results in the partial melting of lower continental crust to generate the trachytes of Donglingtai Formation. And the characteristics of high Sr and Ba for these trachytes suggest that part of underplating basalts should take a share in partial melting at least.
基金part of H.J.'s MSc thesis project (PIPGSTS-16-16)conducted at the Petroleum Institute,UAE.
文摘Several types of felsic granitoid rocks have been recognized, intrusive in both the mantle and the crustal sequence of the Semail ophiolite. Several models have been proposed for the source of this suite of tonalites, granodiorites, trondhjemites intrusions, however their genesis is still not clearly understood.The sampled Dadnah tonalites that intruded in the mantle section of the Semail ophiolite display arctype geochemical characteristics, are high siliceous, low-potassic, metaluminous to weakly peraluminous, enriched in LILE, show positive peaks for Ba, Pb, Eu, negative troughs for U, Ti and occur with low δ^(18)O_(H_2 O), moderate ε_(Sr) and negative eNd values. They have crystallized at temperatures that range from～550 ℃ to ～720 ℃ and pressure ranging from 4.4 kbar to 6.5 kbar. The isotopic ages from our tonalite samples range between 98.6 Ma and 94.9 Ma, slightly older and overlapping with the age of the metamorphic sole. Our field observations, mineralogical, petrological, geochemical, isotopic and melt inclusion data suggest that the Dadnah tonalites formed by partial melting(～10%-15% continuous or ～12% batch partial melting), accumulation of plagioclase, fractional crystallization(～55%-57%), and interaction with their host harzburgites. These tonalites were the end result of partial melting and subsequent contamination and mixing of ～4% oceanic sediments with ～96% oceanic lithosphere from the subducted slab. This MORB-type slab melt composed from ～97% recycled oceanic crust and ～3% of the overlying mantle.We suggest that a possible protolith for these tonalites was the basaltic lavas from the subducted oceanic slab that melted during the initial stages of the supra-subduction zone(SSZ), which was forming synchronously to the spreading ridge axis. The tonalite melts mildly modified due to low degree of mixing and interaction with the overlying lithospheric mantle. Subsequently, the Dadnah tonalites emplaced at the upper part of the mantle sequence of the Semail ophiolite and are geochemically distinct from the other mantle intrusive felsic granitoids to the south.
基金supported by National Key R&D Program of China(2022YFF0800902)National Natural Science Foundation of China(No.42121002)+3 种基金the Open Research Project from the State Key Laboratory for Mineral Deposits Research,Nanjing University(2022-LAMD-K11)SinoProbe Lab 202204,State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences(GPMR202110)the 111 Project(B18048)the Fundamental Research Funds for the Central Universities(2-9-2019-034).
文摘Recent studies have shown that Cu-rich sulfide accumulates in the lower continental crust and serves as a critical reservoir to balance Cu depletion in the upper crust.Recycling of Cu in the lower crust is also assumed to be a major metal source for non-arc setting porphyry Cu deposits.To test this hypothesis and further explore the behavior of Cu in the lower crust,we analyzed the elemental and Cu isotopic compositions of lower crustal rocks from different geological domains.The collected samples include hornblendites from the Kohistan arc,granulite xenoliths and hornblendites from the Gangdese arc,hornblendites and gabbros from the Laiyuan complex in the North China Craton,and hornblendite xenoliths from the western margin of the Yangtze Craton.These lower crustal rocks have experienced varying degrees of primary or secondary sulfide accumulation,with significantly varied Cu contents(11.2 to 145 ppm)andδδ^(65)(1.05‰to 1.40‰).Petrography and geochemistry reveal varying degrees of metasomatism and fluid interaction in these rocks,and based on this,they can be further divided into three groups:Group I includes the Gangdese granulites and Yunnan hornblendites,which perhaps experienced significant metasomatism.This suite of rocks shows enrichment ofδ^(65)(dδ^(65)=0.01‰to 1.40‰),positively correlated with metasomatism(dδ^(65)vs.Ce/Pb).We suggest the secondary sulfides which transformed from sulfates during the interaction between lower crust and arc magma are dominant in these rocks,so the feature of heavy isotope enrichment is inherited.Group II includes Laiyuan hornblendites and gabbros,derived from the same parental magma and emplaced at different depths(hornblendites,23.3–28.1 km;gabbros 8.4–11.1 km).The Cu isotopic compositions are strongly fractionated between these two kinds of rocks,with low dδ^(65)in the hornblendites(0.00‰to 0.28‰)and highly polarized dδ^(65)in the gabbros(1.05‰to 0.81‰).Geochemical indicators and mineral assemblages reveal that fluid interaction is most likely responsible for this feature.Primary sulfides were decomposed by fluids and reprecipitated at shallower depths.Since this process involves multiple redox reactions,the Cu isotopic composition in the shallowed emplaced gabbros was large fractionated.Group III includes the Gangdese hornblendites and Kohistan hornblendites which show negligible impacts of subduction-like metasomatism and fluid interaction.The Gangdese hornblendites show a homogeneous and unfractionated Cu isotopic composition(-0.09‰to 0.18‰)and Cu content(83.4 to 128 ppm),suggesting insignificant Cu migration and isotope fractionation.In contrast,the Cu isotopic composition of the Kohistan hornblendites is strongly fractionated(-0.36‰to 1.27‰).Geochemistry and modeling results suggest partial melting plays a role in the Cu isotope fractionation.The light Cu isotopes are preferentially distributed into sulfide melts and removed from the source region during partial melting of the lower crust,resulting in a decrease in Cu content and enrichment of heavy Cu isotopes in residues.Results suggest that partial melting and fluid interaction are two efficient mechanisms that encourage Cu migration in the lower crust.
基金supported by the National Basic Research Program of China(Grant No.2015CB856101)the National Natural Science Foundation of China(Grant Nos.41172070,41425012)the Ministry of Education of China and the State Administration of Foreign Expert Affairs of China(Grant No.B07039)
文摘It is proposed in the subduction channel model that the plate interface interaction is a basic mechanism for the mass and energy exchange between Earth’s surface and interior.The significant difference in composition and nature between continental lithosphere and oceanic lithosphere inevitably leads to variations in deep physical and chemical processes as well as crust-mantle interaction products in these two settings.Many studies of experimental petrology have provided constraints on the potential partial melting and crust-mantle interaction in oceanic subduction channels for silicate and carbonate rocks.The partial melts of mafic and felsic compositions are adakitic or non-adakitic granitic melts depending on melting pressure or depth.A trivial amount of CO2 can lower significantly the melting temperature of peridotites and lead to pronounced enrichment of incompatible elements in carbonate melt.The silica saturated or unsaturated melts can react with mantle-wedge peridotites in subduction channels to generate complex products.However,the existing experiments are mostly dedicated to island arc settings above oceanic subduction zones rather than dehydration melting above continental subduction zones.It is crucial to conduct high pressure and high temperature experiments to investigate all possible reactions between peridotites and crustal materials and their derivatives under the conditions responsible for the slab-mantle interface in continental subduction channels.Experimental results,combined with natural observations,are possible to elucidate the processes of metamorphic dehydration,partial melting and mantle metasomatism in continental subduction channels.
基金supported by the National Natural Science Foundation of China (No. 40972139)the Foundation of Earthquake Sciences of China Earthquake Administration (No. 2008419012)the Key Basic Research and Development Program of China (No. 2004CB418405)
文摘It is commonly agreed that seismic anisotropy, most likely caused by lattice preferred orientation （LPO） of major minerals, is a very important indicator of intracrustal deformation. Ultra- sonic velocity measurements on the rocks from higher Himalayan crystallines （HHC） and Honghe （红河） strike-slip fault zone in Southwest China showed an average anisotropic magnitude of about 5%. However, a series of seismic measurements conducted in Tibet indicated marked anisotropy with a magnitude ranging from 8% to 18% within middle to lower crust. What causes the anomalously strong anisotropy within Tibetan crust？ Parts of HHC rocks, to some extent, had undergone granulitic-grade metamorphism, the temperature and pressure of which were in excess of their solidus. Additionally, oriented leucocratic portions, which are accepted to be products crystallized from localized melt bands and aligned melt pocket （AMP）, are present in HHC pervasively. If melt is oriented, it is expected to be an extremely important factor to influence anisotropy behavior. Experiments performed on analogue materials composed of plexiglass matrix and chocolate demonstrated that aligned melt could result in an extra anisotropy whose magnitude might increase two to three times. The contribution of AMP on anisotropy is likely comparable to or larger than that induced by LPO of major minerals, possibly amphiboles and micas, in middle to lower crust. It is implied that aligned melt may be a potential factor to induce anomalously strong anisotropy within Tibetan middle to lower crust.
