The Okinawa Trough(OT) is a back-arc basin at an initial spreading stage that is under the influence of subduction of the Philippine Sea Plate. In this study, we analyzed the geochemical compositions of basaltic gla...The Okinawa Trough(OT) is a back-arc basin at an initial spreading stage that is under the influence of subduction of the Philippine Sea Plate. In this study, we analyzed the geochemical compositions of basaltic glass in the OT and discussed the effects of different magmatic sources, evolution, and subducted components in basalts. Our results showed that the middle and southern regions of the OT exhibit characteristics consistent with an iron-rich tholeiite series. Trace element proportions conform to the typical spider diagram pattern characteristic of back-arc basin basalts, rich in large ion lithophile elements(LILEs) including Rb, Ba, Pb, U, and Th, while depleted in high field-strength elements(HFSEs) including Nb, Ta, Zr, Hf, and Ti. The distribution of rare earth elements(REEs) is also consistent with enrichment by right-leaning light rare earth elements(LREEs).The addition of enriched mantle type I(EMI) materials as well as mantle heterogeneity may have led to variable degrees of enrichment in different regions. The magma source of the middle trough has undergone crystallization towards pyroxene, while development of plagioclase was restricted partly, and the crystallization of spinel and olivine ceased altogether. At the same time, crystallization of the southern OT magma source was dominated by olivine and including the formation of plagioclase, pyroxene, and magnetite(or titanomagnetite). Finally, the results of this study showed that 90% Th, 95% Ba in the southern basalt, 50%–70% Th and 70%–90% Ba in the middle basalt originated from subducted component. Different subducted component influence may be due to different subduction zone structural feature.展开更多
Several orogenic cycles of mountain building and subsequent collapse associated with periods of shal- lowing and steepening of subduction zones have been recognized in recent years in the Andes. Most of them are chara...Several orogenic cycles of mountain building and subsequent collapse associated with periods of shal- lowing and steepening of subduction zones have been recognized in recent years in the Andes. Most of them are characterized by widespread crustal delamination expressed by large calderas and rhyolitic flare-up produced by the injection of hot asthenosphere in the subduction wedge. These processes are related to the increase of the subduction angle during trench roll-back. The Payenia paleoflat-slab, in the southern Central Andes of Argentina and Chile (34° -37°S) recorded a complete cycle from crustal thickening and mountain uplift to extensional collapse and normal faulting, which are related to changes in the subduction geometry. The early stages are associated with magmatic expansion and migration, subsequent deformation and broken foreland. New ages and geochemical data show the middle to late Miocene expansion and migration of arc volcanism towards the foreland region was associated with important deformation in the Andean foothills. However, the main difference of this orogenic cycle with the previously described cycles is that the steepening of the oceanic subducted slab is linked to basaltic flooding of large areas in the retroarc under an extensional setting. Crustal delamination is concentrated only in a narrow central belt along the cordilleran axis. The striking differences between the two types of cycles are interpreted to be related to the crustal thickness when steepening the subducting slab, The crustal thickness of the Altiplano is over 60-80 km, whereas Payenia is less than 42 km in the axial part, and near 30 km in the retroarc foothills. The final extensional regime associated with the slab steepening favors the basaltic flooding of more than 8400 km3 in an area larger than 40,000 km2, through 800 central vents and large fissures. These characteristics are unique in the entire present-day Andes.展开更多
Large igneous provinces(LIPs) formed by mantle superplume events have irreversibly changed their composition in the geological evolution of the Earth from high-Mg melts(during Archean and early Paleoproterozoic) t...Large igneous provinces(LIPs) formed by mantle superplume events have irreversibly changed their composition in the geological evolution of the Earth from high-Mg melts(during Archean and early Paleoproterozoic) to Phanerozoic-type geochemically enriched Fe-Ti basalts and picrites at 2.3 Ga.We propose that this upheaval could be related to the change in the source and nature of the mantle superplumes of different generations.The first generation plumes were derived from the depleted mantle,whereas the second generation(thermochemical) originated from the core-mantle boundary(CMB).This study mainly focuses on the second(Phanerozoic) type of LIPs,as exemplified by the midPaleoproterozoic Jatulian-Ludicovian LIP in the Fennoscandian Shield,the Permian-Triassic Siberian LIP,and the late Cenozoic flood basalts of Syria.The latter LIP contains mantle xenoliths represented by green and black series.These xenoliths are fragments of cooled upper margins of the mantle plume heads,above zones of adiabatic melting,and provide information about composition of the plume material and processes in the plume head.Based on the previous studies on the composition of the mantle xenoliths in within-plate basalts around the world,it is inferred that the heads of the mantle(thermochemical) plumes are made up of moderately depleted spinel peridotites(mainly lherzolites)and geochemically-enriched intergranular fluid/melt.Further,it is presumed that the plume heads intrude the mafic lower crust and reach up to the bottom of the upper crust at depths-20 km.The generation of two major types of mantle-derived magmas(alkali and tholeiitic basalts) was previously attributed to the processes related to different PT-parameters in the adiabatic melting zone whereas this study relates to the fluid regime in the plume heads.It is also suggested that a newly-formed melt can occur on different sides of a critical plane of silica undersaturation and can acquire either alkalic or tholeiitic composition depending on the concentration and composition of the fluids.The presence of melt-pockets in the peridotite matrix indicates fluid migration to the rocks of cooled upper margin of the plume head from the lower portion.This process causes secondary melting in this zone and the generation of melts of the black series and differentiated trachytic magmas.展开更多
The basalts within the greenstone belt worldwide serve as an ideal target to decipher the nature of Archean mantle sources and further to extend the understanding of the early stages of Earth's evolution.To provid...The basalts within the greenstone belt worldwide serve as an ideal target to decipher the nature of Archean mantle sources and further to extend the understanding of the early stages of Earth's evolution.To provide important insights into the issues,we carried out a detailed investigation of whole-rock geochemistry and Sm-Nd isotopes,and zircon U-Pb-Hf isotopes for the Late Neoarchean metamorphosed basalts in eastern Hebei,North China Craton.U-Pb isotopic dating using the LA-ICPMS on zircons reveals that the basalts in eastern Hebei erupted at ca.2.48-2.51 Ga and subsequently experienced multiple regional metamorphic events at 2477 and 1798 Ma,respectively.The metamorphosed basalts are featured by low SiO_(2),MgO,K_(2)O+Na_(2)O,and high Fe O contents,endowed with the subalkaline and high-Fe tholeiitic affinities.The radiogenic initial Nd and Hf isotope values and correlations among V,Ni and Cr contents strongly imply that the basalts experienced significant clinopyroxene and olivine fractionation and minor crustal contamination during magma evolution.They are also characterized by the relatively low total REE contents and exhibit significant depletions to moderate enrichments in the LREE contents,indicating the derivation from a deep mantle source in an Archean proto-mantle plume setting.展开更多
基金The National Basic Research Program of China under contract No.2013CB429702the open fund project supported by the Laboratory for Marine Geology,Qingdao National Laboratory for Marine Science and Technology under contract No.MGQNLM-KF201707+4 种基金the National Natural Science Foundation of China under contract Nos 41476044,41325021 and 41306053the Special Fund for the Taishan Scholar Program of Shandong Province under contract No.ts201511061the AoShan Talents Program supported by Qingdao National Laboratory for Marine Science and Technology under contract No.2015ASTP-0S17the Innovative Talent Promotion Program under contract No.2012RA2191the Science and Technology Development Program of Shandong Province under contract No.2013GRC31502
文摘The Okinawa Trough(OT) is a back-arc basin at an initial spreading stage that is under the influence of subduction of the Philippine Sea Plate. In this study, we analyzed the geochemical compositions of basaltic glass in the OT and discussed the effects of different magmatic sources, evolution, and subducted components in basalts. Our results showed that the middle and southern regions of the OT exhibit characteristics consistent with an iron-rich tholeiite series. Trace element proportions conform to the typical spider diagram pattern characteristic of back-arc basin basalts, rich in large ion lithophile elements(LILEs) including Rb, Ba, Pb, U, and Th, while depleted in high field-strength elements(HFSEs) including Nb, Ta, Zr, Hf, and Ti. The distribution of rare earth elements(REEs) is also consistent with enrichment by right-leaning light rare earth elements(LREEs).The addition of enriched mantle type I(EMI) materials as well as mantle heterogeneity may have led to variable degrees of enrichment in different regions. The magma source of the middle trough has undergone crystallization towards pyroxene, while development of plagioclase was restricted partly, and the crystallization of spinel and olivine ceased altogether. At the same time, crystallization of the southern OT magma source was dominated by olivine and including the formation of plagioclase, pyroxene, and magnetite(or titanomagnetite). Finally, the results of this study showed that 90% Th, 95% Ba in the southern basalt, 50%–70% Th and 70%–90% Ba in the middle basalt originated from subducted component. Different subducted component influence may be due to different subduction zone structural feature.
基金supported by different grants received supported by different grants received from CONICET,UBACYT
文摘Several orogenic cycles of mountain building and subsequent collapse associated with periods of shal- lowing and steepening of subduction zones have been recognized in recent years in the Andes. Most of them are characterized by widespread crustal delamination expressed by large calderas and rhyolitic flare-up produced by the injection of hot asthenosphere in the subduction wedge. These processes are related to the increase of the subduction angle during trench roll-back. The Payenia paleoflat-slab, in the southern Central Andes of Argentina and Chile (34° -37°S) recorded a complete cycle from crustal thickening and mountain uplift to extensional collapse and normal faulting, which are related to changes in the subduction geometry. The early stages are associated with magmatic expansion and migration, subsequent deformation and broken foreland. New ages and geochemical data show the middle to late Miocene expansion and migration of arc volcanism towards the foreland region was associated with important deformation in the Andean foothills. However, the main difference of this orogenic cycle with the previously described cycles is that the steepening of the oceanic subducted slab is linked to basaltic flooding of large areas in the retroarc under an extensional setting. Crustal delamination is concentrated only in a narrow central belt along the cordilleran axis. The striking differences between the two types of cycles are interpreted to be related to the crustal thickness when steepening the subducting slab, The crustal thickness of the Altiplano is over 60-80 km, whereas Payenia is less than 42 km in the axial part, and near 30 km in the retroarc foothills. The final extensional regime associated with the slab steepening favors the basaltic flooding of more than 8400 km3 in an area larger than 40,000 km2, through 800 central vents and large fissures. These characteristics are unique in the entire present-day Andes.
基金supported by grants RFBR(projects Nos.14-0500458a and 16-05-00708)
文摘Large igneous provinces(LIPs) formed by mantle superplume events have irreversibly changed their composition in the geological evolution of the Earth from high-Mg melts(during Archean and early Paleoproterozoic) to Phanerozoic-type geochemically enriched Fe-Ti basalts and picrites at 2.3 Ga.We propose that this upheaval could be related to the change in the source and nature of the mantle superplumes of different generations.The first generation plumes were derived from the depleted mantle,whereas the second generation(thermochemical) originated from the core-mantle boundary(CMB).This study mainly focuses on the second(Phanerozoic) type of LIPs,as exemplified by the midPaleoproterozoic Jatulian-Ludicovian LIP in the Fennoscandian Shield,the Permian-Triassic Siberian LIP,and the late Cenozoic flood basalts of Syria.The latter LIP contains mantle xenoliths represented by green and black series.These xenoliths are fragments of cooled upper margins of the mantle plume heads,above zones of adiabatic melting,and provide information about composition of the plume material and processes in the plume head.Based on the previous studies on the composition of the mantle xenoliths in within-plate basalts around the world,it is inferred that the heads of the mantle(thermochemical) plumes are made up of moderately depleted spinel peridotites(mainly lherzolites)and geochemically-enriched intergranular fluid/melt.Further,it is presumed that the plume heads intrude the mafic lower crust and reach up to the bottom of the upper crust at depths-20 km.The generation of two major types of mantle-derived magmas(alkali and tholeiitic basalts) was previously attributed to the processes related to different PT-parameters in the adiabatic melting zone whereas this study relates to the fluid regime in the plume heads.It is also suggested that a newly-formed melt can occur on different sides of a critical plane of silica undersaturation and can acquire either alkalic or tholeiitic composition depending on the concentration and composition of the fluids.The presence of melt-pockets in the peridotite matrix indicates fluid migration to the rocks of cooled upper margin of the plume head from the lower portion.This process causes secondary melting in this zone and the generation of melts of the black series and differentiated trachytic magmas.
基金supported financially by the National Natural Science Foundation of China(Nos.42002238 and 41872057)。
文摘The basalts within the greenstone belt worldwide serve as an ideal target to decipher the nature of Archean mantle sources and further to extend the understanding of the early stages of Earth's evolution.To provide important insights into the issues,we carried out a detailed investigation of whole-rock geochemistry and Sm-Nd isotopes,and zircon U-Pb-Hf isotopes for the Late Neoarchean metamorphosed basalts in eastern Hebei,North China Craton.U-Pb isotopic dating using the LA-ICPMS on zircons reveals that the basalts in eastern Hebei erupted at ca.2.48-2.51 Ga and subsequently experienced multiple regional metamorphic events at 2477 and 1798 Ma,respectively.The metamorphosed basalts are featured by low SiO_(2),MgO,K_(2)O+Na_(2)O,and high Fe O contents,endowed with the subalkaline and high-Fe tholeiitic affinities.The radiogenic initial Nd and Hf isotope values and correlations among V,Ni and Cr contents strongly imply that the basalts experienced significant clinopyroxene and olivine fractionation and minor crustal contamination during magma evolution.They are also characterized by the relatively low total REE contents and exhibit significant depletions to moderate enrichments in the LREE contents,indicating the derivation from a deep mantle source in an Archean proto-mantle plume setting.