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
Compositional peculiarities of the siliceous high-Mg series(SHMS)rocks formed at the Archean-Paleoproterozoic boundary as a function of plume activity are discussed using example of Early Paleoproterozoic mafic volcan...Compositional peculiarities of the siliceous high-Mg series(SHMS)rocks formed at the Archean-Paleoproterozoic boundary as a function of plume activity are discussed using example of Early Paleoproterozoic mafic volcanic rocks of the Vodlozero Domain,Fennoscandian Shield.These rocks are characterized by wide variations in Mg#(33-67)and Cr contents(25-1123 ppm),LREE enrichment,and weakly negative_(εNd)(from-0.7 to-2.9).The high Gd/Yb ratio in the primitive high-Mg rocks of the Vodlozero Domain suggests their generation from a garnet-bearing source.At the same time,their negative_(εNd)in combination with LREE enrichment points to the crustal contamination.A new model was proposed to explain the remarkable global-scale similarity of SHMS.Such rocks can be generated by the contamination of a high-degree(30%)partial melt derived from a depleted mantle.The lower crustal sanukitoid-type rocks can be considered as a universal crustal contaminant.Modeling showed that such mixing can provide the observed narrow_(εNd)variations in Early Paleoproterozoic volcanics.The Neoarchean sanukitoid suites,which are widespread on all cratons,presumably composed the lower crust at the beginning of the Paleoproterozoic.Therefore,this mechanism can be considered universal for the genesis of the SHMS rocks.The high-to low-Cr rock series can be produced by the fractionation of the mafic melt coupled with an insignificant crustal assimilation of felsic end members of the sanukitoid suite of the Vodlozero Domain en route to the surface,as suggested by the positive correlation of_(εNd)with Cr and Mg#,negative correlation with Th,and slight decrease of_(εNd)in the more evolved varieties.展开更多
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 by the Russian Foundation for Basic Research(Project Nos.16-05-00708 and 15-05-01214)
文摘Compositional peculiarities of the siliceous high-Mg series(SHMS)rocks formed at the Archean-Paleoproterozoic boundary as a function of plume activity are discussed using example of Early Paleoproterozoic mafic volcanic rocks of the Vodlozero Domain,Fennoscandian Shield.These rocks are characterized by wide variations in Mg#(33-67)and Cr contents(25-1123 ppm),LREE enrichment,and weakly negative_(εNd)(from-0.7 to-2.9).The high Gd/Yb ratio in the primitive high-Mg rocks of the Vodlozero Domain suggests their generation from a garnet-bearing source.At the same time,their negative_(εNd)in combination with LREE enrichment points to the crustal contamination.A new model was proposed to explain the remarkable global-scale similarity of SHMS.Such rocks can be generated by the contamination of a high-degree(30%)partial melt derived from a depleted mantle.The lower crustal sanukitoid-type rocks can be considered as a universal crustal contaminant.Modeling showed that such mixing can provide the observed narrow_(εNd)variations in Early Paleoproterozoic volcanics.The Neoarchean sanukitoid suites,which are widespread on all cratons,presumably composed the lower crust at the beginning of the Paleoproterozoic.Therefore,this mechanism can be considered universal for the genesis of the SHMS rocks.The high-to low-Cr rock series can be produced by the fractionation of the mafic melt coupled with an insignificant crustal assimilation of felsic end members of the sanukitoid suite of the Vodlozero Domain en route to the surface,as suggested by the positive correlation of_(εNd)with Cr and Mg#,negative correlation with Th,and slight decrease of_(εNd)in the more evolved varieties.
