Palynological study of 2 lacustrine suites by limnological and 14C dating reveals the local evolutionary history of the paleovegetation and the paleoclimate of Zoige Plateau, northeastern Qinghai-Xizang Plateau, since...Palynological study of 2 lacustrine suites by limnological and 14C dating reveals the local evolutionary history of the paleovegetation and the paleoclimate of Zoige Plateau, northeastern Qinghai-Xizang Plateau, since the last 30ka. The territorial paleovegetation evolution experienced 7 stages: alpine desert vegetation, grassland, marsh and sub-alpine meadow, islet forest, marsh and meadow, Picea and Abies forest, marsh and sub-alpine meadow, islet dark conifer, marsh and sub-alpine meadow, and the inferred paleoclimate evolution shared similar variations correspondingly. The major climatic events during the periods of post-glacial and the Holocene left their distinct marks in sediment record.展开更多
The Bafoussam area in western Cameroon is part of the central Cameroon Volcanic Ligne(CVL).This study presents the mineralogy,major and trace element compositions,Sr-Pb-Hf isotopes,and new K-Ar geochronological data a...The Bafoussam area in western Cameroon is part of the central Cameroon Volcanic Ligne(CVL).This study presents the mineralogy,major and trace element compositions,Sr-Pb-Hf isotopes,and new K-Ar geochronological data about mafic and felsic volcanic rocks.These rocks belong to two different series:A transitional series made of basalts,basaltic andesite,and trachytes and an alkaline mafic series with basalts,hawaiites,and basanites.New age data show that the transitional series belongs to the oldest part of the CVL and was emplaced between 47 and 35 Ma.The alkaline volcanism is younger,with ages ranging from10 to 4.5 Ma.Magmatic evolution in both series is accomplished through a fractional crystallization process,with the removal of olivine and clinopyroxene,while plagioclase does not seem to be a major crystallizing phase.All the samples are enriched in incompatible trace elements,but the rocks from the alkaline series have more fractionated REE patterns and high Nb content compared to the transitional mafic lavas.Alkaline lavas have lower initial^(87)Sr/^(86)Sr and higher^(176)Hf/^(177)Hf and Pb isotopic ratios than the transitional lavas.Low La/Nb and high^(87)Sr/^(86)Sriratio are among chemical characteristics that show that some samples from the transitional series have interacted with a crustal component during their evolution in the crust.They cannot be used for discussing the mantle source of the volcanic rocks from this series.Trace elements show that primary magmas for both series formed in a garnet-bearing mantle source,with higher partial melting degrees(3-5%)for the transitional magmas than for the alkaline magmas(<2.5%).Combining trace elements and isotopic ratios,we show that the Bafoussam lavas formed from two different mantle sources.Transitional magmas formed from a pyroxenite-bearing enriched mantle with low Pb isotopic composition.This mantle source is present in all the oldest lavas from the CVL.Alkaline magmas formed from an HIMU-like mantle source,different from the Mt Cameroon HIMU mantle source.The depleted asthenospheric mantle is not involved in the Bafoussam magmatism and the two mantle sources are probably located in the lithospheric mantle,in agreement with recent geophysical models presenting the CVL as a consequence of the partial melting of the lithospheric mantle in response to edge convection along the margin of the Congo craton.展开更多
The paleogeographic reconstruction of fragmented and dispersed continents often poses a challenge due to the lack of information regarding the nature of that extend beneath passive margin basins.To define the width of...The paleogeographic reconstruction of fragmented and dispersed continents often poses a challenge due to the lack of information regarding the nature of that extend beneath passive margin basins.To define the width of the continental crust beneath passive margin basins and its implications for paleogeographic reconstruction of conjugate continental margins,this study investigates the architecture of the stretched continental crust of the southern South Atlantic conjugate margin.The investigated region encompasses South Africa,Namibia,southern Brazil,and Uruguay,which were formed during the Mesozoic rifting of SW Gondwana.Employing a multi-tool approach combining seismic interpretation,gravity,magnetometry,and U-Pb isotopic data,the research aims to quantify the extension of stretched continental crust and its implications for plate reconstructions.The study reveals that the restored stretched crust spans at least 150 km,emphasizing the significance of considering connections between both margins for realistic paleogeographic reconstructions.Furthermore,the distinct U-Pb zircon age distribution patterns between SW Africa and SE South America reinforce the lack of direct connections despite their Gondwanan origin.The missing link estimated in this study is around 150 km,comparable in size to major mountain ranges such as the Andean or Urals.This work sheds light on critical aspects of Earth’s dynamic crustal evolution and emphasizes the need for comprehensive reconstructions considering stretched and eroded crust in the South Atlantic conjugate margin.展开更多
基金Project supported by the National Natural Science Foundation of China and Xi'an Key Laboratory of Loess and Quaternary Geology.
文摘Palynological study of 2 lacustrine suites by limnological and 14C dating reveals the local evolutionary history of the paleovegetation and the paleoclimate of Zoige Plateau, northeastern Qinghai-Xizang Plateau, since the last 30ka. The territorial paleovegetation evolution experienced 7 stages: alpine desert vegetation, grassland, marsh and sub-alpine meadow, islet forest, marsh and meadow, Picea and Abies forest, marsh and sub-alpine meadow, islet dark conifer, marsh and sub-alpine meadow, and the inferred paleoclimate evolution shared similar variations correspondingly. The major climatic events during the periods of post-glacial and the Holocene left their distinct marks in sediment record.
文摘The Bafoussam area in western Cameroon is part of the central Cameroon Volcanic Ligne(CVL).This study presents the mineralogy,major and trace element compositions,Sr-Pb-Hf isotopes,and new K-Ar geochronological data about mafic and felsic volcanic rocks.These rocks belong to two different series:A transitional series made of basalts,basaltic andesite,and trachytes and an alkaline mafic series with basalts,hawaiites,and basanites.New age data show that the transitional series belongs to the oldest part of the CVL and was emplaced between 47 and 35 Ma.The alkaline volcanism is younger,with ages ranging from10 to 4.5 Ma.Magmatic evolution in both series is accomplished through a fractional crystallization process,with the removal of olivine and clinopyroxene,while plagioclase does not seem to be a major crystallizing phase.All the samples are enriched in incompatible trace elements,but the rocks from the alkaline series have more fractionated REE patterns and high Nb content compared to the transitional mafic lavas.Alkaline lavas have lower initial^(87)Sr/^(86)Sr and higher^(176)Hf/^(177)Hf and Pb isotopic ratios than the transitional lavas.Low La/Nb and high^(87)Sr/^(86)Sriratio are among chemical characteristics that show that some samples from the transitional series have interacted with a crustal component during their evolution in the crust.They cannot be used for discussing the mantle source of the volcanic rocks from this series.Trace elements show that primary magmas for both series formed in a garnet-bearing mantle source,with higher partial melting degrees(3-5%)for the transitional magmas than for the alkaline magmas(<2.5%).Combining trace elements and isotopic ratios,we show that the Bafoussam lavas formed from two different mantle sources.Transitional magmas formed from a pyroxenite-bearing enriched mantle with low Pb isotopic composition.This mantle source is present in all the oldest lavas from the CVL.Alkaline magmas formed from an HIMU-like mantle source,different from the Mt Cameroon HIMU mantle source.The depleted asthenospheric mantle is not involved in the Bafoussam magmatism and the two mantle sources are probably located in the lithospheric mantle,in agreement with recent geophysical models presenting the CVL as a consequence of the partial melting of the lithospheric mantle in response to edge convection along the margin of the Congo craton.
基金supported by the UNISINOS-PETROBRAS and UNISINOS-CNODC Cooperation Agreements.
文摘The paleogeographic reconstruction of fragmented and dispersed continents often poses a challenge due to the lack of information regarding the nature of that extend beneath passive margin basins.To define the width of the continental crust beneath passive margin basins and its implications for paleogeographic reconstruction of conjugate continental margins,this study investigates the architecture of the stretched continental crust of the southern South Atlantic conjugate margin.The investigated region encompasses South Africa,Namibia,southern Brazil,and Uruguay,which were formed during the Mesozoic rifting of SW Gondwana.Employing a multi-tool approach combining seismic interpretation,gravity,magnetometry,and U-Pb isotopic data,the research aims to quantify the extension of stretched continental crust and its implications for plate reconstructions.The study reveals that the restored stretched crust spans at least 150 km,emphasizing the significance of considering connections between both margins for realistic paleogeographic reconstructions.Furthermore,the distinct U-Pb zircon age distribution patterns between SW Africa and SE South America reinforce the lack of direct connections despite their Gondwanan origin.The missing link estimated in this study is around 150 km,comparable in size to major mountain ranges such as the Andean or Urals.This work sheds light on critical aspects of Earth’s dynamic crustal evolution and emphasizes the need for comprehensive reconstructions considering stretched and eroded crust in the South Atlantic conjugate margin.
