HR: 08:00h
AN: T31E-01 INVITED     [Abstracts]
TI: Temporal And Spatial Variation In Geochemistry Of Cenozoic Basalts In North China: Implication For The Origin Of Intraplate Volcanism
AU: * Xu, Y
EM: yigangxu@gig.ac.cn
AF: Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou, 510640 China
AB: In North China, Cenozoic basalts were erupted in association with formations of graben basins and their regional boundary faults. Radiometric dating shows that eruption age is between early Tertiary to Quaternary. While Sr-Nd isotopic ratios and relative abundances of incompatible elements are consistent with a dominant asthenospheric origin, Cenozoic basalts from both sides of the Daxin­_anling-Taihang gravity lineament (DTGL) that separates the North China Craton (NCC) into two parts, namely the western and eastern NCC, show contrasting evolution trends. In the western NCC, magmas evolved from xenolith-bearing alkali basalts of late Eocene-Oligocene age to coexisting alkali and tholeiitic basalts of late Miocene-Quaternary age. This change in basalt type is accompanied by a decrease in La/Yb and increase in Yb content. This temporal variation in basalt geochemistry is interpreted as reflecting progressive lithospheric thinning in the western NCC during the Cenozoic. An opposite trend is observed for Cenozoic basalts from the eastern NCC, suggesting lithospheric thickening during this time period. This thickening was probably related to regional thermal decay following peak magmatism in the late Cretaceous-Early Tertiary. Such contrasting lithospheric processes may reflect diachronous extension in the NCC, with extension taking place earlier in the eastern NCC than in western NCC. Recent seismic tomography reveals a flat subducting slab at the transition zone. The west end of this stagnant slab virtually coincides with the location of the DTGL. It is thus possible that Cenozoic volcanism resulted from interaction between two different tectonic regimes. The generation of Cenozoic basalts from the eastern NCC could be related to the back-arc extension owing to late Mesozoic-Paleogene Pacific subduction underneath the Asian continent, whereas subsequent extension in the western NCC may have been induced by the early Tertiary Indian-Eurasian collision. This interpretation also implies that the formation of the DTGL was likely coeval with the late Mesozoic destruction of the Archean lithospheric root beneath the eastern NCC.
DE: 8110 Continental tectonics--general (0905)
DE: 7209 Earthquake dynamics and mechanics
DE: 7218 Lithosphere and upper mantle
DE: 1208 Crustal movements--intraplate (8110)
DE: 1020 Composition of the crust
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting