Contrasting Cenozoic Lithospheric Evolution and Architecture in the Western and Eastern Sino-Korean Craton:Constraints from Geochemistry of Basalts and Mantle Xenoliths

摘要

Cenozoic basalts from both sides of the Daxin'anling-Taihang gravity lineament that separates the Sino-Korean craton(SKC)into western and eastern parts have been studied.In the western SKC,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 an increase in Yb content.Sr-Nd isotopic ratios and relative abundances on incompatible elements are consistent with a prevailing asthenospheric origin,although lithospheric mantle may have also contributed to these basalts.This temporal variation in basalt geochemistry is interpreted as reflecting progressive lithospheric thinning in the western SKC during the Cenozoic.An opposite trend is observed for Cenozoic basalts from the eastern SKC,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 SKC,with initial extension in the eastern part owing to the Late Mesozoic paleo-Pacific subduction and subsequent extension in the western SKC induced by the Early Tertiary Indian-Eurasian collision.An implication is that the lithospheric mantle in the western SKC is relatively old compared with that beneath the eastern SKC,which may be a mixture of old lithospheric relicts and newly accreted mantle.This predicted lithospheric architecture is consistent with Sr-Nd isotopic data and recent Re-Os age determinations from mantle xenoliths included in Cenozoic basalts.