Petrogenesis of the Early Jurassic Nandaling flood basalts in the Yanshan belt,North China Craton:A correlation between magmatic underplating and lithospheric thinning

摘要

Integrated geochemical and Sr-Nd-Pb isotopic studies of the Early Jurassic Nandaling flood basalts(NFB)in the Yanshan belt,northern margin of the North China Craton(NCC),are presented in this paper.These sub-alkaline basalts evolved from a more magnesium-rich parental magma through fractional crystallization of olivine and clinopyroxene.The primitive magma of the NFB originated from 2-5% partial melting of spinel to garnet transitional peridotite at about 70-80 km depth in the Mesozoic lithosphere mantle.The NFB contain a distinctive lithospheric component,characterized by Nb(Ta),Th,U and Ti depletions,LREE enrichments,moderate Sr,and low Nd and Pb initial isotopic ratios,as a result of an interaction between lower crust(15-25%)and primitive magma evoked by magmatic underplating at crust-mantle boundary.The Early Jurassic NFB extruded in an intraplate extensional setting related to post-orogenic collapse in the northern margin of.the NCC,indicating an event of lithospheric modification earlier than that in the southern margin(Early Cretaceous).The temporal similarity of the Jurassic-Cretaceous mantle-derived mafic rocks to lower crust replacement,and the decoupling of surface shortening with lithospheric thinning during the Late Jurassic-Early Cretaceous,suggest the important role of magmatic underplating and subsequent crust-mantle interaction accompanied by asthenosphere upwelling on the evolution of the Mesozoic lithosphere of the NCC.The correlation between lithospheric thinning and magmatic underplating may be an important process in continental rifting.