Geochemistry And Petrogenesis Of Cenozoic Basalts Near Gravity Gradient Zone

The North China Craton is one of the oldest Archean cratons in the world, Lithospheric thinning since Mesozoic as its significant features is different from other ancient cratons (such as African Craton), which attracted many geologists’ attention in recent years. According to the seismic tomography data, the lithospheric thickness of North China Craton isn’t homogeneity at present in Space, which gradually increase from the eastern coast to the western Tibetan plateau, and have a sudden change in gravity gradient zone regions. The Da Hinggan Ling-Taihangshan gravity gradient zone, also called Great Gradient Line, which have a big difference from east to west in topography、gravity anomaly, lithosphere thickness, crustal thickness and heat flow value, The lithosphere thickness of the east block isrelatively thin (< 80 km), while the west block is thick (100-150 km). In response to basalts more widely dispersed in east than west. The Gravity gradient zone is located in the transitional zone of lithosphere differential evolution, which superior geographical position indicating its importance in the study of the North China Craton lithospheric evolution. Detailed studies on petrology, petrography, whole rocks geochemistry and Sr-Nd-Pb-Hf Isotopic geochemistry of Fanshi basalts as a Case are carried out in order to qualified their source and petrogenesis, and discuss the generation of the gravity gradient zone, thelithospheric evolution process and mechanism of the North China Craton.Previous studies gave the Fanshi basalts whole rock K-Ar ages of 26.3-24.3Ma. The Fanshi basalts from Sumengzhuang and Yingxian two locations all show OIB-like trace element and isotopic signatures, i.e., they are enriched in incompatible elements with highly fractionated LREEs and HREEs ([La/Yb]N =8.42-21.60) without negative Sr and Eu anomalies. They show relatively low Sr (87Sr/86Sr=0.703848-0.704870) and high Nd (143Nd/144Nd=0.512617-0.513057)、 Hf (176Hf/177Hf=0.282873-0.283001) isotope ratios, and Pb isotope ratios were 206Pb/204Pb=17.2-17.9,207Pb/204Pb=15.3-15.4 and 208Pb/204Pb=37.5-37.9. All these geochemical features, combined with petrographic observations and major element data, allow us to suggest that the Fanshi basalts were derived from small degree partial melting of asthenosphere mantle with contributions of metasomatized lithospheric mantle, Olivine and clinopyroxene dominated fractional crystallization is an important process in the petrogenesis of these basalts, which may have taken place in magma chamber of lithospheric mantle condition, The rapid ascent explains the lack of crustal contamination. Sumengzhuang samples are characterized by relatively deeper and lower extent of melting, whereas the Yingxian samples have the signature of slightly shallower and higher extent of melting. Together with studies of Cenozoic basalts in other areas near the gravity gradient zone, we suggest that the Cenozoic basalts near the Gravity Gradient zone have a same source, which may be originated from decompression melting of eastward asthenosphere flow when crossing the gravity gradient zone. This study offers some new perspectives on the petrogenesis of the Cenozoic basaltic volcanism in eastern China in general.