Sources and petrogenesis of the Cenozoic alkali basalts in Hebi,eastern North China Craton:Geochemical and Sr-Nd-Hf isotopic evidence

Continental alkali basalts are the key to understand the nature of the subcontinental lithospheric mantle and the interaction among the crust,lithospheric mantle and asthenosphere.Cenozoic alkali basalts,located in the central North China Craton,are main components of Cenozoic basalt province in the North China Craton.Their source and petrogenesis would provide complement evidence for studying the modification of lithospheric mantle beneath the central North China Craton during Cenozoic.The Hebi basalts are basanites and alkali olivine basalts,belonging to alkaline rocks,with SiO2 contents of 45.0%~47.8%,MgO concentrations of 7.3%~8.5%,and Na2O+K2O abundances of 4.6%~6.3%.They are enriched in light rare earth elements(REEs) with highly fractionated LREEs and HREEs((La/Yb)N=15.1~21.2).In primitive mantle-normalized spidergrams,all rocks have evident positive Nb and Ta anomalies without Eu anomalies.They are enriched in LILEs and HFSEs,with Nb/U ratios of 35~48,La/Nb ratios of 0.4~0.7,and Ba/Nb ratios of 3.6~11.They have depleted Sr-Nd-Hf isotopic compositions,with initial 87Sr/86Sr ratios of 0.7036~0.7044,eNd(t) values of +0.4 to +5.4 and eHf(t) values of +5.0 to +9.7.All these characteristics are similar to those of the oceanic island basalts(OIBs).All these geochemical features,combined with petrographic observations,suggest weak crustal contamination and fractional crystallization in their origin.It is concluded that the Hebi alkali basalts were derived from small degree partial melting of asthenospheric mantle at depths more than 80km,with interaction with lithospheric mantle.