Mesozoic lithosphere destruction beneath the North China Craton: evidence from major-, trace-element and Sr–Nd–Pb isotope studies of Fangcheng basalts

摘要

Major- and trace-element as well as Sr–Nd–Pb isotopic data of the Mesozoic Fangcheng basalts provide an insight into the nature of their mantle source and the secular evolution of the lithospheric mantle beneath the North China Craton. Fangcheng basalts include alkali basalt and olivine tholeiite, both characterized by high Mg (Mg#=65–72), Si, and Ca and low K+Na, Ti, and P. They are extremely enriched in LREEs ((La/Yb)N=39.3–49.3) and LILEs (Ce, Rb, Ba, U, Th) and depleted in HFSEs (Nb, Ta, Zr, Hf, Ti), with slightly negative Pb anomaly. Correspondingly, these basalts are exceedingly high in ε Sr (74.0~81.5) and low in ε Nd (–13.1~–14.2) and 206Pb/204Pb (<17.8). Since crustal contamination during the magma ascent is insignificant, the Fangcheng basalts could reflect the nature of its mantle source. The isotopic data of these basalts cannot be explained by mixing of typical mantle components, but can be accounted for by interaction of an old lithospheric mantle with the lower/middle crust. Therefore, we consider that these basalts originated from the Mesozoic lithospheric mantle, which evolved from its Paleozoic counterpart through extensive interaction with a crust-derived melt. We propose that this melt was generated from the melting of the subducted lower crust of the Yangtze Craton. This peculiar Mesozoic lithospheric mantle somehow was in turn replaced later by the hot and thin Cenozoic lithospheric mantle.