Evolution of the lithospheric mantle beneath the southeastern North China Craton: Constraints from mafic dikes in the Jiaobei terrain

Abstract

Two stages of mafic dikes with ages of ~117–116Ma and ~95–87Ma, respectively, are identified in the Jiaobei terrain based on zircon U–Pb ages. The Early Cretaceous mafic-intermediate dikes (ECMDs) and Late Cretaceous mafic dikes (LCMDs) show distinct geochemical and isotopic features. The ECMDs are enriched in large ion lithophile elements (LILE, Rb, Sr, Ba, U and Th) and light rare earth elements (LREE, La, Ce, Pr, Nd, Sm and Eu) with a strong depletion in high field strength elements (HFSE, Nb, Ta, Ti and P). They show uniformly high (87Sr/86Sr)i (0.70841–0.71387), low εNd(t) values (−18.1 to −13.2), positive (206Pb/204Pb)i (17.07–17.90) and negative εHf(t) (−21.0 to −6.1) values. These rocks were derived from partial melting of an enriched lithospheric mantle, and the enrichment of the mantle source resulted from source metasomatism imparted by the melts generated from remelting of the subducted lower crust of the Yangtze Craton (YC). In contrast, the LCMDs show lower SiO2 (41.5–50.1wt.%), lack of HFSE depletion and are depleted in whole rock Nd and zircon Hf isotope. They are interpreted to have sourced from a newly accreted depleted lithospheric mantle generated through continuous erosion of older enriched lithospheric mantle by asthenospheric upwelling, leaving out minor remnants of the older mantle in the newly accreted lithospheric mantle. The geochemical and isotopic characteristics reported in our study demonstrate that the lithospheric mantle beneath the southeastern North China Craton (NCC) was generally enriched during the Early Cretaceous (~120Ma) and the mechanism of the enrichment may be related to source metasomatism caused by the subducted lower crust of the YC. However, in Late Cretaceous, the lithospheric mantle beneath southeastern NCC was highly heterogeneous, which might be related to asthenospheric non-uniform erosion. The ~116–87Ma marks the transition from enriched lithospheric mantle to depleted asthenospheric mantle.