Integrated elemental and Sr-Nd-Pb-Hf isotopic studies of Mesozoic mafic dykes from the eastern North China Craton: implications for the dramatic transformation of lithospheric mantle

Evolution of the lithospheric mantle beneath the North China Craton (NCC) from its Precambrian cratonic architecture until Paleozoic, and the transformation to an oceanic realm during Mesozoic, with implications on the destruction of cratonic root have attracted global attention. Here we present geochemical and isotopic data on a suite of newly identified Mesozoic mafic dyke swarms from the Longwangmiao, Weijiazhuang, Mengjiazhuang, Jiayou, Huangmi, and Xiahonghe areas (Qianhuai Block) along the eastern NCC with an attempt to gain further insights on the lithospheric evolution of the region. The Longwangmiao dykes are alkaline with LILE (Ba and K)- and LREE-enrichment ((La/Yb) (N) (>) 4.3) and EM1-like Sr-Nd-Pb-Hf isotopic signature ((Sr-87/Sr-86) (i >) 0.706; epsilon (Nd) (t) < -6.3, (Pb-206/Pb-204) (i) > 16.6, (Pb-207/Pb-204) (i >) 15.4, (Pb-208/Pb-204) (i >) 36.8, epsilon (Hf) (t) < -22.4). The Weijiazhuang dykes are sub-alkaline with LILE (Ba and K)- and LREE-enrichment ((La/Yb) (N) > 3.7), and display similar EM1-like isotopic features ((Sr-87/Sr-86) i > 0.706; epsilon (Nd) (t) < -7.0, (Pb-206/Pb-204) (i) > 16.7, (Pb-207/Pb-204) (i >) 15.4, (Pb-208/Pb-204) (i >) 36.9, epsilon (Hf) (t) < -23.3). The Mengjiazhuang dykes are also sub-alkaline with LILE (Ba and K)- and LREE-enrichment ((La/Yb) (N) > 2.4) and EM1-like isotopic features((Sr-87/Sr-86) (i >) 0.706; epsilon (Nd) (t) < -18.4, (Pb-206/Pb-204) (i) > 16.7, (Pb-207/Pb-204) (i >) 15.4, (Pb-208/Pb-204) (i >) 36.9, epsilon (Hf) (t) < -8.6). The Jiayou dykes also display sub-alkaline affinity with LILE (Ba and K)- and LREE-enrichment ((La/Yb) (N) > 3.7) and EM1-like Sr-Nd-Pb-Hf isotopic features ((Sr-87/Sr-86) (i >) 0.706; epsilon (Nd)(t) < -15.3, (Pb-206/Pb-204) (i) > 16.7, (Pb-207/Pb-204) (i >) 15.4, (Pb-208/Pb-204) (i >) 36.9, epsilon (Hf) (t) < -18.4). The Huangmi dykes are alkaline (with Na2O + K2O ranging to more than 5.9 wt.%)) with LILE (Ba and K)- and LREE-enrichment ((La/Yb) (N) > 9.3) and EM1-like isotopic composition ((Sr-87/Sr-86) (i >) 0.705; epsilon (Nd) (t) < -15.1, (Pb-206/Pb-204) (i) > 16.9, (Pb-207/Pb-204) (i >) 15.5, (Pb-208/Pb-204) (i >) 36.9, epsilon (Hf) (t) < -12.2). The Xiahonghe dykes are alkaline with LILE (Ba and K)- and LREE-enrichment ((La/Yb) (N) = 2.12-2.84) and similar EM1-like Sr-Nd-Pb-Hf isotopic signature ((Sr-87/Sr-86) (i) > 0.705; epsilon (Nd) (t)<-18.0, (Pb-206/Pb-204) (i) > 16.9, (Pb-207/Pb-204) (i >) 15.5, (Pb-208/Pb-204) (i >) 36.9, epsilon (Hf) (t) < -8.6). Our data from the various mafic dyke suites suggest that the magmas were derived from EM1-like lithospheric mantle, corresponding to lithospheric mantle modified by the previously foundered lower crust beneath the eastern NCC. Our results suggest contrasting lithospheric evolution from Triassic (212 Ma) to Cretaceous (123 Ma) beneath the NCC. These mafic dykes mark an important phase of lithospheric thinning in the eastern North China Craton.