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Geochemistry of Mesozoic Basalts and Mafic Dikes,Southeastern North China Craton,and Tectonic Implications

【摘要】:正Mesozoic basaltic magmatism in the North China craton(NCC) is a key for probing the nature of the Mesozoic lithospheric mantle,including the process of lithosphere thinning over Phanerozoic time.Early Cretaceous calc-alkaline basalts and mafic dikes are widespread in the Fangcheng,Mengyin,Jimo,and Jiaodong regions of the southeastern NCC.Both rock types have similar geochemical signatures;are high in SiO_2 content;moderately high in MgO,CaO,and Al_2O_3; highly enriched in LREE((Ce/Yb)_N10) and LILE(Rb,Ba,U,Th);and depleted in HFSE(Ti and Nb).These rocks are highly enriched in Sr-Nd isotopes(~(87)Sr/~(86)Sr_i0.7072,Nd_((t))=-11.5~-17.5) and depleted in Pb isotopes(~(206)Pb/~(204)Pb17.6.~(207)Pb/~(204)Pb15.6.~(208)Pb/~(204)Pb38.1). Theirgeochemistry demonstrates that these basalts and mafic dikes were derived from enriched lithospheric mantle,which differs considerably from the Paleozoic and Cenozoic lithosphere of the region.Calculations indicate that the Mesozoic lithosphere was extensively modified by silicic melt, derived from the partial fusion of subducted silicic crust.Tectonic underplating of the Yangtze continental crust beneath the base of the NCC lithosphere is well established.Extensive melting of the enriched lithosphere and tectonic collapse of the continental root provide a possible mechanism for lithosphere thinning linked to the Dabie collision.

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