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Magmatism and geochemistry |
1 CAS Key Laboratory of Crust–Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China
2 Department of Geology, University of Illinois at Urbana–Champaign, 1301 W. Green Street, IL 61801, USA
(e-mail: * (Corresponding author e-mail: lsg{at}ustc.edu.cn))
The lithospheric mantle underneath the North China Craton changed completely from the Palaeozoic to the Cenozoic. This study reviews geochemical data from Mesozoic mantle-derived mafic rocks from the North China Craton to investigate the role of mafic lower continental crust in lithosphere replacement. Samples from the North China Craton have typical ‘continental’ geochemical signatures, including depletion of high field strength elements, enrichment of large ion lithophile elements and Pb, unradiogenic Pb isotopes, and enriched Sr–Nd isotopic ratios. Positive correlation between initial 87Sr/86Sr and 206Pb/204Pb, low Ce/Pb and Nb/U, high Ba/Nb and La/Nb, and unradiogenic Pb isotopes of Mesozoic mafic rocks cannot simply be explained by derivation from a lithospheric mantle enriched by ancient (Archaean or Mesoproterozoic) fluid or melt metasomatism. Instead, they more probably result from a lithospheric mantle or upwelling asthenosphere underneath the North China Craton that was modified by the lower continental crust in the Mesozoic. Because oceanic plate subduction zones surrounded the North China Craton during the late Palaeozoic, the lithospheric mantle underneath the North China Craton was weakened by fluids derived from subducted slabs, and thus shortened and thickened by continent–continent collisions of the North China Block with the South China Block and the Siberian plate. Metamorphic reactions occurred in the mafic lower continental crust beneath the North China Craton, creating garnet-bearing assemblages (eclogite and garnet pyroxenite) with densities of up to 3.8 g cm–3, which led to negative buoyancy in the over-thickened lithosphere. The unstable lithosphere was delaminated and subsided into the uppermost mantle. The delaminated lower crust partially melted, producing SiO2-rich melts that metasomatized surrounding asthenospheric mantle, which upwelled and replaced the volume formerly occupied by the delaminated lithospheric mantle, resulting in the ‘continental’ geochemical signatures widely observed in Mesozoic mantle-derived mafic rocks from the North China Craton. The ‘continental’ geochemical signatures of Mesozoic mantle-derived mafic rocks suggest that lithospheric delamination could have occurred by the time of volcanic eruption in the northern margin of the North China Craton in the mid-Jurassic and later in the southern margin and Dabie–Sulu Orogen in the early Cretaceous.
