LATE TRIASSIC ADAKITIC PLUTONS WITHIN THE ARCHEAN TERRANE OF THE NORTH CHINA CRATON: MELTING OF THE ANCIENT LOWER CRUST AT THE ONSET OF THE LITHOSPHERIC DESTRUCTION

We present the results of a geochemical and geochronological study for Late Triassic (220-230 Ma) adakitic plutons within the Archean terrane of the eastern part of the North China Craton (NCC). These plutons show adakitic signatures with high Sr, Sr/Y, (La/Yb)_N, and low Cr and Ni. The evolved Nd-Hf isotopic compositions and old Nd and Hf isotope model ages suggest that the adakitic pluton may be derived from the Paleoproterozoic mafic lower crust. Calculations of equilibrium mineral assemblages and trace element modeling suggest that the petrogenesis of the pluton is consistent with partial melting of the Paleoproterozoic mafic lower crust at 10-12 kbar (35-40 km), with a residue of garnet granulite. Melting of the ancient mafic lower crust may be triggered by excess heating of the upwelling mantle in an extensional setting evoked by the contemporary subduction toward beneath the NCC from both north and south. Complex mantle-crust interaction may have been responsible for the destruction or thinning of the NCC, which might have begun as early as in the late Triassic.