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

Abstract

We present the results of a geochemical and geochronological study for Late Triassic (230 220 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 enriched Nd-Hf isotopic compositions (epsilon(Nd)(t) = -13.3 to -12.9; epsilon(Hf)(t) = -17.4 to -14.6) and old Nd (T-DM2 = 2078-2037 Ma) and Hf (T-DM2 = 2366-2192 Ma) isotope model ages suggest that the adakitic pluton may be derived from the underplated mafic lower crust of Paleoproterozoic age. The relatively low Cr and Ni contents and lower epsilon(Nd)(t) and epsilon(Hf)(t) values of the Taili adaldtic plutons imply negligible input of mantle materials. Calculations of equilibrium mineral assemblages and modeling of trace element partition between melts and residual phases at different pressures confirm the interpretation that the petrogenesis of the Taili adakitic plutons is consistent with partial melting of the Paleoproterozoic mafic lower crust at 10-12 kbar (36-43 km) with a garnet granulite residue. 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, which could serve as one possible mechanism for the destruction or lithospheric thinning of the NCC. Complex mantle-crust interaction through various mechanisms may have been responsible for the long-lived process of destruction or lithospheric thinning, which might have begun as early as in the late Triassic. (C) 2014 Elsevier B.V. All rights reserved.