Recycling of deeply subducted continental crust in the Dabie Mountains, central China

Post-collisional mafic-ultramafic intrusive rocks from the North Dabie zone show evidence for derivation from a mantle source that was overprinted by partial melts derived from subducted crust of the South China Block. All the samples are characterized by enrichment of large ion lithophile elements (LILE, e.g., Ba, Th) and depletion of high field strength elements (HFSE, e.g., Nb, Ti). The Zhujiapu intrusion developed in the North Dabie zone has moderately enriched Sr-87/Sr-86(i) (0.70605-0.70827), negative epsilon(Nd)(T) values (-2.3 to -18.8), and unradiogenic Pb isotopes. Such typical continental geochemical features do not result from crustal contamination or magma mixing, but reflect the properties of the mantle source. Because the trace element and Sr-Nd isotopic compositions of the post-collisional mafic-ultramafic intrusive rocks from the North Dabie zone are similar to those of the Mesozoic mantle-derived rocks and lower crust granulites and xenoliths from the North China Block, it is unclear whether the continental features are due to the contribution of lithospheric mantle and lower crust from the North China Block or deeply subducted materials from the South China Block. However, for a given Pb-206/Ph-204(i) or Delta 7/4, the Pb-208/Pb-204(i) or Delta 8/4 (159-180) of the post-collisional mafic-ultramafic intrusive rocks from the North Dabie zone is close to the Mesozoic granitoids, eclogites, and gray gneisses from the Dabie-Sulu orogen, but significantly higher than the Mesozoic mantle-derived rocks and lower crust granulites and xenoliths from the North China Block (Delta 8/4 < 160). This indicates that the subducted slabs of the South China Block experienced a higher time-integrated Th/U but similar U/Pb as the deep lithosphere of the North China Block. We therefore propose that the mantle source of the post-collisional mafic-ultramafic intrusive rocks from the Dabie-Sulu orogen result from the depleted upper mantle metasomatised by melts from recycled deeply subducted crust. Due to the continent-continent collision between the South China Block and North China Block in the late Triassic, the lithosphere of the Dabie orogen was over-thickened resulting in eclogitic metamorphism in the mafic lower crust, producing a gravitational instability. The subducted eclogitic lower crust foundered and was recycled into the upper mantle beneath the orogen, resulting in mountain root removal, less mafic composition of unusually evolved continental crust, and upper mantle heterogeneity. (C) 2006 Elsevier B.V All rights reserved.