High-Mg adakitic rocks in the Dabie orogen, central China: Implications for foundering mechanism of lower continental crust

The late Mesozoic high-Mg adakitic rocks from Eastern and Central China provide important insight into the foundering mechanism of the over-thickened lower continental crust. The Chituling high-Mg adakites (131 +/- 3 Ma, SHRIMP zircon U-Pb age) from the eastern margin of the Southern Dabie ultrahigh pressure metamorphic zone, adjacent to the Tan-Lu fault, have high Al2O3 and Sr contents, high Sr/Y, La/Yb, and Mg# (44-63), but low Yand Yb contents. The samples also have moderately enriched Sr-87/(86)Sri (0.70691-0.70775), very low epsilon(Nd)(T) (-20.7 to -24.9), unradiogenic Pb isotopes, enrichment of large ion lithophile elements, and depletion of high field strength elements. These geochemical features indicate that they did not result from melting of young oceanic crust, assimilation and fractional crystallization, or magma mixing. Instead, they were derived from partial melting of delaminated lower continental crust, with subsequent reaction with surrounding mantle peridotites during ascent to crustal depths. The reactions between the adakitic melt and peridotites also generated the enriched mantle source of the post-collisional basaltic rocks in the Dabie orogen. Distribution of the late Mesozoic high-Mg adakites in eastern and central China generally forms a high-Mg adakite belt along the southern Tan-Lu fault. Therefore, we propose that the large strike-slip motion of the Tan-Lu fault in eastern and central China due to the western subduction of the Pacific plate in the early Cretaceous might trigger the foundering of some fragments of the over-thickened lithosphere near the Tan-Lu fault, which caused mantle upwelling and partial melting of the thickened lower crust in the Dabie orogen and eastern boundary of the North China Craton. This further weakened the gravitationally instable lithosphere, consequently resulting in delamination and foundering of the mountain root underneath the Dabie orogen, which could be an important foundering mechanism of lower continental crust. (C) 2008 Published by Elsevier B.V.