Approach to the source of the gray gneisses in North Dabie Terrain: Sm-Nd isochron age and isotope composition.

The North Dabie Metamorphic Complex (NDC) is a suite of gray gneiss, mainly of tonalitic gneiss. Sm-Nd isochron age of 1047 +/- 44. 5Ma, epsilon(Nd) = - 8. 99 and r = 0. 998 are obtained. Nd model ages (TDM) and Pb model ages also indicate an ancient crust-derived source for the gray gneisses. Either the Sr-87/Sr-86 present ratios (>0. 707) and the present epsilon(Nd) value (-8. 84 similar to-23. 56), or the epsilon(Nd) value (-8. 99) determined by isochron clearly constrained a crust source for the gray gneisses in NDC. Hence it can be inferred that the gray gneisses of NDC were derived from an reworking ancient middle to lower continental crust. The generation and evolution process is closely related to the subduction of the Yangtze plate beneath the North China plate. The intrusion of the Yanshan granitic magma caused the last and the strongest tectonothermal event for the gray gneisses.