Sm-Nd age and Sr, Nd isotopic characteristics of the Fanshan potassic alkaline ultramafite-syenite complex in Hebei province, China

The intrusive complex in the Fanshan, Hebei province, China, consisting of layered patassic alkaline ultramafite and alkaline syenite, is know in the world for its containing large magnetite apatite deposits, and the rocks an d deposits have been investigated and a lot of data have been accumulated o ver the past twenty years. However, their formation timing is still in disp ute. Most geologists consider that the rocks and deposits in the Fanshan in trusive complex originated in Triassic Period between 250Ma and 200Ma., but some believe that they were formed in Permian Period. Sm-Nd dating of the Fanshan intrusive complex has been performed, and its age have been obtaine d. The Sm-Nd isochron age of four mineral samples and two whole rock sample s from the Fanshan intrusive complex is 243. 4 +/-9.7Ma with I-Nd = 0.51204 5, epsilon (Nd) (t) = - 5.4, and the Sm-Nd isochron age of four mineral sam ples and seven whole rock samples from the Fanshan intrusive complex is 239 +/- 19Ma with I-Nd = 0.512055, epsilon (Nd) (t) = - 5.3 These age data ind icate that emplacement of the Fanshan complex started at early Triassic per iod. However, the Rb-Sr isochron age in the Fanshan complex is 218 +/- 8Ma with I-Sr 0.70554, epsilon (Sr) = 18.4, suggesting possibly that the Fansha n complex was solidified completely in the middle Triassic Period. The Sr, Nd isotope fact of the Fanshan complex above shows that the Fanshan complex was derived from enriched mantle materials.