Partial Melting of Hydrous Lower Continental Crust:Discussion on the Petrogenesis of C-Type Adakites From the Dabie Orogen

On the basis of continental composition,principles of partial melting of hydrous mafic rocks,and the published experimental data,we discuss the partial melting mechanism of lower continental crust.By comparing the Dabie C-type adakites with experimental melts,we provide constraints on the magma source,melting conditions(temperature and pressure),and the melting degree.Overall,the lower continental crust of East China,including the Dabie orogen,is basic to intermediate(50～60 % SiO 2) and slightly hydrous.Thus,fluid-absent dehydration melting of hydrous minerals is a major mechanism for the lower continental crust generating hydrous felsic melts and anhydrous residues.More than ～20% garnet in the residues could be formed by partial melting of amphibolite at a medium pressure(1.0～1.2 GPa,equivalent to 35～40 km),which is essential to make the coexisting melt to possess adakitic signatures(especially low Y,high Sr/Y and La/Yb).Based on a H 2 O activity model and solidus of metamafic rocks,dehydration partial melting,which occurs at ～950 ℃,could lead to a maximum melting proportion of 15～20%,meeting the requirement for efficient melt segregation.The Dabie C-type adakitic rocks,are of high-K calc-alkaline series(3.5～5% K 2 O),and it is very clear,when compared with experimental melts,that they cannot be derived from low-K protoliths with reasonable melting degrees.In the light of the highly incompatible behavior of K in the course of partial melting,our calculation based on the partition coefficients between melt and possible residues indicates that high-K calc-alkaline melts similar to the Dabie C-type adakitic rocks can be generated by 15～20%(mass basis) partial melting of amphibolites with ～1 wt% K 2 O.