The Caledonian low Al-TTD series from the Northern Qinling Orogenic Belt: Rock properties,genetic simulation and geological implication

A belt of low Al-Trondhjemite-Tonalite-Diorite (TTD) series has been recognized along the Northern Qinling Orogenic Belt (NQOB), which is dated as 430―399 Ma and characteristic of positive ε_Nd(t) values. Geochemical features and trace elements simulation indicate that the TTD series was probably originated from differently partial melting of tholeiitic rocks from the Erlangping group. Thermodynamic analysis on the residue phases of the partial melting processes suggests that the TTD series may represent a dynamic process of temperature-increasing and pressure-decreasing, indicating an extension and thinning process of a thickened crust.