Regresar

Mechanism and time of deformation and metamorphism of mylonitic orthogneisses from the Shagou Shear Zone, Qinling Belt, China

Abstract:

The Shagou Shear Zone south of Xian (Shaanxi Province) is a branch of the Shangdan Fault Zone in the Qinling Belt and documents ductile deformation that affected granitoid rocks. Mylonitization took place under conditions of the lower amphibolite facies at temperatures of ca. 530°C, followed by a phase of stagnation, still under the same metamorphic conditions. The last deformation was a weak event during retrogressive greenschist metamorphism. The emplacement of the granitoid precursors of the mylonite is dated at 211 ± 8 Ma by the U/Pb zircon method, much younger than Nd model ages of 1.2-1.3 Ga, which indicates the involvement of older continental crust, probably in an active continental margin environment. The end of mylonitic deformation is given by a Sm/Nd garnet-whole rock age of 126 ± 9 Ma which is interpreted as a minimum age for the main deformation. These geochronological data provide age brackets for the main deformation which must have occurred between the late Triassic and the early Cretaceous. © 1990.