THE STEEP FOLIATION BELT IN THE HIGH-GRADE METAMORPHIC COMPLEX OF THE DAQINGSHAN-WULASHAN AREA, INNER MONGOLIA: Product of orogeny

The high-grade metamorphic complex in Daqingshan-Wulashan area, Inner Mongolia is a part of Huai'an Helanshan terrain of granulite facies, which lies in the western segment of the north margin of North China craton. The complex, characterized by well developed khondalite series, is seriously metamorphosed and deformed with complicated structures. Study shows that the E-W-trending steep foliation belt (ETSFB) is one of the major structural patterns, which is well-preserved in the complex. The large-scaled ETSFB is characterized by well-developed E-W-trending steep-dipping foliations and sub-vertical folds, suggesting pure shearing under the condition of crust contraction. The ETSFB, superimposed by dextral shear belts of deep structural phase, is formed in the tectonic environment of amphibole phase to amphibole-granulite phase during late Paleoproterozoic orogenesis, overprinting on the early sub-horizontal bedding detachment and dome structures.