Abstract:

The characteristics of the ductile shear deformation developed between the Neoarchaeozoic Se’erdengshan Group, and Neoarchaeozoic to Paleoprozoic gneissic tonalite (the basement)，and the Dulahala and Jianshan formation of the Baiyun’ebo Group, and the Shujigou and Zenglongchang formations of the Zha’ertaishan Group, Changcheng System ( the cover) are identical. The mylonitic foliations of the ductile deformation in this region strike nearly east-west, and dip to the north with the angle from 35°to 50°. The kinematic direction inferred from mineral stretching lineations shows that the cover slides to the north(northwest or northeast). The rock system of the basement at contact zone consists of mylonitic schist and mylonite; the lower part of the cover consists of mylonitic quartzite, mylonitic limestone, slate. Recumbent folds are developed in the quartzite of the Shuijigou and Dulahala formation, whereas boudinage and folding layer are developed in the slate and limestone from low amphibolite--high green-schist facies of the basement to low green-schist of the cover. It is shown by the characteristics of sedimentary, metamorphism and deformation that the Changcheng System of the Baiyun’ebo and Zha’ertaishan Group are the sediments of stable passive continental margin, and the ductile shear deformation developed in the Baiyun’ebo, Zha’ertaishan Group is a production of the same structural process in the same stage. The tectonic background of the ductile shear deformation is the extensional detachment of the Changcheng System. The deformation environment belongs to a lower temperature-pressure one, and the age when the ductile shear deformation began is in the end of the Changcheng period (1,400Ma). The ductile-shearing, upwarping and rifting in the Middle-Proterozoic in the middle part of Inner Mongolia were producted at different stages in one extensional tectonic event.

Key words: basement system, Changcheng System, ductile shear deformation, detachament, Middle-Proterozoic, central Inner Mongolia