LARGE DUCTILE SHEAR ZONES IN THE EARLY PRECAMBRIAN CRYSTALLINE BASEMENT IN THE NORTHERN PART OF NORTH CHINA

Some large ductile shear zones are found in the early Precambrian crystalline basement in the northern part of North China. They may be in the main grouped into NE- and nearly E-W-trending systems. They are marked by reworked gneisses, mylonitic gneiss,striped gneiss, augen gneiss, straight gneiss and typical mylonites and so on. Each of them may be traced for 150 km with a width of several tens of km. The meso- and micro-structures in these zones are described. As shown by the associations of deformed minerals in deformed rocks, they have undergone two deformation phases（from lower amphibolite facies to greenschist facies conditions） that took place at the end of the Archean and in the early Proterozoic respectively, accompanied by crutsal uplift and reduction in temperature. These large ductile shear zones and remnant gneissic fragments constitute the main framework of the crystalline basement in the area. Almost all these zones have been subjected to brittle faulting at the erosion surface.