THE DEFORMATION MECHANISM OF MARBLE MYLONITESIN THE DASHANKOU SHEAR ZONE

The Dashankou Ductile Shear Zone (DDSZ) is a typical example of strike slip ductile shear zone with in the crystal line axis of the Dabei orogenic belt parallel to its extension.The marble mylonites of which show a variety of microstructures usually occurring under greenschist facies conditions, such as mortar structure, mica fish, subgrain, S-C structure, asymmetric strain shadow and mineral crystal preferred orientation.The mylonitization is characterized by decreasing grain-size of calcits.Based on a large number of microstructures along with macroscopic kinematic characteristics, It is shown that the DDSZ is a dextral strike-slip ductile shear zone, which was modified by a low-angle ductile shear zone.From the deformed mineral assemblages, microstructures and fluid inclusions thermometry, the DDSZ was determined to have formed at a temperature of 350-450℃, and a pressure of 364-468 MPa with a differential stress of 30-130 MPa, a shear strain of 6.06-7.02, and a strain rate of 2.87×10^-14 1 29×10^-10 s^-1, and a shear strain rate of 1.17×10^-13-2.23×10^-10 S^-1, and the corresponding displacement rate being 25 30 mm/a.From an analysis of macrostructural, microstructural and petrologic data, it is inferred that the Dashankou dextral strike-slip ductile shear zone may be the result of an oblique collision between the North China craton and Yangtze craton during the Indosinian-Yanshanian orogenies.