S-C MYLONITIZATION OF GRANITIC ROCKS ALONG MIDDLE-LOWER CRUST FAULT ZONES-EXEMPLIFIED BY THE HEFANGKOU DETACHMENT FAULT ZONE, YUNMENGSHAN, NORTH CHINA

S-C mylonites, a typical type of tectonites from middle-lower crustal fault zones, provide important clues on rheology and deformation of rocks at the middle-lower crustal level. This paper focuses on the S-C mylonites from the Hefangkou detachment fault zone in Yunmengshan, North China. Microstructural studies and EBSD crystallographic preferred orientation analysis of quartz and biotite from the granitic S-C mylonites have been conducted. Through x-ray diffraction experiment, the space groups and cell parameters of fine-grained biotite grains were determined. In the S-C mylonites microfractures within and bulging recrystallization around feldspar grains are popular in the rocks. Quartz grains were dynamically recrystallized via subgrain rotation recrystallization. The recrystallized quartz grains also show oblique foliations due to progressive shearing. Extremely fine biotite grains were derived from large host crystals and are aligned along C foliation.

The c- axis fabrics of quartz in oblique foliation possess Y-maxima which demonstrate a prism<a> slip system in the dynamically recrystallized quartz grains. A deformation temperature of ca. 550 -650°C is estimated. The c-axis fabric of quartz grains along the S-foliations progressively change from Y-axis maximum to Z-axis maximum resulted from passive rotation of quartz grains instead of activation of a new slip system within quartz grains during formation of the C-foliations. The {001} of the very fine biotite grains are distributed along a great circle normal to the X direction. The {100} and {010} of the biotite grains, however, are randomly distributed.

The microstructural and fabric data suggest that the C-foliations are zones of high strains or narrow channels of brittle-ductile flow. Dynamic recrystallization, frictional fractural slipping, passive grain rotation and channeled flow of extremely fine grains were coevally prevailing during the S-C mylonitization.