Abstract:

The quartz schist from Milin in the IndusYarlung Tsangpo suture zone has undergone extensive mylonization with well developed lineation, foliation; its SC fabric, “σ” type porphyroblasts and asymmetrical folds indicate the northwestward normal slip ductile shear deformation. According to the electron backscattered diffraction (EBSD) test results, lattice preferred orientation (LPO) patterns for quartzes in the matrix foliation (or external foliation, S3) and inclusiontrails at rims of the garnet porphyroblasts (S2) both show a topdowntotheNW (normal) shear sense. While LPO patterns for quartzes in the inclusiontrails at the garnet porphyroblast cores (S1) show an opposite shear sense. The garnet compositional zonation indicates a growth zoning characteristic from energy dispersive spectroscopy (EDS) analysis. By connecting the inclusiontrails in garnet cores, an early stage foliation trace with asymmetrical fold shape can be obtained. It suggests that these garnet porphyroblasts with snowball structure formed on the asymmetrical folds by the rotation of external shear sense rather than the rotation of garnet porphyroblasts. Based on the study of garnet porphyroblast with snowball structure, the authors proposed that the rocks from the west of IndusYarlung Tsangpo suture zone have undergone southeastward thrusting and then the late from southeastward to northwestward detachment event.

Key words:  IndusYarlung suture, garnet porphyoblasts, snowball structure, EBSD