Characteristics of the Nangang-Gaogongdao ductile shear zone in the South Su-Lu high-pressure metamorphic belt and EBSD fabric analysis of quartz

Abstract：The characteristics of the Nangang-Gaogongdao ductile shear zone in the south Su-Lu high-pressure metamorphic belt were studied on the basis of the geological section survey. The results reveal that： rocks in the upper part of the shear zone are deformed weakly, in which the S-C fabric and stretching lineation are developed; rocks in the central part are deformed intensely, in which asymmetric folds, S-C fabrics and σ- and δ-type rolled porphyroblasts, as well as domino structure of plagioclase are developed; and rocks in the lower part are deformed most intensely, in which mylonitized grains accounts for up to 80%-90% and congruous tight folds can also be found. EBSD fabric analysis shows that： mylonitized quartz grains in the upper part are characterized by the low- to medium-temperature rhombohedral face fabric and medium-temperature prismatic face fabric, those in the central and lower parts are characterized by low-temperature basal face fabric and low- to medium-temperature rhombohedral face fabric, and banded quartz grains in the shear zone are dominated by medium-temperature prism face fabric, which indicates that the shear zone experienced a complex deformation process from medium-temperature through low-medium temperature to low temperature deformations. The shear sense of the quartz fabrics is mainly from SE to NW and subordinately from NW to SE, reflecting that thrusting shear deformation is dominant, while ductile déollement shear is subordinate in the shear zone. No apparent trend of variations exists in chemical composition and REE patterns of the rocks from the upper part to lower part of the shear zone, so the composition may be determined mainly by the primary compositions of the rocks. 39Ar-40Ar ages of muscovite and biotite from the shear zone indicate that intensive metamorphism and deformation took place in the period of 253.8-214.2 Ma.