Abstract:

Intra continental orogeny has recorded a complex geological process. The research area, Ann Kang ShiquanHanyinXunyang belt, belongs to South Qinling orogen, and lies on the convergence parts of South Qinling intra continental compound orogen and the northern Yangtze plate. And there develops the early Paleozoic deformed metamorphic rock sheets of the Silurian system, which mainly is overthrust structure and multilevel ductile slip overthrust rock sheet. There widely produces new multistage foliations that can be classified into 3 stages since the Indosinian period; the foliation replacement shows distinctly, and S2 foliations replacing S1 foliations appears dominantly in the covering stratum area. Considering the following facts that the multistage structural deformation, the granite invasion into cap rock, the magma diapirstretch and heat dome rising of the basement, the thermal metamorphism with the late granite veins and high temperature mineral pairs that consist of secondary enlargement biotite porphyroblasts and garnet phenocrysts, the wide development of many types of hydrothermal alteration, the differences between the temperature and pressure characteristics of the fluid inclusions in quartz veins that exist in different stage foliations, that the Ar Ar dating of biotites along the ductile shear deformation foliations is 161169 Ma in Shiquan area, that the ArAr dating of micas along the ductile shear deformation foliations is 178163 Ma in the northern part of Fenghuangshan dome etc., it is suggested that the research area has undergone at least the Indosinian and the Yanshanian, the two tectonic deformation periods. Particularly, S2 foliation roughly corresponds to Yanshanian period, it is typically an intra continental orogenic structural deformation. The characteristics of plastic flow in solid state and brittle ductile shear deformation are significant. Through the structurepetrography mapping, we conclude that the S2 foliations are predominant in the area, their strike is mainly to the northwest; most of the new minerals and rocks are distributed along the S2 foliation, those suggest that Yanshanian period have a close relation to intra continental orogeny. The brittle ductile shear zones can emerge in the intense portion of structure deformation. The late IndosinianYanshanian period has been mainly presented by the combination of folds+S2+3 foliations+overthrust faults, the Cenozoic has been shown by the fault blocks with different degree in strike slip and uplift. The evolution of intra continental orogeny can be divided into three substages: (1)the collapse stage in the late Indosinian early Yanshanian period (T3J1+2); (2)the extrusion overthrust stage in the midlate Yanshanian period (J3K2); (3)the block uplifting and transformation stage in the Himalayan period. Intra continental tectonic convergence and strikeslip induced the condensation of tectonic thermal power in the upper crust, and formed the FenghuangshanNiushan magmaheat dome, leading to the partial melting of crust and the invasion of granite veins. The southern part, Annkang fault belt and the northern part, Ning Shann fault belt has undergone dextral strikneslip deformation and sinistral strike slip deformation respectively in Yanshanian period, which formed the more distinct east extrusion slip shear deformation in the study area, showing that the strikeslip of intra continental orogeny cannot be neglected.

Key words: intra contiental orogeny, new foliation, structural deformation, structure petrography mapping, Indosinian Yanshanian period, South Qinling orogen