The Deformation Characteristic Of The Small-scale Folds And Its Genesis In Luotian Area Of Eastern Dabie Mountains

There are many faults and folds in Dabie orogenic belt, and the faults have been studied by some researchers, but the folds have not been systematically researched. In particular, on the central of Dabie orogenic belt——Luotian gneiss area in North Dabie, it exhibits a gneiss dome nowadays. By field inversitgete, we discover that the folds in Luotian gneiss area are different from the folds by dome. This paper wills carefulness observer the characteristic of small folds first, then applies finite strain analysis and quartz c-axis analysis for structural deformation characteristic of the small folds. On the basis of dating of veins which the small folds parasitized by zircon U-Pb LA-ICP-MS, combined with region data, we can restrict the time of the small folds, and provide new information for the region structural evolvement.In Luotian gneiss area, we can mark out 4 generation leucosome which distribute on the region, and the N₄ leucosome paralleling to the gneissosity and folds to reverse "Z" type. This generation fold leading trait is that the limb is narrow and the core is intense thick. The compression characteristic of these folds suggests that they are not formed by the Luotian dome, they should have other dynamical origin.Strain analysis for this generation fold shows that there is compression deformation and extension deformation in the core and also in the limbs of the Luotian area. This reflects the gneiss of Luotian area have suffered buckling. The x-axis(representation linear bearing) inclined to SE on the region, reflecting the matter move to NW during the deformation; it steep inclinednd distribute for a circlewise near the central of dome, reflecting the gneiss have suffered domed. The z-axis inclined to NW mostly, reflecting the gneiss area have suffered SE-NW compression.Quartz C-axis fabrics, who usually develop high maxima paralleling to the Z-axis, show that the gneiss suffer SE-NW compression. The SE-NW compression forms the bulking fold in the middle-upper crust.The dating of veins in Luotian gneiss area which the small folds parasitized by zircon U-Pb LA-ICP-MS shows that, the age of one sample is 133.5～125.4Ma and the concordant age is 129.0±1.4Ma; and the age of the other sample is 137～134Ma and the concordant age is 135.0±1.4Ma. The zircon U-Pb data indicate that the rocks were migmatized about 137～125.4Ma ago, and the intense migmatization happened in 130Ma±.The veins which the bulking fold parasitized are formed by the migmatization in Yanshanian magmatism, and the age is 137～125.4Ma. Thus, the formed age of bulking fold should after the migmatization or coeval with it. Combined with the age of Luotian dome(150～127Ma), we can restrict the formed age of bulking fold in 137～127.5Ma.The fact of the bulking fold formed in 137～127.5Ma suggests that, the Luotian gneiss area have suffered a deeper compression after the formation of Luotian dome. This compression accelerated the uplifting of Luotian dome during the earlier Cretaceous. The drive origin may be associated with the Pacific Plate subducted to the East Asian continent with high speed.This compression may be concentrated in the central of Dabie orogenic belt——North Dabie complex, by the virtue of the dynamical origin on the deep. The ultra-high pressure metamorphic rock piled upon the North Dabie complex at that time slide to S-SE. This indicates that the structural evolvement history of ultra-high pressure metamorphic rock and the North Dabie complex has comparability but the time is different.