Intrafolia Folds Within Marbles Of The South Liaohe Group In The Tongyuanpu And Adjacent Areas,Eastern Liaoning

The Liaodong Paleoproterozoic Orogenic Belt,composed of granitoids and meta-sedimentary rocks called the Liaohe Group,is located in the east of the North China Craton.There are still arguments on tectonic setting and evolution of the Jiao-Liao-Ji Paleoproterozoic Orogenic Belt so the Liaodong is attractive.A comprehensive research on the lithography,chronology,structural deformation and microstructures,especially folds of marbles of the South Liaohe Group,is conducted to provide evidence for nature of the Liaodong Paleoproterzoic Orogenic Belt.The Tongyuanpu area is characterized by gneiss domes whose cores are the Liaoji Granites,mantles are ductile shear zones composed of gneisses and migmatites and shells are bedded strata characterized as overall ordered but locally disordered folds and shear deformation.There are three intrafolia folds in the marbles of the South Liaohe Group: open folds,close folds and micro-folds.Their inclinations of axodes and foliations are south-north,nearly the same and those of hinges are similar with regional lineations,which are 110°,so all of them are A-type fold.A research on the intrafolia folds shows that they are generated in the setting of thick strata alternating with thin strata or strong ones alternating with weak ones among which weak strata are the main places characterized as easy slide among their boundaries.This paper uses two general methods to calculate kinematic vorticity through which shear components are 49-84% and 33-89%(points are mainly over 50% component with 3 points below 50%),indicating the deformation is dominated by simple shear containing some pure shear.So bedded strata moved eastwards and westwards and flew in the mid-lower crust with vertical deformation instead of simple vertical deformation with uplift of domes.Besides,their temperature of deformation and metamorphism goes lower away from the coreThree zircon U-Pb ages of granitic veins,1886±15 Ma 1884 ± 6 Ma and1874±30 Ma,are obtained to constrain the time of folds.Combining previous chronological framework,pre-orogeny 2250?2100 Ma,diapause 2100?1940 Ma,syn-orogeny 1940?1860 Ma,and post-orogeny 1860?1840 Ma,intrafolia folds resulted from regional orogeny and ended at late syn-orogeny(1870 Ma).