Study On Micro-deformation Characteristics Of Feldspar

The Qinling Orogen is composed of the Southern North China Block(S-NCB),the North Qinling Belt(NQB),the South Qinling Belt(SQB)and the Northern South China Block(N-SCB).It has undergone complex structural superposition from Proterozoic to Mesozoic,exposing middle-lower crustal rocks with high deformation and metamorphism,which is an ideal place to study the deformation of feldspars.The NQB is the main part of the Qinling Orogen,and the contact relationship between the Qinling Group and the Danfeng Group has been controversial for a long time,which belongs to the NQB.In this study,the mylonite between these two groups was taken as the research object and detailed works have been taken.The focus was on the micro-deformation characteristics of feldspars in the mylonite.This study mainly used the Electron Backscatter Diffraction(EBSD)technology to obtain the microstructural photos and the crystallographic preferred orientation(CPO)data of feldspars.In the oriented thin section,feldspar porphyroclasts generally show micro-fractures with plastic deformations such as wave extinction and deformed twin.K-feldspar porphyroclasts are mainly orthoclases and myrmekite-forming reaction occurs.Plagioclase porphyroclasts are mainly anorthites,along with albites,they normally accompanied with bulging recrystallization.Myrmekite-forming reaction and bulging recrystallization obviously cause fine granulation of feldspars.The CPO pole figure of fine-grained feldspars is mainly depends on the type of feldspar end members:(1)The fine-grained albites formed by the myrmekite-forming reaction of orthoclase have different levels of CPO pole figures,their slip system is(010)/(001)[100];(2)Affected by bulging recrystallization,albite porphyroclasts form fine-grained albites and orthoclases,while anorthite porphyroclasts form fine-grained anorthites and albites.Those fine-grained feldspars show pole figures,thereinto,the slip system of plagioclase is(010)/(001)[100],but part of anorthites have no CPO;(3)In the fine-grained matrix,albite have CPO and the slip system is(010)[100],but anorthites have no CPO.By observing the microstructural photos of feldspars in mylonite,analyzing and comparing their CPO data,combined with field geological investigation and chronological data,it is believed that:(1)Under intermediate metamorphic conditions,plagioclases are more susceptible to dynamic recrystallization,whereas K-feldspars are normally affected by the myrmekite-forming reaction,which is the dominant mechanism to cause fine granulation.Under conditions where the deformation temperature is not very high,the influence of chemical activity on the deformation cannot be ignored;(2)Under the strain conditions that are sufficient to cause the myrmekite-forming reaction of the orthoclase,compared with anorthites,albites are more likely to intragranular plastic deformation.Moreover,even the deformation dominated by the chemical metasomatism process is also controlled by the strain localization;(3)According to the microstructural combination characteristics of feldspars,the deformation temperature of feldspars is estimated to be about 450-500 ?,therefore,it is considered that the Qinling Group and the Danfeng Group have undergone ductile shear in the mid-crust environment of about 15-17 Km deep.The collision was earlier than 478 Ma.The Qinling Group is more metamorphic and the collision occurred during the ascending stage.