Study On The Syntectonic Mica Of Ductile Shear Zone In Huangshandian Area,Beijing

The syntectonic mica in this study is the mica in the nappe tectonic belt of Huangshandian in Fanshan district in Beijing.This area has experienced the Indosinian movement and the Yanshanian volcanic activity.Tectonic movements occur frequently in the area.The main part of the study area is ductile shear zoneThe main results of this paper are:1.By observing the ductile shear zone,a profile map on the basis of a large number of field geological phenomena is completed.According to the features of asymmetrical folds,S-C fabrics,and rotational augens,seen from the west side of the mountain,it is indicated that the shear is Left-lateral shear.By the microtectonic method,the phenomenons of mineral dynamic recovery and recrystallization has been collected.The statics about the size of the mineral has been analyzed.The mica formation environment is roughly restored.It is speculated that the syntectonic mica is formed in a geological environment at the pressure of⁰.1GPa and the highest temperature of⁵00°C;2.The mineral species of mica is identified by infrared spectroscopy,scanning electron microscopy,X-ray powder diffraction.It is identified as fluorophlogopite.The alteration of mica strips is serious.The altered minerals are chlorite and talc,and the process of reducing talc alteration:bedding cracking-surface fracture-recrystallization.3.In this study,the crystal structure and crystal chemistry of fluorophlogopite were analyzed by electron probe and X-ray single crystal diffractometer.The crystal chemical formula of fluorophlogopite is:(K_0.82Na_0.06)(Mg_2.59Ti_0.01Fe_0.02)(Si_3.12Al_1.07O₁₀)[F_1.05?OH?_0.95],The unit cell parameters are:a=5.299?7??,b=9.172?1??,c=10.20?9??,?=100.0?3?°,Z=2,C2/m,where R1=0.0527.The b value of the fluorophlogopite is smaller than that of the previous data.The reason for the change of the length of the b-axis is as follows:the<T-O>bond length is short in the formation of fluorophlogopite in this area,and the stress causes the bond length of the<Mg-O>bond length and the thickness of the octahedral layer to decrease;4.This research is revealed that The ductile shearing occurred in the Huangshandian area in the eastern margin of the North China Domain in¹28 Ma.The structural deformation pressure was⁰.1GPa,and the highest temperature reached⁵00°C,and formed a series of structural phenomenas such as asymmetric folds,lens bodies and rotating fragments and so on.The fluorophlogopite grew in the tectonic movement,and due to the influence of stress,knee fracture,wavy extinction and alteration occurred.During the process of stress of fluorophlogopite,a series of changes have taken place in the crystal structure.The length of<Mg-O>bond shorter and the thickness of the octahedral layer decreased.As the result,the b-axis shrinked.The b-axis shortening of fluorophlogopite indicates the stress of the area.It indicates that the tectonic movement in the eastern margin of the North China Craton produces shear stress locally in the Huangshandian area,which has a limiting effect on the growth of fluorophlogopite.