郯庐断裂带西山驿岩体构造变形分析

Analysis on structural deformation of the Xishanyi pluton in the Tancheng—Lujiang fault belt

地处肥东岩群西侧的西山驿岩体,由于郯庐断裂带左行平移活动的影响而发生韧性变形。根据野外调查和室内研究,岩体的西侧边界由于受到郯庐断裂带次级剪切带的影响,导致变形强度比岩体内部强烈,岩性上可分为边界糜棱岩带和内部片麻岩带。本文对边界韧性剪切带进行了变形温度估算,剪切类型分析,应力场的方向、差应力大小、应变速率计算。结果显示剪切带糜棱岩的变形温度约为500～600℃,为低角闪岩相变质,内部片麻岩变形温度约为400～530℃,为高绿片岩相到低角闪岩相变质;糜棱岩样品中矿物变斑晶的涡度值介于0.86～0.94之间,剪切类型为简单剪切为主的一般剪切;古应力方向为164°～171°,最低差应力下限值为20.02～23.08MPa,应变速率为1.35×10^(-13)～4.93×10^(-13)s^(-1)。在中、晚侏罗世到早白垩世期间,由于太平洋板块的斜向汇聚作用,郯庐断裂带发生构造活动,而通过上述西山驿岩体的构造变质变形分析结果,反演郯庐断裂带的变形规律,并且希望借此在以后的研究中归纳该时期中国东部的构造演化过程。

Objective: The Tancheng-Lujiang fault( Tanlu fault) belt,as a giant shear zone in eastern China,has been a hot spot among geologists since it was discovered.The Xishanyi pluton,which protolith was monzogranite,located in the west of Feidong group. However,the pluton occurred ductile deformation due to sinistral strike-slip shearing of the Tanlu fault belt.The mylonite with strong deformation is formed on both sides of the pluton,and the gneiss within the pluton is deformed weakly. Due to the serious overburden,only the west side is exposed. In this paper,the structural analysis of mylonite and gneiss are carried out,to investigate the structural deformation characteristics of the Xishanyi pluton.Methods: Detailed field observation,microstructure analysis,estimation of metamorphic-deformation temperature,analysis of tectonic stress field,kinematic vorticity calculation,the fractal dimension of the quartz,and the calculation of differential stress and strain rate of quartz are used to study the Xishanyi pluton.Results: Through the observation and analysis,the dynamic recrystallization type of quartz and feldspar in mylonite is mainly GBM and BLG-SR,respectively. The dynamic recrystallization type of quartz and feldspar in gneiss is SR-GBM and BLG,respectively. In addition,the fractal dimension of the quartz in mylonite is between1.164 ~ 1.173,indicating the metamorphic-deformation temperature of mylonite is between 500 ~ 600℃.However it is 400 ~ 530℃ in gneiss.Through microscopic analysis and measurement,the vorticity value Wkis between 0.86 ~0.94,which is greater than 0.75 and indicates that the shear zone on the west side of the Xishanyi pluton is mainly simple shear,with little component of pure shearing;The results of stress field show that the direction of maximum compressional stress is between 164°~ 171°. The minimum differential stress is between 20. 02 ~ 23. 08 Mpa,and strain rate of the shear zone is between 1.35×10^-13~ 4.93×10^-13 s^-1.Conclusions: Through detailed field observation and indoor analysis on the Xishanyi pluton,we know the pluton participated in the activities of the Tanlu fault belt as a tectonic lenticles.The pluton and the shear zone on both sides constitute the "S-C"fabric.The pluton recorded the activity characteristics of the Tanlu fault belt.