秦岭洛栾断裂带糜棱岩的变形特征及形成环境分析

摘要

Luoluan fault belt is the boundary between the southern margin of the North China plate and the northern margin of the Qinling Mountains.The deformation details can restore the plate margin deformation during the plate collision process.The trend of Luoluan fault belt is 290°,inclined to NNE inclination for 67°～80°.It is composed of a plurality of subparallel ductile shear belts.The single shear band width varies from a few meters to hundreds of meters,narrow in west,and wide in the east relatively.There are four events the in the Luoluan fault belt,with the second strong plastic deformation forming a large-scale of mylonite.Mylonitic lineation plunging,mineral sigma,delta porphyroclasts and quartz vein morphology kinematics indicate that the fault zone was formed by the kinematic characteristics of the south northward subduction combined with sinistral strike-slip.Mylonite deformation characteristics show the deformation mechanism of the Luoluan fault belt is plasticity in the eastern part,and low temperature diffusion creep in the west,grain boundary sliding and crystal plastic deformation mechanism.Plastic deformation sequence in mylonite minerals:calcite →biotite →quartz →plagioclase →potash feldspar.The felsic mylonite deformation phase of Luoluan fault belt is from east to west followed by two feldspar deformation of quartz phase,plagioclase deformation phase and quartz deformation phase.Mylonite formation temperature was 300～550℃,and the temperature in the east was higher than in the western.Amphibole aluminum pressure gauge obtained pressure:0.75～0.95 GPa and phengite pressure is 0.27～0.87 GPa.The differential stress is 0.32～0.41GPa,and the strain rate is 3.92917 × 10-11 ～3.17713× 10-16.Cigar shaped strain ellipsoid and more than 0.75 of the vorticity show strong shearing deformation.These characteristics show the Luoluan fault belt is a ductile deformation belt developed in deep leveland the deformation condition is medium temperature and pressure but eastern temperature is higher than that of the west.Differential stress is bigger in the west than in the east.The deformation environment of eastern part is equivalent to the middle crust,while the western part to the upper and middle crust,which indicate that the uplift is higher in the east than in the west.The mylonite deformation of Luoluan fault belt responsed to the second phase of tectonic activity,which truly reflects the Luoluan fault belt with the tectonic structure of physical processes and formation environment.It is of great significance to interpret the forming environment of the Luoluan fault belt.%洛栾断裂带是华北板块南缘与秦岭北缘的分界线,其变形作用细节可以恢复板块碰撞过程中板缘的变形作用.洛栾断裂带走向为290.,倾向NNE,倾角多为67.～80.,由多条近平行的韧性剪切带组成,单个剪切带宽度由几米至上百米不等,西部较窄,东部较宽.洛栾断裂带主要构造活动有四期,第二期强烈的塑性变形形成了大规模的糜棱岩.糜棱岩中的线理倾伏向、矿物σ、δ残斑和石英脉形态等运动学特征均指示断裂带具有由南向北俯冲兼有左行平移的运动学特征.糜棱岩的变形特征显示洛栾断裂带东部的变形机制为晶质塑性变形,西部为低温扩散蠕变、颗粒边界滑移以及晶质塑性变形机制.糜棱岩中的矿物塑性变形序列为:方解石→黑云母→石英→斜长石→钾长石.洛栾断裂带长英质糜棱岩的变形相自东向西依次为二长石变形相、石英斜长石变形相和石英变形相.糜棱岩的形成温度为300～550℃,其中东部的温度高于西部.角闪石全铝压力计获得的压力为:0.75～0.95GPa,多硅白云母压力计得出的压力为0.27～0.87 GPa.差异应力为0.32～0.41GPa,应变速率值为3.92917×10-11～3.17713×1016.雪茄状应变椭球体和大于0.75的涡度显示断裂带为强烈剪切拉伸变形,且西段强于东段.上述特征显示洛栾断裂带是发育于中-中深层次的韧性变形,糜棱岩的变形条件总体属于中温、中压环境下的中等应变条件,但东部温度高于西部,西部差异应力大于东部.相当于东部位于中地壳偏深环境,西部位于上地壳下部-中地壳上部环境,说明洛栾断裂带在后期的抬升过程中东部高于西部.这期糜棱岩的变形是对洛栾断裂带第二期构造活动的响应,真实地反映了洛栾断裂带这期构造活动的构造物理作用过程和形成环境,对解读洛栾断裂带的形成环境有着重要意义.