大别造山带东缘韧性剪切带中构造岩的构造地球化学分析

Tectono-geochemistry analyses of tectonites in shear zone of the eastern margin of the Dabie Mountains

安徽省桐城市挂镇（挂车河镇）地区出露郯庐断裂系（北北东—北东向）和大别构造系（近东西向）的韧性剪切带,变质温度分别为400℃～ 450℃和600 ℃～700℃,均发育镁铁质和长英质构造岩,是研究不同变形条件下岩石中元素活化迁移规律的理想对象.质量平衡计算表明长英质剪切带在韧性变形过程中,大别构造系体积基本不变,郯庐断裂系体积亏损且随变形程度的增强而增加;镁铁质性剪切带在脆—韧变形过程中体积增加.长英质构造岩的稀土和微量元素变异远弱于镁铁质构造岩,长英质构造岩基本继承了原岩的特征而镁铁质构造岩与原岩的差异显著,长英质和镁铁质混合的构造岩主要显示镁铁质组分的迁移规律.长英质构造岩的元素迁移种类和迁移率与变形程度和变质温度正相关,其中体积亏损变形的郯庐断裂系表现为重稀土中等迁出、中稀土几乎未迁移和轻稀土微弱迁入;等体积变形的大别构造系为稀土元素迁入,轻稀土、中稀土和重稀土的迁移率依次增加.镁铁质构造岩的稀土元素迁移表现为轻稀土强烈迁入和重稀土中等迁出.体积变化不是元素变异的主因;在原岩和矿物蚀变制约的基础上,元素迁移受到岩石流变分异作用和热液渗流作用的双重作用,低温时以渗透流体作用为主,而高温时岩石流变分异作用增强.

The Tanlu fault joins the Dabie orogenic belt on its eastern margin. There are ductile shear zones of NNE-NE trending of Tanlu fault system and EW trending of Dabie tectonic system in Guazhen（ or Guachehe town） area,Tongcheng city,Anhui Province. Those ductile shear zones of two tectonic systems, which are ideal to research element migration in different deformation conditions, both develop mafic and felsic tectonites with deformation temperature of 400 ℃450 ℃ and 600℃ -700 ℃ respectively. Mass balance calculations suggest that volumes of felsic shear zones in Dabie tectonic system were almost constant while volume-loss of felsic shear zones in Tanlu fault system increased with the deformation intensity during ductile deformation,and mafic shear zones gained in volume during brittle-ductile deformation. The variations of REEs and trace elements in felsic tectonites were far lower than that of mafic tectonites,the former on the whole inherited chief feature of host rocks while the latter were remarkably different with that of host rocks. Under mix of mafic and felsic components,tectonite would mostly reveal the element migration features of mafic component. The kinds and rates of element migration in felsic rocks were positive correlation with intensity and temperature of the ductile shearing deformation. Therein, HREEs, MREEs and LREEs of volume-loss Tanlu fault system were respectively medium losses,little/no change and little gains,while REEs gains of isovolume Dabie tectonic system in turn increased from LREEs, MREEs to HREEs. In mafic tectonites,LREEs were largely gains and HREEs were medium losses. The volume loss is not the real reason for variation of REEs and trace elements in the shear zone. Based on restriction of host rocks and mineral alteration, the joint function of rock rheological differentiation and fluid filtration with high water/rock ratio is the predominant reason for element migration, and the latter was predominant under lower deformation temperature while the former was enhanced with increment of deformation temperature.