江西武功山东区大型韧性剪切带的显微构造特征

武功山东区存在一条大型韧性剪切带。鞘褶皱倒向以及旋转变形构造(如S-C面理组构、旋转碎斑系、雪球构造和粒内显微破裂构造等)显示此剪切带为由南向北逆冲推覆性质。砾石、黄铁矿还原斑和石英斑晶的有限应变分析表明剪切带西段和东段岩石分别以收缩型椭球和压扁型椭球变形为特征。剪切带的主要变形时代是早古生代,可能与早古生代华夏陆块和扬子陆块之间的碰撞造山作用有关。

MICROSTRUCTURES OF A LARGE-SCALE DUCTILE SHEAR ZONE IN EAST WUGONG MOUNTAIN AREA OF JIANGXI

A large-scale ductile shear zone with length of 80km and width of 10-30km was found in East Wugong Mountain area of Jiangxi. The rocks in the shear zone were mylonitized into mica——quartz mylonite, including type ⅡS-C mylonite mostly. The shear zone is inferred to be a thrust shear zone in orientation from S to N according to the occurrence of sheath folds and the rotational microstructures. The deformation markers indicating sense of shearing are:(1)S-C foliation fabrics, (2)mica fish,(3) rotational porphyroclast systems, (4)snowball structures, and(5) intragranular microfractures. The pebbles, quartz grains and pyrite reduction spots were Chosen from different positions of the shear zone for finite-strain analysis. The result shows that the pexbbles and pyrite spots from the deformed rocks in western part of the shear zone exhibit a strain type of constractional ellipsoid. The Flinn parameters of the pebbles and pyrite spots are 3.5 and 15.1, respectively. The finitestrain analysis of quartz grains in eastern part of the shear zone gave their Flinn parameter of 0.60-0.69 being of flattening ellipsoid. The grain-size of the dynamically recrystallized quartz from mylonite in eastern part of the shear zone was measured in order to estimate the differential stress and strain rate. Analysis of three samples shows that the differencial stess is 30.1-49.7 MPa and strain rate is 4.59×10-¹⁴-1.35×10^-12s^-1. They are higher in internal part than in outer of the shear zone. According to the strain value of the deformed rocks on x-axis, the amplitude of total thrust displacement along the shear zone may be estimated. The displacement amount along the eastern and western parts of the shear zone is about 20 and 95 km, respectively, calculated from the ΔX of 78% and 215% of the deformedrocks in the tow parts respectively. It is suggested that the deformation age of the shaer zone is early Paleozoic on the basis of the stratigraphic age of the deformed rocks in the shear zone and their relationship to the granitic intrusions. Discovery of the early Paleozoic ophiolitic melange zone and the paired Caledonian granitoid belts in southeastern China imply a early Paleozoic collisional-orogenic belt existing between Cathaysian Block and Yangtze Block. The study on the large——scale ductile shear zone in East Wugong Mountain area may supply some new evidence for confirming the existence of collisional-orogenic belt in southeastern China.