浙闽碰撞造山带中剪切带的磁性组构与碰撞动力学的研究

A STUDY ON MAGNETIC FABRIC OF THE MYLONITES FROM COLLISIONAL OROGENIC BETWEEN SE.ZHEJING AND NW.FUJIAN TERRANES,SOUTHEASTERN CHINA

糜棱岩是韧性变形的产物,对其形成过程和机理近年来进行了多方面的研究。岩石磁化率各向异性研究。为岩石特别是变质岩的显微构造研究提供了更为有效的新兴工作方法。作者对在闽西北和浙东南地体碰撞带内的庆元济下和龙泉查田韧性剪切带中采集的54个下古生界糜棱岩及与其共生的片麻岩和变质硬砂岩样品进行了磁性组构——磁化率各向异性的实验测定。对于磁化率和各向异度较低的样品(<250×10^(-6)SI),规定了重复测量数据可靠平均值的判定标准:(1)发散度系数Ci小于20°;(2)磁性线理量或面理量应在仪器噪音水平(仪器空转时的信号)的五倍以上。测量结果发现这些糜棱岩的磁面理非常发育,而磁线理发育较弱,其磁化率量值椭球为压偏型,与磁性面理相垂直的最小磁化率主轴的分布收敛,并与野外实地测量岩石的片理(走向NE,向SE缓倾)所得到的片理法线产状基本吻合,显示了向NW方向的挤压应力场特征。剪切带中岩石的应变随着深度的增加,表现为由简单剪切作用向纯剪作用转化,因而其面状构造的发育不断增强。这些结果说明,岩石磁性组构的测定可为研究糜棱岩形成的构造应力作用和应变机制以及与糜棱岩形成有关的构造因素——碰撞造山带的动力学特征提供更为迅速而精确的信息。

Mylonites as products of ductile deformation, as well as the process of deformation and its genesis mechanism, have been studied intensively in recent years. The study of anisotropy of magnetic susceptibility(AMS)in rocks which was initiated in the 1960＇s has offered a new technique to research on the microstructures of rocks, especially on thosc of metamorphic rocks. A measurement has been made of the AMS in 54 specimens of mylonites, gneisses, and metagraywackes from the ductile deformation belts in an early Paleozoic collision zone between the NW. Fujian and SE. Zhejiang terrancs. The criteria for the reliable average orientaion of a principal axis of magnetic susceptibility have been made in reference to repeated measurement of the weak bulk susceptibility(<250×10^(-6)SI)and weak AMS in a sample, i.e.(1)the scattering parameter Ci<20°,(2)the quanta of magnetic lincation or foliation, i.e. the difference between K_1 and K_2 or bctween K_2 and K_3 should be five times higher than background noisc the instrument, It was thus found that the magnetic foliation is well developed but the magnetic lineation is weakly developed in mylonitic rocks, forming an oblate susceptibility amplitude ellipsoid. The converge orientations of minimum principal axes, which are poles of magnetic foliations(gently dipping toward SE with the stikes of NE), coincide basically with the normals to the schistocities of the rocks measured in the field, showing a feature of the NW-ward compressive stress. The strain of the rocks is characterized by the transformation of a simple shcar component to a pure shear strain in the ductile deformation belts as the depth of the belt increased. The schistocity structures in the rocks studied have devel. oped continuously with increase of deformation. Consequently, it is shown that the measurement on magnetic fabric in rocks provides some quick and precise informationabout the features of dynamics in the genesis process of rrylonites, including the techonis stress, strain mechanism and other techonic factors, which were associated with the collisional orogenic belt.