大别山超高压变质岩带的岩石磁学和磁组构研究及其地质意义  

Rock magnetism and magnetic fabric studies of the ultrahigh-pressure(UHP)metamorphic rocks from the Dabie orogenic belt,east-central China:implications for retrograde metamorphism.

作　　者：潘永信[1] 朱日祥[1] 

机构地区：[1]中国科学院地质与地球物理研究所岩石圈演化国家重点实验室,北京100029

出　　处：《岩石学报》2005年第4期1101-1108,共8页Acta Petrologica Sinica

基　　金：国家基础研究计划"大陆深俯冲"项目 G1999075509课题;国家自然科学基金委创新群体(40221402)资助

摘　　要：本文对采自大别山碧溪岭、新店、石马、花凉亭和朱家冲等地点的102块定向超高压和高压变质榴辉岩、片麻岩和大理岩等样品,进行了岩石磁学和磁化率各向异性(AMS)研究。磁化率和磁化强度随温度变化以及磁滞回线参数的分析结果表明,岩石剩磁载体以假单畴-多畴磁铁矿为主。新鲜榴辉岩和大理岩的磁化率很低,经过退变质作用的榴辉岩具有最大磁化率值,片麻岩的磁化率变化范围较大。这表明这些岩石中的磁性矿物含量主要受退变质作用和原岩成分差异的控制。磁化率各向异性度(P)主要受磁面理(F)的控制,显示出在其发育期以挤压构造环境为主;新鲜榴辉岩、退变质榴辉岩和片麻岩的 F 和 P 值依次增大。榴辉岩和片麻岩的 AMS 椭球的展布近似。在地理坐标下,这些 AMS 椭球的最小主轴(K_3)以向北倾为主,最大主轴(K_1)多为南倾。The Dabie ultrahigh-pressure （UHP） Orogenic Belt in east-central China was formed by collision of the North China and Yangtze Blocks during Early Mesozoic. Many geochronological data indicate that UHP metamorphism occurred around 230 Ma. Here we report new results of rock magnetism and magnetic fabrics of these UHP rocks. Total 102 samples including eclogites and gneisses were collected from six locations in the Southern Dabie UHP zone. Low-temperature thermal demagnetization of saturation isothermal remanent magnetization, room-temperature hysteresis loop, high-temperature dependence of low-field magnetic susceptibility and remanent magentization results indicated that pseudo-single domain and multidomain magnetite are dominant remanence carriers. Para- and ferrimagnetic minerals contribute to the low-field magnetic susceptibilities. Fresh eclogite have low values of magnetic susceptibilities while the retrograde eclogites have high values and gneisses have a broad range of values. It suggests that magnetic susceptibilities are associated with the retrograde metamorphism and the protolith. The anisotropy of magnetic susceptibility （AMS） measurements demonstrated that the anisotropy degree （P） are dominated by magnetic foliations （ F）, which were controlled by factors such as rock physic property differences, metomorphism and tectonic stresses. It shows that fresh eclogites have very low F and P values, which is echoed with the low magnetite content; while gneisses have very high values, in consistent to high magnetite content. The retrograde eclogites have higher P and F than fresh eclogites. In addition, the retrograde eclogites and host gneisses have similar orientations of AMS ellipsoids. Thus the AMS is due to the preferred crystallographic orientation of paramagnetic matrix minerals and magnetite, and the shape anisotropy of magnetite grains, forming in the retrograde metamorphic stage and of tectonic origin. Furthermore, we found that magnetic foliations of gneisses and retrograde eclogites

关 键 词：磁化率各向异性 退变质作用 俯冲与折返 大别山 超高压变质岩 

分 类 号：P318.4[天文地球—固体地球物理学]