西秦岭温泉岩体的磁组构特征及其侵位机制意义  

Magnetic fabric studies of Wenquan granite pluton in western Qinling Mountains and implications for emplacement mechanism

作　　者：谢晋强[1] 张国伟[1] 鲁如魁[1] 郭秀峰[1] 程斌[1] 梁文天[1] 陈应涛[1] 

机构地区：[1]大陆动力学国家重点实验室,西北大学地质学系,西安710069

出　　处：《地球物理学报》2010年第5期1187-1195,共9页Chinese Journal of Geophysics

基　　金：国家自然科学重点基金项目(40234041);大陆动力学国家重点实验室自主研究课题“中国南北构造带形成演化与动力学机制研究”资助

摘　　要：结合岩石磁学、磁化率各向异性度与区域构造分析了西秦岭温泉岩体的侵位机制及意义.温泉岩体的样品的平均磁化率k_m值总体很大,岩石磁学表明对于磁化率较低的样品,顺磁性矿物(如黑云母等)对磁化率的贡献较大,而少量铁磁性矿物(如磁铁矿等)可能作为剩磁载体.对于磁化率较高的样品,其主要载磁矿物为磁铁矿;花岗岩样品的校正磁化率各向异性度P_J总体小于1.2,显示了岩体为流动磁组构的特征,磁化率椭球体形态参数T总体大于0,扁率E总体大于1,以压扁椭球体为主;岩体的磁面理同磁线理相比更为发育,样品的磁面理普遍表现出围绕岩体边界分布的特点,且倾角较陡;而在岩体内部磁线理与磁面理分布相对散乱,定向性差,这一特征说明温泉岩体的磁组构主要由侵位时的侧向挤压作用形成的;虽然岩体的磁组构特征总体显示了N-NEE和SW向的挤压作用,但岩体侵位时由商丹缝合带闭合所产生的垂直于缝合带方向的挤压作用已相对较弱.本文认为,温泉岩体侵位时是一种弱挤压环境,或者是一种相对稳定的环境甚至可能是一种相对引张的背景,这与温泉岩体形成于后碰撞环境,秦岭造山带已演化至后碰撞拆沉作用发生的伸展阶段所反映的区域构造背景是一致的.Wenquan pluton is situated in western Qinling Mountains and is adjacent to the south of Shangdan suture zone. Through analyses of regional tectonics and the anisotropy of low-field magnetic susceptibility （AMS） together with rock magnetism of 125 oriented samples, the emplacement mechanism of the pluton is studied. The values of most mean susceptibilities （km） show a wide variation range. Thermomagnetic curves demonstrate that the paramagnetic minerals such as biotite, acting as the main magnetic carriers, made dominant contributions to the magnetic susceptibility of the samples with low values, including a small contribution to remanence from small amount of magnetite. The magnetite, however, acted as the dominant magnetic carrier of the samples with high magnetic susceptibility values. The corrected anisotropy degree （PJ） of less than 1.2 can be regarded as flow magnetic fabrics based on microscopic observations. The shape parameters （T） of most magnetic susceptibility ellipsoid display that the Wenquan pluton is dominated by oblate compressional fabric, and the magnetic foliations are dominant over magnetic lineations. In addition, it is well marked that most magnetic foliations of the samples on the border seem parallel to the edge of the granite pluton, which have relatively steep inclinations, and the interior magnetic lineations and foliations appear relatively disordered. Hence it can be inferred that the magnetic fabrics of the pluton are mainly formed by the lateral compression during the emplacement. Though the magnetic fabrics show N-NEE and SW compression as a whole, the compression caused by the closure of Shangdan suture zone in Indosinian became much weaker than the one in main orogeny stage. Consequently it can be concluded that the weak compression setting can be reflected by the characteristics of magnetic fabrics when the pluton emplaced, and furthermore the relatively sustained and even extension environment can be probable and understandable, which was consistent with the reg

关 键 词：磁组构 岩体 侵位 西秦岭 

分 类 号：P318[天文地球—固体地球物理学]