粤北南雄断裂带糜棱岩岩相学特征及其对构造变形机制的指示意义

Petrographic Characteristics of Mylonites in Nanxiong Fault Zone,Northern Guangdong Province and the Implications for Tectonic Deformation Mechanism

南雄断裂带位于华南板块湘粤桂北海西印支凹陷与闽赣粤后加里东隆起的结合部位,为一条大型拉伸剥离断层,是粤北诸广铀矿集区的控矿构造带,带内糜棱岩广泛发育。糜棱岩作为蕴含着金属成矿带或构造带丰富地质信息的载体,为南雄断裂带构造特征及其变形机制研究提供了可能。在野外构造地质调查基础上,以带内糜棱岩为研究对象,通过详细的构造要素测量和显微构造解析,研究书斜构造、核幔构造、压力影、σ型与δ型旋转碎斑、S-C面理、云母鱼扭折、动态重结晶、机械双晶等典型显微构造类型。其运动学标志分析揭示糜棱岩带的剪切方向以左行剪切为主;有限应变测量表明,富林参数K值为0.54~0.77,剪应变r值为7.4~11.7,指示其属I型韧性剪切位移,应变类型为压缩型变形;石英岩组分析解读出糜棱岩滑移类型主要为底面滑移型和菱面滑移型,以中低温(250~350℃)韧性变形为主。南雄断裂带以左行剪切为主,经历了早期中低温为主的韧性变形和晚期脆性变形的多期变形过程,造就了其力学性质多变的复杂构造面貌,其中韧性、脆性断裂为成矿流体及铀源的活化和运移提供了重要的通道与媒介。

The Nanxiong fault zone is located at the junction of the Hercynian Indosinian depression of the South China plate and the post Caledonian uplift.It is a large tensile detachment fault and the ore controlling structural belt of Zhuguang uranium concentration area in northern Guangdong.Mylonite is widely developed in the belt.Mylonite is the carrier of rich geological information of metal metallogenic belt or structural belt,which is possible to study the structural characteristics and deformation mechanism of Nanxiong fault zone.On the basis of detailed field structural geological survey,the authors of this paper have made a detailed investigation on the geological structure in the field,the mylonite in the belt is taken as the research object,through detailed structural element measurement and microstructure analysis.The typical microstructural types,such as book oblique structure,core mantle structure,pressure shadow,σ-andδ-type rotational speckles,S-C foliation,mica fish kink,dynamic recrystallization and mechanical twinning are identified.The kinematic analysis reveals that the shear direction of mylonite zone is mainly left lateral shear.The finite strain measurement shows that Fulin parameter K is 0.54~0.77 and the shear strain R is 7.4~11.7.This indicates that it belongs to type I ductile shear displacement,and the strain type is compression deformation.According to the analysis of quartzite group,the slip types of mylonite are mainly bottom slip and rhombic slip,and the ductile deformation formed at medium low temperature(250~350℃).The Nanxiong fault zone is dominated by left lateral shear,which experienced with multi-stage deformation process of ductile deformation dominated by early medium low temperature and late brittle deformation,forming a complex structure with changeable mechanical properties.Ductile and brittle faults provide important channels and media for the activation and migration of ore-forming fluids and uranium sources.