The deformation of granitic rocks in the Jinzhou detachment fault zone, southern Liaoning: Microstructures, fabrics and geochronology analysis
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摘要: 花岗岩（脉）在中下地壳韧性剪切带中普遍发育，如何正确鉴别剪切带中剪切前、剪切期及剪切后花岗岩（脉）以及正确理解剪切过程中构造变形与岩浆作用之间的关系一直是一个重要课题。本文以辽南金州拆离断层带为研究对象，选取中部地壳伸展作用过程中具有不同变形表现的花岗岩（脉）开展宏观-微观构造观察、石英EBSD组构分析及锆石LA-ICP-MS年代学测试等工作，从而进一步丰富构造-岩浆关系判别准则。剪切前花岗岩（脉）多变形强烈且具有后期固态变形叠加在早期高温岩浆组构之上的特点，而剪切期的花岗岩由于侵位的时间不同，岩石的变形程度也会不同。剪切晚期侵入的岩脉遭受了较弱的晶内塑性变形，而剪切早期的岩脉可以显示岩浆流动或结晶后高温至中温固态变形。从组构特点上看，剪切前和剪切期花岗质岩石石英c轴组构大多表现为中高温组构叠加有低温组构的特点。剪切后的花岗质岩石仅发生微弱的晶内变形或未变形而显示低温或无规律的组构特征。对五个典型的样品进行年代学测试，其结果符合相应的期次划分类型。应用宏观构造、显微构造与组构分析，结合年代学测试综合分析，对于辽南变质核杂岩构造-岩浆活动性进行了精细划分，包括134~130Ma初始伸展阶段，130~115Ma峰期伸展与强烈岩浆活动阶段，以及115Ma前后伸展作用结束。Abstract: Granitic plutons and dykes are widely distributed in the mid-lower crustal ductile shear zones. How to distinguish pre-, syn- and post-shearing intrusions and the relation between magmatic activity and tectonic deformation are persistent questions. This paper focuses on deformation of granitic rocks which were sheared at different stages of lithospheric extension along the Liaonan Jinzhou detachment fault zone. Macro-microscopic structural analysis, quartz EBSD fabric analysis and zircon LA-ICP-MS U-Pb dating are applied so that we can enrich our understanding of the relation between magma emplacement and regional deformation. The pre-shearing granites were intensely deformed. Solid-state deformation was imposed onto early high temperature magmatic fabrics. The syn-shearing granites vary in strain history because of the relative timing of their emplacement with respect to ductile shearing. The late syn-shearing granites may only show weak intragranular plastic deformation, while the early syn-shearing granites possess magmatic, or high to mid-temperature solid-state deformation features formed during cooling. The quartz CPO's of pre- and syn-shearing granites are characterized by high-temperature fabrics superimposed by low-temperature fabrics. The post-shearing granites show no or weak intracrystalline deformation. The quartz CPO fabrics are either low-temperature fabrics or random fabrics. Zircon U-Pb geochronology dating was conducted on five typical samples and the results support their structural timing. The macro-microstructure and fabric analysis combined with the geochronology dating give us a better understanding of the tectono-magmatic activity stages in Liaonan metamorphic core complex, including the initial extension stage in 134Ma to 130Ma, the peak tectono-magmatic activity stage in 130Ma to 115Ma and the cessation of the detachment faulting after 115Ma.
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