Stability of zircon during mylonitization: a case study of granitic mylonite in Gaoligong and Ximeng, Southwest China
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摘要: 自然条件下锆石的稳定性对于合理解释锆石年龄及深入理解锆石同位素年代学有重要意义。本文对西南三江造山带高黎贡山和西盟地区的花岗质糜棱岩进行了显微构造和锆石U-Pb年代学研究。高黎贡山和西盟糜棱岩中主要造岩矿物长石、石英、云母发生了明显的变形和重结晶，而锆石则以独立晶体或以残留核的形式保存了原始的岩浆结构。高黎贡山和西盟糜棱岩锆石U-Pb年龄分别为513±8 Ma和459±2 Ma，代表花岗质原岩的时代。受新生代构造变质影响，锆石边部发育有窄的海绵状结构和溶蚀结构，指示变质重结晶作用发生在有少量流体参与条件下。石英变质机制为颗粒边界迁移重结晶，指示变形温度为500~700℃。石英C组构EBSD分析表明，糜棱岩经历了早期550~650℃和晚期400~550℃的递进变形作用。岩石显微构造、锆石结构及年代学研究结果表明，岩石糜棱岩化过程中，还有少量流体参与的情况下，温度在550~650℃时锆石晶体结构就受到破坏，发生变质重结晶作用。这为解释自然界复杂条件下锆石年龄提供了重要约束。Abstract: The stability of zircon under natural conditions is of great significance for reasonable interpretation of zircon age and further understanding of zircon isotopic chronology. In this paper, the microstructure and zircon U-Pb chronology of granitic mylonites in Gaoligongshan and Ximeng areas of the Sanjiang orogenic belt in the southwestern China have been studied. Finerals feldspar, which are the main rock-forming quartz and mica in granitic mylonite from the Gaoligong and Ximeng areas were significantly deformed and recrystallized, while zircon preserved the original structure in the form of independent crystal or residual core. The zircon U-Pb ages of the Gaoligongshan and the Ximeng mylonites are 513 ±8 Ma and 459 ±2 Ma, respectively, which represent the age of granitic protolith. Influenced by the Cenozoic tectonic metamorphism, some zircon grains developed spongy structure and dissolution structure at the rim, indicating the metamorphic recrystallization under the condition of a small amount of fluid. The metamorphic mechanism of quartz is grain boundary migration recrystallization, which indicates that the deformation temperature is 500-700℃. Quartz c-axis fabrics revealed by the electron backscatter diffraction (EBSD) technique reflect two stages of metamorphic temperature in the mylonites:the early stage of 550-650℃ and the late stage of 400-550℃. The results of rock microstructure, zircon structure and chronology show that the crystal structure of zircon is destroyed and metamorphic recrystallization occurred at 550-650℃ in the process of mylonization with a small amount of fluid involved. These conditions provide important constraints for the interpretation of zircon ages under complex natural conditions.
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Key words:
- Zircon /
- stability /
- recrystallization temperature /
- mylonitization /
- granitic mylonite /
- Sanjiang orogenic belt
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