摘要
锆石是岩浆岩、变质岩和石英脉型金矿床中的一种常见副矿物,对锆石成因类型的准确判断是正确理解锆石U-Pb年龄意义的关键。本文中笔者对不同成因类型锆石的判别标志及年龄意义进行系统的总结,并认为将锆石的阴极发光图像(CL)、背散射电子图像(BSE)、痕量元素组成及矿物包裹体特征的研究相结合是进行锆石成因鉴定的有效方法。近年来同位素质谱技术的发展使得人们对同一锆石颗粒内部不同成因类型的锆石晶域进行原位年龄测定成为可能。通过微区原位定年技术,能够给出有关寄主岩石的地质演化历史等重要信息,这可以为地质过程的精细年代学格架的建立提供有效的证据。来自不同类型岩石中的锆石可能经历了Pb的扩散丢失作用、晶格损伤导致的蜕晶化作用以及变质重结晶作用。这些过程对锆石计时的准确性和有效性带来了不同程度的影响。为了对测定锆石年龄的地质意义进行合理解释,在进行锆石U-Pb定年前,必需对锆石进行成因矿物学和矿物内部结构的深入研究,特别是阴极发光和背散射电子成像研究,通过内部结构特征确定锆石的成因类型和形成环境。笔者认为,组成单一的岩浆锆石是理想的U-Pb定年对象,变质重结晶锆石域常是重结晶锆石和继承晶质锆石的混合区,容易给出混合年龄,只有变质增生锆石和完全变质重结晶锆石才能给出准确的变质时代,而从继承锆石中鉴别出的热液锆石可以获得可靠的流体活动时间。
Zircon is one of the most important accessory minerals in igneous rocks, metamorphic rocks and quartz- vein-type gold deposit. Carefully study on zircon origin is a key for understanding of geological meaning for zircon U-Pb age. The differentiation criterions for zircon origin and their geological meanings are summarized in this paper. A combined study of cathode luminescence （CL）image, backscattered electron （BSE）imaging, trace elements and mineral inclusions on zircon from rocks is an effective way to correctly distinguish different origins. Recent development in mass spectrometry makes it possible to determine the age of individual growth domains within single crystals. Thus the information on source and the geological history of host rock can be attained. Zircon from various rocks may suffer from Pb diffusion loss, metamictization caused by lattice radiation damage of these processes would affect the accuracy and validity of zircon ages to some extent. meaningful , and recrystallization. All In order to get geological ages, one must pay particular attention to mineral genesis and texture of zircons, and decide firmly zircon genetic type and environment through internal structure observation, especially CL and BSE image before zircon U-Pb dating. In the end, the author get a conclusion that magmatic zircons with simple composition are ideal minerals for zircon U-Pb dating. However, the metamorphic recrystallized zircon domain is often a mixture between recrystallized zircons and inherited crystalline zircons, and often gives a mixing age. Only complete recrystallized zircon domain can yield a meaningful metamorphic age. Hydrothermal zircons to be distinguish from the inherited zircons can provide reliable age constraints of fluids activity.
出处
《地质调查与研究》
2009年第3期161-174,共14页
Geological Survey and Research
基金
国土资源部计划项目(1212010711814)
国家自然科学基金重点项目(40234048)
关键词
岩浆锆石
变质锆石
热液锆石
蜕晶化作用
重结晶作用
年代学制约
magmatic zircon
metamorphic zircon
hydrothermal zircon
metamictization
recrystallization
constraints for geochronology
