大别山锆石喇曼光谱初步研究

Laser Raman spectroscopy study of zircons from Dabie Shan

对北大别地区的鹿吐石、道士冲和南大别地区的三祖寺、双河片麻岩锆石进行了激光喇曼探针、背散射电子显微镜(BSE)观察和U、Th等微量元素的组成分析,发现锆石喇曼光谱波数和半高宽都能指示其蜕晶化程度,而大多数情况下B1g(υ3)峰更特征些。单颗粒锆石内部BSE图像与喇曼光谱能完全对应。通过锆石微区微量元素和喇曼探针分析发现,半高宽和Th/U比正相关,这是由于岩浆锆石在变质过程中不同程度的重结晶作用造成的。锆石重结晶作用是变质岩中锆石年龄和化学组成变化的主要原因。三祖寺片麻岩可能曾经遭受过燕山期的热事件扰动。

Gneissic zircons from Lutushi and Daoshichong, Northern Dabie, and Sanzusi and Shuanghe, Southern Dabie are investigated by Raman microprobe (RMP) analysis and back scattered electron micrograph (BSE) images, and U and Th compositions are measured using ICP MS. It is found that the systematic change in wavenumbers with half widths of the Raman band is a result of metamictization, especially the B1g(ν3) band except for highly metamict zircons, whose wavenumbers are a little random. Within a single zircon, the half widths in different areas change systematically with the color levels of BSE image. Combining RMP and ICP MS analysis, it is found that the wavenumbers correlate with Th/U ratios. It is believed that this relationship is caused by recrystallization of magmatic zircons during high pressure metamorphism. The magmatic zircons suffered different recrystallization during metamorphosis. Recrystallization results in the change in the age and chemical composition of zircon. The gneissic zircons of Sanzusi may have undergone continuous thermal annealing of radiation damage for a long time after the closure of the U Pb isotope system, probably until about 120 Ma.