大别山石榴石中的柯石英包裹体模型化分析

ANALYSIS OF MODEL FOR GARNET-COESITE INCLUSION IN DABIE MOUNTAINS,CHINA

在大别山超高压变质带中发现的柯石英多数已经完全退变为聚晶石英,并伴随着包裹它的外层基质的破裂现象,通过对这一现象的分析可以揭示变质岩形成的一些历史信息。在透射电子显微镜(TEM)下可以观察到石榴石-柯石英包裹体分为石榴石基质层,退变石英层和柯石英层,以此建立一个三层复合球模型。基质石榴石层的p-T轨迹可以看成与已知的外界变质岩轨迹一样,而内部石英和柯石英层在基质发生破裂前可看成降压降温的生长退变过程,这一体积增大过程最终导致石榴石层的破裂。模型根据已知的大别山变质岩的温度变化速率,计算出石榴石-柯石英包体的发生破裂的温度条件,并模拟柯石英不同退变程度所对应的不同路径,从而得出变质岩不同的抬升路径。

Coesites are found as inclusions in garnet and other metamorphic minerals in DaBie Mountains. Most of them have transformed into quartz, partially or all. Radial fractures occurs in the garnet shell and some information may be discovered about the history of metamorphic rocks. By use of transmission electron microscope we found that the sample is composed of garnet, quartz and coesite, so the present paper built a three-shelled composite sphere model under state p=0.05 GPa T=1 000 K→700 K to compute the transition of coesite. By giving crystal growth formulas and pressure condition of rupture in garnet the radius of quartz sphere depending on temperature was calculated. By implying the temperature rate of 5 K/Ma in Dabie mountain we concluded that the transformation of coesite began at 765 K . At last we draw the different track of transiton for different transformation rate.