差应力在超高压变质岩形成过程中的作用——来自石英-柯石英转化的高温高压实验证据

THE EFFECT OF DIFFERENTIAL STRESS ON THE FORMATION OF ULTRA-HIGH PRESSURE METAMORPHIC ROCKS—Evidence from high temperature-high pressure experiments on quartz-coesite transition

文中概述了超高压变质岩在静水压条件下的形成温压范围和存在的问题 ,并重新整理和分析了Hirth等 (1994 )在差应力条件下石英向柯石英转化的高温高压实验数据。结果表明 ,存在较大差应力的条件下柯石英出现需要的围压 (1 2 0～ 1 2 5GPa) ,远小于静水压下的围压 (2 5～ 3GPa) ,显然差应力的影响是显著的。榴辉岩中部分石榴石存在塑性变形 ,这表明在碰撞造山的构造环境中构造差应力是客观存在的 ,而构造差应力的上限受岩石强度制约。因此 ,差应力在超高压变质岩的形成过程中起到了重要作用 ,而系统地进行高温高压实验研究是深入探讨这一问题必要而有效的方法之一。

In summarizing the metamorphic temperature and pressure at hydrostatic condition of ultra high pressure rocks obtained from both field geology and experiments, we find that this problem is needed to further discuss, because the collision tectonic zone is not under hydrostatic conditions but under the action of differential stress. In this paper, we reassess and analyze the data of high temperature high pressure experiment on quartz coesite transition at differential stress condition made by Hirth and Tullis (1994). The results show that coesite is observed in both the semibrittle faulting and semibrittle flow regimes under temperature condition of 500～700℃ and pressure condition of 1.20～1.25GPa. Coesite is present mainly at the top and bottom of the tested samples adjacent to the pistons, as well as along fracture zones and along grain boundaries oriented perpendicular to σ 1 within the sample. The confining pressure (1.20～1.25GPa) required for quartz coesite transition in the presence of large differential stress is much lower than that (2.5～3GPa) at hydrostatic pressure condition. Obviously, the effect of differential stress is of great significance in the experiments. It is found that garnet in eclogite might be plastically deformed, indicating that differential stress do exist in collision tectonic zone, while the upper limit of the tectonic differential stress is constrained by the strength of rocks. Accordingly, differential stress is of great significance to ultra high pressure metamorphism. It is suggested, therefore, that systematic high temperature high pressure experiments are the essential and effective way to further investigate this problem.