关键词: 超高压变质岩, 差应力, 柯石英

Abstract: 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 σ₁ 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.

Key words: ultra-high pressure metamorphic rocks, differential stress, coesite