OBSERVATION OF MICROCRACKS WITHIN GABBRO IN EXPERIMENTS ALONG VARIOUS STRESS PATHS

The mierocracks within crystal grains of Jinan gabbro have been observed in experiments along various stress paths. We load the rock sample to a certain high stress state, the differential stress reaching about 80% of the ultimate strength of the gabbro specimen. Then further stress is applied along various stress paths A, B and C. These paths have been described in a previous paper (Geng et al., 1979). After unloading, the rock samples are cut and polished or thin sections are made. It is observed that there exist obvious differences in microfractures between the three cases. In case B, most of the mierocracks in plagioclase are shortened whose lengths are less than the minor axis of the plagioclase grains. These cracks are concentrated in rather narrow regions. In case C, the number of inclined mierocracks increases, and the mierocracks in the plagioclase grains become much longer and much more in number than that in case A and B. There appear some star-like mierocracks. It is possible that the microscopic feature in gabbro are related to the super-dense and ultra-dilatant state of the rock samples.