CRYSTAL-GROWTH DURING A SINGLE-STAGE OPENING EVENT AND ITS IMPLICATIONS FOR SYNTECTONIC VEINS

The crystallization of a fluid filled vein or cavity and interpretatio n of the microstructure in a deformed rock is an important problem for geologists. Dynamic crystallization experiments using ice and an aque ous solution have been used as an analogue system for single-stage ope ning events and have shown that the presence or absence of a deviatori c stress is an important factor in the development of texture as a res ult of crystallization from a fluid. The textures can vary from (1) co arse crystal aggregates involving static growth where there is no sign ificant macroscopic deviatoric stress at the time of crystallization, the free-face growth model to (2) polycrystalline aggregates involving static growth which develop from a mixture of skeletal crystals and f luid in a stressed environment. In the latter situation, the crystals and fluid coexist until the macroscopic stress can be transmitted betw een impinging crystals and, at this stage, fast dynamic recrystallizat ion and grain boundary migration occurs. This recrystallization has be en termed the contact growth model and the resulting metamorphic textu re is compared with examples in natural quartz veins. During a single stage vein opening, uncomplicated by intragranular deformation, either free-face and/or contact growth textures can develop from the fluid f illed void depending upon the kinematic environment.