ABSTRACT

The quartzofeldspathic gneiss of the Shillong Plateau Gneissic Complex has preserved magnetite-centered ocellar texture or fleck structure. Results presented in this work suggest that the intrusion of hot, volatile-rich pegmatitic magma into the thermally elevated quartzofeldspathic gneiss during the late Pan-African tectonothermal event initiated very localized partial melting in the latter forming neosomes. After their formation, the neosomes were infiltrated by the highly-viscous pegmatite magma leading to chaotic mixing between the two magmatic phases. The occurrence of chaotic mixing enabled the pegmatite magma to venture into the neosomes as veins or filaments by stretching and folding dynamics. As the pegmatitic veins traversed through the partially molten rocks or neosomes due to advection, substantial stretching caused the veins to develop sinuous perturbations. Eventually, the perturbations magnified and nascent swirls began to grow on the sinuous filaments by concentrating the high-viscosity pegmatitic veins into globules and impoverishing the areas in between them. Gradually, the globules separated from each other and commingled with the surrounding neosome, producing individual emulsions or ocelli. The viscous swirling phenomenon produced discrete diffusive elements that significantly enhanced the interfacial area between the two magmatic phases, which promoted diffusion and eased mixing between them.