Mechanisms of dynamic recrystallization: recognition from natural tectonites

As the major research target of geologists, “rock is like book” (K-K-K), which contains abundant information of the Earth’s evolutionary history. Microstructure in tectonites is a “deformation process written in stone”. Observations of microstructure may reveal the deformation mechanism of associated rocks and minerals. Take for example mylonite occurring in the Tianshan orogenic belt, the authors introduce the dynamic recrystallization mechanism of deformed rocks at the middle and lower crustal levels and features of related microstructures. As tectonites are all the product of recrystallization, they can only reveal the mechanism of deformed minerals. At the middle and lower crustal levels three major dynamic recrystallization mechanisms of minerals (from low to high temperatures): bulging (BLG), subgrain rotation (SGR), and high-temperature grain boundary migration (GBM), which result in formation of different microstructures. The typical microstructure formed by BLG is featured by interlobated fine and equigranular structure. SGR gives rise to ribbons consisting of oriented elongated new grains. GBM forms an assemblage of typical new grains with irregular grain boundaries and greatly varied grain size and microstructure with indistinct wavy extinction inside grains. Besides temperature conditions, strain rates also have significant effect on the dynamic recrystallisation mechanism. “Detail” and “completeness” should be the main principles in microtectonic observations and structural data interpretation.