Fluid-enhanced grain-size reduction of K-feldspar from a natural middle crustal shear zone in northern Beijing, China

Abstract

Grain-size reduction may lead to a transition from grain-size insensitive to grain-size sensitive deformation of rocks in the middle crustal level, thereby affecting the lithosphere's rheology significantly. However, the mechanisms of grain-size reduction of sheared rocks have long been debated. In this study, granitic mylonites from the Shuiyu shear zone of the Yunmengshan metamorphic core complex were sampled to investigate the mechanisms of grain-size reduction of K-feldspar during shearing in the middle crust. The mylonites are characterized by a typical microstructure of coarse-grained K-feldspar porphyroclasts surrounded by fine-grained matrix of oligoclase, K-feldspar, and quartz. Deformation temperatures are constrained from 400 °C to 550 °C, estimated using the feldspar and quartz microstructures, crystal-preferred orientation patterns of recrystallized quartz, and a two-feldspar geothermometer. Here, we show that the derivation of fine grains in the matrix from the K-feldspar porphyroclasts was accomplished via fluid-assisted subgrain rotation recrystallization. Walls of either well-organized or tangled dislocations constituted the boundaries of micron-scale subgrains along the margins of porphyroclasts. Hydrolytic weakening enhanced the dislocation motions to form subgrains. Furthermore, the subgrain rotation recrystallization promoted the replacement reaction of K-feldspar by sodium-rich fluid via increasing the areas of fluid/grain interfaces. The newly formed oligoclase and quartz were subsequently deformed by diffusion creep.