Fluid-enhanced diffusive mass transfer combined with GBS as an important process for protracted weakening in the middle-lower crust

Whether dislocation creep or diffusion creep dominates the flow of the middle-lower crust remains an enigma. In this study, integrated micro-analyses on laminated ultramylonites along the Wulian detachment fault zone, North China, were performed to unravel the dominant deformation mechanisms in naturally sheared granitic rocks in the middle-lower crust. The ultramylonites consist of interlayers of quartz ribbons and polymineralic layers of feldspars and quartz. The quartz ribbons are characterized by widespread intracrystalline low-angle grain boundaries and obvious crystallographic preferred orientations (CPOs), indicating the activity of dislocation creep. In contrast, random CPOs and well-mixed phases compatible with the operation of GBS characterize the polymineralic layers. Cathodoluminescence observations reveal zoning structures in the plagioclase and K-feldspar grains from such polymineralic layers. Meanwhile, chemical composition analysis shows that K-feldspar and plagioclase grains within the layers commonly possess potassium-and calcium-enriched rims, respectively. Furthermore, no evidence for intracrystalline distortion is detected in the plagioclase and K-feldspar grains. Together with interstitial K-feldspar grains, the evidence therefore shows that fluid-enhanced diffusive mass transfer played a dominant role in accommodating GBS in the polymineralic layers. In addition, mixed phases due to cavitation-sealing and geometric phase mixing inhibit grain growth and further ensure the maintenance of GBS during progressive deformation. Consequently, we would argue that operation of fluid-enhanced diffusive mass transfer combined with GBS leads to protracted weakening of the middle-lower crust.