Melt-assisted interior to margin switch from dislocation to diffusion creep in coarse grained plagioclase: Evidence from a deformed anorthosite pluton

At Bolangir (Eastern India), massive anorthosites in the pluton interior grade into anorthosites with outward-dipping, margin-parallel foliation neighboring the pluton margin. Sheets and veins of ferrodiorites – residual melts of anorthosite crystallization – concordant and discordant to the margin-parallel foliation suggest pluton deformation at near magmatic conditions, T ∼ 950 °C and P 6–12 kbar. In the pluton interior, the larger-than-centimeter sized magmatic plagioclase grains are replaced by aggregates of smaller (100–600 μm) dynamically recrystallized internally-strained grains with un-equilibrated boundaries. Neighboring the pluton margin, poly-sized (200–2500 μm) plagioclase grains in anorthosites are of two types: strain-free rectangular-shaped plagioclase grains with high-energy An-richer margins indenting neighboring plagioclase grains formed by diffusion creep, whereas unstrained end-to-end touching euhedral plagioclase grains showing tilling represent magmatic flow textures. The pluton interior-to-margin switch in plagioclase deformation from grain boundary migration accommodated dislocation creep to grain boundary diffusion creep is attributed to the increasing melt fraction (melt/crystal ratio) during syn-deformation pluton emplacement. Plagioclase grains in the Bolangir pluton are significantly coarser compared to plagioclase aggregates (<200 μm) in experiments designed to understand deformation mechanisms. The present study demonstrates that presence of melts promoted diffusion creep over dislocation creep, albeit in larger-than-experiment plagioclase grains.