New polygonal grains formed by dissolution-redeposition in quartz mylonite

Abstract

Arrays of polygonal grains are common along original grain margins, subgrain boundaries and intragranular fractures of deformed quartz porphyroclasts. We investigated these internal polygonal grains within quartz porphyroclasts of a partially recrystallized quartz mylonite deformed in the presence of fluid in a greenschist facies shear zone from Quadrilátero Ferrífero, southeastern Brazil. Optical observation showed profuse fluid inclusions and muscovite flakes inside the deformed porphyroclasts. Observation by SEM revealed the presence of crystal faces on the internal polygonal grains. Domainal c-axis analysis showed that the internal polygonal grains have a preferred orientation similar to the host porphyroclasts ( c-axes at low angles to the stretching lineation), but different from the fabric of recrystallized matrix grains (type-1 cross girdle). Based on these observations, we suggest a mechanism of in situ solution-reprecipitation, where the new grains are nucleated in the intergranular fluid-filled pores present along the internal discontinuities of the porphyroclasts. The grains grow with their boundaries largely in contact with fluid, allowing the development of crystal faces, at least during the early growth stages. After the new grain reaches the opposite pore wall, further grain growth should occur through ordinary grain boundary migration mechanisms driven by the strain energy difference between the new grain and the host porphyroclast. We conclude that in these rocks, in situ solution-reprecipitation operated in conjunction with crystal-plastic processes, accounting for the nucleation of new grains during deformation/recrystallization in the presence of fluid under low grade metamorphic conditions.