Growth and concentration of fibrous sillimanite related to heterogeneous deformation in K‐feldspar‐sillimanite metapelites

Abstract In foliated K‐feldspar‐sillimanite metapelites, fibrous sillimanite is commonly concentrated in folia that anastomose between lenticular pods of coarser‐grained aggregates rich in quartz, feldspar and biotite, with or without garnet, cordierite and residual andalusite. Many of the folia appear to be limbs of crenulation microfolds. The sillimanite concentrations may be due largely to the ability of fibrous sillimanite aggregates to undergo strong non‐coaxial deformation by grain‐boundary sliding (‘fibre sliding’;) without appreciable build‐up of dislocations, whereas other minerals are unstable in these zones and so concentrate in lower‐strain interfolial zones. Initiation, and especially concentration of the sillimanite in folia, may be assisted by fluid flow and local base‐cation leaching, whereby minerals unstable in zones of strong non‐coaxial strain are dissolved and removed from these zones.