Reaction relationships during retrograde metamorphism at Olary, South Australia

Abstract In metapelitic schists of the north‐eastern Weekeroo Inliers, Olary Block, Willyama Supergroup, South Australia, syn‐S1 and syn‐S2 assemblages involving staurolite, garnet, biotite and another mineral, most probably cordierite, were overgrown by large syn‐S3 andalusite porphyroblasts, owing to isobaric heating from metamorphic conditions that existed during the development of S2. Conditions during the development of S3 probably just reached the andalusite—sillimanite transition. During the development of S4, at somewhat lower temperatures than those that accompanied the development of S3, the following reaction occurred: staurolite + chlorite + muscovite ± biotite + andalusite + quartz + H₂O. The amount of retrogression is controlled primarily by the amount of H₂O added by infiltration. As the syn‐S3 matrix assemblage was stable during the development of S4, but the andalusite porphyroblasts were no longer stable with the matrix when H₂O was added, the retrogression is focused in and around the porphyroblasts. With enough H₂O available, and if quartz was consumed before biotite in a porphyroblast, then the following reaction occurred: staurolite + chlorite + muscovite + corundum ± biotite + andalusite + H₂O. This reaction allowed corundum inclusions in the andalusite to grow, regardless of the presence of quartz in the matrix assemblage.