Sequential growth of cordierite and andalusite porphyroblasts, Cooma Complex, Australia: microstructural evidence of a prograde reaction

Abstract Low‐pressure prograde metamorphism of pelitic rocks in the Cooma Complex, south‐east Australia, has produced cordierite‐andalusite schists at intermediate grades. The first foliation (S₁) is preserved largely as inclusion trails in cordierite porphyroblasts. Microstructural evidence indicates that the cordierite porphyroblasts grew during the early stages of development of a crenulation‐foliation (S₂) and that andalusite porphyroblasts grew during the development of a later crenulation‐foliation (S₃). Microstructural evidence also indicates that the andalusite was a product of the prograde reaction: cordierite + muscovite ± andalusite + biotite + quartz. The occurrence of the products of this reaction in ‘beard’structures between cordierite microboudins formed by extension in S₃ confirms that the andalusite grew during the development of S₃. The investigation shows that porphyroblast‐matrix relationships can preserve the orientation of an early S‐surface that has been largely obliterated from the matrix, as well as providing relatively direct evidence of sequential mineral growth and metamorphic reactions.