Orthopyroxene–sillimanite–sapphirine granulites from the Bamble granulite terrane, southern Norway

Silica‐deficient sapphirine‐bearing rocks occur as an enclave within granulite facies Proterozoic gneisses and migmatites near Grimstad in the Bamble sector of south‐east Norway (Hasleholmen locality). The rocks contain peraluminous sapphirine, orthopyroxene, gedrite, anthophyllite, sillimanite, sapphirine, corundum, cordierite, spinel, quartz and biotite in a variety of assemblages. Feldspar is absent. Fe 2+ /(Fe 2+ + Mg) in the analysed minerals varies in the order: spinel > gedrite ≥ anthophyllite ≥ biotite > sapphirine>orthopyroxene > cordierite. Characteristic pseudomorph textures indicate coexistence of orthopyroxene and sillimanite during early stages of the reaction history. Assemblages containing orthopyroxene‐sillimanite‐sapphirine‐cordierite‐corundum developed during a high‐pressure phase of metamorphism and are consistent with equilibration pressures of about 9 kbar at temperatures of 750–800°C. Decompression towards medium‐pressure granulite facies generated various sapphirine‐bearing assemblages. The diagnostic assemblage of this stage is sapphirine‐cordierite. Sapphirine occurs in characteristic symplectite textures. The major mineralogical changes can be described by the discontinuous FMAS reaction: orthopyroxene + sillimanite → sapphirine + cordierite + corundum. The disequilibrium textures found in the Hasleholmen rocks are characteristic for reactions which have been in progress but then ceased before they run to completion. Textures such as reaction rims, symplectites, partial replacement, corrosion and dissolution of earlier minerals are characteristic of granulite facies rocks. They indicate that, despite relatively high temperatures (700–800° C), equilibrium domains were small and chemical communication and transport was hampered as a result of dry or H 2 O‐poor conditions.