Two stages of sapphirine formation during prograde and retrograde metamorphism in the palaeoproterozoic Lewisian Complex in South Harris, NW Scotland

Abstract

Two types of sapphirine occurrences were found in the Lewisian complex in South Harris, NW Scotland: (1) inclusions within porphyroblasts; (2) symplectic grains together with secondary cordierite, plagioclase and orthopyroxene. The presence of sapphirine inclusions implies that sapphirine was stable at the early stage of ultra-high-temperature metamorphism, whereas symplectic sapphirine grains were formed during decompressional retrograde metamorphism. The sapphirine occurrences and compositions of associated minerals depend on the host rock composition. Sapphirine inclusions occur only in rocks with high bulk-XMG, and sapphirine is never present as porphyroblastic grains because of its breakdown in response to pressure increase. Sapphirine symplectites can be seen in the relatively low bulk-XMG rocks, and the texture suggests local equilibrium in the Mg-Al-rich domain that is formed by metamorphic segregation of the Mg-Al-rich minerals in response to partial melting. The various sapphirine occurrences observed in South Harris were controlled by not only protolith composition but also local, mineral-scale, composition in a continuous metamorphic history.