Two Stages of Sapphirine Formation During Prograde and Retrograde Metamorphism in the Palaeoproterozoic Lewisian Complex in South Harris, NW Scotland

Two types of sapphirine occurrences were found in the Lewisiancomplex in South Harris, NW Scotland: (1) inclusions withinporphyroblasts; (2) symplectic grains together with secondarycordierite, plagioclase and orthopyroxene. The presence of sapphirineinclusions implies that sapphirine was stable at the early stageof ultra-high-temperature metamorphism, whereas symplectic sapphirinegrains were formed during decompressional retrograde metamorphism.The sapphirine occurrences and compositions of associated mineralsdepend on the host rock composition. Sapphirine inclusions occuronly in rocks with high bulk-X_Mg, and sapphirine is never presentas porphyroblastic grains because of its breakdown in responseto pressure increase. Sapphirine symplectites can be seen inthe relatively low bulk-X_Mg rocks, and the texture suggestslocal equilibrium in the Mg–Al-rich domain that is formedby metamorphic segregation of the Mg–Al-rich mineralsin response to partial melting. The various sapphirine occurrencesobserved in South Harris were controlled by not only protolithcomposition but also local, mineral-scale, composition in acontinuous metamorphic history. KEY WORDS: sapphirine; Lewisian complex; high-Mg garnet; partial melting