Mica-feldspar equilibria in supercritical alkali chloride solutions

The relations between cordiente, hercynite and magnetite in specimens of the garnetiferous migmatites within the osumilite-in isograd from the high-grade polymetamorphic Precambrian of Rogaland, SW Norway, have been studied, using an electron microprobe. In most specimens the primary spinel of the M 2 stage of metamorphism is exsolved into hercynite and magnetite. The exsolution continued during subsequent cooling to moderate temperature conditions of the M 2 stage or even to retrogressive conditions of metamorphism of the M 4 stage. Fe-Mg distributions give evidence that cordierite and hercynite are in equilibrium at these mentioned conditions, while their textural relations belong to the high-grade metamorphic stage M 2. This implies that neither cordierite nor spinel are suitable for use in geothermometry of high-grade metamorphic stage M 2 in Rogaland. It is tentatively concluded that re-equilibration of Mg and Fe continues to lower retrograde temperatures in the Fe-rich cordierite-hercynite pairs than in Mg-rich pairs.