THE DEVELOPMENT OF A MARGARITE CORUNDUM BLACKWALL BY METASOMATIC ALTERATION OF A SLICE OF MICA SCHIST IN ULTRAMAFIC ROCK, KVESJOEN, NORWEGIAN CALEDONIDES

Abstract

An unusual occurrence of blackwall rock occurs on a promontory in Lake Kvesjoen, in the Norwegian Caledonides, at the contact between a lens of ultramafic rock (Fo‐En-rock) 200 m in diameter and amphibolite-facies metapelitic country-rocks. The blackwall contains corundum, staurolite, tourmaline and four micas: muscovite, paragonite, margarite and biotite. Accessories include allanite‐epidote, apatite, ilmenite, rutile, monazite, and zircon. The blackwall evolved from a normal quartz-rich staurolite‐garnet‐biotite metapelite by progressive loss of SiO2 to the silica-deficient Fo-rich ultramafic rock at about 570°C and 6.5 kbar. Thermodynamic modeling of the metasomatic process predicts first the disappearance of garnet and excess quartz, and then the production of corundum from excess Al2O3. Subsequent desilication of plagioclase then results in the formation of paragonite and margarite in addition to the “primary” muscovite and biotite. Abundant development of biotite at the expense of staurolite and muscovite at a late stage of the process leads to the observed dark biotite-rich corundum‐margarite rock. The model process predicts the observed blackwall mineralogy and also suggests that the corundum‐margarite rock represents a mature endproduct of the metasomatic reaction. The SiO2 lost by the metapelite reacted with forsterite and enstatite of the ultramafic rock to form talc. The chemical potential of SiO2 imposed by the talc‐forsterite assemblage was much lower than that given by quartz saturation in the metapelite under the prevailing conditions during the active process. The resulting gradient in chemical potential was the prime driving force in the formation of the blackwall.