Olivines from the Skaergaard intrusion, Kangerdlugssuak, East Greenland

Four olivines separated from the rocks of the Skaergaard intrusion, Kangerdlugssuak, East Greenland, have been analysed. The olivines occur in a layered gabbro intrusion which shows continuous crystal-lisation-differentiation from a lower and earlier hypersthene-olivine-gab-bro to an upper and later fayalite-quartz-gabbro. Two of the three main constituents of this differentiation series, namely plagioclase and monoclinic pyroxene, show a continuous variation in composition, the plagioclase from medium labradorite to oligoclase-andesine, and the monoclinic pyroxene from a diopside-augite to a member of the heden-bergite-clinoferrosilite solid solution series Hypersthene is also an important constituent of the earlier rocks of the intrusion and varies from Fs₄₁ in the lowest exposed rocks to at least Fs₆₀ in rock 2000 m. up; above this it does not occur as separate crystals. Olivine is not found throughout the intrusion being absent for 500 metres at about the middle of the layered series. The olivines in the lower and earlier part of the layered series, of which I is an example, are in equilibrium with labradorite, a common type of pyroxene and hypersthene with about 40% FeSiO₃. The olivines of the second crystallisation period, of which II, III and IV are examples, are in equilibrium with andesine, an unusual iron-rich pyroxene, which we have already described, and quartz. In these upper rocks the iron-rich olivine was among the later minerals to crystallise, while in the lower and earlier rocks of the intrusion the more magnesian olivine, as is usual, was an early mineral. Except close to the olivine free horizon the amount of modal olivine in the average rock is never less than 10% and rarely greater than 20%.