Geikielite and ilmenite in Archaean meta-ultramafic rocks, Fiskenaesset, West Greenland
Geikielite and ilmenite in Archaean meta-ultramafic rocks, Fiskenaesset, West Greenland
European Journal of Mineralogy (June 1989) 1 (3): 427-437
Geikielite and ilmenite occur in 10-15 m thick lenses of ultramafic rocks along the top contact of the Fiskenaesset complex in the Angnertussoq area of W Greenland with an overlying meta- volcanic amphibolite. Several mineral assemblages reflect variations in original bulk X (sub Mg) , the most magnesian ore consists of forsterite (X (sub Mg) 0.99) and spinel (X (sub Mg) 0.92) with minor diopside, clintonite, chlorite, carbonate, titanite, rutile and geikielite (X (sub Mg) 0.75). Where hornblende substitutes for diopside, ilmenite has X (sub Mg) 0.40. Ilmenite has the lowest X (sub Mg) (0.20) where enstatite occurs instead of forsterite. All geikielite/ilmenite has <2 mol % MnTiO (sub 3) . The mineral assemblages of these rocks were formed during sillimanite- grade metamorphism in the amphibolite facies. However, the more Mg- rich geikielite/ilmenite occurring as inclusions in large forsterite porphyroclasts may represent high-T relics, either of the early metamorphic evolution or from a mantle stage.