On sapphirine from West Greenland

Abstract

Two new occurrences of sapphirine from West Greenland are described. In the first of them (from Sukkertoppen District) sapphirine is present as rims of reaction around grains of corundum in patches of anorthite in small lenses of hornblendite. The hornblendites are found in a hydrid zone around a bronzititic ultrabasite. The second occurrence of sapphirine is a boulder of hornblendite found in the Qorqut branch of the Godthaab Fiord. Sapphirine is here found in a similar way as in the first-mentioned occurrence, namely as rims between corundum/spinel and anorthite. New observations from the Fiskenæsset occurrence are published. It is shown that the sapphirine-bearing rock has been formed by retrograde metamorphism of a spinel-bearing bronzitite. Sapphirine is formed at the expense of spinel ; gedrite, hornblende and phlogopite at the expense of the bronzite. Corundum has also been found in this occurrence. Irregular replacement veins are found in the sapphirine-bearing rock. They consist of basic plagioclase, hornblende, biotite and cordierite. It is suggested that this cordierite (and possibly the kornerupine) has been formed at the expense of the sapphirine. The occurrences of sapphirine at Sukkertoppen and at Auvaitsersarfik are briefly mentioned. Corundum in ultrabasic rocks is discussed. The parageneses with sapphirine mentioned in the literature are reviewed and the stability relations of sapphirine are discussed. It is concluded that sapphirine most often appears to be formed at the expense of spinel and corundum and that it often is associated with ultrabasic rocks. Sapphirine is regarded to be formed where concentrations of Al in a milieu rich in Mg and poor in Si have been metamorphosed in a "closed system" under conditions of P,T corresponding to high amphibolite low granulite facies.