On sapphirine-bearing rocks in the vicinity of Sukkertoppen (West Greenland)

Abstract

A sapphirine bearing complex is situated as a large inclusion in the Sukkertoppen granodioritic gneiss some 3-4 km north of the Sukkertoppen colony. The complex has, during the orogeny, undergone mechanical and chemical metamorphism together with the enclosing gneiss. Thereby the minerals of the troctolitic inclusion have been subjected to transitions which have developed hornblende, bronzite, and spinel on the cost of olivine and plagioclase. All the olivine is commonly altered, while bytownitic plagioclase exists in excess. In schlieren with primarily great concentration of olivine the plagioclase is completely altered giving rise to olivine and bronzite-rocks with smaller amounts of hornblende and spinel.
As the last reaction prior to the quenching of the rock before this was elevated too high during the erosion of the crust, some of the minerals interacted during the formation of sapphirine and phlogopite. Perhaps this process was combined with a small conveyance of Si and K from the surrounding gneiss.
During the whole alteration the complex was subjected to a certain metamorphic differentiation with migration of elements inside the complex so that great porphyroblasts of spinel, and different bands and schlieren appeared.
The mechanical deformation was essentially a plastic one, rupture is rarely found. But where fissures are encountered these are filled with quartz along which the Si-metasomatism has been comparativley intense altering the hornblende-spine! rocks into hypersthene, plagioclase and diopside.