Geological investigations in East Greenland. Part VII. The Basistoppen sheet. A differentiated basic intrusion into the upper part of the Skaergaard Complex, East Greenland

Abstract

The Basistoppen sheet is an extensive differentiated gabbroic mass, about 400-500 m. thick, occurring within the upper bord er rocks of the Skaergaard intrusion. Originally, it was considered as a huge inclusion because the underlying Skaergaard rocks vein its base (WAGER and DEER, 1939). It is now considered as resulting from a later injection of basic magma into the still hot, but consolidated, Skaergaard rocks. Along its lower margin there are fine grained rocks which are interpreted as partial chills. Veining of the base of the sheet by the underlying Skaergaard rocks is believed to be back-veining, due to their rheomorphism by heat from the sheet.

The sheet, much of which has been previously described by HUGHES (1956), displays marked cryptic layering, both the continuous and discontinuous (phase change) types. On the basis of the phase change cryptic layering, it has been divided into a gabbro-picrite zone and successive zones of bronzite gabbro, pigeonite gabbro and pyroxene ferrodiorite. A small part of the sequence is missing because the upper contact has not been reached.

Close analogy between the sheet and the Skaergaard intrusion is found in the sequence of mineral phases, strong iron enrichment and in the behaviour of certain trace elements. Particularly slow cooling apparently allowed a high degree of fractionation to occur.