The Eqaloqarfia layered dyke, Nunarssuit, South Greenland

Abstract

The layering in the Eqaloqarfia dyke defines a synform running parallel to the length of the dyke, with a maximum inward dip of the limbs of 36°. The layering is developed in the lower part of a trough of feldspar-phyric gabbro in an otherwise normal dolerite. Homogeneous dolerite both underlies and forms a border to the gabbro. A perpendicular feldspar layer occurs in places at the border between the dolerite and the gabbro: in this layer unzoned plagioclase laths have grown at steep angles to the margin of the dolerite. The gabbro is characterised by platy feldspars up to 2 cm wide which are set in a doleritic matrix. The cores of the plates have a composition in the range An_57-64 and constitute an early generation of plagioclase which crystallised at depth. The second generation of plagioclase, forming the shells of these plates and the smaller crystals in the matrix, began crystallising with a composition An_69-75, zoning down to An_30-40 at the margins of crystals. The layering comprises olivine-rich horizons alternating with layers of the feldspar-phyric gabbro. There is no graded bedding in the olivine-rich layers, nor is there any lamination or packing of the feldspar plates in the gabbro. Intermittent settling of olivine during the early slow crystallisation in the gabbro trough is regarded as the cause of the layering. A slight cryptic variation is shown by the second generation of plagioclase. In view of the lack of evidence for either magmatic currents or feldspar settling, it is thought that diffusion can contribute more to differentiation than is usually allowed. Diffusion is also required to explain the lack of zoning in the plagioclase laths of the perpendicular feldspar layer.