LAMINATED OLIVINE SPINIFEX TEXTURES IN THE MOUNT-AYLIFF INTRUSION, SOUTH-AFRICA

A thin layer of spinifex troctolite occurs in the middle of the lherzo lite unit of the Mount Ayliff Intrusion, South Africa. Olivine crystal s reach 1 cm in length, have an aspect ratio up to 20, and lie paralle l to the contact. This layer is considered to be intrusive into a pre- existing package of cumulates rather than to represent a layer which f ormed at the liquid-crystal interface of the magma chamber. The crysta ls define a planar, not linear, fabric, which is interpreted to indica te that the elongated crystals grew in situ and were not concentrated by flow differentiation as this would produce a linear fabric. It is s uggested that this texture formed as a result of crystallization under conditions of relatively slow cooling and minimal supercooling. Under these conditions growth rate is high, whilst nucleation rate is low, producing few large crystals. Perpendicular growth is inhibited under these conditions by the absence of a supercooled zone within the bound ary layer. Low concentrations of incompatible elements in this rock in dicate that chemically it is a cumulate. This composition was produced as a result of migration of the residual liquid away from the olivine -plagioclase crystal mush, rather than by mechanical accumulation of t he olivine and plagioclase.