THE SHETLAND OPHIOLITE COMPLEX - FIELD EVIDENCE FOR THE INTRUSIVE EMPLACEMENT OF THE CUMULATE LAYERS

Unlike many other ophiolites, the lower-crustal sequence in the Shetla nd ophiolite has not been been formed in situ as a series of cumulate layers in the base of a magma chamber over-lying the harzburgitic mant le. Field evidence shows that the layers were intruded sequentially ab ove the mantle. Dunite rose as a fluid through mantle conduits, relies of which are preserved as invasive pods and sheets within the harzbur gite. It formed an intrusive layer several kilometres thick between th e mantle and an overlying banded wehrlite-clinopyroxenite layer, relie s of which are preserved in places in the roof of the dunite intrusion as partially assimilated xenoliths and screens. A similarly uniform a nd thick intrusive layer of gabbro was emplaced immediately above the dunite layer and contains xenoliths and screens of the wehrlite-clinop yroxenite layer in both its base and its roof. A mixed gabbro-microgab bro layer lies immediately above the gabbro and has been cut repeatedl y by parallel basic sheets giving the layer a quasi-sheeted-dyke appea rance. However, the sheets are more nearly parallel than normal to the gabbro and dunite layers and have been intruded from outside the expo sed layered sequence. The succession listed is lying on its side formi ng a nappe, in which both the layers and the sheets are vertical. It i s not clear whether the succession had been rotated into this position before or after the sheets were emplaced. It is suggested that the du nite in the mantle was formed in the manner proposed by Kelemen et al. in 1995, that is that MORB produced by adiabatic melting in the mantl e was undersaturated in orthopyroxene and reached the top of the mantl e by intergranular flow through the mantle during which the porosity a long the flow path was increased by dissolution of the pyroxenes, thus concentrating the flow into dunite conduits. Under these circumstance s the flow of liquid MORB through the solid dunite conduits, because o f increased porosity and decreased grain adhesion, could possibly lead to fluidization of the dunite in the conduits. The fluidized dunite w ould be forced upwards out of the mantle to form the overlying intrusi ve dunite layer.