Finger structures in the Rhum Complex

Finger-like protrusions of peridotite are developed on Rhum where peridotite is overlain by allivalite. These structures, which were described by Brown as ‘upward-growing pyroxene structures-, are found in the following environments: at the main intra-unit junctions; along the upper surface of subsidiary peridotites in certain allivalites; and along the lower surface of allivalite blocks in some peridotites. The structures generally take the form of parallel-sided or tapering protrusions with circular cross-sections. The tops of fingers are conical or hemispherical in shape. Typical dimensions are: finger amplitude, 2-5 cm; finger diameter, up to 3 cm; and finger wavelength, 5-10 cm. Peridotite in the finger is modally and texturally similar to the underlying layer, varieties range from feldspathic peridotite to dunitic peridotite. In the field the fingers apparently cut through layering, laminae and lamination without any associated disruption of the planar structures. Two contrasting mechanisms of formation are discussed: vertical deformation of crystal mushes, and metasomatic replacement. On balance, we prefer to interpret the fingers as evidence for the replacement of pre-existing allivalite by secondary peridotite. Replacement was achieved by pore magma from the underlying peridotite migrating upwards into the overlying allivalite, in response to compaction. This pore magma was able to resorb plagioclase but crystallize olivine and pyroxene in its place.