Western Gabbro - St. Kilda: an illustrated account of the geology
|From: Harding, R.R. and Nancarrow, P.H.A. 1984. St. Kilda: an illustrated account of the geology. BGS Report Vol. 16, No. 7. Keyworth: British Geological Survey.].|
Chapter 2 Western Gabbro: EW Field description
Keywords: banding, cumulate, alteration
The western cliffs of Hirta and Dun consist largely of dark gabbroic rocks (the Western Gabbro), only superficially weathered but massively jointed and faulted into a spectacular landscape. They are the oldest rocks exposed in the St Kilda island group and are cut by offshoots of major intrusions on the Cambir (by the Cambir Dolerite), in Gleann Mor (by the Glen Bay Gabbro and Mullach Sgar Complex), and on Ruaival (by the Mullach Sgar Complex). In addition there are numerous minor intrusions of dolerite and felsite in the form of inclined sheets and dykes. The western limit of the Western Gabbro lies under the sea and its form is not known but the eastern limit lies north-south in Gleann Mor and northwest-south-east on Ruaival and Dun. The present thickness of the gabbro is more than 360 m but the easterly dip of the banding on the Cambir and the northeasterly dip on Dun indicate that it may be a remnant of a much bigger saucer-shaped intrusion whose centre lay between Hirta and Boreray.
The Western Gabbro is made up of coarse-grained rocks consisting essentially of plagioclase feldspar, calcium-rich pyroxene and olivine, with minor amounts (< 5%) of orthopyroxene, magnetite, ilmenite, amphibole, chlorite or spinels. The definition of this rock as a gabbro (as with the naming of other igneous rocks on St Kilda) follows the scheme proposed by the LUGS Subcommission and described by Streckeisen (1976). Different mineral proportions and different mineral textures, which are visible particularly on weathered slabs, have enabled division of the gabbros into 3 types. Type 1 occurs on the Cambir, lower slopes of Mullach Bi and south east Dun, and consists of grains of plagioclase, pyroxene and olivine of sub-rounded or sub-angular shape up to 1 cm across in relatively consistent proportions, although some feldspar-rich or pyroxene-rich bands are present (Figure 3B). Type 2 gabbro lies above Type 1 on Dun and south of Mullach Bi and constitutes the higher parts of Ruaival. it is characterised by black pyroxene grains up to 3 cm across with inclusions of small white plagioclase crystals, the contrast creating an overall speckled appearance on the weathered surface (Figure 3C). Rounded olivine grains are of similar size to the feldspar in this rock and weather honey brown. The third group of gabbros (Type 3) contains many textural and structural varieties (Figure 3A), (Figure 3D) and it is this variability that distinguishes it from Types 1 and 2. The best exposures are on the upper slopes of Mullach Bi where fine granulitic and coarse pegmatitic rocks are interbanded with granular and poikilitic gabbros similar to Types 1 and 2.
The attitude of the boundary between the gabbros of Types 1 and 2 is indistinct and gradational but broadly coplanar with the sporadic banding and layering which dips at about 45° to the east on the Cambir and to the north-east on Dun. The transition between Types 1 and 2 and Type 3 is also gradational but in the latter the character and attitude of the banding is more disturbed. In Type 3 gabbros, quite different textural types are in close proximity, and many structures visible in the cliffs between Mullach Bi and Claigeann Mor resemble those in sedimentary rocks. Size grading, mineral grading, detached blocks, intraformational slumping and faulting and local breccias are all features visible in this part of the Western Gabbro.
The disposition of the types of gabbro and the smaller scale banding and textural features which they display indicate that the Western Gabbro was formed mainly by crystal accumulation. The consistent textures of Types 1 and 2 suggest that tranquil conditions prevailed during their formation but the variability in Type 3 gabbros indicates rapid (if small) changes in magma composition or in pressure and temperature conditions, with associated differential movement of liquids, mushes and even blocks of gabbro. The layering and textural variation in the Western Gabbro is mainly the result of concentrations of primary or cumulus minerals. This is reflected in the chemical composition of the main rock types which do not correspond to known basaltic lavas. It seems likely that the concentration of variable amounts of olivine, pyroxene or plagioclase was effected firstly by mechanical crystal-sorting during magma flow and intrusion, and secondly by nucleation-diffusion during solidification of the basaltic magma. Both processes probably operated to different degrees in formation of the three gabbro Types: the structures and textures in Type 3 gabbros indicate that crystal-sorting was a dominant factor in their formation, in Type 2 the bulk composition and mineral textures suggest that crystal-sorting and diffusion were equally significant processes, while in Type 1 there is less evidence for either process.
In many parts of the Western Gabbro, gabbro pegmatites and narrow fine-grained green veins stand out on the weathered surfaces. Both features were formed after solidification of the gabbro, the former filling tension cracks and the latter occupying both tension and shear fractures. The shear veins are more abundant near the eastern margin of the gabbro and in places relative rotation of the banded rocks has occurred. At the top of Carn Mor, faulting is probably responsible for the close juxtaposition of shallow and steep banding in the Gabbro. Faults and tension joints with a south-west-north-east trend are preferential sites for erosion and are a major cause of the irregular shape of the west coast.
|Chemical analysis Western Gabbro|
|Major elements (Oxide, wt %)||Type 1||Type 2||Minor elements (ppm)||Type 1||Type 2|
|Analyses by A. N. Morigi, A. E. Davies and K. A. Holmes using β-probe and direct reading emission spectrometry|
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