Cambir Dolerite - 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 4 Cambir Dolerite D
Keywords: mineral analyses, annealed textures, cooling history
At the north west end of the Cambir the Western Gabbro is intruded by sheets and veins of the fine-grained Cambir Dolerite. It is exposed on the Cambir cliffs facing the island of Soay, and also near sea level below Mullach Bi where south-westerly-dipping sheets of similar rock cut the Western Gabbro. On weathered surfaces small variations in grain size give the Dolerite a streaky appearance and altered poikilitic olivine crystals form vague bands of elongate black blotches. Contacts with the Western Gabbro are sharp and transgressive to the banding and lamination but variable in attitude and orientation; overall the Dolerite probably dips steeply south-east. Outcrops of Dolerite near the grass-mantled gully below and west of the Cambir summit are variable in grain size and in places distinction from the Western Gabbro is difficult. Near the top of the gully at about 500 ft (160 m) there is an isolated intrusion of medium-grained gabbro similar to some on Soay. Like the Western Gabbro, the Cambir Dolerite is intruded by coarse gabbroic and pegmatitic veins and affected by the same kinds of faulting and shearing which gave rise to the network of veins.
More than 90% of the Dolerite is made up of plagioclase, augite and opaque minerals, and the rest consists of olivine, orthopyroxene and a small quantity of alteration products. Most plagioclase grains are small and unzoned, ranging in composition from An50 to An68 in different rocks; some larger crystals are more calcic with complex zoning patterns and may be xenocrysts derived from the Western Gabbro. Clinopyroxene grains, making up about 40% of the rock, have a relatively constant composition of En38 Wo42 Fs20, and are generally less magnesian than the augite(s) of the Western Gabbro. the opaque grains are mostly magnetite with variable contents of either ilmenite lamellae or ulvospinel (Fe2TiO4) and spinel (MgFeA14O8) granules, and this is reflected in the TiO2 content which in some grains reaches 25 % . Irregularly shaped ilmenite grains occur in association with magnetite but are not common. Orthopyroxene grains, of smaller size and shape to the granular clinopyroxenes, are found in a few rocks, but this mineral occurs more commonly as poikilitic platy crystals enclosing plagioclase. Olivine also occurs in this form but not generally in the same rock and this mutually exclusive development of olivine in some rocks and orthopyroxene in others may be related to local temperature and oxidation conditions. Commonly, magnetite mantles the poikilitic olivine grains and there is some alteration to chlorite in the interior; fresh grains however range in composition from Fo63Fa37 to Fo53Fa47 (Figure 5). The sheets of dolerite below Mullach Bi have a similar mineralogy but they are coarser, with granular olivines and an overall intergranular texture. Parts of the Dolerite on the Cambir also are intergranular but the main mass is granulitic in texture, resembling a metamorphic rather than an igneous rock. This texture is characteristic of basic rocks that have been heated and maintained at a high enough temperature for sub-solidus recrystallisation (annealing) of the minerals to have occurred. Thus the sharp contacts and the transgressive nature of the veins of Cambir Dolerite in the Western Gabbro indicate that it was a liquid or a crystal mush when intruded, while the textural evidence suggests that much of the Dolerite has suffered a complex cooling history.
Although the chemical analyses of the dolerites on the Cambir (col. 1) and Mullach Bi (col. 2) are similar, the latter is slightly more magnesian and less silicic; there is some variation in the trace elements and it is possible that the example of dolerite from Mullach Bi is related more to the suite of gabbros and dolerites which form parts of Soay, Glacan Mor and Boreray. Certainly the range of sulphides found in this dolerite was greater than in the main Cambir dolerite and these include chalcopyrite, nickeliferous pyrite and a member of the linnaeite–violarite family (Ni1.8Co0.8Fe0.6)S4.
|Chemical analysis Western Gabbro|
|Major elements (Oxide, wt %)||1||2||Minor elements (ppm)||1||2|
|CaO||12.20||12.30||Rb||< 10||< 10|
|Analyses: 1 by A. N. Morigi, A. E. Davis and K. A. Holmes, 2 by R. R. Harding|
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