St Kilda Central Complex, Hebridean Igneous Province
| Emeleus, C H, and Bell, B R. 2005. British regional geology: The Palaeogene volcanic districts of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey. |
Introduction
The St Kilda archipelago comprises a group of precipitous islets consisting entirely of igneous rocks that form a central complex close to the edge of the continental margin (Harding, 1966, 1967; Harding et al., 1984; Stoker et al., 1993; P914119, I). Although no Pre-Paleocene rocks are found above sea level, Lewisian gneisses crop out on the surrounding sea bed. Detrital minerals characteristic of regional and thermal metamorphic rocks occur in stream sediments on Hirta, washed out of drift deposits (Chapter 13). The central complex is the site of a strong positive Bouguer gravity anomaly and the igneous rocks are reversely magnetised, most likely during the magnetic interval C26R. The principal intrusive units comprising the varied geology of Hirta, the largest island, are described below in order of their emplacement (P914138).
The Western Gabbro is a texturally variable olivine-gabbro. An equigranular variant exhibits streaky banding which reflects variation in modal mineral concentrations. A second type is characterised by large black augite crystals (about 2 cm across) containing numerous small white plagioclase crystals, which impart a characteristic speckled appearance to weathered surfaces. A third variety has a particularly marked textural range; in addition to containing elements of the first two types, there are pyroxene-rich and porphyritic facies and the rocks are commonly layered due to variation in both texture and mineral proportions. The dip of this layering, while locally variable, is general easterly to north-easterly, at about 45°. These gabbros may originally have formed part of a large layered intrusion. Gabbro identical to some of the variants in the Western Gabbro occurs (with other gabbros) as blocks in the extensive breccias described below, which therefore postdate the Western Gabbro. On the north end of Hirta, a concentration of fine-grained, olivine–dolerite sheets intruding the Western Gabbro is termed the Cambir Dolerite.
Breccias of gabbro and dolerite crop out at Glacan Mor on Hirta, and on Boreray and Soay; they also form numerous islets and stacks. These spectacular breccias are made of large blocks of coarse-grained gabbro, commonly sheared, which are fragmented, veined and enclosed by dolerite. Similar brecciated gabbros have been recovered from the sea bed between the main islands.
The Glen Bay Gabbro is an olivine-gabbro that crops out either side of Glen Bay and differs chemically from the older Western Gabbro. The age relationship is established by coarse-grained gabbroic veins, most likely derived from the Glen Bay Gabbro, which cut sheared parts of the Western Gabbro. On the eastern side of Glen Bay, the Glen Bay Gabbro has a black, splintery chilled margin against the brecciated gabbros and dolerites of Glacan Mor, in which glass extends for about 10 mm from the contact.
The Glen Bay Granite is medium grained, granophyric in places, and contains microphenocrysts of zoned plagioclase. Ferroaugite and ferropigeonite are variably replaced by amphibole. The rare–earth-bearing, titanium-rich silicate chevkinite is an accessory mineral. The granite is fine grained (probably chilled) adjacent to the Glen Bay Gabbro, and both have been affected by shearing.
The Mullach Sgar Intrusion-complex is one of the most distinctive rock assemblages in the St Kilda Central Complex. It comprises an extremely complicated assemblage of microgranite, microdiorite and dolerite sheets, dykes and igneous breccias. At least four generations of dolerite and microgranite intrusion are recognised and there is abundant evidence for the co-existence and mixing of silicic and basic magmas. Some of the most spectacular examples of this mixing phenomenon in the Hebridean Igneous Province are provided by the pillow-like masses of dolerite and basalt enclosed by microgranite, against which the basic rocks have developed fine grained, glassy selvedges. Members of the Mullach Sgar Intrusion-complex intrude the Western Gabbro and enclose blocks derived from it; similar material intrudes the Glen Bay Granite and the gabbro and dolerite breccias in north Hirta.
The Conachair Granite is the youngest major intrusion on Hirta. It forms the peninsula containing the hills of Conachair and Oiseval. The granite is a somewhat drusy, leucocratic, medium-grained rock, and its exposures have a distinctive slabby appearance caused by prominent horizontal and vertical jointing. The rock consists largely of intergrown quartz and microperthitic alkali feldspar and albite, with small amounts of calcic amphibole and biotite. Chevkinite, sphene and zircon are accessory minerals. The granite is in sharp, but not noticably chilled contact with the Mullach Sgar Intrusion-complex and with the gabbro and dolerite breccias, both of which are altered near the contacts.
Numerous sheets and dykes of basic and silicic composition, and composite (basic—silicic) sheets cut all the plutonic rocks, being most conspicuous in the pale-weathering cliffs formed of Conachair Granite. The dykes and inclined sheets appear to converge on a common focus at depth, east of Hirta.