Conachair Granite - 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 13 Conachair Granite
Keywords: leucocratic drusy rock, textures, chevkinite, chemical analysis
Much of the eastern portion of Hirta, about one third of its area, is occupied by the youngest of the major intrusions comprising the St Kilda Tertiary igneous complex. The Conachair Granite(I) is a cream-coloured medium-grained leucogranite, containing scattered druses vapour cavities which formed as the granite magma cooled and consolidated. Glassy or milky quartz crystals and pale creamy-yellow grains of feldspar are the principal mineral constituents, and both minerals can be found with perfect crystal terminations where they project into the druses. Conachair granite is usually the first rock encountered by visitors to Hirta, for blocks of it have been used in the construction of the jetty and natural outcrops flank the slipway to the jetty. It has also been extensively used in the building of cottages, cleits and walls in the Village Bay area. Above the village, the granite forms the prominent outcrops of Glacan Chonachair, Oiseval and, of course, Conachair. The spectacular cliffs below the Gap, Conachair and Oiseval are also mostly granite, as are the rocky islets of Bradastac and Mina Stac and many smaller islets nearby. The slabby or blocky appearance of granite outcrops is due to the pattern of joints. These are vertical and sub-horizontal planar surfaces which develop as the rock contracts after cooling; weathering tends to enhance the joint pattern.
The granite was intruded into dolerites and microgranites of the Mullach Sgar Complex along a western junction which dips at 30°–70° to the WSW. In the Village Bay this junction is obscured by Quaternary deposits, but it can be followed up the slopes of Glacan Chonachair and, after disappearing in boggy ground, is again found in the cliffs above Bradastac. Contact between granite and gabbro' is seen in crags 300 m bearing 293° from Conachair summit. From this point, at a height of about 300 m, the junction can be followed down the cliffs in a north-westerly direction to about the 200 m contour. It is again seen at the water line in cliffs overhanging Na Cleitean, where medium-grained granite is in contact with mixed acid/basic rocks of the Mullach Sgar Complex. To the south and south-east the submarine extension of the granite may terminate against NW–SE faults, but in any case cannot extend beyond the arcuate exposures of the Western Gabbro represented by Levenish and submarine outcrops of Ew or EK (pp. 34–35). North-eastwards, the occurrence of gabbroic rocks forming submarine outcrops 2.25 km SSW of Stac Lee limits the thickness of this sheet-like granitic intrusion to a maximum of 4 km for an overall south-westerly dip of 50°.
Thin section petrography shows that the granite consists largely of quartz, orthoclase and low-albite, forming respectively 25–30%, 40–50% and 25–30% of the rock. All three minerals are commonly closely intergrown, orthoclase and albite forming a microperthitic feldspar which itself may contain granophyric intergrowths of quartz (Figure 23B), (Figure 23C)). Other minerals occur in accessory amounts only and are commonly found in and adjacent to drusy cavities; they include calcic-amphibole, chlorite replacing amphibole, biotite, titaniferous-magnetite, zircon, rutile, fluorite, sphene, anatase, and chevkinite (Figure 23B). The typical granite of Oiseval and Conachair has a microgranitic texture with up to 20% of granophyre, and consists mostly of anhedral perthite grains, up to 5 mm across, enclosing unstrained, subhedral quartz grains up to 3 mm across. Quartz grains are commonly embayed and serrated relics of stubby bipyramids, indicating that they are inverted β -quartz phenocrysts that have suffered magmatic corrosion (Figure 23A). Granophyric quartz usually forms an optically continuous systems of blebs, stems and wedges whose domain is controlled by the enclosing perthite grain. Finely vermicular quartz characterises interstitial granophyre moulded onto perthite grains or forming outgrowths on corroded quartz phenocrysts. Tablets of lamellar-twinned plagioclase are enclosed in some perthite grains, often occurring as ghost euhedra overgrown and replaced by perthite, and occasionally mantled by vermicular quartz. Late albite An6Ab94 to An8Ab92 occurs as clear rims on perthite grains bordering druses and also as discrete euhedra within the cavities.
No evidence of chilling is seen at the margins of the granite. Contacts with gabbro to the west are sharp and often show microgranite-filled fissures extending 1–30 cm into the country rock. Within 2–3 mm of contacts, metasomatic alteration of the gabbro has resulted in alkali-feldspar overgrowths on plagioclase and replacement of pyroxene by amphibole and biotite. An intrusive contact with microgranite of the Mullach Sgar Complex has led to coarse granophyric intergrowths in the Conachair granite, which develop an elongate habit, 10–15 mm long, perpendicular to the junction (Figure 23C). Passing inwards, the granophyre becomes patchy and equant at 20 mm from the contact, and at 30 mm or more becomes finer-grained and microgranitic in texture. Such contact textures suggest that the country rocks (MSC and EK) may have been hot at the time of intrusion.
Thin veins (up to 15 cm) of pale grey rhyolite and aplite cut the granite and possibly represent fissure-fillings, associated with the episode of late sheet intrusion (p. 24). The rhyolite veins show corroded phenocrysts of inverted β -quartz and perthitic alkali feldspar in a microcrystalline felsitic groundmass; tiny xenoliths of granophyre may also be present. Some veins show flow alignment of groundmass microlites; others display a slight increase in grain size at the margins of the veins, indicating limited annealing, of the chilled groundmass against hot, fissured microgranite. Aplitic veins consist of a saccharoidal intergrowth of quartz and alkali-feldspar with a grain size of 0.2–0.5 mm. Quartz-feldspar grain boundaries are mostly straight and triple-junctions approximate to angles of 120° (Figure 23D). A few larger (up to 1.2 mm) perthite grains enclose blebs of quartz, reminiscent of the granophyre xenoliths noted above. The equilibrium textures of the aplites suggest solid-state recrystallisation, possibly the result of prolonged annealing of rhyolitic vein-filling in hot microgranite.
|Chemical analyses: Conachair Granite
|Major elements (Oxide, wt%)
|* n.r. not reported.
Chemically the Conachair granite shows many similarities with other Tertiary acid intrusions (Bell, 1982). Typically it is more siliceous (mean 76.2% (Table 23)) and alkaline (mean Na2O + K2O = 8.58%), but less aluminous (mean 12.33%), magnesian (mean 0.20%) and calcic (mean 0.31%) than Le Maitre's (1976) average granite, and indeed more closely approaches his average rhyolite. All the analysed rocks are peraluminous and are sparingly corundum-normative. Low normative anorthite (0.13–4.8) and a colour index ranging from 1.2–5.2, emphasise the alkaline and extreme leucocratic nature of the granite. In terms of the synthetic system Ab–Or–Q it plots close to the thermal minimum at 1 kbar PH2O. Such a composition could have arisen from either extreme fractionation or fusion of sialic crust. Chemically the latter is unlikely because the initial 87Sr/86Sr ratio is low (p. 40), and the REE distribution (p. 43) is inconsistent with fusion of Torridonian or Lewisian basement rocks (Thompson, 1983). The geochemical evidence suggests that the Conachair Granite resulted from fractionation of a mafic magma.
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