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The Shiant Isles Main Sill: structure and mineral fractionation trends

  • F. G. F. Gibb (a1) and C. M. B. Henderson (a2)


The Shiant Isles Main Sill, of Tertiary age, is a classic example of a composite, differentiated alkaline basic sill. The first unit to be intruded was a 2 m thick olivine teschenite which was emplaced with phenocrysts of olivine (mg > 83) [mg ≡ Mg#] and, perhaps, plagioclase. This was intruded by a 24 m thick picrite sill consisting of a mush of melt and suspended olivine phenocrysts (mg > 83) with a D-shaped modal profile. The 140 m thick picrodolerite-crinanite unit was formed by a magma carrying ∼ 10% olivine (mg > 83) as the main phenocryst phase, together with some calcic plagioclase phenocrysts, being emplaced into the top of the picrite unit before the host rock was completely solidified. The olivine phenocrysts settled towards the bottom to form the picrodolerites. In-situ differentiation processes occurred under conditions of almost perfect fractional crystallization, during which very strongly zoned ophitic crystals of olivine (fayalitic rims) and clinopyroxene (hedenbergitic rims), and zoned laths of plagioclase (anorthoclase rims), formed. The last unit consists of ∼ 2 m of granular olivine picrodolerite which was intruded into the upper crinanites, again before the host rock was fully solid.

The mineral zoning patterns are interpreted using published cation diffusion coefficient data, and used to show that the picrite unit might have cooled to the blocking temperatures for Mg and Fe diffusion in < 5 years, and that even the relatively thick crinanite unit cooled very fast, so preserving the zoned Fe-Mg olivine and pyroxene compositions. The compositions of coexisting ilmenites and spinels define a redox trend which initially lies close to fayalite-magnetite-quartz buffer conditions, but later becomes more reducing and approaches magnetite-wustite buffer conditions. The final stages of development occurred during sub-solidus deuteric processes and involved formation of analcime and zeolites, as well as localized sulphide mineralization.



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Buddington, A.F. and Lindsley, D.H. (1964) Iron- titanium oxides and synthetic equivalents. J. Petrol., 5, 310–57.
Deer, W.A., Howie, R.A. and Zussman, J. (1978) RockForming Minerals: Vol 2a, Single-Chain Silicates. Longman, London and New York, 668 pp.
Dickin, A.P., Henderson, C.M. and Gibb, F.G. (1984) Hydrothermal Sr contamination of the Dippin sill, Isle of Arran, Western Scotland. Mineral. Mag., 48, 31122.
Drever, H.I. and Johnston, R. (1959) The lower margin of the Shiant Isles Sill. Q. J. Geoi Soc. London, 114, 343–65.
Drever, H.I. and Johnston, R. (1965) New petrographical data on the Shiant Isles picrite. Mineral. Mag., 34, 194–203.
Gamble, J.A. (1984) Petrology and geochemistry of diiferentiated teschenite intrusions from the Hunter Valley, New South Wales, Australia. (1984) Contrib. Mineral. Petrol. 88, 173—87.
Gibb, F.G.F. (1973) The zoned clinopyroxenes of the Shiant Isles Sill, Scotland. J. Petrol, 14, 203–30.
Gibb, F.G. and Gibson, S.A. (1989) The Little Minch sill complex. Scott. J. Geoi, 25, 367–70.
Gibb, F.G. and Henderson, C.M. (1978) The petrology of the Dippin sill, Isle of Arran. Scott. J. GeoL, 14, 1–27.
Gibb, F.G. and Henderson, C.M. (1984) The structure of the Shiant Isles sill complex, Outer Hebrides. Scott. J. Geoi 20, 21—29.
Gibb, F.G. and Henderson, C.M. (1989) Discontinuities between picritic and crinanitic units in the Shiant Isles sill: evidence of multiple intrusion. Geoi. Mag., 126, 127–37.
Gibb, F.G. and Henderson, C.M. (1992) Convection and crystal settling in sills. Contrib. Min. Petrol, 109, 538–45.
Gibson, S.A. and Jones, A.P. (1991) Igneous stratigraphy and internal structure of the Little Minch Sill Complex, Trotternish Peninsula, northern Skye, Scotland. Geoi Mag., 128, 51–66.
Helz, R.T. (1982) Experimental studies of amphibole stability. Reviews in Mineralogy (Mineralogical Society of America, Washington), 9B, 279—353.
Henderson, C.M. and Gibb, F.G. (1983) Felsic mineral crystallization trends in differentiating alkaline basic magmas. Contrib. Min. Petrol., 84, 355–64.
Henderson, C.M. and Gibb, F.G. (1987) The petrology of the Lugar Sill, SW Scotland. Trans. Roy. Soc. Edinbugh: Earth Sciences, 77 (for 1986), 32547.
Huppert, H.E. and Sparks, R.S. (1980) The fluid dynamics of a basaltic magma chamber replenished by influx of hot dense ultrabasic magma. Contrib. Mineral. Petrol., 75, 279–89.
Jaoul, O., Sautter, V. and Abel, F. (1991) Nuclear microanalysis: A powerful tool for measuring low atomic diffusivity with mineralogical applications. In: Diffusion, atomic ordering, and mass transport (Ganguly, J., ed.), Advances in Physical Geochemistry, 8, 198–220, Springer, Dordrecht.
Johnston, R. (1953) The olivines of the Garbh Eilean sill, Shiant Isles. Geoi Mag., 90, 161–71.
Kitchen, D.E. (1985) The parental magma on Rhum — Evidence from alkaline segregations and veins in peridotites from Salisbury Dam. Geoi Mag., 122, 529–37.
Kohn, S.C., Henderson, C.M. and Mason, R.A. (1989) Element zoning trends in olivine phenocrysts from a supposed primary high-magnesian andesite: an electron– and ion–microprobe study. Contrib. Mineral. Petrol, 103, 242–52.
Kramer, M.J. and Seifert, K.E. (1991) Strain enhanced diffusion in feldspars. In: Diffusion, atomic ordering, and mass transport(J. Ganguly, ed.), Advances in Physical Geochemistry, 8, 286–303, Springer, Dordrecht.
MacKenzie, W.S., Donaldson, C.H. and Guilford, C. (1982) Atlas of Igneous Rocks and their Textures. 148 pp. Longmans, Harlow, England.
Marsh, B.D. (1988) Crystal capture, sorting and retention in convecting magma. Geoi. Soc. Amer. Bull., 100, 1720–37.
Marsh, B.D. (1989) On convective style and vigor in sheet-like magma chambers. J. Petrol, 30, 479–530.
Marsh, B. (1996) Solidification fronts and magmatic evolution. Mineral Mag., 60, 5–40 (this volume).
Martin, D., Griffiths, R.W. and Campbell, I.H. (1987) Compositional and thermal convection in magma chambers. Contrib. Mineral. Petrol., 96, 465–75.
Miyamoto, M. and Takeda, H. (1983) Atomic diffusion coefficients calculated for transition metals in olivine. Nature, 303, 602–3.
Murray, R.J. (1954) The clinopyroxenes of the Garbh Eilean sill, Shiant Isles. Geoi Mag., 91, 17–31.
Nash, W.P. and Wilkinson, J.F. (1970) Shonkin Sag laccolith, Montana, I. Mafic minerals and estimates of temperature, pressure, oxygen fugacity and silica activity. Contrib. Mineral. Petrol., 25, 241–69.
Powell, R. and Powell, M. (1977) Geothermometry and oxygen barometry using coexisting iron-titanium oxides: a reappraisal. Mineral Mag., 41, 257–63.
Sparks, R.S., Huppert, H.H. and Turner, J.S. (1984) The fluid dynamics of evolving magma chambers. Phil. Trans. R. Soc. London A310, 511—34.
Sparks, R.S., Huppert, H.E., Koyaguchi, T. and Hallworth, M.A. (1993) Origin of modal and rhythmic igneous layering by sedimentation in a convecting magma chamber. Nature, 361, 246–9.
Tegner, C., Robins, B. and S0rensen, H.S. (1996) Crystallization from stratified magmas in the Honningsvag Intrusive Suite, Northern Norway: a reappraisal. Mineral. Mag., 60, 41–51 (this volume).
Wager, L.R. and Brown, G.M. (1967) Layered Igneous Rocks Oliver and Boyd, Edinburgh and London, 588 pp.
Walker, F. (1930) The geology of the Shiant Isles (Hebrides). Q. J. Geol Soc. London, 86, 355–98.
Wilkinson, J.F. (1956) Clinopyroxenes of alkali olivine basalt magma. Amer. Mineral., 41, 724–43.
Wilkinson, J.F. (1979) The mineralogy and petrography of alkali basaltic rocks. In: The Alkaline Rocks (Sørensen, H., ed.), Wiley Interscience, London, 67—95
Wilkinson, J.F. and Hensel, H.D. (1994) Nephelines and analcimes in some igneous rocks. Contrib. Mineral. Petrol., 118, 79–91,


The Shiant Isles Main Sill: structure and mineral fractionation trends

  • F. G. F. Gibb (a1) and C. M. B. Henderson (a2)


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