Hostname: page-component-74656f9884-2dmjx Total loading time: 0 Render date: 2026-11-08T14:14:21.291Z Has data issue: false hasContentIssue false
  • English
  • Français

Finger structures in the Rhum Complex

Published online by Cambridge University Press:  01 May 2009

Alan R. Butcher ,
Iain M. Young  and
John W. Faithfull
Alan R. Butcher
Affiliation:
Department of Geology, University of Manchester, M13 9PL, U.K.
Iain M. Young
Affiliation:
Department of Geology, University of St Andrews, St Andrews, Fife, Scotland, U.K.
John W. Faithfull
Affiliation:
Department of Geological Sciences, University of Durham, Science Laboratories, Durham, DH1 3LE, U.K.
Get access Rights & Permissions [Opens in a new window]

Abstract

Finger-like protrusions of peridotite are developed on Rhum where peridotite is overlain by allivalite. These structures, which were described by Brown as ‘upward-growing pyroxene structures’, are found in the following environments: at the main intra-unit junctions; along the upper surface of subsidiary peridotites in certain allivalites; and along the lower surface of allivalite blocks in some peridotites.

The structures generally take the form of parallel-sided or tapering protrusions with circular cross-sections. The tops of fingers are conical or hemispherical in shape. Typical dimensions are: finger amplitude, 2–5 cm; finger diameter, up to 3 cm; and finger wavelength, 5–10 cm. Peridotite in the finger is modally and texturally similar to the underlying layer, varieties range from feldspathic peridotite to dunitic peridotite. In the field the fingers apparently cut through layering, laminae and lamination without any associated disruption of the planar structures.

Two contrasting mechanisms of formation are discussed: vertical deformation of crystal mushes, and metasomatic replacement. On balance, we prefer to interpret the fingers as evidence for the replacement of pre-existing allivalite by secondary peridotite. Replacement was achieved by pore magma from the underlying peridotite migrating upwards into the overlying allivalite, in response to compaction. This pore magma was able to resorb plagioclase but crystallize olivine and pyroxene in its place.

Information

Type
Articles
Information
Geological Magazine , Volume 122 , Issue 5 , September 1985 , pp. 491 - 502
Copyright
Copyright © Cambridge University Press 1985

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

Article purchase

Temporarily unavailable

References

Anketell, J. M., Cegla, J. & Dzulynski, S. 1970. On the deformational structures in systems with reversed density gradients. Annales de la Société Géologique de Pologne 40, 3–30.Google Scholar
Bowen, N. L. 1928. The Evolution of the Igneous Rocks. Princeton University Press. 334 pp.Google Scholar
Brown, G. M. 1956. The layered ultrabasic rocks of Rhum, Inner Hebrides. Philosophical Transactions of the Royal Society of London B 240, 1–53.Google Scholar
Butcher, A. R. 1985. Channelled metasomatism in Rhum layered cumulates – evidence from late-stage veins. Geological Magazine 122, 503–18.CrossRefGoogle Scholar
Cameron, E. N. & Desborough, G. A. 1964. Origin of certain magnetite-bearing pegmatites in the eastern Bushveld Complex, South Africa. American Mineralogist 62, 1082–96.Google Scholar
Huppert, H. E. & Sparks, R. S. J. 1980. The fluid dynamics of a basaltic magma chamber replenished by influx of hot, dense ultrabasic magma. Contributions to Mineralogy and Petrology 75, 279–89.CrossRefGoogle Scholar
Irvine, T. N. 1980. Magnetic infiltration metasomatism, double-diffusive fractional crystallization, and adcumulus growth in the Muskox intrusion and other layered intrusions. In Physics of Magmatic Processes (ed. Margraves, R. B.), pp. 325–83. Princeton, N.J.: Princeton University Press.CrossRefGoogle Scholar
Lee, C. A. 1981. Post-deposition structures in the Bushveld Complex mafic sequence. Journal of the Geological Society of London 138, 327–41.CrossRefGoogle Scholar
McBirney, A. R. 1979. Effects of assimilation. In The Evolution of the Igneous Rocks (ed. Yoder, H. S. Jr.,), pp. 307–38. Princeton, N.J.: Princeton University Press.Google Scholar
McKenzie, D. P. 1984. The generation and compaction of partially molten rock. Journal of Petrology 25, 713–65.CrossRefGoogle Scholar
Robins, B. 1982. Finger structures in the Lille Kufjord layered intrusion, Finnmark, Northern Norway. Contributions to Mineralogy and Petrology 81, 290–5.CrossRefGoogle Scholar
Schiffries, C. M. 1982. The petrogenesis of a platiniferous dunite pipe in the Bushveld Complex: infiltration metasomatism by a chloride solution. Economic Geology 77, 1439–53.CrossRefGoogle Scholar
Sparks, R. S. J., Huppert, H. E., Kerr, R. C., McKenzie, D. P. & Tait, S. R. 1985. Postcumulus processes in layered intrusions. Geological Magazine 122, 555–68.CrossRefGoogle Scholar
Sparks, R. S. J., Meyer, P. & Sigurdsson, H. 1980. Density variation amongst mid-ocean ridge basalts: implications for magma mixing and the scarcity of primitive lavas. Earth and Planetary Science Letters 46, 419–30.CrossRefGoogle Scholar
Stolper, E. & Walker, D. 1980. Melt density and the average composition of basalt. Contributions to Mineralogy and Petrology 74, 7–12.CrossRefGoogle Scholar
Thy, P. & Wilson, J. R. 1980. Primary igneous load-cast deformation structures in the Fongen–Hyllingen layered basic intrusion, Trondheim Region, Norway. Geological Magazine 117, 363–74.CrossRefGoogle Scholar
Wadsworth, W. J. 1961. The layered ultrabasic rocks of south-west Rhum, Inner Hebrides. Philosophical Transactions of the Royal Society of London B 244, 21–64.Google Scholar