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The Tertiary lavas and sediments of northwest Rhum, Inner Hebrides

Published online by Cambridge University Press:  01 May 2009

C. H. Emeleus
C. H. Emeleus
Affiliation:
Department of Geological Sciences, University of Durham, South Road, Durham DH1 3LE, U.K.
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Abstract

Several small outliers of Tertiary lavas and sediments rest with strong unconformity on a buried landscape eroded from Torridonian sediments and Tertiary granophyre. Erosion continued during the period of sediment and lava accumulation. Four formations are recognized; these are, in order of increasing age, the Orval Formation (hawaiite and basaltic hawaiite lavas), the Guirdil Formation (icelandite lavas, interbedded conglomerates), the Upper Fionchra Formation (tholeiitic basaltic andesite lavas, hyaloclastite deposits, basal conglomerate) and the Lower Fionchra Formation (alkali and transitional basalt, basaltic hawaiite and hawaiite lava flows, basal conglomerate); each is separated by an erosional interval. Clasts in the conglomerates reveal a history of erosion of a terrain exposing gneisses, Torridonian sediments, igneous rocks derived from the Rhum Tertiary Central Complex (including allivalites), and Tertiary lavas of local origin but also including, in the oldest conglomerates, tholeiitic basalts not now preserved on or near Rhum. Prior to and during lava and sediment accumulation, erosion on Rhum had cut down to a level similar to that of the present day, although not to the extent that high-grade thermally altered rocks, which are a marked feature of the Central Complex, were being eroded in any quantity. A sequence of east–west trending valleys, possibly initiated on the line of the earlier Main Ring Fault, drained the area of the Central Complex which then, as now, must have been high ground. Small lakes occasionally formed in the valleys allowing the accumulation of fine-grained sediment with plant remains, and promoting the formation of hyaloclastite deposits when buried by later flows. No source for any of the lava formations is preserved on Rhum; they are thought to have come from feeders north of Rhum, possibly near Canna, and to have ponded against the hills and valleys near and in the Central Complex.

The oldest tholeiitic lavas, not now found in situ, were followed by alkali and transitional flows compositionally similar to the Skye Main Lava Series but characteristically feldsparphyric; the most mafic also contain phenocrysts of magnesian olivine (with included Cr-Al-rich spinels) and aluminous spinel. Both the early alkalic/transitional basalts and the youngest hawaiites and basaltic hawaiites equilibrated at pressures < 9 kb; the tholeiitic basaltic andesites and icelandites equilibrated at relatively shallows depths.

Apart from a few N–S to NW–SE-trending basalt dykes, the lava formations represent the youngest Tertiary igneous event on Rhum.

Type
Articles
Information
Geological Magazine , Volume 122 , Issue 5 , September 1985 , pp. 419 - 437
Copyright
Copyright © Cambridge University Press 1985

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References

Bailey, E. B. 1945. Tertiary igneous tectonics of Rhum (Inner Hebrides). Quarterly Journal of the Geological Society of London 100, 165–91.CrossRefGoogle Scholar
Bailey, E. B. 1956. Hebridean notes: Rhum and Skye. Liverpool and Manchester Geological Journal 1, 420–26.CrossRefGoogle Scholar
Black, G. P. 1952 a. The age relationship of the granophyre and basalt of Orval, Isle of Rhum. Geological Magazine 89, 106–12.CrossRefGoogle Scholar
Black, G. P. 1952 b. The Tertiary volcanic succession of the Isle of Rhum, Inverness-shire. Transactions of the Edinburgh Geological Society 15, 39–51.CrossRefGoogle Scholar
Brown, G. M. 1956. The layered ultrabasic rocks of Rhum, Inner Hebrides. Philosophical Transactions of the Royal Society of London 240 B, 1–53.Google Scholar
Brown, G. M. 1957. Pyroxenes from the early and middle stage of fractionation of the Skaergaard intrusion, East Greenland. Mineralogical Magazine 31, 511–43.CrossRefGoogle Scholar
Brown, G. M. 1967. Mineralogy of basaltic rocks. In Basalts. The Poldervaart Treatise on Rocks of Basaltic Composition (ed. Hess, H. H. and Poldervaart, A.), pp. 103–62. New York: John Wiley.Google Scholar
Carmichael, I. S. E. 1964. The petrology of Thingmuli, a Tertiary volcano in Eastern Iceland. Journal of Petrology 5, 435–60.CrossRefGoogle Scholar
Dagley, P. & Mussett, A. E. 1981. Palaeomagnetism of the British Tertiary igneous province: Rhum and Canna. Geophysical Journal of the Royal Astronomical Society 65, 475–91.CrossRefGoogle Scholar
Dunham, A. C. 1968. The felsites, granophyre, explosion breccias and tuffisites of the north-eastern margin of the Tertiary igneous complex of Rhum, Inverness-shire. Quarterly Journal of the Geological Society of London 123, 327–52.CrossRefGoogle Scholar
Dunham, A. C. & Wadsworth, W. J. 1978. Cryptic variation in the Rhum layered intrusion. Mineralogical Magazine 42, 347–56.CrossRefGoogle Scholar
Emeleus, C. H. 1973. Granophyre pebbles in Tertiary conglomerate on the Isle of Canna, Inverness-shire. Scottish Journal of Geology 9, 157–59.CrossRefGoogle Scholar
Emeleus, C. H. 1980. 1:20000 Solid Geology Map of Rhum. Scotland: Nature Conservancy Council.Google Scholar
Emeleus, C. H. & Forster, R. M. 1979. Field Guide to the Tertiary Igneous Rocks of Rhum. Newbury: Nature Conservancy Council. 44pp.Google Scholar
Emeleus, C. H., Dunham, A. C. & Thompson, R. N. 1971. Iron-rich pigeonite from acid rocks in the Tertiary Igneous Province, Scotland. American Mineralogist 56, 940–51.Google Scholar
Emeleus, C. H., Wadsworth, W. J. & Smith, N. J. 1985. The early igneous and tectonic history of the Rhum Tertiary Volcanic Centre. Geological Magazine 122, 451–7.CrossRefGoogle Scholar
Geikie, A. 1897. The Ancient Volcanoes of Great Britain. 2 volumes. London: Macmillan.CrossRefGoogle Scholar
Gibb, F. G. F. 1973. The zoned clinopyroxenes of the Shiant Isles Sill, Scotland. Journal of Petrology 14, 203–30.CrossRefGoogle Scholar
Harker, A. 1908. The Geology of the Small Isles of Inverness-shire. Memoir, Geological Survey of Scotland. Edinburgh: H.M.S.O. 210 pp.Google Scholar
Harker, A. & Barrow, G. 1903. One-Inch Geological Map of the Small Isles of Inverness-shire. Sheet 60, Geological Survey, Scotland. Edinburgh: H.M.S.O.Google Scholar
Holland, J. G. & Brown, G. M. 1972. Hebridean tholeiitic magmas: a geochemical study of the Ardnamurchan cone sheets. Contributions to Mineralogy and Petrology 37, 139–60.CrossRefGoogle Scholar
Hughes, C. J. 1960. The Southern Mountains igneous complex, Isle of Rhum. Quarterly Journal of the Geological Society of London 116, 111–38.CrossRefGoogle Scholar
Johnstone, G. S. 1971. One-Inch Geological Map of Rhum. Sheet 60 (2nd (Provisional) Edition). Institute of Geological Sciences, Scotland.Google Scholar
Mattey, D. P., Gibson, I. L., Marriner, G. F. & Thompson, R. N. 1977. The diagnostic geochemistry, relative abundance and spatial distribution of high-calcium, low-alkali olivine tholeiite dykes in the Lower Tertiary regional swarm of the Isle of Skye. Mineralogical Magazine 41, 273–85.CrossRefGoogle Scholar
Mussett, A. E. 1984. Time and duration of Tertiary igneous activity of Rhum and adjacent areas. Scottish Journal of Geology 20, 273–80.CrossRefGoogle Scholar
Richey, J. E. 1961. British Regional Geology; Scotland, the Tertiary Volcanic Districts (3rd ed., revised by MacGregor, A. G. and Anderson, F. W.). Geological Survey. Edinburgh: H.M.S.O. 120 pp.Google Scholar
Ridley, W. I. 1971. The petrology of some volcanic rocks from the British Tertiary province: the islands of Rhum, Eigg, Canna and Muck. Contributions to Mineralogy and Petrology 32, 251–66.CrossRefGoogle Scholar
Ridley, W. I. 1973. The Petrology of Volcanic Rocks from the Small Isles of Inverness-shire. Report of the Institute of Geological Sciences No. 73/10. 55 pp.Google Scholar
Ridley, W. I. 1977. The crystallization trends of spinels in Tertiary basalts from Rhum and Muck and their petrogenetic significance. Contributions to Mineralogy and Petrology 64, 243–55.CrossRefGoogle Scholar
Smith, N. J. 1985. The age and structural setting of limestones and basalts on the Main Ring Fault in southeast Rhum. Geological Magazine 122, 439–45.CrossRefGoogle Scholar
Sweatman, T. R. & Long, J. V. P. 1969. Quantitative electron-probe microanalysis of rock-forming minerals. Journal of Petrology 10, 332–79.CrossRefGoogle Scholar
Thompson, R. N. 1982. Magmatism of the British Tertiary volcanic province. Scottish Journal of Geology 18, 49–107.CrossRefGoogle Scholar
Thompson, R. N., Esson, J. & Dunham, A. C. 1972. Major element chemical variation in the Eocene lavas of the Isle of Skye, Scotland. Journal of Petrology 13, 219–54.CrossRefGoogle Scholar
Tomkeieff, S. I. 1942. The Tertiary lavas of Rhum. Geological Magazine 79, 1–13.CrossRefGoogle Scholar
Tomkeieff, S. I. & Blackburn, K. B. 1942. On the remains of fossil wood enclosed in a Tertiary lava on the Isle of Rhum, Inner Hebrides. Geological Magazine 79, 14–17.CrossRefGoogle Scholar
Wadsworth, W. J. 1961. The ultrabasic rocks of southwest Rhum. Philosophical Transactions of the Royal Society of London 244 B, 21–64.Google Scholar
Wager, L. R., Weedon, D. S. & Vincent, E. A. 1953. A granophyre from Coire Uaigneich, Isle of Skye, containing quartz paramorphs after tridymite. Mineralogical Magazine 30, 263–76.CrossRefGoogle Scholar