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The Southern Mountains Zone, Isle of Rum, Scotland: volcanic and sedimentary processes upon an uplifted and subsided magma chamber roof

Published online by Cambridge University Press:  25 March 2009

E. P. HOLOHAN ,
V. R. TROLL ,
M. ERRINGTON ,
C. H. DONALDSON ,
G. R. NICOLL  and
C. H. EMELEUS
E. P. HOLOHAN*
Affiliation:
Department of Geology, School of Natural Sciences, Trinity College, Dublin 2, Ireland
V. R. TROLL
Affiliation:
Department of Geology, School of Natural Sciences, Trinity College, Dublin 2, Ireland
M. ERRINGTON
Affiliation:
School of Geosciences, University of St Andrews, Scotland, UK
C. H. DONALDSON
Affiliation:
School of Geosciences, University of St Andrews, Scotland, UK
G. R. NICOLL
Affiliation:
Department of Geology, School of Natural Sciences, Trinity College, Dublin 2, Ireland
C. H. EMELEUS
Affiliation:
Department of Earth Sciences, University of Durham, England, UK
*
†Author for correspondence: [email protected]; current address: Fault Analysis Group, UCD School of Geological Sciences, Belfield, Dublin 4, Ireland
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Abstract

The Southern Mountains Zone of the Rum Central Complex lies inside a major ring fault and comprises an intricate association of country-rock outcrops, breccias and rhyodacite. The breccias and rhyodacite were long thought to be products of subterranean explosion and intrusion, respectively. Here, we report new observations that support re-interpretation of these units as mass movement deposits and ignimbrites. The most abundant breccias (Coire Dubh-type) consist mainly of country-rock clasts <1 m in diameter in a sand or silt matrix. Internally bedded and graded, and interlayered with sandstones and lithic tuffs, these breccias are interpreted as debris flow and stream flow deposits. Rhyodacite sheets show gradational or sharp, concordant contacts with Coire Dubh-type breccias, and display graded basal lithic tuffs and graded fiamme swarms. These sheets are interpreted as moderately to densely welded rhyodacite ignimbrites (25–100 m thick). A steep body of fragmented (fiamme-bearing) rhyodacite with intrusive non-fragmented contacts is interpreted as an ignimbrite vent system. The rhyodacite and breccia succession is over 200 m thick and unconformably overlies a structurally uplifted Precambrian basement, within which there is also evidence of later subsidence. Outcrops of potential caldera-collapse ‘megabreccia’ are more structurally consistent than previously thought, and are re-interpreted here as coherent segments of Precambrian country rock (caldera floor). The Southern Mountains Zone breccias and rhyodacites respectively reflect sedimentary and pyroclastic processes acting in response to a complex tectonic interplay of intrusion-related uplift and caldera subsidence.

Keywords

intrusionupliftdebris flowbrecciaignimbritecalderaIsle of Rum
Type
Original Article
Information
Geological Magazine , Volume 146 , Issue 3 , May 2009 , pp. 400 - 418
Copyright
Copyright © Cambridge University Press 2009

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Footnotes

‡

Current address: Department of Earth Sciences, Uppsala University, SE-752 36, Uppsala, Sweden

References

Almond, D. C. 1977. The Sabaloka igneous complex, Sudan. Philosophical Transactions of the Royal Society of London 273, 595–635.Google Scholar
Anderson, E. M. 1936. The dynamics of the formation of cone sheets, ring dykes and cauldron subsidence. Proceedings of the Royal Society of Edinburgh 56, 128–63.CrossRefGoogle Scholar
Bailey, E. B. 1945. Tertiary igneous tectonics of Rhum (Inner Hebrides). Quarterly Journal of the Geological Society of London 100, 165–91.CrossRefGoogle Scholar
Bell, B. R. & Emeleus, C. H. 1988. A review of silicic pyroclastic rocks of the British Tertiary Volcanic Province. In Early Tertiary Volcanism and the Opening of the NE Atlantic (eds Morton, A. C. & Parson, L. M.), pp. 365–79. Geological Society of London, Special Publication no. 39.Google Scholar
Branney, M. J. & Kokelaar, P. 1994. Volcanotectonic faulting, soft-state deformation, and rheomorphism of tuffs during development of a piecemeal caldera, English Lake District. Geological Society of America Bulletin 106, 507–30.2.3.CO;2>CrossRefGoogle Scholar
Brown, D. J. & Bell, B. R. 2007. Debris flow deposits within the Palaeogene lava fields of NW Scotland: evidence for mass wasting of the volcanic landscape during emplacement of the Ardnamurchan Central Complex. Bulletin of Volcanology 69, 847–68.CrossRefGoogle Scholar
Brown, D. J., Holohan, E. P. & Bell, B. R. 2009. Sedimentary and volcano-tectonic processes in the British Paleocene Igneous Province: a review. Geological Magazine 146, 326–52.CrossRefGoogle Scholar
Cas, R. A. F. & Wright, J. V. 1987. Volcanic Successions: modern and ancient. London: Allen and Unwin.CrossRefGoogle Scholar
Collinson, J., Mountney, N. & Thompson, D. 2006. Sedimentary Structures, 3rd ed. Harpenden: Terra Publishing, 292 pp.Google Scholar
Donaldson, C. H., Troll, V. R. & Emeleus, C. H. 2001. Felsites and breccias in the Northern Marginal Zone of the Rum Central Complex: changing views, c. 1900–2000. Proceedings of the Yorkshire Geological Society 53, 167–75.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. Journal of the Geological Society, London 123, 327–52.CrossRefGoogle Scholar
Emeleus, C. H. 1980. 1:20000 solid geology map of Rhum, 1st ed. Newbury: Nature Conservancy Council.Google Scholar
Emeleus, C. H. 1985. The Tertiary lavas and sediments of northwest Rhum, Inner Hebrides. Geological Magazine 122, 419–37.CrossRefGoogle Scholar
Emeleus, C. H. 1994. 1:20000 solid geology map of Rum, 2nd ed. Edinburgh: Scottish Natural Heritage.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
Emeleus, C. H. 1997. Geology of Rum and the adjacent islands: Memoir for 1:50,000 Geological Sheet 60 (Scotland) of the British Geological Survey. London: The Stationery Office.Google Scholar
Fisher, R. V. & Schmincke, H.-U. 1984. Pyroclastic Rocks. Berlin: Springer-Verlag.CrossRefGoogle Scholar
Freundt, A., Wilson, C. J. N. & Carey, S. N. 2000. Ignimbrites and Block-and-Ash Flow Deposits. In Encyclopedia of Volcanoes (eds Sigurdsson, H., Houghton, B. F., McNutt, S. R., Rymer, H. & Stix, J.), pp. 581–99. San Diego: Academic Press.Google Scholar
Geshi, N., Shimano, T., Chiba, T. & Nakada, S. 2002. Caldera collapse during the 2000 eruption of Miyakejima Volcano, Japan. Bulletin of Volcanology 64, 55–68.CrossRefGoogle Scholar
Glicken, H. 1996. Rockslide-Debris Avalanche of May 18, 1980, Mount St. Helens Volcano, Washington. U.S. Geological Survey Open File Report, 96–677, 90 pp.CrossRefGoogle Scholar
Hughes, C. J. 1960. The Southern Mountains Igneous Complex, Isle of Rhum. Quarterly Journal of the Geological Society of London 96, 111–38.CrossRefGoogle Scholar
Kokelaar, B. P. & Moore, I. D. 2006. Classical Areas of British Geology: Glencoe caldera volcano, Scotland. Nottingham: British Geological Survey.Google Scholar
Lipman, P. W. 1976. Caldera-collapse breccias in western San-Juan Mountains, Colorado. Geological Society of America Bulletin 87, 1397–1410.2.0.CO;2>CrossRefGoogle Scholar
Lipman, P. W. 1984. The roots of ash flow calderas in western North America; windows into the tops of granitic batholiths. Journal of Geophysical Research 89B, 8801–41.CrossRefGoogle Scholar
Lipman, P. W. 1997. Subsidence of ash-flow calderas; relation to caldera size and magma-chamber geometry. Bulletin of Volcanology 59, 198–218.CrossRefGoogle Scholar
McPhie, J., Doyle, M. & Allen, R. 1993. Volcanic Textures: A guide to the interpretation of textures in volcanic rocks. Centre for Ore Deposit and Exploration Studies, University of Tasmania Press, 196 pp.Google Scholar
Moore, I. & Kokelaar, P. 1998. Tectonically controlled piecemeal caldera collapse; a case study of Glencoe Volcano, Scotland. Geological Society of America Bulletin 110, 1448–66.2.3.CO;2>CrossRefGoogle Scholar
Nemec, W. & Steel, R. J. 1984. Alluvial and coastal conglomerates: their significant features and some comments on gravely mass-flow deposits. In Sedimentology of Gravels and Conglomerates (eds Koster, E. H. & Steel, R. J.), pp. 1–31. Canadian Society of Petroleum Geologists, Memoir no. 10.Google Scholar
Nicoll, G. R., Holness, M. B., Troll, V. R., Donaldson, C. H., Holohan, E. P., Emeleus, C. H. & Chew, D. 2009. Early mafic magmatism and crustal anatexis on the Isle of Rum: evidence from the Am Màm intrusion breccia. Geological Magazine 146, 368–81.CrossRefGoogle Scholar
Richey, J. E. & Thomas, H. H. 1932. The Tertiary Ring Complex of Slieve Gullion, Ireland. Quarterly Journal of the Geological Society of London 88, 653–88.CrossRefGoogle Scholar
Roche, O., Druitt, T. H. & Merle, O. 2000. Experimental study of caldera formation. Journal of Geophysical Research 105, 395–416.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
Smith, R. L. & Bailey, R. A. 1968. Resurgent cauldrons. Geological Society of America Memoir 116, 613–62.CrossRefGoogle Scholar
Troll, V. R., Emeleus, C. H. & Donaldson, C. H. 2000. Caldera formation in the Rum Central igneous complex, Scotland. Bulletin of Volcanology 62, 301–17.CrossRefGoogle Scholar
Troll, V. R., Donaldson, C. H. & Emeleus, C. H. 2004. Pre-eruptive magma mixing in ash-flow deposits of the Tertiary Rum Igneous Centre, Scotland. Contributions to Mineralogy and Petrology 147, 722–39.CrossRefGoogle Scholar
Vallance, J. W. 2000. Lahars. In Encyclopedia of Volcanoes (eds Sigurdsson, H., Houghton, B. F., McNutt, S. R., Rymer, H. & Stix, J.), pp. 601–16. San Diego: Academic Press.Google Scholar
Williams, P. J. 1985. Pyroclastic rocks in the Cnapan Breaca felsite, Rhum. Geological Magazine 122, 447–50.CrossRefGoogle Scholar
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