Hostname: page-component-76fb5796d-qxdb6 Total loading time: 0 Render date: 2024-04-25T14:25:12.466Z Has data issue: false hasContentIssue false

The Rum Igneous Centre, Scotland

Published online by Cambridge University Press:  05 July 2018

C. H. Emeleus*
Affiliation:
Department of Earth Sciences, Durham University, Science Labs, South Road, Durham DH1 3LE, UK
V. R. Troll*
Affiliation:
Department of Earth Sciences, CEMPEG, Uppsala University, Villavägen 16, SE-752 36, Uppsala, Sweden
Rights & Permissions [Opens in a new window]

Abstract

Core share and HTML view are not available for this content. However, as you have access to this content, a full PDF is available via the ‘Save PDF’ action button.

The publication of the British Geological Survey memoir on Rum and the Small Isles in 1997 was followed by a period of intense petrological and mineralogical research, leading to some 40 papers, books and other publications. The research progress since then is reviewed here and integrated with the information previously available to provide an overview of the current status of understanding of the centre. New data on the acidic and mixed acid/basic magmas of the early Rum caldera demonstrate that frequent mafic replenishments were the main driver for magmatic activity at Rum right from its initial stages. The caldera is bound by the Main Ring Fault, a structure which probably also exercised an influence on the emplacement of the subsequent basic and ultrabasic intrusions. The later emplacement of gabbros and ultrabasic rocks caused only limited thermal metamorphism of the surrounding Torridonian sandstones, contrasting markedly with the crustal isotope signatures of the early intracaldera ignimbrite magmas and the intense alteration of uplifted masses of Lewisian gneiss within the ring fault. Rare picritic dykes provide an indication of the possible parent magma for the mafic and ultrabasic rocks, but these, as with most other magmatic rocks on Rum, have undergone varying degrees of crustal contamination, involving both Lewisian granulite and amphibolite-type crust but, notably, no Moine metasedimentary compositions as is the case at the nearby Ardnamurchan centre. Detailed textural studies on the gabbroic and ultrabasic rocks allow a distinction between intrusive peridotites and peridotite that forms part of the classic layered cumulate units of Rum and, furthermore, this work and that on the chromite seams and veins in these rocks shows that movement of trapped magma and magma derived from later intrusions, may produce textures regarded previously as of primary cumulate origin. Sulfides in the chromitite seams and ultrabasic rocks, in turn, show possible influences from assimilated Mesozoic sediments. Igneous activity on Rum was short-lived, possibly only between 0.5 and 1 m.y. in duration and commenced at ∼60.5 Ma. The Rum Central Complex was extinct by the time the main activity at the nearby Skye Central Complex commenced (∼59 Ma). From recent apatite fission-track studies it seems probable that Rum, in common with other Palaeogene centres, underwent a brief, but significantly later heating event (∼45 Ma).

Type
Research Article
Creative Commons
Creative Common License - CCCreative Common License - BY
Copyright © The Mineralogical Society of Great Britain and Ireland 2014 This is an Open Access article, distributed under the terms of the Creative Commons Attribution license. (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2014

References

Bailey, E.B. (1945) Tertiary igneous tectonics of Rhum (Inner Hebrides). Quarterly Journal of the Geological Society, London, 100, 165191.CrossRefGoogle Scholar
Bédard, J.H., Sparks, R.S.J., Renner, R., Cheadle, M.J. and Hallworth, M.A. (1988) Peridotite sills and metasomatic gabbros in the Eastern Layered Series of the Rhum complex. Journal of the Geological Society, London, 145, 207224.CrossRefGoogle Scholar
Black, G.P. (1952) The age relationship of the granophyre and basalt of Orval, Isle of Rhum. Geological Magazine, 89, 106112.CrossRefGoogle Scholar
Blake, D.H., Elwell, R.W.D., Skelhorn, R.R. and Walker, G.P.L. (1965) Some relationships resulting from the intimate association of acid and basic magmas. Quarterly Journal of the Geological Society, London, 121, 3149.CrossRefGoogle Scholar
British Geological Survey (1994) Rum. Scotland Sheet 60. Solid and Drift Geology. 1:50,000. British Geological Survey, Keyworth, Nottingham, UK.Google Scholar
Brothers, R.N. (1964) Petrofabric analyses of Rhum and Skaergaard rocks. Journal of Petrology, 5, 255274.CrossRefGoogle Scholar
Brown, D.J., Holohan, E.P. and Bell, B.R. (2009) Sedimentary and volcano-tectonic processes in the British Palaeogene Igneous Province: a review. Geological Magazine, 146, 326352.CrossRefGoogle Scholar
Brown, G.M. (1956) The layered ultrabasic rocks of Rhum, Inner Hebrides. Philosophical Transactions of the Royal Society of London, 24B, 1–53.Google Scholar
Brown, P.E., Chambers, A.D. and Becker, S.M. (1986) A large soft-sediment fold in the Lilloise Intrusion, East Greenland. Pp 125–144 in: Origins of Igneous Layering (I. Parsons, editor). NATO ASI Series, Series C: Mathematical and Physical Sciences, Vol. 186. Kluwer Academic, Dordrecht, The Netherlands.Google Scholar
Burchardt, S., Troll, V.R., Mathieu, L., Emeleus, C.H. and Donaldson, C.H. (2013) Ardnamurchan 3D cone-sheet architecture explained by a single elongate magma chamber. Scientific reports, doi:10.1038/srep02891.CrossRefGoogle Scholar
Butcher, A.R., Young, I.M. and Faithfull, J.W. (1985) Finger structures in the Rhum Complex. Geological Magazine, 122, 491502.CrossRefGoogle Scholar
Butcher, A.R., Pirrie, D., Prichard, H.M. and Fisher, P. (1999) Platinum-group mineralization in the Rum layered intrusion, Scottish Hebrides. Journal of the Geological Society, London, 131, 931944.Google Scholar
Chambers, L.M., Pringle, M.S. and Parrish, R.R. (2005) Rapid formation of the Small Isles igneous centre constrained by precise 40Ar/39Ar and U-Pb ages. Lithos, 79, 367384.CrossRefGoogle Scholar
Collinson, J.D. and Thompson, D.B. (1982) Sedimentary Structures. George Allen & Unwin, London, 194 pp.Google Scholar
Dickin, A.P. and Jones, N.W. (1983) Isotopic evidence for the age and origin of pitchstones and felsites, Isle of Eigg, NW Scotland. Journal of the Geological Society, London, 140, 691700.CrossRefGoogle Scholar
Dobson, K.J., Stuart, F.M. and Dempster, T.C. (2010) Constraining the post-emplacement evolution of the Hebridean Igneous Province (HIP) using low-temperature thermochronology: how long has the HIP been cool? Journal of the Geological Society, London, 167, 973984.CrossRefGoogle Scholar
Donaldson, C.H., Drever, H.I. and Johnston, R. (1973) Crystallization of poikilo-macro-spherulitic feldspar in a Rhum peridotite. Nature, Physical Sciences, 243, 6970.CrossRefGoogle Scholar
Donaldson, C.H., Troll, V.R. and 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, 167175.CrossRefGoogle Scholar
Dunham, A.C. (1965) A new type of banding in ultrabasic rocks from central Rhum, Inverrness-shire, Scotland. American Mineralogist, 50, 14101420.Google 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, Scotland. Quarterly Journal of the Geological Society, London, 123, 327352.CrossRefGoogle Scholar
Elias, R.T. (1989) The origin of cyclic layering in the Eastern Layered Series of the Rhum intrusion. Geological Society of London Newsletter, 18, 38.Google Scholar
Emeleus, C.H. (1985) The Tertiary lavas and sediments of northwest Rhum, Inner Hebrides. Geological Magazine, 122, 419437.CrossRefGoogle Scholar
Emeleus, C.H. (1997) Geology of Rum and the adjacent islands. Memoir of the British Geological Survey, Sheet 60 (Scotland). British Geological Survey, Keyworth, Notingham, UK.Google Scholar
Emeleus, C.H. and Bell, B.R. (2005) British Regional Geology: the Palaeogene volcanic districts of Scotland (Fourth Edition). British Geological Survey, Keyworth, Nottingham, UK.Google Scholar
Emeleus, C.H. and Troll, V.R. (2008) A geological excursion guide to Rum: the Palaeocene igneous rocks of the Isle of Rum, Inner Hebrides. Edinburgh Geological Society, Edinburgh.Google Scholar
Emeleus, C.H. and Troll, V.R. (2011) Recent research developments on the Isle of Rum, NW Scotland. Geology Today, 27, 184192.CrossRefGoogle Scholar
Emeleus, C.H., Cheadle, M.J., Hunter, R.H., Upton, B.G.J. and Wadsworth, W.J. (1996) The Rum Layered Suite. Pp. 403–440 in: Layered Igneous Rocks (R.G. Cawthorn, editor). Elsevier, Amsterdam.Google Scholar
Faithfull, J.W. (1985) The Lower Eastern Layered Series of Rhum. Geological Magazine, 122, 459468.CrossRefGoogle Scholar
Faithfull, J.W., Timmerman, M.J., Upton, B.G.J. and Rumsey, M.S. (2012) Mid-Eocene renewal of magmatism in NW Scotland: the Loch Roag dyke, Outer Hebrides. Journal of the Geological Society, London, 169, 115118.CrossRefGoogle Scholar
Geldmacher, J., Troll, V.R., Emeleus, C.H. and Donaldson, C.H. (2002) Pb-isotope evidence for contrasting crustal contamination of primitive to evolved magmas from Ardnamurchan and Rhum: implications for the structure of the underlying crust. Scottish Journal of Geology, 38, 5561.CrossRefGoogle Scholar
Goodenough, K., Jerram, D.J., Emeleus, C.H. and Troll, V.R. (2008) Golden Rum: understanding the Forbidden Isles. Geoscientist, 18, 2224.Google Scholar
Geikie, A. (1897) The Ancient Volcanoes of Great Britain. 2 volumes. Macmillan, London.CrossRefGoogle Scholar
Hamilton, M.A., Pearson, D.G., Thompson, R.N., Kelley, S.P. and Emeleus, C.H. (1998) Rapid eruption of the Skye lavas inferred from precise U-Pb and Ar-Ar dating of the Rum and Cuillin plutonic complexes. Nature, 394, 260263.CrossRefGoogle Scholar
Harker, A. (1908) The geology of the Small Isles of Inverness-shire. Memoirs of the Geological Survey: Scotland, Sheet 60. British Geological Survey, Keyworth, Nottingham, UK.Google Scholar
Holness, M. (1999) Contact metamorphism and anatexis of Torridonian arkose by minor intrusions of the Rum Igneous Complex, Inner Hebrides, Scotland. Geological Magazine, 136, 527542.CrossRefGoogle Scholar
Holness, M.B. (2002) Spherulitic textures formed during crystallization of partially melted arkose, Rum, Scotland. Geological Magazine, 139, 651663.CrossRefGoogle Scholar
Holness, M.B. (2005) Spatial constraints on magma chamber events from textural observations on cumulates: the Rum Layered Intrusion, Scotland. Journal of Petrology, 46, 15851601.CrossRefGoogle Scholar
Holness, M.B. (2007) Textural immaturity of cumulates as an indicator of magma chamber processes: infiltration and crystal accumulation in the Rum layered suite. Journal of the Geological Society, London, 164, 529539.CrossRefGoogle Scholar
Holness, M.B. and Isherwood, C.E. (2003) The aureole of the Rum Tertiary Igneous Complex, Scotland. Journal of the Geological Society, London, 160, 1527.CrossRefGoogle Scholar
Holness, M.B. and Winpenny, B. (2009) The Unit 12 allivalite, Eastern Layered Intrusion, Isle of Rum: a textural and geochemical study of an open-system magma chamber. Geological Magazine, 146, 437460.CrossRefGoogle Scholar
Holness, M.B., Cheadle, M.J. and McKenzie, D.F. (2005) The use of changes in dihedral angle to decode late-stage evolution in cumulates. Journal of Petrology, 46, 15651583.CrossRefGoogle Scholar
Holness, M.B., Hallworth, M.A., Woods, A. and Sides, R.E. (2007) Infiltration metasomatism of cumulates by intrusive magma replenishments: the Wavy Horizon, Isle of Rum, Scotland. Journal of Petrology, 48, 563587.CrossRefGoogle Scholar
Holness, M.B., Humphreys, M.C.S., Sides, R., Helz, R.T. and Tegner, C. (2012a) Towards an understanding of disequilibrium dihedral angles in mafic rocks. Journal of Geophysical Research, 117, 131.CrossRefGoogle Scholar
Holness, M.B., Sides, R., Prior, D.J., Cheadle, M.J. and Upton, B.G.J. (2012b) The peridotite plugs of Rum: crystal settling and fabric development in magma conduits. Lithos, 134, 2340.CrossRefGoogle Scholar
Holohan, E.P., Troll, V.R. and Emeleus, C.H. (2008) The Southern Mountains and Dibidil. Pp. 97–140 in: A geological excursion guide to Rum: The Paleocene igneous rocks of the Isle of Rum, Inner Hebrides (C.H. Emeleus, and V.R. Troll, editors). Edinburgh Geological Society, Edinburgh.Google Scholar
Holohan, E.P., Troll, V.R., Errington, M., Donaldson, C.H., Nicoll, G.R. and Emeleus, C.H. (2009) The Southern Mountains Zone, Isle of Rum, Scotland: volcanic and sedimentary processes upon an uplifted and subsided magma chamber roof. Geological Magazine, 146, 400418.CrossRefGoogle Scholar
Hughes, C.J. (1960) The Southern Mountains Igneous Complex, Isle of Rhum. Quarterly Journal of the Geological Society, London, 116, 111138.CrossRefGoogle Scholar
Huppert, E.H. and Sparks, R.S.J. (1985) Cooling and contamination of mafic and ultramafic magma during ascent through continental crust. Earth and Planetary Science Letters, 74, 371386.CrossRefGoogle Scholar
Irvine, T. (1980) Magmatic infiltration, double diffusive fractional crystallization and adcumulus growth in the Muskox and other layered intrusions. Pp. 325–383 in: Physics of Magmatic Processes (R.B. Hargreaves, editor). Princeton University Press, New Jersey, USA.Google Scholar
Judd, J.W. (1874) The Secondary rocks of Scotland. Second paper. On the ancient volcanoes of the Highlands and the relations of their products to the Mesozoic Strata. Quarterly Journal of the Geological Society, London, 30, 220301.CrossRefGoogle Scholar
Lacasse, C., Sigurdsson, H., Johannesson, H., Paterne, M. and Carey, S. (1995) Source of ash zone 1 in the North Atlantic. Bulletin of Volcanology, 57, 1832.CrossRefGoogle Scholar
Lacasse, C., Sigurdsson, H., Carey, S.N., Johannesson H., Thomas, L.E. and Rogers, N.W. (2007) Bimodal volcanism at the Katla subglacial caldera, Iceland: insight into the geochemistry and petrogenesis of rhyolitic magmas. Bulletin of Volcanology. 69, 373399.CrossRefGoogle Scholar
Larsen, G. (2000) Holocene eruptions within the Katla volcanic system, south Iceland: characteristics and environmental impact. Jökull, 49, 128.Google Scholar
Latypov, R., O’Driscoll, B. and Lavrenchuk, A. (2013) Towards a model for the in situ origin of PGE reefs in layered intrusions: insights from chromitite seams of the Rum Eastern Layered Intrusion, Scotland. Contributions to Mineralogy and Petrology, 166, 309327.CrossRefGoogle Scholar
LeMaitre, R.W. (2002) Igneous Rocks. A Classification and Glossary of Terms (2nd Edition). Cambridge University Press, Cambridge, UK, pp 236.CrossRefGoogle Scholar
McBirney, A.R. and Noyes, R.M. (1979) Crystallisation and layering in the Skaergaard intrusion. Journal of Petrology, 38, 487554.CrossRefGoogle Scholar
McClurg, J. (1982) Petrology and evolution of the northern part of the Rhum ultrabasic complex. Unpublished PhD thesis, University of Edinburgh, Edinburgh.Google Scholar
Meyer, R., Nicoll, G.R., Hertogen, J., Troll, V.R., Ellam, R.M. and Emeleus, C.H. (2009) Trace element and isotope constraints on crustal anatexis by upwelling mantle melts in the North Atlantic Igneous Province: an example from the Isle of Rum, Scotland. Geological Magazine, 146, 382399.CrossRefGoogle Scholar
Nicoll, G.R. (2007) Evolution and crustal contamination of igneous rocks from the Palaeocene Volcanic District of North-West Scotland. Unpublished PhD Thesis, Trinity College, Dublin.Google Scholar
Nicoll, G.R., Holness, M.B., Troll, V.R., Holohan, E.P., Emeleus, C.H. and Chew, D. (2009) Early mafic magmatism and crustal anatexis on the Isle of Rum: evidence from the Am Màm intrusion breccias. Geological Magazine, 146, 368381.CrossRefGoogle Scholar
O’Driscoll, B., Donaldson, C.H., Troll, V.R., Jerram, D.J. and Emeleus, C.H. (2007a) An origin for harrisitic and granular olivine in the Rum Layered Suite, NW Scotland: a Crystal Size Distribution study. Journal of Petrology, 48, 253270.CrossRefGoogle Scholar
O’Driscoll, B., Hargreaves, R.B., Emeleus, C.H., Troll, V.R., Donaldson, C.H. and Reavy, R.J. (2007b). Magmatic lineations inferred from anisotropy of magnetic susceptibility fabrics in Units 8, 9 and 10 of the Rhum Eastern Layered Series, Scotland. Lithos, 98, 2744.CrossRefGoogle Scholar
O’Driscoll, B., Day, J.M.D., Daly, J.S., Walker, R.J. and McDonough, W.F. (2009a) Rhenium-Osmium isotopes and platinum-group elements in the Rum Layered Suite, Scotland: implications for Cr-spinel seam formation and the composition of the Iceland mantle anomaly. Earth and Planetary Science Letters, 286, 4161.CrossRefGoogle Scholar
O’Driscoll, B., Donaldson, C.H., Daly, J.S. and, Emeleus, C.H. (2009b) The roles of melt infiltration and cumulate assimilation in the formation of anorthosites and a Cr-spinel seam in the Rum Eastern Layered Intrusion. Lithos, 111, 620.CrossRefGoogle Scholar
O’Driscoll, B., Emeleus, C.H., Donaldson, C.H. and Daly, J.S. (2010) Cr-spinel petrogenesis in the Rum Layered Suite, NW Scotland: cumulate assimilation and in situ crystallization in a deforming crystal mush. Journal of Petrology, 51, 11711201.CrossRefGoogle Scholar
Óladóttir, B.A., Larsen, G., Thordarson, T. and Sigmarsson, O. (2005) The Katla volcano SIceland: Holocene tephra stratigraphy and eruption frequency, Jökull, 55, 5374.Google Scholar
Óladóttir, B.A., Sigmarsson, O., Larsen, G. and Thordarson, T. (2008) Katla volcano, Iceland: magma composition, dynamics and eruption frequency as recorded by Holocene tephra layers. Bulletin of Volcanology, 70, 475493.CrossRefGoogle Scholar
Petronis, M.S., O’Driscoll, B., Troll, V.R., Emeleus, C.H. and Geissman, J.W. (2009) Palaeomagnetic and anisotropy of magnetic susceptibility data bearing on the emplacement of the Western Granite, Isle of Rum, Scotland. Geological Magazine, 146, 419436.CrossRefGoogle Scholar
Power, M.R., Pirrie, D. and Andersen, J.C.O. (2000a) Testing the validity of chrome spinel as a provenance and petrogenetic indicator. Geology, 28, 10271030.2.0.CO;2>CrossRefGoogle Scholar
Power, M.R., Pirrie, D., Andersen, J.C.O. and Butcher, A.R. (2000b). Stratigraphic distribution of platinumgroup elements in the Eastern Layered Series, Rum, Scotland. Mineralium Deposita, 35, 762775.CrossRefGoogle Scholar
Power, M.R., Pirrie, D. and Andersen, J.C.O. (2003) Diversity of platinum-group element mineralization styles in the North Atlantic Igneous Province: new evidence from Rum, U.K. Geological Magazine, 144, 499512.CrossRefGoogle Scholar
Renner, R. and Palacz, Z.A. (1987) Basaltic replenishment of the Rhum magma chamber: evidence from Unit 14. Journal of the Geological Society, London, 144, 961970.CrossRefGoogle Scholar
Scottish Natural Heritage (1994) Rum – Solid Geology, 1:20,000. Scottish Natural Heritage, Inverness, UK.Google Scholar
Sparks, R.S.J., Huppert, H.E., Kerr, R.C., McKenzie, D.P. and Tait, S.R. (1985) Postcumulus processes in layered intrusions. Geological Magazine, 122, 558568.CrossRefGoogle Scholar
Tepley, F.J. III and Davidson, J.P. (2003) Mineral-scale Sr-isotope constraints on magma evolution and magma chamber dynamics in the Rum layered intrusion, Scotland. Contributions to Mineralogy and Petrology, 145, 628641.CrossRefGoogle Scholar
Tilley, C.E. (1944) A note on the gneisses of Rum. Geological Magazine, 89, 129131.CrossRefGoogle Scholar
Troll, V.R., Emeleus, C.H. and Donaldson, C.H. (2000) Caldera formation in the Rum Central Igneous Complex, Scotland. Bulletin of Volcanology, 62, 301317.CrossRefGoogle Scholar
Troll, V.R., Donaldson, C.H. and Emeleus, C.H. (2004) Pre-eruptive magma mixing in ash-flow deposits in the Tertiary Rum Igneous Centre, Scotland. Contributions to Mineralogy and Petrology, 147, 722739.CrossRefGoogle Scholar
Troll, V.R., Nicoll, G.R., Donaldson, C.H. and Emeleus, C.H. (2008) Dating the onset of volcanism at the Rum Igneous Centre, NW Scotland. Journal of the Geological Society, London, 165, 651659.CrossRefGoogle Scholar
Upton, B.G.J., Skovgaard, A.C., McClurg, J.E., Kirstein, L., Cheadle, M.J., Emeleus, C.H., Wadsworth, W.J. and Fallick, A.E. (2002) Picritic magmas in the Rum ultramafic complex, Scotland. Geological Magazine, 139, 437452.CrossRefGoogle Scholar
Volker, J.A. and Upton, B.G.J. (1990) The structure and petrogenesis of the Trallval and Ruinsival area of the Rhum ultrabasic complex. Transactions of the Royal Society of Edinburgh: Earth Sciences, 81, 6988.CrossRefGoogle Scholar
Volker, J.A. and Upton, B.G.J. (1991) Reply to comments by J.H. Bédard and R.S.J. Sparks. Transactions of the Royal Society of Edinburgh: Earth Sciences, 82, 391.CrossRefGoogle Scholar
Wadsworth, W.J. (1961) The layered ultrabasic rocks of south-west Rhum, Inner Hebrides. Philosophical Transactions of the Royal Society of London. 244B, 21–64.Google Scholar
Wadsworth, W.J. (1985) Layered intrusions – a fluid situation? Geology Today, 2, 5054.CrossRefGoogle Scholar
Wadsworth, W.J. (1992) Ultrabasic igneous breccias of the Long Loch area, Isle of Rhum. Scottish Journal of Geology, 28, 103113.CrossRefGoogle Scholar
Wager, L.R. and Brown, G.M. (1968) Layered Igneous Rock. Oliver and Boyd, Edinburgh.Google Scholar
Wager, L.R., Brown, G.M. and Wadsworth, J.W. (1960) Types of igneous cumulates. Journal of Petrology, 1, 7385.CrossRefGoogle Scholar
Williamson, I.T. and Bell, B.R. (1994) The Palaeocene lava field of west-central Skye, Scotland: stratigraphy, palaeogeography and structure. Transactions of the Royal Society of Edinburgh: Earth Sciences, 89, 3975.CrossRefGoogle Scholar
Worrell, L.M., Cheadle, M.J., Coogan, L.A., Prior, D.J., Toplis, M.J. and Wheeler, J. (2003) Multidisciplinary approach to understanding the origin of peridotite cumulates. American Geophysical Union, Fall Meeting 2003, San Francisco, USA, Abstract #V11F-02. American Geophysical Union, Washington, USA.Google Scholar
Young, I.M. and Donaldson, C.H. (1985) Formation of granular-textured layers and laminae within the Rhum crystal pile. Geological Magazine, 122, 519528.CrossRefGoogle Scholar