Hostname: page-component-78c5997874-g7gxr Total loading time: 0 Render date: 2024-11-09T15:21:11.116Z Has data issue: false hasContentIssue false
  • English
  • Français

The emplacement of geochemically distinct groups of rhyolites during the evolution of the Lower Rhyolitic Tuff Formation caldera (Ordovician), North Wales, U.K.

Published online by Cambridge University Press:  01 May 2009

S. D. G. Campbell ,
A. J. Reedman ,
M. F. Howells  and
A. C. Mann
S. D. G. Campbell
Affiliation:
British Geological Survey (NERC), Bryn Eithyn Hall, Llanfarian, Aberystwyth, Dyfed, Wales, SY23 4BY, U.K.
A. J. Reedman
Affiliation:
British Geological Survey (NERC), Bryn Eithyn Hall, Llanfarian, Aberystwyth, Dyfed, Wales, SY23 4BY, U.K.
M. F. Howells
Affiliation:
British Geological Survey (NERC), Bryn Eithyn Hall, Llanfarian, Aberystwyth, Dyfed, Wales, SY23 4BY, U.K.
A. C. Mann
Affiliation:
Department of Geology, Goldsmith's College, University of London, Rachel McMillan Building, Creek Road, London SE8 3BU, U.K.
Get access Rights & Permissions [Opens in a new window]

Abstract

Rhyolites in the vicinity of Snowdon (North Wales) are intimately associated with the evolution of the Lower Rhyolitic Tuff Formation (LRTF) caldera of Ordovician (Caradoc) age. They occur as deep-seated dykes, sills and small stocks, shallow-level intrusive domes, and domes extruded within a predominantly shallow-marine environment. Extrusion occurred during three main phases, indicating the episodic availability of rhyolite magma. The rhyolites can be divided on their trace element ratios (e.g. Nb/Zr) into five main groups. Extrusive representatives indicate that each group correlates strongly with a single phase of rhyolite extrusion. Within each group, the distribution and variation of intrusive form with stratigraphic level suggests that geochemically similar rocks were emplaced at approximately the same time. Consequently, the groups represent discrete magma compositions tapped from the evolving Snowdon subvolcanic magma system. Differences in distribution of the groups reflect changes in structural controls of emplacement before and after development of the LRTF caldera.

Type
Articles
Information
Geological Magazine , Volume 124 , Issue 6 , November 1987 , pp. 501 - 511
Copyright
Copyright © Cambridge University Press 1987

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.)

References

Allen, P. M. 1982. Lower Palaeozoic volcanism in Wales, the Welsh Borderland. Avon, and Somerset. In Igneous Rocks of the British Isles (ed. Sutherland, D.S.), pp. 65–91. London: Wiley.Google Scholar
Beavon, R. V. 1963. The succession and structure east of the Glaslyn river, North Wales. Quarterly Journal of the Geological Society of London 119, 479–512.CrossRefGoogle Scholar
Beavon, R. V. 1980. A resurgent cauldron in the early Palaeozoic of Wales, U.K. Journal of Volcanology and Geothermal Research 7, 157–74.CrossRefGoogle Scholar
Bevins, R. E. & Rowbotham, G. 1983. Low grade metamorphism within the Welsh sector of the paratectonic Caledonides. Geological Journal 18, 141–67.CrossRefGoogle Scholar
British, Geological Survey. 1986. 1:25000 Sheet, SH65/66, The Passes of ant Ffrancon and Llanberis. London: H.M.S.O.Google Scholar
British, Geological Survey. 1987. 1:25000 Sheet, SH65/66, Snowdon. London: H.M.S.O.Google Scholar
British, Geological Survey. In prep. 1:50000 Sheet, 119, Snowdon. London: H.M.S.O.Google Scholar
Bromley, A. V. 1969. Acid plutonic igneous activity in the Ordovician of North Wales. In The Precambrian and Lower Palaeozoic Rocks of Wales (ed. Wood, A.), pp. 387–408. Cardiff: University of Wales Press.Google Scholar
Crecraft, H. R., Nash, P. W. & EvansS. H., Jn. S. H., Jn. 1981. Late Cenozoic volcanism at Twin Peaks, Utah; geology and petrology. Journal of Geophysical Research 86, 10303–20.CrossRefGoogle Scholar
Eugster, H. P. 1985. Granites and hydrothermal ore deposits: a geochemical framework. Mineralogical Magazine 49, 7–23.CrossRefGoogle Scholar
Francis, E. H. & Howells, M. F. 1973. Transgressive welded ash-flow tuffs among the Ordovician sediments of N.E. Snowdonia, N. Wales. Journal of the Geological Society of London 129, 621–41.CrossRefGoogle Scholar
Howells, M. F., Campbell, S. D. G., Reedman, A. J. & Tunnicliff, S. P. 1987. An acidic fissure-controlled volcanic centre (Ordovician) at Yr Arddu, N. Wales. Geological Journal 21, 133–49.CrossRefGoogle Scholar
Howells, M. F., & Leveridge, B. E. 1980. The Capel Curig Volcanic Formation. Report of the Institute of Geological Sciences, London no. 80/6, 23 pp.Google Scholar
Howells, M. F., Leveridge, B. E., Addison, R., Evans, C. D. R. & Nutt, M. J. C. 1979. The Capel Curig Volcanic Formation, Snowdonia, North Wales; variations in ash-flow tuffs related to emplacement environment. In The Caledonides of the British Isles – Reviewed (ed. Harris, A. L. Holland, C. H., Leake, B. E.), pp. 611–18. Special Publication of the Geological Society of London no. 8. Edinburgh: Scottish Academic Press.Google Scholar
Howells, M. F., Leveridge, B. E., Addison, R. & Reedman, A. J. 1983. The lithostratigraphical subdivision of the Ordovician underlying the Snowdon and Crafnant volcanic groups, North Wales. Report of the Institute of Geological Sciences, London no. 83/1, 11–15.Google Scholar
Howells, M. F., Leveridge, B. E. & Evans, C. D. R. 1973. Ordovician ash flow tuffs in eastern Snowdonia. Report of the Institute of Geological Sciences, London no. 73/3, 33 pp.Google Scholar
Howells, M. F., Leveridge, B. E., Evans, C. D. E. & Nutt, M. J. C. 1981. Dolgarrog: Description of J: 25000 Geological Sheet SH76. Classical areas of British geology. Institute of Geological Sciences. (London: Her Majesty's Stationery Office.)Google Scholar
Howells, M. F., Reedman, A. J. & Campbell, S. D. G. 1986. The submarine eruption and emplacement of the Lower Rhyolithic Tuff Formation (Ordovician), N. Wales. Journal of the Geological Society of London 143, 411–24.CrossRefGoogle Scholar
Howells, M. F., Reedman, A. J. & Leveridge, B. E. 1985. Geology of the country around Bangor. Explanation for 1:50000 Sheet 106 (England and Wales). British Geological Survey. London: H.M.S.O. 34 pp.Google Scholar
Kokelaar, B. P. 1984. Fluidization of wet sediments during the emplacement and cooling of various igneous bodies. Journal of the Geological Society of London 139, 21–33.CrossRefGoogle Scholar
Kokelaar, B. P., Bevins, R. E. & Roach, R. A. 1985. Submarine silicic volcanism and associated sedimentary and tectonic processes, Ramsay Island, SW Wales. Journal of the Geological Society of London 142, 591–614.CrossRefGoogle Scholar
Kokelaar, B. P., Howells, M. F., Bevins, R. E., Roach, R. A. & Dunkley, P. N. 1984. The Ordovician marginal basin of Wales. In Marginal Basin Geology: Volcanic and Associated Sedimentary and Tectonic Processes in Modern and Ancient Marginal Basins (ed. Kokelaar, B. P. Howells, M. F.), pp. 245–69. Special Publication of the Geological Society of London no. 16. Oxford: Blackwell.Google Scholar
Leat, P. T., Jackson, S. E., Thorpe, R. S. & Stillman, C. J. 1986. Geochemistry of bimodal basalt-subalkaline/peralkaline rhyolite provinces associated with volcanogenic mineralization within the Southern British Caledonides. Journal of the Geological Society of London 143, 259–74.CrossRefGoogle Scholar
Leat, P. T. & Thorpe, R. S. 1986. Geochemistry of an Ordovician basalt-trachybasalt-subalkaline/peralkaline rhyolite association from the Lleyn Peninsula, North Wales, U.K. Geological Journal 21, 29–43.CrossRefGoogle Scholar
Mahood, G. A. 1984. Pyroclastic rocks and calderas associated with strongly peralkaline magmatism. Journal of Geophysical Research 89, 8540–52.CrossRefGoogle Scholar
Mahood, G. A. & Hildreth, W. 1983. Large partition coefficients for trace elements in high-silica rhyolites. Geochimica et Cosmochimica Acta 47, 11–30.CrossRefGoogle Scholar
Marsh, B. D. 1984. On the mechanics of caldera resurgence. Journal of Geophysical Research 86, 8245–52.CrossRefGoogle Scholar
Merriman, R. J., Bevins, R. E. & Ball, T. K. 1986. Geochemical variations within the Tal y fan intrusion: implications for element mobility during low-grade metamorphism. Journal of Petrology 27, 1409–36.CrossRefGoogle Scholar
O'Brien, C., Plant, J. A., Simpson, P. R. & Tarney, J. 1985. The geochemistry, metasomatism and petrogenesis of the granites of the English lake District. Journal of the Geological Society of London 142, 1139–58.CrossRefGoogle Scholar
Pearce, J. A. & Cann, J. R. 1973. Tectonic setting of basic volcanic rocks determined by using trace element analyses. Earth and Plantetary Science Letters 19, 290–300.CrossRefGoogle Scholar
Rast, N. 1961. Mid-Ordovician structures in south-western Snowdonia. Liverpool and Manchester Geological Journal 2, 645–51.CrossRefGoogle Scholar
Rast, N. 1969. The relationship between Ordovician structure and volcanicity in Wales. In The Pre-cambrian and Lower Palaeozoic rocks of Wales (ed. Wood, A.), pp. 305–35. Cardiff: University of Wales.Google Scholar
Reedman, A. J., Colman, T. B., Campbell, S. D. G. & Howells, M. F. 1985. Volcanogenic mineralization related to the Snowdon Volcanic Group (Ordovician), North Wales. Journal of the Geological Society of London 142, 875–88.CrossRefGoogle Scholar
Roberts, B. 1981. Low grade and very low grade regional metabasic Ordovician rocks of Llyn and Snowdonia, Gwynedd, North Wales. Geological Magazine 118, 189–200.CrossRefGoogle Scholar
Roberts, B. & Merriman, R. J. 1985. The distinction between Caledonian burial and regional metamorphism in metapelites from North Wales: an analysis of isocryst patterns. Journal of the Geological Society of London 142, 615–24.CrossRefGoogle Scholar