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Cambrian and Ordovician metabentonites and their relevance to the origins of associated mudrocks in the northern sector of the Lower Palaeozoic Welsh marginal basin

Published online by Cambridge University Press:  01 May 2009

B. Roberts  and
R. J. Merriman
B. Roberts
Affiliation:
Geology Department, Birkbeck College, 7-15, Gresse Street, London W1P 1PA, U.K.
R. J. Merriman
Affiliation:
British Geological Survey, Keyworth, Nottingham NG12 5GG, U.K.
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Abstract

Middle Cambrian and Caradoc metabentonites and associated mudrocks are described from the northern part of the Welsh Lower Palaeozoic basin. They occur in areas of slight deformation, distant from thick piles of contemporaneous volcanics. The < 2 μm fraction from anchizonal Middle Cambrian metabentonites consists dominantly of 1Md micas (with up to 5% randomly interstratified smectite) with lesser 1M and 2M1 muscovite, corrensite, quartz, chlorite and kutnohorite. Interbedded mudrocks are similar but contain, additionally, minor paragonite and regular (6:4) paragonite-muscovite. Late diagenetic grade Caradoc bentonite consists of rectorite, pyrophyllite and small amounts of chlorite. Interbedded mudrocks are similar but contain, additionally, 1Md mica with up to 10% smectite interlayers, minor kaolinite and quartz.

Metabentonites contain high concentrations of the trace elements Ce, La, Hf, Nb, Ta, Th, Y and Zr in comparison with associated mudrocks. Discriminant diagrams indicate derivation from trachyandesite–dacite–rhyolite magmas in a within-plate setting. Major element variations in Cambrian metabentonites compare closely with those described in Silurian examples whereas the Caradoc metabentonite is closely similar to rhyolitic tuffs in the Caradoc Llewelyn Volcanic Group.

Both geochemical and field evidence indicate that whereas Middle Cambrian mudrocks were derived from bentonitic material adulterated variably by a terrigenous input. Lower Cambrian mudrocks were derived from a strongly depleted source such as basic Mona Complex rocks. Geochemical and field evidence suggest Caradoc mudrocks southeast of the Aber–Dinlle fault are largely volcanogenic; and the geological setting of Tremadoc, Arenig and Llanvirn mudrocks demands some volcanogenic input.

The Caradoc volcanic succession of Snowdonia is represented by a single bentonite northwest of the Aber–Dinlle fault. The fault system was either a highly effective contemporaneous barrier or, more probably, a site of post-Caradoc transcurrent movement.

Type
Articles
Information
Geological Magazine , Volume 127 , Issue 1 , January 1990 , pp. 31 - 43
Copyright
Copyright © Cambridge University Press 1990

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References

Ball, T. K. & Merriman, R. J. in press. The petrology and geochemistry of the Ordovician Llewelyn Volcanic Group, Snowdonia, North Wales. Research Reports of the British Geological Survey.Google Scholar
Batchelor, R. A. & Weir, J. A. 1988. Metabentonite geochemistry: magmatic cycles and graptolite extinctions at Dob's Linn, southern Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences 79, 19–41.Google Scholar
Bevins, R. E., Kokelaar, B. P. & Dunkley, P. N. 1984. Petrology and geochemistry of lower to middle Ordovician igneous rocks in Wales: a volcanic arc to marginal basin transition. Proceedings of the Geologists' Association 95, 337–47.Google Scholar
Blatt, H., Middleton, G. & Murray, R. 1980. Origin of Sedimentary Rocks. New Jersey: Prentice-Hall.Google Scholar
Cameron, T. D. J. & Anderson, T. B. 1980. Silurian metabentonites in County Down, Northern Ireland. Geological Journal 15, 59–75.CrossRefGoogle Scholar
Campbell, S. D. G., Reedman, A. J., Howells, M. F. & Mann, A. C. 1987. The emplacement of geochemically distinct groups of rhyolites during the evolution of the Lower Rhyolitic Tuff Formation caldera (Ordovician), N. Wales U. K. Geological Magazine 124, 501–11.CrossRefGoogle Scholar
Campbell, S. D. G., Howells, M. F., Smith, M. & Reedman, A. J. 1988. A Caradoc failed-rift within the Ordovician marginal basin of Wales. Geological Magazine 125, 257–66.Google Scholar
Coward, M. P. & Siddans, A. W. B. 1979. The tectonic evolution of the Welsh Caledonides. In The Caledonides of the British Isles – Reviewed (ed. Harris, A. L., Holland, C. H. and Leake, B. E.), pp. 187–98. Special Publication of the Geological Society of London no. 8.Google Scholar
Crimes, T. P. 1970. A facies analysis of the Cambrian of Wales. Palaeogeography, Palaeoclimatology, Palaeoecology 7, 113–70.CrossRefGoogle Scholar
Fisher, R. V. & Schmincke, H.-U. 1984. Pyroclastic Rocks. Berlin: Springer-Verlag.CrossRefGoogle Scholar
Frey, M. 1969. A mixed-layer paragonite/phengite of low grade metamorphic origin. Contributions to Mineralogy and Petrology 24, 63–5.CrossRefGoogle Scholar
Howells, M. F., Francis, E. H., Leveridge, B. E. & Evans, C. D. R. 1978. Capel Curig and Betws-y-Coed: Description of 1:25000 sheet SH 75. Classical areas of British Geology, Institute of Geological Sciences. London: H.M.S.O.Google 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
Jarvis, I. & Jarvis, K. E. 1985. Rare-earth element geochemistry of standard sediments: A study using inductively coupled plasma spectrometry. Chemical Geology 53, 335–44.Google Scholar
Kokelaar, B. P. 1979. Tremadoc to Llanvirn volcanism on the southeast side of the Harlech Dome (Rhobell Fawr), North Wales. In The Caledonides of the British Isles – Reviewed, (eds Harris, A. L. Holland, C. H. and Leake, B. E.), pp. 591–6. Special Publication of the Geological Society of London no. 8.Google 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 (eds Kokelaar, B. P. and Howells, M. F.), pp. 245–69. Special Publication of the Geological Society of London no. 16.Google Scholar
Kolata, D. R., Frost, J. K. & Huff, W. D. 1987. Chemical correlation of K-bentonite beds in the MiddleOrdovician Decorah Subgroup, Upper Mississippi Valley. Geology 15, 208–11.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 43, 259–73.CrossRefGoogle Scholar
Merriman, R. J. & Roberts, B. 1985. A survey of white mica crystallinity and polytypes in pelitic rocks of Snowdonia and Llŷn, North Wales. Mineralogical Magazine 49, 305–19.Google Scholar
Morgan, D. J., Highley, D. E. & Bland, D. J. 1978. A montmorillonite, kaolinite association in the Lower Cretaceous of south-east England. In International Clay Conference 1978 (eds Mortland, M. M. and Farmer, V. C.), pp. 301–10. Elsevier.Google Scholar
Nakamura, N. 1974. Determination of REE, Ba, Fe, Mg, Na and K in carbonaceous and ordinary chondrites. Geochimica el Cosmochimica Acta 38, 757–75.Google Scholar
Nicholas, T. C. 1915. The geology of the St. Tudwal's Peninsula (Caernarvonshire). Quarterly Journal of the Geological Society of London 71, 83–143.Google Scholar
Odin, G. S., Hurford, A. J., Morgan, D. J. & Toghill, P. 1986. K-Ar biotite date for Ludlovian bentonites from Great Britain. Chemical Geology (Isotope Geosciences Section) 59, 127–31.Google Scholar
Pearce, J. A., Harris, N. B. W. & Tindle, A. G. 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of Petrology 25, 956–83.Google Scholar
Reedman, A. J., Leveridge, B. E. & Evans, R. B. 1984. The Arfon Group (‘Arvonian’) of North Wales. Proceedings of the Geologists' Association 95, 313–21.CrossRefGoogle Scholar
Roberts, B. 1979. The Geology of Snowdonia and Llŷn. Bristol: Adam Hilger.Google Scholar
Roberts, B. 1981. Low grade and very low grade regional metabasic Ordovician rocks of Llŷn and Snowdonia, Gwynedd, North Wales. Geological Magazine 118, 189–200.Google 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
Robinson, D. & Bevins, R. E. 1986. Incipient metamorphism in the Lower Palaeozoic marginal basin of Wales. Journal of Metamorphic Geology 4, 101–13.Google Scholar
Schultz, L. G., Tourtelot, H. A. & Flanagan, F. J. 1976. Description and analyses of eight new USGS rock standards: Cody Shale, SCo-1, from Natrona County, Wyoming. United States Geological Survey Professional Paper no. 840, 21–23.Google Scholar
Shackleton, R. M. 1953. The structural evolution of North Wales. Liverpool and Manchester Geological Journal 1, 261–97.Google Scholar
Spears, D. A. & Kanaris-Sotiriou, R. K. 1979. A geochemical and clay mineralogical investigation of some British and other European tonsteins. Sedimentology 26, 407–25.CrossRefGoogle Scholar
Teale, C. T. & Spears, D. A. 1986. The mineralogy and origin of some Silurian bentonites, Welsh Borderland, U.K. Sedimentology 33, 757–65.Google Scholar
Thomson, J., Carpenter, M. S. N., Colley, S., Wilson, T. R. S., Elderfield, H. & Kennedy, H. 1984. Metal accumulation rates in northwest Atlantic pelagic sediments. Geochimica et Cosmochimica Acta 48, 1935–48.CrossRefGoogle Scholar
Trewin, N. H. 1971. Potassium bentonites in the Wenlock Limestone of Shropshire. The Mercian Geologist 4, 1–7.Google Scholar
Winchester, J. A. & Floyd, P. A. 1977. Geochemical discrimination of different magma series and their differentiation products using immobile elements. Chemical Geology 20, 325–43.CrossRefGoogle Scholar