Hostname: page-component-77c89778f8-7drxs Total loading time: 0 Render date: 2024-07-19T23:13:43.007Z Has data issue: false hasContentIssue false
  • English
  • Français

The influence of strain, lithology and stratigraphical depth on white mica (illite) crystallinity in mudrocks from the vicinity of the Corris Slate Belt, Wales: implications for the timing of metamorphism in the Welsh Basin

Published online by Cambridge University Press:  01 May 2009

B. Roberts ,
R. J. Merriman  and
W. Pratt
B. Roberts
Affiliation:
Geology Department, Birkbeck College, Malet Street, London WC1E 7HX, U.K.
R. J. Merriman
Affiliation:
British Geological Survey, Keyworth, Nottingham NG12 5GG, U.K.
W. Pratt
Affiliation:
British Geological Survey, Bryn Eithyn Hall, Llanfarian, Aberystwyth, Dyfed SY23 4BY, U.K.
Get access Rights & Permissions [Opens in a new window]

Abstract

Low and very low grade metamorphism in a mudrock-dominated succession, ranging from Upper Cambrian to Wenlock, is characterized by the white mica (illite) crystallinity technique. Kubler indices indicate that grade ranges from late diagenetic to low epizonal. The prograde reaction most easily observed by XRD study is the progressive ordering of K/Na micas; diagnostic 2M1 polytypic reflections become recognizable at 0.4 Δ 2θ and strengthen as grade advances. Grade generally correlates with thickness of overburden in the > 9 km thick succession. However, strain exercises additionally a profound influence on grade, and the distribution of strain was related to décollement resulting from inhomogeneities and ductility contrasts within the succession. Strain energy is seen as an additional and important variable in the development of white mica crystallinity in slates. The distribution of metamorphic grade implies that the Lower Palaeozoic succession was augmented by several kilometres of Lower Devonian (Old Red Sandstone) strata at the metamorphic culmination. The culmination was syntectonic and represents an Acadian rather than an early diastathermal event. Subsequent dextral movement on the Bala Fault displaced the regional isocryst pattern by c. 4 km. Retrogressive smectite appears to be localized to the vicinity of the Bala Fault and is probably of Variscan age.

Type
Articles
Information
Geological Magazine , Volume 128 , Issue 6 , November 1991 , pp. 633 - 645
Copyright
Copyright © Cambridge University Press 1991

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

Aldahan, A. A. & Morad, S. 1986. Mineralogy and chemistry of diagenetic clay minerals in Proterozoic sandstones from Sweden. American Journal of Science 286, 29–80.CrossRefGoogle Scholar
Allen, P. M. & Jackson, A. A. 1985. Geology of the country around Harlech. Memoir for 1:50000 geological sheet 135 with part of sheet 149 (England and Wales). British Geological Survey.Google Scholar
Austin, G. S., Glass, H. D. & Hughes, R. E. 1989. Resolution of the polytype structure of some illitic clay minerals that appear to be 1Md. Clays and Clay Minerals 37, 128–34.CrossRefGoogle Scholar
Bevins, R. E. & Merriman, R. J. 1988. Compositional controls on co-existing prehnite-actinolite and prehnite-pumpellyite facies assemblages in the Tal y Fan metabasite intrusion, North Wales: implications for Caledonian metamorphic field gradients. Journal of Metamorphic Geology 6, 17–39.CrossRefGoogle Scholar
Bevins, R. E. & Robinson, D. 1988. Low grade metamorphism of the Welsh Basin Lower Palaeozoic succession: an example of diastathermal metamorphism? Journal of the Geological Society, London 145, 363–6.CrossRefGoogle Scholar
Blenkinsop, T. G. 1988. Definition of low grade metamorphic zones using illite crystallinity. Journal of Metamorphic Geology 6, 623–6.CrossRefGoogle Scholar
Bottrell, S. H., Greenwood, P. B., Yardley, B. W. D., Shepherd, T. J. & Spiro, B. 1990. Metamorphic and post-metamorphic fluid flow in the low-grade rocks of the Harlech Dome, north Wales. Journal of Metamorphic Geology 8, 131–43.CrossRefGoogle Scholar
British Geological Survey, 1984. 1:50000 Series Geological Map Sheet 163, Aberystwyth. Keyworth: British Geological Survey.Google Scholar
Cave, R. & Hains, B. A. 1986. Geology of the country between Aberystwyth and Machynlleth. Memoir for 1:50000 geological sheet 163 (England and Wales). British Geological Survey.Google Scholar
Cox, A. H. & Wells, A. K. 1921. The Lower Palaeozoic rocks of the Arthog-Dolgelley district. Quarterly Journal of the Geological Society of London 76,254–324.CrossRefGoogle Scholar
Dunkley, P. N. 1979. Ordovician volcanicity of the SE Harlech Dome. 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
Flehmig, W. 1973. Kristallinität und Infrarotspektroskopie naturlicher dioktaedrischer illite. Neues Jarbuch für Mineralogie Monatshefte 1973, 351–61.CrossRefGoogle Scholar
Flehmig, W. & Langheinrich, G. 1974. Beziehung zwischen tektonischer Deformation und illit-Kristal-linität. Neues Jarbuch für Geologie und Palaontologie Abhandlungen 146, 325–6.Google Scholar
Frey, M. 1969. A mixed layer paragonite/phengite of low grade metamorphic origin. Contributions to Mineralogy and Petrology 24, 63–5.CrossRefGoogle Scholar
Frey, M. 1987. Very low grade metamorphism of clastic sedimentary rocks. In Low Temperature Metamorphism (ed. Frey, M.), pp. 9–58. Chapman & Hall.Google Scholar
Frey, M., Hunziker, J. C., Rogwiller, P. & Schindler, C. 1973. Progressive neidriggradige Metamorphose glaukonitführender Horizonte in den helvetischen Alpen der Ostschweiz. Contributions to Mineralogy and Petrology 39, 185–218.CrossRefGoogle Scholar
James, D. M. D. 1972. Sedimentation across an intra-basinal slope: the Garnedd-wen Formation (Ashgillian), West-central Wales. Sedimentary Geology 6, 255–70.CrossRefGoogle Scholar
James, D. M. D. 1973. The Garnedd-wen Formation (Ashgillian) of the Towyn-Abergynolwyn district, Merion-ethshire. Geological Magazine 110, 145–52.CrossRefGoogle Scholar
Jehu, R. M. 1926. The geology of the district around Towyn and Abergynolwyn. Quarterly Journal of the Geological Society of London 82, 465–89.CrossRefGoogle Scholar
Jiang, W. T., Peacor, D. R., Merriman, R. J. & Roberts, B. 1990. Transmission and analytical electron microscopic study of mixed-layer illite/smectite formed as an apparent replacement product of diagenetic illite. Clays and Clay Minerals 38, 449–68.CrossRefGoogle Scholar
Jones, O. T. & Pugh, W. J. 1916. The geology of the district around Machynlleth and the Llyfnant Valley. Quarterly Journal of the Geological Society of London 71, 343–85.CrossRefGoogle Scholar
Jones, O. T. 1935. The geology of the districts around Machynlleth and Aberystwyth. Proceedings of the Geologists' Association 46, 247–300.CrossRefGoogle Scholar
Kisch, H. J. 1980. Incipient metamorphism of Cambro-Silurian clastic rocks from the Jamtland Supergroup, central Scandinavian Caledonides, western Sweden: illite crystallinity and ‘vitrinite’ reflectance. Journal of the Geological Society, London 137, 271–88.CrossRefGoogle Scholar
Kisch, H. J. 1987. Correlation between indicators of very low grade metamorphism. In Low Temperature Metamorphism (ed. Frey, M.), pp. 227–300. Chapman & Hall.Google Scholar
Knipe, R. J. 1981. The interaction of metamorphism and deformation in slates. Tectonophysics 78, 249–72.CrossRefGoogle 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
Kubler, B. 1967 a. La crystallinité de l'illite et les zones tout à fait supérieures du métamorphisme. In Étages Tectonique, pp. 105–21. Colloque de Neuchâtel, 1966, À la Baconnière, Neuchâtel, Suisse.Google Scholar
Kubler, B. 1967 b. Anchimétamorphisme et schistosit´. Bulletin-Centre de Recherches Pau-SNFA 1, 259–78.Google Scholar
Kubler, B. 1968. Evaluation quantitative du métamorphisme par la crystallinityé de l'illite. Bulletin-Centre de Recherches Pau-SNFA 2, 385–97.Google Scholar
Liou, J. G., Maruyama, S. & Cho, M. 1985. Phase equilibria and mineral parageneses of metabasites in low grade metamorphism. Mineralogical Magazine 49, 321–33.CrossRefGoogle Scholar
Lockley, M. G. 1980. The geology of the Llanuwchllyn to Llanymawddwy area, South Gwynedd, North Wales. Geological Journal 15, 21–41.CrossRefGoogle Scholar
Martin, B. A., Howells, M. F. & Reedman, A. J. 1981. A geological reconnaissance study of the Dyfi Valley region, Gwynedd/Powys, Wales. Natural Environment Research Council Report No. ENPU 81–1, 57 pp.Google 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.CrossRefGoogle Scholar
Merriman, R. J., Roberts, B. & Peacor, D. R. 1990. A transmission electron microscope study of white mica crystallite size distribution in a mudstone to slate transitional sequence, North Wales, U.K. Contributions to Mineralogy and Petrology 106, 27–40.CrossRefGoogle Scholar
North, F. J. 1946. The Slates of Wales. Cardiff: National Museum of Wales, 121 pp.Google Scholar
Nyk, R. 1985. Illite crystallinity in Devonian slates of the Meggen mine (Rhenish Massif). Neues Jarbuch für Mineralogie Monatshefte 1985 (6), 268–76.Google Scholar
Ostwald, W. 1900. Über die vermeintliche Isomerie des roten und gelben Quecksilberoxyds und die Ober-flächenspannung fester Körper. Zeitschriftfür Physikalische Chemie, Stoechiometrie und Verwandschaftslehre 34, 495–503.CrossRefGoogle Scholar
Powell, C. McA. 1979. A morphological classification of rock cleavage. In Microstructural Processes during Deformation and Metamorphism (eds Bell, T. H. and Vernon, R. H.). Tectonophysics 58, 21–34.Google Scholar
Pratt, W. T. 1991. The Nod Glas décollement of the Welsh Basin. Geological Magazine 128, 279–82.CrossRefGoogle Scholar
Pugh, W. J. 1923. The geology of the district around Corris and Aberllefenni. Quarterly Journal of the Geological Society of London 79, 508–41.CrossRefGoogle Scholar
Pugh, W. J. 1928. The geology of the district around Dinas Mawddwy. Quarterly Journal of the Geological Society of London 84, 345–81.CrossRefGoogle Scholar
Pugh, W. J. 1929. The geology of the district around Llanymawddwy and Llanuwchllyn. Quarterly Journal of the Geological Society of London 85, 242–306.CrossRefGoogle Scholar
Ridgway, J. 1975. The stratigraphy of Ordovician volcanic rocks around the south and east of the Harlech Dome in Merionethshire, North Wales. Geological Journal 10, 87–106.CrossRefGoogle Scholar
Roberts, B. & Merriman, R. J. 1985. The distinction between Caledonian burial and regional metamorphism in pelites from North Wales: an analysis of isocryst patterns. Journal of the Geological Society, London 142, 615–24.CrossRefGoogle Scholar
Roberts, B., Evans, J. A., Merriman, R. J. & Smith, M. 1989. Discussion on low grade metamorphism of the Welsh Basin Lower Palaeozoic succession: an example of diastathermal metamorphism? Journal of the Geological Society, London 146, 885–90.Google Scholar
Roberts, B., Morrison, C. & Hirons, S. 1990. Low grade metamorphism of the Manx Group, Isle of Man: a comparative study of white mica crystallinity techniques. Journal of the Geological Society, London 147, 271–77.CrossRefGoogle Scholar
Robinson, D. 1987. The transition from diagenesis to metamorphism in extensional and collisional settings. Geology 15, 966–9.2.0.CO;2>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.CrossRefGoogle Scholar
Robinson, D., Warr, L. & Bevins, R. E. 1990. The illite ‘crystallinity’ technique: a critical appraisal of its precision. Journal of Metamorphic Geology 8, 333–44.CrossRefGoogle Scholar
Smith, B. & George, T. N. 1961. British Regional Geology: North Wales. London: H.M.S.O. 97 pp.Google Scholar
Symkatz-Kloss, W. & Althaus, E. 1974. Experimental investigation of the temperature dependence of the ‘crystallinity’ of illites and glauconites. Bulletin Groupe Français des Argiles 26, 319–25.CrossRefGoogle Scholar