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Low grade and very low grade regional metabasic Ordovician rocks of Llŷn and Snowdonia, Gwynedd, North Wales

Published online by Cambridge University Press:  01 May 2009

B. Roberts
B. Roberts*
Affiliation:
Geology Department, Birkbeck College, 7–15 Gresse Street, London WIP IPA
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Summary

Metabasic assemblages belong to the prehnite-pumpellyite and greenschist facies. The isograds pumpellyite-in, pumpellyite-out/clinozoisite-in and biotite-in have been mapped, and divide the area into four metamorphic zones. Field and petrographic evidence indicate that the metamorphic crystallization was largely syn-and immediately post-tectonic, viz. end-Silurian to Devonian. The use of published Schreinemakers' bundles indicates that most of the prehnite-pumpellyite-bearing assemblages formed at high grades within the facies and that the greenschist facies rocks crystallized in the higher pressure field. As the amount of calcite increases so diagnostic Ca–Al silicates are reduced and eventually eliminated from assemblages. Contrary to recent claims, the evidence indicates that the Gwna Group and the Gwyddel Felsitic Beds of Llŷn were metamorphosed prior to the Arenig.

Type
Research Article
Information
Geological Magazine , Volume 118 , Issue 2 , March 1981 , pp. 189 - 200
Copyright
Copyright © Cambridge University Press 1981

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References

Albee, A. L. 1962. Relationships between the mineral association, chemical composition and physical properties of the chlorite series. Am. Miner. 47 851–70.Google Scholar
Anderson, J. G. C. & Owen, T. R. 1968. The Structure of the British Isles. Oxford: Pergamon.Google Scholar
Baker, J. W. 1969. Correlation problems in metamorphosed Precambrian rocks in Wales and S.E. Ireland. Geol. Mag. 106 249–59.CrossRefGoogle Scholar
Barber, A. J. & Max, M. D. 1979. A new look at the Mona Complex (Anglesey, North Wales). J. geol. Soc. (Lond.) 136 407–32.CrossRefGoogle Scholar
Bevins, R. E. 1978. Pumpellyite-bearing basic igneous rocks from the Lower Ordovician of North Pembrokeshire, Wales. Mineralog. Mag. 42 81–3.CrossRefGoogle Scholar
Brown, E. H. 1971. Phase relations of biotite and stilpnomelane in the greenschist facies. Contrib. Mineral. Petrol. 31 275–99.CrossRefGoogle Scholar
Brown, E. H. 1975. A petrogenetic grid for reactions producing biotite and other Al-Fe-Mg silicates in the greenschist facies. J. Petrology. 16 258–71.Google Scholar
Coombs, D. S., Horodyski, R. J. & Naylor, R. S. 1970. Occurrence of prehnite-pumpellyite facies metamorphism in Northern Maine. Am. J. Sci. 268 142–56.CrossRefGoogle Scholar
Floyd, P. A., Lees, G. J. & Roach, R. A. 1976. Basic intrusions in the Ordovician of North Wales–geochemical data and tectonic setting. Proc. Geol. Ass. Lond. 87 389–400.CrossRefGoogle Scholar
Hawkins, T. R. W. 1970. Hornblende gabbros and picrites at Rhiw, Caernarvonshire. Geol. J. 7 1–24.CrossRefGoogle Scholar
Jenkins, D. A. & Ball, D. F. 1964. Pumpellyite in Snowdonian soils and rocks. Mineralog. Mag. 33 1093–6.Google Scholar
Matley, C. A. 1928. The Pre-Cambrian complex and associated rocks of south-western Lleyn (Caernarvonshire). Q. Jl geol. Soc. Lond. 84, 440–504.CrossRefGoogle Scholar
Muir, M. D., Bliss, G. M., Grant, P. R. & Fisher, M. 1979. Palaentological evidence for the age of some supposedly Precambrian rocks in Anglesey, North Wales. J. Geol. soc. (Lond.) 136 61–4.CrossRefGoogle Scholar
Nicholls, G. D. 1959. Autometasomatism in the Lower Spilites of the Builth Volcanic Series. Q. Jl geol. Soc. Lond. 114 137–61.CrossRefGoogle Scholar
Nitsch, K. H. 1971. Stabilitätsbeziehungen von Prehnit und Pumpellyithaltigen Paragenesen. Contrib. Mineral. Petrol. 30, 250–60.CrossRefGoogle Scholar
Roach, R. A. 1969. The composite nature of the St. David's Head and Carn Llidi intrusions of North Pembrokeshire. In The Pre-Cambrian and Lower Palaeozoic rocks of Wales (ed. Wood, A.), pp. 409–34. Cardiff: Univ. Wales Press.Google Scholar
Roberts, B. 1979. The geology of Snowdonia and Llŷn. Bristol: Adam Hilger.Google Scholar
Shackleton, R. M. 1954. The succession and structure of Anglesey and the Lleyn peninsula. Advmt Sci., Lond. XI (41), 106–8.Google Scholar
Shackleton, R. M. 1956. Notes on the structure and relations of the Pre-Cambrian and Ordovician rocks of south-western Lleyn (Caernarvonshire). Lpool Manchr geol. J. 1 400–9.CrossRefGoogle Scholar
Shackleton, R. M. 1969. The Pre-Cambrian of North Wales. In The Pre-Cambrian and Lower Palaeozoic rocks of Wales (ed. Wood, A.), pp. 1–22. Cardiff: Univ. Wales Press.Google Scholar
Wood, D. S. 1969. The base and correlation of the Cambrian rocks of North Wales. In The Pre-Cambrian and Lower Palaeozoic rocks of Wales (ed. Wood, A.), pp. 47–66. Cardiff: Univ. Wales Press.Google Scholar
Zen, E-an. 1974. Prehnite- and pumpellyite-bearing assemblages, West side of the Appalachian metamorphic belt, Pennsylvania to Newfoundland. J. Petrology 15 197–242.CrossRefGoogle Scholar