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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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