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Ordovician volcanism in the Welsh Borderland

Published online by Cambridge University Press:  01 May 2009

P. T. Leat  and
R. S. Thorpe
P. T. Leat
Affiliation:
Department of Geology, Imperial College of Science and Technology, London SW7 2BP, U.K.
R. S. Thorpe
Affiliation:
Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, U.K.
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Abstract

Basaltic, andesitic and rhyodacitic/rhyolitic volcanism was widespread during Ordovician time in the Welsh Basin. New chemical data are presented for Llanvirn to Caradoc lavas and tuffs from the Welsh Borderland which, during Ordovician time, formed the southeastern margin of the Welsh Basin. In view of the observed chemical alteration, immobile elements are used in the interpretation of the original geochemical character. The data indicate that the Llanvirn Stapeley volcanic group of the Shelve inlier was a bimodal basalt/basaltic andesite – rhyodacite/rhyolite association. The basalts have trace element contents of tholeiitic associations with a subduction-related character. The Caradoc Whittery and Hagley volcanic groups of the Shelve inlier comprise lavas and tuffs of calc-alkaline andesite. Blocks sampled from the Breidden Hills show that these were also derived from a calc-alkaline volcano. Associated Caradoc pumice- and ash-flow deposits from the Breidden Hills are probably of altered calc-alkaline rhyodacite/rhyolite composition. The Sibdon Carwood basalt flow, the only known example of Ordovician volcanism east of the Pontesford–Linley and Church Stretton lineaments, has transitional tholeiitic to alkaline character, with trace element contents influenced by subduction-related processes. The overall tholeiitic to calc-alkaline nature of the magmatism is consistent with the view that, during Llanvirn to Caradoc time, the Welsh Basin was an ensialic marginal basin.

Type
Articles
Information
Geological Magazine , Volume 123 , Issue 6 , November 1986 , pp. 629 - 640
Copyright
Copyright © Cambridge University Press 1986

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