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The emplacement of geochemically distinct groups of rhyolites during the evolution of the Lower Rhyolitic Tuff Formation caldera (Ordovician), North Wales, U.K.

Published online by Cambridge University Press:  01 May 2009

S. D. G. Campbell ,
A. J. Reedman ,
M. F. Howells  and
A. C. Mann
S. D. G. Campbell
Affiliation:
British Geological Survey (NERC), Bryn Eithyn Hall, Llanfarian, Aberystwyth, Dyfed, Wales, SY23 4BY, U.K.
A. J. Reedman
Affiliation:
British Geological Survey (NERC), Bryn Eithyn Hall, Llanfarian, Aberystwyth, Dyfed, Wales, SY23 4BY, U.K.
M. F. Howells
Affiliation:
British Geological Survey (NERC), Bryn Eithyn Hall, Llanfarian, Aberystwyth, Dyfed, Wales, SY23 4BY, U.K.
A. C. Mann
Affiliation:
Department of Geology, Goldsmith's College, University of London, Rachel McMillan Building, Creek Road, London SE8 3BU, U.K.
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Abstract

Rhyolites in the vicinity of Snowdon (North Wales) are intimately associated with the evolution of the Lower Rhyolitic Tuff Formation (LRTF) caldera of Ordovician (Caradoc) age. They occur as deep-seated dykes, sills and small stocks, shallow-level intrusive domes, and domes extruded within a predominantly shallow-marine environment. Extrusion occurred during three main phases, indicating the episodic availability of rhyolite magma. The rhyolites can be divided on their trace element ratios (e.g. Nb/Zr) into five main groups. Extrusive representatives indicate that each group correlates strongly with a single phase of rhyolite extrusion. Within each group, the distribution and variation of intrusive form with stratigraphic level suggests that geochemically similar rocks were emplaced at approximately the same time. Consequently, the groups represent discrete magma compositions tapped from the evolving Snowdon subvolcanic magma system. Differences in distribution of the groups reflect changes in structural controls of emplacement before and after development of the LRTF caldera.

Type
Articles
Information
Geological Magazine , Volume 124 , Issue 6 , November 1987 , pp. 501 - 511
Copyright
Copyright © Cambridge University Press 1987

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