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A re-appraisal of the petrogenesis and tectonic setting of the Ordovician Fishguard Volcanic Group, SW Wales

Published online by Cambridge University Press:  28 July 2015

BETHAN A. PHILLIPS ,
ANDREW C. KERR  and
RICHARD BEVINS
BETHAN A. PHILLIPS*
Affiliation:
School of Earth and Ocean Sciences, Cardiff University, Park Place, Cardiff CF10 3AT, Wales
ANDREW C. KERR
Affiliation:
School of Earth and Ocean Sciences, Cardiff University, Park Place, Cardiff CF10 3AT, Wales
RICHARD BEVINS
Affiliation:
Department of Natural Sciences, National Museum Cardiff, Cathays Park, Cardiff CF10 3NP, Wales
*
†Author for correspondence: [email protected]
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Abstract

The Fishguard Volcanic Group represents an excellently preserved example of a volcanic sequence linked to the closure of the Iapetus Ocean. This study re-examines the petrogenesis and proposed tectonic setting for the Llanvirn (467–458 Ma) Fishguard Volcanic Group, South Wales, UK. New major and trace element geochemical data and petrographic observations are used to re-evaluate the magma chamber processes, mantle melting and source region. The new data reveal that the Fishguard Volcanic Group represents a closely related series of basalts, basaltic andesites, dacites and rhyolites originating from a spinel lherzolite source which had been modified by subduction components. The rocks of the Fishguard Volcanic Group are co-genetic and the felsic members are related to the more primitive basalts mainly by low-pressure fractional crystallization. The geochemistry of the lavas was significantly influenced by subduction processes associated with a coeval arc, while significant amounts of assimilation of continental crust along with fractional crystallization appear to have contributed to the compositions of the most evolved lavas. The Fishguard Volcanic Group was erupted into a back-arc basin where extensive rifting but no true seafloor spreading had occurred.

Keywords

geochemistrymagmatismback-arc basinsubduction
Type
Original Articles
Information
Geological Magazine , Volume 153 , Issue 3 , May 2016 , pp. 410 - 425
Copyright
Copyright © Cambridge University Press 2015 

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