Ordovician Fishguard Volcanic Group, SW Wales

19 The Fishguard Volcanic Group represents an excellently preserved example of a volcanic 20 sequence linked to the closure of the Iapetus Ocean. This study re-examines the petrogenesis 21 and proposed tectonic setting for the Llanvirn (467-458 Ma) Fishguard Volcanic Group, 22 South Wales, UK. New major and trace element geochemical data and petrographic 23 observations are used to re-evaluate the magma chamber processes, mantle melting and 24 source region. The new data reveal that the Fishguard Volcanic Group represents a closely 25 related series of basalts, basaltic andesites, dacites and rhyolites originating from a spinel 26 lherzolite source which had been modified by subduction components. The rocks of the 27 Fishguard Volcanic Group are co-genetic and the felsic members are related to the more 28 primitive basalts mainly by low pressure fractional crystallisation. The geochemistry of the 29 lavas was significantly influenced by subduction processes associated with a coeval arc, 30 while significant amounts of assimilation of continental crust along with fractional 31 crystallisation appear to have contributed to the compositions of the most evolved lavas. The 32 Fishguard Volcanic Group was erupted into a back-arc basin where extensive rifting, but no 33 true seafloor spreading had occurred. 34