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Journal of the Geological Society; 1991; v. 148; issue.4; p. 711-718;
DOI: 10.1144/gsjgs.148.4.0711
© 1991 Geological Society of London

Article

An Ordovician basalt to peralkaline rhyolite fractionation series from Avoca, Ireland

B. J. McCONNELL1,3, C. J. STILLMAN1 & J. HERTOGEN2

1 Department of Geology, Trinity College, Dublin, 2, Ireland
2 Fysico-chemische geologie, University of Leuven, B–3001 Leuven, Belgium
3 Present address: Department of Earth Sciences, University of Liverpool, PO Box 147, L69 3BX, UK

The Upper Ordovician Avoca volcanic rocks host a Kuroko-type massive sulphide deposit in southeast Co. Wicklow, Ireland. Immobile element geochemistry of lavas shows a bimodal subdivision into acid and basic groups. Basalts have characteristics of magmas from both subduction zone and enriched mantle sources, and are interpreted as products of the rifting of a volcanic arc. Rhyolites are highly enriched in HFS-elements and were originally peralkaline. Constant incompatible element ratios throughout the compositional spectrum indicate a fractional crystallization relationship between the basic and acid end members. The basaltic magmas evolved by crystallization of an assemblage in which plagioclase feldspar was an important component, producing large negative Eu anomalies. The transition to peralkalinity is marked by LREE depletion and no further increase in the Eu anomaly, which may represent the interval of minor phase precipitation and fO2 variation seen in other peralkaline series. A group of acid pyroclastic Avoca volcanic rocks are subalkaline and are probably unrelated to the main evolutionary series. The Avoca volcanic rocks were erupted in an extensional tectonic setting within a continental margin volcanic arc, similar to the contemporaneous Welsh marginal basin.




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