The petrogenesis of the Tertiary basaltic and intermediate lavas of northeast Ireland

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Abstract
Synopsis: New trace element results are presented for the Tertiary basaltic and intermediate lavas in Antrim, including REE data available for the first time. The Lower Basalts are characteristically light REE enriched relative to chondritic values, the associated intermediate lavas are substantially enriched with marked negative Eu anomalies and the Upper Basalts are light REE depleted. The Lower Basalts probably formed by low degrees (<10%) of partial melting of a spinel lherzolite mantle with high-level fractionation of mainly olivine and plagioclase to produce the associated intermediate lavas. This mantle source may have been depleted by a previous magmatic episode in Permian times. The Upper Basalts probably result from a second stage fusion of the same mantle source with partial melting greater than 10%. They represent a change to magmas resembling Preshal Mhor type with MORB characteristics and are the final stages of an abortive rifting episode recognized elsewhere in the British Tertiary Volcanic Province.