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

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.

Early Tertiary basalts and tuffaceous sandstones from the Hebrides Shelf and Wyville-Thomson Ridge, NE Atlantic

“…7), with the light REE showing a slightly convex-upward profile and slight depletion of the heavy REE: (La/Yb)N > 1. This pattern is comparable with other rocks of the BTIP, and in particular is very similar to that shown by the Mull Plateau Group ) and the Lower Basalt Formation of Antrim (Lyle 1985). It is also comparable to the upper basalts of Well 163/6-1A in the basinal parts of the northern Rockall Trough (Morton et al 1988b).…”

“…8). Although differing from the main Plateau Lavas of the BTIP, analogues to the 85/7 basalts may be found in the Preshal Mhor lavas of Skye and Mull ) and the Upper Basalts of Antrim (Lyle 1985). They differ from the basalts of 163/6-1A by being more light REE depleted.…”

Traps and Ocean Formation

“…At the same time, there are differences between the traps at the continental margins of the Northern and Southern Atlantic; the most important of these are the presence of subalkaline and alkaline rocks among Early Cenozoic traps of the Britain-Arctic province, a higher total magnesium content in the basalts, and lower contents of titanium, potassium, strontium, and rubidium [38]. All of these features, however, are within the range of compositional parameters for riftogenic igneous rock associations and can be explained by repeated melting of mantle material in the region during epochs preceding the opening of the present-day Atlantic Ocean and the generation of magmas over the course of Paleocene-Eocene riftogenesis [29].…”

“…Almost the only possible pair for such comparison are the Cretaceous traps, formed prior to the opening of the South Atlantic, and Tertiary basalts of the Britain-Arctic province (Great Britain, Ireland, Greenland, Baffin Land, Faroe Islands, etc.) associated with riftogenesis during the formation of the North Atlantic [29].…”

Spinel lherzolite and other xenoliths from a dolerite dyke in southwest Donegal

“…The dyke is considerably richer in K 2 O, Rb, Sr, Nb and Zr than any of the Tertiary Antrim Lava Series basalts (Lyle, 1980(Lyle, , 1985 or Tertiary dolerite intrusions from western Ireland (Mohr, 1982). However, the K 2 O and Rb are closely similar to those of Carboniferous Logmor dykes from West Connacht, as are the high total Fe and low Na 2 O/K 2 O (Mitchell & Mohr, 1987).…”