Field studies on the eruption environment and prevailing cooling conditions of multi-tiered lava flows of the type exemplified by the Giant’s Causeway were carried out at localities on the Columbia River Plateau, southern Iceland, on the islands of Mull and Staffa and around the Giant’s Causeway, NE Ireland. Common features can be recognized in the volcanology of such flows in all the localities examined. These include palaeotopographic evidence of river valleys giving conditions conducive to the ponding of flows and the damming of existing drainage; evidence of high rainfall regimes or abundant surface water; association of interlava lacustrine and fluviatile sediments; association of hyaloclastite flow foot breccias within multi-tiered basalt lava successions and evidence of large volume lava flows and high eruption rates. Examination of the petrography of the upper curvi-columnar parts of the lava (the entablature) shows textural signs of quenching, compared with the lower more regular columnar zone (the colonnade). These common field and petrographic features confirm the model of multi-tiered flow formation by modification of the cooling regime in the upper part of the flow following inundation of the hot lava surface by displaced river drainage.

The columnar jointed basalts of the Giant's Causeway in Northern Ireland are a localized group of flows, the Causeway Tholeiite Member within the Tertiary Antrim Lava Group. Major and trace element analyses, including rare-earth element (REE) data, are presented which show a range of compositions from hy-normative tholeiites with mid-ocean ridge basalt (MORB) characteristics, to qz-normative examples with SiO 2 > 54% and K 2 O > 1.0%. Within the Causeway tholeiites are three well defined chemical sub-groups which can be correlated stratigraphically across the area. Zr/Nb data suggest a single mantle source for the lavas with the range of composition the result of variable degrees of partial melting of a progressively depleted mantle, followed by fractional crystallization of the magmas at higher crustal levels. Mass balance calculations are used to model the fractionation processes involved. The lavas of the Causeway result from tectonically controlled decompression melting with magma equilibration in ephemeral chambers at high crustal levels followed by eruption as sheet flows. Disruption of the contemporaneous drainage system and the subsequent flooding of hot flow surfaces modified the cooling regime and formed the characteristic multi-tiered flows with colonnade and entablature development.

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The lavas of NE Ireland represent the largest remnant of the British and Irish Tertiary Igneous Province. In this paper we present new Sr isotope data which, by virtue of extreme variation, provide evidence of extensive open system behaviour in the petrogenesis of these flood basalts. To some extent the initial 87 Sr/ 86 Sr ratios ( 87 Sr/ 86 Sr (60Ma) = 0.703–0.715) record the effects of late-stage hydrothermal processes. However, leaching experiments allow primary magmatic signatures to be inferred with confidence. Coherent increases in initial 87 Sr/ 86 Sr ratio with differentiation indices and incompatible trace element ratios imply a role for crustal contamination. Very significant differences in the style of crustal interaction are recorded by discrete units within the basalt pile. Further, there are also marked contrasts with Tertiary basalts elsewhere in the United Kingdom. In both cases such variations appear to respond to changes in the basement geology reflecting the important control of crustal wall-rocks on the geochemistry of these basalts.

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.