A tholeiitic andesite flow unit among the Causeway Basalts of North Antrim in Northern Ireland

Abstract

AbstractA tholeiitic andesite flow unit occurs in tholeiitic basalt lava in the Giant's Causeway region of North Antrim, Northern Ireland. It is the first example of an intermediate differentiate to be found among these quartz-normative basalts. Separate magma batches for the preceding and succeeding basalt formations are indicated by their Zr/P2O5 ratios, and by the differing fractionation trends shown by molecular proportion ratio plots. The tholeiitic andesite was probably extruded in a superheated condition with few crystal nuclei, and subsequent undercooling produced an unusual fasciculate/spherulitic texture in contrast to the very fine and even grain of the host basalt. A liquid–liquid interface between the flow units shows small-scale lava mixing.