Geochronological evidence for phased volcanic activity in Fife and Caithness necks, Scotland

Abstract

In an attempt to establish a chronology for volcanic neck emplacement and so elucidate petrogenesis, isotopic studies have been carried out on various cumulate inclusions, blocks and megacrysts which occur chiefly in association with tuffs infilling several Scottish vents. K-Ar ages of 13 samples of low-pressure cumulate minerals (biotite, hornblende and pyroxene) from necks in East Fife indicate crystallisation at shallow depth at 314 Ma. U-Pb analyses of zircons are concordant at 318 Ma suggesting they are also members of this suite and their formation is penecontemporaneous with the Namurian volcanic activity which is welldocumented stratigraphically. By 295 Ma crystallisation of anorthoclase megacrysts had been completed, perhaps from the fractionated residuum. An eruption from considerable depth (within the stability field of garnet precipitation) then broke through to the surface bearing high-pressure megacrysts. This penetrated and disrupted the early cumulates carrying them to the surface and producing the diverse vent assemblages. K-Ar dating of basanites suggest that the Duncansby Ness neck in Caithness was emplaced around 270 Ma in the early Permian. For two Fife necks the balance of evidence favours an age of 290 Ma (Stephanian) for this final explosive activity associated with vent formation.