Abstract
The Black Ball Head diatreme on the Beara Peninsula (Co. Cork, south-west Ireland) intrudes Lower Carboniferous marine siliciclastic sediments, and comprises an inner and an outer zone of pyroclastic infill. The outer zone is dominated by lithoclasts of local sediments and igneous rocks, whereas the inner zone is characterised by an assemblage of kaersutite, phlogopite and Ti-magnetite megacrysts and ultramafic pyroxenite nodules. 40Ar/39Ar step-heating dating of two kaersutite megacrysts from the inner zone yielded a c. 318 ‘ 3Ma plateau age (2ao). This late Namurian age is interpreted to be the age of diatreme emplacement. The pyroclastic infill is foliated and the c. 318Ma age therefore provides an older age limit for the Variscan deformation in this part of the Rhenohercynian fold belt.
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