Geophysical and radiometric studies of the Northern Units of the Leinster Intrusion

The Leinster intrusion, S.E. Ireland, is one of a group of Caledonian granites, sensu lato , of Siluro‐Devonian age, which typically exhibit strong geophysical and geochemical contrasts with their host rocks. New gravity data supplement previous road‐based data and define, over the Northern Units of the intrusion, a closed residual anomaly (‐160 g.u.) with steep gradients. The Northern Units are shown to comprise a large volume of low‐density material extending to a depth of 8 km, and hence to constitute a significant contribution to Caledonian crustal growth. The sub‐surface form of the intrusion is essentially vertical‐sided having no broad root, in contrast with other Siluro‐Devonian granites ( s.l. ). Radioelement concentrations in the Northern Units were mapped quantitatively by in situ gamma‐ray spectrometry. The mean concentrations of 4·2 per cent K 2 O, 3·0 ppm U and 8·1 ppm Th measured are lower than those typical of the Siluro‐Devonian granites ( s.l. ), suggesting depletion associated with high‐level fractionation, and in addition a possible Th‐depleted source region. Complex areal radioelement distributions reflect both the multiple intrusive event, and later secondary remobilization, particularly of U. This is demonstrated using fission‐track radiography which shows that U, at least, is located in both primary and secondary minerals.