In the Irish Caledonides, volcanism has been significant in terrane id
entification and in reconstructions of the Appalachian/Caledonian orog
en. Crucial to these reconstructions is the recognition of ocean margi
ns using obducted ocean floor relies (ophiolites) and supra-subduction
zone volcanic assemblages. The volcanic rocks provide much evidence f
or the affinity of a terrane, however, by analogy with present day exa
mples, the ocean floor sediments may provide the best way of tracing b
oth ocean-floor magmatic activity, and continental source areas. This
investigation shows that the Irish Lower Palaeozoic volcanogenic terra
nes can be discriminated in terms of their shale geochemistry, which a
lso gives information on their provenance and environment of depositio
n. South Mayo shales are dominated by volcaniclastic material derived
both from both an are and from an ophiolitic source. The Northern and
Central belts of the Central Terrane show very similar lithogeo-chemis
tries, apparently derived in part from intermediate to silicic volcani
c complexes. The Ordovician-Silurian inliers that straddle the Suture
Zone, here termed the Southern Domain, show a chemistry close to that
of the Leinster Terrane, which, coupled with a greater degree of sea-f
loor weathering, suggests a terrane with sediment of both volcanic and
continental provenance being deposited in deeper water further from l
and. Across the suture the Leinster Terrane shows a mature chemistry w
hich clearly suggests a continental provenance, together with a volcan
ogenic input from supra-subduction volcanism. This maturity is probabl
y due to slower rates of sedimentation with longer residence times for
volcanic detritus, plus the existence of a deeply weathered continent
al basement.