TERRANE GEOCHEMISTRY CONTRASTS ACROSS THE IAPETUS SUTURE IN IRELAND

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.