Terrane geochemistry contrasts across the Iapetus Suture in Ireland

Abstract

In the Irish Caledonides, volcanism has been significant in terrane identification and in reconstructions of the Appalachian/Caledonian orogen. Crucial to these reconstructions is the recognition of ocean margins using obducted ocean floor relics (ophiolites) and supra-subduction zone volcanic assemblages. The volcanic rocks provide much evidence for the affinity of a terrane, however, by analogy with present day examples, the ocean floor sediments may provide the best way of tracing both ocean-floor magmatic activity, and continental source areas. This investigation shows that the Irish Lower Palaeozoic volcanogenic terranes can be discriminated in terms of their shale geochemistry, which also gives information on their provenance and environment of deposition. South Mayo shales are dominated by volcaniclastic material derived both from both an arc and from an ophiolitic source. The Northern and Central belts of the Central Terrane show very similar lithogeochemistries, apparently derived in part from intermediate to silicic volcanic 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-floor weathering, suggests a terrane with sediment of both volcanic and continental provenance being deposited in deeper water further from land. Across the suture the Leinster Terrane shows a mature chemistry which clearly suggests a continental provenance, together with a volcanogenic input from supra-subduction volcanism. This maturity is probably due to slower rates of sedimentation with longer residence times for volcanic detritus, plus the existence of a deeply weathered continental basement.

Implications for the lithospheric geometry of the Iapetus suture beneath Ireland based on electrical resistivity models from deep-probing magnetotellurics

“…Geochemical analyses of shale samples from selected belts within each terrane by Ryan et al (1995) support our interpretation. The analyses showed that the shales from the central and northern belts (LDM, Fig.…”

“…1) have low maturity, whereas those from southern part and the Leinster belt have relatively high maturity. In our study, although it is difficult to find exact spatial correspondence with the litho-geochemical analysis of Ryan et al (1995), the change in resistivity in the western part of Ireland is attributed to the similar processes, as discussed similarly by Korja et al (2008). Figs 8 and 9 show a prominent conductor in central Ireland at upper mantle depths of 60 km and deeper on profile V below the stations 105-107, that is not seen beneath the other profiles.…”