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Terrane geochemistry contrasts across the Iapetus Suture in Ireland

Published online by Cambridge University Press:  01 May 2009

P. D. Ryan ,
C. J. Stillman ,
C. J. Stillman  and
S. Pow
P. D. Ryan
Affiliation:
Geology Department, University College, Galway, Ireland
C. J. Stillman
Affiliation:
Geology Department, Trinity College, Dublin 2, Ireland
C. J. Stillman
Affiliation:
Geology Department, Trinity College, Dublin 2, Ireland
S. Pow
Affiliation:
Powergen, Powergen Technology Centre, Ratcliffe on Soar, Nottingham NG11 OEE, UK
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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.

Type
Articles
Information
Geological Magazine , Volume 132 , Issue 5 , September 1995 , pp. 581 - 597
Copyright
Copyright © Cambridge University Press 1995

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