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Hidden Archaean and Palaeoproterozoic crust in NW Ireland? Evidence from zircon Hf isotopic data from granitoid intrusions

Published online by Cambridge University Press:  07 September 2009

M. J. FLOWERDEW*
Affiliation:
British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET, UK
D. M. CHEW
Affiliation:
Department of Geology, Trinity College Dublin, Dublin 2, Ireland
J. S. DALY
Affiliation:
UCD School of Geological Sciences, University College Dublin, Belfield, Dublin 4, Ireland
I. L. MILLAR
Affiliation:
NERC isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham, NG12 5GG, UK
*
Author for correspondence: mf@bas.ac.uk

Abstract

The presence of major crystalline basement provinces at depth in NW Ireland is inferred from in situ Hf isotope analysis of zircons from granitoid rocks that cut structurally overlying metasedimentary rocks. Granitoids in two of these units, the Slishwood Division and the Tyrone Central Inlier, contain complex zircons with core and rim structures. In both cases, cores have average ϵHf values that differ from the average ϵHf values of the rims at 470 Ma (the time of granitoid intrusion). The Hf data and similarity in U–Pb age between the inherited cores and detrital zircons from the host metasedimentary rocks suggests local contamination during intrusion rather than transport of the grains from the source region at depth. Rims from the Slishwood Division intrusions have average ϵHf470 values of −7.7, consistent with a derivation from juvenile Palaeoproterozoic crust, such as the Annagh Gneiss Complex or Rhinns Complex of NW Ireland, implying that the deep crust underlying the Slishwood Division is made of similar material. Rims from the Tyrone Central Inlier have extremely negative ϵHf470 values of approximately −39. This isotopic signature requires an Archaean source, suggesting rocks similar to the Lewisian Complex of Scotland, or sediment derived wholly from it, occurs at depth in NW Ireland.

Type
Original Article
Copyright
Copyright © Cambridge University Press 2009

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