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Pre-Caledonian History of the Annagh Gneiss Complex North-Western Ireland, and Correlation with Laurentia-Baltica

J. Stephen Daly
Irish Journal of Earth Sciences
Vol. 15 (1996), pp. 5-18
Published by: Royal Irish Academy
Stable URL: http://www.jstor.org/stable/30002311
Page Count: 14
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Pre-Caledonian History of the Annagh Gneiss Complex North-Western Ireland, and Correlation with Laurentia-Baltica
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Abstract

Precambrian rocks of Britain, Ireland and the adjacent continental shelf and Atlantic plateaux are important in tectonic reconstructions for the north Atlantic region. Until recently, knowledge of these regions has been based on poor-quality geochronology, Nd model ages or geological inferences alone. Now single-grain and small-fraction U-Pb abraded zircon geochronology has established a well-constrained chronology for the Annagh Gneiss Complex of north-west Mayo, Ireland. This chronology supports and extends recent attempts to correlate Precambrian events between Laurentia and Baltica. Much of the Annagh Gneiss Complex originated as juvenile Palaeoproterozoic crust represented by the $1753 \pm 3$ million year (Ma) old calc-alkaline Mullet gneisses. Late Mesoproterozoic Cross Point gneisses with A-type geochemistry and Palaeoproterozoic $t_{DM}$ ages were emplaced as anorogenic granltoids at $1271 \pm 6 Ma$, probably by melting of the pre-existing Palaeoproterozoic Mullet gneisses with the addition of a mantle-derived mafic component. The Doolough gneisses comprise a small volume of juvenile granitoids and associated basic rocks which formed at $1177 \pm 4 Ma$ The Doolough gneisses occur as a tectonic shyver, possibly incorporated along a Grenville high-strain zone. Their origin is unclear They may be a further manifestation of anorogenic magmatism or they may represent a much larger volume of juvenile subduction-related material which may underlie substantial areas of northern Ireland consistent with Sm-Nd data from basement xenoliths and Lower Palaeozoic volcanics. Grenville deformation occurred in two stages, between 1177 and 1015 Ma and from 995 to 960 Ma. These events were separated by the intrusion of the Doolough peralkaline granite at c $1015 \pm 4 Ma$ and by migmatisation and pegmatite emplacement between 995 and 980 Ma Post-Grenville metadolerites help to distinguish Caledonian strain from earlier deformation.

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