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The Rhinns Complex: Proterozoic basement on Islay and Colonsay, Inner Hebrides, Scotland, and on Inishtrahull, NW Ireland

Published online by Cambridge University Press:  03 November 2011

R. J. Muir ,
W. R. Fitches  and
A. J. Maltman
R. J. Muir
Affiliation:
Institute of Earth Studies, University of Wales, Aberystwyth, Dyfed, Wales, SY23 3DB, UK
W. R. Fitches
Affiliation:
Institute of Earth Studies, University of Wales, Aberystwyth, Dyfed, Wales, SY23 3DB, UK
A. J. Maltman
Affiliation:
Institute of Earth Studies, University of Wales, Aberystwyth, Dyfed, Wales, SY23 3DB, UK
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Abstract

The Precambrian basement on the islands of Islay, Colonsay and Inishtrahull comprises a deformed igneous association of mainly syenite and gabbro, with minor mafic and felsic intrusions. This association is collectively referred to as the Rhinns Complex. Isotopic data indicate that the complex represents new addition of material to the crust at c. 1·8 Ga. The igneous protolith was juvenile mantle-derived material, not reworked Archaean crust. Overall, the complex has an alkalic composition, with major and trace element patterns similar to igneous rocks generated in a subduction-related setting: high LILE/HFSE and LREE/HREE ratios, together with negative Nb, P and Ti anomalies.

The formation of the Rhinns Complex was contemporaneous with the Laxfordian tectonothermal cycle in the Lewisian Complex. These Proterozoic events are most likely associated with an extensive 1·9–1·7 Ga mobile belt around the southern margin of Laurentia-Baltica. As part of this belt, the Rhinns Complex forms a link between the Ketilidian province of South Greenland and the Svecofennian of Scandinavia.

Inherited isotopic signatures in the Caledonian granites on the north side of the Highland Boundary Fault may reflect the presence of a large area of Proterozoic basement (?Rhinns Complex) beneath Scotland and NW Ireland. Alternatively, the Proterozoic signature could be derived from the incorporation of Moine or Dalradian sediment into the granitic magmas.

Keywords

alkalic igneous provincesyenitegabbrogeochemistryNd isotopesLewisian ComplexLaxfordian
Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 85 , Issue 1 , 1994 , pp. 77 - 90
Copyright
Copyright © Royal Society of Edinburgh 1994

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