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The structure and petrology of the Cnoc nan Cuilean Intrusion, Loch Loyal Syenite Complex, NW Scotland

Published online by Cambridge University Press:  22 February 2013

HANNAH S. R. HUGHES*
Affiliation:
Cardiff University, School of Earth and Ocean Sciences, Main Building, Park Place, Cardiff CF10 3AT, UK
KATHRYN M. GOODENOUGH
Affiliation:
British Geological Survey, Murchison House, West Mains Road, EdinburghEH9 3LA, UK
ABIGAIL S. WALTERS
Affiliation:
British Geological Survey, Kingsley Dunham Centre, Keyworth, NottinghamNG12 5GG, UK
MICHAEL MCCORMAC
Affiliation:
British Geological Survey, Murchison House, West Mains Road, EdinburghEH9 3LA, UK
A. GUS GUNN
Affiliation:
British Geological Survey, Kingsley Dunham Centre, Keyworth, NottinghamNG12 5GG, UK
ALICJA LACINSKA
Affiliation:
British Geological Survey, Kingsley Dunham Centre, Keyworth, NottinghamNG12 5GG, UK
*
Author for correspondence: HughesH6@cardiff.ac.uk

Abstract

In NW Scotland, several alkaline intrusive complexes of Silurian age intrude the Caledonian orogenic front. The most northerly is the Loch Loyal Syenite Complex, which is divided into three separate intrusions (Ben Loyal, Beinn Stumanadh and Cnoc nan Cuilean). Mapping of the Cnoc nan Cuilean intrusion shows two main zones: a Mixed Syenite Zone (MZ) and a Massive Leucosyenite Zone (LZ), with a gradational contact. The MZ forms a lopolith, with multiple syenitic lithologies, including early basic melasyenites and later felsic leucosyenites. Leucosyenite melts mixed and mingled with melasyenites, resulting in extreme heterogeneity within the MZ. Continued felsic magmatism resulted in formation of the relatively homogeneous LZ, invading western parts of the MZ and now forming the topographically highest terrane. The identification of pegmatites, microgranitic veins and unusual biotite-magnetite veins demonstrates the intrusion's complex petrogenesis. Cross-sections have been used to create a novel 3D GoCad™ model contributing to our understanding of the intrusion. The Loch Loyal Syenite Complex is known to have relatively high concentrations of rare earth elements (REEs), and thus the area has potential economic and strategic value. At Cnoc nan Cuilean, abundant REE-bearing allanite is present within melasyenites of the MZ. Extensive hydrothermal alteration of melasyenites here formed steeply dipping biotite-magnetite veins, most enriched in allanite and other REE-bearing accessories. This study has thus identified the area of greatest importance for further study of REE enrichment processes in the Cnoc nan Cuilean intrusion.

Type
Original Articles
Copyright
Copyright © Natural Environment Research Council. Published by Cambridge University Press 2013 

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