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Armorican Massif |
1 Department of Geology, Portsmouth Polytechnic, Burnaby Road, Portsmouth PO1 3QL, UK
2 Department of Geology, University of Nottingham, Nottingham NG7 2RD, UK
3 School of Geological Sciences, Kingston Polytechnic, Kingston-upon-Thames KT1 2EE, UK
4 Department of Geology, University of Wales at Cardiff, PO Box 78, Cardiff CF1 1XL, UK
5 Department of Geology, University of Maryland, College Park, MD 20742, USA
The foliated granitic rocks of the Channel Islands and La Hague are predominantly quartz dioritic in composition with some granodiorite and minor granite. Their variably developed foliation is of post-solidification origin. Greenschist facies metamorphism affects some rocks as does patchy very low grade metamorphism. Field relations show that the granites are younger than the pre-Icartian metasedimentary gneisses and older than the post-Cadomian igneous complexes but there are no contacts with the Brioverian Supergroup. Isotopic age data are very unsatisfactory but suggest a possible age span of 700–580 Ma.
168 geochemical analyses show that the rocks are calc-alkaline, the majority are metaluminous with Na2O greater than K2O and that they have the characteristics of normal volcanic arc granites. Trace element plots show that there are differences in the suites of analyses from Guernsey, Sark, Alderney and La Hague reflecting minor differences in their source materials. REE plots for 34 rocks all show strong fractionation of LREEs regardless of rock composition and most show HREE depletions. These patterns are interpreted as originating in the source and suggest an origin by high pressure melting of predominantly basaltic compositions.
The evidence supports the model of Dupret et al. (1990) for a major period of Cadomian crustal growth by progressive cratonization of a basin and arc system but extension of the model on a more regional basis is difficult because of the uncertain status of neighbouring terranes.
