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Armorican Massif |
1 Department of Geology, Oxford Polytechnic, Headington, Oxford OX3 0BP, UK
2 Department of Geology, Kingston Polytechnic, Penrhyn Road, Kingston upon Thames KT1 2EE UK
5 > Department of Geology, University of Maryland, College Park, MD20742, USA
3 Department of Geology, Portsmouth Polytechnic, Burnaby Road, Porstmouth PO1 3QL
4 Department of Geology, University of Keele, Keele, Staffs ST5 5BG, UK
The Northern Igneous Complex of Guernsey consists of four intrusive bodies, the St Peter Port Gabbro, the Bordeaux Diorite Complex, the Cobo Granite and the L’Ancresse Granodiorite. Although these bodies can be shown to be emplaced in the order given, evidence suggests that a small time span separated the intrusive events, which occurred around 500 Ma ago. The geochemistry of the complex is calc-alkaline, compositions from olivine gabbro to granite being represented. The granitic rocks have compositions typical of volcanic arc granites. The St Peter Port Gabbro is a layered body, with layering present on three different scales. The layering and the geochemical variation can be explained by invoking magmatic processes. The Bordeaux Diorite Complex consists of a range of lithologies from meladiorite to granodiorite, the magmas of which are shown to have been contemporaneously mobile, although magma mingling as opposed to mixing has occurred. Despite their contemporaneity, the different magmas of the Bordeaux Diorite Complex are not related directly to each other by fractionation. The Cobo Granite is homogenous but has complex relationships with the Diorite Complex. At the northern margin of the Granite a zone of granite/diorite magma mixing occurs. Details of the process involve local chilling, breaking of the chilled carapace followed by comingling and partial mixing. Geochemistry reflects the mixing process. The L’Ancresse Granodiorite is lithologically and geochemically homogeneous. Its western contact with the Diorite Complex is faulted, but the eastern contact again shows a variety of granodioritic and dioritic lithologies which have led others to invoke processes of magma mixing and metasomatism. It is argued here, however, that these lithologies are part of the variation inherent in the Bordeaux Diorites, rather than having been produced during the emplacement of the Granodiorite.
