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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 Department of Geology, Oxford Polytechnic, Headington, Oxford OX3 OBP, UK
In the La Hague region, NW of Cherbourg, France, the post-tectonic Cadomian Northern Granites Complex consists of seven rock units which range in composition from diorites to trondhjemite and K-feldspar granite. With the notable exception of the Ecuty Granite, the units tend to become mineralogically more evolved with decreasing age. They are calc-alkaline and have the characteristics of the products of volcanic arc magmatism. In detail, their geochemical differences support magma formation from several different sources.
The Ecuty Granite gives a Rb-Sr whole rock isochron age of 527 ± 4 Ma, the St Germain Granite, 515 ± 12 Ma and the Cap de la Hague Granodiorite, 477 ± 8 Ma. Thus this igneous activity spanned at least 50 Ma of Cambro-Ordovician time. This, together with evidence of plutonic sequences of similar character and age in Guernsey and Jersey supports the view that post-tectonic Cadomian activity continued well into lower Palaeozoic times in the northern part of the Armorican Massif.
