Magnetic modelling in the French Cadomian belt (northern Armorican Massif)

The French Cadomian belt (northern Armorican Massif) constitutes an excelle nt area for studying Panafrican orogenic processes in Europe for which geop hysical methods are useful for obtaining a realistic interpretation of the geological structures at depth. Magnetic modelling of the area to the west of Saint-Brieuc Bay was carried out to determine the geometry of the Cadomi an geological units. Modelling was computed along profiles extracted from t he regional aeromagnetic map. The shape of the magnetic bodies was determin ed with the help of interpretative geological sections and the total magnet ization of the magnetic bodies was determined from a data base of about 350 rock sample measurements. The measured susceptibility (X) ranges from 0.15 x 10(-3) SI to 221.5 x 10(-3) SI (the induced magnetization IM = 40 A/M X chi.) The natural remanent magnetization (NRM) is weak and shows varied dir ections; it is therefore negligible at the formation scale, and only the ma gnetic susceptibility was taken into account for modelling. A computed mean IM was attributed to each magnetic body in the model. Measurements of the magnetic properties and thin-section observations of fi eld samples show that magnetite is the main carrier of the magnetization. T he strong susceptibilities are consistent with ferromagnetic behaviour of m ultidomain grains, whereas the weak susceptibilities accord with dominantly paramagnetic behaviour. Sample measurements show that the Saint-Quay intru sion, the acid part of the Lanvollon Formation and one of the three lenses of Squiffiec metagabbro are the most highly magnetic formations. Magnetic modelling made it possible to estimate the long-wavelength fold sh ape of the Binic formation. The basin, within this formation, reaches about 2.0-2.5 km depth. The modelling also indicates that the Saint-Quay intrusi on cuts the Binic basin at its centre with vertical contacts. Modelling sho ws that the intrusion is apparently composed of two imbricated magnetic bod ies in its eastern part, and of homogeneous magnetic bodies in its western part, which is consistent with the geological observations. The Lanvollon Formation exhibits an heterogeneous magnetic behaviour consis tent with its lithology of intercalated acid and basic metavolcanic bodies. Magnetic modelling indicates a difference in thickness of the acid part of the Lanvollon Formation between the profiles of the Binic basin area and t hose of the Squiffiec-Plouha area; this we interpret as reflecting an initi al variation in the thickness of the acid volcanic bodies. The Binic and th e Squiffiec-Plouha profiles show that the southern boundary of the basic pa rt of the Lanvollon Formation is probably vertical or steeply dipping to th e north. (C) 2001 Elsevier Science B.V. All rights reserved.