Triassic magnetic overprints related to albitization in granites from the Morvan massif (France).
Résumé
The Morvan massif, located in the northern part of the French Massif Central, shows widespread unusual deep alteration profiles in the Palaeozoic basement rocks (gneisses, granites and rhyolites). They are characterised by overall albitization, chloritisation and haematisation. Albite and chlorite constitute their characteristic mineral paragenesis. Haematite pigmentation comes with this paragenesis and gives an overall pink coloration to the weathered rocks. A paleomagnetic analysis was conducted after detailed petrographical and mineralogical studies of the alteration that lead to the albitized profiles. Special attention was devoted to the occurrence of haematite, the main paleomagnetism carrier, with the though to date this palaeoweathering event. Petrographical checking after heating treatments showed that deshydroxylation of chlorite also led to the formation of secondary haematite. This haematite formed by recrystallization acquired remanence properties and thus superimposed and hided the original magnetic signal. This problem has prevented the extraction of the original magnetic signal especially in the sections made up of rhyolites. Virtual Geomagnetic Poles (VGPs) obtained from the characteristic directions carried by haematite have been compared to the European Apparent Polar Wander Path (APWP). An Early Triassic magnetic overprint has been recorded in the albitized basement rocks of the Morvan massif. The age obtained for this albitization phenomena, strengthens the hypothesis that albitization developed in relation with palaeoweathering processes linked with the Triassic palaeosurface. Therefore, large scale continental weathering could be the main mechanism for Late Permian/Early Triassic remagnetizations which are widespread throughout the Palaeozoic formations of western Europe. As far as we know, this is the first time that deep palaeoweathering phenomena affecting hard and non-disaggregated weathered rocks of the basement have been directly dated. Palaeomagnetism appears as a precious tool to date alteration events developed in widely and long-lasting emerged areas of the basement.