Albitisation related to the Triassic unconformity in igneous rocks of the Morvan Massif (France)

The Carboniferous Morvan Massif, in the northern part of the French Massif Central, consists of granite and some rhyolite. A Triassic erosional unconformity has developed on the massif which is covered by Mesozoic sediments of the Paris Basin. The igneous rocks of the Morvan Massif show a strong alteration with pseudomorphic replacement of the primary plagioclases into albite, pseudomorphic replacement of primary biotite into chlorite and minor precipitation of neogenic minerals like albite, chlorite, apatite, haematite, calcite and titanite. The geometry and arrangement of these alterations give significant constraints about their development. Some of the altered facies develop in a pervasive manner; others are restricted to centimetric to metric-wide joints that imply fluid-flow phenomena. Moreover, the alteration facies are arranged in a clear succession with strongly altered facies at the top and weakly altered facies towards the depth, which point to a genetic relationship with the Triassic unconformity. Regional distribution of the alterations, which affect the Carboniferous igneous and volcanic formations beneath the Jurassic sedimentary cover, also leads to associate these alterations with the Triassic unconformity. Dating of the alterations provides even a further constraint, alterations are of Triassic age, that means the same age as the unconformity. Taking into account all these geological constraints, it is proposed that albitisation of the Morvan Massif was developed under low temperature subsurface conditions in relation to the Triassic palaeosurface.