A magnetic fabric study of the Aigoual-Saint Guiral-Liron granite pluton (French Massif Central) and relationships with its associated dikes

In the southeastern French Massif Central, the Aigoual-Saint Guiral-Liron pluton consists of porphyritic and microgranitic types. The latter is encountered within dikes forming the northern end of the pluton. Both types show prefull crystallization microstructures indicating weak subsolidus deformations. An anisotropy of magnetic susceptibility (AMS) study has been carried out to determine the granite fabric. Biotite, local hornblende, and small grains of magnetite are the main carriers of AMS in both types. Porphyritic granite and dikes display different AMS patterns related to magma dynamics and regional deformation recorded during crystallization. In the porphyritic type, the AMS lineation is mainly consistent with the regional, NW-SE to E-W trending, extensional event coeval with emplacement and crystallization of the pluton indicating an influence of regional tectonics on the linear fabric development. The dome shaped foliation pattern of the Saint Guiral massif likely corresponds to internal deformation-related processes within the pluton. In the microgranite dike swarm, the NNE-SSW trending lineations with shallow plunges record magmatic flow processes within dikes, i.e., magma injection and filling of dikes from a probable source located southwestward. Regional tectonics played a significant role in the pluton geometry and fabric development. For example, the NE-SW trend of the dikes suggests that extensional fractures took place in the same extensional strain field as elsewhere in the pluton. Different fabric development modes were therefore responsible for the contrasted fabric patterns between the microgranite and the porphyritic granite.