Emplacement in an extensional setting of the Mont Lozère–Borne granitic complex (SE France) inferred from comprehensive AMS, structural and gravity studies

Abstract

The emplacement mode and setting of the Late Hercynian Mont Lozère–Borne granitic complex (French Massif Central), which consists of several plutons, is investigated. Structural and anisotropy of magnetic susceptibility (AMS) studies have been carried out to characterize the internal fabrics of the granitic plutons. Throughout the Pont-de-Montvert–Borne pluton, an E–W-trending magnetic lineation is well developed. In the host rock and the thermal aureole, a conspicuous E–W-trending lineation is interpreted as an evidence of a late-orogenic extensional event. To the east of the pluton, the AMS fabric is characterized by values of anisotropy degree ( P′) around 4–5% with a prolate ellipsoid and subsolidus structures, whereas, to the west, the P′ parameter is weaker with an oblate ellipsoid and purely magmatic microstructures. A gravity investigation allows determination of the 3D shape of the pluton. The western part of the granitic complex is thicker than the eastern one and is interpreted as the feeder zone. This suggests an eastward spreading of the magma. The consistency between regional stretching and directions of AMS lineations in the pluton and the shape of the complex deduced from gravity strongly argues that the emplacement mode of the complex was influenced by the regional extensional tectonic setting during the collapse of the Hercynian belt.