Emplacement and fabric-forming conditions of the Alous-En-Tides granite, eastern border of the Tin Seririne/Tin Mersoı̈ basin (Algeria): magnetic and visible fabrics analysis

Abstract

The Alous-En-Tides pluton intruded within one of the most important Late Panafrican N–S shear zones, the 7°30 shear zone in the southern Hoggar shield. A magnetic fabric study points out strong preferred orientations of the magnetite grains. The orientation is coherent in the whole massif. However it cannot be correlated with any of the visible structures in the field or in thin section. Strongly oriented magnetites are thus disseminated within all the rock. The main rock-forming minerals in this pluton were not preferably oriented during their crystallization, showing that magma was emplaced without strongly oriented stresses. Magnetite orientations on the contrary reflect a strong stress field. Strike-slip movements along the 7°30 shear zone probably generated local relative distension, allowing pluton emplacement. During the last magmatic phase, they put the intrusion under the regional compressional stress field, causing orientation of the magnetite. The magnetic fabric then reveals the regional ENE–WSW stress field during Late Panafrican times. In part of the pluton, which was later affected by very intense solid-state deformation during dextral movement, the magnetic fabric remains mainly in connection with this initial magnetite fabric.