Magnetic fabrics and microstructures of the post-collisional aegirine-augite syenite Triunfo pluton, northeast Brazil

Anisotropy of magnetic susceptibility (AMS) and microstructures of the Neoproterozoic Triunfo alkaline pluton, situated in northeast Brazil, have been investigated in order to characterize fabrics formed during its emplacement. Bulk magnetic susceptibility in the pluton varies between 0.15 × 10−3 and 25 × 10t-3 SI, and the mean magnetic anisotropy ratio is around 1.04. Aegirine-augite and secondary magnetite are the main carriers of the magnetic susceptibility, with the former controlling the directions of the anisotropy. Magnetic foliation and lineation are consistently oriented throughout the pluton: foliation typically dips gently to the northeast and lineation plunges gently to the east. These structures were mostly acquired at the magmatic and sub-magmatic stages. The shape of the magnetic ellipsoid is dominantly oblate, a feature that may reflect either the uniaxial oblate AMS ellipsoid of the aegirine-augite crystals and/or a vertical compaction of the magma in its final stages of crystallization. The foliation pattern suggests the pluton has a tabular shape within the basement rocks. Magma emplacement is inferred to have occurred by its lateral migration along a flat-lying crustal structure after fracture propagation and ascension of the magma from a lower crust-mantle source. Late, fine-grained, vertical dykes with the same mineral composition as the Triunfo syenite cross-cut the wall rocks of the pluton and close the alkaline magmatism in the area. The NE-trending alignment of alkaline plutons and the NNE-trending syenitic dykes indicate that the emplacement of such bodies occurred along a crustal extension event with a minimum compressive stress horizontal trending roughly E-W.