Abstract
Shear zone kinematics and anisotropy of magnetic susceptibility (AMS) data have been used in a Brasiliano/Pan-African fayalite-clinopyroxene (±orthopyroxene) syenite of Umarizal (NE Brazil) in order to reveal its emplacement mechanisms. The pluton is located at the intersection of two major shear zones, a NE-trending transcurrent Portalegre shear zone (PSZ) and a NW- to E-trending Frutuoso Gomes shear zone (FGSZ). The FGSZ dips gently NE in its NW-trending branch and N in the E-trending branch. Asymmetric porphyroclasts and S C′ shear planes observed in mylonitic gneisses along the FGSZ point to a top to the East (extensional) movement. Hysteresis and low-field susceptibility measurements on mangerites and charnockites indicate that magnetic susceptibility is principally carried by multidomain magnetite. In the core of the pluton, the magnetic anisotropy is weaker ( P<1.05) and characterized by neutral ( L= F) to rod-shaped ( L> F) AMS ellipsoids. The magnetic anisotropy increases south-westward ( P>1.10) approaching the FGSZ and the AMS ellipsoid becomes dominantly disc shaped ( F> L). Magnetic lineations plunge gently to NW in the western part of the pluton and to ENE on the East. The absence of steep lineations, except around a large xenolith in the center of the pluton, exclude the presence of magmatic feeding zones at the actual level of exposure. Foliations are roughly parallel to the pluton contact, but most of them dip moderately towards North-east. In the SW border the magnetic foliation is parallel to the mylonitic foliation displayed in the FGSZ. Although emplaced within migmatized host rocks, the kinematics of the shear zones and the magmatic fabric pattern rule out a buoyancy-driven diapiric emplacement of the pluton. Syn-magmatic shear deformation associated with EW crustal stretching accounts for the fabrics of emplacement recorded in the Umarizal pluton.
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