Titanohematite lattice-preferred orientation and magnetic anisotropy in high-temperature mylonites

Abstract

Magnetic and crystallographic fabric studies were performed in mylonitic granulites from a km-wide strike-slip shear zone in the Ribeira Belt (southeastern Brazil). In these mylonites, a strong compositional layering underlines the tectonic foliation and the elongation of titanohematite, amphibole and orthopyroxene crystals defines a mineral stretching lineation. Magnetic fabric deduced from anisotropy of magnetic susceptibility (AMS) measurements and tectonic fabric compare favorably. Rock-magnetic studies show that both paramagnetic and ferromagnetic minerals can be carriers of AMS. The anisotropy of isothermal remanent magnetization, which is due to the shape-preferred orientation of magnetite grains, is coaxial with AMS. Lattice-preferred orientation (LPO) measurements using the electron backscattered diffraction technique show that orthopyroxene, amphibole, biotite and titanohematite have a strong LPO tightly related to the tectonic fabric. Among these four minerals, titanohematite is the only mineral present in relatively large proportions (>1.5%) in all studied samples. Titanohematite LPO is characterized by a strong concentration of (0001) poles ( c-axes) sub-perpendicular to the foliation and by a distribution of the poles of the (2 11 0) and (10 1 0) prism planes within the foliation with a maximum close to the lineation. This characteristic LPO is interpreted as resulting from dislocation creep during the mylonitization. Magnetic fabrics and titanohematite LPO fit well: the axis of minimum susceptibility is aligned with the c-axis maximum and the axis of maximum susceptibility coincides with the maximum concentration of poles of the prism planes. Titanohematite LPO may provide a valuable constraint for the kinematic interpretation of the magnetic fabrics.