Magnetic Fabric and Petrofabric of Amphibolites from the Namcha Barwa Complex, Eastern Himalaya

Abstract

The magnetic fabric and petrofabric are often used as tectonic indicators of geological and geodynamic processes that a rock has experienced such as growth, deformation and metamorphism. This study presents the low field anisotropy of magnetic susceptibility (AMS) and the crystallographic preferred orientation (CPO) of constituent minerals in amphibolites from the Namcha Barwa Complex in the eastern Himalayan Syntaxis, Tibet. The bulk magnetic susceptibility varies significantly from 7.29×10−4 to 3.31×10−2 SI, with the Jelinek’s anisotropy values (Pj) ranges from 1.094 to 1.463. The maximum susceptibility is approximately parallel to the lineation while the minimum susceptibility is subnormal to the foliation plane. Electron backscatter diffraction (EBSD) analyses show pronounced CPOs of amphibole in all samples, with a preferred alignment of the [001] axes along the lineation and the [100] axes spreading along a girdle normal to the lineation. Numerical simulations and comparison with laboratory measurements suggest that the magnetic anisotropy of amphibolite is largely controlled by the CPOs of amphibole. If present, the well oriented iron-titanium oxides such as ilmenite along rock foliation and lineation could increase the susceptibility and the anisotropy of a rock. Our results show a strong correlation between the magnetic anisotropy and the petrofabric of amphibolite, which could provide constraint for the interpretation of strong magnetic anomalies observed in the tectonic syntaxes of Tibet.