Characterizing magnetic fabrics from ∼116 Ma old Salma dike, India: implication for magma flow direction

SUMMARY Anisotropy of magnetic susceptibility (AMS) analysis is widely used as an efficient petro-fabric tool to infer magma flow patterns within dikes. However, interpretations of magnetic fabric often get complicated by the occurrence of anomalous (intermediate/inverse) fabrics oriented normal to the dike plane, which may lead to uncertainty, unlike the straightforward normal fabrics along the intrusion plane. In this article, we present a detailed rock-magnetic and magnetic fabric study of India’s ∼116 Ma old Salma dike, which is the most prominent and longest dike related to the early Cretaceous Rajmahal Trap (RT) volcanism. A joint analysis of in-phase and out-of-phase anisotropy of magnetic susceptibility (i.e. ipAMS and opAMS) and anhysteretic remanent magnetization (AARM) fabrics allowed us to identify the different sources of the observed fabric and subfabrics. Rock-magnetic analyses suggest that the magnetic mineralogy consists of at least two types of titanomagnetite with varying Ti-content. FORC suggests that the dike is dominated by PSD grains with varying influence from SD grains. The ipAMS fabric is primarily carried by SD grains of low-Ti titanomagnetite, while opAMS is governed by larger MD/PSD titanomagnetite with higher-Ti content. Results indicate that anomalous, intermediate-type ipAMS fabrics, particularly along the dike margins, are caused as a combined effect of SD grains, late-stage crystallization and mild high-temperature oxidation. At the dike centre, post-emplacement alteration, intense exsolution of the primary titanomagnetite and magma backflow are responsible for anomalous fabrics. In contrast, the majority of normal ipAMS, opAMS and AARM fabrics are coaxial, providing a reliable record of magma flow as confirmed from the long axes trend distribution of the plagioclase laths along the dike plane. These fabrics reveal a dominant subvertical magma flow direction during emplacement, indicating magma ascent from depth. Even though the magnetic fabrics do not unequivocally constrain the deeper processes underneath Moho, they are compatible with the idea of a subcrustal magmatic layer beneath the region, potentially originating from decompression melting or the influence of the Kerguelen plume, to be the feeder source.