Understanding the maturity of columnar joints and its spatial relationship with eruptive centre: A critical appraisal from the Rajmahal basalt, India

Abstract

We investigate the columnar joints of the Rajmahal Trap (India) in detail to understand the maturity with respect to its eruptive centre. The fabrics in the massive columnar basalt of the study area are investigated using Anisotropy of Magnetic Susceptibility (AMS) studies prior to the maturity analysis. Fabric analysis reveals that rocks are devoid of any tectonic deformation and the columnar structures that are observed in them are necessarily primary. The maturity analysis has been implemented using a number of maturity indices, such as Voronoi polygons, hexagonality index, uniformity in mean side length, mean area and mean internal angle of the polygons, and also by the distribution of the number of polygon sides. The Voronoi polygon analysis for Rajmahal Trap suggests that the diameter ratio ranges from 1.38 to 1.52, thus indicating the same maturity as Voronoi polygon sets. The hexagonality index depicts that the maturity increases away from the fissure point towards the periphery, where six sided columns with smaller area and side length are observed. It is found that as the maturity increases, column mean side length as well as column mean top area decreases, while average number of column sides increases. We envisaged that the difference between ambient and the magma temperature played an important role in controlling column maturity. Based on the overall maturity studies, we conclude that maturity of the columnar joint increases away from the eruptive centre, towards the peripheral part of the Rajmahal Trap.