Petrographic and Magnetic Studies of a Columnar Sill and a Dyke from the Deccan Traps

A critical petrographic investigation together with magnetic studies such as the variation of susceptibility with temperature and hysteresis have been carried out on basalt samples drilled across a dyke and a columnar sill from the Deccan Traps. In the case of columnar basalts, not much variation has been found in the grain size but display increase in shattering of the grains towards the edge of the column. In contrast, the dyke samples are tachylitic at the edge grading to fine-grained basalt in the middle.

The magnetic hysteresis studies indicate an increase of single-domain (SO) mineral grains from the margin to the centre of the columnar basalt. Low temperature susceptibility studies clearly indicate Ihe presence of some multidomain (MD) componcnt of pure magnctite in the cdge samples where as it is barely noticeahle in the s.1l1lple from the cenlre. Samples from the dyke show systematic variations of rhe magnetic parameters. These magnetic variations can be undcrstood in terms of titanomagnctite composition and domain states of thc grains among samples from across the dyke.