PETROGRAPHIC AND MAGNETIC STUDIES OF A COLUMNAR SILL AND A DYKE FROM THE DECCAN TRAPS

Abstract

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 (SD) mineral grains from the margin to the centre of the columnar basalt. Low temperature susceptibility studies clearly indicate the presence of some multidomain (MD) component of pure magnetite in the edge samples whereas it is barely noticeable in the sample from the centre. Samples from the dyke show systematic variations of the magnetic parameters. These magnetic variations can be understood in terms of titanomagnetite composition and domain states of the grains among samples from across the dyke.