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

A critical petrographic investigation together with magnetic studies s uch as the variation of susceptibility with temperature and hysteresis have been carried out on basalt samples drilled across a dyke and a c olumnar sill from the Deccan Traps. In the case of columnar basalts, n ot much variation has been found in the grain size but display increas e in shattering of the grains towards the edge of the column. In contr ast, the dyke samples are tachylitic at the edge grading to fine-grain ed basalt in the middle. The magnetic hysteresis studies indicate an i ncrease of single-domain (SD) mineral grains from the margin to the ce ntre of the columnar basalt. Low temperature susceptibility studies cl early 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 variatio ns of the magnetic parameters. These magnetic variations can be unders tood in terms of titanomagnetite composition and domain states of the grains among samples from across the dyke.