Differentiated Hypersthene-Olivine Dolerite Dyke near Harohalli, Mysore State

Textural, mineralogical and petrochemical variations observed across the width of the differentiated hypersthene-olivine dolerite dyke situated near Harohalli have been described and discussed. The salient features of the dyke are (i) the occurrence of mafic minerals rich in iron, and plagioclase less rich in anorthite content at the borders, with a progressive decrease of iron content in the mafic minerals and increase in anorthite content of the plagioclase towards the central zone and (ii) the textural variation from the flow and hyalopilitic at the margins, intersertal, intergranular, ophitic and subophitic at the intermediate zones and poikilitic at the centre. The chilled margins show a higher content of iron, alkalies and silica and a lower content of MgO, CaO and Al₂O₃. than the central portion. These characters indicate that the differentiation is due to thermodiffusion as described by Wahl as the observed features cannot be explained by any other known mechanisms of differentiation.