A petrographic study of the alkaline rocks at Sivamalai

Abstract

Sir Thomas Holland proposed the term Sivamalai Series to include a group of alkaline rocks which are prevalent in the neighbourhood of Sivamalai in the Coimbatore District. Since the publication of Holland's Memoir little attention has been paid to them. As a result of detailed petrographic study of the rock types of this area a geological map has been prepared by the present writer. Three types of rocks and a rose coloured variety of nepheline have been analysed. The rock suite is characterised by soda-rich feldspars and pyroxenes, the feldspars being mostly varieties of crypto and microperthite, of the exsolution and replacement types. The plagioclase feldspars in these rocks have 13-18% 'An' content and invariably exhibit the Albite-Ala twin law. The rose-coloured nepheline was found to be particularly rich in soda and having low indices of refraction. The pyroxene in the pyroxene syenites is pleochroic aegirine-augite, occurring intimately associated with olivine and hypersthene. Three varieties of amphibole were noticed in these rocks, one of which is similar to 'Girnarite', a species named by Mathur. Eight distinct types of rocks have been recognised on the basis of detailed petrographie examination, these being (1) coarse nepheline syenite, (2) aplitesyenite, (3) biotite-nepheline-syenite, (4) hornblende-nepheline-syenite, (5) hornblende-nepheline-syenodiorite, (6) even-grained granulitic nepheline-syenite, (7) aegerine-augite-syenite, and (8) microperthitite. Rock types 3, 5 and 7 have been analysed and these together with an analysis given by Holland have been plotted on the 'QLM' and 'πκ' triangular diagrams after Niggli. These diagrams bring out the undersaturated nature of these rocks and their affinity to the Atlantic Suite. Field evidence suggests a syntectic origin for these rocks, but for a fuller understanding of the genesis of the suite it will be necessary to undertake more extensive chemical studies.