Igneous Complex of Mount Girnar, Saurashtra, Gujarat - A Reappraisal

Mount Girnar is a major igneous plutonic complex which intruded into the basalts towards the close of the Deccan Trap period. The rock types identified in this complex are gabbros (tholeiitic and alkalic), diorites, lamprophyres, alkali-syenites and rhyolites. The parent gabbroic magma is shown to have given rise in sequence to diorites, lamprophyres and alkali-syenites. The rhyolite, though earlier considered a product of differentiation, is now believed to be an independent magma without any genetic link with the gabbro and its variants.

On scanning through available literature, however, it became apparent that the chemical/petrographic data were at variance with the crystallization differentiation hypothesis proposed by earlier workers to account for the varying rock types. To overcome these difficulties it is thought that the 'fenitization' model proposed here can better explain the formation of the different rock types. During the geological history of Mount Girnar, at least three different types of genetically unrelated gabbroic, rhyolitic and syenitic magmas erupted independently at different periods. The highly volatile-charged alkali-syenite magma which came last, was responsible in bringing about metasomatic changes (Na-fenitization) in the earlier formed rocks. It was the process of fenitization that gave rise to rock types such as alkali-gabbro, diorites and lamprophyres. The rhyolitic rocks developed out of an independent rhyolite magma.