Minor intrusions towards the east of the Girnar hills

The basaltic lava flows towards the east of the Girnar Hills are traversed by a number of basic intrusions of both concordant and discordant types. The former is represented by a single sill, which runs in a roughly north-south direction coinciding with the long axis of an ellipsoidal olivine-gabbro mass of the Girnar plutonic igncous complex while the discordant bodies form radial dykes converging at a centre in Girnar. Petrographically, the intrusive rocks are olivine-dolerites exhibiting a little variation in grain size. They show remarkable similarity in mineral and chemical composition to the olivine-gabbro of the main plutonic body, suggesting their consanguinity. The basalts, into which the dolerites are intruded, show much alteration presumably due to the pneumatolytic effect of the solutions and vapours coming from the central volcano. Such alteration is entirely absent in the olivine dolerites indicating that the dykes are younger than the basalts. On the basis of the structural setting of the rocks, it is suggested that tensional fractures were formed in the lava flows due to the effect of laccolithic intrusion of gabbro, and subsequently the fractures have been filled by the same magma giving rise to olivine-dolerites.