A Rb–Sr isochron age of 1265 ± 85 m.y. has been obtained for a group of alkaline rocks (Kunavaram series) which intrude highly folded, metamorphosed igneous and sedimentary rocks of the Eastern Ghats belt of South India. The initial 87Sr/86Sr ratio is 0.7051 ± 0.0013. This age is believed to be the time of intrusion of these rocks. Assuming an initial ratio of 0.710, two mica samples from the nepheline syenite (a biotite and a muscovite) gave an age of 620 m.y. One sample of the country rock (garnet–biotite gneiss) gave an age of 2100 m.y., which agrees with prior estimates for the age of the principal orogenic activity in the Eastern Ghats belt.

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