Abstract
Miaskitic nepheline syenites of the Ilmenogorsk-Vishnevogorsk Alkaline Complex in the Ural Mountains were emplaced approximately 450 m.y. ago, as indicated by Rb-Sr whole rock ages. Their intrusion coincided with a dilatation period along the eastern margin of the Russian platform, and predated the compres-sive stage of the Hercynian orogeny in the Urals which began during the Silurian. Thus these rocks are not orogenic type nepheline syenites as previously postulated. Internal mineral isochrons for two nepheline syenites and an alkali-feldspar syenite from different regions of the complex yield consistent Permian ages of 245 m.y. These ages are interpreted as dating the cooling of the alkaline rocks after a final amphibolite grade Hercynian metamorphism. The initial ratio of the nepheline syenites is low, being around 0.703. Generation of the alkaline melts from the gneissic country rocks is therefore not likely. It is suggested that a division of miaskitic alkaline complexes into orogenic and non-orogenic is misleading and unjustified.
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