RARE-EARTH ELEMENT DISTRIBUTION IN ALKALIC ROCKS OF THE LOWER SAYAN CARBONATITE COMPLEX

Abstract

While studying alkalic rocks of the Lower Sayan pluton, we were able to establish several geologically and geochemically important features which allowed us to take a new look at ijolite and its part in carbonatite-forming processes. The most noteworthy feature here is the unusual order of formation of alkalic rocks: jacupirangite and melteigite → early ijolite → melilitolite → late ijolite → nepheline syenite. Other unusual features of this sequence are the position of melilite rocks (formed mostly before jacupirangite, melteigite and ijolite and the appearance of ijolitic rocks twice, both before and after the melilitolites. The results show that all the alkalic rocks of the complex except nepheline syenite are markedly enriched in REE, whose highest concentrations are observed in 'primary' rocks like jacupirangite, melteigite and melilitolite. The differentiation of rare-earth and petrogenic elements observed in the alkalic rocks of the Lower Sayan pluton suggests that ijolitization of alkalic pyroxenite and melilitolite could have generated a high-temperature carbonatite liquid in equilibrium with the ijolite melt.