2000 Volume 95 Issue 7 Pages 162-172
Sr, Nd, C and O isotopic compositions were determined for carbonatite and associated peralkaline silicate rocks from the Zhidoy complex in the East Sayan province, Russia. The complex consists predominantly of alkali pyroxenite with a small amount of ijolite, nepheline syenite and carbonatite.
Rb-Sr isotopic data for two carbonatite samples, ijolite, and nepheline syenite give a whole rock isochron age of 569 Ma with initial εSr and εNd values of about −6.1 and 1.1, respectively. This indicates that the rocks were derived from a single parental magma by magmatic differentiation. Judging from the mode of occurrence, it is presumed that the carbonatites were produced by carbonate-silicate liquid immiscibility.
Another two carbonatite samples have εSr and εNd values of −9.3 and −11.8, 2.3 and 3.4, respectively. They show a wide range of variation in initial εSr and εNd values. The concentration of Sr in the rock varies from 4660 to 13600 ppm. The chemical and isotopic variations in the carbonatite are not due to crustal contamination but are the result of the mixing of two different carbonatitic components.
It is concluded that alkaline magmatism occurred at least twice in the Zhidoy area: (1) the first magmatism produced the alkali pyroxenite cumulate and the carbonatite with low initial 87Sr/86Sr ratio; (2) the second one produced the ijolite, nepheline syenite and high-εSr carbonatite. The generation of magmas with different 87Sr/86Sr and 143Nd/144Nd ratios may reflect an isotopic heterogeneity of the mantle source region.