Isotopic composition of strontium and neodymium in potassic rocks of the Little Murun complex, Aldan Shield, Siberia

Abstract

This paper presents the first data on the isotopic composition of Nd and Sr in ultrapotassic rocks of the Little Murun complex, Aldan Shield, Siberia. The samples analysed include a SrBa-carbonatite and rocks belonging to the biotite pyroxenite-shonkinite series. The latter are representative of rocks termed “lamproites” by previous investigators of the complex. The rocks have isotopic compositions that indicate derivation from ancient sources that have higher Rb Sr and lower Sm Nd ratios than those of bulk Earth. The samples define an array on a SrNd correlation diagram, suggesting that the isotopic composition of Sr and Nd in the rocks is a mixture of two components: a dominant one derived from an ancient lithospheric mantle source that has been isolated from convecting mantle, and a minor component of asthenospheric origin. We do not consider that any of the samples analysed are lamproites, although our isotopic data suggest that conditions in the lithospheric mantle in this region are suitable for the generation of lamproites. The potassic rocks from the Aldan shield have isotopic compositions that are similar to those of potassic rocks in the Wyoming craton. It is proposed that the geochemical evolution of the two shields was broadly similar.