Radiogenic Isotopic Ratio Variations in Carbonatites and Associated Alkaline Silicate Rocks: Role of Crustal Assimilation

Abstract

Carbonatites commonly occur together with alkaline silicate rocks. It is frequently argued in the literature that the varied Sr/Sr, Nd/Nd and Pb isotopic ratios in the alkaline silicate rocks reflect source heterogeneity, and that in many cases bear no genetic significance to the coexistence of silicates and carbonatites. Such a hypothesis may not be universal as it fails to explain the observations that in numerous carbonatite^alkaline complexes both rock types are contemporaneous, they exhibit complementary trace element patterns, and most importantly their initial radiogenic isotopic ratios overlap, all of which when considered together suggest a common parentage. In addition, the alkaline silicate rocks have more variable and higher Sr/Sr, and lower Nd/Nd and Pb isotopic ratios compared with those of the carbonatites and show hyperbolic trends in isotopic ratio vs concentration plots, hinting at a possible contamination of their parental magma by crustal or lithospheric material. Using a mathematical model that quantifies the isotopic ratios and concentrations of an element during concurrent assimilation and fractional crystallization of silicate rocks combined with immiscible separation of carbonate melt (AFCLI), I propose that the isotopic ratio variations in most carbonatite^alkaline silicate complexes can be explained by assimilation of crustal material by parental carbonated silicate parental magmas. A highly plausible scenario that emerges from this exercise is that not only are the carbonate and associated silicate magmas derived from a single parental magma, but also that the lower crust plays an important role in their diversification.