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Journal of Petrology Advance Access published online on September 7, 2009
Journal of Petrology, doi:10.1093/petrology/egp063
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© The Author 2009. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

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

Jyotiranjan S. Ray*

Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India

Received June 8, 2008; Revised typescript accepted August 14, 2009


  Abstract

Carbonatites commonly occur together with alkaline silicate rocks. It is frequently argued in the literature that the varied 87Sr/86Sr, 143Nd/144Nd 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 87Sr/86Sr, and lower 143Nd/144Nd 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.

KEY WORDS: carbonatites; alkaline silicate rocks; liquid immiscibility; assimilation; radiogenic isotopes

*Corresponding author. Telephone: 91-79-26314165. E-mail: jsray{at}prl.res.in


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