Petrogeneses of Proterozoic carbonates in Madagascar

在马达加斯加南部元古代地体中发现了碳酸盐岩，并对其进行了矿物学、全岩地球化学、稳定的碳、氧同位素和放射性成因的锶、钕同位素研究。碳酸盐岩中的碳、氧稳定同位素显示岩浆从原生地幔碳酸盐场向重的方向分异，表明岩浆同位素分馏作用。碳酸盐辉石岩和伴生辉石岩都具有极富的锶和贫化的钕同位素组成。目前的资料表明，所研究的碳酸盐岩是来自于岩石圈地幔上部的高度富集源的硅碳酸盐。

Carbonatites have been identified in the Proterozoic terrain of southern Madagascar and studied for mineralogy, whole-rock geochemistry, stable C- and O-isotopes, and radiogenic Sr- and Nd-isotopes. Carbon and oxygen stable isotopes in the carbonates show magmatic differentiation form the primary mantle carbonate field toward heavier values, suggesting magmatic isotopic fractionation. Both carbonate and associated pyroxenites are characterized by extremely enriched Sr- and depleted Nd-isotope compositions. The present data imply that the studied carbonates are silicocarbonatite derived from extremely enriched sources, which were located at the upper part of lithospheric mantle.