Petrogeneses of Proterozoic carbonates in Madagascar

在马达加斯加南部的元古代地层中发现了碳酸盐岩，并对其进行了矿物学、全岩地球化学、稳定的C-和o -同位素以及放射性成因的Sr-和nd -同位素的研究。碳酸盐岩的碳氧稳定同位素显示岩浆分异，从原生地幔碳酸盐岩场向重值方向分化，表明岩浆同位素分异。碳酸盐及其伴生辉石岩的特征是Sr-同位素组成极其富集，nd -同位素组成极其贫。目前的资料表明，所研究的碳酸盐为硅碳酸盐，其来源极为丰富，位于岩石圈地幔上部。

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.