Petrogeneses of Proterozoic carbonates in Madagascar

碳酸盐岩已在马达加斯加南部的元古宙地区被发现，并进行了矿物学、全岩地球化学、稳定C-和O-同位素以及放射成因Sr-和Nd-同位素研究。碳酸盐岩中的碳、氧稳定同位素显示岩浆分异作用从原始地幔碳酸盐岩场向重值方向发展，反映岩浆同位素分馏作用。碳酸盐岩和伴生辉石岩的特点是极其丰富的Sr-和亏损Nd-同位素组成。研究结果表明，研究区碳酸盐岩是来源于岩石圈地幔上部的硅碳酸盐岩。

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.