Neodymium and Sr isotopic constraints on the petrogenetic relationships between carbonatites and cancrinite syenites from the Lueshe Alkaline Complex, east Zaire

Field observations, petrographic investigations and age determinations on minerals of cancrinite syenites and carbonatites from the Lueshe Alkaline Complex, east Zaire, provide evidence for a spatial and temporal relationship of the two rock types. The Nd and Sr isotope compositions of these carbonatites and cancrinite syenites, however, reveal a genetic heterogeneity. In particular, the initial Sr data show a large variation in ε_Sr (-15.9 to + 33.8). ε_Nd;'s vary from -1.33 to +0.10. Within the cancrinite syenites, a spatial variation in texture, as well as in mineral and chemical composition is observed. The rocks range from trachytoidal syenites found in the central part of the body to equigranular syenites with mosaic textures, which finally grade into carbosyenites when in contact with carbonatite. The variation in Sr isotope composition of this rock sequence (ε_Sr: + 4.7 to -10.5) is accompanied by an increase in the Ca and the Sr contents. This variation is interpreted as a fenitising effect imported on the syenites in situ by the carbonatite melts. Neglecting this late stage alteration of the syenites with its changing of the Sr isotope composition towards that of the carbonatites, the syenites become isotopically similar to the fenites. Therefore, a palingenetic origin for the cancrinite syenites involving rheomorphism of high-grade fenites which should have been similar in isotopic composition to the surrounding fenites of the Lueshe Complex is suggested.