Plagioclase-Ca-rich-nepheline tergrowths in a syenite from the Marangudzi complex, Rhodesia

SUMMARY. Nepheline in a basic syenite from the Marangudzi complex is relatively Ca-rich with CaO from I'9 to 2-4 (wt) %. This Ca-rich nepheline sometimes occurs in intimate intergrowths with plagioclase (andesine). These intergrowths are thought to have formed at temperatures in the order of 750 ~ o ~ by resorption of plagioclase with simultaneous replacement bynepheline. THE nepheline-bearing rocks from the Marangudzi Ring Complex, Rhodesia, form a differentiated series varying from nepheline monzonite to pulaskite and foyaite. In this series the modal alkali feldspar and nepheline increase and plagioclase and hastingsitic amphibole decrease in amount, the plagioclase becoming more Ab-rich and the hastingsite becoming more Fe-rich (Henderson, I968). The pulaskites and foyaites are hypersolvus yenites containing orthoclase micro- and crypto-perthite and nepheline the compositions of which show typically magmatic tie-lines in the system Ne-Ks-Qz (Tilley, I957). The pulaskites how the nepheline-plagioclase inter-growths described in this paper. Textures. One pulaskite sample (R. 26) is described as it shows the nepheline-plagioclase textures particularly well. The rock is medium-coarse grained, containin