Petrology and geochemistry of the alkaline intrusion, Jombo Hill, Kenya

Summary The field occurrence and the known immiscibility between silicate and carbonate melts suggest the Jombo alkaline intrusion and Mrima carbonatite intrusion have come from the same magmatic source. However, only the ijolite series apparently originated from a magma body at Jombo hill intrusion, while the feldspathoidal syenite series appears to have resulted from metasomatic alteration of the country rock-sandstone. The K/Rb ratios seem to indicate a mantle origin for the ijolites and a crustal one for the feldspathoidal syenites. The feldspathoidal syenite series were presumably formed by alkali metasomatism, resulting from magmatic fluid infiltration through intergranular movement. It is suggested that the partitioning of K and Na components in the metasomatizing fluid and solid feldspar phases at different temperatures, was the cause of two rock types – albite nepheline syenite and orthoclase nepheline syenite.