Metasomatic and Intrusive nepheline-bearing rocks from the Mbozi syenite-gabbro complex, southwestern Tanzania

The Mbozi Complex (20 × 8 miles) is situated on the Great North Road 15 miles east of the Tanzania–Zambia border. It is at least 743 million years old, and is emplaced in Ubendian Gneisses (1800 m.y.) along the northeastern flank of the Tunduma Rift.The oldest rocks in the complex are layered calcic gabbros, with cumulates of iron-rich pyroxenite and bytownite anorthosite. The basic rocks were faulted and deformed prior to the emplacement of alkaline rocks around the perimeter of the complex. The alkaline rocks are saturated with respect to silica near the ends of the complex, but are nepheline-bearing near the center. The larger masses of marginal syenite are magmatic intrusions.Within the basic rocks near the center of the complex is an elliptical zone of feldspathoidal rocks surrounding a core of unaltered gabbro. The rocks in this ring structure are heterogeneous, ranging in composition from slightly altered gabbro, through all intermediate compositions to ijolite, litchfieldite, and rutterite. They have well-developed concentric banding, but vary rapidly in composition and texture along and across strike. Contacts are gradational. The majority of the ring rocks appear to be metasomatized gabbros, but a few persistant, concordant bands of homogeneous litchfieldite are probably intrusive.Assuming no volume change and little change in color index, metasomatism of gabbroic rocks to produce the ring rocks would involve introduction of cations to the standard cell of 160 O ions in the proportion K5 – 9 Na21 – 27 Al−2 – +5 Si−5 – +11 P0 – +4 and removal of Ca14 – 21 Mg11 – 15 Fe2 – 4Ti1 – 2.It is concluded that the ring structure developed above a volatile-rich nepheline syenite, and that the metasomatism was effected by the volatile elements of that magma.