Nachendezwaya

The four hills of Nakalulu, West and East Nachendezwaya and Nangwale are aligned northwest-southeast close to the border with Malawi and 3 km northeast of the Ilomba complex (Malawi No. 103-00-001). The complex comprises a central area of carbonatite which is flanked by foyaite, which it intrudes. Emplaced in metagabbros and amphibolites of the Precambrian Ubendian system, the complex is itself cut by dolerite dykes. Where observed, foliation in the carbonatite and foyaite strikes approximately northwest-southeast parallel to that of the surrounding gneisses. The Nachendezwaya carbonatites are mainly biotite and amphibole sovites. The rock is coarse and granular with calcite, dolomite and some ankerite, and with bands rich in brown biotite, arfvedsonite and a further pale mauve to pale green amphibole. Apatite and titanomagnetite are abundant and there is some pyrite, plagioclase, epidote, pyrochlore and areas of fine-grained silica. There are outcrops of ijolite that appear to be contemporaneous with the carbonatite (Horne, 1961). The ijolites generally have a colour index of about 40 and consist of euhedral to subhedral nepheline, aegirine-augite, partly rimmed by sodic amphibole which is intergrown with brown biotite, a little albite and accessory calcite, titanite, apatite and pyrite. The foyaites are porphyritic rocks of perthite phenocrysts in a matrix of albite, microcline, nepheline, sometimes altered to cancrinite, deep reddish brown biotite, in some varieties aegirine, calcite and accessory apatite, large titanites, zircon, pyrite, titanomagnetite and pyrochlore. There are some rocks containing up to 20% calcite that are transitional to carbonatite; however, in these the feldspar is solely albite. Analyses of three sovites, with chondrite normalized REE plots, are given by van Straaten (1989), and a further analysis of sovite, including a range of trace elements, with REE and Sr and Nd isotopes, is given by Nelson et al. (1988).