Petrological studies of the alkaline complexes of eastern Uganda

Abstract

Of the ten Tertiary and pre-Tertiary alkaline igneous centres in Eastern Uganda, three (Budeda, Tororo and Toror) are described in this thesis, and a summary of a fourth (Napak) is presented. The earliest intrusive rock at Budeda and Napak is melteigite/pyroxenite which in both areas becomes converted to variable ijolite by replacement in the form of diffuse patches and well-defined veins. Melanite, wollastonite and calcite are characteristic of later phases of ijolite at Napak and Tororo. Nepheline syenites are produced by the development of feldspar in the marginal zone of the ijolites. At Budeda, cancrinite and calcite also occur in these rocks. They show intrusive relations with the fenites but are gradational towards the ijolite. At Napak, the cancrinite syenites and related feldspathic types occur as dyke-like masses within the ijolite. Feldspathization also affects the mixed rocks of the agglomerate (including ijolite and nepheline syenite) adjacent to the carbonatite at Tororo, locally producing orthoclasites. At Toror, the gneisses around the carbonatite are in places converted to feldspathic fenites (potash-feldspar rocks). Feldspathic breccias and intrusive rocks of similar composition (about 12% K2O) include trachytes, which evidently represent the mobilized products. The carbonatite at Budeda is a small mass formed by the replacement of melteigite, but at the other complexes it is clearly intrusive. Pyroxene, biotite and magnetite are considered to be derived from assimilated silicate rocks. Around the intrusive complexes fenites were formed by alkali metasomatism of the Basement. At Budeda syenitic fenites are widely developed, and have a foliated character which is related to early crushing. Locally the Budeda fenites are nepheline-bearing; they are also extensively feldspathized. The writer finds no direct evidence (with the exception of the trachytes) of mobilization of fenites to give rocks of the intrusive series. The intrusive rocks are considered to be derived from a melanephelinite magma, which by differentiation and the retention of volatiles led ultimately to the formation of a carbonatite residuum.