Kisingiri And Rangwa

The latitude and longitude given for the extensive Kisingiri volcano is that of the central caldera (Rangwa) but the lavas extend eastwards to at least 34°34’E with the more peripheral lavas of the western flank concealed beneath the waters of Lake Victoria. The volcano is built of gently dipping nephelinitic and melilitic lavas and agglomerates deep erosion of which has revealed a succession >1000 m thick that rests on a domed Pre-cambrian basement (McCall, 1958; Le Bas, 1977). Erosion has also laid bare a central caldera containing an intrusive complex of carbonatite and alkaline rocks. The lavas cover about 2000 km2, but faulting and erosion have broken the outcrop into several units the largest of which forms and lies southeast of the Kaniamwia Escarpment that marks the line of the Kaniamwia fault, which defines the southern margin of the Kavirondo rift. The Gembe and Gwasi Hills of volcanic rocks surround the central group of hills marking the Rangwa complex, from which they are separated by an annular valley. Lavas are also found on several islands in Lake Victoria, the largest of which are Rusinga and Mfanganu, where important hominid fossils have been found, and on the Uyoma Peninsula on the north side of the Kavirondo Gulf. McCall (1958) divided the volcanic succession into lower and upper lavas and a middle pyroclastic group. However, Le Bas (1977, Fig. 7.2) demonstrates in a series of stratigraphic columns across the volcano that this is somewhat simplified. He has calculated that the overall succession comprises 51% nephelinite agglomerate and tuff, 6% melanite nephelinite agglomerate, 21% melanephelinite, 3% olivine and melilite melanephelinite, 4% phonolitic melanephelinite, 4% melilitite and melilite nephelinite, 6% nephelinite and 5% mugearite. Bestland et al. (1995), based on stratigraphic studies on Rusinga and Mfanganu Islands, suggest that there were three periods of doming of the volcano that produced three distinct sedimentary cycles. A survey of dykes cutting the basement and central intrusive complex (Le Bas, 1977) indicated that the most abundant are phonolites, which are not represented amongst the lavas, followed by melanephelinites and phonolitic nephelinites, which do mirror the lava sequence, and alnoitic lamprophyres, which again are not found as lavas. The nephelinite lavas contain phenocrysts of nepheline up to 1 cm in diameter, diopside, often zoned to aegirine-augite, and an opaque phase in a groundmass of the same minerals with a little perovskite and rare poikilitic phlogopite or biotite; scarce fragments of melanite nephelinite occur in agglomerates. Melilite nephelinite lavas have melilite phenocrysts up to 0.5 cm long with nepheline phenocrysts equally abundant or absent. The melanephelinites are characterised by 15-50% phenocrysts of which diopside is predominant with fewer of titanite, apatite and an opaque phase. The groundmass comprises 40-80% pyroxene, an opaque phase and analcime; a brown glass, tiny perovskite granules, poikilitic phlogopite, barkevikite or olivine are present in some rocks. The mugearites generally contain phenocrysts of diopside, oligoclase-andesine and an opaque phase in a groundmass of either glass or abundant plagioclase microlites, an opaque mineral, analcime and carbonate. The pyroclastic parts of the succession include nephelinite agglomerates and tuffs with a maximum thickness of 480 m and a 120 m thick agglomerate on Mfanganu island. A discontinuous limestone horizon outcrops in one area, and water laid sediments are found at a number of places within the volcanic succession. Bestland and Krull (1999) have described the stratigraphy of Rusinga Island in detail and indicate that it includes pyroclastic surge and hydromagmatic pyroclastic deposits. Close to the lavas on the Uyoma Peninsula an intrusion of syenite occurs underlying Ramogi Hill. This occurrence does not appear to have been described but M.J. Le Bas (pers. comm., 1999) found that the intrusion is emplaced amongst Nyanzian basalts and consists principally of coarse-grained syenites of widely differing colour index with much amphibole in places; a sodalite syenite was identified at one locality. It is probably much older (?Precambrian) than the Kisingiri volcano.The deeply eroded central area of the Kisingiri volcano is a low-lying zone of Precambrian basement rocks which is partly fenitized and cut by a number of ijolitic, melilitolitic and carbonatitic intrusions that generally form hills and spurs. In the centre of this zone is the 4 km diameter central caldera of Rangwa. The larger silicate intrusions include the Sagarume, Kiyako, Kiawindu and Rangwa complexes, each of which is described in some detail by Le Bas (1977). Sagarume is an ijolite intrusion of 700x700 m around which is an earlier 50 m wide zone of pyroxenite and micro-ijolite. There is a wide aureole of fenites, in which several zones have been mapped, that has been investigated in terms of mass transfer and volume change during the metasomatism by Rubie (1982), and the role of element speciation in the fenitizing fluids by Rubie and Gunter (1983). The pyroxenite consists of aegirine-augite, up to 10% apatite, and accessory titanite and an opaque phase. Some variants contain wollastonite and in the vicinity of ijolite nepheline and melanite. The ijolites are typically heterogeneous grading into urtites and with pegmatitic varieties and cross-cutting melteigite and ijolite veins. Dykes and veins of nepheline syenite, wollastonite urtite, ijolite and melteigite cut the fenites. Similar rock types are found several kilometres southeast of Sagarume at Omutuma and Kiachuki. Kiyako and Kiawindu are ijolite, some of which contains orthoclase, and foyaite intrusions emplaced in fenites southwest of Rangwa. Aegirine sovite is present within both intrusions as patches and dykes in the ijolite. The Rangwa uncompahgrite (melilitolite) complex lies on the southeastern side of Rangwa immediately outside, and in contact with, the fault bounding the caldera. The earlier and larger part of the intrusion consists of a crescentic body of banded uncompahgrite and turjaite that dips inward towards the caldera. Enveloping this is an outer, near vertical, ring-dyke of ijolite that is also in contact with surrounding fenites. Parallel to the textural banding of the uncompahgrite and turjaite are numerous cone-sheets. The uncompahgrite is a coarse- to medium-grained rock of 60-80% melilite, diopside, perovskite, magnetite, biotite, olivine and alteration products of the melilite. The turjaite is a very coarse-grained rock of melilite, and its alteration products, nephelines up to 2 cm diameter, diopside and mica up to 3 cm in diameter and forming up to 15-20% of the rock. The ijolites of the outer ring-dyke are similar to those of Sagarume and grade into nepheline syenite near parts of the outer margin. After emplacement of the silicate intrusions extensive brecciation of the basement took place accompanied by potassic fenitization and intrusion of potassic trachytes, tuffs, and agglomerates and carbonatites. The carbonatite forms dykes and plug-like bodies southwest and northeast of Rangwa. The dykes seldom exceed 1 m in width, whilst the largest of the plugs, at Nyamgurka northeast of Rangwa, is only about 300 m across. They include sovite, alvikite and ferrocarbonatite. Extrusive tuffs and agglomerates close to the Rukungu vent are up to 50 m thick and contain abundant carbonate, as both carbonatite fragments and a carbonate cement.The Rangwa caldera forms a circular group of hills consisting largely of inward-dipping extrusive agglomerates and tuffs into which breccias, ring dykes of carbonatite, plugs and cone sheets were intruded (Le Bas, 1977). The agglomerates comprise 95% volcanic fragments including nephelinite, melanephelinite and melilitite, together with blocks of basement, mostly fenitized, and intrusive alkaline silicate rocks. In the middle of the succession is a group of bedded and lapilli tuffs which Le Bas (1977) estimates to contain 23-64% carbonate. In the centre of Rangwa is an approximately circular intrusion of carbonatite, although Le Bas (1977) indicates that the boundary between the agglomerates and carbonatite intrusion is not clear cut, and carbonatites in the form of small cone-sheets and radial dykes cut the surrounding pyroclastic rocks. The central carbonatite is built of an outer sovite and carbonatitic breccia, in which the former is given a banded appearance by streaks of phlogopite and apatite; pyrochlore is present. The breccia consists mainly of carbonatite fragments with fewer of melanite ijolite and orthoclasite and is strongly feldspathised. The carbonatite at the centre of the carbonatite intrusion forms an oval body of fine-grained, brown alvikite in which magnetite defines a banding that is concentric about the centre. The carbonatite contains an Fe-bearing calcite, orthoclase, mica and pyrochlore. The central carbonatite also has an associated breccia which comprises well-defined angular fragments of carbonatite. Cone-sheets and dykes of ferrocarbonatite were the last carbonatites emplaced at Rangwa. Whole rock analyses for major elements of all the main rock types will be found in Le Bas (1977, Appendix 2) and Sr and Nd isotopic data for four nephelinites in Norry et al. (1980). C and O isotope data are given by Deines and Gold (1973).