Napak

The Napak volcano holds an important place in carbonatite studies as it was the first locality at which the relationship between intrusive carbonatite and volcanism was first demonstrated (King, 1949). The complex comprises two distinct parts: a central complex of carbonatite, ijolite and fenite and the remnants of a volcanic cone. The surface of the basement gneisses is domed across the volcanic centre. The volcanic rocks now probably comprise less than 10% of the original cone, which is estimated to have had a volume in excess of 1000 km3, a diameter of more than 30 km and to have reached a height in excess of 3000 m. The remaining volcanic rocks form an arc to the east and south of the central intrusive complex and a smaller remnant to the southwest. There are steep, inward-facing slopes and gentle outer slopes approximately parallel to the layering. The central complex, which erosion has completely isolated from the volcanic rocks, consists of a small central knoll of carbonatite, known as Moruangaberu, which is surrounded by an annular ridge of ijolite, called Lokupoi, which is only breached in the northwest. The ijolite complex includes pyroxenite, melteigite, ijolite and urtite and is characterised by great structural and textural heterogeneity (see Trendall. 1965a, Plate 2 and King and Sutherland, 1966, Figs 15 and 16). Pyroxenite invariably appears to be the earliest phase, forming xenolithic masses that often pass outwards into melteigite. Ijolite surrounds and invades the pyroxenite-melteigite, forms banded rocks of differing colour index and also pegmatitic veins. All these rocks are net veined by urtite and consist principally of pyroxene and nepheline, the former varying from colourless diopside in the pyroxenites to green-margined aegirine-augite in the melteigites and ijolites to varieties even richer in aegirine molecule in the later veins. An orange-brown biotite is found in some pyroxenites and an Fe-Ti oxide phase is ubiquitous in the melanocratic rocks; perovskite, apatite and titanite are accessory. Melanite is locally abundant, as is wollastonite, particularly in the urtites, while cancrinite, pectolite, calcite and zeolites are widespread secondary minerals. Feldspar-bearing rocks form late dykes and veins up to 0.5 m wide and contain K-feldspar and aegirine or cancrinite. There are also zones of nepheline syenite which King and Sutherland (1966) interpret as having formed by partial replacement of nepheline in ijolite by large plates of K-feldspar. On the eastern flank of Lokupoi is a mass of turjaite consisting of phenocrysts of biotite, phenocrystal and groundmass diopsidic pyroxene, nepheline and melilite together with accessory Fe-Ti oxides, perovskite and rare melanite. Carbonatite forms a 400 m diameter plug from which dykes extend into the encircling ijolite. The main plug is concentrically and vertically banded and is essentially sovite but dolomitic varieties are also present (King, 1949). Pyrochlore, apatite, ilmenite and biotite have been found in insoluble residues (Trendall, 1965a). The fenites which envelop the ijolite-carbonatite plug are sheared quartz-feldspar gneisses and granulites which are heavily fractured, the fractures being filled principally with aegirine and/or sodic amphibole. Titanite, magnetite and an orange-brown biotite are developed and the original feldspar is replaced by new Na- and K-feldspar: all the original quartz is replaced. Over 90% of the remnants of the volcanic cone are agglomerates with perhaps 3% of lavas and the rest of mixed sediments and tuffs. The agglomerates consist of blocks of lava generally 4-5 cm in diameter, but up to several metres, with at some horizons basement complex rocks, ijolites and fragments of sediments, set in a matrix of crystal fragments and finer-grained pyrochlastic material and secondary calcite and zeolite. The lavas are predominantly melanephelinite and nephelinite consisting of diopside zoned to aegirine-augite, which commonly forms phenocrysts, and euhedral nepheline, the rock name depending on which phase is predominant (King, 1949). Magnetite, rarer ilmenite, in a few lavas biotite and in others a blue-green amphibole occur; perovskite and apatite are rare. Zeolite- and sometimes calcite-filled amygdales are of frequent occurrence. Olivine melanephelinite constitutes 17% of the lavas but olivine nephelinite only 1% according to King (1949); the olivine always forms phenocrysts and in these rocks the pyroxene is always diopsidic. Melilite nephelinite and olivine-melilite nephelinite are uncommon. At the base of the volcanic succession in the northern part is a flow of mugearite consisting of andesine (Ab65), iron oxides and chlorite. Sediments and tuffs in the succession include a basal gravel and bedded tuffs that are thought to have been deposited in temporary lakes formed behind lava dams. Limestones are also found at several localities, the origins of which are not, perhaps, clear (King, 1949; Trendall, 1965a). A reconstruction of the volcanic events at Napak is explored in some detail by Trendall (1965a). Analyses of lavas will be found in King (1949) and King and Sutherland (1966), of ijolitic rocks in King (1965) and of carbonatites in King (1949). A carbonatite analysis with trace element data, including REE and Sr and Nd isotopes, is in Nelson et al. (1988) and data for U, Th and Pb isotopes of a carbonatite are presented by Lancelot and Allegre (1974), and oxygen and carbon isotopes by Denaeyer (1970) and Pineau et al. (1973). Whole rock analyses for eleven nephelinites and Nd, Pb and Sr isotopic data for the same rocks, two carbonatites and for nepheline and pyroxene from an ijolite are given, and discussed in detail, by Simonetti and Bell (1994). Numerous analyses of pyroxenes from the central ijolite complex will be found in Tyler and King (1967). Simonetti et al. (1996) demonstrate that there are populations of chromian and titanium-bearing aluminian diopside phenocrysts in the nephelinite lavas, it having been shown earlier (Simonetti and Bell, 1993), by a study of the Pb, Sr and Nd isotopes of the same pyroxenes, that they are not in isotopic equilibrium with their host rocks and have a complex evolutionary history.