Usaki

The Usaki ijolite complex lies immediately south of, and in contact with, the Nyamaji complex (No. 085-00-041) and has maximum dimensions of 4x2 km, but is somewhat irregular in outline because it is partly obscured by recent deposits. The earliest igneous rocks are pyroxenites and melteigites that are preserved as xenoliths up to 50 m across in all the later rocks. Micro-ijolites were then intruded and these in turn cut by ijolite, which occupies the greater part of the central area. The ijolite is cut by three stocks of wollastonite urtite, the whole complex being surrounded by a broad fenite aureole. This order of emplacement, as determined by Le Bas (1977), is contrary to that proposed by Pulfrey (1950). The ijolite has extensive mineral banding which takes the form of schlieren of fenite, rhythmic banding, intrusions of urtite and veins with comb structure. The urtite is also banded but on a lesser scale. The pyroxenite blocks consist of aegirine-augite, apatite and nepheline with accessory perovskite, titanite, biotite and melanite. These rocks with increasing nepheline grade into melteigites, but the nepheline appears to be a metasomatic addition from the enveloping rocks (Pulfrey, 1950; Le Bas, 1977). The micro-ijolite and ijolite consist of about 50% nepheline, 25% aegirine-augite and 15% melanite with a little wollastonite, particularly in the ijolite adjacent to the wollastonite ijolite intrusions, and accessory apatite, titanite, biotite and, in the micro-ijolite, K-feldspar. Pulfrey (1950) illustrates an account of the textures of the ijolites with numerous drawings and Le Bas et al. (1977) describe sodic and potassic glassy inclusions in apatite in a pegmatitic ijolite. The urtites vary from rocks with >80% nepheline to varieties with 70% nepheline and 24% wollastonite; pyroxene and melanite are the other principal minerals. Le Bas (1977) distinguishes five fenite zones at Usaki but considers that they were formed ahead of the emplacement of the Usaki intrusions because they are found as xenoliths in the micro-ijolite. He also suggests that part of the micro-ijolite represents rheomorphic ijolitic fenite. The fenite zones grade into each other and distinctions are based on mineralogy. Zone I fenites are only marginally metamorphosed but Zone II is characterised by absence of quartz. Zone III contains more aegirine than II together with clear K-feldspar, Zone IV includes nepheline and Zone V fenites are micro-ijolites. Rock analyses of all rock types will be found in Le Bas (1977, Appendix 2) and rock and some mineral analyses are given by Pulfrey (1950). Sr, Nd and Pb isotope data for two ijolites are given by Kalt et al. (1997). Le Bas and Aspden (1981) describe fluid inclusions in apatite from ijolites.