ULTRAMAFIC XENOLITHS AND MEGACRYSTS FROM A MELILITITE TUFF CONE, DEETI, NORTHERN TANZANIA

Deeti, a similar to 50 m high 'ubehebe' tuff ring is situated in a sma ll field of similar tuff cones of Quaternary age, next to the two neph elinite-carbonatite volcanoes of Kerimasi and Oldoinyo Lengai, south o f Lake Natron. The tuff cones, lying on small parallel faults, may hav e been closely associated and possibly triggered by the same events th at have given rise to the volcanic activity at Oldoinyo Lengai and Ker imasi. The host extrusive at Deeti is a melilitite (SiO2 37.20%, MgO 1 5.0%, TiO2 3.93%, CaO 9.26%, K2O 2.76% and Mg-66.4); it is mica porphy ritic and contains ijolitic xenolith fragments. The upper part of the cone is formed of spectacular carbonate-cemented, coarse bedded deposi ts of cored, golf ball size lapilli. A distinctive megacryst suite is composed of phlogopitic mica (FeO 9.5%, TiO2 4.8%, Mg-80), pargasitic amphibole (Mg-71) and diopsidic clinopyroxene (Mg-80). Numerous ultram afic xenoliths dominated by amphibole-mica peridotites and pyroxenites , form the cores of larger lapilli and exist as bombs up to 30 cm acro ss. This amphibole is chromian-pargasite, with very rare relies of ric hterite (Mg-88). The xenoliths show abundant evidence of multiple vein ing, overgrowths and substantial fabric modification. On the basis of electron microprobe data, we show that these petrographic textures pro bably developed as a result of metasomatism by alkaline silicate, and possibly carbonatite melts. The original protoliths include more primi tive spinel peridotites (Fo(>88)) that have been significantly Fe-enri ched. The lava has sampled upper mantle wall rock to depths of origin of the melilitite (> 60 km) and these xenoliths may constrain possible mantle source compositions for the adjacent larger carbonatite volcan oes. (C) 1997 Elsevier Science Limited.