The Ambatofinandrahana intrusive complex consists of an intricate association of gabbroïc, syenitic and granitic rocks characterized by their potassic to sodipotassic affinities. It belongs to the alkaline cafemic varieties, folIowing Debon and al. classification (1988). Syenites are bearing sodic ferromagnesian mineraI phases along with REE-rich mineraIs such as monazite and titanite. Field relationships indicate that syenites are subsequent to the gabbro. The REE distribution suggests that the syenites are cogenetic, considering in particular the high amount of LREE, and a REE spectrum devoid of the Eu anomaly . Besides, the granite seems to belong to a different magmatic Iineage. Absolute dating of the syenites by the Rb / Sr isochron method gives an age of 455 m.y. Therefore these syenitic intrusions represent the last magmatic activity in Madagascar before those of the cretaceous period, in the NW of the country. According to the Sr, these syenites provide evidence of a crustal contribution in their genesis. Within alkaline bodies, siliceous veins as weIl as eluvial deposits are bearing some REE-mineraIlizations (bastnaesite, monazite and chevkinite) which are not of the pegmatitic type, but are instead resuIting from a kind ofhydrothermal process. The chemistry of the REE bearing mineraIs demonstrates an alkaline source for the mineralization. Indeed, in the se mineraIs, REE are differentiaIly fractionated according to the geochemical environments in which they were formed ..
