Home – Home – Jornals – Russian Geology and Geophysics 2003 number 6

Alkali granites with abnormally high contents of Na-pyroxenes and Na-amphiboles (rockallite, lindinosite, etc.) have been found in deeply differentiated alkaline complexes all over the world. Dikes of rare-metal taxitic or foliated aegirine-riebeckite granites developed in the Ampasindava province of Madagascar are called fasibitikites. These are intricate obliquely cutting veined bodies, each consisting of alternating interbeds parallel to the plane of contact and made up of pegmatoid riebeckite granites and fine-grained quartz-aegirine rock poor in feldspar. The thickness of ultimately veined fasibitikite bodies varies from 20 cm to 2.5 m. The proximal veins are grouped into veined series, which form a 300 m wide and 2 km long zone. Fasibitikites have commercially significant contents of Ta, Nb, Zr, and REE; the average contents are as follows (%): Ta₂O₅ - 0.037, Nb₂O₅ - 0.34, ZrO₂- 2.31, and REE₂O₃ - 0.6%. The aegirine granites have extremely inhomogeneous composition, structure, and texture and are ultimately enriched in rare-metal minerals such as eudialyte, zircon, pyrochlore, and chevkinite. The content of commercially important components in them is 1.5-2 times higher than that in the riebeckite varieties. The REE pattern of fasibitikites shows the maximum enrichment in light lanthanides and a serious Eu deficit.
The extremely high content of modal silica in fasibitikites is inconsistent with the hypothesis of their fenite nature, though these rocks are likely to be related to an undersaturated foid source. Obviously, they resulted from a residual Si-oversaturated melt rich in superstoichiometric Na and incompatible Fe, Zr, Ti, Nb, Ta, and Y with participation of the fluid phase. By analogy with the foliated aplite-pegmatite bodies, we suggest that the interbeds of pegmatoid riebeckite granites crystallized through separation of melt portions enriched in volatiles. Crystallization from disequilibrated overcooled melt with possible precrystallization segregation is not ruled out either.