Ti-Nb-Ta oxides in the Anjanabonoina pegmatite, Central Madagascar: A record of magmatic and late-postmagmatic events

The Anjanabonoina rare-element-enriched granitic pegmatite dikes, located about 40 km southwest of Betafo, in central Madagascar, are emplaced in a complex environment characterized by paragneiss, marble and orthogneiss of the Neoproterozoic Itremo Group, of medium metamorphic grade. The subhorizontal bodies of pegmatite are strongly zoned and, in the mineralized area called Sarodivotra, they show a rich paragenetic assemblage in which Ti–Nb–Ta-bearing minerals occur. We recognize two generations of pyrochlore-group and columbite-group minerals. They have a high Nb:Ta ratio, with predominance of Nb- and Ti-dominant phases, and a rare late development of Ta-bearing minerals in fractures. Crystals of the fi rst generation, Nb-dominant (1.20–1.23 apfu), are pyrochlore, whereas those of the second generation, mainly Ti-dominant, are U-rich betafite (0.29–0.57 apfu U) and locally pyrochlore. The primary minerals are typical of an NYF-type pegmatite, and contrast with the LCT-type mineralogy (which includes abundant elbaite–liddicoatite, a pink variety of beryl, spodumene, and hambergite) typical of much of the Anjanabonoina dike. Large crystals of pyrochlore of the fi rst generation are affected by a primary alteration, at high temperature, due to circulation of hydrothermal fl uids, represented by the substitution ACaYO → AY at the core, and by a secondary alteration (at low temperature) along fractures and at the rim, during which the vacancies increased in the A and Y sites. Crystals of the second generation are even more intensely altered. The columbite-group mineral, also formed in two generations and by exsolution in pyrochlore, is fersmite. Rarely, fersmite replaces pyrochlore in the second-generation crystals. We use pyrochlore- and columbite-group minerals to monitor magmatic evolution and postmagmatic events related to circulation of the hydrothermal fl uids and weathering-type reactions