EVOLUTION OF NB,TA-OXIDE MINERALS IN THE PRASIVA GRANITIC PEGMATITES,SLOVAKIA - 1 - PRIMARY FE,TI-RICH ASSEMBLAGE

Accessory Nb,Ta-oxide minerals occur in two narrow veins of relatively poorly fractionated granitic pegmatite that crosscut the parent Hercy nian pluton, the Prasiva biotite granodiorite-granite, in central Slov akia. High Ti contents are typical of all members of this magmatic sui te, and of their relevant minerals. In the pegmatites, niobian to tant alian rutile (less than or equal to 24 wt.% Nb2O5, less than or equal to 30 wt.% Ta2O5) and ferrocolumbite are most widespread. Ferrocolumbi te (to manganocolumbite) contains 2.5 to 5 wt.% TiO2, and titanian ixi olite has 11 to 21 wt.% TiO2. Tiny lamellae of niobian-tantalian armal colite to pseudobrookite (6 to 10 wt.% Nb2O5, 0.5 to 5.5 wt.% Ta2O5) e xsolved from the rutile matrix; complex exsolution-induced domains of niobian ilmenite rimming intergrowths of ilmenite and titanian hematit e are rare. Exsolution of armalcolite - pseudobrookite probably took p lace at magmatic temperatures, and its subsolidus survival is possibly metastable.